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The polarizable continuum model ( PCM ) is a commonly used method in computational chemistry to model solvation effects. When it is necessary to consider each solvent molecule as a separate molecule, the computational cost of modeling a solvent-mediated chemical reaction becomes prohibitively high. Modeling the solvent as a polarizable continuum, rather than individual molecules, makes ab initio computation more readily achievable. Two types of PCMs have been popularly used: the dielectric PCM (D-PCM), in which the continuum is polarizable (see dielectrics ), and the conductor-like PCM (C-PCM), in which the continuum is conductor-like, similar to the COSMO Solvation Model . [ 1 ] [ 2 ]
The molecular free energy of solvation is computed as the sum of three terms:
The Charge-transfer effect is also considered as a part of solvation in cases. [ 1 ]
The PCM solvation model is available for calculating energies and gradients at the Hartree–Fock and density functional theory (DFT) levels in several quantum chemical computational packages such as Gaussian , GAMESS [ 3 ] and JDFTx .
The authors of a 2002 paper observe that PCM has limitations where non-electrostatic effects dominate the solute-solvent interactions. They write in the abstract: "Since only electrostatic solute-solvent interactions are included in the PCM, our results lead to the conclusion that, for the seven molecules studied, in cyclohexane , acetone , methanol , and acetonitrile electrostatic effects are dominant while in carbon tetrachloride , benzene , and chloroform other nonelectrostatic effects are more important." [ 4 ]
There is an integral equation formalism (IEF) version of the PCM which is very commonly used. [ 5 ]
PCM is also used to model outer solvation layers in multi-layered solvation approach. [ 6 ] | https://en.wikipedia.org/wiki/Polarizable_continuum_model |
In classical electromagnetism , polarization density (or electric polarization , or simply polarization ) is the vector field that expresses the volumetric density of permanent or induced electric dipole moments in a dielectric material. When a dielectric is placed in an external electric field , its molecules gain electric dipole moment and the dielectric is said to be polarized.
Electric polarization of a given dielectric material sample is defined as the quotient of electric dipole moment (a vector quantity, expressed as coulombs *meters (C*m) in SI units ) to volume (meters cubed). [ 1 ] [ 2 ] Polarization density is denoted mathematically by P ; [ 2 ] in SI units, it is expressed in coulombs per square meter (C/m 2 ).
Polarization density also describes how a material responds to an applied electric field as well as the way the material changes the electric field, and can be used to calculate the forces that result from those interactions. It can be compared to magnetization , which is the measure of the corresponding response of a material to a magnetic field in magnetism .
Similar to ferromagnets , which have a non-zero permanent magnetization even if no external magnetic field is applied, ferroelectric materials have a non-zero polarization in the absence of external electric field.
An external electric field that is applied to a dielectric material, causes a displacement of bound charged elements.
A bound charge is a charge that is associated with an atom or molecule within a material. It is called "bound" because it is not free to move within the material like free charges . Positive charged elements are displaced in the direction of the field, and negative charged elements are displaced opposite to the direction of the field. The molecules may remain neutral in charge, yet an electric dipole moment forms. [ 3 ] [ 4 ]
For a certain volume element Δ V {\displaystyle \Delta V} in the material, which carries a dipole moment Δ p {\displaystyle \Delta \mathbf {p} } , we define the polarization density P : P = Δ p Δ V {\displaystyle \mathbf {P} ={\frac {\Delta \mathbf {p} }{\Delta V}}}
In general, the dipole moment Δ p {\displaystyle \Delta \mathbf {p} } changes from point to point within the dielectric. Hence, the polarization density P of a dielectric inside an infinitesimal volume d V with an infinitesimal dipole moment d p is:
The net charge appearing as a result of polarization is called bound charge and denoted Q b {\displaystyle Q_{b}} .
This definition of polarization density as a "dipole moment per unit volume" is widely adopted, though in some cases it can lead to ambiguities and paradoxes. [ 5 ]
Let a volume d V be isolated inside the dielectric. Due to polarization the positive bound charge d q b + {\displaystyle \mathrm {d} q_{b}^{+}} will be displaced a distance d {\displaystyle \mathbf {d} } relative to the negative bound charge d q b − {\displaystyle \mathrm {d} q_{b}^{-}} , giving rise to a dipole moment d p = d q b d {\displaystyle \mathrm {d} \mathbf {p} =\mathrm {d} q_{b}\mathbf {d} } . Substitution of this expression in (1) yields P = d q b d V d {\displaystyle \mathbf {P} ={\mathrm {d} q_{b} \over \mathrm {d} V}\mathbf {d} }
Since the charge d q b {\displaystyle \mathrm {d} q_{b}} bounded in the volume d V is equal to ρ b d V {\displaystyle \rho _{b}\mathrm {d} V} the equation for P becomes: [ 3 ]
where ρ b {\displaystyle \rho _{b}} is the density of the bound charge in the volume under consideration. It is clear from the definition above that the dipoles are overall neutral and thus ρ b {\displaystyle \rho _{b}} is balanced by an equal density of opposite charges within the volume. Charges that are not balanced are part of the free charge discussed below.
For a given volume V enclosed by a surface S , the bound charge Q b {\displaystyle Q_{b}} inside it is equal to the flux of P through S taken with the negative sign, or
Let a surface area S envelope part of a dielectric. Upon polarization negative and positive bound charges will be displaced. Let d 1 and d 2 be the distances of the bound charges d q b − {\displaystyle \mathrm {d} q_{b}^{-}} and d q b + {\displaystyle \mathrm {d} q_{b}^{+}} , respectively, from the plane formed by the element of area d A after the polarization. And let d V 1 and d V 2 be the volumes enclosed below and above the area d A .
It follows that the negative bound charge d q b − = ρ b − d V 1 = ρ b − d 1 d A {\displaystyle \mathrm {d} q_{b}^{-}=\rho _{b}^{-}\ \mathrm {d} V_{1}=\rho _{b}^{-}d_{1}\ \mathrm {d} A} moved from the outer part of the surface d A inwards, while the positive bound charge d q b + = ρ b d V 2 = ρ b d 2 d A {\displaystyle \mathrm {d} q_{b}^{+}=\rho _{b}\ \mathrm {d} V_{2}=\rho _{b}d_{2}\ \mathrm {d} A} moved from the inner part of the surface outwards.
By the law of conservation of charge the total bound charge d Q b {\displaystyle \mathrm {d} Q_{b}} left inside the volume d V {\displaystyle \mathrm {d} V} after polarization is:
d Q b = d q in − d q out = d q b − − d q b + = ρ b − d 1 d A − ρ b d 2 d A {\displaystyle {\begin{aligned}\mathrm {d} Q_{b}&=\mathrm {d} q_{\text{in}}-\mathrm {d} q_{\text{out}}\\&=\mathrm {d} q_{b}^{-}-\mathrm {d} q_{b}^{+}\\&=\rho _{b}^{-}d_{1}\ \mathrm {d} A-\rho _{b}d_{2}\ \mathrm {d} A\end{aligned}}}
Since ρ b − = − ρ b {\displaystyle \rho _{b}^{-}=-\rho _{b}} and (see image to the right) d 1 = ( d − a ) cos ( θ ) d 2 = a cos ( θ ) {\displaystyle {\begin{aligned}d_{1}&=(d-a)\cos(\theta )\\d_{2}&=a\cos(\theta )\end{aligned}}}
The above equation becomes d Q b = − ρ b ( d − a ) cos ( θ ) d A − ρ b a cos ( θ ) d A = − ρ b d d A cos ( θ ) {\displaystyle {\begin{aligned}\mathrm {d} Q_{b}&=-\rho _{b}(d-a)\cos(\theta )\ \mathrm {d} A-\rho _{b}a\cos(\theta )\ \mathrm {d} A\\&=-\rho _{b}d\ \mathrm {d} A\cos(\theta )\end{aligned}}}
By ( 2 ) it follows that ρ b d = P {\displaystyle \rho _{b}d=P} , so we get: d Q b = − P d A cos ( θ ) − d Q b = P ⋅ d A {\displaystyle {\begin{aligned}\mathrm {d} Q_{b}&=-P\ \mathrm {d} A\cos(\theta )\\-\mathrm {d} Q_{b}&=\mathbf {P} \cdot \mathrm {d} \mathbf {A} \end{aligned}}}
And by integrating this equation over the entire closed surface S we find that
which completes the proof.
By the divergence theorem, Gauss's law for the field P can be stated in differential form as: − ρ b = ∇ ⋅ P , {\displaystyle -\rho _{b}=\nabla \cdot \mathbf {P} ,} where ∇ · P is the divergence of the field P through a given surface containing the bound charge density ρ b {\displaystyle \rho _{b}} .
By the divergence theorem we have that − Q b = ∭ V ∇ ⋅ P d V , {\displaystyle -Q_{b}=\iiint _{V}\nabla \cdot \mathbf {P} \ \mathrm {d} V,} for the volume V containing the bound charge Q b {\displaystyle Q_{b}} . And since Q b {\displaystyle Q_{b}} is the integral of the bound charge density ρ b {\displaystyle \rho _{b}} taken over the entire volume V enclosed by S , the above equation yields − ∭ V ρ b d V = ∭ V ∇ ⋅ P d V , {\displaystyle -\iiint _{V}\rho _{b}\ \mathrm {d} V=\iiint _{V}\nabla \cdot \mathbf {P} \ \mathrm {d} V,} which is true if and only if − ρ b = ∇ ⋅ P {\displaystyle -\rho _{b}=\nabla \cdot \mathbf {P} }
In a homogeneous , linear, non-dispersive and isotropic dielectric medium, the polarization is aligned with and proportional to the electric field E : [ 7 ] P = χ ε 0 E , {\displaystyle \mathbf {P} =\chi \varepsilon _{0}\mathbf {E} ,}
where ε 0 is the electric constant , and χ is the electric susceptibility of the medium. Note that in this case χ simplifies to a scalar, although more generally it is a tensor . This is a particular case due to the isotropy of the dielectric.
Taking into account this relation between P and E , equation ( 3 ) becomes: [ 3 ]
The expression in the integral is Gauss's law for the field E which yields the total charge, both free ( Q f ) {\displaystyle (Q_{f})} and bound ( Q b ) {\displaystyle (Q_{b})} , in the volume V enclosed by S . [ 3 ] Therefore,
− Q b = χ Q total = χ ( Q f + Q b ) ⇒ Q b = − χ 1 + χ Q f , {\displaystyle {\begin{aligned}-Q_{b}&=\chi Q_{\text{total}}\\&=\chi \left(Q_{f}+Q_{b}\right)\\[3pt]\Rightarrow Q_{b}&=-{\frac {\chi }{1+\chi }}Q_{f},\end{aligned}}}
which can be written in terms of free charge and bound charge densities (by considering the relationship between the charges, their volume charge densities and the given volume): ρ b = − χ 1 + χ ρ f {\displaystyle \rho _{b}=-{\frac {\chi }{1+\chi }}\rho _{f}}
Since within a homogeneous dielectric there can be no free charges ( ρ f = 0 ) {\displaystyle (\rho _{f}=0)} , by the last equation it follows that there is no bulk bound charge in the material ( ρ b = 0 ) {\displaystyle (\rho _{b}=0)} . And since free charges can get as close to the dielectric as to its topmost surface, it follows that polarization only gives rise to surface bound charge density (denoted σ b {\displaystyle \sigma _{b}} to avoid ambiguity with the volume bound charge density ρ b {\displaystyle \rho _{b}} ). [ 3 ]
σ b {\displaystyle \sigma _{b}} may be related to P by the following equation: [ 8 ] σ b = n ^ out ⋅ P {\displaystyle \sigma _{b}=\mathbf {\hat {n}} _{\text{out}}\cdot \mathbf {P} } where n ^ out {\displaystyle \mathbf {\hat {n}} _{\text{out}}} is the normal vector to the surface S pointing outwards. (see charge density for the rigorous proof)
The class of dielectrics where the polarization density and the electric field are not in the same direction are known as anisotropic materials.
In such materials, the i -th component of the polarization is related to the j -th component of the electric field according to: [ 7 ]
P i = ∑ j ε 0 χ i j E j , {\displaystyle P_{i}=\sum _{j}\varepsilon _{0}\chi _{ij}E_{j},}
This relation shows, for example, that a material can polarize in the x direction by applying a field in the z direction, and so on. The case of an anisotropic dielectric medium is described by the field of crystal optics .
As in most electromagnetism, this relation deals with macroscopic averages of the fields and dipole density, so that one has a continuum approximation of the dielectric materials that neglects atomic-scale behaviors. The polarizability of individual particles in the medium can be related to the average susceptibility and polarization density by the Clausius–Mossotti relation .
In general, the susceptibility is a function of the frequency ω of the applied field. When the field is an arbitrary function of time t , the polarization is a convolution of the Fourier transform of χ ( ω ) with the E ( t ) . This reflects the fact that the dipoles in the material cannot respond instantaneously to the applied field, and causality considerations lead to the Kramers–Kronig relations .
If the polarization P is not linearly proportional to the electric field E , the medium is termed nonlinear and is described by the field of nonlinear optics . To a good approximation (for sufficiently weak fields, assuming no permanent dipole moments are present), P is usually given by a Taylor series in E whose coefficients are the nonlinear susceptibilities:
P i ε 0 = ∑ j χ i j ( 1 ) E j + ∑ j k χ i j k ( 2 ) E j E k + ∑ j k ℓ χ i j k ℓ ( 3 ) E j E k E ℓ + ⋯ {\displaystyle {\frac {P_{i}}{\varepsilon _{0}}}=\sum _{j}\chi _{ij}^{(1)}E_{j}+\sum _{jk}\chi _{ijk}^{(2)}E_{j}E_{k}+\sum _{jk\ell }\chi _{ijk\ell }^{(3)}E_{j}E_{k}E_{\ell }+\cdots }
where χ ( 1 ) {\displaystyle \chi ^{(1)}} is the linear susceptibility, χ ( 2 ) {\displaystyle \chi ^{(2)}} is the second-order susceptibility (describing phenomena such as the Pockels effect , optical rectification and second-harmonic generation ), and χ ( 3 ) {\displaystyle \chi ^{(3)}} is the third-order susceptibility (describing third-order effects such as the Kerr effect and electric field-induced optical rectification).
In ferroelectric materials, there is no one-to-one correspondence between P and E at all because of hysteresis .
The behavior of electric fields ( E , D ), magnetic fields ( B , H ), charge density ( ρ ) and current density ( J ) are described by Maxwell's equations in matter .
In terms of volume charge densities, the free charge density ρ f {\displaystyle \rho _{f}} is given by
ρ f = ρ − ρ b {\displaystyle \rho _{f}=\rho -\rho _{b}}
where ρ {\displaystyle \rho } is the total charge density. By considering the relationship of each of the terms of the above equation to the divergence of their corresponding fields (of the electric displacement field D , E and P in that order), this can be written as: [ 9 ]
D = ε 0 E + P . {\displaystyle \mathbf {D} =\varepsilon _{0}\mathbf {E} +\mathbf {P} .}
This is known as the constitutive equation for electric fields. Here ε 0 is the electric permittivity of empty space. In this equation, P is the (negative of the) field induced in the material when the "fixed" charges, the dipoles, shift in response to the total underlying field E , whereas D is the field due to the remaining charges, known as "free" charges. [ 5 ] [ 10 ]
In general, P varies as a function of E depending on the medium, as described later in the article. In many problems, it is more convenient to work with D and the free charges than with E and the total charge. [ 1 ]
Therefore, a polarized medium, by way of Green's theorem can be split into four components.
When the polarization density changes with time, the time-dependent bound-charge density creates a polarization current density of
J p = ∂ P ∂ t {\displaystyle \mathbf {J} _{p}={\frac {\partial \mathbf {P} }{\partial t}}}
so that the total current density that enters Maxwell's equations is given by
J = J f + ∇ × M + ∂ P ∂ t {\displaystyle \mathbf {J} =\mathbf {J} _{f}+\nabla \times \mathbf {M} +{\frac {\partial \mathbf {P} }{\partial t}}}
where J f is the free-charge current density, and the second term is the magnetization current density (also called the bound current density ), a contribution from atomic-scale magnetic dipoles (when they are present).
In a simple approach the polarization inside a solid is not, in general, uniquely defined. Because a bulk solid is periodic, one must choose a unit cell in which to compute the polarization (see figure). [ 11 ] [ 12 ] In other words, two people, Alice and Bob, looking at the same solid, may calculate different values of P , and neither of them will be wrong. For example, if Alice chooses a unit cell with positive ions at the top and Bob chooses the unit cell with negative ions at the top, their computed P vectors will have opposite directions. Alice and Bob will agree on the microscopic electric field E in the solid, but disagree on the value of the displacement field D = ε 0 E + P {\displaystyle \mathbf {D} =\varepsilon _{0}\mathbf {E} +\mathbf {P} } .
Even though the value of P is not uniquely defined in a bulk solid, variations in P are uniquely defined. [ 11 ] If the crystal is gradually changed from one structure to another, there will be a current inside each unit cell, due to the motion of nuclei and electrons. This current results in a macroscopic transfer of charge from one side of the crystal to the other, and therefore it can be measured with an ammeter (like any other current) when wires are attached to the opposite sides of the crystal. The time-integral of the current is proportional to the change in P . The current can be calculated in computer simulations (such as density functional theory ); the formula for the integrated current turns out to be a type of Berry's phase . [ 11 ]
The non-uniqueness of P is not problematic, because every measurable consequence of P is in fact a consequence of a continuous change in P . [ 11 ] For example, when a material is put in an electric field E , which ramps up from zero to a finite value, the material's electronic and ionic positions slightly shift. This changes P , and the result is electric susceptibility (and hence permittivity ). As another example, when some crystals are heated, their electronic and ionic positions slightly shift, changing P . The result is pyroelectricity . In all cases, the properties of interest are associated with a change in P .
In what is now called the modern theory of polarization , the polarization is defined as a difference. Any structure which has inversion symmetry has zero polarization; there is an identical distribution of positive and negative charges about an inversion center. If the material deforms there can be a polarization due to the charge in the charge distribution. [ 12 ]
Another problem in the definition of P is related to the arbitrary choice of the "unit volume", or more precisely to the system's scale . [ 5 ] For example, at microscopic scale a plasma can be regarded as a gas of free charges, thus P should be zero. On the contrary, at a macroscopic scale the same plasma can be described as a continuous medium, exhibiting a permittivity ε ( ω ) ≠ 1 {\displaystyle \varepsilon (\omega )\neq 1} and thus a net polarization P ≠ 0 . | https://en.wikipedia.org/wiki/Polarization_density |
Polarization gradient cooling (PG cooling), or Sisyphus cooling, is a technique in laser cooling of atoms by dampening the motion of the trapped particles via photon momentum. It was proposed to explain the experimental observation of cooling below the Doppler limit [ 1 ] observed in cesium atom-related laser cooling experiments in 1985. Shortly after the theory was introduced, experiments were performed that verified the theoretical predictions. [ 2 ] While Doppler cooling allows atoms to be cooled to hundreds of microkelvin, PG cooling allows atoms to be cooled to a few microkelvin or less. [ 3 ] [ 4 ]
True to its name, PG cooling involves the use of a polarization gradient typically generated by the superposition of two counter propagating beams of light with orthogonal polarizations . This creates a gradient where the polarization varies in space, with the gradient depending on which type of polarization is used. Orthogonal linear polarizations (the lin⊥lin configuration) results in the polarization varying between linear and circular polarization in the range of half a wavelength. However, if orthogonal circular polarizations (the σ + σ − configuration) are used, the result is a linear polarization that rotates along the axis of propagation. Both configurations can be used for cooling and yield similar results, however, the physical mechanisms involved are very different. For the lin⊥lin case, the polarization gradient causes periodic light shifts in Zeeman sublevels of the atomic ground state that allows for a Sisyphus effect to occur. In the σ + -σ − configuration, the rotating polarization creates a motion-induced population imbalance in the Zeeman sublevels of the atomic ground state, resulting in an imbalance in the radiation pressure that opposes the motion of the atom. Both configurations achieve sub-Doppler cooling and instead reach the recoil limit . While the limit of PG cooling is lower than that of Doppler cooling, the capture range of PG cooling is lower and thus an atomic gas must be pre-cooled before PG cooling.
When laser cooling of atoms was first proposed in 1975, the only cooling mechanism considered was Doppler cooling. [ 5 ] As such the limit on the temperature was predicted to be the Doppler limit: [ 6 ]
k B T = ℏ Γ 2 {\displaystyle k_{B}T={\frac {\hbar \Gamma }{2}}}
Here k b is the Boltzmann constant , T is the temperature of the atoms, and Γ is the inverse of the excited state's radiative lifetime.
Early experiments seemed to be in agreement with this limit, and it was understood to be the main method of laser cooling atoms. [ 7 ] However, in 1988 experiments began to report temperatures below the Doppler limit. [ 1 ] These observations would take the theory of PG cooling to explain.
There are two different configurations that form polarization gradients: lin⊥lin and σ + σ − . Both configurations provide cooling, but the type of polarization gradient and the physical mechanism for cooling are different between the two.
In the lin⊥lin configuration cooling is achieved via a Sisyphus effect. Consider two counterpropagating electromagnetic plane waves with equal amplitude and orthogonal linear polarizations E 1 → = E 0 e i k z x ^ {\displaystyle {\vec {E_{1}}}=E_{0}e^{ikz}{\hat {x}}} and E 2 → = E 0 e − i k z y ^ {\displaystyle {\vec {E_{2}}}=E_{0}e^{-ikz}{\hat {y}}} , where k is the wavenumber k = 2 π λ {\displaystyle k=\textstyle {\frac {2\pi }{\lambda }}} . The superposition of E 1 → {\displaystyle {\vec {E_{1}}}} and E 2 → {\displaystyle {\vec {E_{2}}}} is given as:
E → t o t = E 0 2 ( cos ( k z ) x ^ + y ^ 2 − i sin ( k z ) − x ^ + y ^ 2 ) {\displaystyle {\vec {E}}_{tot}={\frac {E_{0}}{\sqrt {2}}}\left(\cos(kz){\frac {{\hat {x}}+{\hat {y}}}{\sqrt {2}}}-i\sin(kz){\frac {-{\hat {x}}+{\hat {y}}}{\sqrt {2}}}\right)}
Introducing a new pair of coordinates x ^ ′ = x ^ + y ^ 2 {\displaystyle {\hat {x}}'=\textstyle {\frac {{\hat {x}}+{\hat {y}}}{\sqrt {2}}}} and y ^ ′ = − x ^ + y ^ 2 {\displaystyle {\hat {y}}'=\textstyle {\frac {-{\hat {x}}+{\hat {y}}}{\sqrt {2}}}} the field can be written as:
E → t o t = E 0 2 ( cos ( k z ) x ^ ′ − i sin ( k z ) y ^ ′ ) {\displaystyle {\vec {E}}_{tot}={\frac {E_{0}}{\sqrt {2}}}\left(\cos(kz){\hat {x}}'-i\sin(kz){\hat {y}}'\right)}
The polarization of the total field changes with z. For example: we see that at z = 0 {\displaystyle z=0} the field is linearly polarized along x ^ ′ {\displaystyle {\hat {x}}'} , at z = λ 8 {\displaystyle z=\textstyle {\frac {\lambda }{8}}} the field has left circular polarization, at z = λ 4 {\displaystyle z=\textstyle {\frac {\lambda }{4}}} the field is linearly polarized along y ^ ′ {\displaystyle {\hat {y}}'} , at z = 3 λ 8 {\displaystyle z=\textstyle {\frac {3\lambda }{8}}} the field has right circular polarization, and at z = λ 2 {\displaystyle z=\textstyle {\frac {\lambda }{2}}} the field is again linearly polarized along x ^ ′ {\displaystyle {\hat {x}}'} .
Consider an atom interacting with the field detuned below the transition from atomic states F g = 1 2 {\displaystyle F_{g}=\textstyle {\frac {1}{2}}} and F e = 3 2 {\displaystyle F_{e}=\textstyle {\frac {3}{2}}} ( ℏ ω f i e l d < E e g {\displaystyle \hbar {}\omega {}_{field}<E_{eg}} ). The variation of the polarization along z results in a variation in the light shifts of the atomic Zeeman sublevels with z. The Clebsch-Gordan coefficient connecting the | g , m F = − 1 2 ⟩ {\displaystyle |g,m_{F}=-\textstyle {\frac {1}{2}}\rangle } state to the | e , m F = − 3 2 ⟩ {\displaystyle |e,m_{F}=-\textstyle {\frac {3}{2}}\rangle } state is 3 times larger than connecting the | g , m F = 1 2 ⟩ {\displaystyle |g,m_{F}=\textstyle {\frac {1}{2}}\rangle } state to the | e , m F = − 1 2 ⟩ {\displaystyle |e,m_{F}=-\textstyle {\frac {1}{2}}\rangle } state. Thus for σ − {\displaystyle \sigma ^{-}} polarization the light shift is three times larger for the | g , m F = − 1 2 ⟩ {\displaystyle |g,m_{F}=-\textstyle {\frac {1}{2}}\rangle } state than for the | e , m F = 1 2 ⟩ {\displaystyle |e,m_{F}=\textstyle {\frac {1}{2}}\rangle } state. The situation is reversed for σ + {\displaystyle \sigma ^{+}} polarization, with the light shift being three times larger for the | g , m F = 1 2 ⟩ {\displaystyle |g,m_{F}=\textstyle {\frac {1}{2}}\rangle } state than the | e , m F = − 1 2 ⟩ {\displaystyle |e,m_{F}=-\textstyle {\frac {1}{2}}\rangle } state. When the polarization is linear, there is no difference in the light shifts between the two states. Thus the energies of the states will oscillate in z with period λ 2 {\displaystyle \textstyle {\frac {\lambda }{2}}} .
As an atom moves along z, it will be optically pumped to the state with the largest negative light shift. However, the optical pumping process takes some finite time τ {\displaystyle \tau } . For field wavenumber k and atomic velocity v such that k v ≈ τ − 1 {\displaystyle kv\approx \tau {}^{-1}} , the atom will travel mostly uphill as it moves along z before being pumped back down to the lowest state. In this velocity range, the atom travels more uphill than downhill and gradually loses kinetic energy, lowering its temperature. This is called the Sisyphus effect after the mythological Greek character. Note that this initial condition for velocity requires the atom to be cooled already, for example through Doppler cooling.
Representing the total electric field as E → ( z , t ) = E + ( z ) e − i ω L t + c . c . {\displaystyle {\vec {E}}(z,t)=E^{+}(z)e^{-i\omega _{L}t}+c.c.} , we can make the argument that the positive-frequency component is expressed as E → + ( z ) = ϵ → E 0 e i k z + ϵ → ′ E 0 ′ e − i k z {\displaystyle {\vec {E}}^{+}(z)={\vec {\epsilon }}E_{0}e^{ikz}+{\vec {\epsilon }}'E_{0}'e^{-ikz}} , where ϵ → {\displaystyle {\vec {\epsilon }}} and ϵ → {\displaystyle {\vec {\epsilon }}} ' are polarization vectors along some axes. In this case, we consider the Cartesian coordinate system for familiarity. Then, we consider the case where we have two opposing circular polarizations , or:
ϵ → = − 1 2 ( ϵ x + i ϵ y ) {\displaystyle {\vec {\epsilon }}=-{\frac {1}{\sqrt {2}}}(\epsilon _{x}+i\epsilon _{y})}
ϵ → ′ = 1 2 ( ϵ x − i ϵ y ) {\displaystyle {\vec {\epsilon }}'={\frac {1}{\sqrt {2}}}(\epsilon _{x}-i\epsilon _{y})}
Where ϵ x {\displaystyle {\epsilon }_{x}} and ϵ y {\displaystyle {\epsilon }_{y}} are the amplitudes of the polarization vectors across the x- and y- axis, respectively. Substituted into our positive-frequency electric field expression, we note:
E → + ( z ) = ϵ 1 1 2 ( E 0 ′ − E 0 ) − ϵ 2 1 2 ( E 0 ′ + E 0 ) {\displaystyle {\vec {E}}^{+}(z)=\epsilon _{1}{\frac {1}{\sqrt {2}}}(E_{0}'-E_{0})-\epsilon _{2}{\frac {1}{\sqrt {2}}}(E_{0}'+E_{0})}
Where we utilize Euler's identity to simplify the polarization vectors ϵ → {\displaystyle {\vec {\epsilon }}} and ϵ → {\displaystyle {\vec {\epsilon }}} ' into the following forms:
ϵ 1 = ϵ x cos ( k z ) − ϵ y sin ( k z ) {\displaystyle \epsilon _{1}=\epsilon _{x}\cos {(kz)}-\epsilon _{y}\sin(kz)}
ϵ 2 = ϵ x sin ( k z ) + ϵ y cos ( k z ) {\displaystyle \epsilon _{2}=\epsilon _{x}\sin {(kz)}+\epsilon _{y}\cos(kz)}
This results in a total electric field that is elliptically polarized . It falls from elliptical polarization that when one vector moves along the propagation axis, the axes of the ellipse rotate accordingly an angle -kz. This preserves the elliptical polarization of the total electric field regardless of the position along the propagation axis.
As a result, there is no Sisyphus effect. The rotating polarization instead leads to motion-induced population imbalances in the Zeeman levels that cause imbalances in radiation pressure leading to a damping of the atomic motion. These population imbalances are only present for states with F = 1 {\displaystyle F=1} or higher.
Consider two EM waves detuned from an atomic transition F g = 1 → F e = 2 {\displaystyle F_{g}=1\rightarrow F_{e}=2} with equal amplitudes. Now, consider an atom moving along the z -axis with some velocity v . The atom sees the polarization rotating with a frequency of k v {\displaystyle kv} . In the rotating frame, the polarization is fixed, however, there is an inertial field due to the frame rotating. This inertial term appears in the Hamiltonian as follows.
H ^ r o t = k v F z {\displaystyle {\hat {H}}_{rot}=kvF_{z}}
Here we see the inertial term looks like a magnetic field along z ^ {\displaystyle {\hat {z}}} with an amplitude such that the Larmor precession frequency is equal to rotation frequency in the lab frame. For small v , this term in Hamiltonian can be treated using perturbation theory .
Choosing the polarization in the rotating frame to be fixed along y ^ {\displaystyle {\hat {y}}} , the unperturbed atomic eigenstates are the eigenstates of F ^ y {\displaystyle {\hat {F}}_{y}} . The rotating term in the Hamiltonian causes perturbations in the atomic eigenstates such that the Zeeman sublevels become contaminated by each other. For F g = 1 {\displaystyle F_{g}=1} the | g , m f = 0 ⟩ y {\displaystyle |g,m_{f}=0\rangle _{y}} is light shifted more than the | g , m f = ± 1 ⟩ y {\displaystyle |g,m_{f}=\pm {1}\rangle _{y}} states. Thus, the steady state population of the | g , m f = 0 ⟩ y {\displaystyle |g,m_{f}=0\rangle _{y}} is higher than that of the other states. The populations are equal for the | g , m f = ± 1 ⟩ y {\displaystyle |g,m_{f}=\pm {1}\rangle _{y}} states. Thus, states are balanced with ⟨ F ^ y ⟩ = 0 {\displaystyle \langle {\hat {F}}_{y}\rangle =0} . However, when we change basis, we see that populations are not balanced in the z-basis and there is a non-zero value of ⟨ F ^ z ⟩ {\displaystyle \langle {\hat {F}}_{z}\rangle } proportional to the atom's velocity: [ 8 ]
⟨ F ^ z ⟩ = 40 ℏ k v 17 Δ 0 ′ {\displaystyle \langle {\hat {F}}_{z}\rangle ={\frac {40\hbar {}kv}{17\Delta _{0}^{'}}}}
Where Δ 0 ′ {\displaystyle \Delta _{0}^{'}} is the light shift for the m F = 0 {\displaystyle m_{F}=0} state. There is a motion induced population imbalance in the Zeeman sublevels in the z basis. For red detuned light, Δ 0 ′ {\displaystyle \Delta _{0}^{'}} is negative, and thus there will be a higher population in the | g , m f = − 1 ⟩ {\displaystyle |g,m_{f}=-1\rangle } state when the atom is moving to the right (positive velocity) and a higher population in the | g , m f = 1 ⟩ {\displaystyle |g,m_{f}=1\rangle } state when the atom is moving to the left (negative velocity). From the Clebsch-Gordan coefficients , we see that the | g , m f = − 1 ⟩ {\displaystyle |g,m_{f}=-1\rangle } state has a six times greater probability of absorbing a σ − {\displaystyle \sigma ^{-}} photon moving to the left than a σ + {\displaystyle \sigma ^{+}} photon moving to the right. The opposite is true for the | g , m f = 1 ⟩ {\displaystyle |g,m_{f}=1\rangle } state. When the atom moves to the right it is more likely to absorb a photon moving to the left and likewise when the atom moves to the left it is more likely to absorb a photon moving to the right. Thus, there is an unbalanced radiation pressure when the atom moves which dampens the motion of the atom, lowering its velocity and therefore its temperature by virtue of the kinetic theory .
Note the similarity to Doppler cooling in the unbalanced radiation pressures due to the atomic motion. The unbalanced pressure in PG cooling is not due to a Doppler shift but an induced population imbalance. Doppler cooling depends on the parameter k v Γ {\displaystyle \textstyle {\frac {kv}{\Gamma }}} where Γ {\displaystyle \Gamma } is the scattering rate, whereas PG cooling depends on k v Δ 0 ′ {\displaystyle \textstyle {\frac {kv}{\Delta _{0}^{'}}}} . At low intensity, Δ 0 ′ ≪ Γ {\displaystyle \Delta _{0}^{'}\ll \Gamma } , indicating PG cooling works at lower atomic velocities and temperatures than Doppler Cooling.
Both methods of PG cooling surpass the Doppler limit and instead are limited by the one-photon recoil limit:
k T r e c o i l = ℏ 2 k 2 2 M {\displaystyle kT_{recoil}={\frac {\hbar {}^{2}k^{2}}{2M}}}
Where M is the atomic mass.
For a given detuning δ {\displaystyle \delta } and Rabi frequency Ω {\displaystyle \Omega } , dependent on the light intensity, both configurations display a similar scaling at low intensity ( Ω ≪ | δ | {\displaystyle \Omega \ll |\delta |} ) and large detuning ( δ ≫ Γ {\displaystyle \delta \gg \Gamma } ):
k T = α ℏ Ω 2 | δ | {\displaystyle kT=\alpha {}{\frac {\hbar {}\Omega ^{2}}{|\delta |}}}
Where α {\displaystyle \alpha } is a dimensionless constant dependent on the configuration and atomic species. See ref [ 8 ] for a full derivation of these results.
Therefore, in order to reduce the temperature, it is advised to have the Rabi frequency Ω {\displaystyle \Omega } be substantially larger than the detuning δ {\displaystyle \delta } (i.e. the detuning should be minimized).
PG cooling is typically performed using a 3D optical setup with three pairs of perpendicular laser beams with an atomic ensemble in the center. Each beam is prepared with an orthogonal polarization to its counterpropagating beam. The laser frequency detuned from a selected transition between the ground and excited states of the atom. Since the cooling processes rely on multiple transitions between ground and excited states, care must be taken such that the atomic state does not fall out of these two states. This is done by using a second, "repumping", laser to pump any atoms that fall out back into the ground state of the transition. For example: in cesium cooling experiments, the cooling laser is typically chosen to be detuned from the | 6 2 S 1 / 2 F = 4 ⟩ {\displaystyle |6^{2}S_{1/2}F=4\rangle } to | 6 2 P 3 / 2 F ′ = 5 ⟩ {\displaystyle |6^{2}P_{3/2}F^{'}=5\rangle } transition and a repumping laser tuned to the | 6 2 S 1 / 2 F = 3 ⟩ {\displaystyle |6^{2}S_{1/2}F=3\rangle } to | 6 2 P 3 / 2 F ′ = 4 ⟩ {\displaystyle |6^{2}P_{3/2}F^{'}=4\rangle } transition is also used to prevent the Cs atoms from being pumped into the | 6 2 S 1 / 2 F = 3 ⟩ {\displaystyle |6^{2}S_{1/2}F=3\rangle } state.
The atoms must be cooled before the PG cooling, this can be done using the same setup via Doppler cooling. If the atoms are precooled with Doppler cooling, the laser intensity must be lowered and the detuning increased for PG cooling to be achieved.
The atomic temperature can be measured using the time of flight (ToF) technique. In this technique, the laser beams are suddenly turned off and the atomic ensemble is allowed to expand. After a set time delay t, a probe beam is turned on to image the ensemble and obtain the spatial extent of the ensemble at time t. By imaging the ensemble at several time delays, the rate of expansion is found. By measuring the rate of expansion of the ensemble the velocity distribution is measured and from this, the temperature is inferred. [ 1 ] [ 9 ]
An important theoretical result is that in the regime where PG cooling functions, the temperature only depends on the ratio of Ω 2 {\displaystyle \Omega ^{2}} to | γ | {\displaystyle |\gamma |} and that the cooling approaches the recoil limit. These predictions were confirmed experimentally in 1990 when W.D. Phillips et al. observed such scaling in their cesium atoms as well as a temperature of 2.5 μ {\displaystyle \mu } K, [ 2 ] 12 times the recoil temperature of 0.198 μ {\displaystyle \mu } K for the D2 line of cesium used in the experiment. [ 10 ]
Recently, PG cooling has been important in research topics such as Bose-Einstein condensates , [ 11 ] optical dipole traps , [ 12 ] and integrated photonics . [ 13 ] As an important aspect of atom trapping, there is substantial interest in achieving PG cooling for 3D magneto-optical traps . However, such traps typically require large volumes due to necessitating the use of multiple collimated lasers within an atomic vacuum cell. Thus, there is an active research scene in PICMOTs, or photonic integrated circuit magneto-optical traps. One proposed avenue through which such small form factors can be achieved is via metasurfaces for devices orders of magnitude smaller. [ 14 ] If this were to be successful, PG cooling could be achieved at a much smaller form factor than currently possible, and deployed in the use of PICMOTs for higher levels of system integration, reduced optical losses, and compact magnetic field generation. [ 15 ]
With regards to optical dipole traps, it was recently shown that PG cooling operating under the σ + σ − configuration is able to probe an optical trap's trapping field (i.e. the dependency of the cooling limit of its polarization). [ 12 ] Currently, the efficiency of such an idea is vastly unexplored by literature and thus provides a promising field of interest for further research. | https://en.wikipedia.org/wiki/Polarization_gradient_cooling |
In linear algebra , a branch of mathematics , the polarization identity is any one of a family of formulas that express the inner product of two vectors in terms of the norm of a normed vector space .
If a norm arises from an inner product then the polarization identity can be used to express this inner product entirely in terms of the norm. The polarization identity shows that a norm can arise from at most one inner product; however, there exist norms that do not arise from any inner product.
The norm associated with any inner product space satisfies the parallelogram law : ‖ x + y ‖ 2 + ‖ x − y ‖ 2 = 2 ‖ x ‖ 2 + 2 ‖ y ‖ 2 . {\displaystyle \|x+y\|^{2}+\|x-y\|^{2}=2\|x\|^{2}+2\|y\|^{2}.} In fact, as observed by John von Neumann , [ 1 ] the parallelogram law characterizes those norms that arise from inner products.
Given a normed space ( H , ‖ ⋅ ‖ ) {\displaystyle (H,\|\cdot \|)} , the parallelogram law holds for ‖ ⋅ ‖ {\displaystyle \|\cdot \|} if and only if there exists an inner product ⟨ ⋅ , ⋅ ⟩ {\displaystyle \langle \cdot ,\cdot \rangle } on H {\displaystyle H} such that ‖ x ‖ 2 = ⟨ x , x ⟩ {\displaystyle \|x\|^{2}=\langle x,\ x\rangle } for all x ∈ H , {\displaystyle x\in H,} in which case this inner product is uniquely determined by the norm via the polarization identity. [ 2 ] [ 3 ]
Any inner product on a vector space induces a norm by the equation ‖ x ‖ = ⟨ x , x ⟩ . {\displaystyle \|x\|={\sqrt {\langle x,x\rangle }}.} The polarization identities reverse this relationship, recovering the inner product from the norm.
Every inner product satisfies: ‖ x + y ‖ 2 = ‖ x ‖ 2 + ‖ y ‖ 2 + 2 Re ⟨ x , y ⟩ for all vectors x , y . {\displaystyle \|x+y\|^{2}=\|x\|^{2}+\|y\|^{2}+2\operatorname {Re} \langle x,y\rangle \qquad {\text{ for all vectors }}x,y.}
Solving for Re ⟨ x , y ⟩ {\displaystyle \operatorname {Re} \langle x,y\rangle } gives the formula Re ⟨ x , y ⟩ = 1 2 ( ‖ x + y ‖ 2 − ‖ x ‖ 2 − ‖ y ‖ 2 ) . {\displaystyle \operatorname {Re} \langle x,y\rangle ={\frac {1}{2}}\left(\|x+y\|^{2}-\|x\|^{2}-\|y\|^{2}\right).} If the inner product is real then Re ⟨ x , y ⟩ = ⟨ x , y ⟩ {\displaystyle \operatorname {Re} \langle x,y\rangle =\langle x,y\rangle } and this formula becomes a polarization identity for real inner products.
If the vector space is over the real numbers then the polarization identities are: [ 4 ] ⟨ x , y ⟩ = 1 4 ( ‖ x + y ‖ 2 − ‖ x − y ‖ 2 ) = 1 2 ( ‖ x + y ‖ 2 − ‖ x ‖ 2 − ‖ y ‖ 2 ) = 1 2 ( ‖ x ‖ 2 + ‖ y ‖ 2 − ‖ x − y ‖ 2 ) . {\displaystyle {\begin{alignedat}{4}\langle x,y\rangle &={\frac {1}{4}}\left(\|x+y\|^{2}-\|x-y\|^{2}\right)\\[3pt]&={\frac {1}{2}}\left(\|x+y\|^{2}-\|x\|^{2}-\|y\|^{2}\right)\\[3pt]&={\frac {1}{2}}\left(\|x\|^{2}+\|y\|^{2}-\|x-y\|^{2}\right).\\[3pt]\end{alignedat}}}
These various forms are all equivalent by the parallelogram law : [ proof 1 ] 2 ‖ x ‖ 2 + 2 ‖ y ‖ 2 = ‖ x + y ‖ 2 + ‖ x − y ‖ 2 . {\displaystyle 2\|x\|^{2}+2\|y\|^{2}=\|x+y\|^{2}+\|x-y\|^{2}.}
This further implies that L p {\displaystyle L^{p}} class is not a Hilbert space whenever p ≠ 2 {\displaystyle p\neq 2} , as the parallelogram law is not satisfied. For the sake of counterexample, consider x = 1 A {\displaystyle x=1_{A}} and y = 1 B {\displaystyle y=1_{B}} for any two disjoint subsets A , B {\displaystyle A,B} of general domain Ω ⊂ R n {\displaystyle \Omega \subset \mathbb {R} ^{n}} and compute the measure of both sets under parallelogram law.
For vector spaces over the complex numbers , the above formulas are not quite correct because they do not describe the imaginary part of the (complex) inner product.
However, an analogous expression does ensure that both real and imaginary parts are retained.
The complex part of the inner product depends on whether it is antilinear in the first or the second argument.
The notation ⟨ x | y ⟩ , {\displaystyle \langle x|y\rangle ,} which is commonly used in physics will be assumed to be antilinear in the first argument while ⟨ x , y ⟩ , {\displaystyle \langle x,\,y\rangle ,} which is commonly used in mathematics, will be assumed to be antilinear in its second argument.
They are related by the formula: ⟨ x , y ⟩ = ⟨ y | x ⟩ for all x , y ∈ H . {\displaystyle \langle x,\,y\rangle =\langle y\,|\,x\rangle \quad {\text{ for all }}x,y\in H.}
The real part of any inner product (no matter which argument is antilinear and no matter if it is real or complex) is a symmetric bilinear map that for any x , y ∈ H {\displaystyle x,y\in H} is always equal to: [ 4 ] [ proof 1 ] R ( x , y ) : = Re ⟨ x ∣ y ⟩ = Re ⟨ x , y ⟩ = 1 4 ( ‖ x + y ‖ 2 − ‖ x − y ‖ 2 ) = 1 2 ( ‖ x + y ‖ 2 − ‖ x ‖ 2 − ‖ y ‖ 2 ) = 1 2 ( ‖ x ‖ 2 + ‖ y ‖ 2 − ‖ x − y ‖ 2 ) . {\displaystyle {\begin{alignedat}{4}R(x,y):&=\operatorname {Re} \langle x\mid y\rangle =\operatorname {Re} \langle x,y\rangle \\&={\frac {1}{4}}\left(\|x+y\|^{2}-\|x-y\|^{2}\right)\\&={\frac {1}{2}}\left(\|x+y\|^{2}-\|x\|^{2}-\|y\|^{2}\right)\\[3pt]&={\frac {1}{2}}\left(\|x\|^{2}+\|y\|^{2}-\|x-y\|^{2}\right).\\[3pt]\end{alignedat}}}
It is always a symmetric map , meaning that [ proof 1 ] R ( x , y ) = R ( y , x ) for all x , y ∈ H , {\displaystyle R(x,y)=R(y,x)\quad {\text{ for all }}x,y\in H,} and it also satisfies: [ proof 1 ] R ( i x , y ) = − R ( x , i y ) for all x , y ∈ H , {\displaystyle R(ix,y)=-R(x,iy)\quad {\text{ for all }}x,y\in H,} which in plain English says that to move a factor of i {\displaystyle i} to the other argument, introduce a negative sign. These properties can be proven either from the properties of inner products directly or from properties of norms by using the polarization identity.
Let R ( x , y ) := 1 4 ( ‖ x + y ‖ 2 − ‖ x − y ‖ 2 ) . {\displaystyle R(x,y):={\frac {1}{4}}\left(\|x+y\|^{2}-\|x-y\|^{2}\right).} Then R ( y , x ) = 1 4 ( ‖ y + x ‖ 2 − ‖ y − x ‖ 2 ) = 1 4 ( ‖ x + y ‖ 2 − ‖ x − y ‖ 2 ) = R ( x , y ) , {\displaystyle R(y,x)={\frac {1}{4}}\left(\|y+x\|^{2}-\|y-x\|^{2}\right)={\frac {1}{4}}\left(\|x+y\|^{2}-\|x-y\|^{2}\right)=R(x,y),}
which proves that R ( x , y ) = R ( y , x ) {\displaystyle R(x,y)=R(y,x)} .
Additionally,
R ( i x , y ) = 1 4 ( ‖ i x + y ‖ 2 − ‖ i x − y ‖ 2 ) {\displaystyle R(ix,y)={\frac {1}{4}}\left(\|ix+y\|^{2}-\|ix-y\|^{2}\right)}
= 1 4 ( ‖ x + ( 1 / i ) y ‖ 2 − ‖ x − ( 1 / i ) y ‖ 2 ) {\displaystyle ={\frac {1}{4}}\left(\|x+(1/i)y\|^{2}-\|x-(1/i)y\|^{2}\right)}
= 1 4 ( ‖ x − i y ‖ 2 − ‖ x + i y ‖ 2 ) {\displaystyle ={\frac {1}{4}}\left(\|x-iy\|^{2}-\|x+iy\|^{2}\right)}
= − 1 4 ( ‖ x + i y ‖ 2 − ‖ x − i y ‖ 2 ) {\displaystyle =-{\frac {1}{4}}\left(\|x+iy\|^{2}-\|x-iy\|^{2}\right)}
= − R ( x , i y ) , {\displaystyle =-R(x,iy),}
which proves that R ( i x , y ) = − R ( x , i y ) . {\displaystyle R(ix,y)=-R(x,iy).} ◼ {\displaystyle \blacksquare }
Unlike its real part, the imaginary part of a complex inner product depends on which argument is antilinear.
Antilinear in first argument
The polarization identities for the inner product ⟨ x | y ⟩ , {\displaystyle \langle x\,|\,y\rangle ,} which is antilinear in the first argument, are
where x , y ∈ H . {\displaystyle x,y\in H.} The second to last equality is similar to the formula expressing a linear functional φ {\displaystyle \varphi } in terms of its real part: φ ( y ) = Re φ ( y ) − i ( Re φ ) ( i y ) . {\displaystyle \varphi (y)=\operatorname {Re} \varphi (y)-i(\operatorname {Re} \varphi )(iy).}
Antilinear in second argument
The polarization identities for the inner product ⟨ x , y ⟩ , {\displaystyle \langle x,\ y\rangle ,} which is antilinear in the second argument, follows from that of ⟨ x | y ⟩ {\displaystyle \langle x\,|\,y\rangle } by the relationship: ⟨ x , y ⟩ := ⟨ y | x ⟩ = ⟨ x | y ⟩ ¯ for all x , y ∈ H . {\displaystyle \langle x,\ y\rangle :=\langle y\,|\,x\rangle ={\overline {\langle x\,|\,y\rangle }}\quad {\text{ for all }}x,y\in H.} So for any x , y ∈ H , {\displaystyle x,y\in H,} [ 4 ]
This expression can be phrased symmetrically as: [ 5 ] ⟨ x , y ⟩ = 1 4 ∑ k = 0 3 i k ‖ x + i k y ‖ 2 . {\displaystyle \langle x,y\rangle ={\frac {1}{4}}\sum _{k=0}^{3}i^{k}\left\|x+i^{k}y\right\|^{2}.}
Summary of both cases
Thus if R ( x , y ) + i I ( x , y ) {\displaystyle R(x,y)+iI(x,y)} denotes the real and imaginary parts of some inner product's value at the point ( x , y ) ∈ H × H {\displaystyle (x,y)\in H\times H} of its domain, then its imaginary part will be: I ( x , y ) = { R ( i x , y ) if antilinear in the 1 st argument R ( x , i y ) if antilinear in the 2 nd argument {\displaystyle I(x,y)~=~{\begin{cases}~R({\color {red}i}x,y)&\qquad {\text{ if antilinear in the }}{\color {red}1}{\text{st argument}}\\~R(x,{\color {blue}i}y)&\qquad {\text{ if antilinear in the }}{\color {blue}2}{\text{nd argument}}\\\end{cases}}} where the scalar i {\displaystyle i} is always located in the same argument that the inner product is antilinear in.
Using R ( i x , y ) = − R ( x , i y ) {\displaystyle R(ix,y)=-R(x,iy)} , the above formula for the imaginary part becomes: I ( x , y ) = { − R ( x , i y ) if antilinear in the 1 st argument − R ( i x , y ) if antilinear in the 2 nd argument {\displaystyle I(x,y)~=~{\begin{cases}-R(x,{\color {black}i}y)&\qquad {\text{ if antilinear in the }}{\color {black}1}{\text{st argument}}\\-R({\color {black}i}x,y)&\qquad {\text{ if antilinear in the }}{\color {black}2}{\text{nd argument}}\\\end{cases}}}
In a normed space ( H , ‖ ⋅ ‖ ) , {\displaystyle (H,\|\cdot \|),} if the parallelogram law ‖ x + y ‖ 2 + ‖ x − y ‖ 2 = 2 ‖ x ‖ 2 + 2 ‖ y ‖ 2 {\displaystyle \|x+y\|^{2}~+~\|x-y\|^{2}~=~2\|x\|^{2}+2\|y\|^{2}} holds, then there exists a unique inner product ⟨ ⋅ , ⋅ ⟩ {\displaystyle \langle \cdot ,\ \cdot \rangle } on H {\displaystyle H} such that ‖ x ‖ 2 = ⟨ x , x ⟩ {\displaystyle \|x\|^{2}=\langle x,\ x\rangle } for all x ∈ H . {\displaystyle x\in H.} [ 4 ] [ 1 ]
We will only give the real case here; the proof for complex vector spaces is analogous.
By the above formulas, if the norm is described by an inner product (as we hope), then it must satisfy ⟨ x , y ⟩ = 1 4 ( ‖ x + y ‖ 2 − ‖ x − y ‖ 2 ) for all x , y ∈ H , {\displaystyle \langle x,\ y\rangle ={\frac {1}{4}}\left(\|x+y\|^{2}-\|x-y\|^{2}\right)\quad {\text{ for all }}x,y\in H,} which may serve as a definition of the unique candidate ⟨ ⋅ , ⋅ ⟩ {\displaystyle \langle \cdot ,\cdot \rangle } for the role of a suitable inner product. Thus, the uniqueness is guaranteed.
It remains to prove that this formula indeed defines an inner product and that this inner product induces the norm ‖ ⋅ ‖ . {\displaystyle \|\cdot \|.} Explicitly, the following will be shown:
(This axiomatization omits positivity , which is implied by (1) and the fact that ‖ ⋅ ‖ {\displaystyle \|\cdot \|} is a norm.)
For properties (1) and (2), substitute: ⟨ x , x ⟩ = 1 4 ( ‖ x + x ‖ 2 − ‖ x − x ‖ 2 ) = ‖ x ‖ 2 , {\textstyle \langle x,x\rangle ={\frac {1}{4}}\left(\|x+x\|^{2}-\|x-x\|^{2}\right)=\|x\|^{2},} and ‖ x − y ‖ 2 = ‖ y − x ‖ 2 . {\displaystyle \|x-y\|^{2}=\|y-x\|^{2}.}
For property (3), it is convenient to work in reverse.
It remains to show that ‖ x + z + y ‖ 2 − ‖ x + z − y ‖ 2 = ? ‖ x + y ‖ 2 − ‖ x − y ‖ 2 + ‖ z + y ‖ 2 − ‖ z − y ‖ 2 {\displaystyle \|x+z+y\|^{2}-\|x+z-y\|^{2}{\overset {?}{=}}\|x+y\|^{2}-\|x-y\|^{2}+\|z+y\|^{2}-\|z-y\|^{2}} or equivalently, 2 ( ‖ x + z + y ‖ 2 + ‖ x − y ‖ 2 ) − 2 ( ‖ x + z − y ‖ 2 + ‖ x + y ‖ 2 ) = ? 2 ‖ z + y ‖ 2 − 2 ‖ z − y ‖ 2 . {\displaystyle 2\left(\|x+z+y\|^{2}+\|x-y\|^{2}\right)-2\left(\|x+z-y\|^{2}+\|x+y\|^{2}\right){\overset {?}{=}}2\|z+y\|^{2}-2\|z-y\|^{2}.}
Now apply the parallelogram identity: 2 ‖ x + z + y ‖ 2 + 2 ‖ x − y ‖ 2 = ‖ 2 x + z ‖ 2 + ‖ 2 y + z ‖ 2 {\displaystyle 2\|x+z+y\|^{2}+2\|x-y\|^{2}=\|2x+z\|^{2}+\|2y+z\|^{2}} 2 ‖ x + z − y ‖ 2 + 2 ‖ x + y ‖ 2 = ‖ 2 x + z ‖ 2 + ‖ z − 2 y ‖ 2 {\displaystyle 2\|x+z-y\|^{2}+2\|x+y\|^{2}=\|2x+z\|^{2}+\|z-2y\|^{2}} Thus it remains to verify: ‖ 2 x + z ‖ 2 + ‖ 2 y + z ‖ 2 − ( ‖ 2 x + z ‖ 2 + ‖ z − 2 y ‖ 2 ) = ? 2 ‖ z + y ‖ 2 − 2 ‖ z − y ‖ 2 {\displaystyle {\cancel {\|2x+z\|^{2}}}+\|2y+z\|^{2}-({\cancel {\|2x+z\|^{2}}}+\|z-2y\|^{2}){\overset {?}{{}={}}}2\|z+y\|^{2}-2\|z-y\|^{2}} ‖ 2 y + z ‖ 2 − ‖ z − 2 y ‖ 2 = ? 2 ‖ z + y ‖ 2 − 2 ‖ z − y ‖ 2 {\displaystyle \|2y+z\|^{2}-\|z-2y\|^{2}{\overset {?}{=}}2\|z+y\|^{2}-2\|z-y\|^{2}}
But the latter claim can be verified by subtracting the following two further applications of the parallelogram identity: ‖ 2 y + z ‖ 2 + ‖ z ‖ 2 = 2 ‖ z + y ‖ 2 + 2 ‖ y ‖ 2 {\displaystyle \|2y+z\|^{2}+\|z\|^{2}=2\|z+y\|^{2}+2\|y\|^{2}} ‖ z − 2 y ‖ 2 + ‖ z ‖ 2 = 2 ‖ z − y ‖ 2 + 2 ‖ y ‖ 2 {\displaystyle \|z-2y\|^{2}+\|z\|^{2}=2\|z-y\|^{2}+2\|y\|^{2}}
Thus (3) holds.
It can be verified by induction that (3) implies (4), as long as α ∈ Z . {\displaystyle \alpha \in \mathbb {Z} .} But "(4) when α ∈ Z {\displaystyle \alpha \in \mathbb {Z} } " implies "(4) when α ∈ Q {\displaystyle \alpha \in \mathbb {Q} } ".
And any positive-definite, real-valued , Q {\displaystyle \mathbb {Q} } -bilinear form satisfies the Cauchy–Schwarz inequality , so that ⟨ ⋅ , ⋅ ⟩ {\displaystyle \langle \cdot ,\cdot \rangle } is continuous.
Thus ⟨ ⋅ , ⋅ ⟩ {\displaystyle \langle \cdot ,\cdot \rangle } must be R {\displaystyle \mathbb {R} } -linear as well.
Another necessary and sufficient condition for there to exist an inner product that induces a given norm ‖ ⋅ ‖ {\displaystyle \|\cdot \|} is for the norm to satisfy Ptolemy's inequality , which is: [ 6 ] ‖ x − y ‖ ‖ z ‖ + ‖ y − z ‖ ‖ x ‖ ≥ ‖ x − z ‖ ‖ y ‖ for all vectors x , y , z . {\displaystyle \|x-y\|\,\|z\|~+~\|y-z\|\,\|x\|~\geq ~\|x-z\|\,\|y\|\qquad {\text{ for all vectors }}x,y,z.}
If H {\displaystyle H} is a complex Hilbert space then ⟨ x ∣ y ⟩ {\displaystyle \langle x\mid y\rangle } is real if and only if its imaginary part is 0 = R ( x , i y ) = 1 4 ( ‖ x + i y ‖ 2 − ‖ x − i y ‖ 2 ) {\displaystyle 0=R(x,iy)={\frac {1}{4}}\left(\Vert x+iy\Vert ^{2}-\Vert x-iy\Vert ^{2}\right)} , which happens if and only if ‖ x + i y ‖ = ‖ x − i y ‖ {\displaystyle \Vert x+iy\Vert =\Vert x-iy\Vert } .
Similarly, ⟨ x ∣ y ⟩ {\displaystyle \langle x\mid y\rangle } is (purely) imaginary if and only if ‖ x + y ‖ = ‖ x − y ‖ {\displaystyle \Vert x+y\Vert =\Vert x-y\Vert } .
For example, from ‖ x + i x ‖ = | 1 + i | ‖ x ‖ = 2 ‖ x ‖ = | 1 − i | ‖ x ‖ = ‖ x − i x ‖ {\displaystyle \|x+ix\|=|1+i|\|x\|={\sqrt {2}}\|x\|=|1-i|\|x\|=\|x-ix\|} it can be concluded that ⟨ x | x ⟩ {\displaystyle \langle x|x\rangle } is real and that ⟨ x | i x ⟩ {\displaystyle \langle x|ix\rangle } is purely imaginary.
If A : H → Z {\displaystyle A:H\to Z} is a linear isometry between two Hilbert spaces (so ‖ A h ‖ = ‖ h ‖ {\displaystyle \|Ah\|=\|h\|} for all h ∈ H {\displaystyle h\in H} ) then ⟨ A h , A k ⟩ Z = ⟨ h , k ⟩ H for all h , k ∈ H ; {\displaystyle \langle Ah,Ak\rangle _{Z}=\langle h,k\rangle _{H}\quad {\text{ for all }}h,k\in H;} that is, linear isometries preserve inner products.
If A : H → Z {\displaystyle A:H\to Z} is instead an antilinear isometry then ⟨ A h , A k ⟩ Z = ⟨ h , k ⟩ H ¯ = ⟨ k , h ⟩ H for all h , k ∈ H . {\displaystyle \langle Ah,Ak\rangle _{Z}={\overline {\langle h,k\rangle _{H}}}=\langle k,h\rangle _{H}\quad {\text{ for all }}h,k\in H.}
The second form of the polarization identity can be written as ‖ u − v ‖ 2 = ‖ u ‖ 2 + ‖ v ‖ 2 − 2 ( u ⋅ v ) . {\displaystyle \|{\textbf {u}}-{\textbf {v}}\|^{2}=\|{\textbf {u}}\|^{2}+\|{\textbf {v}}\|^{2}-2({\textbf {u}}\cdot {\textbf {v}}).}
This is essentially a vector form of the law of cosines for the triangle formed by the vectors u {\displaystyle {\textbf {u}}} , v {\displaystyle {\textbf {v}}} , and u − v {\displaystyle {\textbf {u}}-{\textbf {v}}} .
In particular, u ⋅ v = ‖ u ‖ ‖ v ‖ cos θ , {\displaystyle {\textbf {u}}\cdot {\textbf {v}}=\|{\textbf {u}}\|\,\|{\textbf {v}}\|\cos \theta ,} where θ {\displaystyle \theta } is the angle between the vectors u {\displaystyle {\textbf {u}}} and v {\displaystyle {\textbf {v}}} .
The equation is numerically unstable if u and v are similar because of catastrophic cancellation and should be avoided for numeric computation.
The basic relation between the norm and the dot product is given by the equation ‖ v ‖ 2 = v ⋅ v . {\displaystyle \|{\textbf {v}}\|^{2}={\textbf {v}}\cdot {\textbf {v}}.}
Then ‖ u + v ‖ 2 = ( u + v ) ⋅ ( u + v ) = ( u ⋅ u ) + ( u ⋅ v ) + ( v ⋅ u ) + ( v ⋅ v ) = ‖ u ‖ 2 + ‖ v ‖ 2 + 2 ( u ⋅ v ) , {\displaystyle {\begin{aligned}\|{\textbf {u}}+{\textbf {v}}\|^{2}&=({\textbf {u}}+{\textbf {v}})\cdot ({\textbf {u}}+{\textbf {v}})\\[3pt]&=({\textbf {u}}\cdot {\textbf {u}})+({\textbf {u}}\cdot {\textbf {v}})+({\textbf {v}}\cdot {\textbf {u}})+({\textbf {v}}\cdot {\textbf {v}})\\[3pt]&=\|{\textbf {u}}\|^{2}+\|{\textbf {v}}\|^{2}+2({\textbf {u}}\cdot {\textbf {v}}),\end{aligned}}} and similarly ‖ u − v ‖ 2 = ‖ u ‖ 2 + ‖ v ‖ 2 − 2 ( u ⋅ v ) . {\displaystyle \|{\textbf {u}}-{\textbf {v}}\|^{2}=\|{\textbf {u}}\|^{2}+\|{\textbf {v}}\|^{2}-2({\textbf {u}}\cdot {\textbf {v}}).}
Forms (1) and (2) of the polarization identity now follow by solving these equations for u ⋅ v {\displaystyle {\textbf {u}}\cdot {\textbf {v}}} , while form (3) follows from subtracting these two equations.
(Adding these two equations together gives the parallelogram law.)
The standard Jordan – von Neumann theorem, as stated previously, is that the if a norm satisfies the parallelogram law, then it can be induced by an inner product defined by the polarization identity. There are variants of the theorem. [ 7 ]
Define various senses of orthogonality:
Let V {\textstyle V} be a vector space over the real or complex numbers. Let ‖ ⋅ ‖ {\textstyle \|\cdot \|} be a norm over V {\textstyle V} . We consider conditions for which the norm is induced by an inner product. In the following statements, whenever a scalar appears, the scalar may be restricted to be merely real, even when V {\textstyle V} is over the complex numbers.
For the real vector space, there is also the condition:
Since the internal John ellipse E {\textstyle E} is unique, for any bijective linear map T {\textstyle T} that preserves the unit circle, it must have T E = E {\textstyle TE=E} . Since E {\textstyle E} must touch the circle at some point x {\textstyle x} , we may map x {\textstyle x} to any other point y {\textstyle y} on the circle, thus every point y {\textstyle y} touches the ellipse E {\textstyle E} . Thus the circle is the ellipse.
The Banach-Mazur rotation problem: Given a separable Banach space V {\textstyle V} such that for any two unit vectors x , y , {\textstyle x,y,} there exists a linear surjective isometry T {\textstyle T} such that T ( x ) = y {\textstyle T(x)=y} or T ( y ) = x {\textstyle T(y)=x} , is V {\textstyle V} isometrically isomorphic to a Hilbert space?
The general case of the problem is open . When the space is parable finite-dimensional, the answer is yes. In other words, given a finite-dimensional normed vector space over the real or complex numbers, if any point on the unit sphere can be mapped (rotated) to any other point by a linear isometry, then the norm is induced by an inner product. [ 8 ]
The polarization identities are not restricted to inner products.
If B {\displaystyle B} is any symmetric bilinear form on a vector space, and Q {\displaystyle Q} is the quadratic form defined by Q ( v ) = B ( v , v ) , {\displaystyle Q(v)=B(v,v),} then 2 B ( u , v ) = Q ( u + v ) − Q ( u ) − Q ( v ) , 2 B ( u , v ) = Q ( u ) + Q ( v ) − Q ( u − v ) , 4 B ( u , v ) = Q ( u + v ) − Q ( u − v ) . {\displaystyle {\begin{aligned}2B(u,v)&=Q(u+v)-Q(u)-Q(v),\\2B(u,v)&=Q(u)+Q(v)-Q(u-v),\\4B(u,v)&=Q(u+v)-Q(u-v).\end{aligned}}}
The so-called symmetrization map generalizes the latter formula, replacing Q {\displaystyle Q} by a homogeneous polynomial of degree k {\displaystyle k} defined by Q ( v ) = B ( v , … , v ) , {\displaystyle Q(v)=B(v,\ldots ,v),} where B {\displaystyle B} is a symmetric k {\displaystyle k} -linear map. [ 9 ]
The formulas above even apply in the case where the field of scalars has characteristic two, though the left-hand sides are all zero in this case.
Consequently, in characteristic two there is no formula for a symmetric bilinear form in terms of a quadratic form, and they are in fact distinct notions, a fact which has important consequences in L-theory ; for brevity, in this context "symmetric bilinear forms" are often referred to as "symmetric forms".
These formulas also apply to bilinear forms on modules over a commutative ring , though again one can only solve for B ( u , v ) {\displaystyle B(u,v)} if 2 is invertible in the ring, and otherwise these are distinct notions. For example, over the integers, one distinguishes integral quadratic forms from integral symmetric forms, which are a narrower notion.
More generally, in the presence of a ring involution or where 2 is not invertible, one distinguishes ε {\displaystyle \varepsilon } -quadratic forms and ε {\displaystyle \varepsilon } -symmetric forms ; a symmetric form defines a quadratic form, and the polarization identity (without a factor of 2) from a quadratic form to a symmetric form is called the " symmetrization map", and is not in general an isomorphism. This has historically been a subtle distinction: over the integers it was not until the 1950s that relation between "twos out" (integral quadratic form) and "twos in" (integral symmetric form) was understood – see discussion at integral quadratic form ; and in the algebraization of surgery theory , Mishchenko originally used symmetric L -groups, rather than the correct quadratic L -groups (as in Wall and Ranicki) – see discussion at L-theory .
Finally, in any of these contexts these identities may be extended to homogeneous polynomials (that is, algebraic forms ) of arbitrary degree , where it is known as the polarization formula , and is reviewed in greater detail in the article on the polarization of an algebraic form . | https://en.wikipedia.org/wiki/Polarization_identity |
Polarization of electromagnetic radiation is a useful tool for detecting various astronomical phenomenon. For example, energy can become polarized by passing through interstellar dust or by magnetic fields . Microwave energy from the primordial universe can be used to study the physics of that environment.
The polarization of starlight was first observed by the astronomers William Hiltner and John S. Hall in 1949. Subsequently, Jesse Greenstein and Leverett Davis, Jr. developed theories allowing the use of polarization data to trace interstellar magnetic fields.
Though the integrated thermal radiation of stars is not usually appreciably polarized at source, scattering by interstellar dust can impose polarization on starlight over long distances. Net polarization at the source can occur if the photosphere itself is asymmetric, due to limb polarization . Plane polarization of starlight generated at the star itself is observed for Ap stars (peculiar A type stars). [ 1 ]
Both circular and linear polarization of sunlight has been measured. Circular polarization is mainly due to transmission and absorption effects in strongly magnetic regions of the Sun's surface. Another mechanism that gives rise to circular polarization is the so-called "alignment-to-orientation mechanism". Continuum light is linearly polarized at different locations across the face of the Sun (limb polarization) though taken as a whole, this polarization cancels. Linear polarization in spectral lines is usually created by anisotropic scattering of photons on atoms and ions which can themselves be polarized by this interaction. The linearly polarized spectrum of the Sun is often called the second solar spectrum . Atomic polarization can be modified in weak magnetic fields by the Hanle effect . As a result, polarization of the scattered photons is also modified providing a diagnostics tool for understanding stellar magnetic fields . [ 2 ]
Polarization is also present in radiation from coherent astronomical sources due to the Zeeman effect (e.g. hydroxyl or methanol masers ).
The large radio lobes in active galaxies and pulsar radio radiation (which may, it is speculated, sometimes be coherent) also show polarization.
Apart from providing information on sources of radiation and scattering, polarization also probes the interstellar magnetic field in our galaxy as well as in radio galaxies via Faraday rotation . [ 3 ] : 119, 124 [ 4 ] : 336–337 In some cases it can be difficult to determine how much of the Faraday rotation is in the external source and how much is local to our own galaxy, but in many cases it is possible to find another distant source nearby in the sky; thus by comparing the candidate source and the reference source, the results can be untangled.
The polarization of the cosmic microwave background (CMB) is also being used to study the physics of the very early universe . [ 5 ] [ 6 ] CMB exhibits 2 components of polarization: B-mode (divergence-free like magnetic field) and E-mode (curl-free gradient-only like electric field) polarization. The BICEP2 telescope located at the South Pole initially claimed the detection of B-mode polarization in the CMB, though the initially claimed result was later retracted. The polarization modes of the CMB may provide more information about the influence of gravitational waves on the development of the early universe.
It has been suggested that astronomical sources of polarised light caused the chirality found in biological molecules on Earth. [ 7 ] | https://en.wikipedia.org/wiki/Polarization_in_astronomy |
In mathematics , in particular in algebra , polarization is a technique for expressing a homogeneous polynomial in a simpler fashion by adjoining more variables. Specifically, given a homogeneous polynomial, polarization produces a unique symmetric multilinear form from which the original polynomial can be recovered by evaluating along a certain diagonal.
Although the technique is deceptively simple, it has applications in many areas of abstract mathematics: in particular to algebraic geometry , invariant theory , and representation theory . Polarization and related techniques form the foundations for Weyl's invariant theory .
The fundamental ideas are as follows. Let f ( u ) {\displaystyle f(\mathbf {u} )} be a polynomial in n {\displaystyle n} variables u = ( u 1 , u 2 , … , u n ) . {\displaystyle \mathbf {u} =\left(u_{1},u_{2},\ldots ,u_{n}\right).} Suppose that f {\displaystyle f} is homogeneous of degree d , {\displaystyle d,} which means that f ( t u ) = t d f ( u ) for all t . {\displaystyle f(t\mathbf {u} )=t^{d}f(\mathbf {u} )\quad {\text{ for all }}t.}
Let u ( 1 ) , u ( 2 ) , … , u ( d ) {\displaystyle \mathbf {u} ^{(1)},\mathbf {u} ^{(2)},\ldots ,\mathbf {u} ^{(d)}} be a collection of indeterminates with u ( i ) = ( u 1 ( i ) , u 2 ( i ) , … , u n ( i ) ) , {\displaystyle \mathbf {u} ^{(i)}=\left(u_{1}^{(i)},u_{2}^{(i)},\ldots ,u_{n}^{(i)}\right),} so that there are d n {\displaystyle dn} variables altogether. The polar form of f {\displaystyle f} is a polynomial F ( u ( 1 ) , u ( 2 ) , … , u ( d ) ) {\displaystyle F\left(\mathbf {u} ^{(1)},\mathbf {u} ^{(2)},\ldots ,\mathbf {u} ^{(d)}\right)} which is linear separately in each u ( i ) {\displaystyle \mathbf {u} ^{(i)}} (that is, F {\displaystyle F} is multilinear), symmetric in the u ( i ) , {\displaystyle \mathbf {u} ^{(i)},} and such that F ( u , u , … , u ) = f ( u ) . {\displaystyle F\left(\mathbf {u} ,\mathbf {u} ,\ldots ,\mathbf {u} \right)=f(\mathbf {u} ).}
The polar form of f {\displaystyle f} is given by the following construction F ( u ( 1 ) , … , u ( d ) ) = 1 d ! ∂ ∂ λ 1 … ∂ ∂ λ d f ( λ 1 u ( 1 ) + ⋯ + λ d u ( d ) ) | λ = 0 . {\displaystyle F\left({\mathbf {u} }^{(1)},\dots ,{\mathbf {u} }^{(d)}\right)={\frac {1}{d!}}{\frac {\partial }{\partial \lambda _{1}}}\dots {\frac {\partial }{\partial \lambda _{d}}}f(\lambda _{1}{\mathbf {u} }^{(1)}+\dots +\lambda _{d}{\mathbf {u} }^{(d)})|_{\lambda =0}.} In other words, F {\displaystyle F} is a constant multiple of the coefficient of λ 1 λ 2 … λ d {\displaystyle \lambda _{1}\lambda _{2}\ldots \lambda _{d}} in the expansion of f ( λ 1 u ( 1 ) + ⋯ + λ d u ( d ) ) . {\displaystyle f\left(\lambda _{1}\mathbf {u} ^{(1)}+\cdots +\lambda _{d}\mathbf {u} ^{(d)}\right).}
A quadratic example. Suppose that x = ( x , y ) {\displaystyle \mathbf {x} =(x,y)} and f ( x ) {\displaystyle f(\mathbf {x} )} is the quadratic form f ( x ) = x 2 + 3 x y + 2 y 2 . {\displaystyle f(\mathbf {x} )=x^{2}+3xy+2y^{2}.} Then the polarization of f {\displaystyle f} is a function in x ( 1 ) = ( x ( 1 ) , y ( 1 ) ) {\displaystyle \mathbf {x} ^{(1)}=(x^{(1)},y^{(1)})} and x ( 2 ) = ( x ( 2 ) , y ( 2 ) ) {\displaystyle \mathbf {x} ^{(2)}=(x^{(2)},y^{(2)})} given by F ( x ( 1 ) , x ( 2 ) ) = x ( 1 ) x ( 2 ) + 3 2 x ( 2 ) y ( 1 ) + 3 2 x ( 1 ) y ( 2 ) + 2 y ( 1 ) y ( 2 ) . {\displaystyle F\left(\mathbf {x} ^{(1)},\mathbf {x} ^{(2)}\right)=x^{(1)}x^{(2)}+{\frac {3}{2}}x^{(2)}y^{(1)}+{\frac {3}{2}}x^{(1)}y^{(2)}+2y^{(1)}y^{(2)}.} More generally, if f {\displaystyle f} is any quadratic form then the polarization of f {\displaystyle f} agrees with the conclusion of the polarization identity .
A cubic example. Let f ( x , y ) = x 3 + 2 x y 2 . {\displaystyle f(x,y)=x^{3}+2xy^{2}.} Then the polarization of f {\displaystyle f} is given by F ( x ( 1 ) , y ( 1 ) , x ( 2 ) , y ( 2 ) , x ( 3 ) , y ( 3 ) ) = x ( 1 ) x ( 2 ) x ( 3 ) + 2 3 x ( 1 ) y ( 2 ) y ( 3 ) + 2 3 x ( 3 ) y ( 1 ) y ( 2 ) + 2 3 x ( 2 ) y ( 3 ) y ( 1 ) . {\displaystyle F\left(x^{(1)},y^{(1)},x^{(2)},y^{(2)},x^{(3)},y^{(3)}\right)=x^{(1)}x^{(2)}x^{(3)}+{\frac {2}{3}}x^{(1)}y^{(2)}y^{(3)}+{\frac {2}{3}}x^{(3)}y^{(1)}y^{(2)}+{\frac {2}{3}}x^{(2)}y^{(3)}y^{(1)}.}
The polarization of a homogeneous polynomial of degree d {\displaystyle d} is valid over any commutative ring in which d ! {\displaystyle d!} is a unit . In particular, it holds over any field of characteristic zero or whose characteristic is strictly greater than d . {\displaystyle d.}
For simplicity, let k {\displaystyle k} be a field of characteristic zero and let A = k [ x ] {\displaystyle A=k[\mathbf {x} ]} be the polynomial ring in n {\displaystyle n} variables over k . {\displaystyle k.} Then A {\displaystyle A} is graded by degree, so that A = ⨁ d A d . {\displaystyle A=\bigoplus _{d}A_{d}.} The polarization of algebraic forms then induces an isomorphism of vector spaces in each degree A d ≅ Sym d k n {\displaystyle A_{d}\cong \operatorname {Sym} ^{d}k^{n}} where Sym d {\displaystyle \operatorname {Sym} ^{d}} is the d {\displaystyle d} -th symmetric power .
These isomorphisms can be expressed independently of a basis as follows. If V {\displaystyle V} is a finite-dimensional vector space and A {\displaystyle A} is the ring of k {\displaystyle k} -valued polynomial functions on V {\displaystyle V} graded by homogeneous degree, then polarization yields an isomorphism A d ≅ Sym d V ∗ . {\displaystyle A_{d}\cong \operatorname {Sym} ^{d}V^{*}.}
Furthermore, the polarization is compatible with the algebraic structure on A {\displaystyle A} , so that A ≅ Sym ∙ V ∗ {\displaystyle A\cong \operatorname {Sym} ^{\bullet }V^{*}} where Sym ∙ V ∗ {\displaystyle \operatorname {Sym} ^{\bullet }V^{*}} is the full symmetric algebra over V ∗ . {\displaystyle V^{*}.} | https://en.wikipedia.org/wiki/Polarization_of_an_algebraic_form |
Polarization spectroscopy comprises a set of spectroscopic techniques based on polarization properties of light (not necessarily visible one; UV , X-ray , infrared , or in any other frequency range of the electromagnetic radiation ). By analyzing the polarization properties of light, decisions can be made about the media that emitted the light (or the media the light passes/scatters through). Alternatively, a source of polarized light may be used to probe a media; in this case, the changes in the light polarization (compared to the incidental light) allow inferences about the media's properties.
In general, any kind of anisotropy in the media results in some sort of change in polarization. Such an anisotropy can be either inherent to the media (e.g., in the case of a crystal substance), or imposed externally (e.g., in the presence of magnetic field in plasma or by another laser beam).
This spectroscopy -related article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Polarization_spectroscopy |
The polarized targets are used as fixed targets in scattering experiments. In high energy physics they are used to study the nucleon spin structure of simple nucleons like protons , neutrons or deuterons . In deep inelastic scattering the hadron structure is probed with electrons , muons or neutrinos .
Using a polarized high energy muon beam, for example, on a fixed target with polarized nucleons it is possible to probe the spin dependent part of the structure functions . [ 1 ] [ 2 ]
In the simple parton model the nucleon consists of quarks and gluons and their interaction is governed by quantum chromodynamics . An alternative method to the fixed targets is to use two colliding polarized beams . Several institutes and laboratories work in this field. [ 3 ] [ 4 ] [ 5 ] [ 6 ] [ 7 ] [ 8 ]
An international workshop on "Polarized Sources, Targets and Polarimetry" takes place every two years. [ 9 ] [ 10 ] [ 11 ] [ 12 ] [ 13 ] [ 14 ]
The nuclear spins in the solid targets are polarized with dynamic nuclear polarization method typically in 2.5 or 5 T magnetic field. [ 15 ] [ 16 ]
The magnetic field can be generated with a superconducting magnet filled with liquid helium . The more traditional iron magnets are not preferred due to their large mass and limited geometrical acceptance for the produced particles. The target polarization during the experiment is determined with the nuclear magnetic resonance method. The integrated enhanced NMR-signals are compared to the signals taken in superfluid helium-4 bath at well known calibration temperatures around 1 K, where the spin magnetization follows the Curie law and the nuclear polarization can be calculated from the temperature by using the Brillouin function . During the polarization build up a microwave generator is used to pump the paramagnetic centers in the target material close to the electron spin resonance frequency (about 70 GHz in 2.5 T field).
In the helium-3 gas targets [ 17 ] [ 18 ] [ 19 ] optical pumping is used to polarize the nucleons.
In the frozen spin targets low temperatures are needed to preserve the polarization for long data taking periods (for the highest possible integrated luminosity )
and to reach maximum nuclear polarization for the best figure of merit. Usually a dilution refrigerator with high cooling power is used to reach temperatures below 300 mK during the polarization build up and below 50 mK in frozen spin mode. [ 20 ] [ 21 ] [ 22 ]
To preserve the paramagnetic centers in the target material it has to be kept all the time at cryogenic temperatures typically below 100 K. A horizontal dilution cryostat with the possibility to load directly the target material into the helium-3/4 mixing chamber from a liquid nitrogen bath is needed for this reason. While the beam should interact with the target material scattering from the target construction materials is not desired. This leads to an additional requirement of small material budget in terms of radiation length . Thin and low density construction materials are used for this reason in the region of the incoming beam and the scattering products.
The properties of a good polarized target material [ 4 ] are high number of polarizable nucleons compared to the total amount of nucleons, high polarization degree, short polarization build up time, slow polarization loss rate in frozen spin mode, good resistance against radiation damage and easy handling of the target material. For the dynamic nuclear polarization the material has to be doped with free radicals . Two different ways are usual: chemical doping by mixing with free radicals and creation of F-centers by irradiation in an intensive electron beam . Commonly used target materials are butanol , ammonia , [ 23 ] [ 24 ] [ 25 ] lithium hydrides [ 26 ] and their deuterated counterparts. A very interesting material is hydrogen deuteride , because it has the maximal content of polarizable nucleons. High proton polarizations have been reached in a large naphthalene single crystal using optically excited triplet states of fully deuterated pentacene guest molecules. [ 27 ] at temperatures around 100 K and magnetic field of 0.3 T. Hyperpolarized carbon-13 has been studied for medical imaging applications [ 28 ] | https://en.wikipedia.org/wiki/Polarized_target |
A polaron is a quasiparticle used in condensed matter physics to understand the interactions between electrons and atoms in a solid material. The polaron concept was proposed by Lev Landau in 1933 [ 1 ] and Solomon Pekar in 1946 [ 2 ] to describe an electron moving in a dielectric crystal where the atoms displace from their equilibrium positions to effectively screen the charge of an electron, known as a phonon cloud. This lowers the electron mobility and increases the electron's effective mass .
The general concept of a polaron has been extended to describe other interactions between the electrons and ions in metals that result in a bound state , or a lowering of energy compared to the non-interacting system. Major theoretical work has focused on solving Fröhlich and Holstein Hamiltonians . This is still an active field of research to find exact numerical solutions to the case of one or two electrons in a large crystal lattice , and to study the case of many interacting electrons.
Experimentally, polarons are important to the understanding of a wide variety of materials. The electron mobility in semiconductors can be greatly decreased by the formation of polarons. Organic semiconductors are also sensitive to polaronic effects, which is particularly relevant in the design of organic solar cells that effectively transport charge. Polarons are also important for interpreting the optical conductivity of these types of materials.
The polaron, a fermionic quasiparticle , should not be confused with the polariton , a bosonic quasiparticle analogous to a hybridized state between a photon and an optical phonon.
The energy spectrum of an electron moving in a periodical potential of a rigid crystal lattice is called the Bloch spectrum , which consists of allowed bands and forbidden bands. An electron with energy inside an allowed band moves as a free electron but has an effective mass that differs from the electron mass in vacuum. However, a crystal lattice is deformable and displacements of atoms (ions) from their equilibrium positions are described in terms of phonons . Electrons interact with these displacements, and this interaction is known as electron-phonon coupling. One possible scenario was proposed in the seminal 1933 paper by Lev Landau , which includes the production of a lattice defect such as an F-center and a trapping of the electron by this defect. A different scenario was proposed by Solomon Pekar that envisions dressing the electron with lattice polarization (a cloud of virtual polar phonons). Such an electron with the accompanying deformation moves freely across the crystal, but with increased effective mass. [ 3 ] Pekar coined for this charge carrier the term polaron .
Landau [ 4 ] and Pekar [ 5 ] constructed the basis of polaron theory. A charge placed in a polarizable medium will be screened. Dielectric theory describes the phenomenon by the induction of a polarization around the charge carrier. The induced polarization will follow the charge carrier when it is moving through the medium. The carrier together with the induced polarization is considered as one entity, which is called a polaron (see Fig. 1).
While polaron theory was originally developed for electrons, there is no fundamental reason why it could not be any other charged particle interacting with phonons. Indeed, other charged particles such as (electron) holes and ions generally follow the polaron theory. For example, the proton polaron was identified experimentally in 2017 [ 6 ] and on ceramic electrolytes after its existence was hypothesized. [ 7 ]
Usually, in covalent semiconductors the coupling of electrons with lattice deformation is weak and polarons do not form. In polar semiconductors the electrostatic interaction with induced polarization is strong and polarons are formed at low temperature, provided that their concentration is not large and the screening is not efficient. Another class of materials in which polarons are observed is molecular crystals , where the interaction with molecular vibrations may be strong. In the case of polar semiconductors, the interaction with polar phonons is described by the Fröhlich Hamiltonian. On the other hand, the interaction of electrons with molecular phonons is described by the Holstein Hamiltonian. Usually, the models describing polarons may be divided into two classes. The first class represents continuum models where the discreteness of the crystal lattice is neglected. In that case, polarons are weakly coupled or strongly coupled depending on whether the polaron binding energy is small or large compared to the phonon frequency. The second class of systems commonly considered are lattice models of polarons. In this case, there may be small or large polarons, depending on the relative size of the polaron radius to the lattice constant a .
A conduction electron in an ionic crystal or a polar semiconductor is the prototype of a polaron. Herbert Fröhlich proposed a model Hamiltonian for this polaron through which its dynamics are treated quantum mechanically (Fröhlich Hamiltonian). [ 10 ] [ 11 ] The strength of electron-phonon interaction is determined by the dimensionless coupling constant α = ( e 2 / κ ) ( m / 2 ℏ 3 ω ) 1 / 2 {\displaystyle \alpha =(e^{2}/\kappa )(m/2\hbar ^{3}\omega )^{1/2}} . Here m {\displaystyle m} is electron mass, ω {\displaystyle \omega } is the phonon frequency and κ − 1 = ϵ ∞ − 1 − ϵ 0 − 1 {\displaystyle \kappa ^{-1}={\epsilon }_{\infty }^{-1}-{\epsilon }_{0}^{-1}} , ϵ 0 {\displaystyle {\epsilon }_{0}} , ϵ ∞ {\displaystyle {\epsilon }_{\infty }} are static and high frequency dielectric constants. In table 1 the Fröhlich coupling constant is given for a few solids. The Fröhlich Hamiltonian for a single electron in a crystal using second quantization notation is:
The exact form of γ depends on the material and the type of phonon being used in the model. In the case of a single polar mode γ ( q ) = i ℏ ω ( 4 π α V 0 ( ℏ m ω ) 1 / 2 ) 1 / 2 1 q {\displaystyle \gamma (q)=i\hbar \omega ({\frac {4\pi \alpha }{V_{0}}}({\frac {\hbar }{m\omega }})^{1/2})^{1/2}{\frac {1}{q}}} , here V 0 {\displaystyle V_{0}} is the volume of the unit cell. In the case of molecular crystal γ is usually momentum independent constant. A detailed advanced discussion of the variations of the Fröhlich Hamiltonian can be found in J. T. Devreese and A. S. Alexandrov. [ 12 ] The terms Fröhlich polaron and large polaron are sometimes used synonymously since the Fröhlich Hamiltonian includes the continuum approximation and long range forces. There is no known exact solution for the Fröhlich Hamiltonian with longitudinal optical (LO) phonons and linear γ {\displaystyle \gamma } (the most commonly considered variant of the Fröhlich polaron) despite extensive investigations. [ 5 ] [ 9 ] [ 10 ] [ 11 ] [ 13 ] [ 14 ] [ 15 ] [ 16 ] [ 17 ] [ 18 ]
Despite the lack of an exact solution, some approximations of the polaron properties are known.
The physical properties of a polaron differ from those of a band-carrier. A polaron is characterized by its self-energy Δ E {\displaystyle \Delta E} , an effective mass m ∗ {\displaystyle m^{*}} and by its characteristic response to external electric and magnetic fields (e. g. dc mobility and optical absorption coefficient).
When the coupling is weak ( α {\displaystyle \alpha } small), the self-energy of the polaron can be approximated as: [ 19 ]
and the polaron mass m ∗ {\displaystyle m*} , which can be measured by cyclotron resonance experiments, is larger than the band mass m {\displaystyle m} of the charge carrier without self-induced polarization: [ 20 ]
When the coupling is strong (α large), a variational approach due to Landau and Pekar indicates that the self-energy is proportional to α² and the polaron mass scales as α ⁴. The Landau–Pekar variational calculation [ 5 ] yields an upper bound to the polaron self-energy E < − C P L α 2 {\displaystyle E<-C_{PL}\alpha ^{2}} , valid for all α , where C P L {\displaystyle C_{PL}} is a constant determined by solving an integro-differential equation . It was an open question for many years whether this expression was asymptotically exact as α tends to infinity. Finally, Donsker and Varadhan, [ 21 ] applying large deviation theory to Feynman's path integral formulation for the self-energy, showed the large α exactitude of this Landau–Pekar formula. Later, Lieb and Thomas [ 22 ] gave a shorter proof using more conventional methods, and with explicit bounds on the lower order corrections to the Landau–Pekar formula.
Feynman [ 23 ] introduced the variational principle for path integrals to study the polaron. He simulated the interaction between the electron and the polarization modes by a harmonic interaction between a hypothetical particle and the electron. The analysis of an exactly solvable ("symmetrical") 1D-polaron model, [ 24 ] [ 25 ] Monte Carlo schemes [ 26 ] [ 27 ] and other numerical schemes [ 28 ] demonstrate the remarkable accuracy of Feynman's path-integral approach to the polaron ground-state energy. Experimentally more directly accessible properties of the polaron, such as its mobility and optical absorption, have been investigated subsequently.
In the strong coupling limit, α ≫ 1 {\displaystyle \alpha \gg 1} , the spectrum of excited states of a polaron begins with polaron-phonon bound states with energies less than ℏ ω 0 {\displaystyle \hbar \omega _{0}} , where ω 0 {\displaystyle \omega _{0}} is the frequency of optical phonons. [ 29 ]
In the lattice models the main parameter is the polaron binding energy: E p = 1 2 N ∑ q | γ ( q ) | 2 / ℏ ω {\displaystyle E_{p}={\frac {1}{2N}}\sum _{q}|\gamma (q)|^{2}/\hbar \omega } , [ 30 ] here summation is taken over the Brillouin zone. Note that this binding energy is purely adiabatic, i.e. does not depend on the ionic masses. For polar crystals the value of the polaron binding energy is strictly determined by the dielectric constants ϵ 0 {\displaystyle \epsilon _{0}} , ϵ ∞ {\displaystyle \epsilon _{\infty }} , and is of the order of 0.3-0.8 eV. If polaron binding energy E p {\displaystyle E_{p}} is smaller than the hopping integral t the large polaron is formed for some type of electron-phonon interactions. In the case when E p > t {\displaystyle E_{p}>t} the small polaron is formed. There are two limiting cases in the lattice polaron theory. In the physically important adiabatic limit t ≫ ℏ ω {\displaystyle t\gg \hbar \omega } all terms which involve ionic masses are cancelled and formation of polaron is described by nonlinear Schrödinger equation with nonadiabatic correction describing phonon frequency renormalization and polaron tunneling. [ 18 ] [ 31 ] [ 32 ] In the opposite limit t ≪ ℏ ω {\displaystyle t\ll \hbar \omega } the theory represents the expansion in t / ℏ ω {\displaystyle t/\hbar \omega } . [ 18 ]
The expression for the magnetooptical absorption of a polaron is: [ 33 ]
Here, ω c {\displaystyle \omega _{c}} is the cyclotron frequency for a rigid-band electron. The magnetooptical absorption Γ(Ω) at the frequency Ω takes the form Σ(Ω) is the so-called "memory function", which describes the dynamics of the polaron. Σ(Ω) depends also on α, β (β = 1 / k T {\displaystyle =1/kT} , where k {\displaystyle k} is the Boltzmann constant and T {\displaystyle T} is the temperature) and ω c {\displaystyle \omega _{c}} .
In the absence of an external magnetic field ( ω c = 0 {\displaystyle \omega _{c}=0} ) the optical absorption spectrum (3) of the polaron at weak coupling is determined by the absorption of radiation energy, which is reemitted in the form of LO phonons. At larger coupling, α ≥ 5.9 {\displaystyle \alpha \geq 5.9} , the polaron can undergo transitions toward a relatively stable internal excited state called the "relaxed excited state" (RES) (see Fig. 2). The RES peak in the spectrum also has a phonon sideband, which is related to a Franck–Condon-type transition.
A comparison of the DSG results [ 34 ] with the optical conductivity spectra given by approximation-free numerical [ 35 ] and approximate analytical approaches is given in ref. [ 36 ]
Calculations of the optical conductivity for the Fröhlich polaron performed within the Diagrammatic Quantum Monte Carlo method, [ 35 ] see Fig. 3, fully confirm the results of the path-integral variational approach [ 34 ] at α ≲ 3. {\displaystyle \alpha \lesssim 3.} In the intermediate coupling regime 3 < α < 6 , {\displaystyle 3<\alpha <6,} the low-energy behavior and the position of the maximum of the optical conductivity spectrum of ref. [ 35 ] follow well the prediction of Devreese. [ 34 ] There are the following qualitative differences between the two approaches in the intermediate and strong coupling regime: in ref., [ 35 ] the dominant peak broadens and the second peak does not develop, giving instead rise to a flat shoulder in the optical conductivity spectrum at α = 6 {\displaystyle \alpha =6} . This behavior can be attributed to the optical processes with participation of two [ 37 ] or more phonons. The nature of the excited states of a polaron needs further study.
The application of a sufficiently strong external magnetic field allows one to satisfy the resonance condition Ω = ω c + Re Σ ( Ω ) {\displaystyle \Omega =\omega _{\mathrm {c} }+\operatorname {Re} \Sigma (\Omega )} , which {(for ω c < ω {\displaystyle \omega _{c}<\omega } )} determines the polaron cyclotron resonance frequency. From this condition also the polaron cyclotron mass can be derived. Using the most accurate theoretical polaron models to evaluate Σ ( Ω ) {\displaystyle \Sigma (\Omega )} , the experimental cyclotron data can be well accounted for.
Evidence for the polaron character of charge carriers in AgBr and AgCl was obtained through high-precision cyclotron resonance experiments in external magnetic fields up to 16 T. [ 38 ] The all-coupling magneto-absorption calculated in ref., [ 33 ] leads to the best quantitative agreement between theory and experiment for AgBr and AgCl. This quantitative interpretation of the cyclotron resonance experiment in AgBr and
AgCl [ 38 ] by the theory of Peeters [ 33 ] provided one of the most convincing and clearest demonstrations of Fröhlich polaron features in solids.
Experimental data on the magnetopolaron effect, obtained using far-infrared photoconductivity techniques, have been applied to study the energy spectrum of shallow donors in polar semiconductor layers of CdTe. [ 39 ]
The polaron effect well above the LO phonon energy was studied through cyclotron resonance measurements, e. g., in II–VI semiconductors, observed in ultra-high magnetic fields. [ 40 ] The resonant polaron effect manifests itself when the cyclotron frequency approaches the LO phonon energy in sufficiently high magnetic fields.
In the lattice models the optical conductivity is given by the formula: [ 30 ]
Here E a {\displaystyle E_{a}} is the activation energy of polaron, which is of the order of polaron binding energy E p {\displaystyle E_{p}} . This formula was derived and extensively discussed in [ 41 ] [ 42 ] [ 43 ] and was tested experimentally for example in photodoped parent compounds of high temperature superconductors. [ 44 ]
The great interest in the study of the two-dimensional electron gas (2DEG) has also resulted in many investigations on the properties of polarons in two dimensions. [ 45 ] [ 46 ] [ 47 ] A simple model for the 2D polaron system consists of an electron confined to a plane, interacting via the Fröhlich interaction with the LO phonons of a 3D surrounding medium. The self-energy and the mass of such a 2D polaron are no longer described by the expressions valid in 3D; for weak coupling they can be approximated as: [ 48 ] [ 49 ]
It has been shown that simple scaling relations exist, connecting the physical properties of polarons in 2D with those in 3D. An example of such a scaling relation is: [ 47 ]
where m 2 D ∗ {\displaystyle m_{\mathrm {2D} }^{*}} ( m 3 D ∗ {\displaystyle m_{\mathrm {3D} }^{*}} ) and m 2 D {\displaystyle m_{\mathrm {2D} }} ( m 3 D {\displaystyle m_{\mathrm {3D} }} ) are, respectively, the polaron and the electron-band masses in 2D (3D).
The effect of the confinement of a Fröhlich polaron is to enhance the effective polaron coupling. However, many-particle effects tend to counterbalance this effect because of screening. [ 45 ] [ 50 ]
Also in 2D systems cyclotron resonance is a convenient tool to study polaron effects. Although several other effects have to be taken into account (nonparabolicity of the electron bands, many-body effects, the nature of the confining potential, etc.), the polaron effect is clearly revealed in the cyclotron mass. An interesting 2D system consists of electrons on films of liquid He. [ 51 ] [ 52 ] In this system the electrons couple to the ripplons of the liquid He, forming "ripplopolarons". The effective coupling can be relatively large and, for some values of the parameters, self-trapping can result. The acoustic nature of the ripplon dispersion at long wavelengths is a key aspect of the trapping.
For GaAs/Al x Ga 1−x As quantum wells and superlattices, the polaron effect is found to decrease the energy of the shallow donor states at low magnetic fields and leads to a resonant splitting of the energies at high magnetic fields. The energy spectra of such polaronic systems as shallow donors ("bound polarons"), e. g., the D 0 and D − centres, constitute the most complete and detailed polaron spectroscopy realised in the literature. [ 53 ]
In GaAs/AlAs quantum wells with sufficiently high electron density, anticrossing of the cyclotron-resonance spectra has been observed near the GaAs transverse optical (TO) phonon frequency rather than near the GaAs LO-phonon frequency. [ 54 ] This anticrossing near the TO-phonon frequency was explained in the framework of the polaron theory. [ 55 ]
Besides optical properties, [ 9 ] [ 17 ] [ 56 ] many other physical properties of polarons have been studied, including the possibility of self-trapping, polaron transport, [ 57 ] [ 58 ] magnetophonon resonance, etc.
Significant are also the extensions of the polaron concept: acoustic polaron, piezoelectric polaron, electronic polaron, bound polaron, trapped polaron, spin polaron, molecular polaron, solvated polarons, polaronic exciton, Jahn-Teller polaron, small polaron, bipolarons and many-polaron systems. [ 9 ] These extensions of the concept are invoked, e. g., to study the properties of conjugated polymers, colossal magnetoresistance perovskites, high- T c {\displaystyle T_{c}} superconductors, layered MgB 2 superconductors, fullerenes, quasi-1D conductors, semiconductor nanostructures.
The possibility that polarons and bipolarons play a role in high- T c {\displaystyle T_{c}} superconductors has renewed interest in the physical properties of many-polaron systems and, in particular, in their optical properties. Theoretical treatments have been extended from one-polaron to many-polaron systems. [ 9 ] [ 59 ] [ 60 ]
A new aspect of the polaron concept has been investigated for semiconductor nanostructures : the exciton-phonon states are not factorizable into an adiabatic product Ansatz, so that a non-adiabatic treatment is needed. [ 61 ] The non-adiabaticity of the exciton-phonon systems leads to a strong enhancement of the phonon-assisted transition probabilities (as compared to those treated adiabatically) and to multiphonon optical spectra that are considerably different from the Franck–Condon progression even for small values of the electron-phonon coupling constant as is the case for typical semiconductor nanostructures. [ 61 ]
In biophysics Davydov soliton is a propagating along the protein α-helix self-trapped amide I excitation that is a solution of the Davydov Hamiltonian. The mathematical techniques that are used to analyze Davydov's soliton are similar to some that have been developed in polaron theory. In this context the Davydov soliton corresponds to a polaron that is (i) large so the continuum limit approximation in justified, (ii) acoustic because the self-localization arises from interactions with acoustic modes of the lattice, and (iii) weakly coupled because the anharmonic energy is small compared with the phonon bandwidth. [ 62 ]
It has been shown that the system of an impurity in a Bose–Einstein condensate is also a member of the polaron family. [ 63 ] This allows the hitherto inaccessible strong coupling regime to be studied, since the interaction strengths can be externally tuned through the use of a Feshbach resonance . This was recently realized experimentally by two research groups. [ 64 ] [ 65 ] The existence of the polaron in a Bose–Einstein condensate was demonstrated for both attractive and repulsive interactions, including the strong coupling regime and dynamically observed. [ 66 ] | https://en.wikipedia.org/wiki/Polaron |
Polatuzumab vedotin , sold under the brand name Polivy , is a CD79b -directed antibody-drug conjugate medication used for the treatment of diffuse large B-cell lymphoma ( cancer ). [ 6 ] It was developed by the Genentech subsidiary of Roche . [ 8 ]
The most common side effects include low levels of white blood cells (neutropenia), platelets (thrombocytopenia) and red blood cells (anemia); nerve damage (peripheral neuropathy); fatigue; diarrhea; fever; decreased appetite; and pneumonia. [ 9 ]
Polatuzumab vedotin was approved for medical use in the United States in June 2019, [ 9 ] [ 10 ] [ 11 ] in Australia in October 2019, [ 1 ] in the European Union in January 2020, [ 7 ] and in Canada in November 2020. [ 3 ]
Polatuzumab vedotin is indicated in combination with bendamustine and a rituximab product for the treatment of adults with relapsed or refractory diffuse large B-cell lymphoma. [ 6 ] [ 7 ]
Polivy in combination with rituximab, cyclophosphamide, doxorubicin, and prednisone (R-CHP) is indicated for the treatment of adults with previously untreated diffuse large B-cell lymphoma (DLBCL). [ 6 ] [ 7 ]
In June 2019, polatuzumab vedotin was approved in the United States in combination with the chemotherapy bendamustine and a rituximab product, to treat adults with diffuse large B-cell lymphoma (DLBCL) that has progressed or returned after at least two prior therapies. [ 9 ] Polatuzumab vedotin is a novel antibody-drug conjugate, and DLBCL is the most common type of non-Hodgkin lymphoma. [ 9 ] The US Food and Drug Administration (FDA) granted accelerated approval to polatuzumab vedotin used in combination with the chemotherapy bendamustine and a rituximab product. [ 9 ]
The FDA approved polatuzumab vedotin based primarily on evidence from one clinical trial (NCT02257567) that was conducted in the United States, Canada, Europe, and Asia. [ 11 ] Participants in the trial had lymphoma that came back or did not improve after prior treatment. [ 11 ] The FDA granted the application of polatuzumab vedotin breakthrough therapy , priority review , and orphan drug designations. [ 9 ] The FDA granted the approval of Polivy to Genentech. [ 9 ]
Polatuzumab vedotin was approved for medical use in the European Union in January 2020, as a second-line treatment. [ 7 ] The European Medicines Agency (EMA) designated polatuzumab vedotin an orphan medicine in April 2018. [ 7 ] In March 2022, the European Medicines Agency's Committee for Medicinal Products for Human Use recommended Polatuzumab vedotin, in combination with R-CHP or R-CHOP , as a primary treatment. [ 12 ]
In February 2023, polatuzumab vedotin was recommended by the National Institute for Health and Care Excellence (NICE) to be used in combination with rituximab, cyclophosphamide , doxorubicin and prednisolone (R-CHP) for untreated diffuse large B-cell lymphoma ( DLBCL ). [ 13 ]
In April 2023, the FDA approved polatuzumab vedotin in combination with rituximab, cyclophosphamide , doxorubicin and prednisone as first-line therapy for people with previously untreated diffuse large B-cell lymphoma, not otherwise specified or high-grade B-cell lymphoma who have an International Prognostic Index score of two or greater. [ 14 ] [ 15 ] [ 16 ] [ 17 ] [ 18 ]
Polatuzumab vedotin is the international nonproprietary name [ 19 ] [ 20 ] and the United States Adopted Name . [ 21 ] | https://en.wikipedia.org/wiki/Polatuzumab_vedotin |
A polder ( Dutch pronunciation: [ˈpɔldər] ⓘ ) is a low-lying tract of land that forms an artificial hydrological entity, enclosed by embankments known as dikes . The three types of polder are:
The ground level in drained marshes subsides over time. All polders will eventually be below the surrounding water level some or all of the time. Water enters the low-lying polder through infiltration and water pressure of groundwater , or rainfall, or transport of water by rivers and canals. This usually means that the polder has an excess of water, which is pumped out or drained by opening sluices at low tide . Care must be taken not to set the internal water level too low. Polder land made up of peat (former marshland) will sink in relation to its previous level, because of peat decomposing when exposed to oxygen from the air.
Polders are at risk of flooding at all times, and care must be taken to protect the surrounding dikes. Dikes are typically built with locally available materials, and each material has its own risks: sand is prone to collapse owing to saturation by water; dry peat is lighter than water and potentially unable to retain water in very dry seasons. Some animals dig tunnels in the barrier, allowing water to infiltrate the structure; the muskrat is known for this activity and hunted in certain European countries because of it. Polders are most commonly, though not exclusively, found in river deltas, former fenlands , and coastal areas.
Flooding of polders has also been used as a military tactic in the past. One example is the flooding of the polders along the Yser River during World War I . Opening the sluices at high tide and closing them at low tide turned the polders into an inaccessible swamp, which allowed the Allied armies to stop the German army .
The Netherlands has a large area of polders: as much as 20% of the land area has at some point in the past been reclaimed from the sea, thus contributing to the development of the country. IJsselmeer is the most famous polder project of the Netherlands. Some other countries which have polders are Bangladesh , Belgium , Canada and China . Some examples of Dutch polder projects are Beemster , Schermer , Flevopolder and Noordoostpolder .
The Dutch word polder derives successively from Middle Dutch polre , from Old Dutch polra , and ultimately from pol- , a piece of land elevated above its surroundings, with the augmentative suffix -er and epenthetical -d- . The word has been adopted in thirty-six languages. [ 1 ]
The Netherlands is frequently associated with polders, as its engineers became noted for developing techniques to drain wetlands and make them usable for agriculture and other development. This is illustrated by the saying "God created the world, but the Dutch created the Netherlands". [ 2 ]
The Dutch have a long history of reclamation of marshes and fenland, resulting in some 3,000 polders [ 3 ] nationwide. By 1961, about half of the country's land, 18,000 square kilometres (6,800 sq mi), was reclaimed from the sea. [ 4 ] [ better source needed ] About half the total surface area of polders in northwest Europe is in the Netherlands. The first embankments in Europe were constructed in Roman times. The first polders were constructed in the 11th century. The oldest extant polder is the Achtermeer polder, from 1533.
As a result of flooding disasters, water boards called waterschap (when situated more inland) or hoogheemraadschap (near the sea, mainly used in the Holland region) [ 5 ] [ 6 ] were set up to maintain the integrity of the water defences around polders, maintain the waterways inside a polder, and control the various water levels inside and outside the polder. Water boards hold separate elections, levy taxes, and function independently from other government bodies. Their function is basically unchanged even today. As such, they are the oldest democratic institutions in the country. The necessary cooperation among all ranks to maintain polder integrity gave its name to the Dutch version of third-way politics —the Polder Model .
The 1953 flood disaster prompted a new approach to the design of dikes and other water-retaining structures, based on an acceptable probability of overflowing. Risk is defined as the product of probability and consequences. The potential damage in lives, property, and rebuilding costs is compared with the potential cost of water defences. From these calculations follows an acceptable flood risk from the sea at one in 4,000–10,000 years, while it is one in 100–2,500 years for a river flood. The particular established policy guides the Dutch government to improve flood defences as new data on threat levels become available.
Major Dutch polders and the years they were laid dry include Beemster (1609–1612), Schermer (1633–1635), and Haarlemmermeerpolder (1852). Polders created as part of the Zuiderzee Works include Wieringermeerpolder (1930), Noordoostpolder (1942) and Flevopolder (1956–1968)
Several cities on the Paraíba Valley region (in the state of São Paulo ) have polders on land claimed from the floodplains around the Paraíba do Sul river.
Bangladesh has 139 polders, of which 49 are sea-facing, while the rest are along the numerous distributaries of the Ganges-Brahmaputra-Meghna River delta. These were constructed in the 1960s to protect the coast from tidal flooding and reduce salinity incursion. [ 7 ] They reduce long-term flooding and waterlogging following storm surges from tropical cyclones . They are also cultivated for agriculture. [ 8 ]
The Jiangnan region, at the Yangtze River Delta , has a long history of constructing polders. Most of these projects were performed between the 10th and 13th centuries. [ 10 ] The Chinese government also assisted local communities in constructing dikes for swampland water drainage. [ 11 ] The Lijia (里甲) self-monitoring system of 110 households under a lizhang (里长) headman was used for the purposes of service administration and tax collection in the polder, with a liangzhang (粮长, grain chief) responsible for maintaining the water system and a tangzhang (塘长, dike chief) for polder maintenance. [ 12 ]
In Germany, land reclaimed by diking is called a koog . The German Deichgraf system was similar to the Dutch and is widely known from Theodor Storm 's novella The Rider on the White Horse .
In southern Germany, the term polder is used for retention basins recreated by opening dikes during river floodplain restoration , a meaning somewhat opposite to that in coastal context. | https://en.wikipedia.org/wiki/Polder |
The Polder tensor is a tensor introduced by Dirk Polder [ 1 ] for the description of magnetic permeability of ferrites . [ 2 ] The tensor notation needs to be used because ferrimagnetic material becomes anisotropic in the presence of a magnetizing field.
The tensor is described mathematically as: [ 3 ]
Neglecting the effects of damping, the components of the tensor are given by
where
γ = 1.11 × 10 5 ⋅ g {\displaystyle \gamma =1.11\times 10^{5}\cdot g\,\,} (rad / s) / (A / m) is the effective gyromagnetic ratio and g {\displaystyle g} , the so-called effective g-factor (physics) , is a ferrite material constant typically in the range of 1.5 - 2.6, depending on the particular ferrite material. f {\displaystyle f} is the frequency of the RF/microwave signal propagating through the ferrite, H 0 {\displaystyle H_{0}} is the internal magnetic bias field, M {\displaystyle M} is the magnetization of the ferrite material and μ 0 {\displaystyle \mu _{0}} is the magnetic permeability of free space .
To simplify computations, the radian frequencies of ω 0 , ω m , {\displaystyle \omega _{0},\,\omega _{m},\,} and ω {\displaystyle \omega } can be replaced with frequencies (Hz) in the equations for μ {\displaystyle \mu } and κ {\displaystyle \kappa } because the 2 π {\displaystyle 2\pi } factor cancels. In this case, γ = 1.76 × 10 4 ⋅ g {\displaystyle \gamma =1.76\times 10^{4}\cdot g\,\,} Hz / (A / m) = 1.40 ⋅ g {\displaystyle =1.40\cdot g\,\,} MHz / Oe. If CGS units are used, computations can be further simplified because the μ 0 {\displaystyle \mu _{0}} factor can be dropped. | https://en.wikipedia.org/wiki/Polder_tensor |
Pole framing or post-frame construction [ 1 ] ( pole building framing , pole building , pole barn ) is a simplified building technique that is an alternative to the labor-intensive traditional timber framing technique. It uses large poles or posts buried in the ground or on a foundation to provide the vertical structural support, along with girts to provide horizontal support. The method was developed and matured during the 1930s as agricultural practices changed, including the shift toward engine-powered farm equipment and the demand for cheaper, larger barns and storage areas.
Pole building design was pioneered in the 1930s in the United States originally using utility poles for horse barns and agricultural buildings. The depressed value of agricultural products in the 1920s, and 1930s and the emergence of large, corporate farming in the 1930s, created a demand for larger, cheaper agricultural buildings. [ 2 ] As the practice took hold, rather than using utility poles, materials such as pole barn nails were developed specifically for this type of construction, making the process more affordable and reliable. Today, almost any low-rise structure can be quickly built using the post-frame construction method. [ 3 ]
Pole barn construction was a quick and economical method of adding outbuildings on a farm as agriculture shifted to equipment-dependent and capital-intensive agriculture—necessitating shelter for tractors, harvesters, wagons and the like in much greater quantities and sizes. Around North America, many pole-built structures are still readily seen in rural and industrial areas.
Poles, from which these buildings get their name, are natural shaped or round wooden timbers 4 to 12 inches (100 to 300 mm) in diameter. [ 4 ] The structural frame of a pole building is made of tree trunks , utility poles , engineered lumber or chemically pressure-treated squared timbers which may be buried in the ground or anchored to a concrete slab . Generally the posts are evenly spaced 8 to 12 feet (2.4 to 3.7 m) apart except to allow for doors. Buried posts have the benefit of providing lateral stability [ 5 ] so no braces are needed. Buried posts may be driven into the ground or set in holes then filled with soil, crushed stone, or concrete.
Pole buildings do not require walls but may be open shelters , such as for farm animals or equipment or for use as picnic shelters.
Enclosed pole buildings have exterior curtain walls formed by girts fastened to the exterior of the posts at intervals about 2 feet (0.61 m) on center that carry the siding and any interior load. The walls may be designed as a shear wall to provide structural stability. Other girt systems include framing in between the posts rather than on the outer side of the posts. [ 6 ] Siding materials for a pole building are most commonly rolled-rib 29-gauge enameled steel cut to length in 32-or-36-inch (813 or 914 mm) widths attached using color-matched screws with rubber washers to seal the holes. However, any standard siding can be used, including T1-11 , vinyl , lap siding, cedar and even brick . Using sidings other than metal may require first installing sheathing, such as plywood , oriented strand board or boards.
On two walls, usually the long walls, the dimensional lumber girts at the top of the walls are doubled, one on the inside and one on the outside of the posts, and usually through-bolted with large carriage bolts to support the roof load . The roof structure is frequently a truss roof supporting purlins or laths , or built using common rafters . Wide buildings with common rafters need interior rows of posts. Sometimes rafters may be attached directly to the poles. The roof pitch of pole buildings is usually low and the roof form is usually gable or lean-to . Metal roofing is commonly used as the roofing and siding material on pole buildings.
The floor may be soil, gravel, concrete slab , or framed of wood.
In modern developments the pole barns of the 1930s have become pole buildings for use as housing, commercial use, churches, picnic shelters or storage buildings. In the process more often than not, the poles have become posts of squared-off, pressure-treated timbers. These structures have the potential to replicate the functionality of other buildings, but they may be more affordable and require less time to construct. The most common use for pole buildings is storage buildings as it was on the farms, but today they may be for the storage of automobiles, boats, and RVs along with many other household items that would normally be found in a residential garage, or commercially as the surroundings for a light industry or small corporate offices with attached shops. [ 7 ] | https://en.wikipedia.org/wiki/Pole_building_framing |
A pole figure is a graphical representation of the orientation of objects in space. For example, pole figures in the form of stereographic projections are used to represent the orientation distribution of crystallographic lattice planes in crystallography and texture analysis in materials science .
Consider an object with a basis attached to it. The orientation of the object in space can be determined by three rotations to transform the reference basis of space to the basis attached to the object; these are the Euler angles .
If we consider a plane of the object, the orientation of the plane can be given by its normal line . If we draw a sphere with the center on the plane, then
A single pole is not enough to fully determine the orientation of an object: the pole stays the same if we apply a rotation around the normal line. The orientation of the object is fully determined by the use of poles of two planes that are not parallel.
The upper sphere is projected on a plane using the stereographic projection .
Consider the ( x , y ) plane of the reference basis; its trace on the sphere is the equator of the sphere. We draw a line joining the South pole with the pole of interest P .
It is possible to choose any projection plane parallel to the equator (except the South pole): the figures will be proportional (property of similar triangles ). It is usual to place the projection plane at the North pole.
A Wulff net is used to read a pole figure.
The stereographic projection of a trace is an arc. The Wulff net is arcs corresponding to planes that share a common axis in the ( x , y ) plane.
If the pole and the trace of a plane are represented on the same diagram, then
Consider an axis Δ, and planes belonging to the zone of this axis, i.e. Δ is in all these planes, the intersection of all the planes is Δ. If we call P the plane that is perpendicular to Δ, then the normals to the planes all belong to P . Thus, the poles of the planes belonging to the same zone are on the trace of the plane P perpendicular to the axis.
The structure of a crystal is often represented by the pole figure of its crystallographic planes.
A plane is chosen as the equator, usually the (001) or (011) plane; its pole is the center of the figure. Then, the poles of the other planes are placed on the figure, with the Miller indices for each pole. The poles that belong to a zone are sometimes linked with the related trace.
" Texture " in the context of Materials Science means "crystallographic preferred orientation". If a polycrystalline material (i.e. a material composed of many different crystals or grains, like most metals, ceramics or minerals) has "texture" then that means that the crystal axes are not randomly (or, more correctly, uniformly) distributed.
To draw a pole figure , one chooses a particular crystal direction (e.g. the normal to the (100) plane) and then plots that direction, called a pole, for every crystal relative to a set of directions in the material. In a rolled metal, for example, the directions in the material are the rolling direction, transverse direction and rolling plane normal.
If a large number of crystals are involved, then it is typical to make a contour plot , rather than plotting individual poles.
The full determination of the texture requires the plot of two pole figures corresponding to planes that are not parallel and that do not have the same diffraction angle (thus different interplanar distances).
Consider the diffraction pattern obtained with a single crystal, on a plane that is perpendicular to the beam, e.g. X-ray diffraction with the Laue method , or electron diffraction in a transmission electron microscope . The diffraction figure shows spots.
The position of the spots is determined by the Bragg's law . It gives the orientation of the plane.
If the parameters of the optics are known (especially the distance between the crystal and the photographic film), it is possible to build the stereographic diagram from the diffraction diagram, i.e. to transform the diffraction pattern into a pole figure. [ citation needed ] | https://en.wikipedia.org/wiki/Pole_figure |
The Polenske value (also known as the Polenske number ) is a value determined when examining fat . It is an indicator of how much volatile fatty acid can be extracted from fat through saponification . It is equal to the number of milliliters of 0.1 normal alkali solution necessary for the neutralization of the water-insoluble volatile fatty acids distilled and filtered from 5 grams of a given saponified fat. (The hydroxide solution used in such a titration is typically made from sodium hydroxide , potassium hydroxide , or barium hydroxide .) [ 1 ] It is measure of the steam volatile and water insoluble fatty acids, chiefly caprylic, capric and lauric acids, present in oil and fat. The value is named for the chemist who developed it, Eduard Polenske. [ 2 ]
The Reichert value and Kirschner value are related numbers based on similar tests.
This organic chemistry article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Polenske_value |
The poles of astronomical bodies are determined based on their axis of rotation in relation to the celestial poles of the celestial sphere . Astronomical bodies include stars , planets , dwarf planets and small Solar System bodies such as comets and minor planets (e.g., asteroids ), as well as natural satellites and minor-planet moons .
The International Astronomical Union (IAU) defines the north pole of a planet or any of its satellites in the Solar System as the planetary pole that is in the same celestial hemisphere, relative to the invariable plane of the Solar System, as Earth's north pole. [ 1 ] This definition is independent of the object's direction of rotation about its axis. This implies that an object's direction of rotation, when viewed from above its north pole, may be either clockwise or counterclockwise. The direction of rotation exhibited by most objects in the solar system (including Sun and Earth) is counterclockwise. Venus rotates clockwise, and Uranus has been knocked on its side and rotates almost perpendicular to the rest of the Solar System. The ecliptic remains within 3° of the invariable plane over five million years, [ 2 ] but is now inclined about 23.44° to Earth's celestial equator used for the coordinates of poles. This large inclination means that the declination of a pole relative to Earth's celestial equator could be negative even though a planet's north pole (such as Uranus's) is north of the invariable plane.
In 2009 the responsible IAU Working Group decided to define the poles of dwarf planets, minor planets, their satellites, and comets according to the right-hand rule . [ 1 ] To avoid confusion with the "north" and "south" definitions relative to the invariable plane, the poles are called "positive" and "negative." The positive pole is the pole toward which the thumb points when the fingers of the right hand are curled in its direction of rotation. The negative pole is the pole toward which the thumb points when the fingers of the left hand are curled in its direction of rotation. This change was needed because the poles of some asteroids and comets precess rapidly enough for their north and south poles to swap within a few decades using the invariable plane definition.
The projection of a planet's north pole onto the celestial sphere gives its north celestial pole . The location of the celestial poles of some selected Solar System objects is shown in the following table. [ 1 ] The coordinates are given relative to Earth's celestial equator and the vernal equinox as they existed at J2000 (2000 January 1 12:00:00 TT ) which is a plane fixed in inertial space now called the International Celestial Reference Frame (ICRF). Many poles precess or otherwise move relative to the ICRF, so their coordinates will change. The Moon's poles are particularly mobile.
Some bodies in the Solar System, including Saturn 's moon Hyperion and the asteroid 4179 Toutatis , lack a stable north pole. They rotate chaotically because of their irregular shape and gravitational influences from nearby planets and moons, and as a result the instantaneous pole wanders over their surface, and may momentarily vanish altogether (when the object comes to a standstill with respect to the distant stars).
Planetary magnetic poles are defined analogously to the Earth's North and South magnetic poles : they are the locations on the planet's surface at which the planet's magnetic field lines are vertical. The direction of the field determines whether the pole is a magnetic north or south pole, exactly as on Earth. The Earth's magnetic axis is approximately aligned with its rotational axis, meaning that the geomagnetic poles are relatively close to the geographic poles. However, this is not necessarily the case for other planets; the magnetic axis of Uranus , for example, is inclined by as much as 60°.
In addition to the rotational pole, a planet's orbit also has a defined direction in space. The direction of the angular momentum vector of that orbit can be defined as an orbital pole . Earth's orbital pole, i.e. the ecliptic pole, points in the direction of the constellation Draco .
In the particular (but frequent) case of synchronous satellites, four more poles can be defined. They are the near , far , leading , and trailing poles. For example, Io , one of the moons of Jupiter , rotates synchronously, so its orientation with respect to Jupiter stays constant. There will be a single, unmoving point of its surface where Jupiter is at the zenith , exactly overhead – this is the near pole , also called the sub- or pro-Jovian point. At the antipode of this point is the far pole , where Jupiter lies at the nadir ; it is also called the anti-Jovian point. There will also be a single unmoving point which is farthest along Io's orbit (best defined as the point most removed from the plane formed by the north-south and near-far axes, on the leading side) – this is the leading pole . At its antipode lies the trailing pole . Io can thus be divided into north and south hemispheres, into pro- and anti-Jovian hemispheres, and into leading and trailing hemispheres. These poles are mean poles because the points are not, strictly speaking, unmoving: there is continuous libration about the mean orientation, because Io's orbit is slightly eccentric and the gravity of the other moons disturbs it regularly. | https://en.wikipedia.org/wiki/Poles_of_astronomical_bodies |
The Polevskoy Copper Smelting Plant ( Russian : Полевской медеплавильный завод , romanized : Polevskoj medeplavilnyj zavod ), also known as Polevaya or Poleva , [ 1 ] was one of the major metallurgical facilities located in Polevskoy , in Sverdlovsk Oblast of Russia .
It was established by the decree of Peter the Great to process the local copper deposits. The Polevskoy Plant was named after the local river Polevaya. The plant became the basis for the settlement which later grew into the town of Polevskoy.
The place for a new plant was chosen by Vasily Tatishchev . Georg Wilhelm de Gennin was in charge of the construction. The Plant became active in 1724. The copper was branded with the symbol of the Roman goddess Venus . [ 2 ] The Venus symbol (♀), which represents copper as a chemical element, is now displayed in the Polevskoy town coat of arms.
In 1757 the Polevskoy Plant was purchased by the Ural merchant Alexei Turchaninov along with the Seversky Pipe Plant . Along with the Seversky and Sysertsky Plants, the Polevskoy Plant represented the Sysert Mining District. It was extremely profitable. [ 3 ] Turchaninov's descendants managed the plant til 1912. For a long time the main ore supplier for the plant was the Gumyoshevsky mine , called "The Copper Mountain" by the local populace. [ 1 ] However in the 19th century the Gumyoshevsky mine was exhausted. After it was shut down in 1870, the Polevskoy Copper Smelting Plant was reorganized to the iron plant. In this capacity the plant stayed active until 1923, but then shut down due to the lack of prospects. | https://en.wikipedia.org/wiki/Polevskoy_Copper_Smelting_Plant |
The details of a spinning body may impose restrictions on the motion of its angular velocity vector, ω . The curve produced by the angular velocity vector on the inertia ellipsoid , is known as the polhode , coined from Greek meaning "path of the pole". The surface created by the angular velocity vector is termed the body cone .
The concept of polhode motion dates back to the 17th century, and Corollary 21 to Proposition 66 in Section 11, Book 1, of Isaac Newton 's Principia Mathematica . Later Leonhard Euler derived a set of equations that described the dynamics of rigid bodies in torque-free motion. In particular, Euler and his contemporaries Jean d’Alembert , Louis Lagrange , and others noticed small variations in latitude due to wobbling of the Earth around its polar spin axis . A portion of this wobble (later to be called the Earth’s polhode motion) was due to the natural, torque -free behavior of the rotating Earth. Assuming that the Earth was a completely rigid body , they calculated the period of Earth’s polhode wobble to be about 9–10 months . [ citation needed ]
During the mid 19th century, Louis Poinsot developed a geometric interpretation of the physics of rotating bodies that provided a visual counterpart to Euler’s algebraic equations. Poinsot was a contemporary of Léon Foucault , who invented the gyroscope and whose pendulum experiments provided incontrovertible evidence that the Earth rotates. In the fashion of the day, Poinsot coined the terms polhode and its counterpart, herpolhode , to describe this wobble in the motion of rotating rigid bodies. Poinsot derived these terms from the ancient Greek πόλος pólos (pivot or end of an axis) + ὁδός hodós (path or way)—thus, polhode is the path of the pole .
Poinsot’s geometric interpretation of Earth’s polhode motion was still based on the assumption that the Earth was a completely rigid rotating body. It was not until 1891 that the American astronomer, Seth Carlo Chandler , made measurements showing that there was a periodic motion of 14 months in the Earth’s wobble and suggesting that this was the polhode motion. Initially, Chandler’s measurement, now referred to as the “ Chandler wobble ”, was dismissed because it was significantly greater than the long-accepted 9–10 month period calculated by Euler, Poinsot, et al. and because Chandler was unable convincingly to explain this discrepancy. However, within months, another American astronomer, Simon Newcomb , realized that Chandler was correct and provided a plausible reason for Chandler’s measurements. Newcomb realized that the Earth’s mass is partly rigid and partly elastic , and that the elastic component has no effect on the Earth’s polhode period, because the elastic part of the Earth’s mass stretches so that it is always symmetrical about the Earth’s spin axis. The rigid part of the Earth’s mass is not symmetrically distributed, and this is what causes the Chandler Wobble, or more precisely, the Earth’s polhode path.
Every solid body inherently has three principal axes through its center of mass , and each of these axes has a corresponding moment of inertia . The moment of inertia about an axis is a measurement of how difficult it is to accelerate the body about that axis. The closer the concentration of mass to the axis, the smaller the torque required to get it spinning at the same rate about that axis.
The moment of inertia of a body depends on the mass distribution of the body and on the arbitrarily selected axis about which the moment of inertia is defined. The moments of inertia about two of the principal axes are the maximum and minimum moments of inertia of the body about any axis. The third is perpendicular to the other two and has a moment of inertia somewhere between the maximum and minimum.
If energy is dissipated while an object is rotating, this will cause the polhode motion about the axis of maximum inertia (also called the major principal axis ) to damp out or stabilize, with the polhode path becoming a smaller and smaller ellipse or circle , closing in on the axis.
A body is never stable when spinning about the intermediate principal axis, and dissipated energy will cause the polhode to start migrating to the object’s axis of maximum inertia . The transition point between two stable axes of rotation is called the separatrix along which the angular velocity passes through the axis of intermediate inertia.
Rotation about the axis of minimum inertia (also called the minor principal axis ) is also stable, but given enough time, any perturbations due to energy dissipation or torques would cause the polhode path to expand, in larger and larger ellipses or circles, and eventually migrate through the separatrix and its axis of intermediate inertia to its axis of maximum inertia.
It is important to note that these changes in the orientation of the body as it spins may not be due to external torques, but rather result from energy dissipated internally as the body is spinning. Even if angular momentum is conserved (no external torques), internal energy can be dissipated during rotation if the body is not perfectly rigid, and any rotating body will continue to change its orientation until it has stabilized around its axis of maximum inertia, where the amount of energy corresponding to its angular momentum is least. [ 1 ] | https://en.wikipedia.org/wiki/Polhode |
A policeman is a hand-held flexible natural-rubber or plastic scraper. The common type of it is attached to a glass rod and used in chemical laboratories to transfer residues of precipitate or solid on glass surfaces when performing gravimetric analysis . This equipment works well under gentle, delicate and precise requirement. A policeman also comes in various sizes, shapes, and types. Some of them come in one-piece flexible plastic version and some in stainless. The origin of the policeman and its name cannot be identified for sure but some clues led back to the 19th century from German chemist Carl Remigius Fresenius .
A policeman is generally a flexible natural-rubber blade attached to a glass rod, which is typically 5 mm to 6 mm diameter and 150 mm long. However, it also comes in various sizes and shapes depending on its uses. The rubber material provides chemical resistance. In some designs, there is no glass rod, but instead the whole item is made of plastic or stainless steel and is shaped into a spatula or scraper shape at the end.
A policeman can be used for cleaning the inside of glassware, or for getting the last bit of precipitate out of a vessel. Especially in chemical laboratories it is often used to transfer residues of precipitate or solid on glass surfaces when performing the gravimetric analysis. It also used in biological laboratories, to transfer tissue culture cells from a plate to a suspension. It feature is to prevent the glass rod from scratching or breaking glassware.
There is no answer on where the name "policeman" comes from, though it may be related to the function of the instrument.
In chemistry, gravimetric analysis is essential. After precipitating the chemical element of interest, successfully transferring all of the precipitate to the filtration funnel for separation from the supernatant liquid is required. This can be done by using a stream of distilled water from a wash bottle . This is less effective because dense precipitates may become compacted at the bottom of the beaker, while light precipitates may be dispersed on the walls of the beaker. A glass rod can be used to remove the precipitate but this risks poking a hole in the bottom of the beaker or scratching the beaker wall. In the 19th century, German chemist, Carl Remigius Fresenius suggested a solution to overcome this problem using a device similar to the rubber policeman. [ 1 ]
Then rubber policeman was also recorded in 1910 edition of J. C. Olsen's textbook of quantitative analysis that states " particles adhering to the glass must be removed by means of a so-called policeman, which is made by inserting the end of a rather thick large-sized glass stirring-rod into a short piece of rubber tubing. The rubber tube should be left protruding slightly beyond the end of the glass tube and sealed together with a little bicycle [i.e. rubber] cement." [ 1 ]
However, it seems that Olsen has nothing to do with the mass production and sales of this invention. Instead, Oesper Collections catalog indicated that Henry Heil Company of St. Louis sold policemen as early as 1904. [ 1 ]
The second speculation is the most likely the one since in the 1937 edition of Hackh's Chemical Dictionary "platinum policeman," defined as "a platinum-iridium claw that fits over a glass rod and is used to hold a quantitative filter during ignition," [ 2 ] which the purpose of the policeman was to prevent the escape of stray filter paper from the crucible during the ignition process that causes thermal updrafts from the burner. Therefore, for policeman, it likely means to prevent the escape of stray precipitate. | https://en.wikipedia.org/wiki/Policeman_(laboratory) |
Policies promoting wireless broadband are policies, rules, and regulations supporting the "National Wireless Initiative", a plan to bring wireless broadband Internet access to 98% of Americans. [ 1 ]
Spectrum is limited and much of it already in use. This raises the issue of space and strength of supporting the network. The infrastructure has to reach across the entire United States in areas that normally do not have Internet access. The main concept is to bring wireless service to residents in areas that may otherwise not have access to it. [ 2 ] The public's interest in this plan is important as the people are the ones who will utilize this service. Network neutrality raises issues on freedom of information and who will have control over how the information is released, or even lack of control.
The Memorandum on Unleashing the Wireless Broadband Revolution claimed that wireless Internet access had the potential to enhance economic competition and improve the quality of life. [ 3 ] The Internet is considered an important part of the economy and advanced business opportunities as it is a vital infrastructure . [ 3 ] The Code of Federal Regulations says that this is the beginning of the next transformation in information technology , as we encounter the wireless broadband revolution. [ 3 ]
The initial plan by President George W. Bush was to have broadband availability for all Americans by 2007. [ 4 ] In February, 2011, President Obama announced details of the "National Wireless Initiative" or "Wireless Innovation and Infrastructure Initiative".
Federal Communications Commission (FCC) Chairperson Michael K. Powell created the Wireless Broadband Access Task Force to help bring the plan together. The members study existing wireless broadband policies, making recommendations in the FCC's policies for acceleration in the deployment of the wireless technologies and services. [ 4 ] [ 5 ] This is completed by seeking out the expertise, experience, and advice of consumers, state and local governments, the industry, and other stakeholders . [ 4 ] These recommendations are intended to assist with how to make policies and further progress the process for the national wireless plan. They are based on the inquiry of the state of wireless broadband as well as the FCC's policies that impact these services. [ 4 ]
Powell commented in a statement that this broadband plan is a catalyst for positive change, bringing resources and jobs to communities across the country. [ 5 ] The CTIA The Wireless Association encouraged legislative action that recognizes the unique and invaluable role of wireless in providing Americans Internet access . [ 6 ]
This plan included issues such as:
The plan is to free up enough of the radio spectrum from licensed and unlicensed space. To free up space, the FCC intends to hold incentive auctions , spurring innovation, by current licensees voluntarily giving up their spectrum space. [ 1 ] [ 7 ] [ 8 ] The CTIA has requested the FCC to obtain more capacity on the spectrum by placing priority on additional spectrum through the national wireless plan. [ 6 ] This hopes to ensure space on the spectrum for the wireless broadband to work. The auction would also require legislation to conduct for the spectrum to be reassigned and reallocated. In this plan, legislation is needed for the FCC to hold these auctions to enable the current spectrum holders to realize a portion of the revenues if they participate. [ 1 ] These voluntary incentive auctions for licensees are a critical part of freeing the spectrum, as well as encouraging the government to more efficiently use the spectrum. [ 1 ] [ 3 ] The auction is intended to bring profit back to the United States and new licensees. However, it was brought up in February 2011 that it is not clear on whether these incentive auctions will take place. [ 2 ] One goal of the plan is to reduce the national deficit by approximately $10 billion through these license auctions and other business opportunities. [ 1 ] The auctions and increased spectrum efficiency from the government would raise $27.8 billion over the next ten years. [ 1 ] The government will be expected to use the spectrum more efficiently using new financial-compensation tools with commitments to using advanced technologies. [ 1 ] [ 6 ] There is also the idea to spur innovation by using $3 billion of the spectrum proceeds for research and development of newer wireless technologies and applications. [ 1 ]
Wireless requires bandwidth , and because of this, there would need to be enough of the spectrum obtained to sustain the bandwidth of the network. It also needs to be considered that upload links and downstream communication can require a difference amount of space. In the FCC News in 2005, it is mentioned that there needs to be enough spectrum to account for the unbalancing of broadband services. The service typically needs a larger amount of bandwidth for downstream than for upload links. [ 4 ] Wireless will only work with adequate spectrum to support the initiative and the many devices, networks, and applications that it will run. [ 3 ] President Obama has set a goal of freeing 500 MHz of the spectrum for any wireless device within a decade. [ 1 ] [ 3 ] The CTIA also supports this within the next ten years. [ 6 ] The space would also need to be within the stronger part of the spectrum. According to the 112th United States Congress , the Public Safety Spectrum and Wireless Innovation Act calls for this to be within the 700 MHz D block spectrum for rural and urban areas and was originally requested to be done before the Digital TV transition . [ 4 ] [ 7 ] [ 8 ] The 700 MHz D block refers to the portion of the spectrum between the following frequencies
In its pursuit to support the FCC, the CTIA also recommended the spectrum in the following ranges
With 4G deployment rising, it is critical to have the airwave space to support future innovation and to avoid the spectrum crunch. [ 1 ] This provides clearance in the spectrum that is already allocated to wireless carriers. The CTIA also have requested access to existing utility poles where new construction is not possible. [ 6 ] Although the spectrum is wide, the science and physics of the spectrum still create limited amounts of space. [ 6 ] [ 10 ]
President Obama also plans to increase public safety by reallocating the D block of the spectrum and $500 million within the Wireless Innovation (WIN) Fund. [ 1 ] [ 4 ] The 700 MHz D block spectrum would be reallocated and integrated for public safety entities. [ 7 ] [ 8 ] The Communications Act of 1934 would be amended to increase the electromagnetic spectrum by 10 MHz for public safety. [ 7 ] [ 8 ] [ 9 ] An important part of this spectrum plan is that there is already space for public safety. One piece that needs to be determined is how to integrate the 700 MHz D block that will be reallocated with the existing public safety spectrum. [ 7 ] The current 20 MHz of public safety spectrum needs to be determined how to be licensed as well. The considerations are nationwide, regional, statewide or some combination in accordance with the public interest. [ 7 ]
In Section 205 of the Public Safety Spectrum and Wireless Innovation Act of the 112th United States Congress , it is required to have a review of the use of the spectrum after a certain period of time of the deployment. After no more than 5 years of the implementation of the wireless network plan, the Commission must conduct a survey and submit a report regarding the public safety spectrum. [ 7 ] This includes how the spectrum is being used, recommendations on whether more spectrum is needed and determine if there is opportunity for some of the spectrum to be returned for other commercial purposes. [ 7 ] The report intends to make sure that there is the right amount of spectrum and ensure it is being used for the correct purposes, as there is only a limited amount of spectrum overall. From the Administration of Barack Obama , they also would like to test the value of underutilized spectrum to be able to open new avenues of use. [ 3 ] Since spectrum space is limited, looking at other spectrum could allow other licensees to move elsewhere, perhaps freeing up more for wireless. Utilizing other spectrum would allow development of advanced technologies. [ 3 ] [ 4 ] The Secretary of Commerce , National Science Foundation (NSF), Department of Defense , Department of Justice , NASA and other organizations have been designated to create a plan to explore these innovative spectrum-sharing technologies. [ 3 ]
In the 1950s, the town of Ten Sleep, Wyoming , had just set up their phone service using federal subsidies and stringing copper wire to every home. [ 11 ] In 2005, they upgraded to fiber optic cable, giving the residents high-speed Internet access. [ 11 ] This scenario caught President Obama's eye in terms of success, which he hopes to duplicate with the national wireless broadband. On February 10, 2011, he pointed to this example of what he wants to replicate and hopes it will help progress more economic development by providing Internet to almost all Americans. [ 11 ] Brendan Greenley from Business Week magazine does not believe Obama's plan will create another success story. [ 11 ] While examples are helpful to reference when creating a plan, not all plans react the same way. There are many different factors involved, such as geography and the type of users involved.
How a technology is designed and built is just as important as the technology itself. Without a proper infrastructure, a national wireless broadband network would not benefit the country. George Ford from the Phoenix Center commented that a reasonable target for broadband would be 95% Internet availability to Americans in five years and questioned the need for coverage across the entire country. [ 12 ] There has been a large permeation of Internet users over the last five years. [ when? ] However, there are still reasons to have wired networking. As stated by Brendan Greeley, call centers and data storage facilities placed in smaller towns need the speed and capacity that a wired fiber optic network can provide. [ 11 ] Wireless networks pose challenges that wired networks do not. [ 6 ] One challenge is the infrastructure of a wireless network, like the spectrum, versus a wired network. For Ten Sleep, they installed fiber optic cable to increase their network speed. Wireless does not have this capability. Fiber optic cable has more capacity than the electromagnetic spectrum, meaning even if the entire spectrum was allocated to the national wireless network, there still would not be the same capacity in fiber optic. [ 10 ]
Technology is growing at an incredible speed, with one important technology being the speed of information. The newest generation of speed at " 4G " is being deployed at rapid speeds throughout the United States by the leading carriers, and promises to be greatly beneficial to the economy and society. [ 1 ] Next-generation technology is ten times faster than current speeds and is capable of benefiting all Americans, helping public safety increase, and further progressing the innovation for wireless applications, equipment and services. [ 1 ] The advancement of technology is intended to move us [ who? ] forward and catch up with other nations that have already implemented these technologies. The technological advances in wireless broadband, like mobile broadband, provide a solid foundation for improved delivery of services. [ 4 ]
A supported infrastructure is important for any network, whether technologically based or socially based. The national wireless network is no different. Under Section 105 Interoperability of the bill S.28, the Commission must establish the technical and operational rules to ensure the national wireless networks are interoperable . [ 7 ] [ 8 ] [ 9 ] It has so far yet to be established as to who will actually support the network, being the government or a private Internet service provider . Rules are to be established to permit a public safety licensee to authorize a service provider to construct and operate the wireless network. [ 9 ] It is also in the plan to have the service provider use their licensee spectrum if the authorization would expedite broadband communications. [ 9 ] The supporting parties will also have to ensure the safety of the network by protecting and monitoring against cyber attacks and any other form of security threat. [ 7 ] [ 9 ] It is imperative to have a secure network that is accessible for the nation to use. A safe environment is needed for new capabilities to be secure, trustworthy and provide necessary safeguards for privacy of users and public safety. [ 3 ]
President Obama estimated a one-time investment of $5 billion and a reformation of the Universal Service Fund to help millions of Americans get access to these technologies. [ 1 ] Another estimated cost of the wireless broadband plan is $7.2 billion from stimulus funds. [ 12 ] Another plan is calling for $10.7 billion to commit to the support, development and deployment of the wireless broadband. [ 1 ] Despite the cost, wireless would help with public safety affordance of greater levels of effectiveness and interoperability . [ 1 ] With broadband technologies developing, equipment and services are getting faster and cheaper. Again, Obama proposes to pay for the wireless network by having broadcasters give back their privilege to the spectrum for government auction. [ 11 ] The auction would then be mostly profit so costs would come from the infrastructure and maintenance of the network, not the spectrum space. It is still questioned whether the costs are too high and if the end benefits outweigh those costs. [ 12 ] In a report by George Ford at Fox News Channel , he stated that spending money on the last frontier of broadband has small incremental value. [ 12 ] Obama not only estimated a one-time investment, but also stated a public safety cost. He called for a $10.7 billion investment to ensure public safety benefits from the technology and to have $3.2 billion to reallocate the D block of the spectrum as mentioned before. [ 1 ] This band of the spectrum would be reserved solely for public safety as stated under current law. Another $7 billion would be needed to support the deployment of the network, and then $500 million from the Wireless Innovation (WIN) Fund for research and development, and to tailor the network to public safety needs. [ 1 ] Although many billions of dollars will go towards building this plan, reducing the national deficit by billions of dollars can be considered worthwhile. Again, President Obama does hope that this plan will cut the nation deficit by $9.6 billion over the next ten years. [ 1 ]
The thought of whether the cost is too high also raises other points in the media . John Horrigan of the Pew Research Center commented that the high cost of broadband now is why more Americans are not already using it. [ 12 ] There is also the consideration that not every American has access to a computer. [ 12 ] Although smartphones with Internet capabilities have been on the rise for many years now, there is still a reason the entire nation is not "online". Whether it is the cost of an Internet-ready device or computer, or obtaining Internet service, cost is still a big factor for this technology. In terms of cost, one-third of Americans who do not have broadband access say cost prohibits them from purchasing it. [ 12 ]
Public interest is important for policies promoting wireless broadband for Americans. The interest of the public is important because if the public does not condone the cost and the utility, the nation will have to cover the failure of the deployment. However, the public may also consider this to be an excellent development for our country. Part of the national wireless broadband goal is to enable businesses to grow faster, help students learn more, and assist public safety officers with having the best, state-of-the-art technology and communications available. [ 1 ] During his State of the Union address , President Obama announced a National Wireless Initiative to make available high speed wireless Internet services for 98% of Americans. [ 1 ] This plan was primarily designed to get this technology to reach more rural areas that otherwise did not have the opportunity of obtaining the service. [ 12 ] [ 13 ] On February 10, 2011 President Obama was commended for his proposal on pursuing the plan with the idea that this would greatly increase jobs and innovation. [ 2 ] The concept of the " last mile " is often brought up for Internet Service Providers (ISP) as they try to expand their network, often time having to stop before the last house on the block because of cost. However, even though this issue happens throughout rural areas, 57% of Americans use broadband services with 91% already having access according to the Pew Internet & American Life Project. [ 12 ] Those that have Internet can access an incredible amount of information at any time as long as they have an Internet-ready device. There has also been an increase in the applications that utilize Internet services. The proliferation of wireless applications is on the rise and continues to empower users and communities. [ 4 ]
A national wireless broadband network is not only about providing Internet access for personal computers in the home, but for anyone with a wireless Internet-ready device. In 2006, the number of households passed over for high-speed Internet was 119 million, and over the past two years, the cable industry has invested $23 billion into their networks. [ 13 ] As the number of homes serviced declines, broadband technology is able to develop. Commissioner Robert M. McDowell commented that the broadband technology has been the fastest in penetration of any technology in modern history. [ 13 ] With the broadband technology, the number of devices that are Internet-ready has been increasing year after year. Both cell phone and laptops with wireless capabilities have increased Internet usage dramatically and have each grown more prevalent since 2009. [ 14 ] It is no longer working professionals with high Internet use, but young adults as well. About 47% of adults go online with a laptop, up from 39% as of April 2009. Also, 40% of adults use a mobile phone, up from 32% in 2009. [ 14 ] The world has seen great technologies, and the Internet is one of the fastest growing because of the number of devices designed to utilize it. According to Pew Research, 59% of adults access the Internet wirelessly through some type of wireless device. Again, this is an increase from the 51% in April 2009. [ 14 ] Internet access has become a critical part of our lives. The deployment and development of wireless broadband as well as other technologies is critical to ensuring this reliable and ubiquitous service is available to Americans. [ 4 ]
Some have not only considered personal or business related uses of national wireless, but the health related uses for hospitals and their patients. As Blair Levin from the Technology, Innovation and Government Reform Team of President Obama states, this will create a world-class broadband platform allowing modernizing of health care records and reforming education. [ 3 ] [ 12 ] Being able to have the instant availability to health records for doctors and patients, and being able to teach and be taught from anywhere in the United States is a concept some may never have considered, or thought possible. As John Horrigan from the Pew Research Center stated, people not understanding the technology being of relevancy to them is a bigger barrier than the cost of the technology itself. [ 12 ] Michael Powell stated Americans benefit most when policies enable consumers and businesses to fully utilize the benefits of emerging technology. [ 4 ]
Public safety has been mentioned in terms of the spectrum use and cost. Public safety is also important in regards to public interest. With the implementation of national wireless broadband, this initiative helps improve public safety communications. The Commission for 9/11 has noted that our homeland security is vulnerable due to lack of interoperable wireless communication among first responders . [ 1 ] The plan would allow all public safety officials to be on the same network, and get the correct information quicker and safer. With 4G networking, they can be provided with a unique opportunity to deploy a system in conjunction with the commercial infrastructure already available. [ 1 ]
Network neutrality is becoming a big issue with the policies involved in the national wireless plan. If the plan comes together, the question of restricting access to the Internet is reason for concern. Network neutrality is the concept of having no rules and regulations for what consumers are able to access through the Internet by their ISPs. As stated by the CTIA, our economic conditions make it hard to understand why people want to impose network neutrality rules, and inject uncertainty in an industry that seems to be working well for the U.S. [ 6 ] This is for both wired and wireless broadband networks. These types of infrastructures cannot be managed for customers and expectations with a one-size-fits-all approach. [ 6 ] The debate is due to the types of restrictions ISPs should be allowed to have if consumers are paying for the service they want. As with wired Internet access, the CTIA has stated that they strongly believe that regulation is not necessary and may do more harm than good. [ 10 ]
More users are obtaining access to the internet and have the wireless devices to access it. It is no surprise that wireless is the fastest growing broadband service. [ 10 ] There is also an increase in the number of users that rely solely on wireless instead of wired connections. Wireless service providers are constantly competing to create the best network with the best service and quality. [ 10 ] With more towers and increased advanced technologies, wireless has become a convenient and widely accessible mode of communication. The negative, however, is the technology which wireless Internet uses. As previously stated, the spectrum itself is limited and wireless data networks rely on the finite source. [ 10 ]
Since the FCC has developed the plan to open the spectrum for the wireless network, the issue of network neutrality is cause for concern for some. The CTIA stated that the imposition of network neutrality will inject uncertainty in the market. [ 10 ] Since this concept supports users having access to the information they want through the Internet, it raises the problem of consumers having limited options. This could ultimately harm consumers and hamper innovation. [ 10 ]
Pros:
Cons: | https://en.wikipedia.org/wiki/Policies_promoting_wireless_broadband_in_the_United_States |
Policresulen is the polycondensation product of meta -cresolsulfonic acid and phenol . [ 1 ] It is used as a topical hemostatic and antiseptic [ 2 ] in infectious and other lesions of the mucous membranes , like gynecological infections, anal hemorrhoids as well as ulcers of the oral cavity including canker sores . In some countries it is marketed under the trade name Albothyl or Polilen (Taiwan) or Faktu (combination with Cinchocaine ).
Policresulen is used in the treatment of gynecological infections since the 1950s. [ 3 ] The range of applications soon widened to include the therapy of other mucous membrane and skin lesions. The mechanism of action is twofold: next to its antiseptic effect, policresulen promotes the selective coagulation of necrotic and pathologically altered tissues while leaving healthy tissues intact. [ 1 ] The shedding of necrotic tissues is accompanied by the reepithelialization of the mucosal (or dermal) wound tissues. [ 1 ]
This dermatologic drug article is a stub . You can help Wikipedia by expanding it .
This drug article relating to the genito-urinary system is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Policresulen |
Policy and Charging Rules Function ( PCRF ) is the software node designated in real-time to determine policy rules in a multimedia network. [ 1 ] As a policy tool, the PCRF plays a central role in next-generation networks. [ 2 ] Unlike earlier policy engines that were added onto an existing network to enforce policy, the PCRF is a software component that operates at the network core and accesses subscriber databases and other specialized functions, such as a charging system, in a centralized manner. [ 3 ] Because it operates in real time, the PCRF has an increased strategic significance and broader potential role than traditional policy engines. This has led to a proliferation of PCRF products since 2008. [ 4 ]
The PCRF is the part of the network architecture that aggregates information to and from the network, operational support systems , and other sources (such as portals) in real time, supporting the creation of rules and then automatically making policy decisions for each subscriber active on the network. Such a network might offer multiple services, quality of service (QoS) levels, and charging rules. [ 5 ] PCRF can provide a network agnostic solution (wire line and wireless) and can also enable multi-dimensional approach which helps in creating a lucrative and innovative platform for operators. [ 6 ] PCRF can also be integrated with different platforms like billing, rating, charging, and subscriber database or can also be deployed as a standalone entity. [ 6 ]
PCRF plays a key role in VoLTE as a mediator of network resources for the IP Multimedia Systems network for establishing the calls and allocating the requested bandwidth to the call bearer with configured attributes. [ citation needed ] This enables an operator to offer differentiated voice services to their user(s) by charging a premium. Operators also have an opportunity to use PCRF for prioritizing the calls to emergency numbers in the next-gen networks. [ citation needed ] | https://en.wikipedia.org/wiki/Policy_and_charging_rules_function |
Polidocanol is a local anaesthetic and antipruritic component of ointments and bath additives. It relieves itching caused by eczema and dry skin. [ 2 ] It has also been used to treat varicose veins , [ 3 ] hemangiomas , and vascular malformations . [ 4 ] It is formed by the ethoxylation of dodecanol .
Polidocanol is also used as a sclerosant , an irritant injected to treat varicose veins , under the trade names Asclera , Aethoxysklerol [ 5 ] and Varithena . [ 6 ] Polidocanol causes fibrosis inside varicose veins, occluding the lumen of the vessel, and reducing the appearance of the varicosity.
The FDA has approved polidocanol injections for the treatment of small varicose (less than 1 mm in diameter) and reticular veins (1 to 3 mm in diameter). Polidocanol works by damaging the cell lining of blood vessels , causing them to close and eventually be replaced by other types of tissue. [ 7 ] [ 8 ] Polidocanol in the form of Varithena injected in the greater saphenous vein can cause the eruption of varicose and spider veins throughout the lower leg. This procedure should be done with caution and with the knowledge that the appearance of the leg may be forever compromised.
This drug article relating to the cardiovascular system is a stub . You can help Wikipedia by expanding it .
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A metallurgical method employed in the purification of copper which contains copper oxide as an impurity and also in the purification of tin which contains tin oxide (stannic oxide or "SnO 2 ") as an impurity. The impure metal, usually in the form of molten blister copper, is placed in an anode furnace for two stages of refining. [ 1 ] In the first stage, sulphur and iron are removed by gently blowing air through the molten metal to form iron oxides and sulfur dioxide . [ 2 ] The iron oxides are skimmed or poured off the top of the copper and the gaseous sulfur dioxide exits the furnace via the off-gas system. Once the first oxidation stage is complete, the second stage ( reduction or poling ) begins. This involves using a reducing agent, normally natural gas or diesel (but ammonia , [ 2 ] liquid petroleum gas, [ 2 ] and naphtha [ 3 ] can also be used), to react with the oxygen in the copper oxide to form copper . In the past, freshly cut ("green") trees were used as wooden poles. [ 2 ] [ 4 ] The sap in these poles acted as the reducing agent. The heat of the copper makes the pole emit wood gas(CO 2 and H 2 ) that reduces the cuprous oxide to copper.
It was the use of these greenwood poles gave rise to the term "poling."
Care must be taken to avoid removing too much of the oxygen from the anode copper, as this will cause other impurities to change from their oxide to metallic states and they will remain in solid solution in the copper, reduce its conductivity and change its physical properties. Also upper surface can be covered with coke to prevent reoxidation of metal. [ 2 ]
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Polio eradication , the goal of permanent global cessation of circulation of the poliovirus and hence elimination of the poliomyelitis (polio) it causes, is the aim of a multinational public health effort begun in 1988, led by the World Health Organization (WHO), the United Nations Children's Fund (UNICEF) and the Rotary Foundation . [ 1 ] These organizations, along with the U.S. Centers for Disease Control and Prevention (CDC) and The Gates Foundation , have spearheaded the campaign through the Global Polio Eradication Initiative (GPEI). Successful eradication of infectious diseases has been achieved twice before, with smallpox in humans [ 2 ] and rinderpest in ruminants. [ 3 ]
Prevention of disease spread is accomplished by vaccination . There are two kinds of polio vaccine —oral polio vaccine (OPV), which uses weakened poliovirus , and inactivated polio vaccine (IPV), which is injected. OPV is less expensive and easier to administer, and can spread immunity beyond the person vaccinated, creating contact immunity . It has been the predominant vaccine used. However, under conditions of long-term vaccine virus circulation in under-vaccinated populations, mutations can reactivate the virus to produce a polio-inducing strain, while OPV can also, in rare circumstances, induce polio or persistent asymptomatic infection in vaccinated individuals, particularly those who are immunodeficient . IPV, being inactivated, does not carry these risks, but does not induce contact immunity. IPV is more costly and the logistics of its delivery are more challenging.
Nigeria is the latest country to have officially stopped endemic transmission of wild poliovirus, with its last reported case in 2016. [ 4 ] Of the three strains of WPV, the last recorded wild case caused by type 2 (WPV2) was in 1999, and WPV2 was declared eradicated in 2015. Type 3 (WPV3) is last known to have caused polio in 2012, and was declared eradicated in 2019. [ 5 ] All wild-virus cases since that date have been due to type 1 (WPV1). [ 6 ]
As of August 2024 [update] , Afghanistan and Pakistan are the only two countries where the disease is still classified as endemic. [ 7 ] [ 8 ] Recent polio cases arise from two sources, the original " wild " poliovirus (WPV), and the much more prevalent mutated oral vaccine strains, known as circulating vaccine-derived poliovirus (cVDPV) or variant poliovirus. Vaccines against each of the three wild strains of polio have given rise to strains of cVDPV, with cVDPV2 being most prominent. cVDPV caused 312 confirmed paralytic polio cases worldwide in 2024, and was detected in 21 countries. [ 9 ] [ 10 ]
Eradication of polio has been defined in various ways:
In theory, if the right tools were available, it would be possible to eradicate all infectious diseases that reside only in a human host. In reality, there are distinct biological features of the organisms and technical factors of dealing with them that make their potential eradicability more or less likely. Three indicators, however, are considered of primary importance in determining the likelihood of successful eradication: that effective interventional tools are available to interrupt transmission of the agent, such as a vaccine ; that diagnostic tools, with sufficient sensitivity and specificity , be available to detect infections that can lead to transmission of the disease; and that humans are required for the life-cycle of the agent, which has no other vertebrate reservoir and cannot amplify in the environment. [ 14 ]
The most important step in eradication of polio is interruption of endemic transmission of poliovirus. Stopping polio transmission has been pursued through a combination of routine immunization , supplementary immunization campaigns, and surveillance of possible outbreaks. Several key strategies have been outlined for stopping polio transmission: [ 15 ]
There are two distinct types of polio vaccine . Oral polio vaccine (OPV, or Sabin vaccine) contains attenuated poliovirus. [ 18 ] OPV is delivered as oral drops or infused into sugar cubes. It is highly effective and inexpensive (about US$0.15 to 0.20 per dose in 2024 [ 18 ] ) and its availability has bolstered efforts to eradicate polio. The vaccine confers long-term, possibly lifelong, immunity to the virus. [ 19 ] Because of its route of administration, it induces an immunization of the intestinal mucosa that protects against subsequent infection, though multiple doses are necessary to achieve effective prophylaxis. [ 18 ] Attenuated poliovirus derived from the oral polio vaccine is excreted, infecting and indirectly inducing immunity in unvaccinated individuals, and thus amplifying the effects of the doses delivered. [ 20 ] The oral administration does not require special medical equipment or training. Taken together, these advantages have made it the favored vaccine of many countries, and it has long been preferred by the global eradication initiative. [ 18 ]
The primary disadvantage of OPV derives from its inherent nature. As an attenuated but active virus, it can induce vaccine-associated paralytic poliomyelitis (VAPP) in approximately one individual per every 2.7 million doses administered. [ 18 ] The live virus can circulate in under-vaccinated populations (circulating vaccine-derived poliovirus, cVDPV) and over time can revert to a neurovirulent form causing paralytic polio. [ 18 ] Until recent times, a trivalent OPV containing all three virus strains was used, but with the eradication of wild poliovirus type 2 this was phased out in 2016 and replaced with bivalent vaccine containing just types 1 and 3, supplemented with monovalent type 2 OPV in regions with documented cVDPV2 circulation. [ 18 ] A novel OPV2 vaccine (nOPV2) genetically modified to reduce the likelihood of disease-causing activating mutations was granted emergency licencing in 2021, and subsequently full licensure in December 2023. [ 21 ] Genetically stabilsed vaccines targeting poliovirus types 1 and 3 are in development, with the intention that these will eventually completely replace the Sabin vaccines. [ 22 ]
The inactivated polio vaccine (IPV, or Salk ) contains trivalent fully inactivated virus, administered by injection. This vaccine cannot induce VAPP nor do cVDPV strains arise from it, but it likewise cannot induce contact immunity and thus must be administered to every individual. Added to this are greater logistical challenges. Though a single dose is sufficient for protection, administration requires medically trained vaccinators armed with single-use needles and syringes. Taken together, these factors result in substantially higher delivery costs. [ 23 ] Original protocols involved intramuscular injection in the arm or leg, but recently subcutaneous injection using a lower dose (so-called fractional-dose IPV, fIPV) has been found to be effective, lowering costs and also allowing for more convenient and cost-effective delivery systems. [ 24 ] [ 25 ] The use of IPV results in serum immunity, but no intestinal immunity arises. As a consequence, vaccinated individuals are protected from contracting polio, but their intestinal mucosa may still be infected and serve as a reservoir for the excretion of live virus. For this reason, IPV is ineffective at halting ongoing outbreaks of WPV or cVDPV, but it has become the vaccine of choice for industrialized, polio-free countries. [ 23 ]
While IPV does not itself induce mucosal immunity, it has been shown to boost the mucosal immunity from OPV, [ 26 ] and the WHO now favors a combined protocol. It is recommended that vulnerable children receive a dose of OPV at birth, then beginning at the age of six weeks a 'primary series' consisting of three OPV doses at least four weeks apart, along with one dose of IPV after 14 weeks. [ 27 ] This combined IPV/OPV approach has also been used in outbreak suppression. [ 28 ]
Polio vaccination is also important in the development of herd immunity . [ 29 ] For polio to occur in a population, there must be an infecting organism ( poliovirus ), a susceptible human population, and a cycle of transmission . There is no animal reservoir for poliovirus, and the transmission cycle of polio is from one infected person to another person susceptible to the disease, usually through the fecal-oral route. [ 20 ] If the vast majority of the population is immune to a particular agent, the ability of that pathogen to infect another host is reduced; the cycle of transmission is interrupted, and the pathogen cannot reproduce and dies out. This concept, called community immunity or herd immunity, is important to disease eradication, because it means that it is not necessary to inoculate 100% of the population—a goal that is often logistically very difficult—to achieve the desired result. If the number of susceptible individuals can be reduced to a sufficiently small number through vaccination , then the pathogen will eventually die off. [ 30 ]
Herd immunity is an important supplement to vaccination. According to the concept of herd immunity, the population for whom the vaccine fails is still protected by the immunity of those around them, and it can only be achieved when vaccination levels are high. It is estimated that the minimum herd immunity threshold for poliovirus eradication ranges from approximately 75% in wealthy high-hygiene populations to 97% in poorer environments. If routine immunization were to be stopped, the number of unvaccinated, susceptible individuals would soon exceed the capability of herd immunity to protect them. [ 31 ]
While vaccination has played an instrumental role in the reduction of polio cases worldwide, the use of attenuated virus in the oral vaccine carries with it an inherent risk. The oral vaccine is a powerful tool in fighting polio in part because of its person-to-person transmission and resulting contact immunity. However, under conditions of long-term circulation in undervaccinated populations, the virus can accumulate mutations that reverse the attenuation and result in vaccine virus strains that themselves cause polio. As a result of such circulating vaccine-derived poliovirus (cVDPV) strains, polio outbreaks have periodically recurred in regions that have long been free of the wild virus, but where vaccination rates have fallen. Oral vaccines can also give rise to persistent infection in immunodeficient individuals, with the virus eventually mutating into a more virulent immunodeficiency-associated vaccine-derived poliovirus (iVDPV). In particular, the type 2 strain seems prone to reversions, so in 2016 the eradication effort abandoned the trivalent oral vaccine containing attenuated strains of all three virus types and replaced it with a bivalent oral vaccine lacking the type 2 virus, while a separate monovalent type 2 vaccine (mOPV2) was to be used only to target existing cVDPV2 outbreaks. A novel oral vaccine targeting type 2 (nOPV2) that has been genetically stabilized to make it less prone to give rise to circulating vaccine-derived strains received full licensure in December 2023, with similar vaccines targeting types 1 and 3 undergoing clinical trials . [ 32 ] [ 33 ] Eradication efforts will eventually require all oral vaccination to be discontinued in favor of the use of injectable vaccines. These vaccines are more expensive and more difficult to deliver, and they lack the ability to induce contact immunity because they contain only killed virus, but they likewise are incapable of giving rise to vaccine-derived viral strains. [ 34 ] [ 35 ]
A global program of surveillance for the presence of polio and the poliovirus plays a critical role in assessment of eradication and in outbreak detection and response. Two distinct methods are used in tandem: acute flaccid paralysis (AFP) surveillance and environmental surveillance. [ 36 ]
Monitoring for AFP aims at identifying outbreaks of polio by screening patients displaying symptoms consistent with, but not exclusive to, severe poliovirus infection. Stool samples are collected from children presenting with AFP and evaluated for the presence of poliovirus by accredited laboratories in the Global Polio Laboratory Network. Since rates of non-polio AFP are expected to be constant and large compared to the number of polio cases, the frequency of non-polio AFP reported in a population is indicative of the effectiveness of surveillance, as is the proportion of AFP patients from whom high-quality stool samples are collected and tested, with a target of at least 80%. [ 36 ]
Environmental surveillance is used to supplement AFP surveillance. This entails the routine testing of sewage samples for the presence of virus, which not only allows the effectiveness of vaccination efforts to be evaluated in countries with active transmission, but also allows the detection of new outbreaks in countries without known transmission. In 2018, the GPEI conducted environmental surveillance in 44 countries, 24 of which are in Africa. [ 36 ]
Among the greatest obstacles to global polio eradication are the lack of basic health infrastructure, which limits vaccine distribution and delivery, the crippling effects of civil war and internal strife, and the sometimes oppositional stance that marginalized communities take against what is perceived as a potentially hostile intervention by outsiders. Another challenge has been maintaining the potency of live (attenuated) vaccines in extremely hot or remote areas. The oral polio vaccine must be kept at 2 to 8 °C (36 to 46 °F) for vaccination to be successful. [ 37 ]
An independent evaluation of obstacles to polio eradication requested by the WHO and conducted in 2009 considered the major obstacles in detail by country. In Afghanistan and Pakistan, the researchers concluded that the most significant barrier was insecurity, but that managing human resources, political pressures, the movement of large populations between and within both countries, and inadequately resourced health facilities also posed problems, as did technical issues with the vaccine. In India , the major challenge appeared to be the high efficiency of transmission within the populations of Bihar and Uttar Pradesh states, set against the low (~80% after three doses against type 1) seroconversion response seen from the vaccine. In Nigeria, the most critical barriers identified were management issues, in particular the highly variable importance ascribed to polio by different authorities at the local government level, although funding issues, community perceptions of vaccine safety, inadequate mobilisation of community groups, and issues with the cold chain also played a role. In those countries where international spread from endemic countries had resulted in the reestablishment of transmission, namely Angola , Chad , and South Sudan , the key issues identified were underdeveloped health systems and low routine vaccine coverage, although the low level of resources committed to Angola and South Sudan for the purpose of curtailing the spread of polio and climatic factors was also identified as playing a role. [ 38 ]
Two additional challenges are found in unobserved polio transmission and in vaccine-derived poliovirus. First, most individuals infected with poliovirus are asymptomatic or exhibit minor symptoms, with fewer than 1% of infections leading to paralysis, [ 39 ] and most infected people are unaware that they carry the disease, allowing polio to spread widely before cases are seen. [ 40 ] In 2000, using new screening techniques for the molecular characterization of outbreak viral strains, it was discovered that some of the outbreaks were actually caused by circulating vaccine-derived poliovirus, following mutations or recombinations in the attenuated strain used for the oral polio vaccine. This discovery altered the strategy for the discontinuation of vaccination following polio eradication, [ 41 ] necessitating an eventual switch to the more expensive and logistically more problematic inactivated polio vaccine, as continued use of the oral inactivated virus would continue to produce such revertant infection-causing strains. [ 42 ] The risk of vaccine-derived polio will persist long after the switch to inactivated vaccine, as a small number of chronic excretors continue to produce active virus for years (or even decades) after their initial exposure to the oral vaccine. [ 43 ]
In a 2012 interview with Pakistani newspaper Dawn , Dr. Hussain A. Gezari, the WHO's special envoy on global polio eradication and primary healthcare, gave his views on obstacles to eradication. He said that the biggest hurdle preventing Pakistan from becoming polio-free was holding district health officials properly accountable—in national eradication campaigns officials had hired their own relatives, even young children. Gezari asked, "How do you expect a seven-year-old thumb-sucking kid to implement a polio campaign of the government?" and added that, in spite of this, "the first national campaign was initiated by your government in 1994 and that year Pakistan reported 25,000 polio cases, and the number was just 198 last year, which clearly shows that the programme is working." [ 44 ]
One factor contributing to the continued circulation of polio immunization programs has been opposition to vaccination in some countries. [ 45 ] In one country, Pakistan, "more than 200 polio team workers" have been killed (team members include not only vaccinators but police and security personnel) from "targeted killing and terrorism" while working on polio campaigns, [ 46 ] (the killers motivated by the belief, spread by "radical clerics", that the vaccine is part of "a Western plot to sterilize Muslims"). [ 47 ]
In the context of the United States invasion of Afghanistan and the subsequent 2003 invasion of Iraq , rumours arose in the Muslim world that immunization campaigns were using intentionally-contaminated vaccines to sterilize local Muslim populations or to infect them with HIV . In Nigeria these rumours fit in with a longstanding suspicion of modern biomedicine, which since its introduction during the era of colonialism has been viewed as a projection of the power of western nations. Refusal of vaccination came to be viewed as resistance to western expansionism, and when the contamination rumours led the Nigerian Supreme Council for Sharia to call for a region-wide boycott of polio vaccination, polio cases in the country increased more than five-fold between 2002 and 2006, with the uncontrolled virus then spreading across Africa and globally. [ 48 ] [ 49 ] In Afghanistan and Pakistan, fears that the vaccine contained contraceptives were one reason given by the Taliban in issuing fatwas against polio vaccination. [ 49 ] [ 45 ] [ 50 ] Religious boycotts based on contamination concerns have not been limited to the Muslim world. In 2015, after claiming that a tetanus vaccine contained a contraceptive, a group of Kenyan Catholic bishops called on their followers to boycott a planned round of polio vaccination. This did not have a major effect on vaccination rates, and dialog along with vaccine testing forestalled further boycott calls. [ 49 ]
Other religion-inspired refusals arise from concern over whether the virus contains pig-derived products, and hence are haram (forbidden) in Islam, [ 51 ] concern that vaccine production may have involved a prohibited taking of animal life, [ 49 ] or a resistance to interfering with disease processes perceived to be divinely-directed. [ 48 ] [ 49 ] Concerns were addressed through extensive outreach, directed both toward the communities involved and respected clerical bodies, as well as promoting local ownership of the eradication campaign in each region. In early 2012, some parents refused to get their children vaccinated in Khyber Pakhtunkhwa (KPK) and in the Federally Administered Tribal Areas (FATA) but religious refusals in the rest of the country had "decreased manifold". [ 44 ] Even with the express support of political leaders, polio workers or their accompanying security guards have been kidnapped, beaten, or assassinated. [ 45 ]
Skepticism in the Muslim world was exacerbated when it was learned that in 2011 the Central Intelligence Agency (CIA) had conducted a fake hepatitis B immunization campaign, hoping to collect blood samples from Osama bin Laden's compound in Abbottabad in order to confirm the genetic identity of the children living there, and by implication his own presence, leading directly to his killing . [ 52 ] [ 53 ] In a letter written to CIA director Leon Panetta , the InterAction Alliance, a union of about 200 U.S.-based non-government organizations, deplored the actions of the CIA in using a vaccination campaign as a cover. [ 54 ] Pakistan reported the world's highest number of polio cases (198), [ 44 ] [ 55 ] in 2011. [ 56 ] and more than 60 polio vaccination workers were killed between December 2012 and April 2014. In May 2014, CIA director John Brennan prohibited his agency both from using vaccinations to cover operations and from testing samples collected by authentic vaccination campaigns. [ 57 ]
Polio vaccination efforts have also faced resistance in another form. The priority placed on vaccination by national authorities has turned it into a bargaining chip, with communities and interest groups resisting vaccination, not due to direct opposition, but to leverage other concessions from governmental authorities. In Nigeria this has taken the form of 'block rejection' of vaccination that is only resolved when state officials agree to repair or improve schools and health-care facilities, pave roads or install electricity. [ 58 ] There have been several instances of threatened boycotts by health workers in Pakistan over payment disputes. [ 59 ] [ 60 ] Some governments have been accused of withholding vaccination or the necessary accompanying infrastructure from regions where opposition to their rule is high. [ 61 ]
A country is regarded as polio-free or non-endemic if no cases have been detected for a year. [ 62 ] [ 63 ] However, it is still possible for polio to circulate under these circumstances, as was the case for Nigeria, where a particular strain of virus resurfaced after five years in 2016. [ 64 ] This can be due to chance, limited surveillance and under-vaccinated populations. [ 65 ] Moreover, for WPV1—the only type of the virus which is currently circulating following the eradication of WPV2 and WPV3—only 1 in 200 infection cases exhibit symptoms of polio paralysis in non-vaccinated children. [ 66 ] Therefore, even a single case is indicative of an epidemic. [ 67 ]
Certification of wild poliovirus eradication requires three consecutive years without case reports, in the presence of reliable disease surveillance networks. [ 68 ] [ 69 ] Wild poliovirus type 2 was certified eradicated in 2015, the last case having been detected in 1999. [ 70 ] Wild poliovirus type 3 has not been detected since 2012, and was certified eradicated in 2019. [ 71 ]
Beginning at certification of complete WPV eradication, the post-certification strategy will come into effect, [ 72 ] : 6 with program priorities shifting towards outbreak preparedness and containment of existing virus material rather than circulating disease. [ 73 ] : 40 Vaccination for WPV1 will cease one year following eradication certification, and eradication of vaccine-derived poliovirus is then targeted for 2030. [ 73 ] : 21
Where possible, the number of facilities conducting operations with poliovirus is to be reduced significantly; and the use of wild poliovirus is to be abandoned altogether in favour of genetically stabilized attenuated strains where possible. [ 74 ] As of May 2021, [update] there are 74 designated Poliovirus Essential Facilities (PEFs) across 24 countries [ 75 ] : 41 where work on poliovirus can be carried out. These facilities are currently involved in producing materials to assist the eradication initiative, and perform crucial research which may prevent future outbreaks. After certification, these facilities retain the equipment and materials to continue vaccine production in the event of deliberate or accidental release. [ 75 ] : 17 Work on Poliovirus outside of the PEFs will be prohibited, and stocks in laboratories that are not designated as PEFs must either be transferred to a PEF or destroyed. [ 75 ] : 26
Following the widespread use of poliovirus vaccine in the mid-1950s, the incidence of poliomyelitis declined rapidly in many industrialized countries. [ 80 ] Czechoslovakia became the first country in the world to scientifically demonstrate nationwide eradication of poliomyelitis in 1960. [ 81 ] In 1962—just one year after Sabin 's oral polio vaccine (OPV) was licensed in most industrialized countries— Cuba began using the oral vaccine in a series of nationwide polio campaigns. The early success of these mass vaccination campaigns suggested that polioviruses could be globally eradicated. [ 82 ] The Pan American Health Organization ( PAHO ), under the leadership of Ciro de Quadros , launched an initiative to eradicate polio from the Americas in 1985. [ 83 ]
Much of the work towards eradication was documented by Brazilian photographer Sebastião Salgado , as a UNICEF Goodwill Ambassador , in the book The End of Polio: Global Effort to End a Disease . [ 84 ]
In 1988, the World Health Organization (WHO), together with Rotary International , UNICEF , and the U.S. Centers for Disease Control and Prevention (CDC) passed the Global Polio Eradication Initiative (GPEI), with the goal of eradicating polio by the year 2000. The initiative was inspired by Rotary International 's 1985 pledge to raise $120 million toward immunising all of the world's children against the disease. [ 83 ] The last case of wild poliovirus poliomyelitis in the Americas was reported in Peru , August 1991. [ 83 ]
On 20 August 1994, the Americas were certified as polio-free. [ 85 ] This achievement was a milestone in efforts to eradicate the disease.
In 1994, the Indian Government launched the Pulse Polio Campaign to eliminate polio. The campaign involves annual vaccination of all children ages 0–5. [ 86 ]
In 1995, Operation MECACAR ( Mediterranean , Caucasus , Central Asian Republics, and Russia ) was launched; National Immunization Days were coordinated in 19 European and Mediterranean countries. [ 87 ] In 1998, Melik Minas of Turkey became the last case of polio reported in Europe. [ 88 ] In 1997, Mum Chanty of Cambodia became the last person to contract polio in the Indo-West Pacific region. [ 89 ] In 2000, the Western Pacific Region (including China ) was certified polio-free. [ 89 ]
In October 1999, the last isolation of type 2 poliovirus occurred in India. This type of poliovirus was subsequently declared eradicated. [ 80 ] [ 70 ]
Also in October 1999, The CORE Group—with funding from the United States Agency for International Development (USAID)—launched its effort to support national eradication efforts at the grassroots level. The CORE Group commenced this initiative in Bangladesh , India, and Nepal in South Asia, and in Angola , Ethiopia , and Uganda in Africa. [ 90 ]
By 2001, 575 million children (almost one-tenth the world's population) had received some two billion doses of oral polio vaccine. [ 91 ] The World Health Organization announced that Europe was polio-free on 21 June 2002, in the Copenhagen Glyptotek . [ 92 ]
In 2002, an outbreak of polio occurred in India. The number of planned polio vaccination campaigns had recently been reduced, and populations in northern India, particularly from the Islamic background, engaged in mass resistance to immunization. [ 93 ] At this time, the Indian state Uttar Pradesh accounted for nearly two-thirds of total worldwide cases reported. [ 94 ] (See the 2002 Global polio incidence map .) However, by 2004, India had adopted strategies to increase ownership of polio vaccinations in marginalized populations, and the immunity gap in vulnerable groups rapidly closed. [ 93 ]
In August 2003, rumors spread in some states in Nigeria, especially Kano , that the vaccine caused sterility in girls. This resulted in the suspension of immunization efforts in the state, causing a dramatic rise in polio rates in the already endemic country. [ 95 ] On 30 June 2004, the WHO announced that after a 10-month ban on polio vaccinations, Kano had pledged to restart the campaign in early July. During the ban the virus spread across Nigeria and into 12 neighboring countries that had previously been polio-free. [ 83 ] By 2006, this ban would be blamed for 1,500 children being paralyzed, [ 96 ] and had cost $450 million for emergency activities. [ 97 ] In addition to the rumors of sterility and the ban by Nigeria's Kano state, civil war and internal strife in Sudan and Côte d'Ivoire have complicated WHO's polio eradication goal. In 2004, 63% of all the polio cases in the world occurred in Nigeria (792 out of 1,265 total). [ 98 ] [ 99 ]
In May 2004, the first case of the polio outbreak in Sudan was detected. The reemergence of polio led to stepped up vaccination campaigns. In the city of Darfur , 78,654 children were immunized and 20,432 more in southern Sudan (Yirol and Chelkou). [ 100 ]
In 2005, there were 1,979 cases of wild poliovirus (excluding vaccine -derived poliovirus). [ 101 ] Most cases were located in two areas: the Indian subcontinent and Nigeria . Eradication efforts in the Indian sub-continent met with a large measure of success. Using the Pulse Polio campaign to increase polio immunization rates, India recorded just 66 cases in 2005, down from 135 cases reported in 2004, 225 in 2003, and 1,600 in 2002. [ 102 ]
Yemen , Indonesia , and Sudan , countries that had been declared polio-free since before 2000, each reported hundreds of cases—probably imported from Nigeria. [ 103 ] On 5 May 2005, news reports broke that a new case of polio was diagnosed in Java , Indonesia , and the virus strain was suspected to be the same as the one that has caused outbreaks in Nigeria. New public fears over vaccine safety, which were unfounded, impeded vaccination efforts in Indonesia. In summer 2005, the WHO, UNICEF and the Indonesian government made new efforts to lay the fears to rest, recruiting celebrities and religious leaders in a publicity campaign to promote vaccination. [ 104 ]
In the United States on 29 September 2005, the Minnesota Department of Health identified the first occurrence of vaccine-derived poliovirus (VDPV) transmission in the United States since OPV was discontinued in 2000. The poliovirus type 1 infection occurred in an unvaccinated, immunocompromised infant girl aged seven months (the index patient) in an Amish community whose members predominantly were not vaccinated for polio. [ 105 ]
In 2006, only four countries in the world (Nigeria, India, Pakistan , and Afghanistan ) were reported to have endemic polio. Cases in other countries are attributed to importation. A total of 1,997 cases worldwide were reported in 2006; of these the majority (1,869 cases) occurred in countries with endemic polio. [ 101 ] Nigeria accounted for the majority of cases (1,122 cases) but India reported more than ten times more cases in 2006 than in 2005 (676 cases, or 30% of worldwide cases). Pakistan and Afghanistan reported 40 and 31 cases respectively in 2006. Polio re-surfaced in Bangladesh after nearly six years of absence with 18 new cases reported. "Our country is not safe, as neighbours India and Pakistan are not polio free", declared Health Minister ASM Matiur Rahman. [ 106 ] (See: Map of reported polio cases in 2006 )
In 2007, there were 1,315 cases of poliomyelitis reported worldwide. [ 101 ] Over 60% of cases (874) occurred in India. In Nigeria, the number of polio cases fell dramatically, from 1,122 cases reported in 2006 to 285 cases in 2007. Officials credit the drop in new infections to improved political control in the southern states and resumed immunisation in the north, where Muslim clerics led a boycott of vaccination in late 2003. Local governments and clerics allowed vaccinations to resume on the condition that the vaccines be manufactured in Indonesia, a majority Muslim country, and not in the United States. [ 103 ] Turai Yar'Adua , wife of recently elected Nigerian president Umaru Yar'Adua , made the eradication of polio one of her priorities. Attending the launch of immunization campaigns in Birnin Kebbi in July 2007, Turai Yar'Adua urged parents to vaccinate their children and stressed the safety of oral polio vaccine. [ 107 ]
In July 2007, a student traveling from Pakistan imported the first polio case to Australia in over 20 years. [ 108 ] Other countries with significant numbers of wild polio virus cases include the Democratic Republic of the Congo , which reported 41 cases, Chad with 22 cases, and Niger and Myanmar , each of which reported 11 cases. [ 101 ]
In 2008, 18 countries reported cases and the total number of cases was 1,651. Of these, 1,505 occurred in the four endemic countries and 146 elsewhere. The largest numbers were in Nigeria (798 cases) and India (559 cases): these two countries combined had 82.2 percent of all cases. Outside endemic countries, Chad reported the greatest number (37 cases). [ 101 ]
In 2009, a total of 1,604 cases were reported across 23 countries. Four endemic countries accounted for 1,256 of these, with the remaining 348 in 19 sub-Saharan countries with imported cases or re-established transmission. Once again, the largest numbers were in India (741) and Nigeria (388). [ 101 ] All other countries had less than one hundred cases: Pakistan had 89 cases, Afghanistan 38, Chad 64, Sudan 45, Guinea 42, Angola 29, Côte d'Ivoire 26, Benin 20, Kenya 19, Burkina Faso 15, Niger 15, the Central African Republic 14, Mauritania 13, and Liberia and Sierra Leone both had 11. The following countries had single-digit numbers of cases: Uganda with 8 cases, Togo 6, Cameroon 3, the Democratic Republic of the Congo 3, Burundi 2, and Mali 2.
According to figures updated in April 2012, the WHO reported that there were 1,352 cases of wild polio across 20 countries in 2010. Reported cases of polio were down 95% in Nigeria (to a historic low of 21 cases) and 94% in India (to a historic low of 42 cases) compared to the previous year, with little change in Afghanistan (from 38 to 25 cases) and an increase in cases in Pakistan (from 89 to 144 cases). An acute outbreak in Tajikistan gave rise to 460 cases (34% of the global total), and was associated with a further 18 cases across Central Asia ( Kazakhstan and Turkmenistan ) and the Russian Federation, with the most recent case from this region being reported from Russia 25 September. These were the first cases in the WHO European region since 2002. The Republic of the Congo ( Brazzaville ) saw an outbreak with 441 cases (30% of the global total). At least 179 deaths were associated with this outbreak, which is believed to have been an importation from the ongoing type 1 outbreak in Angola (33 cases in 2010) and the Democratic Republic of the Congo (100 cases). [ 101 ] [ 109 ]
In 2011, 650 WPV cases were reported across sixteen countries: the four endemic countries—Pakistan, Afghanistan, Nigeria, and India—as well as twelve others. [ 110 ] [ 111 ] Polio transmission recurred in Angola, Chad and the Democratic Republic of the Congo. [ 112 ] Kenya reported its first case since 2009, [ 113 ] while China reported 21 cases, mostly among the Uyghurs of Hotan prefecture, Xinjiang , the first non-imported cases since 1994. [ 114 ] [ 115 ]
The total number of wild-virus cases reported in 2012 was 223, lower than any previous year. These were limited to five countries—Nigeria, Pakistan, Afghanistan, Chad, and Niger—of which all except Nigeria had fewer cases than in 2011. [ 110 ] Several additional countries, Chad, the Democratic Republic of the Congo, Somalia, and Yemen, saw outbreaks of circulating vaccine-derived polio. The last reported type 3 case of polio worldwide had its onset 11 November 2012 in Nigeria; the last wild case outside Nigeria was in April 2012 in Pakistan, [ 110 ] and its absence from sewage monitoring in Pakistan suggests that active transmission of this strain has ceased there. [ 116 ] A total of 416 wild-virus cases were reported in 2013, almost double the previous year. [ 110 ] [ 117 ] : 6 Of these, cases in endemic countries dropped from 197 to 160, while those in non-endemic countries jumped from 5 to 256. [ 110 ] This was due to outbreaks in Central Africa (focused in Cameroon ), the Horn of Africa , and the Middle East (focused in Syria ). [ 117 ] : 6
In April 2013, a case of wild polio in Mogadishu was reported, the first in Somalia since 2007. [ 118 ] By October, over 170 cases had been reported in the country, [ 119 ] with more cases in neighboring Kenya and the Somali Region of Ethiopia.
Routine sewage monitoring in 2012 had detected a WPV1 strain of Pakistani origin in Cairo , sparking a major vaccination push there. [ 120 ] The strain spread to Israel, where there was widespread environmental detection, but like Egypt, no paralysis cases. [ 121 ] [ 122 ] [ 123 ] It had more severe consequences when it spread to neighboring Syria, with the total number of cases eventually reaching 35, the first outbreak there since 1999. [ 124 ] [ 125 ]
In April 2013, the WHO announced a new $5.5 billion, 6-year cooperative plan (called the 2013–18 Polio Eradication and Endgame Strategic Plan) to eradicate polio from its last reservoirs. The plan called for mass immunization campaigns in the three remaining endemic countries, and also dictated a switch to inactivated virus injections, to avoid the risk of the vaccine-derived outbreaks that occasionally occur from use of the live-virus oral vaccine. [ 126 ]
In 2014, there were 359 reported cases of wild poliomyelitis, spread over twelve countries. Pakistan had the most with 306, an increase from 93 in 2013, which was blamed on Al Qaeda and Taliban militants preventing aid workers from vaccinating children in rural regions of the country. [ 127 ] [ 128 ] On 27 March 2014, the WHO announced the eradication of poliomyelitis in the South-East Asia Region, in which the WHO includes eleven countries: Bangladesh , Bhutan , North Korea , India , Indonesia , Maldives , Myanmar , Nepal , Sri Lanka , Thailand , and Timor-Leste . [ 129 ] With the addition of this region, the proportion of world population living in polio-free regions reached 80%. [ 129 ] The last case of wild polio in the South-East Asia Region was reported in India on 13 January 2011. [ 130 ]
During 2015, 74 cases of wild poliomyelitis were reported worldwide, 54 in Pakistan and 20 in Afghanistan. There were 32 circulating vaccine-derived poliovirus (cVDPV) cases in 2015. [ 131 ] [ 132 ]
On 25 September 2015, the WHO declared that Nigeria was no longer considered endemic for wild polio virus, [ 62 ] with no reported case of wild polio virus having been reported since 24 July 2014. [ 110 ] A WPV1 strain not seen in five years resurfaced in Nigeria the following year. [ 64 ]
The WPV2 virus was declared eradicated in September 2015 as it had not been detected in circulation since 1999, [ 70 ] and WPV3 was declared eradicated in October 2019, [ 5 ] having last been detected in 2012. However, both types persist as circulating vaccine-derived strains, the product of years of evolution of the attenuated "live" virus (which makes up the oral vaccine) as it transmits from the vaccinated to the unvaccinated and circulates in an under-immunized community. [ 133 ]
Because cVDPV2 strains continued to arise from trivalent oral vaccine that included attenuated PV2, during 2016 this vaccine was replaced with a bivalent version lacking WPV2 as well as trivalent injected inactivated vaccine that cannot lead to cVDPV cases. This was expected to prevent new strains of cVDPV2 from arising and allow eventual cessation of WPV2 vaccination. [ 135 ] The resulting global use of the injectable vaccine caused shortages, [ 136 ] and led the WHO in April 2017 to recommend general use of the fIPV vaccination protocol, involving subcutaneous injection of a lower dose than used in the standard intramuscular delivery. [ 137 ]
In 2016 there were 37 reported WPV1 cases, half as many as in 2015, with the majority of the cases in Pakistan and Afghanistan. [ 138 ] A small number of additional cases in Nigeria, caused by WPV1, were viewed as a setback, the first being detected there in almost two years, yet the virus had been circulating undetected in regions inaccessible due to the activities of Boko Haram . [ 64 ] [ 139 ] There was also a cVDPV1 outbreak in Laos, [ 140 ] while new strains of cVDPV2 arose separately in Nigeria's Borno and Sekoto states, and in the Quetta area of Pakistan, [ 141 ] [ 142 ] collectively causing five cases.
In 2017 there were 22 reported WPV1 polio cases with onset of paralysis in 2017, down from 37 in 2016. Eight of the cases were in Pakistan and 14 in Afghanistan, [ 79 ] where genetic typing showed repeated introduction from Pakistan as well as local transmission. [ 145 ] In Pakistan, transmission of several genetic lineages of WPV1 seen in 2015 had been interrupted by September 2017, though at least two genetic clusters remain. In spite of a significant drop in detected cases in Pakistan, there was an increase in the percentage of environmental samples that test positive for the polio virus, suggesting gaps in identification of infected individuals. [ 146 ] [ 147 ] In the third country where polio remained endemic, Nigeria, there were no cases, though as few as 7% of infants were fully vaccinated in some districts. [ 79 ] [ 148 ] An April 2017 spill at a vaccine production facility in the Netherlands only resulted in one asymptomatic WPV2 infection, despite release into the sewer system. [ 149 ]
Laos was declared free of cVDPV1 in March 2017, [ 63 ] [ 151 ] but three distinct cVDPV2 outbreaks occurred in the Democratic Republic of the Congo, one of them of recent origin, the other two having circulated undetected for more than a year. Together they caused 20 cases by year's end. [ 152 ] [ 153 ] [ 154 ] [ 155 ] In Syria, a large outbreak began at Mayadin , Deir ez-Zor Governorate , a center of fighting in the Syrian Civil War , and also spreading to neighboring districts saw 74 confirmed cases from a viral strain that had circulated undetected for about two years. [ 156 ] [ 157 ] Circulation of multiple genetic lines of cVDPV2 was also detected in Banadir province, Somalia, but no infected individuals were identified.
[ 158 ] WHO's Strategic Advisory Group of Experts on Immunization recommended that cVDPV2 suppression be prioritized over targeting WPV1, [ 159 ] and according to protocol OPV2 is restricted to this purpose.
There were 33 reported WPV1 paralysis cases with an onset of paralysis in 2018 – 21 in Afghanistan and 12 in Pakistan. [ 79 ] [ 161 ] Polio in Pakistan resurged in the latter part of the year, [ 162 ] due in part to the rates of parental refusal for vaccination increasing, [ 163 ] with wild poliovirus detected in 20% of the year's environmental samples. [ 164 ] In Afghanistan different strains were largely responsible for the cases in the northeast and south of the country. [ 165 ] In Nigeria, the third country classified as having endemic transmission, security concerns continued to limit access to some areas of the country, though migration and novel vaccination approaches would reduce the number of unreached children. [ 148 ] [ 166 ] The nation passed two full years without a detected wild-virus case, though elimination of WPV transmission could not be confirmed. [ 167 ]
Cases caused by vaccine-derived poliovirus were reported in seven countries in 2018, with over 100 total cases, representing nine strains of cVDPV. [ 36 ] In the Democratic Republic of the Congo, one of the outbreaks of cVDPV2 first detected in 2017 caused no additional cases, but suppression of the other two with OPV2 proved insufficient: not only did they continue, but the vaccination efforts gave rise to a novel cVDPV2 outbreak. [ 155 ] The country experienced a total of 20 cases in 2018. [ 154 ] [ 168 ] Two separate cVDPV2 outbreaks in northern Nigeria produced 34 cases, [ 168 ] [ 169 ] as well as giving rise to 10 cases in the neighboring Niger. In Somalia, cVDPV2 continued to circulate, causing several polio cases and detected in environmental samples from as far as Nairobi , Kenya. This virus, along with newly detected cVDPV3, caused twelve total cases in the country, including one patient infected by both strains. [ 168 ] [ 158 ] [ 170 ] The large number of children residing in areas inaccessible to health workers represent a particular risk for undetected cVDPV outbreaks. [ 155 ] A cVDPV2 outbreak in Mozambique resulted in a single case. [ 168 ] [ 171 ] Response to the Syrian cVDPV2 outbreak continued into 2018, and virus transmission was successfully interrupted. [ 172 ] In Papua New Guinea , a cVDPV1 strain arose, causing twenty-six polio cases across nine provinces, [ 173 ] while a single diagnosed cVDPV1 case in neighboring Indonesia [ 168 ] resulted from a distinct outbreak.
In 2019 there were 176 WPV1 paralysis cases detected: 29 in Afghanistan and 147 in Pakistan. [ 79 ] In particular, in Pakistan the number of cases was surging [ 174 ] [ 175 ] and cross-border migration played a role in polio transmission between the two countries. [ 176 ] [ 177 ] While itself problematic, this also fostered a dangerous false narrative in both nations, blaming the other for the presence and spread of polio in their own country. [ 178 ] Environmental sampling in Pakistan showed the virus' presence in eight urban areas, a setback officials attributed primarily to vaccine refusal. [ 179 ] Opponents to vaccination in Pakistan launched a series of attacks in April that left a vaccinator and two security men dead, while false rumors and hoax videos reporting vaccine toxicity also disrupted vaccination efforts there. [ 175 ] [ 180 ] Wild poliovirus of Pakistani origin [ 181 ] also spread to Iran where it was detected in several environmental samples. [ 79 ] Overall, the eradication efforts in Pakistan and Afghanistan have been characterized as having become a "horror show", undermined by "public suspicion, political infighting, mismanagement and security problems". [ 162 ] [ 182 ]
In the third remaining country in which polio was classified as endemic, Nigeria, wild poliovirus has not been detected since October 2016, and levels of AFP surveillance are sufficient, even in security-compromised regions, to suggest transmission of WPV may have been interrupted. [ 166 ] Global WPV3 eradication was certified in October 2019, the virus not having been seen since 2012. [ 71 ]
In addition to the WPV resurgence in Pakistan and Afghanistan, 2019 saw a resurgence of cVDPV, with 378 cases. [ 168 ] The majority of cases were caused by cVDPV2 strains that were able to arise or spread as a consequence of the withdrawal of the PV2 strain from the standard vaccination regimen. Previous cVDPV2 outbreaks in Nigeria, the Democratic Republic of Congo, and Somalia continued into 2019 and spread to neighboring countries, while several countries experienced new outbreaks. [ 183 ] In addition to eighteen reported paralysis cases in Nigeria, the cVDPV2 outbreaks there spread to Benin, Burkina Faso, Chad, Ghana, Niger, and Togo, while the virus was also detected in environmental samples from Cameroon and Ivory Coast. Somalia's continuing outbreaks caused a half-dozen cases there and in neighboring Ethiopia, with a separate Ethiopia outbreak adding one case. The Democratic Republic of the Congo had numerous new and continuing outbreaks, producing more than 80 cases, while multiple new cVDPV2 outbreaks in Angola and the Central African Republic resulted in more than a hundred cases. [ 184 ] Individual new outbreaks of cVDPV2 also caused more than a dozen paralysis cases each in Pakistan [ 168 ] and the Philippines, [ 185 ] [ 186 ] while smaller outbreaks struck Chad, China, and Zambia. [ 168 ] A separate cVDPV1 outbreak in the Philippines also caused cases in Malaysia, where cVDPV2 of Filipino origin was also detected in environmental samples, while additional cVDPV1 outbreaks caused six cases in Myanmar and one case in Yemen. [ 168 ]
In March 2020, the GPEI announced polio eradication resources were being redeployed against the COVID-19 pandemic , recognizing that this would adversely affect its efforts at eradicating polio. [ 187 ] All vaccination efforts, both routine and targeted, were postponed for several months, [ 188 ] with staff reassigned to COVID-19 control. Subsequent statistical analysis indicated that the COVID pandemic resulted in decreases of more than 30% globally in both AFP and environmental surveillance, [ 189 ] with only 23 of 43 priority countries meeting their surveillance targets for 2020. [ 190 ] Additional challenges were a conspiracy theory circulating on social media claiming that the polio vaccine contained coronavirus, and moves by President Donald Trump of the United States to cut funding for the World Health Organization. [ 191 ]
Nigeria was removed from the list of countries with endemic wild poliovirus in June 2020, [ 192 ] four years after the last recorded case in the Northern State of Borno in 2016. [ 193 ] Two months later, the World Health Organization declared the African continent free of wild poliovirus. [ 4 ] This certification came after extensive assessments of the certifications of National Polio Certification Commissions (NCCs) [ 194 ] and confirmation that at least 95% of Africa's population had been immunised. [ 4 ] WHO Director-General Tedros Adhanom called it a "great day... but not the end of polio", [ 195 ] as there remain major continuing outbreaks of vaccine-derived poliovirus in West Africa and Ethiopia in addition to wild cases in Afghanistan and Pakistan. [ 196 ]
Over 1000 cases caused by both continuing and novel outbreaks of cVDPV2 were reported in 2020 across twenty-four countries, with 31 additional cases of cVDPV1, and environmental detections in several additional countries with no diagnosed polio cases. [ 10 ] While in the past cVDPV outbreaks tended to remain localized, significant international spread of these strains was observed. [ 196 ]
In 2021, there were just six confirmed cases of wild poliovirus — one in Pakistan, four in Afghanistan, and one in Malawi. [ 9 ]
2021 saw a partial recovery from the challenges to monitoring brought on by the COVID pandemic, with 74% of high-priority countries meeting surveillance targets, an improvement of over 20% from the previous year. [ 190 ] March 2021 also saw the first use of the modified nOPV2 vaccine in selected countries, engineered to allow vaccination against strain 2 poliovirus without the frequent spawning of cVDPV2 seen with the original OPV2. Full rollout was not expected until 2023. [ 197 ]
Despite previous resistance to eradication efforts, after their takeover of Afghanistan in 2021 the Taliban agreed to allow United Nations healthcare workers to carry out door-to-door vaccination nationwide for the first time in three years, including a commitment to allowing women to participate in the effort, and to provide safety guarantees to eradication staff. [ 198 ] Pakistan's lone case dated from January, but the virus continued to be detected in environmental samples through December, [ 9 ] and was present in most provinces of the country during the year. [ 199 ] The case in Malawi, the country's first in almost three decades and the first in Africa in five years, was seen as a significant setback to the eradication effort. [ 200 ] [ 201 ] [ 202 ] Based on similarity to a strain last detected in Pakistan in 2019, it is thought that WPV1 has been circulating undetected in the country for some time. [ 202 ]
In 2021, reported cVDPV2 cases declined to 685 across 22 countries, over half occurring in Nigeria. The virus was also found in environmental samples or in those from symptom-free people in several additional African and Asian nations without reported cases. [ 10 ] An analysis of cVDPV2 strains from 2020 and the first half of 2021 attributed them to 38 distinct emergences, representing a mix of novel strains and previously detected strains that continued to circulate, while several previously circulating strains were no longer found. [ 203 ] 14 cases of cVDPV1 were identified in Madagascar and 3 in Yemen. No cases of cVDPV3 were observed, though it was detected in environmental samples from China, Israel, and its adjacent occupied territories. [ 10 ] [ 203 ]
In 2022, there were 30 confirmed cases of WPV1 reported to WHO, with two and 20 cases in Pakistan and Afghanistan respectively, while eight non-endemic cases were recorded in Mozambique, [ 9 ] the first cases in the country since 1992. The Mozambique cases derived from the strain of Pakistani origin that caused two confirmed cases in Malawi in 2021. [ 204 ] The cases in Pakistan were the first seen in the country in 15 months, [ 205 ] though the virus had been detected in environmental samples during the intervening period. [ 206 ]
The residual effects of the COVID-19 pandemic continued to be a cause for concern, due to an increased risk of undetected outbreaks due to COVID's interference with routine health care, disease detection, and childhood vaccinations, as well as a concern that an increased fear of vaccines, caused at least in part by politicization of vaccination and bad governance during the pandemic, may result in a general pattern of undervaccination, including for polio. [ 207 ] In Pakistan, a contributing factor to the resurgence of wild virus cases in the country has been threats of violence both from those with anti-vaccine sentiment and from religious extremists. A vaccination worker was assassinated in March 2022, [ 208 ] and a vaccinator and two accompanying police officers providing security for a door-to-door vaccination campaign were murdered in June. [ 209 ] However, the situation had improved compared to previous years as the government and law-enforcement increased their focus. By October 2022, endemic transmission was restricted to seven districts in southern Khyber Pakhtunkhwa , and the number of circulating genetic clusters was down to one, from eleven in 2020. [ 210 ] The situation improved in Afghanistan as well, as between 2021 and 2022 the country became significantly more peaceful as the government consolidated their control, though with remaining pockets of violence. [ 211 ]
Vaccine-derived cases continue to circulate, particularly in the African and Eastern Mediterranean WHO regions. Concern is focused on Ukraine, where cVDPV had been detected in fall 2021, vaccination efforts were halted due to the Russian invasion , [ 212 ] and on poor quality surveillance extant in Southeast Africa [ 213 ] 684 confirmed cVDPV2 cases were seen across twenty countries during 2022, in addition to 189 cases of cVDPV1 and one case of cVDPV3 in Israel. The first case since 2013 was reported in the United States, [ 214 ] tied to cVDPV2 strains found in Israel and the United Kingdom. [ 215 ] In Burundi and the Democratic Republic of the Congo, two outbreaks of cVDPV2 that caused a combined seven cases were identified as having a link to the nOPV2 vaccine, the first outbreaks with ties to nOPV2 after its initial deployment in 2021; [ 216 ] [ 217 ] according to an assessment reported by the GPEI, an estimated 30–40 additional outbreaks of cVDPV2 would have occurred from March 2021 – March 2023 without the nOPV2 rollout. [ 216 ] Vaccine-derived viral strains were also detected in environmental samples from a number of additional countries without diagnosed cases. [ 10 ] [ 218 ] From February to May, traces of cVDPV2 were discovered in sewage in London, United Kingdom. This led the United Kingdom to declare a "national incident". However, authorities have said that the risk to the general public is "extremely low", and no cases have been reported. [ 219 ] [ 220 ]
Twelve cases of WPV1 have been reported in 2023, six in Afghanistan and Pakistan each. Additionally, 187 positive environmental samples were reported in those countries. [ 9 ]
There have been 391 confirmed cases of cVDPV2 reported to the WHO in 2023, across 22 countries, nineteen of which lie in Sub-Saharan Africa , with only a handful of cases in Indonesia, Yemen, and Israel being the exception.
cVDPV1 is now concentrated in three countries, 105 cases in the Democratic Republic of the Congo, 24 in Madagascar and four in Mozambique.
No cVDPV3 virus has been detected for two years now. [ 10 ]
The Independent Monitoring Board of the GPEI in its September 2023 Report [ 221 ] mentioned a continued, geographically restricted, endemic transmission in the east of Afghanistan and in the southern districts of Pakistan's Khyber Pakhtunkhwa province.
Only one wild polio genetic cluster remains in Afghanistan in a much improved immunity (>90%) of the population. Levels of access in the country are the best they have been since 2018.
However, a large pool of unvaccinated children remain in the Kandahar province potentially leading to a large outbreak, if the virus reappears there.
In Pakistan, they estimate that the long-standing and intractable poliovirus circulation in most of the traditional endemic reservoirs has been eliminated. However, a positive environmental probe in Karachi , after no cases for over a year, raised concerns about the vaccination campaign quality in Pakistan.
The report found it was highly likely that the goal of interrupting wild poliovirus transmission in 2023 would be missed and it was certain that ending transmission of vaccine-derived poliovirus will not be possible this year. [ 221 ]
As of 12 May 2025, [update] there have been 99 reported cases of WPV1 with 74 from Pakistan and 25 from Afghanistan. Additionally, 649 positive environmental samples have been collected in Pakistan and 119 in Afghanistan. [ 9 ]
297 cases of cVDPV2 cases have been found across twenty different countries, all of which are in Sub-Saharan Africa except for Algeria in Northern Africa , Indonesia in Southeast Asia , and Yemen and Palestine in West Asia . In addition seventeen countries have had positive environmental samples of cVDPV2 but no confirmed cases: Bahrain, Djibouti, Egypt, Equatorial Guinea, Finland, Gambia, Germany, Ghana, Kenya, Mozambique, Poland, Senegal, Sierra Leone, Spain, Uganda, United Kingdom and Zimbabwe. France had one positive environmental sample of cVDPV3. [ 10 ]
Ten cVDPV1 cases have been detected in the Democratic Republic of the Congo and one in Mozambique. [ 10 ]
After one and a half years without any cases, cVDPD3 has reappeared with four new cases in Guinea between September and November. [ 10 ]
As of 12 May 2025, [update] there have been 10 reported cases of WPV1 with 8 from Pakistan and 2 from Afghanistan. Additionally, 246 positive environmental samples have been collected in Pakistan and 25 in Afghanistan. [ 9 ]
49 cases of cVDPV2 have been found across eight different countries, all of which are in Sub-Saharan Africa. In addition seven countries from Africa, three from Western Europe, two from West Asia and one country from South East Asia have had positive environmental samples of cVDPV2 but no confirmed cases: Algeria, Cameroon, Central African Republic, Cote d'Ivoire, Germany, Israel, Palestine, Papau New Guinea, Poland, Senegal, Somalia, United Kingdom, and Tanzania. [ 10 ]
There have been no cases of cVDPV1 so far this year. cVDPV1 has last been detected in September 2024 in the Democratic Republic of Congo. [ 10 ]
cVDPD3 has reappeared in March 2025 with a case in Guinea after the last one in November 2024. [ 10 ] | https://en.wikipedia.org/wiki/Polio_eradication |
The Polish Chemical Society ( Polish : Polskie Towarzystwo Chemiczne, PTCHem ) is a professional learned society of Polish chemists founded in 1919 to represent the interests of Polish chemists on the local, national and international levels.
The society was founded of 118 Charter Members on 29 June 1919 [ 1 ] on the initiative of Leon Marchlewski , Stanisław Bądzyński and Ignacy Mościcki , future President of Poland who was a chemist himself. The initial aim of the organization was to bring together Polish chemists previously working under different partitions as well as from abroad. It was founded in three Polish cities: Lwów (today Lviv in Ukraine), Kraków , and Warsaw and the first scientific meeting was organized in Warsaw on 1 November 1919 by the executive board of the society. [ 2 ]
The Polish Chemical Society initiated a series of scientific conferences as well as founded Poland's first chemistry journal Roczniki Chemii . [ 3 ]
After the Second World War , the society was reactivated in 1946 and continues its activities until today. It has 1,959 members, who work in 20 regional centres. In 2006, the Polish Chemical Society became a public benefit organization . [ 3 ]
The statute states that one of the goals of the society is ‘‘the encouragement of progress of chemical science and propagation thereof among the public, as well as representation of the professional interests of chemists, both researchers and those industrially employed’’. [ 1 ]
Currently, the offices of the society are located in the 18th-century tenement building at Freta Street 16 in the historic city center of Warsaw. The building is the birthplace of Marie Curie and also houses the Maria Skłodowska-Curie Museum (MMSC). [ 4 ]
The society confers the following awards:
Currently there are 148 honorary members of the society including: [ 5 ] | https://en.wikipedia.org/wiki/Polish_Chemical_Society |
The Political Economy of Research and Innovation (PERI) (or sometimes political economy of technoscience ) is an emerging academic field at the interface of science and technology studies and political economy. It focuses on the production, distribution, and consumption of knowledge , and how these shape and are shaped by different political economies. [ 1 ] Most scholars in this field have so-far focused on the two-way relationship between science, technology, and innovation and political economic processes, practices, and logics.
It has its origins in the critique of neoclassical or orthodox economics of science by scholars like Philip Mirowski , [ 2 ] the 'economic turn' in science and technology studies (see social studies of finance and valuation studies ), [ 3 ] and innovation studies or science policy . [ 4 ] [ 5 ]
Examples of the field include: | https://en.wikipedia.org/wiki/Political_Economy_of_Research_and_Innovation |
Political ecology is the study of the relationships between political, economic and social factors with environmental issues and changes. Political ecology differs from apolitical ecological studies by politicizing environmental issues and phenomena.
The academic discipline offers wide-ranging studies integrating ecological social sciences with political economy [ 1 ] in topics such as degradation and marginalization , environmental conflict , conservation and control, and environmental identities and social movements. [ 2 ]
In international perspective, the origins of political ecology can be traced through different traditions, including an Anglo-American tradition as well as the Latin American and French ecología política and écologie politique . [ 3 ] The English term "political ecology" was first coined by Frank Thone in an article published in 1935. [ 4 ] It has been widely used since then in the context of human geography and human ecology , but with no systematic definition. Anthropologist Eric R. Wolf gave it a second life in 1972 in an article entitled "Ownership and Political Ecology", in which he discusses how local rules of ownership and inheritance "mediate between the pressures emanating from the larger society and the exigencies of the local ecosystem", but did not develop the concept further. [ 5 ] Other origins include other early works of Eric R. Wolf , Michael J. Watts , Susanna Hecht , and others in the 1970s and 1980s.
The origins of the field in the 1970s and 1980s were a result of the development of development geography and cultural ecology , [ 6 ] particularly the work of Piers Blaikie on the sociopolitical origins of soil erosion. [ 7 ] Historically, political ecology has focused on phenomena in and affecting the developing world; since the field's inception, "research has sought primarily to understand the political dynamics surrounding material and discursive struggles over the environment in the third world". [ 8 ]
Scholars in political ecology are drawn from a variety of academic disciplines, including geography, anthropology, development studies, political science, economics, sociology, forestry, and environmental history.
While the broad scope and interdisciplinary nature of political ecology lends itself to multiple definitions and understandings, common assumptions across the field have given the term relevance. Political ecology is frequently seen as an approach to the study of environmental issues, but it can also be understood to define a lived reality and a praxis for change. [ 9 ] Raymond L. Bryant and Sinéad Bailey developed three fundamental assumptions in practising political ecology:
In addition, political ecology attempts to provide critiques and alternatives in the interplay of the environment and political, economic and social factors. Paul Robbins asserts that the field has a "normative understanding that there are very likely better, less coercive, less exploitative, and more sustainable ways of doing things". [ 11 ]
From these assumptions, political ecology can be used to:
Political ecology's movement as a field since its inception in the 1970s has complicated its scope and goals. Through the discipline's history, certain influences have grown more and less influential in determining the focus of study. Peter A. Walker traces the importance of the ecological sciences in political ecology. [ 12 ] He points to the transition, for many critics, from a ‘structuralist’ approach through the 1970s and 1980s, in which ecology maintains a key position in the discipline, to a 'poststructuralist' approach with an emphasis on the 'politics' in political ecology. [ 13 ] This turn has raised questions as to the differentiation with environmental politics as well as the field's use of the term of 'ecology'. Political ecological research has shifted from investigating political influence on the earth's surface to the focus on spatial-ecological influences on politics and power—a scope reminiscent of environmental politics .
Much has been drawn from cultural ecology, a form of analysis that showed how culture depends upon, and is influenced by, the material conditions of society (political ecology has largely eclipsed cultural ecology as a form of analysis according to Walker.) [ 14 ] As Walker states, "whereas cultural ecology and systems theory emphasize[s] adaptation and homeostasis, political ecology emphasize[s] the role of political economy as a force of maladaptation and instability". [ 12 ]
Political ecologists often use political economy frameworks to analyze environmental issues. Early and prominent examples of this were Silent Violence: Food, Famine and Peasantry in Northern Nigeria by Michael Watts in 1983, which traced the famine in northern Nigeria during the 1970s to the effects of colonialism, rather than an inevitable consequence of the drought in the Sahel, and The Political Economy of Soil Erosion in Developing Countries by Piers Blaikie in 1985, which traced land degradation in Africa to colonial policies of land appropriation , rather than over-exploitation by African farmers.
Originating in the 18th and 19th centuries with philosophers such as Adam Smith, Karl Marx, and Thomas Malthus, political economy attempted to explain the relationships between economic production and political processes. [ 15 ] [ 16 ] It tended toward overly structural explanations, focusing on the role of individual economic relationships in the maintenance of social order. [ 17 ] Eric Wolf used political economy in a neo-Marxist framework which began addressing the role of local cultures as a part of the world capitalist system, refusing to see those cultures as "primitive isolates". [ 18 ] But environmental effects on political and economic processes were under-emphasised. [ 16 ]
Conversely, Julian Steward and Roy Rappaport 's theories of cultural ecology are sometimes credited with shifting the functionalist-oriented anthropology of the 1950s and 1960s and incorporating ecology and environment into ethnographic study. [ 19 ]
Geographers and anthropologists, working with their respective strengths, formed the basis of political ecology. [ 20 ] [ 21 ] [ 22 ] [ 23 ] [ 24 ] [ 25 ] [ 26 ] PE focuses on issues of power, recognizing the importance of explaining environmental impacts on cultural processes without separating out political and economic contexts.
The application of political ecology in the work of anthropologists and geographers differs. While any approach will take both the political/economic and the ecological into account, the emphasis can be unequal. Some, such as geographer Michael Watts, focus on how the assertion of power impacts on access to environmental resources. His approach tends to see environmental harm as both a cause and an effect of “social marginalization ”. [ 27 ]
Political ecology has strengths and weaknesses. At its core, it contextualizes political and ecological explanations of human behavior. AS Walker [ 28 ] points out, though, it has failed to offer “compelling counter-narratives” to “widely influential and popular yet deeply flawed and unapologetic neo-Malthusian rants such as Robert Kaplan 's (1994) 'The coming anarchy' and Jared Diamond 's (2005) Collapse (385). Ultimately, Walker holds, applying political ecology to policy decisions – especially in the US and Western Europe – will prove difficult as long there is resistance to Marxist and neo-Marxist thinking. [ 29 ]
Andrew Vayda and Bradley Walters (1999) criticize political ecologists for presupposing “the importance ... of certain kinds of political factors in the explanation of environmental changes” (167). Vayda and Walter's response to overly political approaches in political ecology is to encourage what they call “event ecology”, [ 30 ] focusing on human responses to environmental events without presupposing the impact of political processes on environmental events. The critique has not been taken up widely. One example of work that builds on event ecology, in order to add a more explicit focus on the role of power dynamics and the need for including local peoples' voices is Penna-Firme (2013) "Political and Event Ecology: critiques and opportunities for collaboration".
There is a divergence of ideas between conservation science and political ecology. With conservationists establishing protected areas to conserve biodiversity , "political ecologists have devoted some energy to the study of protected areas, which is unsurprising given political ecology's overall interest in forms of access to, and control over, resources". [ 31 ] Political ecologists argue against enclosure of land for conservation because it usually denies local people access to the land, harming them and hampering their livelihood systems. As Dove and Carpenter state, "indigenous people have important environmental knowledge which could contribute to conservation". [ 32 ] Political ecologists object that it is NGOs and governments who generally impose land use regulations; in effect, this denies indigenous and other local people the ability themselves to conserve species and areas, rendering such people more vulnerable to dispossession.
In a few cases, perhaps especially tragic local groups have been displaced to create national parks and reserves to ‘conserve’ the forest. Fortunately, most conservation bodies are now aware that, if a group has been using and managing a forest for several thousand years, throwing it off the land is more apt to destroy the forest ecosystem than to preserve it. (Sutton 2004: 302)
Power is at the core of political ecology. According to Greenberg and Park, political ecology is a way of creating a synergy between a political economy that aligns power distribution with ecological analysis and economic activities in a wider version of bio-environmental relations. [ 33 ] Bryant explains political ecology as the dynamic in politics that is associated with " discursive struggle " and material in the environment of less developed nations, showing how unequal relations in power make up a political environment. [ 34 ] In the view of Robbins, political ecology is a term for empirical explorations that show changes occurring in an environment in clear connection to power . [ 35 ]
Given the central role of power in political ecology, it is necessary to clarify the field's diverse perspectives on power.
According to the actor-oriented power perspectives, power is exercised by actors which are contrary to the presumption of power being perceived as a force likely to pass individuals with no consciousness. Fredrick Engelstad , a Norwegian sociologist explained the concept of power as the combination of relationality , causality , and intentionality . [ 36 ] The implication of this is that actors are perceived as power carriers in a significant way by which through action a certain intention (intentionality) is achieved, action occurs between at least two actors (relationality), and intended results are produced by action (causality). Viewing the power perspective from the angle of actor-oriented, Dowding submitted that power is linked to the agency, and this does not take away the importance of structure. [ 37 ] Rather, while seen actor's use of power as a constraint, it is also propelled by structures.
The contributions made by actor-oriented power theory were given by Max Weber in 1964, when he explained power to be people’s ability to the realization of their wills irrespective of the resistance posed by others. An instance given by Robert Dahl is the case where actor A exercises power over actor B by getting actor B to execute a task that actor B will otherwise not do. [ 38 ] The extreme case of this is when some group of individuals is mandated to carry out the task contrary to their thought or will.
Svarstad, Benjaminsen, and Overå held that the theory of actor-oriented power help in providing conceptual distinctions with useful insight into the theoretical elements that are vital in studying political ecology. [ 39 ] While there are actors who either exercise or try to put power into use in diverse ways, there are also actors who encounter resistance from their oppositions and other forces. An instance of these forces is resisting the fulfilment of actors' intentions by other opposition who are more powerful. It can also come in the form of institutional structural constraints emanating from the outcome of intended actions.
The use of power by actors who exercise environmental interventions and actors who resist such interventions are oftentimes the emphasis of scholars of political ecology. However, when environmental interventions result in environmental degradations , scholars of political ecology throw their support to actors who resist such exercise of environmental interventions. Actors exercising environmental interventions include corporate organizations, governmental and non-governmental organizations [ 40 ] [ 41 ] [ 42 ] while actors that resist them include groups such as peasants, fishermen, or pastoralists, by exercising counter-power using various kinds of resistance, or active involvement. [ 43 ] [ 44 ] [ 45 ] [ 46 ]
Amongst the foundations of political ecology is the political economy thought of Marxist which centered on the inequalities that emerged from global capitalism. However, the power perspectives of Marx are most likely highlighted even though there are several perspectives of power in political ecology influenced directly or indirectly by Marx. [ 47 ] The Marxist main focus under capitalism is in relation to class and the stability of reproducing this class relation. [ 48 ] Marx also placed human agency as the most important of his power concept with the human agency being socially conditioned as seen in his quote below:
"Men make their history, but they do not make it as they please; they do not make it under self-selected circumstances, but under circumstances existing already, given and transmitted from the past (Marx 1852:5)". [ 49 ]
Thus, Marx's power theory which formed his perspective of power is the understanding of human agency as being constrained by social structure. As structure produces the potential and extent for power exertion, the human agency is reproducing the structure. This is illustrated by Isaac (1987) using the powerful David Rockefeller (1915 to 2017) [ 48 ] as quoted below:
"But a social theory of power must explain what kinds of social relations exist and how power is distributed by these relations, such that it is possible for David Rockefeller to have the power that he has. To do this is not to deny that it is he who possesses this power, nor to deny those personal attributes determining the particular manner in which he exercises it. It is simply to insist that the power individuals possess has social conditions of existence and that it is these conditions that should be the primary focus of theoretical analysis". [ 48 ]
The poststructuralist power perspective is the domain of Michel Foucault ’s work with its application in political ecology. The poststructuralist power perspectives can be in three dimensions such as; biopower , governmentality, and discursive power .
Biopower indicates that to secure life, governments are concerned with the improvement of health and quality of life among populations. Foucault in his work explained how through the knowledge of power, people have learned how they should behave. In so doing, Foucault separates sovereign power from bio-power. Where sovereign power is termed "take life or let live", the bio-power "make life or let die". [ 50 ] While human as specie is continuously elaboration in conformity to nature, the superior one will intervene, acting on the environmental condition if the species of human are to be altered. Therefore, bio-power aim in terms of governance and knowledge is to ascertain environmental issues as core concerns.
Political ecology emphasized that understanding how power works in environmental governance follows Foucault’s notion of “governmentality”. [ 51 ] [ 52 ] [ 53 ] Foucault sees governmentality as the means employed by the government to make its citizens behave in line with the priorities of government. [ 54 ] Fletcher separates governmentality into four kinds. First is " discipline " which ensures that the citizens internalize specific manners like ethical standards and social norms. [ 53 ] The second is the " truth " which is a way of governing citizens using truth-defining standards like religion. The third is " Neoliberal rationality " which is a motivational structure formed and used to improve outcomes. The fourth is "Sovereign power" used to govern based on rules and punishment for faulting the rules. According to Fletcher, these governmentalities may conflict, work alone, or overlap. Also, the first two are dependent on humans believing government priorities, the second two do not but are seen as of importance. [ 53 ]
Lastly, "discursive power" manifest when actors (corporate organization, governmental, and non-governmental organizations) make people or groups imbibe and add to the reproduction of the discourses they produce. Unlike in other fields, in political ecology, discourses are studied in line with a critical realist epistemology. [ 55 ] [ 56 ] [ 57 ] [ 39 ] There are instances where the formation of discursive power is traced to a state’s colonial era when efforts are made in the appropriation of new territories. Going by the basis of Foucault's political-ecological discursive power, it becomes imperative to mention that, there exist various perspectives to those of Foucault with wider space for human agency.
Comparing between bio-power, governmentality, and discursive power, both governmentality, and discursive power can be regarded as a theoretical perspective with significant importance while bio-power can be regarded as a topical concern identified by Foucault as the core of modern-day governments.
Some prominent contemporary scholars include:
Scholarly journals that have been key to the development (and critique) of this field include: | https://en.wikipedia.org/wiki/Political_ecology |
Political representation of nature refers to the concept and practice of granting political or institutional standing to nonhuman entities —such as animals, plants, and ecosystems—within governance systems. [ 3 ] This concept has developed to incorporate natural entities in political decision-making, reflecting debates about human-centered governance's effectiveness in environmental protection. [ 4 ] [ 5 ]
The debate in this field is driven by emerging trends, including the political turn in environmental ethics and the representative turn in political theory . Following progress in legal representation for nonhuman nature, such as the granting of legal personhood to animals, [ 6 ] rivers [ 2 ] and ecosystems, [ 7 ] the conversation has broadened to consider how nature can be represented within legislative and executive branches of government. [ 8 ]
Historically, early thinkers, including Thomas Hobbes , framed nature as passive and separate from human society. Later, theorists such as Bruno Latour introduced concepts, such as a 'parliament of things,' proposing that democratic processes consider nonhuman actors. Latour’s approach has been cited as influential in political theories addressing ecological issues, emphasizing the interconnectedness of human and environmental systems. [ 9 ]
Since the early 2000s, frameworks such as Earth Jurisprudence and Rights of Nature have further influenced the concept of nonhuman representation. These frameworks propose that nature has intrinsic value deserving of rights both legally and morally. [ 10 ] The works of political theorists like Andrew Dobson, [ 11 ] Robyn Eckersley , [ 12 ] Robert Goodin , [ 13 ] and John O'Neill [ 14 ] have contributed to this shift. They propose that humans have an ethical responsibility toward nature, suggesting that democratic systems might evolve to account for nonhuman interests.
Various institutional mechanisms have been proposed and put into practice to incorporate nonhuman entities and ecosystems into formal political decision-making processes.
Electoral reforms like proportional representation and lowering electoral thresholds have been suggested to facilitate the entry of green parties into parliaments. These changes could help increase the political influence of parties dedicated to environmental advocacy, further amplifying the representation of nature in governance. [ 8 ] [ 15 ]
Deliberative initiatives , such as citizen assemblies and deliberative mini-publics, engage randomly selected citizens in structured discussions on environmental issues, providing a platform for nonhuman interests to be included in political discourse. While these bodies typically do not have formal decision-making power, they can influence policy indirectly by informing legislators. [ 8 ] [ 16 ]
Dedicated ombudspersons or commissioners are independent offices that investigate and advocate for the rights of natural entities, operating similarly to human rights commissioners. [ 8 ] [ 17 ]
The allocation of seats in existing parliaments to specific representatives of nature mirrors systems of political reservations used to ensure representation for marginalised human groups. These nature representatives would advocate for the interests of ecosystems and nonhuman entities within legislative bodies. [ 8 ] [ 18 ]
On a global level, the establishment of new supranational institutions has been proposed. Suggested frameworks include regional ecosystem assemblies for different biomes and an Earth System Council to coordinate global environmental action. Both institutions aim to include representation from states, indigenous communities, and proxy guardians for the non-human. [ 3 ] [ 19 ]
Examples of nonhuman representation exist in several countries. [ 20 ]
In New Zealand, the Parliamentary Commissioner for the Environment is responsible for monitoring environmental issues. This office conducts investigations, reviews, and reports on environmental conditions and evaluates the effectiveness of government policies. Its role is intended to support transparency and accountability in environmental governance. [ 21 ] [ 22 ]
The European Commissioner for Health and Animal Welfare is responsible for updating animal welfare regulations within the European Union. The position also promotes the One Health approach, which recognizes the interdependence of people, animals, plants, and their shared environment. [ 23 ] [ 24 ]
In Wales, the Future Generations Commissioner ensures that the interests of future generations are considered in policy-making. The office evaluates the long-term impacts of current policies and promotes sustainable development practices aimed at safeguarding both human and environmental health. [ 25 ] [ 26 ]
In Germany, the Federal Animal Welfare Officer provides oversight on animal welfare matters and advises the federal government across sectors such as agriculture, research, and industry. [ 27 ] [ 28 ]
Australia’s Inspector-General of Animal Welfare and Live Animal Exports advises the federal government on issues related to animal welfare and oversees the development of policies to improve the treatment and protection of animals. [ 29 ] [ 30 ]
In Spain, the General Director of Animal Rights manages animal welfare policies at the federal level, developing strategies to enhance animal rights and incorporating these into broader social and legislative frameworks. [ 31 ] [ 32 ]
The Commissioner for Animal Welfare in Malta enforces animal welfare laws and oversees the proper treatment of animals, ensuring compliance with national regulations. [ 33 ] [ 34 ]
In New York City, the Office of Animal Welfare focuses on policies related to the health, safety, and welfare of wild animals and pets. [ 35 ] [ 36 ] | https://en.wikipedia.org/wiki/Political_representation_of_nature |
The politics of outer space includes space treaties, law in space , international cooperation and conflict in space exploration , international economics , and the hypothetical political impact of any contact with extraterrestrial intelligence .
Astropolitics has its foundations in geopolitics and is a theory that is used for space in its broadest sense.
International cooperation on space projects has resulted in the creation of new national space agencies. By 2005, there were 35 national civilian space agencies. [ 1 ]
The Outer Space Treaty , formally the Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies, is a multilateral treaty that forms the basis of international space law . Negotiated and drafted under the auspices of the United Nations , it was opened for signature in the United States , the United Kingdom , and the Soviet Union on 27 January 1967, entering into force on 10 October 1967. As of March 2024 [update] , 115 countries are parties to the treaty—including all major spacefaring nations —and another 22 are signatories. [ 2 ] [ 3 ] [ 4 ]
The Outer Space Treaty was spurred by the development of intercontinental ballistic missiles (ICBMs) in the 1950s, which could reach targets through outer space . [ 5 ] The Soviet Union's launch of Sputnik , the first artificial satellite, in October 1957, followed by a subsequent arms race with the United States, hastened proposals to prohibit the use of outer space for military purposes. On 17 October 1963, the U.N. General Assembly unanimously adopted a resolution prohibiting the introduction of weapons of mass destruction in outer space. Various proposals for an arms control treaty governing outer space were debated during a General Assembly session in December 1966, culminating in the drafting and adoption of the Outer Space Treaty the following January. [ 5 ]
Key provisions of the Outer Space Treaty include prohibiting nuclear weapons in space; limiting the use of the Moon and all other celestial bodies to peaceful purposes; establishing that space shall be freely explored and used by all nations; and precluding any country from claiming sovereignty over outer space or any celestial body. Although it forbids establishing military bases, testing weapons and conducting military maneuvers on celestial bodies, the treaty does not expressly ban all military activities in space, nor the establishment of military space forces or the placement of conventional weapons in space. [ 6 ] [ 7 ] From 1968 to 1984, the OST gave birth to four additional agreements: rules for activities on the Moon; liability for damages caused by spacecraft; the safe return of fallen astronauts; and the registration of space vehicles . [ 8 ]
The Agreement Governing the Activities of States on the Moon and Other Celestial Bodies, [ 15 ] [ 16 ] better known as the Moon Treaty or Moon Agreement, is a multilateral treaty that turns jurisdiction of all celestial bodies (including the orbits around such bodies) over to the participant countries. Thus, all activities would conform to international law, including the United Nations Charter .
The Artemis Accords build on a number of treaties that affect the conduct of states and their commercial industries in the exploration and use of space, including the 1967 Outer Space Treaty , the 1972 Liability Convention , and the 1975 Registration Convention . NASA has stated that in leading the Artemis program, international partnerships will prepare for a historic human mission to Mars while playing a key role in achieving a sustainable and robust presence on the Moon. The core of the Artemis agreement requires that all activities be conducted for peaceful purposes, consistent with the principles of the Outer Space Treaty . International cooperation under the Artemis Agreement aims not only to promote space exploration , but also to strengthen peaceful relations between nations. [ 19 ]
A post-detection policy (PDP), also known as a post-detection protocol, is a set of structured rules, standards, guidelines, or actions that governmental or other organizational entities plan to follow for the "detection, analysis, verification, announcement, and response to" confirmed signals from extraterrestrial civilizations . [ 20 ] Though no PDPs have been formally and openly adopted by any governmental entity, there is significant work being done by scientists and nongovernmental organizations to develop cohesive plans of action to utilize in the event of detection. The most popular and well known of these is the "Declaration of Principles Concerning Activities Following the Detection of Extraterrestrial Intelligence", which was developed by the International Academy of Astronautics (IAA), with the support of the International Institute of Space Law. [ 21 ] The theories of PDPs constitute a distinct area of research but draw heavily from the fields of SETI (the Search for Extra-Terrestrial Intelligence) , METI (Messaging to Extra-Terrestrial Intelligence) , and CETI (Communication with Extraterrestrial Intelligence) .
US Air Force judge advocate Matthew King, writing in a personal capacity, has looked at the major powers' cooperation in space matters in the light of international geo-political relationships on earth. He holds that joint engagement in space matters, especially for the United States, China and Russia, can be viewed via two alternative lenses: it can facilitate peace and understanding (a lens he refers to as an olive branch) or it can hide true relationships (the lens of a fig leaf). [ 23 ]
In 1972, a milestone was reached in co-operation between the United States and the Soviet Union in space with the Apollo-Soyuz Test Project . The project occurred during a period of détente between the two superpowers, and led in July 1975 to Soyuz 19 docking with an Apollo spacecraft.
From 1978 to 1987, the USSR's Interkosmos program included allied Warsaw Pact countries and countries which were not Soviet allies, such as India, Syria, and France, in crewed and uncrewed missions to Space stations Salyut 6 and 7. In 1986, the USSR extended its co-operation to a dozen countries in the Mir program. From 1994 to 1998, NASA Space Shuttles and crew visited Mir in the Shuttle–Mir program .
In 1998, assembly of the space station began. [ 24 ] On 28 January 1998, the Space Station Intergovernmental Agreement (IGA) was signed. This governs ownership of modules, station usage by participant nations, and responsibilities for station resupply. The signatories were the United States of America, Russia, Japan, Canada, and eleven member states of the European Space Agency (Belgium, Denmark, France, Germany, Italy, The Netherlands, Norway, Spain, Sweden, Switzerland, and the United Kingdom). [ 25 ] [ 26 ] With the exception of the United Kingdom, all of the signatories went on to contribute to the Space Station project. A second layer of agreements was then achieved, four memoranda of understanding between NASA and ESA, CSA, RKA and JAXA. These agreements are then further split, such as for the contractual obligations between nations, and trading of partners' rights and obligations. [ 26 ] Use of the Russian Orbital Segment is also negotiated at this level. [ 27 ]
In 2010, the ESA announced that European countries which were not already part of the program would be allowed access to the station in a three-year trial period. [ 28 ]
In March 2012, a meeting in Quebec City between the leaders of the space agencies of Canada, Japan, Russia, the United States, and involved European nations resulted in a renewed pledge to maintain the space station until at least 2020. NASA reports to be still committed to the principles of the mission but also to use the station in new ways, which were not elaborated. CSA President Steve MacLean stated his belief that the station's Canadarm will continue to function properly until 2028, alluding to Canada's likely extension of its involvement beyond 2020. [ 29 ]
On 28 March 2015, Russian sources announced that Roscosmos and NASA had agreed to collaborate on the development of a replacement for the current ISS. [ 30 ] Igor Komarov , the head of Russia's Roscosmos, made the announcement with NASA administrator Charles Bolden at his side. [ 31 ] In a statement provided to SpaceNews on 28 March, NASA spokesman David Weaver said the agency appreciated the Russian commitment to extending the ISS, but did not confirm any plans for a future space station. [ 32 ]
On 30 September 2015, Boeing's contract with NASA as prime contractor for the ISS was extended to 30 September 2020. Part of Boeing's services under the contract related to extending the station's primary structural hardware past 2020 to the end of 2028. [ 33 ]
There have also been suggestions in the commercial space industry that the station could be converted to commercial operations after it is retired by government entities. [ 34 ]
In July 2018, the Space Frontier Act of 2018 was intended to extend operations of the ISS to 2030. This bill was unanimously approved in the Senate, but failed to pass in the U.S. House. [ 35 ] [ 36 ] In September 2018, the Leading Human Spaceflight Act was introduced with the intent to extend operations of the ISS to 2030, and was confirmed in December 2018. [ 37 ] [ 38 ] [ 39 ]
On 12 April 2021, at a meeting with Russian President Vladimir Putin , then-Deputy Prime Minister Yury Borisov announced he had decided that Russia might withdraw from the ISS programme in 2025. [ 40 ] [ 41 ] According to Russian authorities, the timeframe of the station’s operations has expired and its condition leaves much to be desired. [ 40 ]
In January 2022, NASA announced a planned date of January 2031 to de-orbit the ISS and direct any remnants into a remote area of the South Pacific Ocean. [ 42 ]
On 24 February 2022, NASA said that American and Russian astronauts currently aboard the ISS would continue normal operations despite the 2022 Russian invasion of Ukraine . [ 43 ] British Prime Minister Boris Johnson commented on the current status of cooperation, saying "I have been broadly in favour of continuing artistic and scientific collaboration, but in the current circumstances it's hard to see how even those can continue as normal." [ 44 ] On the same day, Roscosmos Director General Dmitry Rogozin insinuated that Russian withdrawal could cause the International Space Station to de-orbit due to lack of reboost capabilities, writing in a series of tweets, "If you block cooperation with us, who will save the ISS from an unguided de-orbit to impact on the territory of the US or Europe? There's also the chance of impact of the 500-ton construction in India or China. Do you want to threaten them with such a prospect? The ISS doesn't fly over Russia, so all the risk is yours. Are you ready for it?" [ 45 ] (The last claim is not true, as the ISS' orbital inclination of 51.66° allows it to overfly the latitude of Saratov .) Rogozin later tweeted that normal relations between ISS partners could only be restored once sanctions have been lifted, and indicated that Roscosmos would submit proposals to the Russian government on ending cooperation. [ 46 ] NASA stated that, if necessary, US corporation Northrop Grumman has offered a reboost capability that would keep the ISS in orbit. [ 47 ]
On 26 July 2022, Borisov, who had become head of Roscosmos, submitted to Putin his plans for withdrawal from the programme after 2024. [ 48 ] However, Robyn Gatens, the NASA official in charge of space station operations, responded that NASA had not received any formal notices from Roscosmos concerning withdrawal plans. [ 49 ] The United States Congress, in its CHIPS and Science Act signed by President Joe Biden on 9 August, approved extending NASA's funding for the ISS through 2030. [ 50 ] [ 51 ]
In recent years, the advancement in technology and engineering have made mining asteroid more plausible. The global space mining market is estimated to be worth $14.71 billion by 2025, as indicated by market research. [ 54 ] Although the industry could be years away from successfully mining asteroids, this renewed interest in asteroid mining for metal extraction has the potential to influence the global market of rare metal and create a new geopolitical order. [ 55 ] [ 56 ] [ 57 ] [ 58 ]
Outer space has been a ground for geopolitical competition since the Cold War , and the expected growth in asteroid mining could bring about a new geopolitical order organized around resources extraction. [ 59 ] [ 57 ] [ 60 ] Access to rare metals from asteroids could position nation-states and their private sectors competitively. [ 56 ] Countries with technological capacity to explore space and finance a new and costly asteroid mining endeavour are better positioned to dominate the global supply chain for such metals, while a handful of countries will lag behind. This threatens to drive a resource race in outer space and could create similar patterns of conflict around resources extraction to what has been experienced on earth. [ 56 ] [ 57 ]
The expansion of resource extraction in outer space will require the development of a legal regulatory framework that adequately governs asteroid mining activities. The two current treaties that govern activities in outer space are the Outer Space Treaty and the Moon Treaty . Ratified by 98 countries in 1976, the Outer Space Treaty prevents sovereign or private ownership of outer space and its resources, asserting it belongs to all mankind but does not prevent exploitation of its resources. Ratified in 1979, the Moon Treaty clears some of the vague language surrounding the heritage for humankind outline in the Outer Space Treaty. Similar to the Outer Space Treaty, its provisions outline the Moon and other celestial bodies are “ not subject to national appropriation by claim of sovereignty, by means of use or occupation, or by any other means” [ 61 ] It also introduces a number of provisions that limit activities in outer space. While those provisions have implications for the development of space mining industry, Article 11.7 of the treaty has serious implications for countries that wish to assert monopoly over the emerging asteroid mining industry.
Article 11.7: The main purposes of the international regime to be established shall include [ 62 ] :
1. The orderly and safe development of the natural resources of the moon;
2. The rational management of those resources;
3. The expansion of opportunities in the use of those resources;
4. An equitable sharing by all States Parties in the benefits derived from those resources, whereby the interests and needs of the developing countries, as well as the efforts of those countries which have contributed either directly or indirectly to the exploration of the moon, shall be given special consideration.
Given the limitation the Moon treaty place on states, it has not been ratified by key players in the space frontier, such as the US , China , and Russia . [ 63 ]
Both the Outer Space Treaty and the Moon Treaty lack a robust enforcement mechanism that holds states and private entities accountable for their violation of the agreements. [ 57 ] The lack of clarity over claims for ownership could result in conflicts among countries and private companies. [ 64 ] [ 56 ] While International efforts to reconsolidate a regulatory framework to govern future mining activities are much needed, they have been slow. Establishing rules on transparency, resource sharing, and mechanisms to guide conflict resolutions are needed to support the growing economy of asteroid mining. [ 64 ]
Several countries have conducted research missions to asteroids . While the US, EU , Japan , Russia, and China have all had successful asteroid missions, only the US and Japan were able to bring samples from an asteroid. With geological surveys for metals often preceding the militarization of territories and the expansion of nation-states in terrestrial context, research missions to asteroids allude to the expansion of state ambition to dominate a large-scale extraterrestrial extractive regime. [ 57 ]
While asteroid mining remains in its infancy, countries are competing to dominate it. However, the difficulty of reaching consensus on global treaties has led countries to branch out to legitimatize the economic exploration and exploitation of asteroids, through passing national law, and relying on loopholes in the international law. [ 64 ] [ 65 ]
In 2015, the US passed the U.S Commercial Space Launch Competitiveness Act . While the act does not position the US as a state to have authority or ownership in outer space, it positions its citizens to have ownership over resources acquired from space. [ 66 ] [ 67 ] This allows the U.S. to adhere to Outer Space Treaty but also allow to its private entities to carry out mining activities on Asteroids, once feasible. [ 66 ] AstroForge , a US start-up with a mission focused on developing technology for asteroid mining, announced two commercial missions to asteroids that were launched in 2023. [ 68 ] [ 69 ]
In 2016, the government of Luxembourg introduced a legal framework that support and guide the private activities of mining asteroids. [ 70 ] To grant certainty investors, it passed a law that explicitly permits private entities to own and sell resources extracted from asteroids. The government also pledged to support research and start-ups focused on space exploration and extraterrestrial resource extraction with a funding of approximately US$225 million. [ 71 ] Relative to the US, the Luxembourg Space Act provides more clarity and position the country more competitively to engage in asteroid mining. Given its strategic location in Europe, Luxembourg intends to establish itself as a Silicon Valley for space activities. [ 71 ]
Despite the differences in passed legislation, the goal remains the same: to emerge as a leader in the new asteroid mining frontier and obtain the economic benefits associated with it. Current and future developments in asteroid mining do not indicate whether competition in this frontier will lead to positive changes in international law or allow for the harmonization of national laws among states. Nevertheless, the steady growth in asteroid mining will have implications for the geopolitics of terrestrial and extraterrestrial resource extraction.
Participation and representation of humanity in space is an issue of human access to and presence in space ever since the beginning of spaceflight. [ 72 ] Different space agencies , space programs and interest groups such as the International Astronomical Union have been formed supporting or producing humanity's or a particular human presence in space. Representation has been shaped by the inclusiveness, scope and varying capabilities of these organizations and programs.
Some rights of non-spacefaring countries to partake in spaceflight have been secured through international space law , declaring space the " province of all mankind ", understanding spaceflight as its resource, though sharing of space for all humanity is still criticized as imperialist and lacking, [ 72 ] [ 73 ] particularly regarding regulation of private spceflight. [ 74 ]
Space colonization has been discussed as postcolonial [ 83 ] continuation of imperialism and colonialism , [ 84 ] [ 85 ] [ 86 ] [ 87 ] calling for decolonization instead of colonization. [ 88 ] [ 86 ] Critics argue that the present politico-legal regimes and their philosophic grounding, advantage imperialist development of space, [ 87 ] that key decisionmakers in space colonization are often wealthy elites affiliated with private corporations, and that space colonization would primarily appeal to their peers rather than ordinary citizens. [ 89 ] [ 90 ] Furthermore, it is argued that there is a need for inclusive [ 91 ] and democratic participation and implementation of any space exploration, infrastructure or habitation. [ 92 ] [ 93 ] According to space law expert Michael Dodge, existing space law , such as the Outer Space Treaty , guarantees access to space, but does not enforce social inclusiveness or regulate non-state actors. [ 88 ]
Particularly the narrative of the " New Frontier " has been criticized as unreflected continuation of settler colonialism and manifest destiny , continuing the narrative of exploration as fundamental to the assumed human nature . [ 94 ] [ 95 ] [ 85 ] [ 89 ] [ 86 ] Joon Yun considers space colonization as a solution to human survival and global problems like pollution to be imperialist; [ 96 ] others have identified space as a new sacrifice zone of colonialism. [ 97 ]
Furthermore, the understanding of space as empty and separate is considered a continuation of terra nullius . [ 98 ] [ 99 ]
Natalie B. Trevino argues that not colonialism but coloniality will be carried into space if not reflected on. [ 100 ]
More specifically the advocacy for territorial colonization of Mars has been called surfacism , in contrast to habitation in the atmospheric space of Venus , [ 101 ] [ 102 ] a concept similar to Thomas Golds surface chauvinism .
More generally space infrastructure such as the Mauna Kea Observatories have also been criticized and protested against as being colonialist. [ 103 ] Guiana Space Centre has also been the site of anti-colonial protests, connecting colonization as an issue on Earth and in space. [ 83 ]
In regard to the scenario of extraterrestrial first contact , it has been argued that the employment of colonial language would endanger such first impressions and encounters. [ 88 ]
Space activities entail an environmental costs, which also affect countries that don't participate, prompting questions of environmental justice . [ 104 ]
The historical dominance of nations in space activities has progressively been challenged by private companies, such as SpaceX and Blue Origin , which have established profitable businesses primarily by providing satellite launching services. This fast-paced new market led to the expression "NewSpace", to contrast it with the "OldSpace" of the cold war era. [ 105 ]
Since the mid-20th century, it has become possible for a different actors, such as national armies and government agencies, scientists and private companies, to carry out a variety of space activities, such as the regulation of outer space through international law, the deployment of missile and anti-satellite weapons , the establishment of exploration, communication and navigation satellites , and space travel for tourism and habitat expansion. [ 106 ] | https://en.wikipedia.org/wiki/Politics_of_outer_space |
poll is a POSIX system call to wait for one or more file descriptors to become ready for use. [ 1 ]
On * BSD and macOS , it has been largely superseded by kqueue in high performance applications. [ 2 ] On Linux , it has been superseded by ppoll and epoll . [ 2 ]
This computer-programming -related article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Poll_(Unix) |
Poll Everywhere is a privately held company headquartered in San Francisco , California. The company, founded in April 2007 [ 1 ] is an online service for classroom response and audience response systems . Poll Everywhere's product allows audiences and classrooms in over 100 countries [ 2 ] to use mobile phones, thereby "plotting the obsolescence" [ 3 ] of proprietary hardware response devices otherwise known as clickers.
The company raised $20,000 in venture funding from Y Combinator in 2008. [ 4 ]
Jeff Vyduna, Brad Gessler, and Sean Eby were coworkers at Deloitte Consulting charged with giving internal presentations. One day, to keep the audience awake, they decided to pull a text message "out of a hat". [ 5 ] Code was started in April 2007, and the online site launched in September 2007. During that time, Vyduna matriculated at the Sloan School of Management at MIT . On May 14, 2008, the company placed as a semi finalist in the MIT $100K Entrepreneurship Challenge. [ 6 ] Vyduna subsequently took a leave of absence when Poll Everywhere was accepted into Y Combinator in mid-2008; In 2010 the company moved to San Francisco . | https://en.wikipedia.org/wiki/Poll_Everywhere |
In computational number theory and computational algebra , Pollard's kangaroo algorithm (also Pollard's lambda algorithm , see Naming below) is an algorithm for solving the discrete logarithm problem. The algorithm was introduced in 1978 by the number theorist John M. Pollard , in the same paper as his better-known Pollard's rho algorithm for solving the same problem. [ 1 ] [ 2 ] Although Pollard described the application of his algorithm to the discrete logarithm problem in the multiplicative group of units modulo a prime p , it is in fact a generic discrete logarithm algorithm—it will work in any finite cyclic group .
Suppose G {\displaystyle G} is a finite cyclic group of order n {\displaystyle n} which is generated by the element α {\displaystyle \alpha } , and we seek to find the discrete logarithm x {\displaystyle x} of the element β {\displaystyle \beta } to the base α {\displaystyle \alpha } . In other words, one seeks x ∈ Z n {\displaystyle x\in Z_{n}} such that α x = β {\displaystyle \alpha ^{x}=\beta } . The lambda algorithm allows one to search for x {\displaystyle x} in some interval [ a , … , b ] ⊂ Z n {\displaystyle [a,\ldots ,b]\subset Z_{n}} . One may search the entire range of possible logarithms by setting a = 0 {\displaystyle a=0} and b = n − 1 {\displaystyle b=n-1} .
1. Choose a set S {\displaystyle S} of positive integers of mean roughly b − a {\displaystyle {\sqrt {b-a}}} and define a pseudorandom map f : G → S {\displaystyle f:G\rightarrow S} .
2. Choose an integer N {\displaystyle N} and compute a sequence of group elements { x 0 , x 1 , … , x N } {\displaystyle \{x_{0},x_{1},\ldots ,x_{N}\}} according to:
3. Compute
Observe that:
4. Begin computing a second sequence of group elements { y 0 , y 1 , … } {\displaystyle \{y_{0},y_{1},\ldots \}} according to:
and a corresponding sequence of integers { d 0 , d 1 , … } {\displaystyle \{d_{0},d_{1},\ldots \}} according to:
Observe that:
5. Stop computing terms of { y i } {\displaystyle \{y_{i}\}} and { d i } {\displaystyle \{d_{i}\}} when either of the following conditions are met:
Pollard gives the time complexity of the algorithm as O ( b − a ) {\displaystyle O({\sqrt {b-a}})} , using a probabilistic argument based on the assumption that f {\displaystyle f} acts pseudorandomly. Since a , b {\displaystyle a,b} can be represented using O ( log b ) {\displaystyle O(\log b)} bits, this is exponential in the problem size (though still a significant improvement over the trivial brute-force algorithm that takes time O ( b − a ) {\displaystyle O(b-a)} ). For an example of a subexponential time discrete logarithm algorithm, see the index calculus algorithm .
The algorithm is well known by two names.
The first is "Pollard's kangaroo algorithm". This name is a reference to an analogy used in the paper presenting the algorithm, where the algorithm is explained in terms of using a tame kangaroo to trap a wild kangaroo. Pollard has explained [ 3 ] that this analogy was inspired by a "fascinating" article published in the same issue of Scientific American as an exposition of the RSA public key cryptosystem . The article [ 4 ] described an experiment in which a kangaroo's "energetic cost of locomotion, measured in terms of oxygen consumption at various speeds, was determined by placing kangaroos on a treadmill ".
The second is "Pollard's lambda algorithm". Much like the name of another of Pollard's discrete logarithm algorithms, Pollard's rho algorithm , this name refers to the similarity between a visualisation of the algorithm and the Greek letter lambda ( λ {\displaystyle \lambda } ). The shorter stroke of the letter lambda corresponds to the sequence { x i } {\displaystyle \{x_{i}\}} , since it starts from the position b to the right of x. Accordingly, the longer stroke corresponds to the sequence { y i } {\displaystyle \{y_{i}\}} , which "collides with" the first sequence (just like the strokes of a lambda intersect) and then follows it subsequently.
Pollard has expressed a preference for the name "kangaroo algorithm", [ 5 ] as this avoids confusion with some parallel versions of his rho algorithm, which have also been called "lambda algorithms". | https://en.wikipedia.org/wiki/Pollard's_kangaroo_algorithm |
Pollen DNA barcoding is the process of identifying pollen donor plant species through the amplification and sequencing of specific, conserved regions of plant DNA . Being able to accurately identify pollen has a wide range of applications though it has been difficult in the past due to the limitations of microscopic identification of pollen. [ 1 ]
Pollen identified using DNA barcoding involves the specific targeting of gene regions that are found in most to all plant species but have high variation between members of different species. The unique sequence of base pairs for each species within these target regions can be used as an identifying feature.
The applications of pollen DNA barcoding range from forensics , to food safety , to conservation . Each of these fields benefits from the creation of plant barcode reference libraries. [ 2 ] These libraries range largely in size and scope of their collections as well as what target region(s) they specialize in.
One of the main challenges of identifying pollen is that it is often collected as a mixture of pollen from several species. Metabarcoding is the process of identifying the individual species DNA from a mixed DNA sample and is commonly used to catalog pollen in mixed pollen loads found on pollinating animals and in environmental DNA (also called eDNA) which is DNA extracted straight from the environment such as in soil or water samples. [ 3 ]
Some of the principle constraints of microscopic identification are the expertise and time requirements. Identifying pollen via microscopy requires a high level of expertise in the pollen characteristics of the specific plants being studied. With expertise it can still be extremely difficult to identify pollen accurately with high taxonomic resolution . [ 1 ] The skills required to do DNA barcoding are much more common [ 4 ] making the approach easier to adopt. Pollen DNA barcoding is a technique that has grown in popularity due to the decreased costs associated with "next generation sequencing" (NGS) techniques [ 5 ] and is being continually improved in efficiency including through the use of a dual-indexing approach. [ 6 ] Some of the other major advantages include the savings in time and resources compared to microscopic identification. Identifying pollen is time-consuming, involving spreading pollen on a slide, staining the pollen to improve visibility, then focusing in on individual pollen grains and identifying them based on size, shape, as well as the shape and number of pores. [ 7 ] If a pollen reference library is not available, then pollen has to be collected from wild specimens or from herbarium specimens and is then added to a pollen reference library.
Rare plants visited by some pollinators can be difficult to determine, [ 8 ] by using pollen DNA barcoding researchers can uncover "invisible" interactions between plants and pollinators. [ 9 ]
There are many challenges when it comes to genetic barcoding of pollen. The amplification process of DNA can mean that even small pieces of plant DNA can be detected included those from contaminants to a sample. Strict procedures to prevent contamination are important and can be facilitated by the hardiness of the pollen coat allowing the pollen to be washed of contaminants without damaging the internal pollen DNA.
DNA barcode reference libraries are still being built and standardized target regions are being gradually adopted. These challenges are likely due to the newness of DNA barcoding and will likely improve with the wider adoption of DNA barcoding as a tool used by taxonomists.
Determining the amount of each contributor to a mixed pollen load can be difficult to determine through the use of DNA barcoding. However, scientists have been able to compare pollen amounts via rank order. [ 10 ]
Innovations in automated microscopy and imagining software offer one potential alternative in the identification of pollen. Through the use of pattern-recognition software, researchers have developed software that can characterize microscopic pollen images based on texture analyzes. [ 11 ]
There have been several different regions of plant DNA that have been used as targets for genetic barcoding including rbcL, [ 2 ] matK, [ 12 ] trnH-psbA, [ 13 ] ITS1 [ 14 ] and ITS2. A combination of rbcL and matK has been recommended for use in plant DNA barcoding. It has been found that trnL is better for degraded DNA and ITS1 is better for differentiating species within a genus. [ 15 ]
Being able to identify pollen is especially important in the study of pollination networks which are made up of all the interactions between plants and the animals that facilitate their pollination. [ 16 ] [ 17 ] Identifying the pollen carried on insects helps scientists understand what plants are being visited by which insects. Insects can also have homologous features making them difficult to identify and are themselves sometimes identified through genetic barcoding [ 18 ] (usually of the CO1 region [ 19 ] [ 20 ] ). Every insect that visits a flower is not necessarily a pollinator. [ 21 ] Many lack features such as hairs allowing them to carry pollen while others avoid the pollen-laden anthers to steal nectar. Pollination networks are made more accurate by including what pollen is being carried by which insects. Some scientists argue that pollination effectiveness (PE), which is measured by studying the germination rates of seeds produced from flowers visited only once by a single animal, is the best way to determine which animals are important pollinators [ 22 ] though other scientists have used DNA barcoding to determine the genetic origin of pollen found on insects and have argued that this in conjunction with other traits is a good indication of pollination effectiveness. [ 23 ] By studying the composition and structure of pollination networks, conservationists can understand the stability of a pollination network and identify which species are most important and which are at most risk to perturbation [ 24 ] leading to pollinator declines. [ 25 ]
Another advantage of pollen DNA barcoding is that it can be used to determine the source of pollen found on museum specimens of insects, [ 26 ] and these records of insect-plant interactions can then be compared to modern-day interactions to see how pollination networks have changed over time [ 27 ] due to global warming, land use change, and other factors.
Being accurately able to identify pollen found on evidence helps forensic investigators identify which regions evidence originated from based on the plants that are endemic to those regions. [ 28 ] In addition to this, atmospheric pollen originating from illegal cannabis farms were successfully detected by scientists [ 29 ] which in the future could allow law enforcement officials to narrow down the search areas for illegal farms.
Due to the hardy structure of pollen which has evolved to survive being transported sometimes great distances while keeping the internal genetic information intact, the origin of pollen found mixed in ancient substrates can often be determined through DNA barcoding.
Honeybees carry pollen as well as the nectar used in their production of honey. For food quality and safety concerns it is important to understand the plant providence of human-consumed bee products including honey, royal jelly, and pollen pellets. Investigators can test which plants honeybees foraged on and thus the origin of the nectar used in honey by collecting pollen packets from honeybees' corbicular loads and identify the pollen via DNA metabarcoding . [ 30 ] | https://en.wikipedia.org/wiki/Pollen_DNA_barcoding |
A pollen calendar is used to show the peak pollen times for different types of plant pollen, which causes allergic reactions in certain people. [ 1 ] [ 2 ] [ 3 ]
A pollen calendar can be a very useful tool in forensic science , because it can be used to place the month, or week, or date of death . [ 4 ] [ 5 ] The use of pollen for criminal investigation purposes is called "forensic palynology ". [ 6 ] [ 4 ]
However, the use of a pollen calendar to set the date of death should be used with extreme caution, and only by a carefully trained expert witness . [ 7 ] The CSI effect has put pressure on some police officers and district attorneys to provide pollen-based evidence , but such evidence "appear[s] to be of limited use in the forensic context where outcomes are scrutinised in court ." [ 7 ] | https://en.wikipedia.org/wiki/Pollen_calendar |
A pollen tube is a tubular structure produced by the male gametophyte of seed plants when it germinates. Pollen tube elongation is an integral stage in the plant life cycle. The pollen tube acts as a conduit to transport the male gamete cells from the pollen grain —either from the stigma (in flowering plants ) to the ovules at the base of the pistil or directly through ovule tissue in some gymnosperms . In maize , this single cell can grow longer than 12 inches (30 cm) to traverse the length of the pistil .
Pollen tubes were first discovered by Giovanni Battista Amici in the 19th century.
They are used as a model for understanding plant cell behavior. Research is ongoing to comprehend how the pollen tube responds to extracellular guidance signals to achieve fertilization.
Pollen tubes are unique to seed plants and their structures have evolved over their history since the Carboniferous period. Pollen tube formation is complex and the mechanism is not fully understood. [ 1 ]
The male reproductive organ of the flower, the stamen , produces pollen. The opening of anthers makes pollen available for subsequent pollination (transfer of pollen grains to the pistil , the female reproductive organ). Each pollen grain contains a vegetative cell , and a generative cell that divides to form two sperm cells . Abiotic vectors such as wind , water , or biotic vectors such as animals carry out the pollen distribution.
Once a pollen grain settles on a compatible pistil, it may germinate in response to a sugary fluid secreted by the mature stigma. Lipids at the surface of the stigma may also stimulate pollen tube growth for compatible pollen. Plants that are self-sterile often inhibit the pollen grains from their own flowers from growing pollen tubes. The presence of multiple grains of pollen has been observed to stimulate quicker pollen tube growth in some plants. [ 2 ] The vegetative cell then produces the pollen tube, a tubular protrusion from the pollen grain, which carries the sperm cells within its cytoplasm . The sperm cells are the male gametes that will join with the egg cell and the central cell in double fertilization . The first fertilization event produces a diploid zygote and the second fertilization event produces a triploid endosperm .
The germinated pollen tube must drill its way through the nutrient-rich style and curl to the bottom of the ovary to reach an ovule. Once the pollen tube reaches an ovule, it bursts to deliver the two sperm cells. One of the sperm cells fertilizes the egg cell which develops into an embryo, which will become the future plant. The other one fuses with both polar nuclei of the central cell to form the endosperm , which serves as the embryo's food supply. Finally, the ovary will develop into a fruit and the ovules will develop into seeds .
Gymnosperm pollen is produced in microsporangia borne on the scales of the male cone or microstrobilus . In most species, the plants are wind-pollinated, and the pollen grains of conifers have air bladders that provide buoyancy in air currents. The grains are deposited in the micropyle of the ovule of a female cone or megastrobilus, where they mature for up to a year. In conifers and gnetophytes , the pollen germinate to produce a pollen tube that penetrates the megasporangium or nucellus carrying with it sperm nuclei that are transferred to the egg cell in the developing archegonia of the female plant. [ 3 ] [ 4 ]
The female sporophyte must recognize the pollen stuck to the stigma. Often, only pollen of the same species can successfully grow. Outcrossed pollen grows more successfully. [ 5 ] [ 6 ] With self-incompatibility systems, outcrossed pollen grows and outcompetes self pollen. The interaction between the style and the pollen detects compatibility and influences growth rate of the pollen tube. [ 7 ] This selection process relies on gene level regulation in which gene loci of the gynoecium allow either self-pollen to grow slowly, stop growing or burst while faster growth of outcrossed pollen occurs. Self-incompatibility systems maintain genetic diversity. [ 8 ] [ 9 ] As for gymnosperms, they do not contain a pistil with a stigma. Therefore, pollen must submerge into the pollination droplet, bringing the male gametophyte to the egg of the exposed ovule. However, pollen of different species will not submerge into the droplet; the pollen is left floating on top, while the droplet retracts back into the micropyle. [ 10 ]
Once the pollen grain is recognized and hydrated, the pollen grain germinates to grow a pollen tube. [ 11 ] There is competition in this step as many pollen grains may compete to reach the egg. The stigma plays a role in guiding the sperm to a receptive ovule, in the case of many ovules. [ 11 ] Only compatible pollen grains are allowed to grow as determined by signaling with the stigma.
In the pollen grain, the generative cell gives rise to the sperm, whereas the vegetative cells have a tube cell that grows the pollen tube. Some plants have mechanisms in place to prevent self pollination, such as having stigma and anther mature at different times or being of different lengths, which significantly contributes to increasing genetic diversity of the next generation. [ 12 ] [ 13 ]
There is great variation in the rate of growth of pollen tubes and many studies have focused on signaling. [ 12 ] The gene expression in the pollen grain has been identified as that of the gametophyte and not of the parental sporophyte, as it expresses its own unique mRNA and enzymes. [ 12 ] In the peach tree, the style environment which the pollen tube grows through provides nutrition for the tube's growth to the ovule. [ 11 ] Pollen tubes are tolerant and even pollen damaged by X-rays and gamma rays can still grow pollen tubes. [ 12 ]
Pollen tube growth is influenced by the interaction between the stigma-style and the pollen grain. The elongation of the tube is achieved with elongation of the cytoskeleton and it extends from the tip, which is regulated by high levels of calcium in the cytosol. [ 9 ] The calcium levels help the synaptic vesicles in the membranes grow and extend at the tip. [ 6 ] Polypeptides found in the style also regulate growth of tube and specific peptides that play a role in signaling for growth have been identified.
The LURE peptides that are secreted from the synergids, which occupy the space adjacent to the egg cell, can use attractants . In mutant Arabidopsis plant embryos, specifically in those without the synergids, the pollen tubes were unable to grow [ citation needed ] . Pollen tube growth is toward eggs of the same species as the pollen. Intraspecific signaling helps fertilize egg and sperm of the same species. The signaling in the style is important as pollen tubes can grow without the presence of an embryo sac with just interaction with the style. [ 9 ] [ 5 ] Other parts in the ovary include cytoplasmic factors like miRNA and chemical gradients that attract the pollen tube to grow toward the synergids. [ 5 ] [ 14 ]
Calcium and ethylene in Arabidopsis thaliana were involved in termination of the pollen tube when it grows near the ovary. The increase in calcium allowed release of the two sperm cells from the tube as well as degeneration of a synergid cell. [ 5 ] The chemical gradient of calcium can also contribute to termination early on in tube growth or at the appropriate time. [ 14 ]
The length of the pollen tube varies by species. It grows in an oscillating fashion until it is ready to release the sperm near the egg for fertilization to take place. [ 15 ] [ 16 ] Some fast-growing pollen tubes have been observed in lily, tobacco, and Impatiens sultanii. [ 16 ] [ 17 ] The rate of growth confers advantage to the organism but it is not clear whether the variation in growth rate exists in the population or has been selected for over generations due to increased fitness . [ 12 ]
Many transitional features have been identified that show correlation between the evolution of the pollen tube with that of a non-motile sperm. [ 13 ] Early seed plants like ferns have spores and motile sperm that swim in a water medium, called zooidogamy . [ 18 ] The angiosperm pollen tube is simple, unbranched, and fast growing, however this is not the case for ancestral plants.
In gymnosperms like Ginkgo biloba and cycadophyta, a haustorial pollen tube forms. The tube simply soaks up nutrients from the female nucellus and grows in two stages. The pollen tube is highly branched and grows on the female sporophyte tissues. First, it grows the main tube followed by a more spherical tip at the end to allow the sperm to burst near the archegonia. [ 18 ] The binucleated , multiflagellated sperm can then swim to the egg. [ 13 ] Cycads have a less branched structured and the tip end swells the same way as in the ginkgo . In cycads, however, various enzymes have been identified in the pollen tube that direct growth and the nucellus tissues are more damaged with the tube growth. [ 18 ]
In other phyla of gymnosperms, the Coniferophyta and Gnetophyta , the sperm is non motile, and the pollen tube delivers the sperm to the egg directly, in a process called siphonogamy . Conifers can be branched or unbranched and they cause degeneration of the female tissue as it grows through more tissue. [ clarification needed ] [ 18 ] Pines, for instance discharge cytoplasm of the sperm and union of the one sperm occurs as the other sperm degenerates. Yet, in Gnetophyta, there are features more similar to angiosperm pollen tubes where the tube reaches the egg with an early form of double fertilization. However, the endosperm does not form and the second fertilization is aborted. [ 13 ]
In angiosperms, the mechanism has been studied more extensively as pollen tubes in flowering plants grow very fast through long styles to reach the well-protected egg. There is great variation in pollen tubes in angiosperms and many model plants like petunia, Arabidopsis , lily and tobacco plants have been studied for intraspecific variation and signaling mechanisms. [ 12 ] In flowering plants, a phenomenon called polyamory can occur where many ovules are fertilized and overall fitness of the organism is yet to be studied with respect to rate of pollen tube growth. [ clarification needed ] [ 13 ] [ 12 ]
Pollen tubes are an excellent model for the understanding of plant cell behavior. [ 19 ] They are easily cultivated in vitro and have a very dynamic cytoskeleton that polymerizes at very high rates, providing the pollen tube with interesting mechanical properties. [ 20 ] The pollen tube has an unusual kind of growth; it extends exclusively at its apex. Extending the cell wall only at the tip minimizes friction between the tube and the invaded tissue. This tip growth is performed in a pulsating manner rather than in a steady fashion. [ 9 ] The pollen tube's journey through the style often results in depth-to-diameter ratios above 100:1 and up to 1000:1 in certain species. In maize , this single cell can grow longer than 12 inches (30 cm) to traverse the length of the pistil . The internal machinery and the external interactions that govern the dynamics of pollen tube growth are far from being fully understood.
The actin cytoskeleton has proven to be critical in assisting pollen tube growth. [ 21 ] In terms of spatial distribution, actin filaments are arranged into three different structures within the pollen tube. [ 21 ] Each unique arrangement, or pattern, contributes to the maintenance of polarized cell growth characteristic of the pollen tube. In the apical region - the site of tip-directed growth- actin filaments are less abundant, however they are highly dynamic. Furthermore, small vesicles accumulate in the apex, indicating that this region is the site of critical vesicle targeting and fusing events. Such events are essential for regulating the velocity and direction of pollen tube growth. [ 22 ] In the subapical region, actin filaments are arranged into a collar-like structure. Reverse-fountain cytoplasmic streaming occurs at the subapex; the direction of cytoplasmic streaming is reversed and continues along the axial actin cables comprising the shank. The shank region comprises the central part of the pollen tube. In this region, actin filaments are arranged into axial bundles of uniform polarity, thereby enabling the transport of various organelles and vesicles from the base of the pollen tube to the tip, propelling overall tube growth. [ 22 ]
Both the spatial distribution and dynamics of the actin cytoskeleton are regulated by actin-binding proteins (ABPs). In order to experimentally observe distributional changes that take place in the actin cytoskeleton during pollen tube growth, green fluorescent proteins (GFPs) have been put to use. [ 21 ] GFPs were mainly selected for the purposes of dynamic visualization due to the fact that they provided an efficient means for the non-invasive imaging of actin filaments in plants. Amongst the various GFPs employed during experimentation were GFP-mTalin, LIM-GFP and GFP-fimbrin/ABD2-GFP. [ 23 ] However, each of these markers either disrupted the natural structure of the actin filaments or unfavorably labeled such filaments. For example, GFP-mTalin resulted in excessive filament bundling and GFP-fimbrin/ABD2-GFP did not label actin filaments located in the apical or subapical regions of the pollen tube. [ 23 ] In light of these drawbacks, Lifeact-mEGFP has been designated as the prominent marker of choice for actin filaments in the pollen tube; Lifeact-mEGFP is able to detect all three arrangements of actin filaments, and it has minimal effects on the natural structure of actin filaments. [ 23 ] Lifeact-mEGFP has been used as a marker to study the dynamics of actin filaments in the growing pollen tubes of tobacco, lilies and Arabidopsis . [ 23 ]
Through studies conducted with GFP, it has been confirmed that the dynamic state of actin filaments located in the apical region are essential for pollen tube growth. Experimentation of actin filaments stained with GFP-mTalin have yielded results confirming that tip-localized actin filaments are highly dynamic. [ 24 ] Such experimentation has made a connection between the dynamics of tip-localized actin filaments and their role in the formation of actin structures in the subapical region. [ 24 ] Furthermore, experimentation of actin filaments located in the apical dome of Arabidopsis indicates that actin filaments are continuously produced from the apical membrane of the pollen tube; the production of these actin filaments are mediated by formins . These findings have provided evidence supporting the theory that actin filaments located in the apical region are highly dynamic and are the site of vesicle targeting and fusing events. Experimentation of etiolated hypocotyl cells as well as BY-2 suspension cells show that highly dynamic actin filaments produced from the apical membrane can either be turned over by filament severing and depolarizing events, or they can move from the apex to the apical flank, resulting in decreased accumulation of actin filaments in the apical region of the pollen tube. [ 25 ]
Experimentation of actin filament dynamics in the shank region were also conducted with the use of GFP. Findings indicated that maximum filament length in this region significantly increased, and the severing frequency significantly decreased. Such findings indicate that actin filaments located in the shank region are relatively stable compared to actin filaments located in the apical and subapical regions. [ 25 ]
ABPs regulate the organization and dynamics of the actin cytoskeleton. [ 25 ] As stated previously, actin filaments are continuously synthesized from the apical membrane. This indicates the presence of membrane-anchored actin nucleation factors. Through experimentation, it has been theorized that formins are representative of such actin nucleation factors. For example, formin AtFH5 has been identified as a major regulator of actin filament nucleation, specifically for actin filaments synthesized from the apical membrane of the pollen tube. Genetic knockouts of AtFH5 resulted in a decreased abundance of actin filaments in both apical and subapical regions of the pollen tube, thereby providing more evidence to support the theory that AtFH5 nucleates actin filament assembly in apical and subapical regions of the pollen tube. [ 25 ]
Class I formin AtFH3 is another actin nucleation factor. AtFH3 nucleates actin filament assembly of the longitudinal actin cables located in the shank region of the pollen tube. More specifically, AtFH3 uses the actin/profilin complex in order to interact with the end of actin filaments, thereby initiating actin filament nucleation. [ 25 ]
Extensive work has been dedicated to comprehend how the pollen tube responds to extracellular guidance signals to achieve fertilization. [ 26 ] [ 19 ] [ 27 ] [ 28 ] Pollen tubes react to a combination of chemical, electrical, and mechanical cues during their journey through the pistil. [ 29 ] [ 30 ] [ 31 ] However, it is not clear how these external cues work or how they are processed internally. Moreover, sensory receptors for any external cue have not been identified yet. Nevertheless, several aspects have already been identified as central in the process of pollen tube growth. The actin filaments in the cytoskeleton, the peculiar cell wall , secretory vesicle dynamics, and the flux of ions , to name a few, are some of the fundamental features readily identified as crucial, but whose role has not yet been completely elucidated.
During pollen tube growth, DNA damages that arise need to be repaired in order for the male genomic information to be transmitted intact to the next generation. In the plant Cyrtanthus mackenii , bicellular mature pollen contains a generative cell and a vegetative cell. [ 32 ] Sperm cells are derived by mitosis of the generative cell during pollen tube elongation. The vegetative cell is responsible for pollen tube development. Double-strand breaks in DNA that arise appear to be efficiently repaired in the generative cell, but not in the vegetative cell, during the transport process to the female gametophyte . [ 32 ]
In order for fertilization to occur, there is rapid tip growth in pollen tubes which delivers the male gametes into the ovules. A pollen tube consists of three different regions: the apex which is the growth region, the subapex which is the transition region, and the shank which acts like normal plant cells with the specific organelles. [ 33 ] [ 34 ] The apex region is where tip growth occurs and requires the fusion of secretory vesicles. There is mostly pectin and homogalacturonans (part of the cell wall at the pollen tube tip) inside these vesicles. [ 35 ] The pectin in the apex region contains methylesters which allow for flexibility, before the enzyme pectin methylesterase removes the methylester groups allowing calcium to bind between pectins and give structural support. [ 36 ] The homogalacturonans accumulate in the apex region via exocytosis in order to loosen the cell wall. A thicker and softer tip wall with a lower stress yield will form and this allows cell expansion to occur, which leads to an increase in tip growth. Reverse-fountain cytoplasmic streaming occurs during the tip growth which is essential for the cellular expansion, because it is transporting organelles and vesicles between the shank region and subapex region.
The actin cytoskeleton is an important factor in pollen tube growth, because there are different patterns of actin cytoskeleton within the different regions of the pollen tube for the maintenance of polarized cell growth. For instance, there are longitudinal actin cables in the shank region in order to regulate reverse-fountain cytoplasmic streaming. [ 37 ] The F-actin controls the accumulation of the homogalacturonans full vesicles- essentially mediating tip growth- in the subapex region. [ 38 ] The actin filaments controls the apical membrane and cytoplasm interactions while the pollen tube is growing in the apex region. [ 39 ] The F-actin from the apical membrane makes an actin binding protein called formin which is essential for pollen tube tip growth. Formins are expressed in the tip growth cells and are divided into two subgroups: type I and type II. The type I formins make the actin structures and partake in cytokinesis. The type II formins on the other hand contribute to the growth of polarized cells which is necessary for tip growth. [ 40 ] Tip growth is a form of extreme polarized growth and this polarized process requires actin-binding protein-mediated organization of actin cytoskeleton. An essential protein required for this tip growth is the actin-organizing protein and type II formin protein called Rice Morphology Determinant (RMD). RMD is localized in the tip of the pollen tube and controls pollen tube growth by regulating the polarity and organization of F-actin array. [ 41 ] [ 42 ]
RMD promotes pollen germination and pollen tube growth, and this is proven through numerous experiments. The first experiment compares the features of the pistil and the stigma of rmd-1 mutant (rice plant without a functional RMD) and the wild-type rice plant (with a functional RMD). The anther and pistil were shorter in the rmd-1 mutants than the wild-type. This experiment showed that RMD is critical for pollen development. Wild-type rice plants have increased germination rates while rmd-1 mutants have decreased germination rates. This was seen when both were germinated in a liquid germination medium. After the germination rates were tested, there was a comparison of the lengths and widths of the pollen tubes between the two plants. The pollen tubes of the wild-type plants had a greater pollen tube length than the mutants, but the mutants had a greater tube width. This greater pollen tube width within the mutants indicates the decrease in the growth of polarized cells and thus decrease in tip growth. Next, pollen grains from the wild type and mutants were collected to compare the pollination activities between the wild types and mutants. There was decreased activity and minimal penetration within the mutants whereas an increased activity and penetration through the style and to the bottom of the pistils within the wild types. These observations indicated the delayed pollen tube growth in the rmd-1 mutants. Additionally, there was no effect on fertilization rates between the wild type and the mutant and this was tested by measuring the seed-setting rates between the wild type and mutant. It was found that both had similar seed-setting rates. Therefore, RMD does not affect fertilization and has an effect only on tip growth. [ 42 ]
Total RNA extractions from the whole flower, lemma, palea, lodicule, pistil, anther, and mature pollen grains of the wild type plants took place in order to discover where RMD is specifically expressed in the plant as a whole. Using RT-qPCR (reverse transcription quantitative PCR), it was evident that there were different amounts of RMD transcripts within each part of the plant. And then it was evident where RMD was present in each part of the plant using RT-PCR (reverse transcription PCR) and using UBIQUITIN as a control. These two methods demonstrated that there was an abundant presence of the RMD transcripts in the lemma, pistil, anther, and mature pollen grains. In order to confirm these results, another method was performed. This method used transgenic plants that had an RMD promoter region fused with a reporter gene encoding GUS. [ 42 ] Histochemical staining of the tissues of these transgenic plants then showed high GUS activity within the pistil, anther wall, and mature pollen grains. Therefore, these combined results demonstrated that RMD is expressed in these specific organs of the plant.
Detection of GUS signals were employed once again in order to study where RMD is specifically expressed within the pollen tube. First, pollen grains were collected from proRMD::GUS trangenic plants, and it was noted that there was a strong GUS signal within these mature pollen grains. These pollen grains were then germinated in vitro and GUS signals were observed within the tip growth of the pollen tubes. However, the strength of these GUS signals varied at different germination stages. The GUS signals were weak within the pollen tube tip at the early germination stage, but stronger at the later germination stages. Therefore, these results support that RMD is involved in pollen germination and pollen tube growth.
RMD, which are type II formins, consist of a phosphatase, (PTEN)-like domain (responsible for protein localization), and FH1 and FH2 domains (promotes actin polymerization). [ 43 ] [ 44 ] [ 42 ] In order to discover the localization of RMD in the pollen tube, transient assays of growing pollen tubes of tobacco was performed and the fluorescent protein-GFP was used. Many confocal images of various pollen tubes under specific conditions were observed: pLat52::eGFP (single eGFP driven by the pollen specific Lat52 promoter and this acts as a control); pLat52::RMD-eGFP (RMD protein fused with eGFP); pLat52::PTEN-eGFP (the PTEN domain fused with eGFP); and pLat52::FH1FH2-eGFP (the FH1 and FH2 domains fused with eGFP). By comparing the images of the control with pLat52::RMD-eGFP, it is observed that the single GFP was spread throughout the entire tube whereas RMD-eGFP accumulated in the tip region of the tube. Therefore, this shows that RMD is localized within the tip of the pollen tube.
In order to discover whether the PTEN-like domain is responsible for the localization of RMD, there was a comparison between the confocal images of GFP fused with PTEN domain and shortened RMD without the PTEN domain (pLat52::FH1FH2-eGFP). The PTEN-eGFP signals were localized in the tip of the pollen tubes like the RMD-eGFP signals, whereas the FH1FH2-eGFP signals were present throughout the pollen tube and not localized in a polar manner. Therefore, these combined results demonstrate that the PTEN-like domain is responsible for the tip localization of RMD in the pollen tubes.
In order to determine if RMD controls F-actin organization within the pollen tube, F-actin arrays in wild type and rmd-1 mature pollen grains were observed using Alexa Fluor 488-phalloidin staining. Strongly bundled actin filaments were present around the apertures of the wild type pollen grains although there was no accumulation of actin filaments around the apertures in the rmd-1 pollen grains. Additionally, there were weak signals and random organization of the actin filaments within the rmd-1 pollen grain. Therefore, these results support that RMD is essential for controlling pollen germination.
Fluorescent intensity was measured using statistical analysis in order to observe the actin filament densities within the pollen tubes. [ 45 ] There was greater fluorescence intensity in the shank region of the rmd-mutant tubes which means there was a higher density of F-actin within this region. But, there was a lower density of F-actin observed in the tip region of the rmd-mutant tubes compared to the wild type tubes. This demonstrates that the F-actin distribution pattern of pollen tubes is altered without a functional RMD.
In order to determine the polarity of the actin cables, the angles between the actin cables and elongation axis of the pollen tube were measured. The angles in the shank region of the wild type pollen tubes were predominantly less than 20° whereas the angles for the rmd-mutant pollen tubes were greater than 60°. These results support the fact that RMD is essential for polarized tip growth, because the rmd-mutant pollen tubes (without a functional RMD) exhibited an increased width, and thus a decrease in tip growth. The maximum length of the single cables of F-actin filaments from the apical to the shank region of elongating pollen tubes were also measured to test the polarity within the pollen tube. The maximum length of the F-actin cables were shorter in the rmd-mutant pollen tubes compared to those in the wild type tubes. Therefore, these combined results support that the proper organization of actin cables as well as normal F-actin densities within the tip of the tube can only be achieved if RMD is present. | https://en.wikipedia.org/wiki/Pollen_tube |
A pollination network is a bipartite mutualistic network in which plants and pollinators are the nodes , and the pollination interactions form the links between these nodes. [ 1 ] The pollination network is bipartite as interactions only exist between two distinct, non-overlapping sets of species , but not within the set: a pollinator can never be pollinated, unlike in a predator-prey network where a predator can be depredated. [ 2 ] A pollination network is two-modal, i.e., it includes only links connecting plant and animal communities. [ 3 ]
A key feature of pollination networks is their nested design. A study of 52 mutualist networks (including plant-pollinator interactions and plant-seed disperser interactions) found that most of the networks were nested. [ 4 ] This means that the core of the network is made up of highly connected generalists (a pollinator that visits many different species of plant), while specialized species interact with a subset of the species that the generalists interact with (a pollinator that visits few species of plant, which are also visited by generalist pollinators). [ 5 ] As the number of interactions in a network increases, the degree of nestedness increases as well. [ 4 ] One property that results from nested structure of pollination networks is an asymmetry in specialization, where specialist species are often interacting with some of the most generalized species. This is in contrast to the idea of reciprocal specialization, where specialist pollinators interact with specialist plants. [ 6 ] Similar to the relationship between network complexity and network nestedness, the amount of asymmetry in specialization increases as the number of interactions increases. [ 6 ]
Another feature that is common in pollination networks is modularity . Modularity occurs when certain groups of species within a network are much more highly connected to each other than they are with the rest of the network, with weak interactions connecting different modules. [ 7 ] [ 8 ] Within modules it has been shown that individual species play certain roles. Highly specialized species often only interact with individuals within their own module and are known as ‘peripheral species’; more generalized species can be thought of as ‘hubs’ within their own module, with interactions between many different species; there are also species which are very generalized which can act as ‘connectors’ between their own module and other modules. [ 7 ] A study of three separate networks, all of which showed modularity, revealed that hub species were always plants and not the insect pollinators. [ 8 ] Previous work has found that networks will become nested at a smaller size (number of species) than that where networks frequently become modular. [ 7 ]
There is substantial interest into the robustness of pollination networks to species loss and collapse, especially due to anthropogenic factors such as habitat destruction . The structure of a network is thought to affect how long it is able to persist after species decline begins. In particular, the nested structure of networks has been shown to protect against complete destruction of the network, because the core group of generalists are the most robust to extinction by habitat loss. [ 9 ] [ 10 ] Models specifically focused on the effects of habitat loss have shown that specialist species tend to go extinct first, while the last species to go extinct are the most generalized of the network. [ 9 ] Other studies focusing specifically on the removal of different types of species showed that species decline is the fastest when removing the most generalized species. However, there have been contrasting results on how rapidly decline occurs with removal of these species. One study showed that even at the fastest rate, the decline was still linear. [ 10 ] Another study revealed that with the removal of the most common pollinator species, the network showed a drastic collapse. [ 11 ] In addition to focusing on the removal of species themselves, other work has emphasized the importance of studying the loss of interactions, as this will often precede species loss and may well accelerate the rate at which extinction occurs. [ 12 ] | https://en.wikipedia.org/wiki/Pollination_network |
The pollination of orchids represents a complex aspect of the biology of this plant family , characterized by intricate flower structures and diverse ecological interactions with pollinator . Notably, the topic has garnered significant scientific interest over time, including the attention of Charles Darwin , who is recognized for his contributions to the theory of evolution by natural selection . In 1862, Darwin published his observations on the essential role of insects in orchid pollination in his work The Fertilization of Orchids . [ 1 ] [ note 1 ] He noted that the various strategies employed by orchids to attract their pollinators are complex.
Approximately 97% of orchid species rely on pollinator for the transfer of pollen from one plant to the pistils of another, which is essential for fertilization and seed formation. [ 3 ] The pollen of orchids is organized into compact masses known as pollinia (singular: "pollinium"), preventing dispersal by wind and necessitating the presence of pollinators for sexual reproduction . [ 3 ] These pollinators vary widely and may include flies , mosquitos , bees , wasps , butterflies , coleopterans , and birds , particularly hummingbirds . [ 4 ] [ 5 ] [ 3 ]
The phenomenon of zoophily in orchids requires that pollinating animals frequently visit the flowers and remain long enough to contact both the anthers and stigma. For successful pollen transfer, it is crucial that the pollen adheres effectively to the pollinators, enabling it to reach the stigmas of other flowers. The effectiveness of zoophily depends on the ability of these animals to recognize flowers from a distance and their attraction to flowers of the same species. Consequently, zoophilous flowers typically possess "attractive products" such as pollen and nectar , "means of attraction" like scents and colors, and pollen that is viscous or adhesive. [ 4 ]
Throughout the evolution of angiosperms, there has been significant differentiation in the means of attraction and flower morphology, allowing a broader range of animals to participate in pollination. This evolutionary process has led to the establishment of close relationships between pollinating animals and zoophilous flowers, benefiting both groups. For plants, this relationship has resulted in more precise attraction of specific pollinators, facilitating the transfer of pollen to the stigmas of other plants and reducing the overall production of pollen. In contrast to anemophilous plants, which may produce around one million pollen grains per ovule, orchids typically produce a one-to-one ratio. For specialized pollinators, this mutualism has reduced competition from other anthophilous animals, making targeted pollination advantageous. [ 3 ] [ 4 ]
The evolutionary development of zoophilous angiosperms and their adapting animal partners is best understood as a process of coevolution characterized by reciprocal relationships. In some cases, orchids and their pollinators have become so interdependent that their existence is mutually exclusive. [ 4 ] Pollination mechanisms resulting from this coevolution generally benefit both parties: pollinators obtain nectar from the flowers, while orchids gain pollen transfer. However, in numerous instances, the attraction of pollinators to orchids may rely on deceptive strategies that do not offer any rewards. [ 3 ]
Orchid flowers are predominantly hermaphroditic , with unisexual forms being rare, and they typically exhibit zygomorphic (bilateral) symmetry. Most genera feature three outer elements known as sepals —two lateral and one dorsal—and three inner elements called petals , with the lower petal often modified into a lip or labellum . This labellum is usually larger and more vividly colored than the other petals, often exhibiting a trilobed or uniquely shaped structure, sometimes adorned with fleshy bumps or ridges, and may feature a basal spur with distinct color patterns. [ 6 ]
The androecium of orchids generally comprises one or two stamens , occasionally three, and is fused with the style and stigma to form a structure known as the column (or gynostema/gynostegium). The pollen is organized into masses called pollinia , which can vary in number from one to twelve, though two or four are most common. These pollinia, combined with a sticky stalk derived from the anther or stigma, form the transport unit during pollination . The gynoecium consists of three fused carpels and is situated below the calyx. It features a highly modified style that is solitary and terminal, forming a key component of the column. Near the apex, the stigma has an elongated lobe known as the rostellum, which is typically non-receptive and positioned above the stigmatic region. A part of the rostellum may develop into a sticky platform called the viscidium, which attaches to the pollinium stem. [ 6 ]
Orchids generally produce nectar as a reward for pollinators, with nectaries varying in their location and type. They may be situated on the lip spur, at the tips of the sepals, or on the septa of the gynoecium. Additionally, some orchid species are capable of self-pollinatings or are apomictics , meaning they can produce seeds without the need for pollinators. [ 7 ]
Many orchid species provide various rewards to pollinators, including nectar, food hairs, oils, and other compounds such as waxes, resins, and fragrances. These rewards serve to reinforce pollinator behavior, enhancing the likelihood of effective pollen transfer. Over time, this specialization on a single type of pollinator has led to increased morphological and structural adaptations in orchid flowers, aimed at attracting specific insect species. [ 3 ] [ 8 ] [ 9 ] [ 10 ]
This evolutionary aspect of the interspecific relationships between orchids and their pollinators was notably examined by Charles Darwin in his studies of both British and exotic orchid species. A prominent example is the Madagascar species Angraecum sesquipedale , which features a spur exceeding 30 cm in length that contains nectar. Darwin's attempts to extract the pollinia from this flower using needles were unsuccessful; he was only able to do so by inserting a cylinder with a diameter of 2.5 mm into the spur and pulling it out, causing the viscidium to adhere to the cylinder. Darwin proposed that when a butterfly reached the bottom of the spur to access the nectar, the pollinia would attach to its head as it withdrew its proboscis. Upon visiting another flower, the butterfly would then transfer the pollinia to its stigma. Based on this reasoning, Darwin suggested that the pollinator of Angraecum sesquipedale would need to have a proboscis longer than 30 cm, an idea that seemed implausible to contemporary biologists. However, this prediction was confirmed in 1903 with the discovery of the moth Xanthopan morganii praedicta [ 11 ] in Madagascar, which possesses a proboscis of the predicted length. The subspecific epithet "praedicta" reflects the foretelling of its existence by Darwin. [ 3 ]
The moth is attracted to Angraecum sesquipedale by its fragrance, particularly during the night. Upon approaching the flower, the moth unrolls its proboscis and inserts it into a crevice of the rostellum that leads to the spur. After accessing the nectar at the base of the spur, the moth lifts its head while withdrawing its proboscis, causing the viscidium to adhere to its head or another part of its body. The viscidium contains a small pedicel, known as the caudicle, which carries the pollinia. When the moth finishes feeding and moves to another flower, the caudicle dehydrates, altering its angle relative to the insect’s body. This positioning ensures that when the moth inserts its proboscis into the next flower, the pollinia are properly aligned to attach to the stigma. After successful pollination, the flowers stop producing fragrance, and their tepals wither shortly thereafter. Nonetheless, the processes of pollen transfer, fertilization, and the formation of numerous new individuals have already been secured. [ 3 ]
Many orchids, including Angraecum sesquipedale , are pollinated by nocturnal butterflies and, as a result, tend to have light-colored or nearly white flowers that emit fragrance in the evening or night. Other examples of such orchids include Bonatea speciosa , Habenaria epipactidea , species in the genus Satyrium , Disa cooperi and D. ophrydea. [ 12 ] In contrast, some orchid genera have evolved to be pollinated by diurnal butterflies, exhibiting bright colors and providing nectar as a reward. [ 12 ]
Melitophilous orchids, which are pollinated by bees, typically produce a strong fragrance during the day and are robustly colored. Examples include Satyrium erectum and Disa versicolor . These bee-pollinated orchids often offer not only nectar but also oils—a relatively rare reward in the plant kingdom—used by various bee species to nourish their larvae. Notable genera that provide oil rewards include Disperis , Pterygodium , Corycium , Ceratandra , Evotella , Satyrium , and Pachites . [ 12 ]
Pollination by flies, known as myophily, is the second most prevalent method of pollination among orchids, involving pollinators from twenty different dipteran families. [ 13 ] These flowers typically emit scents reminiscent of decaying organic materials, excrement, or carrion, which attract flies seeking food or suitable sites for egg deposition. [ 8 ] Various floral parts produce putrescent or carrion-like odors and often incorporate traps to retain the pollinators, alongside appendages and colors that may mimic flesh or other aspects of rotting matter. [ 14 ] [ 15 ] [ 16 ] [ 17 ] For example, species in the genus Bulbophyllum , which are daciniphilous, attract true fruit flies from the Dacini tribe (Tephritidae) using pleasant or spicy scents as floral synomones, establishing mutualistic relationships that confer reproductive benefits to both the orchids and the male flies. Notably, B. hortorum has coevolved with male fruit flies to develop a unique pollination mechanism that selects for optimal-sized individuals as potential pollinators. [ 18 ]
Stelis hymenantha produces a strong aroma of sweet menthol. It secretes a sticky substance at the base of the labellum that resembles nectar. Another species, S. immersa , also emits a fragrance, described as melon-like, with its sticky substance located on both the petals and the labellum’s base. The primary visitors to these species are dipterans from various families. Typically, flies remain outside the flowers, investigating the viscous liquid on the petal surfaces. In the case of Stelis immersa , female flies from the genus Megaselia ( Phoridae ) are specifically adapted for effective pollen transfer. After inspecting the nectar-like substance, these insects enter the flower laterally and land on the downward-facing labellum. When this occurs, the labellum rises, pressing the fly against the viscid pollinium and trapping it. To exit, the fly backs up, causing the viscidium to adhere to its thorax. The labellum then returns to its original position, releasing the fly, which has now facilitated pollination. [ 19 ]
The flowers of the subtribes Stanhopeinae and Catasetinae exhibit specialized pollination mechanisms. These species are exclusively pollinated by male euglossine bees, which collect perfumes from the flowers. The reasons behind this behavior are not entirely understood, as the perfumes do not provide nutrition or protection but may play a role in the mating rituals of these bees.
The process of scent collection is quite similar across species. The male bee approaches the osmophore, the scent-producing part of the flower, and perches on the labellum . Using the long, dense hairs on its front legs, the bee gathers the aromatic substances, which are typically liquid but can also be found in crystalline form. If the scent compounds are solid, the bee dissolves them with secretions from its salivary glands. Once saturated with the aroma, the bee transfers the scents using its middle legs to storage cavities in its hind legs, where they can be preserved for extended periods. Different orchid genera attach pollinia to various parts of the bee’s body. [ 20 ]
Orchids, like many plants, attract specific groups of male euglossine bees by producing species-specific scent mixtures, which likely act as reproductive isolation mechanisms. Some orchids exhibit morphological adaptations that ensure pollinia are only released upon visitation by particular bee species, depending on their size and behavior. Consequently, not all euglossine bees that visit a given orchid species are effective pollinators. [ 21 ]
Additionally, certain species of Bulbophyllum attract specific male Dacini fruit flies through particular male attractants such as methyl eugenol (ME), raspberry ketone (RK), or zingerone (ZN). Methyl eugenol serves as a sex pheromone precursor for several quarantine pest species of Bactrocera , including the oriental fruit fly ( B. dorsalis ), B. carambolae , B. occipitalis , and B. umbrosa . Similarly, raspberry ketone and zingerone function as sex pheromone components for male fruit flies like Zeugodacus caudatus , Z. cucurbitae , and Z. tau . [ 22 ] [ 23 ] [ 24 ] [ 25 ] [ 26 ] [ 27 ]
Many orchids employ various deceptive tactics to attract pollinators by mimicking scents, shapes, colors, or movements associated with resources of interest to the pollinators, without providing any actual rewards. The mechanisms of deception are diverse and can be categorized as follows: [ 28 ] [ 29 ]
Among these mechanisms, generalized feeding deception is the most commonly observed in orchids, reported in 38 genera, followed by sexual deception, which has been identified in 18 genera. [ 28 ] [ 29 ]
The ability to attract pollinators without offering rewards has evolved independently across several angiosperm lineages, although it typically occurs in only a few species within each family. [ 32 ] In contrast, it is estimated that approximately one-third of orchid species employ a food-deceptive mechanism. [ 33 ] [ 34 ] This strategy involves signaling the presence of food, such as nectar or pollen, to attract pollinators without providing any actual reward. Orchids often achieve this by resembling species that do offer rewards and that cohabit in the same environment. [ 35 ] Feeding deception commonly manifests as a general resemblance to rewarding species, with orchids featuring large, brightly colored flowers that exploit pollinators' innate preferences for such floral characteristics. [ 29 ] [ 34 ] [ 36 ]
Another method employed by orchids to attract pollinators involves mimicking the flowers of other plant species. A notable example is Epidendrum ibaguense , a terrestrial or lithophilous orchid found from Mexico to Bolivia and Brazil . This orchid features orange flowers with an intense yellow labellum, closely resembling the flowers of Asclepias curassavica , a member of the Asclepiadaceae family. The butterfly Agraulis vanillae , which typically visits Asclepias curassavica to collect nectar in exchange for pollen transport, is frequently drawn to the mimetic flowers of Epidendrum ibaguense . When the butterfly approaches the orchid, it inserts its proboscis into the narrow duct ( gynostemium ) of the flower. The small diameter of this duct can cause the butterfly's spiracles to become temporarily trapped. In its struggle to escape, the insect inadvertently picks up the orchid's pollinia. After being released, the butterfly may visit another Epidendrum flower, transferring the pollinia and facilitating pollination without receiving nectar for its efforts. [ 37 ]
Certain orchids have evolved deceptive flowers that mimic the shape, hairiness, and scent of female wasps or bees to attract male pollinators. A well-known example is Ophrys insectifera , found in southern Europe, which is exclusively visited by two species of wasps from the genus Argogorytes . Male wasps emerge in spring, weeks before females, and are attracted to the fragrance of Ophrys flowers, which resembles the pheromones secreted by female wasps. Additionally, the labellum of Ophrys insectifera closely resembles the shape, color, and texture of female wasps. This interaction is referred to as pseudocopulation , as male wasps attempt to mate with the flower, during which they come into contact with the anther and transfer pollinia between flowers. [ 38 ]
The phenomenon of pollination by pseudocopulation was first documented by A. Pouyanne and H. Correvon in 1916 and 1917 while studying the relationship between the orchid Ophrys speculum and the scoliidae wasp Campsoscolia ciliata in Algeria . [ 39 ] [ 40 ] Their findings initially went unnoticed until Robert Godfrey validated their observations in 1925, prompting increased interest in this area of study. [ note 3 ] Following this, Australian biologist Edith Coleman published numerous papers on the pollination of orchids from the genus Cryptostylis by males of the ichneumonoidea wasp Lissopimpla excelsa . [ 41 ] [ 42 ] [ 43 ]
Several genera of terrestrial orchids employ this pseudocopulation mechanism, including Ophrys , Cryptostylis , Drakaea , Caladenia , [ 44 ] Chiloglottis , [ 45 ] Geoblasta , [ 46 ] Arthrochilus , Calochilus , [ 47 ] Leporella , [ 48 ] and Spiculaea . Most terrestrial orchid genera utilizing pseudocopulation are found in Australia, with Ophrys being the largest and best-documented genus in Europe. This mechanism is not limited to a specific continent; it has also been observed in the South American species Geoblasta penicillata [ 49 ] and in two South African orchids of the genus Disa . [ 50 ]
A similar mechanism is found in Tolumnia henekenii , whose flowers mimic the female of the bee species Centris insularis . The resemblance is so convincing that male bees attempt to copulate with the flower, thereby facilitating its pollination.
In the genus Caleana , commonly known as the duck orchid due to its labellum resembling the head of a duck and the overall flower resembling a flying duck, a distinct mechanism of pseudocopulation has been observed. This species utilizes a spring mechanism to trap pollinating insects in a pouch, with their only means of escape being through the pollinium and stigma. When male insects land on the labellum, they activate a two-hinged relaxation mechanism involving the labellum-lamellum and the lamina-perianth, which flips the insect into the pouch containing the stigma and pollinia. [ 51 ] [ 52 ]
Pseudocopulation is not limited to pollinators from the Hymenoptera order (such as bees and wasps); it has also been documented in certain Diptera . For instance, male mosquitoes of the genus Bradysia have been observed to pseudocopulate with species of Lepanthes , a large genus of orchids found in neotropical rainforests. [ 53 ]
In Ophrys orchids, the flowers not only mimic the shape, size, and color of female pollinators but also emit fragrances that include compounds found in female sex pheromones . This reinforces the sexual attraction of males to the flowers. Chemical and electrophysiological comparisons have been made between the volatile compounds emitted by Ophrys iricolor and the female pheromones of its pollinator, Andrena morio . More than 40 compounds have been identified, including alkanes and alkenes with 20 to 29 carbon atoms, aldehydes with 9 to 24 carbons, and two esters . Most of these compounds are present in similar proportions in both floral extracts of O. iricolor and extracts from the cuticular surface of A. morio females. [ 54 ]
The biologically active volatile compounds in Ophrys species are largely similar to those utilized by other Ophrys species that engage in pseudocopulation with males of the genera Andrena and Colletes . [ 55 ] [ 56 ] [ 57 ] [ 58 ] [ 59 ] | https://en.wikipedia.org/wiki/Pollination_of_orchids |
Pollination syndromes are suites of flower traits that have evolved in response to natural selection imposed by different pollen vectors , which can be abiotic (wind and water) or biotic, such as birds, bees, flies, and so forth through a process called pollinator-mediated selection . [ 1 ] [ page needed ] [ 2 ] [ page needed ] These traits include flower shape, size, colour, odour , reward type and amount, nectar composition, timing of flowering, etc. For example, tubular red flowers with copious nectar often attract birds; foul smelling flowers attract carrion flies or beetles, etc.
The "classical" pollination syndromes were first studied in the 19th century by the Italian botanist Federico Delpino . Although they are useful in understanding of plant-pollinator interactions , sometimes the pollinator of a plant species cannot be accurately predicted from the pollination syndrome alone, and caution must be exerted in making assumptions. [ 3 ]
The naturalist Charles Darwin surmised that the flower of the orchid Angraecum sesquipedale was pollinated by a then undiscovered moth with a proboscis whose length was unprecedented at the time. His prediction had gone unverified until 21 years after his death, when the moth was discovered and his conjecture vindicated. The story of its postulated pollinator has come to be seen as one of the celebrated predictions of the theory of evolution . [ 4 ]
Abiotically pollinated flowers do not attract animal pollinators. Nevertheless, they often have suites of shared traits.
Wind-pollinated flowers may be small and inconspicuous, as well as green and not showy. They produce enormous numbers of relatively small pollen grains (hence wind-pollinated plants may be allergens , but seldom are animal-pollinated plants allergenic). Their stigmas may be large and feathery to catch the pollen grains. Insects may visit them to collect pollen; in some cases, these are ineffective pollinators and exert little natural selection on the flowers, but there are also examples of ambophilous flowers which are both wind and insect pollinated. Anemophilous, or wind pollinated flowers, are usually small and inconspicuous, and do not possess a scent or produce nectar. The anthers may produce a large number of pollen grains, while the stamens are generally long and protrude out of flower. [ citation needed ]
Water-pollinated plants are aquatic and pollen is released into the water. Water currents therefore act as a pollen vector in a similar way to wind currents. Their flowers tend to be small and inconspicuous with many pollen grains and large, feathery stigmas to catch the pollen. However, this is relatively uncommon (only 2% of pollination is hydrophily) and most aquatic plants are insect-pollinated, with flowers that emerge into the air. Vallisneria is an example. [ citation needed ]
Bee -pollinated flowers can be very variable in their size, shape and colouration. They can be open and bowl-shaped (' actinomorphic ', radially symmetrical) or more complex and non-radially symmetric (' zygomorphic '), as is the case with many peas and foxgloves .
Some bee flowers tend to be yellow or blue, often with ultraviolet nectar guides and scent. Nectar , pollen, or both are offered as rewards in varying amounts. The sugar in the nectar tends to be sucrose -dominated. A few bees collect oil from special glands on the flower. [ 5 ]
Butterfly -pollinated flowers tend to be large and showy, pink or lavender in colour, frequently have a landing area, and are usually scented. Since butterflies do not digest pollen (with one exception), more nectar is offered than pollen. The flowers have simple nectar guides with the nectaries usually hidden in narrow tubes or spurs, reached by the long tongue of the butterflies.
Among the more important moth pollinators are the hawk moths ( Sphingidae ). Their behaviour is similar to hummingbirds : they hover in front of flowers with rapid wingbeats. Most are nocturnal or crepuscular . So moth-pollinated flowers tend to be white, night-opening, large and showy with tubular corollas and a strong, sweet scent produced in the evening, night or early morning. Much nectar is produced to fuel the high metabolic rates needed to power their flight.
Other moths ( Noctuids , Geometrids , Pyralids , for example) fly slowly and settle on the flower. They do not require as much nectar as the fast-flying hawk moths, and the flowers tend to be small (though they may be aggregated in heads). [ 6 ]
Myophilous plants, those pollinated by flies, tend not to emit a strong scent, are typically purple, violet, blue, and white, and have open dishes or tubes. [ 7 ]
Sapromyophilous plants attract flies which normally visit dead animals or dung . Flowers mimic the odor of such objects. The plant provides them with no reward and they leave quickly unless it has traps to slow them down. Such plants are far less common than myophilous ones. [ 8 ]
Beetle -pollinated flowers are usually large, greenish or off-white in color and heavily scented. Scents may be spicy, fruity, or similar to decaying organic material. Most beetle-pollinated flowers are flattened or dish shaped, with pollen easily accessible, although they may include traps to keep the beetle longer. The plant's ovaries are usually well protected from the biting mouthparts of their pollinators. [ 9 ] A number of cantharophilous plants are thermogenic , with flowers that can increase their temperature. This heat is thought to help further spread the scent, but the infrared light produced by this heat may also be visible to insects during the dark night, and act as a shining beacon to attract them. [ 10 ]
Flowers pollinated by specialist nectarivores tend to be large, red or orange tubes with a lot of dilute nectar, secreted during the day. Since birds do not have a strong response to scent, they tend to be odorless. Flowers pollinated by generalist birds are often shorter and wider. Hummingbirds are often associated with pendulous flowers, whereas passerines (perching birds) need a landing platform so flowers and surrounding structures are often more robust. Also, many plants have anthers placed in the flower so that pollen rubs against the birds head/back as the bird reaches in for nectar.
There are major differences between bat pollination in the New World as opposed to the Old World . In the Old World pollinating bats are large fruit bats of the family Pteropodidae which do not have the ability to hover and must perch in the plant to lap the nectar; these bats furthermore do not have the ability to echolocate . [ 11 ] Bat-pollinated flowers in this part of the world tend to be large and showy, white or light coloured, open at night and have strong musty odours. They are often large balls of stamens.
In the Americas pollinating bats are tiny creatures called glossophagines which have both the ability to hover as well as echolocate, and have extremely long tongues. Plants in this part of the world are often pollinated by both bats and hummingbirds, and have long tubular flowers. [ 11 ] Flowers in this part of the world are typically borne away from the trunk or other obstructions, and offer nectar for extended periods of time. In one essay, von Helversen et al . speculate that maybe some bell-shaped flowers have evolved to attract bats in the Americas, as the bell-shape might reflect the sonar pulses emitted by the bats in a recognisable pattern. [ 12 ] A number of species of Marcgravia from Caribbean islands have evolved a special leaf just above the inflorescence to attract bats. The leaf petiole is twisted so the leaf sticks upwards, and the leaf is shaped like a concave disc or dish reflector. The leaf reflects echolocation signals from many directions, guiding the pollinating bats towards the flowers. The epiphytic bean Mucuna holtonii employs a similar tactic, but in this species it is a specialised petal that acts as a sonar reflector. [ 13 ] In the New World bat pollinated flowers often have sulphur -scented compounds. [ 14 ]
Bat-pollinated plants have bigger pollen than their relatives. [ 15 ]
The characteristics of the pollination syndrome associated with pollination by mammals which are not bats are: a yeasty odour; cryptic, drab, axillary, geoflorous flowers or inflorescences often obscured from sight; large and sturdy flowers, or grouped together as multi-flowered inflorescences; either sessile flowers or inflorescences or subtended by a short and stout peduncle or pedicel; bowl-shaped flowers or inflorescences; copious, sucrose-rich nectar usually produced during the night; tough and wiry styles; an adequate distance between the stigma and nectar to fit the rostrum of the pollinating animal; and potentially a winter–spring flowering period. [ 16 ] [ 17 ]
Many non-flying mammals are nocturnal and have an acute sense of smell, so the plants tend not to have bright showy colours, but instead excrete a strong odour. These plants also tend to produce large amounts of pollen because mammals are larger than some other pollinators, and lack the precision smaller pollinators can achieve. [ 18 ]
The Western-Australian endemic Honey possum ( Tarsipes rostratus ) is an unusual non-flying mammal pollinator in that it has adapted to feeding exclusively on pollen and nectar. It is known to forage on a wide variety of plants (particularly in the families Proteaceae and Myrtaceae ) including many with typical bird-pollinated flowers such as Calothamnus quadrifidus [ 19 ] and many species of Banksia . [ 20 ]
Pollination syndromes reflect convergent evolution towards forms ( phenotypes ) that limit the number of species of pollinators visiting the plant. [ 21 ] They increase the functional specialization of the plant with regard to pollination, though this may not affect the ecological specialization (i.e. the number of species of pollinators within that functional group). [ 22 ] They are responses to common selection pressures exerted by shared pollinators or abiotic pollen vectors, which generate correlations among traits. That is, if two distantly related plant species are both pollinated by nocturnal moths, for example, their flowers will converge on a form which is recognised by the moths (e.g. pale colour, sweet scent, nectar released at the base of a long tube, night-flowering).
These honeybees selectively visit flowers from only one species for a period of time, as can be seen by the colour of the pollen in their baskets.
Pollinators fluctuate in abundance and activity independently of their plants, [ 22 ] [ 27 ] and any one species may fail to pollinate a plant in a particular year. Thus a plant may be at an advantage if it attracts several species or types of pollinators, ensuring pollen transfer every year. [ 28 ] Many species of plants have the back-up option of self-pollination , if they are not self-incompatible.
Whilst it is clear that pollination syndromes can be observed in nature, there has been much debate amongst scientists as to how frequent they are and to what extent we can use the classical syndromes to classify plant-pollinator interactions. [ 29 ] Although some species of plants are visited only by one type of animal (i.e. they are functionally specialized), many plant species are visited by very different pollinators. [ 28 ] [ 30 ] For example, a flower may be pollinated by bees, butterflies, and birds. Strict specialization of plants relying on one species of pollinator is relatively rare, probably because it can result in variable reproductive success across years as pollinator populations vary significantly. [ 28 ] In such cases, plants should generalize on a wide range of pollinators, and such ecological generalization is frequently found in nature. A study in Tasmania found the syndromes did not usefully predict the pollinators. [ 31 ]
A critical re-evaluation of the syndromes suggests that on average about one third of the flowering plants can be classified into the classical syndromes. [ 3 ] This reflects the fact that nature is much less predictable and straightforward than 19th-century biologists originally thought. Pollination syndromes can be thought of as extremes of a continuum of greater or lesser specialization or generalization onto particular functional groups of pollinators that exert similar selective pressures" [ 21 ] and the frequency with which flowers conform to the expectations of the pollination syndromes is relatively rare. In addition, new types of plant-pollinator interaction, involving "unusual" pollinating animals are regularly being discovered, such as specialized pollination by spider hunting wasps ( Pompilidae ) and fruit chafers ( Cetoniidae ) in the eastern grasslands of South Africa. [ 32 ] These plants do not fit into the classical syndromes, though they may show evidence of convergent evolution in their own right.
An analysis of flower traits and visitation in 49 species in the plant genus Penstemon found that it was possible to separate bird- and bee- pollinated species quite well, but only by using floral traits which were not considered in the classical accounts of syndromes, such as the details of anther opening. [ 33 ] Although a recent review concluded that there is "overwhelming evidence that functional groups exert different selection pressures on floral traits", [ 21 ] the sheer complexity and subtlety of plant-pollinator interactions (and the growing recognition that non-pollinating organisms such as seed predators can affect the evolution of flower traits) means that this debate is likely to continue for some time. | https://en.wikipedia.org/wiki/Pollination_syndrome |
A pollinator is an animal that moves pollen from the male anther of a flower to the female stigma of a flower. [ 1 ] This helps to bring about fertilization of the ovules in the flower by the male gametes from the pollen grains.
Insects are the major pollinators of most plants, and insect pollinators include all families of bees and most families of aculeate wasps ; ants ; many families of flies ; many lepidopterans (both butterflies and moths ); and many families of beetles . Vertebrates, mainly bats and birds, but also some non-bat mammals ( monkeys , lemurs , possums , rodents ) and some lizards pollinate certain plants. Among the pollinating birds are hummingbirds , honeyeaters and sunbirds with long beaks; they pollinate a number of deep-throated flowers. Humans may also carry out artificial pollination .
A pollinator is different from a pollenizer , a plant that is a source of pollen for the pollination process.
Plants fall into pollination syndromes that reflect the type of pollinator being attracted. These are characteristics such as: overall flower size, the depth and width of the corolla, the color (including patterns called nectar guides that are visible only in ultraviolet light), the scent , amount of nectar, composition of nectar, etc. [ 2 ] For example, birds visit red flowers with long, narrow tubes and much nectar, but are not as strongly attracted to wide flowers with little nectar and copious pollen, which are more attractive to beetles. When these characteristics are experimentally modified (altering colour, size, orientation), pollinator visitation may decline. [ 3 ] [ 4 ]
Although non-bee pollinators have been seen to be less effective at depositing pollen than bee pollinators [ 5 ] one study showed that non-bees made more visits than bees resulting in non-bees performing 38% of visits to crop flowers, outweighing the ineffectiveness of their ability to pollinate. [ 6 ] [ 5 ]
It has recently been discovered that cycads , which are not flowering plants , are also pollinated by insects. [ 7 ] In 2016, researchers showed evidence of pollination occurring underwater, which was previously thought not to happen. [ 8 ] [ 9 ]
The most recognized pollinators are the various species of bees , [ 10 ] which are plainly adapted to pollination. Bees typically are fuzzy and carry an electrostatic charge. Both features help pollen grains adhere to their bodies, but they also have specialized pollen-carrying structures; in most bees, this takes the form of a structure known as the scopa , which is on the hind legs of most bees, and/or the lower abdomen (e.g., of megachilid bees), made up of thick, plumose setae . Honey bees , bumblebees , and their relatives do not have a scopa, but the hind leg is modified into a structure called the corbicula (also known as the " pollen basket "). Most bees gather nectar , a concentrated energy source, and pollen, which is high protein food, to nurture their young, and transfer some among the flowers as they are working. [ 11 ] Euglossine bees pollinate orchids, but these are male bees collecting floral scents rather than females gathering nectar or pollen. Female orchid bees act as pollinators, but of flowers other than orchids. Eusocial bees such as honey bees need an abundant and steady pollen source to multiply.
Honey bees travel from flower to flower, collecting nectar (later converted to honey ), and pollen grains. The bee collects the pollen by rubbing against the anthers. The pollen collects on the hind legs, in a structure referred to as a "pollen basket". As the bee flies from flower to flower, some of the pollen grains are transferred onto the stigma of other flowers. Nectar provides the energy for bee nutrition ; pollen provides the protein . When bees are rearing large quantities of brood (beekeepers say hives are "building"), bees deliberately gather pollen to meet the nutritional needs of the brood.
Good pollination management seeks to have bees in a "building" state during the bloom period of the crop, thus requiring them to gather pollen, and making them more efficient pollinators. Thus, the management techniques of a beekeeper providing pollination services are different from, and to some extent in tension with, those of a beekeeper who is trying to produce honey. Millions of hives of honey bees are contracted out as pollinators by beekeepers , and honey bees are by far the most important commercial pollinating agents, but many other kinds of pollinators, from blue bottle flies, to bumblebees, orchard mason bees , and leaf cutter bees are cultured and sold for managed pollination .
Other species of bees differ in various details of their behavior and pollen-gathering habits, and honey bees are not native to the Western Hemisphere ; all pollination of native plants in the Americas and Australia historically has been performed by various native bees. It has also been found that non-native plants may have positive effects on native bee pollinators while also influencing their foraging patterns and bee–plant networks. [ 12 ]
Lepidoptera ( butterflies and moths ) may also pollinate to various degrees. [ 13 ] They are not major pollinators of food crops , but various moths are important pollinators of other commercial crops such as tobacco . Pollination by certain moths may be important, however, or even crucial, for some wildflowers mutually adapted to specialist pollinators. Spectacular examples include orchids such as Angraecum sesquipedale , dependent on a particular hawk moth , Morgan's sphinx . Yucca species provide other examples, being fertilised in elaborate ecological interactions with particular species of yucca moths .
Many bee flies , and some Tabanidae and Nemestrinidae are particularly adapted to pollinating fynbos and Karoo plants with narrow, deep corolla tubes , such as Lapeirousia species. Part of the adaptation takes the form of remarkably long probosces. This also applies to empidine dance flies ( Empidinae ) that visit a wide range of flowering plants, some species of which can pollinate the woodland geranium ( Geranium sylvaticum L. ) as effectively as bees . [ 14 ]
Carrion flies and flesh flies in families such as Calliphoridae and Sarcophagidae are important for some species of plants whose flowers exude a fetid odor . The plants' ecological strategy varies; several species of Stapelia , for example, attract carrion flies that futilely lay their eggs on the flower, where their larvae promptly starve for lack of carrion . Other species do decay rapidly after ripening, and offer the visiting insects large masses of food, as well as pollen and sometimes seed to carry off when they leave.
Hoverflies are important pollinators of flowering plants worldwide. [ 15 ] Often hoverflies are considered to be the second most important pollinators after wild bees. [ 15 ] Although hoverflies as a whole are generally considered to be nonselective pollinators, some species have more specialized relationships. The orchid species Epipactis veratrifolia mimics alarm pheromones of aphids to attract hover flies for pollination. [ 16 ] Another plant, the slipper orchid in southwest China, also achieves pollination by deceit by exploiting the innate yellow colour preference of syrphids. [ 17 ]
Some male dacine fruit flies are exclusive pollinators of some wild Bulbophyllum orchids that lack nectar and have a specific chemical attractant and reward (methyl eugenol, raspberry ketone or zingerone) present in their floral fragrances. [ 18 ] [ 19 ] [ 20 ]
Some flies, especially Anthomyiidae , Empididae and Muscidae , may be the main pollinators at higher elevations of mountains, [ 21 ] [ 22 ] whereas bumblebee species are typically the only other pollinators in alpine regions at timberline and beyond.
Some adult mosquitoes , if they feed on nectar, may act as pollinators; Aedes communis , a species found in North America, is known to pollinate Platanthera obtusata , commonly referred as the blunt-leaved orchid. [ 23 ] [ 24 ]
Biting midges ( Ceratopogonidae ) pollinate Theobroma cacao (Malvaceae), whose flowers have pollen inaccessible to larger pollinators. [ 25 ]
Many insects other than bees accomplish pollination by visiting flowers for nectar or pollen, or commonly both. Many do so adventitiously, but the most important pollinators are specialists for at least parts of their life cycles for at least certain functions.
Prominent among Hymenoptera other than bees are wasps, especially Crabronidae , Chrysididae , Ichneumonidae , Sphecidae and Vespidae . [ 26 ] Some wasps are comparable to or even superior to some bees as pollinators. [ 26 ] The term " pollen wasps ", in particular, is widely applied to the Masarinae, a subfamily of the Vespidae; they are remarkable among solitary wasps in that they specialise in gathering pollen for feeding their larvae, carried internally and regurgitated into a mud chamber prior to oviposition. Also, males of many species of bees and wasps, though they do not gather pollen, rely on flowers as sources of energy (in the form of nectar) and also as territories for meeting fertile females that visit the flowers.
Beetles of species that specialise in eating pollen, nectar, or flowers themselves, may be important cross-pollinators of some plants such as members of the Araceae and Zamiaceae , that produce prodigious amounts of pollen. Others, for example the Hopliini , specialise on flowers of Asteraceae and Aizoaceae .
Thrips pollinate plants such as elderflower Sambucus nigra (Adoxaceae) [ 27 ] and pointleaf manzanita, Arctostaphylos pungens ( Ericaceae ). [ 28 ] Ants also pollinate some kinds of flowers, but for the most part they are parasites, consuming nectar and/or pollen without conveying useful amounts of pollen to a stigma. Other insect orders are rarely pollinators, and then typically only incidentally (e.g., Hemiptera such as Anthocoridae and Miridae ).
A strategy of great biological interest is that of sexual deception, where plants, generally orchids, produce remarkably complex combinations of pheromonal attractants and physical mimicry that induce male bees or wasps to attempt to mate with them, conveying pollinia in the process. Examples are known from all continents apart from Antarctica , though Australia appears to be exceptionally rich in examples. [ 29 ]
Whole groups of plants, such as certain fynbos Moraea and Erica species produce flowers on sticky peduncles or with sticky corolla tubes that only permit access to flying pollinators, whether bird, bat, or insect.
Experimental evidence has shown invertebrates (mostly small crustaceans [ 9 ] ) acting as pollinators in underwater environments. Beds of seagrass have been shown to reproduce this way in the absence of currents. It is not yet known how important invertebrate pollinators might be for other species. [ 8 ] [ 30 ] Later, Idotea balthica was discovered to help Gracilaria gracilis reproduce – the first known case of an animal helping algae reproduce. [ 31 ] [ 32 ]
Bats are important pollinators of some tropical flowers, visiting to take nectar. [ 33 ] Birds, particularly hummingbirds , honeyeaters and sunbirds also accomplish much pollination, especially of deep-throated flowers. Other vertebrates , such as kinkajous , monkeys , lemurs , possums , rodents , lizards , [ 34 ] [ 35 ] and canids [ 36 ] have been recorded pollinating some plants.
Humans can be pollinators, as many gardeners have discovered that they must hand pollinate garden vegetables , whether because of pollinator decline or simply to keep a strain genetically pure. This can involve using a small brush or cotton swab to move pollen, or to simply tap or shake tomato blossoms to release the pollen for the self-pollinating flowers. Tomato blossoms are self-fertile, but (with the exception of potato-leaf varieties) have the pollen inside the anther , and the flower requires shaking to release the pollen through pores . This can be done by wind, by humans, or by a sonicating bee (one that vibrates its wing muscles while perched on the flower), such as a bumblebee. Sonicating bees are extremely efficient pollinators of tomatoes, and colonies of bumblebees are quickly replacing humans as the primary pollinators for greenhouse tomatoes.
Pollinators require a variety of resources. Most native bees in North America are solitary, ground-nesting species that collect a variety of natural resources including pollen, nectar, leaves, petals and resins to be used as sources of food, supplies for their larva, or nest linings. [ 37 ] Floral diet diversity has been seen to increase immunocompetence levels in honeybees (Apis mellifera) where diets that consisted of a wide variety of flowering species induced higher glucose oxidase activity, which honeybees' produce to sterilize their colony. [ 38 ] More than 30% of global bee species depend on non-floral resources for nest building, protection, health, pest resistance, and alternative food sources. [ 39 ] Non-floral resources include leaves, soil, plant resins and secretions, and are often provided by woody-vegetation.
Pollinators provide a key ecosystem service vital to the maintenance of both wild and agricultural plant communities. In 1999 the Convention on Biological Diversity issued the São Paulo Declaration on Pollinators, recognizing the critical role that these species play in supporting and maintaining terrestrial productivity as well as the survival challenges they face due to anthropogenic change. Today pollinators are considered to be in a state of decline; [ 40 ] some species, such as Franklin's bumble bee ( Bombus franklini ) have been red-listed and are in danger of extinction. Although managed bee hives are increasing worldwide, these can not compensate for the loss of wild pollinators in many locations.
A 2017 report done for the Center of Biological Diversity utilized data documented in the United States on native bee species and found that nearly 1 in 4 (347 species of 1,437 species) is imperiled and at increasing risk of extinction. More than half of the native bee species is in decline and 40% of global insect pollinators (primarily native bees) are highly threatened. [ 37 ]
Declines in the health and population of pollinators pose what could be a significant threat to the integrity of biodiversity, to global food webs, and to human health. At least 80% of our world's crop species require pollination to set seed. A 2021 study estimated that without pollinators, fertility would be reduced by 80% in half all wild plant species and one-third of all wild plant species would fail to produce any seeds at all. [ 41 ]
An estimated one out of every three bites of food comes to us through the work of animal pollinators. The quality of pollinator service has declined over time and this had led to concerns that pollination will be less resistant to extinction in the future.
A 2022 study concludes that the decline of pollinator populations is responsible for 500,000 early human deaths per year by reducing the supply of healthy foods. A decline of pollinators has caused 3-5% loss of fruits, vegetables and nuts. Lower consumption of these healthy foods translates to 1% of all deaths, according to the authors. [ 42 ] [ 43 ]
Neonicotinoids (Neonics) are a class of synthetic insecticides that are the most widely applied pesticides today due to its water solubility and ability to treat a wide variety of pests. Neonics are highly environmentally persistent, and may contaminate terrestrial and aquatic habitats for as much as six years. Exposed honeybees' (Apis mellifera) have been seen to have lower reproductive output, reduction in nest building or failed to build nests, reduced foraging abilities, and weakened immunity. [ 44 ]
Researchers are still trying to determine how to scientifically best restore and maintain the diverse pollinator habitats found around the world. Many studies conclude that restoration and conservation are key to maintaining biodiversity and pollinator populations. According to the Kansas National Park Service, native tallgrass prairie was widespread through North America and home to over 300 species of flowering plants. This habitat is crucial to wild pollinators and now only covers 4% of its original 170-million acre range. [ 45 ] By restoring wild pollinators natural habitat and maintaining Earth's biodiversity, populations are assumed to increase. In recent times, environmental groups have put pressure on the Environmental Protection Agency to ban neonicotinoids , a type of insecticide.
On June 20, 2014, President Barack Obama issued a presidential memorandum entitled "Creating a Federal Strategy to Promote the Health of Honey Bees and Other Pollinators". The President's memorandum established a Pollinator Health Task Force, to be co-chaired by the Secretary of Agriculture and the Administrator of the Environmental Protection Agency . The memorandum stated:
Pollinators contribute substantially to the economy of the United States and are vital to keeping fruits, nuts, and vegetables in our diets. Honey bee pollination alone adds more than $15 billion in value to agricultural crops each year in the United States. Over the past few decades, there has been a significant loss of pollinators, including honey bees, native bees, birds, bats, and butterflies, from the environment. The problem is serious and requires immediate attention to ensure the sustainability of our food production systems, avoid additional economic impact on the agricultural sector, and protect the health of the environment. Pollinator losses have been severe. The number of migrating Monarch butterflies sank to the lowest recorded population level in 2013-14, and there is an imminent risk of failed migration. The continued loss of commercial honey bee colonies poses a threat to the economic stability of commercial beekeeping and pollination operations in the United States, which could have profound implications for agriculture and food. Severe yearly declines create concern that bee colony losses could reach a point from which the commercial pollination industry would not be able to adequately recover. The loss of native bees, which also play a key role in pollination of crops, is much less studied, but many native bee species are believed to be in decline. Scientists believe that bee losses are likely caused by a combination of stressors, including poor bee nutrition, loss of forage lands, parasites, pathogens, lack of genetic diversity, and exposure to pesticides. [ 46 ]
In May 2015, the Pollinator Health Task Force issued a "National Strategy to Promote the Health of Honey Bees and Other Pollinators". The national strategy outlined a comprehensive approach to tackling and reducing the impact of multiple stressors on pollinator health, including pests and pathogens, reduced habitat, lack of nutritional resources, and exposure to pesticides. [ 47 ] [ 48 ]
The national strategy laid out federal actions to achieve three goals:
Many of the priority projects that the national strategy identified focused on the I-35 corridor, which extends for 1,500 miles (2,400 km) from Texas to Minnesota. The area through which that highway travels provides spring and summer breeding habitats in the United States' key monarch migration corridor. [ 47 ] [ 48 ]
The Pollinator Health Task Force simultaneously issued a "Pollinator Research Action Plan". The Plan outlined five main action areas, covered in ten subject-specific chapters. The action areas were: (1) Setting a Baseline; (2) Assessing Environmental Stressors; (3) Restoring Habitat; (4) Understanding and Supporting Stakeholders; (5) Curating and Sharing Knowledge. [ 48 ] [ 49 ]
In June 2016, the Task Force issued a "Pollinator Partnership Action Plan". That Plan provided examples of past, ongoing, and possible future collaborations between the federal government and non-federal institutions to support pollinator health under each of the national strategy's goals. [ 50 ]
The North American Pollinator Protection Campaign (NAPPC) aims to promote pollinator health across the North America and has organized annual conferences since 1997, creates task forces to implement specific objectives that includes public education and policy research, and is developing strategic plans for conservation that looks to establish partnership between government entities. 11 pollinator-protection agreements have been signed between NAPPC and federal government agencies, responsible for more than 1.5 billion acres of land protections and management. [ 51 ]
Along with the European Green Deal , which contains initiatives that support pollinator populations, the European Union has implemented the EU Biodiversity Strategy for 2030 which includes the EU Pollinators Initiative that sets long-term objectives to reverse pollinator decline in diversity and numbers by 2030. This initiative includes: (1) improving knowledge of pollinator decline, its causes and consequences; (2) tackling the causes of pollinator decline; and (3) raising awareness, engaging society-at-large and promoting collaboration. [ 52 ]
The Healthy Hives Latin America 2020 (Salud Apícola 2020 Latinoamérica) program is a collaboration between the Bayer Bee Care Center and the Fraunhofer Chile Research Foundation, that works alongside local researchers at universities and beekeepers' associations. The program focuses on increasing the number of healthy worker bees and their colonies by monitoring honey bee health and the contributing factors. This includes educating beekeepers and research collaborations to jointly work on honey bee health. Founded in 2015 with a preliminary project in Chile, the program has expanded to Colombia, Argentina, and Costa Rica. [ 53 ]
The ‘Coalition of the Willing on Pollinators' (Promote Pollinators) was initiated in 2016 during the Convention on Biological Diversity's Conference of the Parties (CBD COP13) and is a growing alliance of countries and observers who support the notion that country-led politics can lead to policy measures and innovative action to protect pollinators'. Their supporters are growing steadily, in which 30 countries currently participate. [ 54 ]
Wild pollinators often visit many plant species and plants are visited by many pollinator species. All these relations together form a network of interactions between plants and pollinators. Surprising similarities were found in the structure of networks consisting out of the interactions between plants and pollinators. This structure was found to be similar in very different ecosystems on different continents, consisting of entirely different species. [ 55 ]
The structure of plant-pollinator networks may have large consequences for the way in which pollinator communities respond to increasingly harsh conditions. Mathematical models, examining the consequences of this network structure for the stability of pollinator communities suggest that the specific way in which plant-pollinator networks are organized minimizes competition between pollinators [ 56 ] and may even lead to strong indirect facilitation between pollinators when conditions are harsh. [ 57 ] This allows pollinator species to survive together under harsh conditions. But it also means that pollinator species collapse simultaneously when conditions pass a critical point. This simultaneous collapse occurs, because pollinator species depend on each other when surviving under difficult conditions. [ 57 ]
Such a community-wide collapse, involving many pollinator species, can occur suddenly when increasingly harsh conditions pass a critical point and recovery from such a collapse might not be easy. The improvement in conditions needed for pollinators to recover, could be substantially larger than the improvement needed to return to conditions at which the pollinator community collapsed. [ 57 ] | https://en.wikipedia.org/wiki/Pollinator |
Pollinator-mediated selection is an evolutionary process occurring in flowering plants , in which the foraging behavior of pollinators differentially selects for certain floral traits. [ 1 ] Flowering plant are a diverse group of plants that produce seeds. Their seeds differ from those of gymnosperms in that they are enclosed within a fruit. These plants display a wide range of diversity when it comes to the phenotypic characteristics of their flowers, which attracts a variety of pollinators that participate in biotic interactions with the plant. Since many plants rely on pollen vectors, their interactions with them influence floral traits and also favor efficiency since many vectors are searching for floral rewards like pollen and nectar. Examples of pollinator-mediated selected traits could be those involving the size, shape, color and odor of flowers, corolla tube length and width, size of inflorescence , floral rewards and amount, nectar guides , and phenology . Since these types of traits are likely to be involved in attracting pollinators, they may very well be the result of selection by the pollinators themselves. [ 2 ]
Having a floral display that either attracts a variety of pollinators or is efficient in the exchanges that occur during pollination can have advantages for the reproductive success of plants. Thus, pollinator behavior is important to understand in relation to the evolution of flowering plants and in some cases pollinator behavior is thought to lead to specialized pollination syndromes where floral traits have co-evolved with their pollinators in a way that are a direct response to the selection occurring from their pollen vectors. [ 3 ] However, many flowering plants don't display morphology that excludes all pollinators except the one they co-evolved with. The most effective pollinator principle posits that floral traits reflect the adaptation to the pollinator that is efficient at transferring the most pollen. Selection might actually favor some degree of generalization while some flowers can also retain particular traits that allow them to adapt to a certain type of pollinator, but will ultimately be molded by the pollinators that are the most effective and visit the most frequently. [ 4 ] This leads to shifts in pollination syndromes and to some genera having a high diversity of pollination syndromes among species, suggesting that pollinators are a primary selective force driving diversity and speciation . [ 5 ] [ 6 ]
Pollinator-mediated selection requires isolation and therefore cannot function in sympatry . Floral isolation is a consequence of pollinator behavior that reduces inter-lineage pollen transfer, which reduces gene flow and increases the possibility for a transition to different syndromes. [ 5 ] Isolation with no gene flow between populations allows for the development of distinct species, thus speciation is a result of reproductive isolation and can be driven by pollinator-mediated selection. [ 1 ] | https://en.wikipedia.org/wiki/Pollinator-mediated_selection |
A pollutant or novel entity [ 1 ] is a substance or energy introduced into the environment that has undesired effect, or adversely affects the usefulness of a resource. These can be both naturally forming (i.e. minerals or extracted compounds like oil ) or anthropogenic in origin (i.e. manufactured materials or byproducts ). Pollutants result in environmental pollution or become of public health concern when they reach a concentration high enough to have significant negative impacts.
A pollutant may cause long- or short-term damage by changing the growth rate of plant or animal species, or by interfering with resources used by humans, human health or wellbeing, or property values. Some pollutants are biodegradable and therefore will not persist in the environment in a long term. However, the degradation products of some pollutants are themselves pollutants such as DDE and DDD produced from the degradation of DDT .
Pollution has widespread negative impact on the environment. [ 1 ] When analyzed from a planetary boundaries perspective, human society has released novel entities that well exceed safe levels. [ 1 ]
Pollutants can be categorized in a variety of different ways. For example, it is sometimes useful to distinguish between stock pollutants and fund pollutants . Another way is to group them together according to more specific properties, such as organic, particulate, pharmaceutical, et cetera. The environment has some capacity to absorb many discharges without measurable harm, and this is called “ assimilative capacity (or absorptive capacity); a pollutant actually causes pollution when the assimilative capacity is exceeded. [ 2 ]
Pollutants, towards which the environment has low absorptive capacity are called stock pollutants . [ 3 ] Examples include persistent organic pollutants like PCBs , non- biodegradable plastics and heavy metals . Stock pollutants accumulate in the environment over time. The damage they cause increases as more pollutant is emitted, and persists as the pollutant accumulates. Stock pollutants can create a burden for the future generations , bypassing on the damage that persists well after the benefits received from incurring that damage, have been forgotten. [ 3 ] Scientists have officially deemed that the planetary boundaries safe chemical pollutant levels (novel entities) have been surpassed. [ 1 ]
In contrast to stock pollutants, for which the environment has low absorptive capacity, fund pollutants are those for which the environment has a moderate absorptive capacity. Fund pollutants do not cause damage to the environment unless the emission rate exceeds the receiving environment's absorptive capacity (e.g. carbon dioxide, which is absorbed by plants and oceans). [ 3 ] Fund pollutants are not destroyed, but rather converted into less harmful substances, or diluted/dispersed to non-harmful concentrations. [ 3 ]
Many pollutants are within the following notable groups:
Light pollution is the impact that anthropogenic light has on the visibility of the night sky. It also encompasses ecological light pollution which describes the effect of artificial light on individual organisms and on the structure of ecosystems as a whole. [ 5 ]
Pollutants can also be defined by their zones of influence, both horizontally and vertically. [ 3 ]
The horizontal zone refers to the area that is damaged by a pollutant. Local pollutants cause damage near the emission source. Regional pollutants cause damage further from the emission source. [ 3 ]
The vertical zone refers to whether the damage is ground-level or atmospheric. Surface pollutants cause damage by accumulating near the Earth's surface. Global pollutants cause damage by concentrating on the [atmosphere].
Measures of pollutant concentration are used to determine risk assessment in public health .
Industry is continually synthesizing new chemicals, the regulation of which requires evaluation of the potential danger for human health and the environment . Risk assessment is nowadays considered essential for making these decisions on a scientifically sound basis.
Measures or defined limits include:
Pollutants can cross international borders and therefore international regulations are needed for their control. The Stockholm Convention on Persistent Organic Pollutants , which entered into force in 2004, is an international legally binding agreement for the control of persistent organic pollutants. Pollutant Release and Transfer Registers (PRTR) are systems to collect and disseminate information on environmental releases and transfers of toxic chemicals from industrial and other facilities.
The European Pollutant Emission Register is a type of PRTR providing access to information on the annual emissions of industrial facilities in the Member States of the European Union , as well as Norway. [ 7 ]
Clean Air Act standards. Under the Clean Air Act , the National Ambient Air Quality Standards (NAAQS) are developed by the Environmental Protection Agency (EPA) for six common air pollutants, also called "criteria pollutants": particulates ; smog and ground-level ozone ; carbon monoxide ; sulfur oxides ; nitrogen oxides ; and lead . [ 8 ] The National Emissions Standards for Hazardous Air Pollutants are additional emission standards that are set by EPA for toxic air pollutants. [ 9 ]
Clean Water Act standards. Under the Clean Water Act , EPA promulgated national standards for municipal sewage treatment plants, also called publicly owned treatment works , in the Secondary Treatment Regulation. [ 10 ] National standards for industrial dischargers are called Effluent guidelines (for existing sources) and New Source Performance Standards , and currently cover over 50 industrial categories. [ 11 ] In addition, the Act requires states to publish water quality standards for individual water bodies to provide additional protection where the national standards are insufficient. [ 12 ]
RCRA standards. The Resource Conservation and Recovery Act (RCRA) regulates the management, transport and disposal of municipal solid waste , hazardous waste and underground storage tanks . [ 13 ] | https://en.wikipedia.org/wiki/Pollutant |
Pollutant-induced abnormal behaviour refers to the abnormal behaviour induced by pollutants . Chemicals released into the natural environment by humans impact the behaviour of a wide variety of animals. The main culprits are endocrine-disrupting chemicals (EDCs), which mimic, block, or interfere with animal hormones. A new research field, integrative behavioural ecotoxicology, is emerging. [ 1 ] However, chemical pollutants are not the only anthropogenic offenders. Noise and light pollution also induce abnormal behaviour.
This topic is of special concern for its conservation and human health implications and has been studied greatly by animal behaviourists, environmental toxicologists, and conservation scientists. Behaviours serve as potential indicators for ecological health. Behaviour can be more sensitive to EDCs than developmental and physiological traits, and it was the behaviour of eagles that first drew attention to the now well-known dangers of DDT. [ 2 ] However, behaviour is generally difficult to measure and can be highly variable.
Behaviours which are critical for survival, such as reproductive and social behaviours, and cognitive abilities like learning can be affected directly or indirectly by chemical pollutants— many examples have been documented, and their chemical culprits have been identified. These same behaviours can also be altered by anthropogenic noise and light, although their mechanisms are relatively unknown.
Source: [ 2 ]
Determining the link between such pollutants and altered behaviours often requires both field studies and laboratory studies. Field studies are useful in determining whether behavioural changes appear with pollution levels occurring in the environment, while laboratory studies can be used to clarify the mechanisms connecting an environmental pollutant to specific behavioural changes.
EDCs affect the synthesis, storage, release, transport, clearance, receptor recognition, binding, or post-receptor responses of hormones. This results in either stimulative or inhibitive effects, resulting in overproduction or underproduction of hormones. The effects of hormones on behaviour have been well studied, and often produce direct behavioural effects by acting on the central nervous system. Indirectly, behaviours may be altered by hormones influencing an animal's metabolism or other important processes. [ 2 ]
Since behaviours also influence hormones, chemical pollutants that induce behavioural changes may also affect hormone levels, which could result in more behavioural or other changes. [ 2 ]
Source: [ 1 ]
Studies into the mechanisms underlying behavioural adjustments fall into a category of animal behaviour research described by Tinbergen.
Studies of animal behaviour typically pertain to one of Tinbergen’s four questions , and these can be applied to studies regarding chemical pollution. Questions of causation focus on how pollutant-exposure disrupts the mechanisms behind normal behaviour. For example, when differences in sexual behaviours were noted in wildlife after the introduction of DDT, biochemical experiments on rats were able to show that the pollutant was inhibiting androgen binding to androgen receptors. [ 3 ]
Secondly, questions of ontogeny consider how exposure disrupts the development of behaviours. An example is when researchers examined the effects of an aerosol on the spatial learning of mice. [ 4 ] Thirdly, questions of adaptation consider how behavioural modifications resulting from exposure will influence fitness. Scientists have investigated the reproductive success of white ibises exposed to methylmercury , for instance. [ 5 ] Lastly, questions of phylogeny consider how phylogenetic history might predetermine sensitivity or resistance to pollutants in a particular behaviour. This could include investigating how animals that are better at learning might be better at avoiding toxins in the environment.
Reproductive behaviour effects may involve changes in courtship and mating behaviours, mate choice , or changes in nest building. [ 2 ] Most studies on this topic have been conducted on fish and birds. For example, treating adult male zebra fish with biphenol A for 7 weeks resulted in decreased courtship behaviour of females. [ 6 ] 17β-trenbolone exposure in adult guppies and mosquitofish also altered female mate selection, as they preferred unexposed males. [ 6 ] Guppies treated with atrazine during breeding and through gestation were less likely to engage in and showed fewer numbers of courtship displays and other reproductive behaviours. Additionally, females preferred untreated males. [ 6 ]
Studies on birds show significant effects of EDCs on mating songs and displays. For example, treating female zebra finches with PCBs before egg laying resulted in a size reduction in the song centres of the chick’s brains. [ 6 ] Methylmercury exposure at environmental levels for 3 years in male white ibises resulted in increased homosexual behaviour, decreased rates of key courtship behaviours, and less attractiveness to females. [ 5 ] Mammals are also susceptible, and effects on individuals have been shown to have transgenerational and even population-level consequences. Illustrating this, female rats three generations removed from vinclozolin exposure show changes in mate preference, preferring unexposed mates, while male rats do not, and this could have complex effects on the population. [ 7 ]
Chemical-induced changes in animal behaviour often have consequences for wild populations. The effects of concern aren’t limited to reproductive effects, which have obvious implications for population vitality. For example, frogs exposed to pesticide-levels found in the environment demonstrate hyperactivity, whip-like convulsions, and depressed avoidance behaviour, which may increase their vulnerability to predation. [ 2 ]
As well, guppies from crude oil-polluted environments are less exploratory after both short-term and long-term exposure. This may weaken their foraging efficiency and resource-use diversity, thus posing a threat to the population viability. [ 8 ] This topic is therefore quite important for understanding how human-impacts on the environment may threaten populations. Additionally, if abnormal behaviours can be used as indicators of toxic pollution, then this provides a much more accessible mode of toxicology science. Therefore, there is potential for engaging citizen scientists in environmental research.
Pollutants are not always chemicals. They can be other unnatural stimuli introduced to the environment by humans, such as noise and light pollution. Anthropogenic noise and light can result in altered antipredator behaviour, reproductive behaviour, communication, foraging behaviour, population distribution, male-male competition and more. However, the mechanisms behind these altered behaviours is relatively unknown within the literature.
Noise pollution is widespread, which is mostly a result of transportation networks. [ 9 ] Although there are many effects of noise pollution, two specific consequences are adjustments in population distribution and modified animal communication. Birds provide a clear example of both of these consequences. Altered population distributions can affect inter-species interactions. For example, the diversity of birds in the woodlands of New Mexico was significantly reduced in areas with unnatural noise levels. [ 10 ] This change in diversity resulted in less nest predation, which was explained by the decreased presence of the dominant predator - the scrub-jay. This effect has also been observed in owls. A negative correlation was found between the noise intensity of an area and the probability that a long-eared owl will reside in that region. [ 11 ] Suggested explanations for this outcome are that hunting efficiency was diminished and communication was less effective.
Noise pollution also affects conspecific communication. High noise levels may require animals to adjust their vocalizations in order for communication to remain effective. Bird songs are a well-studied component of animal communication. The use of adjusted songs was observed in Savannah sparrows residing in noisy environments. [ 12 ] Adjusted songs were so distinct that their use in the control environment did not result in responses (i.e. aggressive territorial behaviour) typically observed when non-adjusted songs were vocalized. Failure to recognize conspecific vocalizations can also be detrimental to male-male competition. Male urban white-crowned sparrows approached stimulus songs of intruders more closely, regardless of song type, when in the presence of noise pollution. [ 13 ] A higher frequency of harmful fights is a suggested consequence of this behaviour because of the relatively small distance between the intruding and defending males.
Noise pollution can also affect marine animals. There are many sources of noise in the world's oceans, such as the sounds produced by commercial shipping, sonars and acoustic deterrents. [ 14 ] Unnatural noise levels can negatively affect reproductive behaviour, such as courtship behaviours. For example, painted goby males did not take part in visual courtship behaviour when in a noisy environment. [ 15 ] Female painted gobies in this experiment were also less likely to spawn in a noisy environment. Noise pollution can also affect foraging behaviour in marine animals, which results in less effective strategies. Porpoises have been found to make fewer prey capture attempts, dive deeper, and cut their foraging behaviour short when a vessel passes by, which results in a higher energy expenditure. [ 16 ] Shore crabs were also observed interrupting their foraging behaviour when in the presence of ship noise. [ 17 ] Antipredator behaviour in marine animals has also been known to change when noise levels are high. Shore crabs took longer to return to their shelters when in the presence of ship noise. [ 17 ] Neolamprologus pulcher (a cichlid fish) females defended their nest less against predators when boat noise was present. [ 18 ]
Specific mitigation strategies and recommendations have been presented by the Marine Mammal Commission (2007). They offer multiple ways to alleviate noise pollution in the ocean. Some strategies include removing the source of noise, employing sound attenuation devices, limiting the use of the sound source and monitoring operational requirements. They recommend improving research programs, creating consistent regulation standards that are better enforced and improving mitigation strategies.
Light pollution affects multiple aspects of animal behaviour, such as reproductive behaviour, foraging behaviour and antipredator behaviour. Altered reproductive behaviour has been observed in multiple taxa. Female crickets were less captious of males when they were raised in bright artificial light. [ 19 ] Male crickets that were raised under continuous artificial light were discriminated against more than male crickets raised in darkness or moonlight. Female fireflies also altered their reproductive behaviour by failing to flash when placed under artificial light and males never flashed in response to these females. [ 20 ] However, it is not only insects that are affected by light pollution. Male green frogs made less calls and moved more often when in the presence of artificial light. [ 21 ] According to Baker and Richardson (2006), these behavioural changes negatively affect breeding success.
Another consequence of light pollution is the disruption of foraging behaviour in wildlife, such as where and when they forage or hunt. Beach mice used foraging patches near sodium vapor lights or yellow bug lights less often than non-lit patches and they also harvested fewer seeds from these lit patches. [ 22 ] This study also hypothesizes that artificial light may alter the movement of mice because of predation risks. Bats are another animal that are greatly affected by light pollution. The presence of artificial lights is associated with a delayed emergence of bats from their dwellings and less time spent emerged. [ 23 ] This alteration in foraging behaviour causes bats to miss the most optimal hunting time for insects. [ 23 ] According to this study, as little as one hour of artificial light exposure after dusk disrupts the bats foraging behaviour, as well as growth rates.
Light pollution can also alter antipredator behaviour in wildlife. When moths come within a close proximity of a hunting bat, they make a powerdive towards the ground [ 24 ] (Roeder & Treat, 1961). If moths are in the presence of artificial light, they are less likely to make this powerdive manoeuver, which results in a reduced ability to evade bat predation. [ 25 ] A suggested explanation for this behaviour, according to this study, is that moths turn their ultrasound detection off in daylight (or simulated daylight). Predator-prey interactions are also altered by noise pollution. An example of this is in loggerhead turtles and ghost crabs. [ 26 ] Ghost crabs are attracted to artificial lights and begin to exhibit more aggressive predatory behaviour in the presence of the light. This study hypothesizes that loggerhead hatchling predation would also increase as a result of this modified predatory behaviour and the predator-prey relationship would, therefore, be altered. | https://en.wikipedia.org/wiki/Pollutant-induced_abnormal_behaviour |
Polly and Molly (born 1997), two ewes , were the first mammals to have been successfully cloned from an adult somatic cell and to be transgenic animals at the same time. [ 1 ] This is not to be confused with Dolly the Sheep , the first animal to be successfully cloned from an adult somatic cell where there wasn’t modification carried out on the adult donor nucleus. Polly and Molly, like Dolly the Sheep, were cloned at the Roslin Institute in Edinburgh , Scotland .
The creation of Polly and Molly built on the somatic nuclear transfer experiments that led to the cloning of Dolly the Sheep. The crucial difference was that in creating Polly and Molly, scientists used cells into which a new gene had been inserted. The gene chosen was a therapeutic protein to demonstrate the potential of such recombinant DNA technology combined with animal cloning. This could hopefully be used to produce pharmacological and therapeutic proteins to treat human diseases. The protein in question was the human blood clotting factor IX . [ 2 ] Another difference from Dolly the Sheep was the source cell type of the nucleus that was transferred. Although Polly and Molly were nuclear clones, they had different mtDNA that was different from the nuclear cells where they received their DNA. [ 3 ]
Prior to the production of Polly and Molly, the only demonstrated way to make a transgenic animal was by microinjection of DNA into the pronuclei of fertilized oocytes (eggs). However, only a small proportion of the animals will integrate the injected DNA into their genome. In the rare cases that they do integrate this new genetic information, the pattern of expression of the injected transgene's protein due to the random integration is very variable. As the aim of such research is to produce an animal that expresses a particular protein in high levels in, for example, its milk, microinjection is a very costly procedure that does not usually produce the desired animal.
In mice, there is an additional option for genetic transfer that is not available in other animals. Embryonic stem cells provide a means to transfer new DNA into the germline . They also allow precise genetic modifications by gene targeting . Modified embryonic stem cells can be selected in vitro before the experiment moves on further for the production of an animal. Embryonic stem cells capable of contributing to the germline of livestock species such as sheep have not been isolated.
The production of Dolly the Sheep and also Megan and Morag , the two sheep that led to the production of Dolly, demonstrated that viable sheep can be produced by nuclear transfer from a variety of somatic cell types which have been cultured in vitro. Polly and Molly represented the further step in which somatic cells were cultured in vitro, just as in the case with the previous sheep. However, in this case they were transfected with foreign DNA, and the transfected cells which stably integrated this new piece of genetic information were selected. The nuclei of these somatic cells was then transferred into an empty oocyte, as in the procedure of nuclear transfer, and this was used to produce several transgenic animals. A cell type PDFF was used. PDFF5 would produce male animals and were not transduced. Cell type PDFF2 produced female animals and were transduced. Of the gestations that occurred, three PDFF2 animals were born, two of which survived birth, 7LL8 and 7LL12. These animals were transfected but contained a marker gene not the cloned gene of interest. These were named "Holly" and "Olly". [ 4 ] Two more subsets of female-producing PDFF2 cells, PDFF2-12 and PDFF2-13, also produced animals which had the cell of interest together with the marker. Of these lambs, 7LL12, 7LL15, and 7LL13 were born alive and healthy. Two of these were named Polly and Molly.
The transgene that was inserted in the donor somatic cells was designed to express the human clotting factor IX protein in the milk of sheep. This protein plays an essential role in blood coagulation, and deficiency leads to the disease haemophilia B of which treatment requires intravenous infusion of factor IX. The production of this protein in livestock milk, a process known as pharming , would provide a source of this therapeutic protein that would reduce the cost and also would be free of potential infectious risk associated with the current source of this protein (human blood). | https://en.wikipedia.org/wiki/Polly_and_Molly |
Polonium-210 ( 210 Po, Po-210, historically radium F ) is an isotope of polonium . It undergoes alpha decay to stable 206 Pb with a half-life of 138.376 days (about 4 + 1 ⁄ 2 months), the longest half-life of all naturally occurring polonium isotopes ( 210–218 Po). [ 1 ] First identified in 1898, and also marking the discovery of the element polonium, 210 Po is generated in the decay chain of uranium-238 and radium-226 . 210 Po is a prominent contaminant in the environment, mostly affecting seafood and tobacco . Its extreme toxicity is attributed to intense radioactivity, mostly due to alpha particles , which easily cause radiation damage, including cancer in surrounding tissue. The specific activity of 210 Po is 166 TBq/g, i.e., 1.66 × 10 14 Bq/g . At the same time, 210 Po is not readily detected by common radiation detectors, because its gamma rays have a very low energy. Therefore, 210 Po can be considered as a quasi-pure alpha emitter.
In 1898, Marie and Pierre Curie discovered a strongly radioactive substance in pitchblende and determined that it was a new element; it was one of the first radioactive elements discovered. Having identified it as such, they named the element polonium after Marie's home country, Poland . Willy Marckwald discovered a similar radioactive activity in 1902 and named it radio-tellurium , and at roughly the same time, Ernest Rutherford identified the same activity in his analysis of the uranium decay chain and named it radium F (originally radium E ). By 1905, Rutherford concluded that all these observations were due to the same substance, 210 Po. Further discoveries and the concept of isotopes, first proposed in 1913 by Frederick Soddy , firmly placed 210 Po as the penultimate step in the uranium series . [ 3 ]
In 1943, 210 Po was studied as a possible neutron initiator in nuclear weapons , as part of the Dayton Project . In subsequent decades, concerns for the safety of workers handling 210 Po led to extensive studies on its health effects. [ 4 ]
In the 1950s, scientists of the United States Atomic Energy Commission at Mound Laboratories , Ohio explored the possibility of using 210 Po in radioisotope thermoelectric generators (RTGs) as a heat source to power satellites. A 2.5- watt atomic battery using 210 Po was developed by 1958. However, the isotope plutonium-238 was chosen instead, as it has a longer half-life of 87.7 years. [ 5 ]
Polonium-210 was used to kill Russian dissident and ex- FSB officer Alexander V. Litvinenko in 2006, [ 6 ] [ 7 ] and was suspected as a possible cause of Yasser Arafat's death , following exhumation and analysis of his corpse in 2012–2013. [ 8 ] The radioisotope may also have been used to kill Yuri Shchekochikhin , Lecha Islamov and Roman Tsepov . [ 9 ]
210 Po is an alpha emitter that has a half-life of 138.376 days; [ 1 ] it decays directly to stable 206 Pb . The majority of the time, 210 Po decays by emission of an alpha particle only, not by emission of an alpha particle and a gamma ray ; about one in 100,000 decays results in the emission of a gamma ray. [ 10 ]
This low gamma ray production rate makes it more difficult to find and identify this isotope. Rather than gamma ray spectroscopy , alpha spectroscopy is the best method of measuring this isotope.
Owing to its much shorter half-life, a milligram of 210 Po emits as many alpha particles per second as 5 grams of 226 Ra . [ 11 ] A few curies of 210 Po emit a blue glow caused by excitation of surrounding air.
210 Po occurs in minute amounts in nature, where it is the penultimate isotope in the uranium series decay chain . It is generated via beta decay from 210 Pb and 210 Bi .
The astrophysical s-process is terminated by the decay of 210 Po, as the neutron flux is insufficient to lead to further neutron captures in the short lifetime of 210 Po. Instead, 210 Po alpha decays to 206 Pb, which then captures more neutrons to become 210 Po and repeats the cycle, thus consuming the remaining neutrons. This results in a buildup of lead and bismuth, and ensures that heavier elements such as thorium and uranium are only produced in the much faster r-process . [ 12 ]
Although 210 Po occurs in trace amounts in nature, it is not abundant enough (0.1 ppb ) for extraction from uranium ore to be feasible. Instead, most 210 Po is produced synthetically, through neutron bombardment of 209 Bi in a nuclear reactor . This process converts 209 Bi to 210 Bi, which beta decays to 210 Po with a five-day half-life. [ 13 ] Through this method, approximately 8 grams (0.28 oz) of 210 Po are produced in Russia and shipped to the United States every month for commercial applications. [ 4 ] By irradiating certain bismuth salts containing light element nuclei such as beryllium, a cascading (α,n) reaction can also be induced to produce 210 Po in large quantities. [ 14 ]
The production of polonium-210 is a downside to reactors cooled with lead-bismuth eutectic rather than pure lead. However, given the eutectic properties of this alloy, some proposed Generation IV reactor designs still rely on lead-bismuth.
A single gram of 210 Po generates 140 watts of power. [ 15 ] Because it emits many alpha particles , which are stopped within a very short distance in dense media and release their energy, 210 Po has been used as a lightweight heat source to power thermoelectric cells in artificial satellites . A 210 Po heat source was also in each of the Lunokhod rovers deployed on the surface of the Moon , to keep their internal components warm during the lunar nights. [ 16 ] Some anti-static brushes, used for neutralizing static electricity on materials like photographic film, contain a few microcuries of 210 Po as a source of charged particles. [ 17 ] 210 Po was also used in initiators for atomic bombs through the (α,n) reaction with beryllium . [ 18 ] Small neutron sources reliant on the (α,n) reaction also usually use polonium as a convenient source of alpha particles due to its comparatively low gamma emissions (allowing easy shielding) and high specific activity .
210 Po is extremely toxic; it and other polonium isotopes are some of the most radiotoxic substances to humans. [ 6 ] [ 19 ] With one microgram of 210 Po being more than enough to kill the average adult, it is 250,000 times more toxic than hydrogen cyanide by weight. [ 20 ] One gram of 210 Po would hypothetically be enough to kill 50 million people and sicken another 50 million. [ 6 ] This is a consequence of its ionizing alpha radiation , as alpha particles are especially damaging to organic tissues inside the body. However, 210 Po does not pose a radiation hazard when contained outside the body. [ 21 ] The alpha particles it produces cannot penetrate the outer layer of dead skin cells. [ 22 ]
The toxicity of 210 Po stems entirely from its radioactivity. It is not chemically toxic in itself, but its solubility in aqueous solution as well as that of its salts poses a hazard because its spread throughout the body is facilitated in solution. [ 6 ] Intake of 210 Po occurs primarily through contaminated air, food, or water, as well as through open wounds. Once inside the body, 210 Po concentrates in soft tissues (especially in the reticuloendothelial system ) and the bloodstream . Its biological half-life is approximately 50 days. [ 23 ]
In the environment, 210 Po can accumulate in seafood. [ 24 ] It has been detected in various organisms in the Baltic Sea , where it can propagate in, and thus contaminate, the food chain. [ 19 ] 210 Po is also known to contaminate vegetation, primarily originating from the decay of atmospheric radon-222 and absorption from soil. [ 25 ]
In particular, 210 Po attaches to, and concentrates in, tobacco leaves. [ 4 ] [ 23 ] Elevated concentrations of 210 Po in tobacco were documented as early as 1964, and cigarette smokers were thus found to be exposed to considerably greater doses of radiation from 210 Po and its parent 210 Pb. [ 25 ] Heavy smokers may be exposed to the same amount of radiation (estimates vary from 100 µSv [ 19 ] to 160 mSv [ 26 ] per year) as individuals in Poland were from Chernobyl fallout traveling from Ukraine. [ 19 ] As a result, 210 Po is most dangerous when inhaled from cigarette smoke. [ 27 ] | https://en.wikipedia.org/wiki/Polonium-210 |
Polonium hydride (also known as polonium dihydride , hydrogen polonide , or polane ) is a chemical compound with the formula Po H 2 . It is a liquid at room temperature, the second hydrogen chalcogenide with this property after water . It is very unstable chemically and tends to decompose into elemental polonium and hydrogen . It is a volatile and very labile compound, from which many polonides can be derived. Additionally, it is radioactive. [ 2 ]
Polonium hydride cannot be produced by direct reaction from the elements upon heating. Other unsuccessful routes to synthesis include the reaction of polonium tetrachloride (PoCl 4 ) with lithium aluminium hydride (LiAlH 4 ), which only produces elemental polonium, and the reaction of hydrochloric acid with magnesium polonide (MgPo). The fact that these synthesis routes do not work may be caused by the radiolysis of polonium hydride upon formation. [ 3 ]
Trace quantities of polonium hydride may be prepared by reacting hydrochloric acid with polonium-plated magnesium foil. In addition, the diffusion of trace quantities of polonium in palladium or platinum that is saturated with hydrogen (see palladium hydride ) may be due to the formation and migration of polonium hydride. [ 3 ]
Polonium hydride is a more covalent compound than most metal hydrides because polonium straddles the border between metals and metalloids and has some nonmetallic properties. It is intermediate between a hydrogen halide like hydrogen chloride and a metal hydride like stannane .
It should have properties similar to that of hydrogen selenide and hydrogen telluride , other borderline hydrides . It is expected to be an endothermic compound, like the lighter hydrogen telluride and hydrogen selenide, and therefore would decompose into its constituent elements, releasing heat in the process. The amount of heat given off in the decomposition of polonium hydride is over 100 kJ/mol , the largest of all the hydrogen chalcogenides .
It is predicted that, like the other hydrogen chalcogenides, polonium may form two types of salts : polonide (containing the Po 2− anion ) and one from polonium hydride (containing –PoH, which would be the polonium analogue of thiol , selenol and tellurol ). However, no salts from polonium hydride are known. An example of a polonide is lead polonide (PbPo), which occurs naturally as lead is formed in the alpha decay of polonium. [ 4 ]
Polonium hydride is difficult to work with due to the extreme radioactivity of polonium and its compounds and has only been prepared in very dilute tracer quantities. As a result, its physical properties are not definitely known. [ 3 ] It is also unknown if polonium hydride forms an acidic solution in water like its lighter homologues, or if it behaves more like a metal hydride (see also hydrogen astatide ). | https://en.wikipedia.org/wiki/Polonium_hydride |
Polony is a contraction of "polymerase colony," a small colony of DNA .
Polonies are discrete clonal amplifications of a single DNA molecule, grown in a gel matrix. This approach greatly improves the signal-to-noise ratio. Polonies can be generated using several techniques that include solid-phase polymerase chain reaction (PCR) in polyacrylamide gels. However, other earlier patented technologies, such as that from Manteia Predictive Medicine (acquired by Solexa), which generate DNA on a solid-phase surface by bridge amplification, are generally referred to as "clusters".
The terminology and distinction between 'polony' and 'cluster' have become confused recently. Growth of clonal copies of DNA on bead surfaces remains to be generically named although some also seek to name this technique as a "polony" method. The concept of localizing and analyzing regions containing clonal nucleic acid populations was first described in patents by Brown, et al.. (assigned to Genomic Nanosystems), however these are in liquid phase. Clusters are distinct in that they are based on solid-phase amplification of single DNA molecules where the DNA has been covalently attached to a surface. This technology, initially coined "DNA colony generation", had been invented and developed in late 1996 at Glaxo-Welcome's Geneva Biomedical Research Institute (GBRI), by Dr Pascal Mayer and Dr Laurent Farinelli, [ 1 ] and was publicly presented for the first time in 1998. [ 2 ] It was finally brought to market by Solexa. Solexa Ltd/INC (Bentley et al.).
This cell biology article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Polony_(biology) |
Polony sequencing is an inexpensive but highly accurate multiplex sequencing technique that can be used to “read” millions of immobilized DNA sequences in parallel. This technique was first developed by Dr. George Church 's group at Harvard Medical School . Unlike other sequencing techniques, Polony sequencing technology is an open platform with freely downloadable, open source software and protocols. Also, the hardware of this technique can be easily set up with a commonly available epifluorescence microscopy and a computer-controlled flowcell/fluidics system. Polony sequencing is generally performed on paired-end tags library that each molecule of DNA template is of 135 bp in length with two 17–18 bp paired genomic tags separated and flanked by common sequences. The current read length of this technique is 26 bases per amplicon and 13 bases per tag, leaving a gap of 4–5 bases in each tag.
The protocol of Polony sequencing can be broken into three main parts, which are the paired end-tag library construction, template amplification and DNA sequencing .
This protocol begins by randomly shearing the tested genomic DNA into a tight size distribution. The sheared DNA molecules are then subjected for the end repair and A-tailed treatment. The end repair treatment converts any damaged or incompatible protruding ends of DNA to 5’-phosphorylated and blunt-ended DNA, enabling immediate blunt-end ligation, while the A-tailing treatment adds an A to the 3’ end of the sheared DNA. DNA molecules with a length of 1 kb are selected by loading on the 6% TBE PAGE gel. In the next step, the DNA molecules are circularized with T-tailed 30 bp long synthetic oligonucleotides (T30), which contains two outward-facing MmeI recognition sites, and the resulting circularized DNA undergoes rolling circle replication . The amplified circularized DNA molecules are then digested with MmeI (type IIs restriction endonuclease) which will cuts at a distance from its recognition site, releasing the T30 fragment flanked by 17–18 bp tags (≈70 bp in length). The paired-tag molecules need to be end-repaired prior to the ligation of ePCR (emulsion PCR) primer oligonucleotides (FDV2 and RDV2) to their both ends. The resulting 135 bp library molecules are size-selected and nick translated . Lastly, amplify the 135 bp paired end-tag library molecules with PCR to increase the amount of library material and eliminate extraneous ligation products in a single step. The resulted DNA template consists of a 44 bp FDV sequence, a 17–18 bp proximal tag, the T30 sequence, a 17-18 bp distal tag, and a 25 bp RDV sequence.
The mono-sized, paramagnetic streptavidin –coated beads are pre-loaded with dual biotin forward primer. Streptavidin has a very strong affinity for biotin , thus the forward primer will bind firmly on the surface of the beads. Next, an aqueous phase is prepared with the pre-loaded beads, PCR mixture, forward and reverse primers, and the paired end-tag library. This is mixed and vortexed with an oil phase to create the emulsion. Ideally, each droplet of water in the oil emulsion has one bead and one molecule of template DNA, permitting millions of non-interacting amplification within a milliliter-scale volume by performing PCR.
After amplification, the emulsion from preceding step is broken using isopropanol and detergent buffer (10 mM Tris pH 7.5, 1 mM EDTA pH 8.0, 100 mM NaCl, 1% (v/v) Triton X‐100, 1% (w/v) SDS), following with a series of vortexing, centrifuging, and magnetic separation. The resulted solution is a suspension of empty, clonal and non-clonal beads, which arise from emulsion droplets that initially have zero, one or multiple DNA template molecules, respectively. The amplified bead could be enriched in the following step.
The enrichment of amplified beads is achieved through hybridization to a larger, low density, non-magnetic polystyrene beads that pre-loaded with a biotinylated capture oligonucleotides (DNA sequence that complementary to ePCR amplicon sequence). The mixture is then centrifuged to separate the amplified and capture beads complex from the unamplified beads. The amplified, capture bead complex has a lower density and thus will remain in the supernatant while the unamplified beads form a pellet. The supernatant is recovered and treated with NaOH which will break the complex. The paramagnetic amplified beads are separated from the non-magnetic capture beads by magnetic separation. This enrichment protocol is capable in enriching five times of amplified beads.
The purpose of bead capping is to attach a “capping” oligonucleotide to the 3’ end of both unextended forward ePCR primers and the RDV segment of template DNA. The cap that being use is an amino group that prevents fluorescent probes from ligating to these ends and at the same time, helping the subsequent coupling of template DNA to the aminosilanated flow cell coverslip.
First, the coverslips are washed and aminosilane-treated, enabling the subsequent covalent coupling of template DNA on it and eliminating any fluorescent contamination. The amplified, enriched beads are mixed with acrylamide and poured into a shallow mold formed by a Teflon -masked microscope slide. Immediately, place the aminosilane-treated coverslip on top of the acrylamide gel and allow to polymerize for 45 minutes. Next, invert the slide/coverslip stack and remove the microscope slide from gel. The silane-treated coverslips will bind covalently to the gel while the Teflon on the surface of microscope slide will enable the better removal of slide from the acrylamide gel. The coverslips then bonded to the flow cell body and any unattached beads will be removed.
The biochemistry of Polony sequencing mainly relies on the discriminatory capacities of ligases and polymerases. First, a series of anchor primers are flowed through the cells and hybridize to the synthetic oligonucleotide sequences at the immediate 3’ or 5’ end of the 17-18 bp proximal or distal genomic DNA tags. Next, an enzymatic ligation reaction of the anchor primer to a population of degenenerate nonamers that are labeled with fluorescent dyes is performed. Differentially labeled nonamers:
The fluorophore-tagged nonamers anneal with differential success to the tag sequences according to a strategy similar to that of degenerate primers , but instead of submission to polymerases, nonamers are selectively ligated onto adjoining DNA- the anchor primer. The fixation of the fluorophore molecule provides a fluorescent signal that indicates whether there is an A, C, G, or T at the query position on the genomic DNA tag. After four-colour imaging, the anchor primer/nonamer complexes are stripped off and a new cycle is begun by replacing the anchor primer. A new mixture of the fluorescently tagged nonamers is introduced, for which the query position is shifted one base further into the genomic DNA tag.
Seven bases from the 5’ to 3’ direction and six bases from the 3’ end could be queried in this fashion. The ultimate result is a read length of 26 bases per run (13 bases from each of the paired tags) with a 4-base to 5-base gap in the middle of each tag.
The polony sequencing generates millions of 26 reads per run and this information needed to be normalized and converted to sequence. These can be done by the software that has been developed by Church Lab. All of the software is free and could be downloaded from the website. [ 1 ]
The sequencing instrument used in this technique could be set up by the commonly available fluorescence microscope and a computer controlled flowcell. The required instruments cost around US$130,000 in 2005. [ citation needed ] A dedicated polony sequencing machine, Polonator , was developed in 2009 and sold at US$170,000 by Dover . [ 2 ] [ 3 ] It and featured open source software, reagents, and protocols and was intended for use in the Personal Genome Project . [ 4 ]
Polony sequencing allows for a high throughput and high consensus accuracies of DNA sequencing based on a commonly available, inexpensive instrument. Also, it is a very flexible technique that enables variable application including BAC (bacterial artificial chromosome) and bacterial genome resequencing, as well as SAGE (serial analysis of gene expression) tag and barcode sequencing. Furthermore, the polony sequencing technique is emphasized as an open system that shares everything including the software that have been developed, protocol and reagents.
However, although the raw data acquisition could be achieved as high as 786 gigabits but only 1 bit of information out of 10,000 bits collected is useful. Another challenge of this technique is the uniformity of the relative amplification of individual targets. The non-uniform amplification could lower the efficiency of sequencing and posted as the biggest obstacle in this technique.
Polony sequencing is a development of the polony technology from the late 1990s and 2000s. [ 5 ] Methods were developed in 2003 to sequence in situ polonies using single-base extension which could achieve 5-6 bases reads. [ 6 ] By 2005, these early attempts had been overhauled to develop the existing polony sequencing technology. [ 7 ] The highly parallel sequencing-by-ligation concepts of polony sequencing has contributed the basis for later sequencing technologies such as ABI Solid Sequencing . | https://en.wikipedia.org/wiki/Polony_sequencing |
Polophylax ( Greek : guardian of the pole ) was a southern constellation that lay where Tucana and Grus now are.
During the Renaissance several new constellations were created for recorded stars that were outside the boundaries of the existing Ptolemaic constellations. [ 1 ] Polophylax was introduced (along with the constellation Columba ) by Petrus Plancius in the small celestial planispheres on his large wall map of 1592. [ 2 ] It is also shown on his smaller world map of 1594 and on world maps copied from Plancius. [ 3 ]
It was superseded by the twelve constellations which Petrus Plancius formed in late 1597 or early 1598 from the southern star observations of Pieter Dircksz Keyser and Frederik de Houtman . [ 3 ]
This constellation -related article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Polophylax |
Poloxamers are nonionic triblock copolymers composed of a central hydrophobic chain of polyoxypropylene (poly(propylene oxide)) flanked by two hydrophilic chains of polyoxyethylene (poly(ethylene oxide)). The word poloxamer was coined by BASF inventor, Irving Schmolka, who received the patent for these materials in 1973. [ 1 ] Poloxamers are also known by the trade names Pluronic , [ 2 ] Kolliphor (pharma grade), [ 3 ] and Synperonic . [ 4 ]
Because the lengths of the polymer blocks can be customized, many different poloxamers exist that have slightly different properties. For the generic term poloxamer , these copolymers are commonly named with the letter P (for poloxamer) followed by three digits: the first two digits multiplied by 100 give the approximate molecular mass of the polyoxypropylene core, and the last digit multiplied by 10 gives the percentage polyoxyethylene content (e.g. P407 = poloxamer with a polyoxypropylene molecular mass of 4000 g/mol and a 70% polyoxyethylene content). For the Pluronic and Synperonic tradenames, coding of these copolymers starts with a letter to define its physical form at room temperature (L = liquid, P = paste, F = flake (solid)) followed by two or three digits, The first digit (two digits in a three-digit number) in the numerical designation, multiplied by 300, indicates the approximate molecular weight of the hydrophobe; and the last digit x 10 gives the percentage polyoxyethylene content (e.g., L61 indicates a polyoxypropylene molecular mass of 1800 g/mol and a 10% polyoxyethylene content). In the example given, poloxamer 181 (P181) = Pluronic L61 and Synperonic PE/L 61.
An important characteristic of poloxamer solutions is their temperature dependent self-assembling and thermo-gelling behavior. Concentrated aqueous solutions of poloxamers are liquid at low temperature and form a gel at higher temperature in a reversible process. The transitions that occur in these systems depend on the polymer composition (molecular weight and hydrophilic / hydrophobic molar ratio).
At low temperatures and concentrations (below the critical micelle temperature and critical micelle concentration ) individual block copolymers (unimers) are present in solution. Above these values, aggregation of individual unimers occurs in a process called micellization . This aggregation is driven by the dehydration of the hydrophobic polyoxypropylene block that becomes progressively less soluble as the polymer concentration or temperature increases. The aggregation of several unimers occurs to minimize the interactions of the PPO blocks with the solvent. Thus, the core of the aggregates is made from the insoluble blocks (polyoxypropylene) while the soluble portion ( polyoxyethylene ) forms the shell of the micelles.
The mechanisms on the micellization at equilibrium have shown to depend on two relaxation times: (1) the first and fastest (tens of the microseconds scale) corresponds to the unimers exchange between micelles and the bulk solution and follows the Aniansson-Wall model (step-by-step insertion and expulsion of single polymer chains), [ 11 ] and (2) the second and much slower one (in the millisecond range) is attributed to the formation and breakdown of whole micellar units leading to the final micellar size equilibration.
Besides spherical micelles, elongated or worm-like micelles can also be formed. The final geometry will depend on the entropy costs of stretching the blocks, which is directly related to their composition (size and polyoxypropylene/polyoxyethylene ratio). [ 12 ] The mechanisms involved in the shape transformation are different compared to the dynamics of micellization. Two mechanisms were proposed for the sphere-to-rod transitions of block copolymer micelles, in which the micellar growth can occur by (A) fusion/fragmentation of micelles or (B) concomitant fusion/fragmentation of micelles and unimer exchange, followed by smoothing of the rod-like structures. [ 13 ]
With higher increments of the temperature and/or concentration, other phenomena can occur such as the formation of highly ordered mesophases (cubic, hexagonal and lamellar). Eventually, a complete dehydration of the polyoxypropylene blocks and the collapse of the polyoxyethylene chains will lead to clouding and/or macroscopic phase separation. This is due to the fact that hydrogen bonding between the polyoxyethylene and the water molecules breaks down at high temperature and polyoxyethylene becomes also insoluble in water.
The phase transitions can also be largely influenced by the use of additives such as salts and alcohols. The interactions with salts are related to their ability to act as water structure makers ( salting-out ) or water structure breakers (salting-in). Salting-out salts increase the self-hydration of water through hydrogen bonding and reduce the hydration of the copolymers, thus reducing the critical micelle temperature and critical micelle concentration . Salting-in electrolytes reduce the water self-hydration and increase the polymer hydration, therefore increasing the critical micelle temperature and critical micelle concentration . The different salts have been categorized by the Hofmeister series according to their ‘salting-out’ power. Different phase diagrams characterizing all these transitions have been constructed for most poloxamers using a great variety of experimental techniques (e.g. SAXS, Differential scanning calorimetry, viscosity measurements, light scattering).
Because of their amphiphilic structures, the polymers have surfactant properties that make them useful in industrial applications. Among other things, they can be used to increase the water solubility of hydrophobic, oily substances or otherwise increase the miscibility of two substances with different hydrophobicities. For this reason, these polymers are commonly used in industrial applications, cosmetics, and pharmaceuticals. They have also been evaluated for various drug delivery applications and were shown to sensitize drug-resistant cancers to chemotherapy.
In bioprocess applications, poloxamers are used in cell culture media for their cell cushioning effects because their addition leads to less stressful shear conditions for cells in reactors. There are grades of poloxamers commercially available specifically for cell culture, including Kolliphor P 188 Bio. [ 14 ]
In materials science, the poloxamer P123 has recently been used in the synthesis of mesoporous materials, including SBA-15 .
In colloidal science , certain poloxamers such as Pluronic F-108 or Pluronic F-127, are used as steric stabilizers to prevent coalescence and/or reduce aggregation. [ 15 ] In the case of hydrophobic colloids, the poloxamer's interior hydrophobic block is absorbed into the colloid while the two hydrophilic tails remain suspended in solution, creating a steric barrier.
When mixed with water, concentrated solutions of poloxamers can form hydrogels. These gels can be extruded easily, acting as a carrier for other particles, and used for robocasting . [ 16 ]
Work led by Kabanov has recently shown that some of these polymers, originally thought to be inert carrier molecules, have a very real effect on biological systems independently of the drug they are transporting. [ 17 ] [ 18 ] [ 19 ] [ 20 ] The poloxamers have been shown to incorporate into cellular membranes affecting the microviscosity of the membranes. The polymers seem to have the greatest effect when absorbed by the cell as an unimer rather than as a micelle . [ 21 ]
Poloxamers have been shown to preferentially target cancer cells, due to differences in the membrane of these cells when compared to noncancer cells. Poloxamers have also been shown to inhibit MDR proteins and other drug efflux transporters on the surface of cancer cells; the MDR proteins are responsible for the efflux of drugs from the cells and hence increase the susceptibility of cancer cells to chemotherapeutic agents such as doxorubicin.
Another effect of the polymers upon cancer cells is the inhibition of the production of ATP in multi-drug resistant (MDR) cancer cells. The polymers seem to inhibit respiratory proteins I and IV, and the effect on respiration seems to be selective for MDR cancer cells, which may be explained by the difference in fuel sources between MDR and sensitive cells (fatty acids and glucose respectively).
The poloxamers have also been shown to enhance proto-apoptotic signaling, decrease anti-apoptoic defense in MDR cells, inhibit the glutathione/glutathione S-transferase detoxification system, induce the release of cytochrome C, increase reactive oxygen species in the cytoplasm, and abolish drug sequestering within cytoplasmic vesicles.
Certain poloxamers such as P85 have been shown not only to be able to transport target genes to target cells, but also to increase gene expression. Certain poloxamers, such as P85 and L61, have also been shown to stimulate transcription of NF kappaB genes, although the mechanism by which this is achieved is currently unknown, bar that P85 has been shown to induce phosphorylation of the inhibitory kappa.
Wang et al. reported that aqueous solutions of poloxamer 188 (Pluronic F-68) and poloxamer 407 (Pluronic F-127) sonicated in the presence or absence of multi-walled carbon nanotubes (MWNTs) can became highly toxic to cultured cells. Moreover, toxicity correlated with the sonolytic degradation of the polymers. [ 22 ] | https://en.wikipedia.org/wiki/Poloxamer |
In computer programming , a poltergeist (or gypsy wagon ) is a short-lived, typically stateless object used to perform initialization or to invoke methods in another, more permanent class. It is considered an anti-pattern . The original definition is by Michael Akroyd at the 1996 Object World West Conference: [ citation needed ]
As a gypsy wagon or a poltergeist appears and disappears mysteriously, so does this short lived object. As a consequence the code is more difficult to maintain and there is unnecessary resource waste. The typical cause for this anti-pattern is poor object design.
A poltergeist can often be identified by its name; they are often called "manager_", "controller_", "supervisor", "start_process", etc.
Sometimes, poltergeist classes are created because the programmer anticipated the need for a more complex architecture. For example, a poltergeist arises if the same method acts as both the client and invoker in a command pattern , and the programmer anticipates separating the two phases. However, this more complex architecture may actually never materialize.
Poltergeists should not be confused with long-lived, state-bearing objects of a pattern such as model–view–controller , or tier-separating patterns such as business delegate pattern .
To remove a poltergeist, delete the class and insert its functionality in the invoked class, possibly by inheritance or as a mixin .
There have been proposed methods in detecting poltergeists in code for refactoring. [ 1 ]
This computer-programming -related article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Poltergeist_(computer_programming) |
Polux is a measuring device for checking the condition of wooden electricity and telephone poles. This is a specific non-destructive testing tool for wooden poles.
Polux technology is used for safety and maintenance diagnostics. The measurements enable operators to climb poles safely and check the condition of the wood to estimate its remaining lifespan. [ 1 ] [ 2 ] [ 3 ] [ 4 ]
The Polux technology was developed in the early 90s by Professor Jean-Luc Sandoz , following on from the Sylvatest , at the initial request of EDF , which had the dual problem of the safety and lifespan of their poles. [ 5 ] In 2003, the technology was presented at the 17th International Congress on Electrical Distribution Networks in Barcelona. [ 6 ] In 2017, a report by Orange 's CGT insisted on the danger of unmaintained poles and demonstrated, through several large-scale comparative tests, the scientific contribution of Polux technology to increasing the safety of personnel climbing poles, drawing on the experience of Enedis . [ 7 ] It works with Picus software, which uses data consisting of local densitometry measurements at the ground line and hygroscopic measurements. [ 8 ] [ 9 ] [ 10 ] The Polux technology continues to be developed at the CBS-Lifteam Research and Development Centre in Switzerland. [ 11 ] [ 12 ] The 5th version enables faster data collection and processing, with a miniaturised tool and the Picus application downloadable to smartphones . [ 13 ] [ 14 ] Polux technology is used in a number of countries, including the United States, Canada, European Union, Africa and Asia. [ 15 ] [ 16 ] [ 17 ]
• (en) Flávio L. R. Vidor, Marçal Pires, Berenice A. Dedavid, Pedro D. B. Montani, and Adriano Gabiatti, « », Rapport scientifique, 2010; [ 20 ]
• (br) M. A. O. Cruz1; M. Pires; B. A. Dedavid; F. L. R. Vidor; W. S. Oliveira; R. C.Abruzzi, « », revue scientifique,2010 (lire en ligne [PDF]). | https://en.wikipedia.org/wiki/Polux |
3GOY , 3Q6Z , 3Q71 , 3SE2 , 3SMI , 3SMJ , 3VFQ , 4ABK , 4ABL , 4D86 , 4F1L , 4F1Q , 4PY4
54625
547253
ENSG00000173193
ENSMUSG00000034422
Q460N5 Q8N546
Q2EMV9
NM_017554
NM_001039530
NP_060024 NP_060024.2
NP_001034619
Poly(ADP-ribose) polymerase family member 14 is a protein that, in humans , is encoded by the PARP14 gene . [ 5 ]
Poly(ADP-ribosyl)ation is an immediate DNA damage-dependent post-translational modification of histones and other nuclear proteins that contributes to the survival of injured proliferating cells. PARP14 belongs to the superfamily of enzymes that perform this modification (Ame et al., 2004 [PubMed 15273990]).
This article incorporates text from the United States National Library of Medicine , which is in the public domain .
This article on a gene on human chromosome 3 is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Poly(adp-ribose)_polymerase_family_member_14 |
Poly(amidoamine) , or PAMAM , is a class of dendrimer which is made of repetitively branched subunits of amide and amine functionality . PAMAM dendrimers, sometimes referred to by the trade name Starburst, have been extensively studied since their synthesis in 1985, [ 1 ] and represent the most well-characterized dendrimer family as well as the first to be commercialized. [ 2 ] Like other dendrimers, PAMAMs have a sphere-like shape overall, and are typified by an internal molecular architecture consisting of tree-like branching, with each outward 'layer', or generation, containing exponentially more branching points. This branched architecture distinguishes PAMAMs and other dendrimers from traditional polymers , as it allows for low polydispersity and a high level of structural control during synthesis, and gives rise to a large number of surface sites relative to the total molecular volume. Moreover, PAMAM dendrimers exhibit greater biocompatibility than other dendrimer families, perhaps due to the combination of surface amines and interior amide bonds; these bonding motifs are highly reminiscent of innate biological chemistry and endow PAMAM dendrimers with properties similar to that of globular proteins . [ 2 ] The relative ease/low cost of synthesis of PAMAM dendrimers (especially relative to similarly-sized biological molecules such as proteins and antibodies), along with their biocompatibility, structural control, and functionalizability, have made PAMAMs viable candidates for application in drug development, biochemistry, and nanotechnology . [ 2 ] [ 3 ] [ 4 ]
Divergent synthesis refers to the sequential "growth" of a dendrimer layer by layer, starting with a core "initiator" molecule which contains functional groups capable of acting as active sites in the initial reaction. Each subsequent reaction in the series increases the number of available surface groups exponentially. Core molecules which give rise to PAMAM dendrimers can vary, but the most basic initiators are ammonia and ethylene diamine. [ 6 ] Outward growth of PAMAM dendrimers is accomplished by alternating between two reactions:
Each round of reactions forms a new "generation", and PAMAM dendrimers are often classified by generation number; the common shorthand for this classification is "GX" or "GX PAMAM", where X is a number referring to the generation number. The first full cycle of Michael addition followed by coupling with ethylene diamine forms Generation 0 PAMAM, with subsequent Michael additions giving rise to "half" generations, and subsequent amide coupling giving rise to "full" (integer) generations.
With divergent synthesis of dendrimers, it is extremely important to allow each reaction to proceed to completion; any defects caused by incomplete reaction or intramolecular coupling of new surface amines with unreacted methyl ester surface groups could cause "trailing" generations, stunting further growth for certain branches. These impurities are difficult to remove when using the divergent synthetic approach because the molecular weight, physical size, and chemical properties of the defective dendrimers are very similar in nature to the desired product. As generation number increases, it becomes more difficult to produce pure products in a timely fashion due to steric constraints. As a result, synthesis of higher-generation PAMAM dendrimers can take months.
Convergent synthesis of a dendrimer begins with what will eventually become the surface of the dendrimer and proceeds inward. The convergent synthetic approach makes use of orthogonal protecting groups (two protecting groups whose deprotection conditions will not remove one another); this is an additional consideration not present when using a divergent approach. The figure below depicts a general scheme for a convergent synthetic approach.
Convergent synthesis as shown above begins with the dendritic subunit composed of reactive "focal group" A and branched group B ( B can be multiply branched in the most generalized scenario, but PAMAMs only split once at each branching point). First, A is orthogonally protected and set aside for further reactions. B is also orthogonally protected, leaving the unprotected A on this molecule to couple with each of the unprotected B groups from the initial compound. This results in a new higher-generation species that is protected on both A and B . Selective deprotection of A yields a new molecule which can again be coupled onto the original monomer, thus forming another new generation. This process can then be repeated to form more and more layers.
It has been established that cationic macromolecules in general destabilize the cell membrane , which can lead to lysis and cell death . [ 9 ] The common conclusion present in current work echoes this observation: increasing dendrimer molecular weight and surface charge (both being generation-dependent) increases their cytotoxic behavior. [ 10 ] [ 11 ] [ 12 ] [ 13 ] [ 14 ] [ 15 ]
Initial studies on PAMAM toxicity showed that PAMAM was less toxic (in some cases, much less so) than related dendrimers, [ 16 ] exhibiting minimal cytotoxicity across multiple toxicity screens, including tests of metabolic activity ( MTT assay ), cell breakdown ( LDH assay ), and nucleus morphology ( DAPI staining). [ 10 ] However, in other cell lines , the MTT assay and several other assays revealed some cytotoxicity. [ 12 ] [ 13 ] These disparate observations could be due to differences in sensitivity of the various cell lines used in each study to PAMAM; although cytotoxicity for PAMAM varies among cell lines, they remain less toxic than other dendrimer families overall.
More recently, a series of studies by Mukherjee et al. [ 13 ] [ 14 ] [ 15 ] have shed some light on the mechanism of PAMAM cytotoxicity, providing evidence that the dendrimers break free of their encapsulating membrane ( endosome ) after being absorbed by the cell, causing harm to the cell's mitochondria and eventually leading to cell death. Further elucidation of the mechanism of PAMAM cytotoxicity would help resolve the dispute as to precisely how toxic the dendrimers are.
In relation to neuronal toxicity, fourth generation PAMAM has been shown to break down calcium transients, altering neurotransmitter vesicle dynamics and synaptic transmission. All of the above can be prevented by replacing the surface amines with folate or polyethylene glycol. [ 17 ]
It has also been shown that PAMAM dendrimers cause rupturing of red blood cells , or hemolysis . [ 12 ] Thus, if PAMAM dendrimers are to be considered in biological applications that involve dendrimers or dendrimer complexes traveling through the bloodstream, the concentration and generation number of unmodified PAMAM in the bloodstream should be taken into account.
To date, few in-depth studies on the in vivo behavior of PAMAM dendrimers have been carried out. This could be in part due to the diverse behavior of PAMAMs depending on surface modification (see below), which make characterization of their in vivo properties largely case-dependent. Nonetheless, the fate and transport of unmodified PAMAM dendrimers is an important case study as any biological applications could involve unmodified PAMAM as a metabolic byproduct. In the only major systematic study of in vivo PAMAM behavior, injections of high levels of bare PAMAMs over extended periods of time in mice showed no evidence of toxicity up through G5 PAMAM, and for G3-G7 PAMAM, low immunogenicity was observed. [ 11 ] These systemic-level observations seem to align with the observation that PAMAM dendrimers are not extremely cytotoxic overall; however, more in-depth studies of the pharmacokinetics and biodistribution of PAMAM are required before a move toward in vivo applications can be made.
One unique property of dendrimers such as PAMAM is the high density of surface functional groups , which allow many alterations to be made to the surface of each dendrimer molecule. In putative PAMAM dendrimers, the surface is rife with primary amines, with higher generations expressing exponentially greater densities of amino groups. Although the potential to attach many things to each dendrimer is one of their greatest advantages, the presence of highly localized positive charges can be toxic to cells. Surface modification via attachment of acetyl [ 18 ] and lauroyl [ 19 ] groups help mask these positive charges, attenuating cytotoxicity and increasing permeability to cells. Thus, these types of modifications are especially beneficial for biological applications. Secondary and tertiary amino surface groups are also found to be less toxic than primary amino surface groups, [ 10 ] suggesting it is charge shielding which has major bearing on cytotoxicity and not some secondary effect from a particular functional group. Furthermore, other studies point to a delicate balance in charge which must be achieved to obtain minimal cytotoxicity. Hydrophobic interactions can also cause cell lysis, and PAMAM dendrimers whose surfaces are saturated with nonpolar modifications such as lipids or polyethylene glycol (PEG) suffer from higher cytotoxicity than their partially substituted analogues. [ 19 ] PAMAM dendrimers with nonpolar internal components have also been shown to induce hemolysis. [ 12 ]
Applications involving dendrimers in general take advantage of either stuffing cargo into the interior of the dendrimer (sometimes referred to as the "dendritic box"), or attaching cargo onto the dendrimer surface. PAMAM dendrimer applications have generally focused on surface modification, taking advantage of both electrostatic and covalent methods for binding cargo. Currently, major areas of study using PAMAM dendrimers and their functionalized derivatives involve drug delivery and gene delivery.
Since PAMAM dendrimers have shown penetration capability to a wide range of cell lines, simple PAMAM-drug complexes would affect a broad spectrum of cells upon introduction to a living system. Thus, additional targeting ligands are required for the selective penetration of cell types. For example, PAMAM derivatized with folic acid is preferentially taken up by cancer cells , which are known to overexpress the folate receptor on their surfaces. Attaching additional treatment methods along with the folic acid, such as boron isotopes , [ 20 ] cisplatin , [ 21 ] and methotrexate have proven quite effective. [ 22 ] In the future, as synthetic control over dendrimer surface chemistry becomes more robust, PAMAM and other dendrimer families may rise to prominence alongside other major approaches to targeted cancer therapy.
In a study of folic acid functionalized PAMAM, methotrexate was combined either as an inclusion complex within the dendrimer or as a covalent surface attachment. In the case of the inclusion complex, the drug was released from the dendrimer interior almost immediately when subjected to biological conditions and acted similarly to the free drug. The surface attachment approach yielded stable, soluble complexes which were able to selectively target cancer cells and did not prematurely release their cargo. [ 22 ] Drug release in the case of the inclusion complex could be explained by the protonation of surface and interior amines under biological conditions, leading to unpacking of the dendrimer conformation and consequent release of the inner cargo. A similar phenomenon was observed with complexes of PAMAM and cisplatin. [ 23 ]
PAMAM dendrimers have also demonstrated intrinsic drug properties. One quite notable example is the ability for PAMAM dendrimers to remove prion protein aggregates, [ 24 ] the deadly protein aggregates responsible for bovine spongiform encephalopathy ("mad cow disease") and Creutzfeldt–Jakob disease in humans. The solubilization of prions is attributed to the polycationic and dendrimeric nature of the PAMAMs, with higher generation (>G3) dendrimers being the most efficient; hydroxy-terminated PAMAMs as well as linear polymers showed little to no effect. Since there are no other known compounds capable of dissolving prions which have already aggregated, PAMAM dendrimers have offered a bit of reprieve in the study of such fatal diseases, and may offer additional insight into the mechanism of prion formation.
The discovery that mediating positive charge on PAMAM dendrimer surfaces decreases their cytotoxicity has interesting implications for DNA transfection applications. Because the cell membrane has a negatively charged exterior, and the DNA phosphate backbone is also negatively charged, the transfection of free DNA is not very efficient simply due to charge repulsion. However, it would be reasonable to expect charged interactions between the anionic phosphate backbone of DNA and the amino-terminated surface groups of PAMAM dendrimers, which are positively ionized under physiological conditions. This could result in a PAMAM-DNA complex, which would make DNA transfection more efficient due to neutralization of the charges on both elements, while the cytotoxicity of the PAMAM dendrimer would also be reduced. Indeed, several reports have confirmed PAMAM dendrimers as effective DNA transfection agents. [ 16 ] [ 26 ] [ 27 ] [ 28 ]
When the charge balance between DNA phosphates and PAMAM surface amines is slightly positive, the maximum transfection efficiency is obtained; [ 23 ] this finding supports the idea that the complex binds to the cell surface via charge interactions. A striking observation is that "activation" of PAMAM by partial degradation via hydrolysis improves transfection efficiency by 2-3 orders of magnitude, [ 23 ] providing further evidence supporting the existence of an electrostatically coupled complex. The fragmentation of some branches of the dendrimer is thought to loosen up the overall structure (fewer amide bonds and space constraints), which would theoretically result in better contact between the dendrimer and DNA substrate because the dendrimer is not forced into a rigid spherical conformation due to sterics . This in turn results in more compact DNA complexes which are more easily endocytosed. After endocytosis, the complexes are subjected to the acidic conditions of the cellular endosome . The PAMAM dendrimers act as a buffer in this environment, soaking up the excess protons with multitudes of amine residues, leading to the inhibition of pH-dependent endosomal nuclease activity and thus protecting the cargo DNA. The tertiary amines on the interior of the dendrimer can also participate in the buffering activity, causing the molecule to puff up; additionally, as the PAMAMs take on more and more positive charge, fewer of them are required for the optimal PAMAM-DNA interaction, and free dendrimers are released from the complex. Dendrimer release and swelling can eventually lyse the endosome, resulting in release of the cargo DNA. The activated PAMAM dendrimers have less spatial barrier to interior amine protonation, which is thought to be a major source of their advantage over non-activated PAMAM. [ 25 ]
In the context of existing approaches to gene transfer, PAMAM dendrimers hold a strong position relative to major classical technologies such as electroporation , microinjection , and viral methods . Electroporation, which involves pulsing electricity through cells to create holes in the membrane through which DNA can enter, has obvious cytotoxic effects and is not appropriate for in vivo applications. On the other hand, microinjection, the use of fine needles to physically inject genetic material into the cell nucleus, offers more control but is a high-skill, meticulous task in which a relatively low number of cells can be transfected. Although viral vectors can offer highly specific, high-efficiency transfection, the generation of such viruses is costly and time-consuming; furthermore, the inherent viral nature of the gene transfer often triggers an immune response, thus limiting in vivo applications. In fact, many modern transfection technologies are based on artificially assembled liposomes (both liposomes and PAMAMs are positively charged macromolecules). [ 25 ] Since PAMAM dendrimers and their complexes with DNA exhibit low cytotoxicity, higher transfection efficiencies than liposome-based methods, and are effective across a broad range of cell lines, [ 16 ] they have taken an important place in modern gene therapy methodologies. The biotechnology company Qiagen currently offers two DNA transfection product lines (SuperFect and PolyFect) based on activated PAMAM dendrimer technology.
Much work lies ahead before activated PAMAM dendrimers can be used as in vivo gene therapy agents. Although the dendrimers have proved to be highly efficient and non-toxic in vitro , the stability, behavior, and transport of the transfection complex in biological systems has yet to be characterized and optimized. As with drug delivery applications, specific targeting of the transfection complex is ideal and must be explored as well. | https://en.wikipedia.org/wiki/Poly(amidoamine) |
Poly(ethylene adipate) or PEA is an aliphatic polyester . [ 3 ] It is most commonly synthesized from a polycondensation reaction between ethylene glycol and adipic acid . [ 4 ] PEA has been studied as it is biodegradable through a variety of mechanisms and also fairly inexpensive compared to other polymers. [ 5 ] Its lower molecular weight compared to many polymers aids in its biodegradability. [ 6 ]
Poly(ethylene adipate) can be synthesized through a variety of methods. First, it could be formed from the polycondensation of dimethyl adipate and ethylene glycol mixed in equal amounts and subjected to increasing temperatures (100 °C, then 150 °C, and finally 180 °C) under nitrogen atmosphere. Methanol is released as a byproduct of this polycondensation reaction and must be distilled off. [ 4 ] Second, a melt condensation of ethylene glycol and adipic acid could be carried out at 190-200 °C under nitrogen atmosphere. [ 7 ] Lastly, a two-step reaction between adipic acid and ethylene glycol can be carried out. A polyesterification reaction is carried out first followed by polycondensation in the presence of a catalyst. Both of these steps are carried out at 190 °C or above. [ 8 ] Many different catalysts can be used such as stannous chloride and tetraisopropyl orthotitanate. Generally, the PEA is then dissolved in a small amount of chloroform followed by precipitation out in methanol. [ 8 ] [ 9 ]
An alternate and less frequently used method of synthesizing PEA is ring-opening polymerization . Cyclic oligo(ethylene adipate) can be mixed with di- n -butyltin in chloroform. This requires temperatures similar to melt condensation. [ 4 ]
PEA has a density of 1.183 g/mL at 25 °C and it is soluble in benzene and tetrahydrofuran . [ 2 ] PEA has a glass transition temperature of -50 °C. [ 10 ] PEA can come in a high molecular weight or low molecular weight variety, i.e.10,000 or 1,000 Da. [ 11 ] Further properties can be broken down into the following categories.
In general, most aliphatic polyesters have poor mechanical properties and PEA is no exception. Little research has been done on the mechanical properties of pure PEA but one study found PEA to have a tensile modulus of 312.8 MPa, a tensile strength of 13.2 MPa, and an elongation at break of 362.1%. [ 3 ] Alternate values that have been found are a tensile strength of ~10 MPa and a tensile modulus of ~240 MPa. [ 6 ]
IR spectra for PEA show two peaks at 1715–1750 cm −1 , another at 1175–1250 cm −1 , and a last notable peak at 2950 cm −1 . These peaks can be easily determined to be from ester groups, COOC bonds, and CH bonds respectively. [ 12 ]
PEA has been shown to be able to form both ring-banded and Maltese-cross (or ring-less) type spherulites . Ring-banded spherulites most notably form when crystallization is carried out between 27 °C and 34 °C whereas Maltese-cross spherulites form outside of those temperatures. [ 11 ] Regardless of the manner of banding, PEA polymer chains pack into a monoclinic crystal structure (some polymers may pack into multiple crystal structures but PEA does not). [ 13 ] The length of the crystal edges are given as follows: a = 0.547 nm, b = 0.724 nm, and c = 1.55 nm. The monoclinic angle, α, is equal to 113.5°. [ 11 ] The bands formed by PEA have been said to resemble corrugation, much like a butterfly wing or Pollia fruit skin. [ 14 ]
Conductivity of films made of PEA mixed with salts was found to exceed that of PEO 4.5 LiCF 3 SO 3 and of poly(ethylene succinate)/LiBF 4 suggesting it could be a practical candidate for use in lithium-ion batteries. [ 15 ] Notably, PEA is used as a plasticizer and therefore amorphous flows occur at fairly low temperatures rendering it less plausible for use in electrical applications. Blends of PEA with polymers such as poly(vinyl acetate) showed improved mechanical properties at elevated temperatures. [ 15 ]
PEA is miscible with a number of polymers including: poly(L-lactide) (PLLA), poly(butylene adipate) (PBA), poly(ethylene oxide) , tannic acid (TA), and poly(butylene succinate) (PBS). [ 2 ] [ 16 ] [ 17 ] PEA is not miscible with low density polyethylene (LDPE). [ 6 ] Miscibility is determined by the presence of only a single glass transition temperature being present in a polymer mixture. [ 18 ]
Aliphatic copolyesters are well known for their biodegradability by lipases and esterases as well as some strains of bacteria. PEA in particular is well degraded by hog liver esterase, Rh. delemar, Rh. arrhizus , P. cepacia, R. oryzae, and Aspergillus sp . [ 8 ] [ 19 ] [ 20 ] An important property in the speed of degradation is the crystallinity of the polymer. Neat PEA has been shown to have a slightly lower degradation rate than copolymers due to a loss in crystallinity. PEA/poly(ethylene furanoate) (PEF) copolymers at high PEA concentrations were shown to degrade within 30 days while neat PEA had not fully degraded, however, mixtures approaching 50/50 mol% hardly degrade at all in the presence of lipases. [ 8 ] Copolymerizing styrene glycol with adipic acid and ethylene glycol can result in phenyl side chains being added to PEA. Adding phenyl side chains increases steric hindrance causing a decrease in the crystallinity in the PEA resulting in an increase in biodegradability but also a notable loss in mechanical properties. [ 3 ]
Further work has shown that decreasing crystallinity is more important to degradation carried out in water than whether or not a polymer is hydrophobic or hydrophilic. PEA polymerized with 1,2-butanediol or 1,2-decanediol had an increased biodegradability rate over PBS copolymerized with the same side branches. Again, this was attributed to a greater loss in crystallinity as PEA was more affected by steric hindrance, even though it is more hydrophobic than PBS. [ 21 ]
Poly(ethylene adipate) urethane combined with small amounts of ligin can aid in preventing degradation by acting as an antioxidant. Additionally, the mechanical properties of the PEA urethane increased by ligin addition. This is thought to be due to the rigid nature of ligin which aids in reinforcing soft polymers such as PEA urethane. [ 20 ]
When PEA degrades, it has been shown that cyclic oligomers are the highest fraction of formed byproducts. [ 22 ]
Using toluene as a solvent, the efficacy of degrading PEA through ultrasonic sound waves was examined. Degradation of a polymer chain occurs due to cavitation of the liquid leading to scission of chemical chains. In the case of PEA, degradation was not observed due to ultrasonic sound waves. This was determined to be likely due to PEA not having a high enough molar mass to warrant degradation via these means. [ 23 ] A low molecular weight has been indicated as being necessary for the biodegradation of polymers. [ 21 ]
Poly(ethylene adipate) can effectively be used as a plasticizer reducing the brittleness of other polymers. Adding PEA to PLLA was shown to reduce the brittleness of PLLA significantly more than poly(butylene adipate) (PBA), poly(hexamethylene adipate) (PHA), and poly(diethylene adipate) (PDEA) but reduced the mechanical strength. [ 10 ] [ 18 ] The elongation at break was increased approximately 65x over neat PLLA. [ 18 ] The thermal stability of PLLA also showed a significant increase with an increasing concentration of PEA. [ 9 ]
PEA has also been shown to increase the plasticity and flexibility of the terpolymer maleic anhydride-styrene-methyl metacrylate (MAStMMA). Observing the changes in thermal expansion coefficient allowed for the increasing in plasticity to be determined for this copolymer blend. [ 12 ]
Self-healing polymers is an effective method of healing microcracks caused by an accumulation of stress. Diels-Alder (DA) bonds can be incorporated into a polymer allowing microcracks to occur preferentially along these weaker bonds. Furyl-telechelic poly(ethylene adipate) (PEAF 2 ) and tris-maleimide (M 3 ) can be combined through a DA reaction in order to bring about self-healing capabilities in PEAF 2 . PEAF 2 M 3 was found to have some healing capabilities after 5 days at 60 °C, although significant evidence of the original cut appeared and the original mechanical properties were not fully restored. [ 24 ]
PEA microbeads intended for drug delivery can be made through water/oil/water double emulsion methods. By blending PEA with Poly-ε-caprolactone, beads can be given membrane porosity. Microbeads were placed into a variety of solutions including a synthetic stomach acid, pancreatin, Hank's buffer, and newborn calf serum. [ 5 ] The degradation of the microcapsules and therefore the release of the drug was the greatest in newborn calf serum, followed by pancreatin, then synthetic stomach acid, and lastly Hank's buffer. The enhanced degradation in newborn calf serum and pancreatin was attributed to the presence of enzyme activity and that simple ester hydrolysis was able to be carried out. Additionally, an increase in pH is correlated with higher degradation rates. [ 5 ] | https://en.wikipedia.org/wiki/Poly(ethylene_adipate) |
Poly( p -phenylene) ( PPP ) is made of repeating p - phenylene units, which act as the precursor to a conducting polymer of the rigid-rod polymer family. The synthesis of PPP has proven challenging, but has been accomplished through excess polycondensation with the Suzuki coupling method. [ 1 ] [ 2 ]
Early efforts typically produced black, insoluble powders that were difficult to characterize. For example, a 1962 paper reports "The solid glowed red-hot in a Bunsen flame, with no evidence of flame formation, and disappeared only slowly." [ 3 ] (J. Polym. Sci. (1960), 47, 45) Initially, the chemical and thermal stability of the material drove interest in its synthesis. It was used in rocket nozzles and some fabrics requiring high thermal stability.
Oxidation or the use of dopants is used to convert the non-conductive form to a semiconductor .
This article about polymer science is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Poly(p-phenylene) |
Poly( p -phenylene oxide) ( PPO ), poly( p -phenylene ether) ( PPE ), poly(oxy-2,6-dimethyl-1,4-phenylene) , often referred to simply as polyphenylene oxide , is a high-temperature thermoplastic with the general formula (C 8 H 8 O) n . It is rarely used in its pure form due to difficulties in processing. It is mainly used as blend with polystyrene, high impact styrene-butadiene copolymer or polyamide . PPO is a registered trademark of SABIC Innovative Plastics B.V. under which various polyphenylene ether resins are sold.
Polyphenylene ether was discovered in 1959 [ 1 ] by Allan Hay , and was commercialized by General Electric in 1960.
While it was one of the cheapest high-temperature resistant plastics, processing was difficult, while the impact and heat resistance gradually decreased with time. Mixing it with polystyrene in any ratio could compensate for the disadvantages. In the 1960s, modified PPE came into the market under the trademark Noryl . [ 2 ]
PPE is an amorphous high-performance plastic. The glass transition temperature is 215 °C, but it can be varied by mixing with polystyrene . Through modification and the incorporation of fillers such as glass fibers, the properties can be extensively modified.
PPE blends are used for structural parts, electronics, household and automotive items that depend on high heat resistance, dimensional stability and accuracy. They are also used in medicine for sterilizable instruments made of plastic. [ 3 ] The PPE blends are characterized by hot water resistance with low water absorption, high impact strength, halogen-free fire protection and low density.
This plastic is processed by injection molding or extrusion; depending on the type, the processing temperature is 260–300 °C. The surface can be printed, hot-stamped, painted or metallized. Welds are possible by means of heating element, friction or ultrasonic welding. It can be glued with halogenated solvents or various adhesives.
This plastic is also used to produce air separation membranes for generating nitrogen. [ 4 ] The PPO is spun into a hollow fiber membrane with a porous support layer and a very thin outer skin. The permeation of oxygen occurs from inside to out across the thin outer skin with an extremely high flux. Due to the manufacturing process, the fiber has excellent dimensional stability and strength. Unlike hollow fiber membranes made from polysulfone, the aging process of the fiber is relatively quick so that air separation performance remains stable throughout the life of the membrane. PPO makes the air separation performance suitable for low temperature (35–70 °F, 2–21 °C) applications where polysulfone membranes require heated air to increase permeation.
Natural phenols can be enzymatically polymerized. Laccase and peroxidase induce the polymerization of syringic acid to give a poly(1,4-phenylene oxide) bearing a carboxylic acid at one end and a phenolic hydroxyl group at the other. [ 5 ]
Translated from the article Polyphenylenether on the German Wikipedia. | https://en.wikipedia.org/wiki/Poly(p-phenylene_oxide) |
Poly( p -phenylene vinylene) ( PPV , or polyphenylene vinylene ) is a conducting polymer of the rigid-rod polymer family. PPV is the only polymer of this type that can be processed into a highly ordered crystalline thin film. PPV and its derivatives are electrically conducting upon doping. Although insoluble in water, its precursors can be manipulated in aqueous solution. The small optical band gap and its bright yellow fluorescence makes PPV a candidate in applications such as light-emitting diodes (LED) and photovoltaic devices. [ 1 ] Moreover, PPV can be doped to form electrically conductive materials. [ citation needed ] Its physical and electronic properties can be altered by the inclusion of functional side groups.
PPVs can be synthesized by a variety of methods, the details of which determine purity and molecular weight. The most popular methods proceed via p- xylylene intermediates after a base induced elimination from α,α'-disubstituted para-xylenes . [ 1 ]
Although xylylene-based routes dominate the synthetic methodology, many other routes have been evaluated. [ citation needed ]
PPV can be synthesized by Wittig-type couplings between the bis(ylide) derived from an aromatic bisphosphonium salt and dialdehyde, especially 1,4-benzenedialdehyde.
Step growth coupling reactions , such as this Wittig condensation, usually yield low molecular weight oligomer with 5-10 repeat units. Incorporation of various side groups (alkyl, alkoxy, or phenyl) increases the solubility of the polymer and gives higher molecular weights. An advantage of the step-polymerization approach is that ortho-, meta-, and para-xylylene linkages can be incorporated in the main chain. Copolymers of defined stereoregularity can also be easily made in this way. [ citation needed ]
PPV derivatives can be also produced via the Knoevenagel condensation between a benzylic nitrile and an aromatic dialdehyde. Since this method produces many side reactions, such as hydrolysis of nitrile group, careful optimization of the reaction conditions was needed.
The couplings of ethylene with a variety of aromatic dibromides via a Heck reaction give reasonable molecular weights (3,000-10,000) when solubilizing groups present. However, this method requires one of the gaseous starting materials to be added in precise amounts, In excess polyethylene could be formed.
A bicyclooctadiene compound has been coupled by ring-opening metathesis polymerization (ROMP) to give a precursor polymer of high molecular weight and soluble in organic solvents. This polymer can be deposited as thin films and converted thermally to PPV. Lower conversion temperatures could be employed with the presence of an amine catalyst.
A modification of the ROMP route to PPV used a silyl-substituted paracyclophane derivative. Transformation into PPV could be achieved by elimination of the silyloxy group followed by thermal treatment or treating the precursor polymer with acid. The advantage of this method is that polymers and block copolymers of well-defined molecular weight can be easily prepared.
Highly oriented PPV films obtained by the soluble polymeric precursor route usually have P21 symmetry with a monoclinic unit cell containing two monomer units: c (chain axis) = 0.658, a = 0.790, b = 0.605 nm, and α (monoclinic angle) = 123o (Figure 1). The structural organization of PPV chains resembles that found in other highly oriented rigid-rod polymers, where the molecules are oriented along the fiber axis (often the stretching direction) but with partial axial translational disorder. [ 2 ]
PPV is a diamagnetic material and has a very low intrinsic electrical conductivity, on the order of 10 −13 S/cm. [ 1 ] The electrical conductivity increases upon doping with iodine, ferric chloride, alkali metals, or acids. However, the stability of these doped materials is relatively low. In general, unaligned, unsubstituted PPV presents only moderate conductivity with doping, ranging from <<10 −3 S/cm (I2 doped) to 100 S/cm (H 2 SO 4 -doped). [ 1 ] Draw ratios of up to 10 are possible. Alkoxy-substituted PPVs are generally easier to oxidize than the parent PPV and hence have much higher conductivities. Longer side chains lower the conductivity and hinder interchain hopping of charge carriers.
Due to its stability, processability, and electrical and optical properties, PPV has been considered for a wide variety of applications. [ 1 ] In 1989 the first polymer-based light emitting diode (LED) was discovered using PPV as the emissive layer. [ 3 ] Polymers are speculated to have advantages over molecular materials in LEDs, such as ease of processing, reduced tendency for crystallization, and greater thermal and mechanical stability. Ever since the first breakthrough in 1989, a large number of PPV derivatives have been synthesized and used for LED applications. Although solid-state lasing has yet to be demonstrated in an organic LED, poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) has been proven to be a promising laser dye due to its high fluorescence efficiency in solution. [ 4 ]
Polyphenylene vinylene is electroluminescent , suggesting applications in polymer-based organic light emitting diodes . PPV was used as the emissive layer in the first polymer light-emitting diodes. [ 3 ] Devices based on PPV emit yellow-green light, and derivatives of PPV obtained by substitution are often used when light of a different color is required. In presence of even a small amount of oxygen , singlet oxygen is formed during operation, by energy transfer from the excited polymer molecules to oxygen molecules. These oxygen radicals then attack the structure of the polymer, leading to its degradation. [ citation needed ]
PPV has also been investigated as an electron-donor in organic solar cells . [ 5 ] PPV-based devices however suffer from poor absorption and photodegradation . [ 6 ] | https://en.wikipedia.org/wiki/Poly(p-phenylene_vinylene) |
Poly(pentafluorophenyl acrylate) (variously abbreviated PPFPA , PolyPFPA , PPfpA , or PolyPfpA ) is a highly fluorinated polymer . It features the pentafluorophenyl ester functionality, from which its properties and applications result. It is most commonly used in post- polymerization modification to synthesize functional polyacrylamides or polyacrylates . [ 1 ] As such, it is advantageous to poly( N -acryloyl succinimide) due to its broader solubility in organic solvents as well as its higher stability towards hydrolysis . [ citation needed ]
Commonly poly(pentafluorophenyl acrylate) is synthesized by free radical polymerization of the monomer pentafluorophenyl acrylate. [ 2 ]
Additionally, pentafluorophenyl acrylate can be successfully polymerized by RAFT polymerization yield homopolymers, copolymers , or block copolymers. [ 3 ]
It has been shown that poly(pentafluorophenyl acrylate) can also be prepared by pulsed plasma deposition. [ 4 ]
Poly(pentafluorophenyl acrylate) is a polymeric active ester and hence features an inherent reactivity towards nucleophiles such as amines. [ 5 ] It is therefore used in the preparation of polyacrylamides by reacting it with amines.
Poly(pentafluorophenyl acrylate) can also be used in a transesterification by reacting it with alcohols when auxiliary DMAP and DMF are used, allowing for the synthesis of polyacrylate homopolymers and copolymers. [ 5 ]
Polymers are known for featuring low refractive indices typically in the range from 1.3 to 1.8, with fluorinated polymers exhibiting refractive indices in the range from 1.3 to 1.4.
As such, poly(pentafluorophenyl acrylate) has been explored as a crosslinkable cladding for optical fibers when copolymerized with glycidyl methacrylate . [ 6 ]
Poly(pentafluorophenyl acrylate) finds application in the synthesis of functional polymers by post-polymerization modification. [ 7 ] Applications of the resulting polyacrylamides can be found in drug delivery, functional surfaces, and nanoparticles. | https://en.wikipedia.org/wiki/Poly(pentafluorophenyl_acrylate) |
Poly(phthalaldehyde) , abbreviated as PPA, is a metastable stimuli-responsive polymer first synthesized in 1967. [ 1 ] It has garnered significant attention during the past couple of years due to its ease of synthesis and outstanding transient and mechanical properties. [ 2 ] For this reason, it has been exploited for a variety of applications including sensing, drug delivery, and EUV lithography . As of 2023, it is considered the only aromatic aldehyde polymerized through a living chain growth polymerization . [ 3 ]
Poly(phthalaldehyde) was first reported in 1967 by Chuji Aso and Sanae Tagami from the department of Organic Synthesis at Kyushu University by an addition homopolymerization reaction of aromatic o -phthalaldehyde . [ 1 ] This polymer, consisting of a polyacetal main chain, is still to date, the only aromatic aldehyde that can be homopolymerized through a chain-growth polymerization method. It is a white brittle solid with a low ceiling temperature and significant self-immolative properties. [ 4 ] It has gathered significant attention in recent [ when? ] years especially in the development of novel responsive materials and applications.
Since its first inception in 1967, many synthesis techniques have been developed and employed for the polymerization of o -phthalaldehyde . Most notably, living polymerization methods are among the most common and promising techniques used, as can be seen in the high number of publications in the literature depicting their usage in poly(phthalaldehyde) preparation. [ 5 ]
Aso and Tagami were the first to report the polymerization of o -phthalaldehyde in 1967 using the cationic living polymerization technique. [ 1 ] This technique, which was initially thought to require the usage of a strong Brönsted acid to initiate polymerization in addition to a strong nucleophile to depress polymerization and endcap the polymer chain was proven successful in a number of polymerization processes reported earlier. [ 6 ] [ 7 ] [ 8 ] Interestingly, the authors were able to produce this polymer without using an initiator nor a terminator and determined the polymer's structure to be cyclic. In fact, they worked at liquid nitrogen temperature and relied on Boron trifluoride etherate catalyst which was sufficient to produce a polymer stable enough at room temperature for a few days.
In the following years, polymer chemists started studying the characteristics of this polymer and worked on enhancing its thermal stability and mechanical properties. [ 9 ] In particular, Moore and coworkers conducted rigorous mechanistic studies on poly(phthalaldehyde) by modifying the type of catalyst used, as well as the starting monomer concentration in an effort to control the molar mass, decrease the polydispersity index, and increase the polymer's purity. [ 10 ] Among the catalysts used were triethyloxonium borofluoride, tin chloride , and triphenylmethylium tetrafluoroborate.
While LCP was the first and sole method used to produce poly(phthalaldehyde), its usage nowadays has dramatically decreased in favor of other polymerization techniques which allow a better control over the polymer properties including molar mass and thermal stability.
While this polymerization technique did not typically gain fame and popularity until 2010, it was also reported by Aso and Tagami in 1969. [ 11 ] In general, LAP involves the usage of a strong nucleophile to initiate polymerization in addition to the employment of an electrophile as a terminator to endcap the polymer chain. [ 12 ] In Tagami's article, PPA was prepared by utilizing tert -butyllithium as an initiator and acetic anhydride as a terminator. [ 11 ] However, the drawbacks faced when utilizing LCP ( low polydispersity index (PDI), low yield, and no control over molecular weight) were also encountered in this polymerization technique.
It was not until 1987 when two chemists, Hedrick and Schlemper, [ 13 ] from the University of Freiburg proposed the use of phosphazene bases to speed up the reaction and lower the polydispersity index . Up until 2023, three different phosphazene bases have been used in PPA polymerization. Moreover, most of the published research articles describing PPA synthesis between 2008 and 2023 revolve around the usage of LAP , rendering it the most common and effective polymerization technique.
The major advantage this polymerization technique presents over LCP lies in the fact that the polymer can be end capped on both sides of the chain with stimuli-responsive groups. [ 14 ] The tuning process of PPA by these functional groups have not only expanded the set of applications this polymer can be used in, but has also improved its properties and attributes. For instance, by controlling the o -phthalaldehyde monomer/alcohol initiator concentration ratio, ultra-high molecular weights (50-150 KDa) PPA can be obtained. [ 15 ] Furthermore, PPA synthesized through LAP is more thermally and mechanically stable. Generally, the presence of endcaps on both ends stabilizes the polymer and results in a more flexible chain with a high thermal stability. And because linear polymers synthesized by LAP method can be end capped whereas cyclic polymers prepared via LCP method cannot be end capped with functional groups, LAP results in more thermally stable polymers. It has a much lower PDI ranging between 1.3 and 1.9 as opposed to PPA synthesized through LCP which has a PDI ranging between 2 and 4.5. This is because of the ability to control the character, molecular weight, and end group of the polymer. [ 3 ] Furthermore, the initiator used in LAP synthesis method, which is a strong nucleophile , acts as the first endcap, and hence by controlling the amount of initiator used, a control over the molar mass and PDI can be obtained. This is in contrary to cyclic PPA which is synthesized through LCP where the initiator ( Lewis acid ) will not be part of the final PPA product, and hence, controlling the amount of Lewis acid used will have no to little effect on the final molar mass and PDI of cyclic PPA polymer.
Although a less known polymerization technique, coordinative polymerization has been used a few times in PPA preparation. It mostly requires the activation of transition metal catalysts with trimethylaluminum or diethyl aluminum chloride and allows a control over the stereoselectivity of the compound. [ 3 ] Another advantage of this technique lies within the usage of water as a co-catalyst in PPA synthesis which is deemed impossible in other polymerization methods. Professor Hisaya Tani from the Department of Polymer Science at Osaka University was the first to report a stereospecific polymerization of o -phthalaldehyde by employing dimeric dimethylaluminumoxybenzylideneaniline [Me 2 AlOCMeNPh] 2 as catalyst and water as a co-catalyst. [ 16 ] He was able to synthesize a fibrous PPA in exclusively trans-configuration which had never been reported before. Nonetheless, due to the inability to endcap the polymer with functional groups, this technique is rarely utilized at present and the mechanism of formation of PPA remains ambiguous and not well studied.
Depending on the polymerization technique applied, two different types of poly(phthalaldehyde) can be acquired, linear and cyclic.
Linear PPA is produced by anionic polymerization methods using a strong nucleophile as an initiator. [ 17 ] This technique prevents the cyclization of the polymer chain as the propagating species have only one charged terminus that cannot backbite the other terminus which, in turn, is neutral in charge. Although processing linear PPA requires highly sensitive reaction conditions and is more time demanding, this type of polymer has many advantages over its cyclic counterpart. [ 18 ] For instance, a control over the polymer's molar mass can easily be achieved by controlling the monomer and alcohol initiator ratios. Furthermore, it has been proven to be more thermally stable than its cyclic counterpart due to the presence of functionalized endcaps that stabilizes the polymer chain from depolymerization. [ 19 ] For these reasons, it has been studied to a far greater extent than cyclic PPA. Various linear PPA with distinct end groups have been reported and studied for a variety of applications including sensing, drug delivery, and lithography . [ 15 ] For instance, once these end groups are cleaved as a response to the exposure of PPA to a specific stimulus, the polymer will sequentially disassemble from head to tail through an unzipping reaction to form the monomer in short times that can be as low as a few minutes.
Cyclic PPA is obtained through a cationic polymerization of o -phthalaldehyde using a Lewis acid , typically Boron trifluoride etherate , as an initiator. [ 20 ] When Aso and Tagami first reported the successful synthesis of PPA using this technique in 1967, [ 1 ] they were unaware of the fact that the polymer they prepared was cyclic and instead reported the structure as linear in their research paper. It was not until 2013 that polymer chemists proved that the structure is cyclic using a combination of characterization techniques including Nuclear Magnetic Resonance (NMR), Fourier Transform Infrared Spectroscopy (FT-IR), Gel Permeation Chromatography (GPC), and Mass Spectrometry (MS). [ 10 ] Cyclic PPA is easy to synthesize; it is reported by Prof. Jeffrey Moore that the cationic polymerization of o -phthalaldehyde is very fast, yielding cyclic PPA within few minutes. [ 20 ] Furthermore, the polymer can be isolated without the addition of pyridine nor methanol nor a strong base terminator, which in general makes this polymerization technique easy, fast, and cheap. [ 21 ] Nevertheless, a known issue of this technique is the fact that the molecular weight cannot be controlled based on the initial concentration of the monomer used, which has led typically to cyclic PPA with a wide variety of molecular weights ranging between 3 kDa to 100 kDa using the same starting conditions. Furthermore, because of its cyclic structure, no end caps are used or needed. The absence of functionalized end caps in the structure has limited the usage of cyclic PPA especially in stimuli responsive applications. [ 22 ]
PPA is a metastable polymer known for its ease of synthesis and rapid depolymerization. In addition, its properties can be easily influenced and manipulated upon either functionalizing the phthalaldehyde monomer with different groups, most efficiently, electron withdrawing groups , or employing different functional groups as end caps . 3
PPA is known to have a rigid and brittle backbone which limits its flexibility and usage in some applications. However, it can be easily tuned by adding additives rendering it a soft material. [ 23 ] The mechanical properties of cyclic PPA films drop cast using different solvents have recently been investigated. [ 22 ] The study showed the polymer to possess a large elastic modulus of 2.5-3 GPa which was also previously reported in another study, in addition to tensile strength values ranging between 25 and 35 MPa and a failure strain of 1-1.5% that is highly dependent of the solvent used. [ 9 ]
With the insurgence in the usage of PPA during the past few years for various applications, the need to ameliorate the transient properties and enhance the mechanical features of this polymer has come to surface. PPA is known to be brittle ; it possesses a large storage modulus , and a glass transition temperature that is above its thermal degradation point, which renders the polymer unsuitable for a broad range of applications. [ 9 ] One way to ameliorate its intrinsic properties is via the addition of a plasticizing agent that can disrupt the polymer's intermolecular packing, and thus making it more flexible, decreasing its storage modulus , depressing its glass transition temperature , and increasing its shear strength . [ 2 ] A few examples of plasticizers that have been used with PPA include dimethyl phthalate , bis(2-ethylhexyl) phthalate , diethyl adipate , and tri-isononyl trimellitate (TINTM). In a recent study, the effect of two ether-ester plasticizers on the mechanical flexibility and photo-transience speed of cyclic PPA was investigated. [ 9 ] The authors were able to show that the addition of these additives broadened the storage modulus range and decreased it from 2300 MPa in the case of pure PPA down to 19 MPa in the PPA/plasticizer mixture, hence making the polymer more flexible and in need of less energy to be distorted. [ 9 ] In another study published by the same research group, the effect of diethyl adipate (DEA) plasticizer on the glass transition temperature of cyclic PPA was investigated. [ 2 ] After determining the glass transition temperature of pure PPA to be 187 °C, PPA films with various DEA concentrations were prepared. By varying DEA concentration, the authors were able depress T g to 12.5 °C demonstrating the importance of plasticizers in enhancing the mechanical flexibility and thermal properties of PPA. Similar results were previously observed where the thermal transitions were depressed from 95 °C for cPPA to 24 °C for diethyl phthalate (DEP)-plasticized cPPA. [ 20 ] Among the few studies that have been reported on the usage of plasticizers with PPA, it has been noted that the usage of plasticizers results in a decrease in the tensile stress of the polymers which indicate that PPA is becoming more flexible and hence the film can fold more easily. Nevertheless, a control on the amount of plasticizer used is important. For instance, in the study discussed above, it has been reported that the usage of a large amount of plasticizer (more than 50% w/w in comparison with PPA polymer) results in phase segregation and a decrease in the flexibility of the PPA film. [ 9 ] Furthermore, the nature of the used solvent can highly affect the mechanical properties of PPA as well. In particular, in another study published in 2019, both the elastic modulus and tensile strength increase when dichloromethane was used as a solvent to drop-cast PPA in comparison to dioxane and chloroform . [ 22 ]
The thermal stability of PPA is highly dependent on whether the polymer is end-capped or isolated without end groups. Cyclic PPA, in addition to functionalized linear PPA chains are known to be thermally stable for up to 150 o C as determined by both Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA). [ 24 ] Moreover, the polymer is known for its long-term shelf life wherein it can be stored at room-temperature for a significant amount of time. [ 24 ] Various chemists have studied substitution effects on the thermal stability of PPA. For instance, scientists at The International Business Machines Corporation (IBM) concluded, after extensive studies, that o -phthalaldehyde monomers functionalized with chloro, bromo, and 4-trimethylsilyl functional groups result in highly stable PPA compared to the unsubstituted polymer. [ 25 ] [ 26 ] Similarly, Phillips et al. proved that the substituted and end-capped poly(4,5-dichlorophthalaldehyde) possesses higher thermal degradation temperatures than its unsubstituted counterparts. [ 27 ]
By means of controlling the identity and reactivity of the endcaps, PPA can withstand harsh chemical conditions with no significant changes in its structure. For instance, while functionalizing PPA with an allyl acetate and tert -butyldimethylsilyl ether functional groups can lead to its rapid depolymerization in the presence of Pd(0) and F − respectively, a simple change in the nature of the endcaps will preserve the chain even in the presence of both corrosive agents. [ 28 ] On a separate note, while PPA is insoluble in aqueous solvents and alcohols, it is highly soluble in organic solvents such as THF , DCM , and DMSO where it can be dissolved for days without triggering depolymerization. [ 10 ]
Due to its unique [ editorializing ] stability, chemical properties, and outstanding tunability and reactivity, PPA has been employed in a variety of applications.
The high solubility and stability of PPA in organic solvents have allowed its investigation as a base material in first generation amplified photoresist for lithography in the early 80s by three scientists, Grant Willson, Jean Fréchet , and Hiroshi Ito who were working at IBM at the time. The story of how this successful achievement started and progressed can be found in the review paper written by Hiroshi Ito. [ 29 ] Because PPA by itself does not undergo complete depolymerization upon its subjection to light, it is usually end-capped or used along photoacid generators (PAGs) for enhanced sensitivity. [ 3 ] In this case, depolymerization is triggered upon irradiation either by end-cap removal and self-immolation or by the generated acid. Ober et al. stated that the use of PPA as photoresist under extreme ultraviolet (EUV) irradiation is yet to be successful due to the instability of PPA and the volatility of its monomers. 30 However, they were able to report one of the first PPA derivatives without the use of PAGs with enhanced photoresist properties upon EUV exposure. 19
Owing to its high reactivity and the ability to tune its endcap groups, PPA has been lately utilized in drug delivery applications. In one recent study, UV-sensitive PPA microcapsules containing different types of drugs were prepared. [ 30 ] Once the capsules were subjected to a UV-light trigger, an unzipping reaction took place and the shell ruptured which led to the release of the core containing the microcapsules. A unique advantage of these microcapsules is that they allow the immediate release of the drug upon exposure to the trigger rather than its continuous release over a period of time ranging from minutes to hours as other common microcapsules function. [ 31 ] In an earlier publication, DiLauro et al. reported the ability to predesign and control the thickness of the microcapsule shells and length of the PPA used to form the shell, which have stimuli-responsive endcaps allowing head-to-tail fluoride-triggered depolymerization. [ 32 ]
PPA is known as a self-immolative material which depolymerizes through endcap cleavage in response to a specific stimulus. For this reason, several PPA polymers with different endcaps have been synthesized and used as self-immolative materials for sensing toxic and specific compounds.
Due to the presence of two types of oxygen atoms in the PPA backbone , in addition to the fact that H + tends to protonate oxygen atoms easily, depolymerization can occur through both endcap cleavage and protonation of oxygen atoms present in the backbone . For this reason, polymer chemists tend to use endcaps rich in oxygen atoms to accelerate depolymerization rate. For example, Moore and co-workers reported the use of a specific ion coactivation (SICA) effect that allowed the ion and acid coactivated-triggered depolymerization of a cyclic PPA microcapsules at the solid/liquid interface of the polymer and solution. [ 33 ]
Silyl groups can be deprotected with fluoride ions resulting in a strong Si-F bond that is hard and challenging to break. For this reason, different polymer chemists started to employ PPA in fluoride sensing by using t -butyldimethylsily l (TBS) containing initiators and terminators. The fluoride sensing ability of PPA has been previously used in applications such as drug release, as previously reported by DiLauro et al. [ 32 ] Another application studied by Phillips and co-workers includes the use of fluoride-triggered PPA depolymerization in changing the structure of plastics in a predetermined way. [ 34 ]
To demonstrate its capability in rapidly depolymerizing in presence of UV-light, DiLauro et al. synthesized a PPA polymer with two UV-sensitive endcaps, 2-nitro-4,5-dimethoxybenzyl alcohol and 1-[[(chlorocarbonyl)oxy]methyl]-4,5-dimethoxy-2 nitrobenzene, and were able to achieve complete depolymerization in a few minutes. [ 15 ] In a practical application in organic electronics, cyclic PPA in the presence of 2-(4-methoxystyryl)-4,6-bis(trichloromethyl)-1,3,5-triazine (MBTT used as PAG) undergoes depolymerization upon exposure to UV-light, which in turn deactivates the transient electronics. [ 35 ] Another similar application in transient electronics was reported where an organic light-emitting diode (OLED) was integrated on the PPA substrate and can cause depolymerization in the presence of a PAG. [ 36 ]
Apart from its usage in sensing acids and fluoride anions, PPA has been used in sensing Pd(0) metal by employing allyl chloroformate as a terminating end cap. This has been reported by Phillips and his research group, where they used an allyl formate endcap that stoichiometrically depolymerized within minutes upon its exposure to a catalytic amount of tetrakis(triphenylphosphine)palladium(0) (Pd(PPh 3 ) 4 ). [ 34 ]
According to the safety data sheet of PPA, it should not be allowed in contact with the skin or eyes as it may lead to skin, eye, and respiratory irritations or allergic reactions. In addition, as some unfunctionalized PPA are unstable at temperatures even lower than room temperature, it is important to note that PPA should be stored at temperatures below -10 °C under inert atmosphere and away from sunlight, moisture, and heat, but with proper ventilation.
Since the depolymerization of PPA is greatly studied in its applications, it is important to also note the possible safety concerns of its monomer. In addition to the abovementioned hazards of PPA, phthalaldehyde is very toxic if swallowed and for aquatic life. | https://en.wikipedia.org/wiki/Poly(phthalaldehyde) |
PolyAnalyst is a data science software platform developed by Megaputer Intelligence that provides an environment for text mining , data mining , machine learning , and predictive analytics . It is used by Megaputer to build tools with applications to health care , business management , insurance , and other industries. PolyAnalyst has also been used for COVID-19 forecasting and scientific research.
PolyAnalyst's graphical user interface contains nodes that can be linked into a flowchart to perform an analysis. The software provides nodes for data import , data preparation , data visualization , data analysis , and data export . [ 2 ] [ 3 ] PolyAnalyst includes features for text clustering , sentiment analysis , extraction of facts, keywords , and entities , and the creation of taxonomies and ontologies . Polyanalyst supports a variety of machine learning algorithms, as well as nodes for the analysis of structured data and the ability to execute code in Python and R . [ 4 ] [ 5 ] PolyAnalyst also acts as a report generator , which allows the result of an analysis to be made viewable by non-analysts. [ 6 ] It uses a client–server model and is licensed under a software as a service model. [ 6 ]
PolyAnalyst was used to build a subrogation prediction tool which determines the likelihood that a claim is subrogatable, and if so, the amount that is expected to be recovered. [ citation needed ] The tool works by categorizing insurance claims based on whether or not they meet the criteria that are needed for successful subrogation. [ citation needed ] PolyAnalyst is also used to detect insurance fraud. [ 7 ]
PolyAnalyst is used by pharmaceutical companies to assist in pharmacovigilance . The software was used to design a tool that matches descriptions of adverse events to their proper MedDRA codes, determines if side effects are serious or non-serious, and to set up cases for ongoing monitoring if needed. [ 8 ] PolyAnalyst has also been applied to discover new uses for existing drugs by text mining ClinicalTrials.gov , [ 9 ] and to forecast the spread of the COVID-19 virus in the United States and Russia. [ 10 ] [ 11 ]
PolyAnalyst is used in business management to analyze written customer feedback including product review data, warranty claims, and customer comments. [ 12 ] In one case, PolyAnalyst was used to build a tool which helped a company monitor its employees' conversations with customers by rating their messages for factors such as professionalism, empathy, and correctness of response. The company reported to Forrester Research that this tool had saved them $11.8 million annually. [ 13 ]
PolyAnalyst is run on the SKIF Cyberia Supercomputer at Tomsk State University , where it is made available to Russian researchers through the Center for Collective Use (CCU). Researchers at the center use PolyAnalyst to perform scientific research and to management the operations of their universities. [ 14 ] In 2020, researchers at Vyatka State University (in collaboration with the CCU) performed a study in which PolyAnalyst was used to identify and reach out to victims of domestic violence through social media analysis . The researchers scraped the web for messages containing descriptions of abuse, and then classified the type of abuse as physical, psychological, economic, or sexual. They also constructed a chatbot to contact the identified victims of abuse and to refer them to specialists based on the type of abuse described in their messages. The data collected in this study was used to create the first ever Russian-language corpus on domestic violence. [ 15 ] [ 16 ] | https://en.wikipedia.org/wiki/PolyAnalyst |
Polydiallyldimethylammonium chloride (shortened polyDADMAC or polyDDA ), also commonly polyquaternium -6, is a homopolymer of diallyldimethylammonium chloride (DADMAC). The molecular weight of polyDADMAC is typically in the range of hundreds of thousands of grams per mole, and even up to a million for some products. PolyDADMAC is usually delivered as a liquid concentrate having a solids level in the range of 10 to 50%. It is a high charge density cationic polymer . The charge density makes it well suited for flocculation . Actually, pDADMAC or DMDAAC, is used as a coagulant - a charge neutralization process that precedes flocculation.
PolyDADMAC polymers were first prepared and studied in 1957 by Professor George Butler at the University of Florida. [ 1 ] It was remarkable as it was soluble in water in contrast at the time to other known synthetic polymers formed by polymerization of monomers containing more than one vinyl functionality. The structure and reaction path was determined in 2002 with NMR studies. Much of what we know about pDADMAC in water treatment and personal care is from Jerry Boothe of Calgon Corporation .
The monomer DADMAC is formed by reacting two equivalents of allyl chloride with dimethylamine .
PolyDADMAC is then synthesized by radical polymerization of DADMAC with an organic peroxide used as a catalyst . Two polymeric structures are possible when polymerizing DADMAC: N -substituted piperidine structure or N -substituted pyrrolidine structure. The pyrrolidine structure is favored. [ 2 ]
PolyDADMAC is used in waste water treatment as a primary organic coagulant which neutralizes negatively charged colloidal material and reduces sludge volume compared with inorganic coagulants.
PolyDADMAC is used for controlling disturbing substances in the papermaking process. It provides superior fixing of pitch from mechanical pulp and of latex from coated broke. Used in the short circulation of a paper mill to enhance retention and dewatering . In addition, it can be used to improve the efficiency of disk filters and flotators, and for cationization of fillers to provide maximal filler retention.
PolyDADMAC is used as a coagulant in water purification . It is effective in coagulating and flocculating inorganic and organic particles such as silt, clay, algae, bacteria and viruses. At high concentrations the organic polymer can remove natural organic matter such as humic and fulvic acids resulting in fewer disinfection byproduct precursors and less color. [ 3 ] | https://en.wikipedia.org/wiki/PolyDADMAC |
Poly Implant Prothèse ( PIP ) was a French company founded in 1991 that produced silicone gel breast implants . The company was preemptively liquidated in 2010 following the revelation that they had been illegally manufacturing and selling breast implants made from cheaper industrial-grade silicone since 2001 (instead of the mandated medical-grade silicone they had previously used).
The hundreds of thousands of unapproved implants sold globally by PIP from 2001 to 2010 were found to have a 500% higher risk of rupturing or leaking than approved models, as well as being implicated in several deaths due to systemic toxicity and several cases of induced breast cancer . The scandal, which produced fears of a massive health disaster, prompted a full recall of the company's implants by the French health ministry in 2010, by which time the company was already defunct. [ 2 ]
PIP was founded in 1991 [ 2 ] by the Frenchman [ 3 ] Jean-Claude Mas, born in 1939, a former butcher and later medical sales representative for the Bristol Myers company [ 4 ] for 15 years. [ 5 ] Mas had previously teamed up with plastic surgeon Henri Arion, who had introduced breast implants to France in 1965. [ 4 ] After Arion died in a plane crash, Mas went on alone and launched PIP in 1991. [ citation needed ]
Starting in 1991, the company produced approximately 2 million sets of silicone breast implants over a 20-year period. The implants were exported to Latin American countries such as Brazil , [ 2 ] Venezuela [ 6 ] and Argentina , Western European markets including Britain (25,000), Germany , Spain and Italy , as well as Australia (8900). [ 2 ]
Following the FDA ruling in 2000 which banned silicone breast implants in the US market (leading to a slump in sales worldwide), Mas sought to "tighten PIP's belt" and recoup some of its lost market share by severely cutting costs. In particular, Mas was credited with the idea that by switching PIP's silicone from externally purchased medical-grade to in-house produced industrial-grade, huge savings (on the order of 90%) could be generated that would ensure profits remained high no matter the market. As one PIP engineer later noted, this decision involved a relatively small change in the formula for the silicone, enough so that only superficial differences in the end product were noted. However, none of the relevant regulations were followed for manufacturing medical implants, and no pre-launch tests were conducted.
2000: Implant sales in the United States halted by an FDA launched moratorium on silicone implants. [ 7 ]
2001: PIP began to use unapproved in-house manufactured industrial-grade instead of medical grade silicone in the majority of its implants. [ 2 ]
2003: PIP is acquired by Heritage Worldwide Inc. [ 8 ] Regulatory filings show the first signs of legal problems and financial losses. [ 2 ]
2009: Concerns surfaced in France first in 2009 when surgeons started reporting an abnormally high rupture rates. Which resulted in a flood of legal complaints and the company's bankruptcy. [ 2 ]
2010: TÜV Rheinland , headquartered in Germany , gave a quality certificate to the production process used by the company until March 2010. However, this didn't apply to the type of silicone used. A former PIP worker and union chief, Eric Mariaccia said, "You had to have been a chemist to have noticed anything". [ 9 ] He also said that "The responsible ones aren't the workers but the heads of the company, notably the four who were linked to production and thus responsible for their quality". [ 9 ]
2010: In March 2010 PIP was placed into liquidation with losses of 9 million EUR after the French medical safety agency recalled its implants. In a subsequent inspection of the manufacturing site, the company was found to use unapproved industrial-grade, silicone , with a cost of only 10% of an approved gel. [ 2 ]
2011: On 20 December French officials say that an action plan is underway [ 10 ] following the death of a woman from ALCL . [ 11 ] The French government recommended on 23 December 2011 [ 12 ] that 30,000 women [ 2 ] in France seek removal of breast implants made of a suspect silicone gel by the worldwide exporting PIP firm. [ 12 ]
2013: Criminal trial is held in France, and Jean-Claude Mas was sent to prison for four years.
2021: French court found TUV Rheinland negligent and partially liable for damages.
Medical problems arise when breast implants rupture and cause inflammation and irritation . Further, impure gel may also release toxic substances. [ 13 ] Removing implants "carries risks in itself", says the French government. [ 2 ] By the end of 2011, eight cases of breast tumours had been uncovered in women after removing the implants, yet, while of concern, [ 13 ] according to the French government, there is no evidence of any increased cancer risk. [ 2 ]
The UK medical watchdog, the Medicines and Healthcare products Regulatory Authority (MHRA), says France has reported rupture rates of around 5% for PIP implants, compared with 1% in the UK. [ 14 ] But a "leading cosmetic surgery firm" has privately warned ministers that the proportion of women at risk is as high as 8%. [ 15 ] Symptoms can include lumps around the implant or in the under-arm, inflammation in the breast tissue and a hardening of the breast. [ 15 ] Patient UK says French regulatory authority has reported a rupture rate of up to 10%, with a gel leak in 11% of cases. [ 16 ]
Nigel Mercer of the British Association of Aesthetic and Plastic Surgeons (BAAPS) says the French move was "certainly not unreasonable" but the British cosmetic surgeon Kevin Hancock said the divergent government responses would cause distress for British women. Plastic surgeon Hancock, of the Liverpool Women's Hospital , says there were concerns in the profession over a high rupture rate and that "We are worried about the rupture risk because it is the rupture that brings the contents into direct contact with the body's tissues... We know that the contents were not what they were supposed to be. So in general we agree with the (French) decision to remove them". [ 2 ]
Patients are used as "guinea pigs", said Nigel Mercer, a former president of the British Association of Aesthetic and Plastic Surgeons (BAAPS), because medical implants lack the requirement of an independent clinical trial . [ 17 ]
The company sold a high-end product that used approved silicone for wealthier clients and which cost five times more than products with silicon gel manufactured in-house. Most PIP implants were manufactured with the in-house silicon. The lawyer, Haddad, representing the founder, said "There is a product made by PIP which did not formally receive the [regulators'] approval and in this regard there was a violation of regulations,". The company did not ask France's medical device safety regulators for approval of their in-house manufactured product, but expected it would have received approval if the company asked for it. [ 1 ]
Cancer has been found in 20 women with the allegedly faulty PIP breast implants but French health authorities ( Agence française de sécurité sanitaire des produits de santé (AFSSAPS)) insisted there was still no proven link with the disease. [ 18 ]
AFSSAPS has as of 28 December 2011, registered 15 cases of breast adenocarcinoma , the most frequent form of breast cancer, one case of breast lymphoma , two cases of other lymphoma, one case of lung cancer and one case of acute myelogenous leukemia in women with the implants. [ 18 ]
A French woman with a PIP breast implant where capsular scar tissue formed in the breast, which is a common occurrence with implants. [ 16 ] This tissue developed anaplastic large-cell lymphoma (ALCL), a rare form of cancer that affects the immune system cells. [ 11 ] The cancer caused death in this case and caused the French authorities to act. [ 11 ]
During the 672 registered preventive extractions, in 43 cases (6.4%) the implants had ruptured and in 14 cases (2.1%) they were "oozing", AFSSAPS said. [ 18 ]
The PIP implants have been resold as "Rofil M-implants". [ 19 ] British Association of Aesthetic and Plastic Surgeons (BAAPS) believes many British women may have had the bad Rofil M-implants used when they bought cut-price breast enlargement surgery from clinics in Eastern and Central Europe . [ 19 ]
The PIP factory was shut down and their products banned after it was found they had used the chemicals Baysilone, Silopren and Rhodorsil in their implants. [ 19 ] These chemicals are normally used as fuel additives or in the manufacture of industrial rubber tubing. [ 19 ]
In 1993, there was an availability crisis of silicone materials for use in medical long-term implant applications. [ 20 ] In 1993, Mas ordered the staff to "hide the truth" and "We did it for 13 years without a problem". [ 21 ] The PIP gel cost 5 EUR per litre while the authorized American gel Nusil cost 35 EUR per litre. The PIP implants were made with 75% non-authorised gel and 25% Nusil. [ 21 ] "The material was better than that used to make the officially authorised gel," Mas said. "Usually, from 1997 onwards we hid the products used to make the PIP gel. I wasn't allowed to buy these products because they were not authorised. We organised everything to escape being monitored." [ 21 ]
Investigators interviewed Mas on 18 and 19 November 2010 after the French health authority AFSSAPS discovered the PIP breast implants did not conform to regulations. They passed the case to a judge who ordered a police inquiry for "aggravated deception". [ 21 ]
The bad silicone has been used to make male chest , testicle [ 19 ] [ 22 ] [ 23 ] and buttock implants as well. [ 23 ] "Three people were specially trained to work with a machine that made silicon testicles'" said a former PIP worker. [ 23 ]
Most of the male chest, buttock and testicle implants were exported to Latin America . [ 23 ]
The French government recommended per 23 December 2011 [ 12 ] that 30,000 women [ 2 ] in France seek removal of breast implants made by PIP. [ 12 ] For women that want to keep their implants, the state will pay for a six-month ultrasound scan. [ 14 ]
French public health care funds will be used to finance the recommended implant removals, at a cost estimated at 60 million EUR . New implants will be paid for in cases where they were used initially for medical reasons, others will need to finance a new implant by themselves. Associations representing women with PIP implants demand that public funds cover all cases. "This announcement is just a smokescreen and the victims of PIP are angry," said Alexandra Blachere, head of the association of PIP implant users in France. [ 2 ]
A government hotline has been set up by December 2011. [ 1 ]
In Britain, there are an estimated 30,000–40,000 affected women. The chief Medical Officer Dame Sally Davies said, "Women with PIP implants should not be unduly worried. We have no evidence of a link to cancer or an increased risk of rupture. If women are concerned they should speak to their surgeon". Removing the implant "carries risks in itself," she said. [ 2 ]
Concerns were filed with the UK MHRA in 2009 when a lump was found during a mammogram and ultrasound scan in a female patients breast but no action was taken by MHRA at the time. [ 17 ]
Shadow Health Secretary Andy Burnham said private clinics in Britain that fitted PIP implants must be forced to pick up the cost of removing them. [ 19 ]
On 6 January 2012, NHS officials began to offer examinations and possible removal – in instances where a doctor ruled that necessary. They also called on private medical centers to offer the same deal to patients who had paid for private sector cosmetic surgery. [ 24 ]
On 8 January 2012, The Sunday Times reported that leading cosmetic surgeon Jan Stanek had conducted a study with colleague Mike Berry into PIP implants. Their report to the Journal of Plastic and Aesthetic Surgery revealed that, in an audit of 453 patients, between 16% and 34% of the PIP implants ruptured. This compared to a failure rate of less than 1% in other implants. [ 25 ]
Around 1,500 Irish women received PIP implants in Shandon Street Hospital in Cork and Clane General Hospital in County Kildare. The implants were removed free-of-charge by the Health Service Executive . [ 26 ]
Germany's medical safety board and Brazil's health watchdog called for users of PIP implants to visit their doctor for checks. [ 2 ]
On 6 January 2012 Germany's Federal Institute for Drugs and Medical Devices recommended the removal of PIP breast implants as a precaution. [ 24 ]
The bad breast implants were sold to about 1,000 Dutch women under the name "M-implants" by the Dutch importer Rofil. [ 27 ] The Dutch healthcare spokeswoman Diana Bouhys said, "We have advised them to consult their physician", but also declined to disclose the name of the company. PIP was the third largest maker of breast implants in the world. [ 5 ]
Approximately 5 000 implants have already been sold, corresponding to an estimated 2,500 Swedish women with the controversial implants. Nine clinics in Sweden has sold them says Gert Bruse of the Swedish Medical Products Agency (Läkemedelsverket) told the Dagens Nyheter (DN) newspaper this week. The agency also recommends that the implants to be removed.
Sweden banned the company's implants in March 2010, along with many other European countries, when it was discovered that substandard silicone gel was causing unusually many implants to burst. "We've had cases where the implants have burst in Sweden, but it's fewer than in France in relation to how many women have these implants in both countries... Implants shouldn't burst and they shouldn't have an irritating effect that spreads throughout the body. What this company has done is criminal, simply put." said Bruse. PIP's implants have only been used in cosmetic breast augmentation surgery, not in reconstructive surgery. [ 13 ]
In Finland only seven women were given PIP implants; all implants were imported from Sweden by the same surgeon. [ 28 ] The Society of Finnish Plastic Surgeons was advising to remove the implants in January 2012. [ 29 ]
Patients from Finland often visit Tallinn in Estonia to get cosmetic surgery. In October 2011 (two months before the ban on 22 December 2011), a Finnish woman in her 30s had received a different implant than what was agreed to in advance. An M-implant with the same type code as the implant from PIP was inserted instead of a promised Allergan implant. At least two Estonian clinics operating in Tallinn have offered M-implants as recently as December 2011. [ 30 ] [ 31 ]
According to the Estonian Health Board, 364 patients were installed with PIP and M-implants. PIP-implants were used by two clinics: Villa Medica and KT Kliinik OÜ, where these breast implants were installed to 163 patients. M-implant implants were installed in 2004–2009 in KT Kliinik OÜ and Plastilise Kirurgia OÜ clinics to 139 patients. In 2011, M-implants have been installed by KT Kliinik OÜ to 61 patients. [ 32 ]
Documents filed with the US government show that implants were sold until May 2000 through Heritage Worldwide, when the FDA launched a moratorium on silicone implants. At the time, the US market accounted for 40% of Heritage Worldwide's revenues, or 3 million EUR , according to corporate documents filed in 2009 with the U.S. Securities and Exchange Commission (SEC). In 2007 significant losses started as both users and distributors filed complaints. Between 1996 and 2009, PIP was targeted by several dozen lawsuits in the United States, filed by users and business partners, claiming breach of contract or unmet payments. Starting in 2003, dozens of women began filing lawsuits mainly for product liability against PIP. However, as of 2009, no trial date had yet been set. Many of the lawsuits were later dismissed. [ 7 ]
On 29 December 2011, Health Minister Renato Balduzzi said that hospitals and clinics are required to compile a list of women who has received breast implants from PIP. Clinics that did not use any PIP implants should also be required to send a declaration stating that. [ 33 ]
The Czech Republic health ministry said about 2,000 women with potentially faulty implants should have them removed. [ 24 ]
According to the Icelandic Directorate of Health, 440 women underwent breast surgery where PIP breast implants were used. The Ministry of Welfare have stated that health insured women will be offered counsel and ultrasound examination, free of charge. The Ministry of Welfare also stated that in the event that the implant is leaking, the government will pay a portion of the cost to have it removed. [ 34 ]
Ministry of Health and Medical Education banned the breast implants and took them off the market in 2012. They advised women that used the products to consult with a doctor and if deemed a health threat, to have them removed. [ 36 ]
The adequacy of regulations on silicone-filled breast implants, investigation by the regulatory authorities and advice given to women implanted with PIP silicone pre-filled breast implants was criticized in December 2012 in a report examining information publicly available from the regulatory authorities. [ 37 ]
A new firm has been registered under the name France Implant Technologie (FIT), with Mas' son Nicolas Lucciardi, 27, and daughter, Peggy Lucciardi, 24, at the address of their mother, Dominique Lucciardi, who was Mas' former civil partner. [ 6 ]
The local paper Nice-Matin obtained the business plan in which Mas is named as a "technical-commercial consultant" to the company and described as "a creative genius". Two former PIP managers were listed in senior positions in the company. With the objective to export implants to the "European, South American and Chinese markets".
An investment of 2 million EUR was planned to put the former PIP plant back into operation, aiming to manufacture 400 implants a day with about 20 workers. However, Nicolas Lucciardi said that the project had collapsed due to media coverage of the scandal, adding, "That's obvious". [ 6 ]
The lawyer Yves Haddad represented Mas. [ 2 ] Legal complaints have been filed by 2,500 women [ 6 ] in France and 250 women in Britain. Mas was wanted in Costa Rica for "life and health" offenses, a warrant issued by Interpol . [ 2 ]
By February 2011, TÜV Rheinland sued PIP, claiming it had been "thoroughly and continuously misled" regarding the silicone used. [ 2 ] Many executives are expected to face charges of aggravated fraud in an ongoing court case in France, expected to be scheduled for October 2012, which carries the possibility of prison terms of up to five years. [ 2 ] The death of a cancer victim who had such implants may result in more serious charges of involuntary manslaughter . [ 2 ]
Frédéric Van Roekeghem, general director for La Caisse Nationale d'Assurance Maladie (French state social insurance agency, CNAM), will go to court regarding the complaints of bad implants. A criminal charge will be made within a few days he said to TF1 on 25 December 2011. The decision has been made in consultation with minister of health, Xavier Bertrand . [ 9 ]
Mas underwent a difficult surgery regarding a vascular problem that prevents him from walking said his lawyer Haddad. [ 5 ] Sources say a Marseilles court could soon announce fraud charges against four to six former PIP employees. [ 5 ]
A fraud case was filed against PIP on 30 December 2011 by France's state-run health insurance fund (CNAM) officials in Marseille announced. Which is near the company's laboratory at La Seyne-sur-Mer . [ 6 ] On 26 January 2012 [ 38 ] [ 39 ] at just before 07:00 [ 39 ] the founder Mas was arrested in the home of a friend [ 38 ] [ 39 ] in the Six-Fours in the Var [ 39 ] located in South of France and taken into custody . [ 38 ] [ 39 ] Investigators searched the house. [ 39 ] A deputy chief executive was also arrested at his home. [ 39 ]
Philippe Courtois, a lawyer represents 1 300 women in France. Laurent Gaudon, represents four women and say a surgeon should inform about complications from both the surgery and devices used. [ 1 ]
Nathalie Lozano, lawyer, represents more than 1 400 women from Colombia, Argentina, United Kingdom and Venezuela. She claims TÜV Rheinland's gross negligence in the process of certifying the quality process. [ 40 ]
On 17 April 2013, the trial of 5 executives in begun in Marseille , France. Those charged are the company founder Mr Mas, who was booed in the court, his deputy Claude Couty, the quality director Hannelore Font, technical director Loic Gossart and products chief Thierry Brinon. They face up to five years in prison. The trial was expected to last until 17 May. [ 41 ]
On 10 December 2013, Jean-Claude Mas was sent to prison for four years, and was fined 75,000 euros (£63,000) by a court in Marseille, France. [ 42 ] The court also ordered TÜV Rheinland to pay 4,000 Euros to each victim. TUV Rheinland was responsible for issuing safety certificates to PIP following annual inspections. [ 43 ] Mas died in 2019. [ 44 ]
In May 2021, a French court affirmed that German firm TÜV Rheinland was partially liable for damages. TUV Rheinland was responsible for issuing safety certificates to PIP following annual inspections. Mas told police that "... employees would remove evidence of the industrial silicone gel before TUV Rheinland made its annual inspections." The court held that TUV Rheinland was negligent in not detecting the fraud. [ 45 ]
British Association of Aesthetic Plastic Surgeons (BAAPS) says that poor postmarketing surveillance is the root of the crisis, and proposes significantly more stringent monitoring of all medical devices including breast implants and all cosmetic injectables, via compulsory, regular reporting of adverse effects and mystery shopping . Professor Sir Bruce Keogh , the NHS Medical Director, will look into whether the cosmetic surgery industry needs to be more effectively regulated. [ 46 ] | https://en.wikipedia.org/wiki/Poly_Implant_Prothèse |
Polyacetylene ( IUPAC name: polyethyne ) usually refers to an organic polymer with the repeating unit [C 2 H 2 ] n . The name refers to its conceptual construction from polymerization of acetylene to give a chain with repeating olefin groups. This compound is conceptually important, as the discovery of polyacetylene and its high conductivity upon doping helped to launch the field of organic conductive polymers . The high electrical conductivity discovered by Hideki Shirakawa , Alan Heeger , and Alan MacDiarmid for this polymer led to intense interest in the use of organic compounds in microelectronics ( organic semiconductors ). This discovery was recognized by the Nobel Prize in Chemistry in 2000. [ 2 ] [ 3 ] Early work in the field of polyacetylene research was aimed at using doped polymers as easily processable and lightweight "plastic metals". [ 4 ] Despite the promise of this polymer in the field of conductive polymers, many of its properties such as instability to air and difficulty with processing have led to avoidance in commercial applications.
Compounds called polyacetylenes also occur in nature, although in this context the term refers to polyynes , compounds containing multiple acetylene groups ("poly" meaning many ), rather than to chains of olefin groups ("poly" meaning polymerization of ). [ 5 ]
Polyacetylene consists of a long chain of carbon atoms with alternating single and double bonds between them, each with one hydrogen atom. The double bonds can have either cis or trans geometry . The controlled synthesis of each isomer of the polymer, cis -polyacetylene or trans -polyacetylene, can be achieved by changing the temperature at which the reaction is conducted. The cis form of the polymer is thermodynamically less stable than the trans isomer. Despite the conjugated nature of the polyacetylene backbone, not all of the carbon–carbon bonds in the material are equal: a distinct single/double alternation exists. [ 6 ] Each hydrogen atom can be replaced by a functional group . Substituted polyacetylenes tend to be more rigid than saturated polymers. [ 4 ] Furthermore, placing different functional groups as substituents on the polymer backbone leads to a twisted conformation of the polymer chain to interrupt the conjugation.
One of the earliest reported acetylene polymers was named cuprene. Its highly cross-linked nature led to no further studies in the field for quite some time. [ 7 ] Linear polyacetylene was first prepared by Giulio Natta in 1958. [ 8 ] The resulting polyacetylene was linear, of high molecular weight, displayed high crystallinity, and had a regular structure. X-ray diffraction studies demonstrated that the resulting polyacetylene was trans -polyacetylene. [ 8 ] After this first reported synthesis, few chemists were interested in polyacetylene because the product of Natta's preparation was an insoluble, air sensitive, and infusible black powder.
The next major development of polyacetylene polymerization was made by Hideki Shirakawa ’s group who were able to prepare silvery films of polyacetylene. They discovered that the polymerization of polyacetylene could be achieved at the surface of a concentrated solution of the catalyst system of Et 3 Al and Ti(OBu) 4 in an inert solvent such as toluene. [ 6 ] In parallel with Shirakawa's studies, Alan Heeger and Alan MacDiarmid were studying the metallic properties of polythiazyl [(SN) x ], a related but inorganic polymer. [ 9 ] Polythiazyl caught Heeger's interest as a chain-like metallic material, and he collaborated with Alan MacDiarmid who had previous experience with this material. By the early 1970s, this polymer was known to be superconductive at low temperatures. [ 9 ] Shirakawa, Heeger, and MacDiarmid collaborated on further development of polyacetylene. [ 8 ]
Upon doping polyacetylene with I 2 , the conductivity increased seven orders of magnitude. [ 6 ] Similar results were achieved using Cl 2 and Br 2 . These materials exhibited the largest room temperature conductivity observed for a covalent organic polymer, and this seminal report was key in furthering the development of organic conductive polymers . [ 10 ] Further studies led to improved control of the cis / trans isomer ratio and demonstrated that cis -polyacetylene doping led to higher conductivity than doping of trans -polyacetylene. [ 6 ] Doping cis -polyacetylene with AsF 5 further increased the conductivities, bringing them close to that of copper. Furthermore, it was found that heat treatment of the catalyst used for polymerization led to films with higher conductivities. [ 11 ]
To account for such an increase in conductivity in polyacetylene, J. R. Schrieffer and Heeger considered the existence of topologically protected solitonic defects, their model is now known as the Su–Schrieffer–Heeger model , which has served as model in other contexts to understand topological insulators . [ 12 ]
A variety of methods have been developed to synthesize polyacetylene. One of the most common methods is via passing acetylene gas over a Ziegler–Natta catalyst , such as Ti(O i Pr) 4 / Al(C 2 H 5 ) 3 . This method allows control over the structure and properties of the final polymer by varying temperature and catalyst loading. [ 13 ] Mechanistic studies suggest that this polymerization involves metal insertion into the triple bond of acetylene. [ 14 ]
By varying the apparatus and catalyst loading, Shirakawa and coworkers were able to synthesize polyacetylene as thin films, rather than insoluble black powders. They obtained these films by coating the walls of a reaction flask under inert conditions with a solution of the Ziegler–Natta catalyst and adding gaseous acetylene resulting in immediate formation of a film. [ 15 ] Enkelmann and coworkers further improved polyacetylene synthesis by changing the catalyst to a Co(NO 3 ) 2 / NaBH 4 system, which was stable to both oxygen and water. [ 7 ]
Polyacetylene can also be produced by photopolymerization of acetylene. Glow-discharge , gamma , and ultraviolet irradiation have all been used. This method avoid the use of catalysts and solvents, but requires cryogenics to produce usable polymer. Gas-phase polymerization typically produces irregular cuprene, whereas liquid-phase polymerization, conducted at −78 °C produces linear cis -polyacetylene, and solid-phase polymerization, conducted at still lower temperature, produces trans -polyacetylene. [ 8 ]
Polyacetylene can be synthesized by ring-opening metathesis polymerisation (ROMP) from cyclooctatetraene , a precursor that is more expensive but easier to handle than the acetylene monomer . [ 16 ] This synthetic route also provides a means for introducing solubilizing groups to the polymer while maintaining the conjugation. [ 4 ] Polymers with linear groups such as n - octyl had high conductivity but low solubility, while highly branched tert - butyl groups increased solubility but decreased conjugation due to polymer twisting to avoid steric crowding. They obtained soluble and conductive polymers with sec -butyl and neopentyl groups, because the methylene (CH 2 ) unit directly connected to the polymer reduces steric crowding and prevents twisting. [ 4 ]
Polyacetylene can also be synthesized from other polymers. This method enables modification and processing of the polymer before conversion into the highly insoluble polyacetylene. Short, irregular segments of polyacetylene can be obtained by dehydrohalogenation of poly(vinyl chloride) : [ 17 ]
More efficient methos for synthesizing long polyacetylene chains exist and include the Durham precursor route in which precusor polymers are prepared by ring-opening metathesis polymerization, and a subsequent heat-induced reverse Diels–Alder reaction yields the final polymer, as well as volatile side products. [ 7 ]
When polyacetylene films are exposed to vapors of electron-accepting compounds ( p-type dopants ), the electrical conductivity of the material increases by orders of magnitude over the undoped material. [ 18 ] [ 19 ] p-Type dopants include Br 2 , I 2 , Cl 2 , and AsF 5 . These dopants act by abstracting an electron from the polymer chain. The conductivity of these polymers is believed to be a result of the creation of charge-transfer complexes between the polymer and halogen . [ 10 ] Charge transfer occurs from the polymer to the acceptor compound; the polyacetylene chain acts as a cation and the acceptor as an anion . The "hole" on the polymer backbone is weakly associated with the anionic acceptor by Coulomb potential . [ 18 ] Polyacetylene doped with ( p-type ) dopants retain their high conductivity even after exposure to air for several days. [ 8 ]
Electron-donating ( n-type ) dopants can also be used to create conductive polyacetylene. [ 19 ] n-Type dopants for polyacetylene include lithium, sodium, and potassium. [ 8 ] As with p-type dopants, charge-transfer complexes are created, where the polymer backbone is anionic and the donor is cationic . The increase in conductivity upon treatment with an n-type dopant is not as significant as those achieved upon treatment with a p-type dopant. Polyacetylene chains doped with n-type dopants are extremely sensitive to air and moisture. [ 8 ]
Polyacetylene can also be doped electrochemically. [ 19 ]
The conductivity of polyacetylene depends on structure and doping. Undoped trans -polyacetylene films have a conductivity of 4.4×10 −5 Ω −1 cm −1 , while cis -polyacetylene has a lower conductivity of 1.7×10 −9 Ω −1 cm −1 . [ 19 ] Doping with bromine causes an increase in conductivity to 0.5 Ω −1 cm −1 , while a higher conductivity of 38 Ω −1 cm −1 is obtained through doping with iodine. [ 10 ] Doping of either cis - or trans -polyacetylene leads to an increase in their conductivities by at least six orders of magnitude. Doped cis -polyacetylene films usually have conductivities two or three times greater than doped trans -polyacetylene even though the parent film has lower conductivity. [ 20 ]
The structure of polyacetylene films have been examined by both infrared spectroscopy [ 21 ] and Raman spectroscopy , [ 22 ] and found that the structure depends on synthetic conditions. When the synthesis is performed below −78 °C, the cis form predominates, while above 150 °C the trans form is favored. At room temperature, the polymerization yields a ratio of 60:40 cis : trans . [ 20 ] Films containing the cis form appear coppery, while the trans form is silvery. [ 20 ] Films of cis -polyacetylene are very flexible and can be readily stretched, while trans -polyacetylene is much more brittle.
The synthesis and processing of polyacetylene films affects the properties. Increasing the catalyst ratio creates thicker films with a greater draw ratio, allowing them to be stretched further. [ 8 ] Lower catalyst loadings leads to the formation of dark red gels , which can be converted to films by cutting and pressing between glass plates. [ 20 ] A foam-like material can be obtained from the gel by displacing the solvent with benzene , then freezing and subliming the benzene. [ 8 ] Polyacetylene has a bulk density of 0.4 g/cm 3 , while density of the foam is significantly lower, at 0.02–0.04 g/cm 3 . [ 8 ] The morphology consists of fibrils , with an average width of 200 Å. These fibrils form an irregular, web-like network, with some cross-linking between chains. [ 8 ] The insolubility of polyacetylene makes it difficult to characterize this material and to determine the extent of cross-linking in the material.
For applications, polyacetylenes suffer from many drawbacks. They are insoluble in solvents, making it essentially impossible to process the material. While both cis and trans -polyacetylene show high thermal stability, [ 20 ] exposure to air causes a large decrease in the flexibility and conductivity. [ 8 ] When polyacetylene is exposed to air, oxidation of the backbone by O 2 occurs. Infrared spectroscopy shows formation of carbonyl groups, epoxides , and peroxides . [ 8 ] [ 23 ] Coating with polyethylene or wax can slow the oxidation temporarily, while coating with glass increases stability indefinitely. [ 8 ]
Polyacetylene has no commercial applications, although the discovery of polyacetylene as a conductive organic polymer led to many developments in materials science. Conducting polymers are of interest for solution-processing for film-forming conductive polymers. [ 6 ] Therefore, attention has shifted to other conductive polymers for application purposes including polythiophene and polyaniline . Molecular electronics could also be a potential application of conductive polyacetylene. | https://en.wikipedia.org/wiki/Polyacetylene |
Polyacrylamide gel electrophoresis ( PAGE ) is a technique widely used in biochemistry , forensic chemistry , genetics , molecular biology and biotechnology to separate biological macromolecules , usually proteins or nucleic acids , according to their electrophoretic mobility . Electrophoretic mobility is a function of the length, conformation, and charge of the molecule. Polyacrylamide gel electrophoresis is a powerful tool used to analyze RNA samples. When polyacrylamide gel is denatured after electrophoresis, it provides information on the sample composition of the RNA species. [ 1 ]
Hydration of acrylonitrile results in formation of acrylamide molecules ( C 3 H 5 NO ) by nitrile hydratase . [ 2 ] Acrylamide monomer is in a powder state before addition of water. Acrylamide is toxic to the human nervous system, therefore all safety measures must be followed when working with it. Acrylamide is soluble in water and upon addition of free-radical initiators it polymerizes resulting in formation of polyacrylamide. [ 2 ] It is useful to make polyacrylamide gel via acrylamide hydration because pore size can be regulated. Increased concentrations of acrylamide result in decreased pore size after polymerization. Polyacrylamide gel with small pores helps to examine smaller molecules better since the small molecules can enter the pores and travel through the gel while large molecules get trapped at the pore openings.
As with all forms of gel electrophoresis , molecules may be run in their native state , preserving the molecules' higher-order structure. This method is called native PAGE. Alternatively, a chemical denaturant may be added to remove this structure and turn the molecule into an unstructured molecule whose mobility depends only on its length (because the protein-SDS complexes all have a similar mass-to-charge ratio). This procedure is called SDS-PAGE . Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) is a method of separating molecules based on the difference of their molecular weight. At the pH at which gel electrophoresis is carried out the SDS molecules are negatively charged and bind to proteins in a set ratio, approximately one molecule of SDS for every 2 amino acids. [ 3 ] : 164–79 In this way, the detergent provides all proteins with a uniform charge-to-mass ratio. By binding to the proteins the detergent destroys their secondary, tertiary and/or quaternary structure denaturing them and turning them into negatively charged linear polypeptide chains. When subjected to an electric field in PAGE, the negatively charged polypeptide chains travel toward the anode with different mobility. Their mobility, or the distance traveled by molecules, is inversely proportional to the logarithm of their molecular weight. [ 4 ] By comparing the relative ratio of the distance traveled by each protein to the length of the gel (Rf) one can make conclusions about the relative molecular weight of the proteins, where the length of the gel is determined by the distance traveled by a small molecule like a tracking dye. [ 5 ]
For nucleic acids, urea is the most commonly used denaturant. For proteins, sodium dodecyl sulfate (SDS) is an anionic detergent applied to protein samples to coat proteins in order to impart two negative charges (from every SDS molecule) to every two amino acids of the denatured protein. [ 3 ] : 161–3 2-Mercaptoethanol may also be used to disrupt the disulfide bonds found between the protein complexes, which helps further denature the protein. In most proteins, the binding of SDS to the polypeptide chains impart an even distribution of charge per unit mass, thereby resulting in a fractionation by approximate size during electrophoresis. Proteins that have a greater hydrophobic content – for instance, many membrane proteins, and those that interact with surfactants in their native environment – are intrinsically harder to treat accurately using this method, due to the greater variability in the ratio of bound SDS. [ 6 ] Procedurally, using both Native and SDS-PAGE together can be used to purify and to separate the various subunits of the protein. Native-PAGE keeps the oligomeric form intact and will show a band on the gel that is representative of the level of activity. SDS-PAGE will denature and separate the oligomeric form into its monomers, showing bands that are representative of their molecular weights. These bands can be used to identify and assess the purity of the protein. [ 3 ] : 161–3
Samples may be any material containing proteins or nucleic acids. These may be biologically derived, for example from prokaryotic or eukaryotic cells, tissues, viruses, environmental samples, or purified proteins. In the case of solid tissues or cells, these are often first broken down mechanically using a blender (for larger sample volumes), using a homogenizer (smaller volumes), by sonicator or by using cycling of high pressure, and a combination of biochemical and mechanical techniques – including various types of filtration and centrifugation – may be used to separate different cell compartments and organelles prior to electrophoresis. Synthetic biomolecules such as oligonucleotides may also be used as analytes.
The sample to analyze is optionally mixed with a chemical denaturant if so desired, usually SDS for proteins or urea for nucleic acids. SDS is an anionic detergent that denatures secondary and non–disulfide–linked tertiary structures, and additionally applies a negative charge to each protein in proportion to its mass. Urea breaks the hydrogen bonds between the base pairs of the nucleic acid, causing the constituent strands to separate. Heating the samples to at least 60 °C further promotes denaturation. [ 7 ] [ 8 ] [ 9 ] [ 10 ]
In addition to SDS, proteins may optionally be briefly heated to near boiling in the presence of a reducing agent, such as dithiothreitol (DTT) or 2-mercaptoethanol (beta-mercaptoethanol/BME) , which further denatures the proteins by reducing disulfide linkages, thus overcoming some forms of tertiary protein folding, and breaking up quaternary protein structure (oligomeric subunits). This is known as reducing SDS-PAGE.
A tracking dye may be added to the solution. This typically has a higher electrophoretic mobility than the analytes to allow the experimenter to track the progress of the solution through the gel during the electrophoretic run.
The gels typically consist of acrylamide , bisacrylamide , the optional denaturant (SDS or urea), and a buffer with an adjusted pH. The solution may be degassed under a vacuum to prevent the formation of air bubbles during polymerization. Alternatively, butanol may be added to the resolving gel (for proteins) after it is poured, as butanol removes bubbles and makes the surface smooth. [ 11 ] A source of free radicals and a stabilizer, such as ammonium persulfate and TEMED are added to initiate polymerization. [ 12 ] The polymerization reaction creates a gel because of the added bisacrylamide , which can form cross-links between two acrylamide molecules. The ratio of bisacrylamide to acrylamide can be varied for special purposes, but is generally about 1 part in 35. The acrylamide concentration of the gel can also be varied, generally in the range from 5% to 25%. Lower percentage gels are better for resolving very high molecular weight molecules, while much higher percentages of acrylamide are needed to resolve smaller proteins. The average pore diameter of polyacrylamide gels is determined by the total concentration of acrylamides (% T with T = Total concentration of acrylamide and bisacrylamide) and the concentration of the cross-linker bisacrylamide (%C with C = bisacrylamide concentration). [ 13 ] The pore size is reduced reciprocally to the %T. Concerning %C, a concentration of 5% produces the smallest pores, since the influence of bisacrylamide on the pore size has a parabola -shape with a vertex at 5%.
Gels are usually polymerized between two glass plates in a gel caster, with a comb inserted at the top to create the sample wells. After the gel is polymerized the comb can be removed and the gel is ready for electrophoresis.
Various buffer systems are used in PAGE depending on the nature of the sample and the experimental objective. The buffers used at the anode and cathode may be the same or different. [ 9 ] [ 14 ] [ 15 ]
An electric field is applied across the gel, causing the negatively charged proteins or nucleic acids to migrate across the gel away from the negative electrode (which is the cathode being that this is an electrolytic rather than galvanic cell) and towards the positive electrode (the anode). Depending on their size, each biomolecule moves differently through the gel matrix: small molecules more easily fit through the pores in the gel, while larger ones have more difficulty. The gel is run usually for a few hours, though this depends on the voltage applied across the gel; migration occurs more quickly at higher voltages, but these results are typically less accurate than at those at lower voltages. After the set amount of time, the biomolecules have migrated different distances based on their size. Smaller biomolecules travel farther down the gel, while larger ones remain closer to the point of origin. Biomolecules may therefore be separated roughly according to size, which depends mainly on molecular weight under denaturing conditions, but also depends on higher-order conformation under native conditions. The gel mobility is defined as the rate of migration traveled with a voltage gradient of 1V/cm and has units of cm 2 /sec/V. [ 3 ] : 161–3 For analytical purposes, the relative mobility of biomolecules, R f , the ratio of the distance the molecule traveled on the gel to the total travel distance of a tracking dye is plotted versus the molecular weight of the molecule (or sometimes the log of MW, or rather the M r , molecular radius). Such typically linear plots represent the standard markers or calibration curves that are widely used for the quantitative estimation of a variety of biomolecular sizes. [ 3 ] : 161–3
Certain glycoproteins , however, behave anomalously on SDS gels. Additionally, the analysis of larger proteins ranging from 250,000 to 600,000 Da is also reported to be problematic due to the fact that such polypeptides move improperly in the normally used gel systems. [ 16 ]
Following electrophoresis, the gel may be stained (for proteins, most commonly with Coomassie brilliant blue R-250 or autoradiography; for nucleic acids, ethidium bromide ; or for either, silver stain ), allowing visualization of the separated proteins, or processed further (e.g. Western blot ). After staining, different species biomolecules appear as distinct bands within the gel. It is common to run molecular weight size markers of known molecular weight in a separate lane in the gel to calibrate the gel and determine the approximate molecular mass of unknown biomolecules by comparing the distance traveled relative to the marker.
For proteins, SDS-PAGE is usually the first choice as an assay of purity due to its reliability and ease. The presence of SDS and the denaturing step make proteins separate, approximately based on size, but aberrant migration of some proteins may occur. Different proteins may also stain differently, which interferes with quantification by staining. PAGE may also be used as a preparative technique for the purification of proteins. For example, preparative native PAGE is a method for separating native metalloproteins in complex biological matrices.
Polyacrylamide gel (PAG) had been known as a potential embedding medium for sectioning tissues as early as 1964, and two independent groups employed PAG in electrophoresis in 1959. [ 17 ] [ 18 ] It possesses several electrophoretically desirable features that make it a versatile medium. It is a synthetic, thermo-stable, transparent, strong, chemically relatively inert gel, and can be prepared with a wide range of average pore sizes. [ 19 ] The pore size of a gel and the reproducibility in gel pore size are determined by three factors, the total amount of acrylamide present (%T) (T = Total concentration of acrylamide and bisacrylamide monomer), the amount of cross-linker (%C) (C = bisacrylamide concentration), and the time of polymerization of acrylamide (cf. QPNC-PAGE). Pore size decreases with increasing %T; with cross-linking, 5%C gives the smallest pore size. Any increase or decrease in %C from 5% increases the pore size, as pore size with respect to %C is a parabolic function with vertex as 5%C. This appears to be because of non-homogeneous bundling of polymer strands within the gel. This gel material can also withstand high voltage gradients, is amenable to various staining and destaining procedures, and can be digested to extract separated fractions or dried for autoradiography and permanent recording.
Polyacrylamide gels are composed of a stacking gel and separating gel. Stacking gels have a higher porosity relative to the separating gel, and allow for proteins to migrate in a concentrated area. Additionally, stacking gels usually have a pH of 6.8, since the neutral glycine molecules allow for faster protein mobility. Separating gels have a pH of 8.8, where the anionic glycine slows down the mobility of proteins. Separating gels allow for the separation of proteins and have a relatively lower porosity. Here, the proteins are separated based on size (in SDS-PAGE) and size/ charge (Native PAGE). [ 20 ]
Chemical buffer stabilizes the pH value to the desired value within the gel itself and in the electrophoresis buffer. The choice of buffer also affects the electrophoretic mobility of the buffer counterions and thereby the resolution of the gel. The buffer should also be unreactive and not modify or react with most proteins. Different buffers may be used as cathode and anode buffers, respectively, depending on the application. Multiple pH values may be used within a single gel, for example in DISC electrophoresis. Common buffers in PAGE include Tris , Bis-Tris, or imidazole .
Counterion balance the intrinsic charge of the buffer ion and also affect the electric field strength during electrophoresis. Highly charged and mobile ions are often avoided in SDS-PAGE cathode buffers, but may be included in the gel itself, where it migrates ahead of the protein. In applications such as DISC SDS-PAGE the pH values within the gel may vary to change the average charge of the counterions during the run to improve resolution. Popular counterions are glycine and tricine . Glycine has been used as the source of trailing ion or slow ion because its pKa is 9.69 and mobility of glycinate are such that the effective mobility can be set at a value below that of the slowest known proteins of net negative charge in the pH range. The minimum pH of this range is approximately 8.0.
Acrylamide ( C 3 H 5 NO ; mW: 71.08) when dissolved in water, slow, spontaneous auto polymerization of acrylamide takes place, joining molecules together by head on tail fashion to form long single-chain polymers. The presence of a free radical -generating system greatly accelerates polymerization. This kind of reaction is known as vinyl addition polymerisation . A solution of these polymer chains becomes viscous but does not form a gel, because the chains simply slide over one another. Gel formation requires linking various chains together. Acrylamide is carcinogenic , [ 21 ] a neurotoxin , and a reproductive toxin. [ 22 ] It is also essential to store acrylamide in a cool dark and dry place to reduce autopolymerisation and hydrolysis .
Bisacrylamide ( N , N ′-Methylenebisacrylamide ) ( C 7 H 10 N 2 O 2 ; mW: 154.17) is the most frequently used cross linking agent for polyacrylamide gels. Chemically it can be thought of as two acrylamide molecules coupled head to head at their non-reactive ends. Bisacrylamide can crosslink two polyacrylamide chains to one another, thereby resulting in a gel.
Sodium dodecyl sulfate (SDS) ( C 12 H 25 NaO 4 S ; mW: 288.38) (only used in denaturing protein gels) is a strong detergent agent used to denature native proteins to individual polypeptides . This denaturation, which is referred to as reconstructive denaturation, is not accomplished by the total linearization of the protein, but instead, through a conformational change to a combination of random coil and α helix secondary structures. [ 6 ] When a protein mixture is heated to 100 °C in presence of SDS, the detergent wraps around the polypeptide backbone. It binds to polypeptides in a constant weight ratio of 1.4 g SDS/g of polypeptide. In this process, the intrinsic charges of polypeptides become negligible when compared to the negative charges contributed by SDS. Thus polypeptides after treatment become rod-like structures possessing a uniform charge density, that is same net negative charge per unit weight. The electrophoretic mobilities of these proteins is a linear function of the logarithms of their molecular weights. Without SDS, different proteins with similar molecular weights would migrate differently due to differences in mass-charge ratio, as each protein has an isoelectric point and molecular weight particular to its primary structure . This is known as native PAGE . Adding SDS solves this problem, as it binds to and unfolds the protein, giving a near uniform negative charge along the length of the polypeptide.
Urea ( CO(NH 2 ) 2 ; mW: 60.06) is a chaotropic agent that increases the entropy of the system by interfering with intramolecular interactions mediated by non- covalent forces such as hydrogen bonds and van der Waals forces . Macromolecular structure is dependent on the net effect of these forces, therefore it follows that an increase in chaotropic solutes denatures macromolecules,
Ammonium persulfate (APS) ( N 2 H 8 S 2 O 8 ; mW: 228.2) is a source of free radicals and is often used as an initiator for gel formation. An alternative source of free radicals is riboflavin , which generated free radicals in a photochemical reaction.
TEMED ( N , N , N ′, N ′-tetramethylethylenediamine) ( C 6 H 16 N 2 ; mW: 116.21) stabilizes free radicals and improves polymerization. The rate of polymerisation and the properties of the resulting gel depend on the concentrations of free radicals. Increasing the amount of free radicals results in a decrease in the average polymer chain length, an increase in gel turbidity and a decrease in gel elasticity. Decreasing the amount shows the reverse effect. The lowest catalytic concentrations that allow polymerisation in a reasonable period of time should be used. APS and TEMED are typically used at approximately equimolar concentrations in the range of 1 to 10 mM.
The following chemicals and procedures are used for processing of the gel and the protein samples visualized in it.
Tracking dye; as proteins and nucleic acids are mostly colorless, their progress through the gel during electrophoresis cannot be easily followed. Anionic dyes of a known electrophoretic mobility are therefore usually included in the PAGE sample buffer. A very common tracking dye is Bromophenol blue (BPB, 3',3",5',5" tetrabromophenolsulfonphthalein). This dye is coloured at alkali and neutral pH and is a small negatively charged molecule that moves towards the anode . Being a highly mobile molecule it moves ahead of most proteins. As it reaches the anodic end of the electrophoresis medium electrophoresis is stopped. It can weakly bind to some proteins and impart a blue colour. Other common tracking dyes are xylene cyanol , which has lower mobility, and Orange G , which has a higher mobility.
Loading aids; most PAGE systems are loaded from the top into wells within the gel. To ensure that the sample sinks to the bottom of the gel, sample buffer is supplemented with additives that increase the density of the sample. These additives should be non-ionic and non-reactive towards proteins to avoid interfering with electrophoresis. Common additives are glycerol and sucrose .
Coomassie brilliant blue R-250 (CBB)( C 45 H 44 N 3 NaO 7 S 2 ; mW: 825.97) is the most popular protein stain. It is an anionic dye, which non-specifically binds to proteins. The structure of CBB is predominantly non-polar, and it is usually used in methanolic solution acidified with acetic acid. Proteins in the gel are fixed by acetic acid and simultaneously stained. The excess dye incorporated into the gel can be removed by destaining with the same solution without the dye. The proteins are detected as blue bands on a clear background. As SDS is also anionic, it may interfere with staining process. Therefore, large volume of staining solution is recommended, at least ten times the volume of the gel.
Ethidium bromide (EtBr) is a popular nucleic acid stain. EtBr allows one to easily visualize DNA or RNA on a gel as EtBr fluoresces an orange color under UV light. [ 23 ] Ethidium bromide binds nucleic acid chains through the process of Intercalation. [ 3 ] While Ethidium bromide is a popular stain it is important to exercise caution when using EtBr as it is a known carcinogen . Because of this fact, many researchers opt to use stains such as SYBR Green and SYBR Safe which are safer alternatives to EtBr. [ 24 ] EtBr is used by simply adding it to the gel mixture. Once the gel has run, the gel may be viewed through the use of a photo-documentation system. [ 3 ]
Silver staining is used when more sensitive method for detection is needed, as classical Coomassie Brilliant Blue staining can usually detect a 50 ng protein band, Silver staining increases the sensitivity typically 10-100 fold more. This is based on the chemistry of photographic development. The proteins are fixed to the gel with a dilute methanol solution, then incubated with an acidic silver nitrate solution. Silver ions are reduced to their metallic form by formaldehyde at alkaline pH. An acidic solution, such as acetic acid stops development. [ 25 ] Silver staining was introduced by Kerenyi and Gallyas as a sensitive procedure to detect trace amounts of proteins in gels . [ 26 ] The technique has been extended to the study of other biological macromolecules that have been separated in a variety of supports. [ 27 ] Many variables can influence the colour intensity and every protein has its own staining characteristics; clean glassware, pure reagents and water of highest purity are the key points to successful staining. [ 28 ] Silver staining was developed in the 14th century for colouring the surface of glass. It has been used extensively for this purpose since the 16th century. The colour produced by the early silver stains ranged between light yellow and an orange-red. Camillo Golgi perfected the silver staining for the study of the nervous system . Golgi's method stains a limited number of cells at random in their entirety. [ 29 ]
Autoradiography, also used for protein band detection post gel electrophoresis, uses radioactive isotopes to label proteins, which are then detected by using X-ray film. [ 30 ]
Western blotting is a process by which proteins separated in the acrylamide gel are electrophoretically transferred to a stable, manipulable membrane such as a nitrocellulose , nylon , or PVDF membrane. It is then possible to apply immunochemical techniques to visualise the transferred proteins, as well as accurately identify relative increases or decreases of the protein of interest. | https://en.wikipedia.org/wiki/Polyacrylamide_gel_electrophoresis |
Poly(acrylic acid) ( PAA ; trade name Carbomer ) is a polymer with the formula (CH 2 −CHCO 2 H) n . It is a derivative of acrylic acid (CH 2 =CHCO 2 H). In addition to the homopolymers , a variety of copolymers and crosslinked polymers, and partially deprotonated derivatives thereof, are known and of commercial value. In a water solution at neutral pH , PAA is an anionic polymer , i.e., many of the side chains of PAA lose their protons and acquire a negative charge. Partially or wholly deprotonated PAAs are polyelectrolytes , with the ability to absorb and retain water and swell to many times their original volume. These properties – acid–base and water-attracting – are the basis of many applications.
PAA, like any acrylate polymer , is usually synthesized through a process known as free radical polymerization, [ 3 ] [ 4 ] though graft polymerization may also be used. [ 5 ] [ 6 ] Free radical polymerization involves the conversion of monomers, in this case, acrylic acid (CH 2 =CHCO 2 H), into a polymer chain through the action of free radicals. [ 7 ] [ 8 ] [ 9 ] The process typically follows these steps: [ 10 ]
The global market was estimated to be worth $3.4 billion in 2022. [ 15 ] [ 16 ]
Polyacrylic acid is a weak anionic polyelectrolyte, whose degree of ionisation is dependent on solution pH. In its non-ionised form at low pHs, PAA may associate with various non-ionic polymers (such as polyethylene oxide, poly-N-vinyl pyrrolidone, polyacrylamide, and some cellulose ethers) and form hydrogen-bonded interpolymer complexes . [ 17 ] In aqueous solutions PAA can also form polycomplexes with oppositely charged polymers such as chitosan, surfactants, and drug molecules (for example, streptomycin). [ 18 ]
Dry PAAs are sold as white, fluffy powders.
In the dry powder form of sodium polyacrylate , the positively charged sodium ions are bound to the polyacrylate , however, in aqueous solutions the sodium ions can dissociate. The presence of sodium cations allows the polymer to absorb a high amount of water.
PAA is widely used in dispersants. Its molecular weight has a significant impact on the rheological properties and dispersion capacity, and hence applications. The dominant application for PAA is as a superabsorbent . About 25% of PAA is used for detergents and dispersants.
Polyacrylic acid and its derivatives (particularly sodium polyacrylate ) are used in disposable diapers . Acrylic acid is also the main component of Superabsorbent Polymers (SAPs), which are cross-linked polyacrylates that can absorb and retain more than 100 times of their own weight in liquid. The US Food and Drug Administration authorised the use of SAPs in packaging with indirect food contact. [ 19 ] [ 20 ]
Detergents often contain copolymers of acrylic acid that assist in sequestering dirt. Cross-linked polyacrylic acid has also been used in the production of household products, including floor cleaners.
PAA may inactivate the antiseptic chlorhexidine gluconate . [ 21 ]
The neutralized polyacrylic acid gels are suitable biocompatible matrices for medical applications such as gels for skin care products. PAA films can be deposited on orthopaedic implants to protect them from corrosion. Crosslinked hydrogels of PAA and gelatin have also been used as medical glue.
Other applications involve paints and cosmetics . They stabilize suspended solid in liquids, [ 22 ] prevent emulsions from separating, and control the consistency in flow of cosmetics. Carbomer codes (910, 934, 940, 941, and 934P) are an indication of molecular weight and the specific components of the polymer. For many applications PAAs are used in form of alkali metal or ammonium salts, e.g. sodium polyacrylate .
Hydrogels derived from PAA have attracted much study for use as bandages and aids for wound healing. [ 23 ]
A few reports were made on PAA use as deflocculant (so called alkaline polyacrylates ) for oil drilling industry. [ 24 ] [ 25 ]
It was also reported to be used for metal quenching in metalworking (see Sodium polyacrylate ). [ 26 ] | https://en.wikipedia.org/wiki/Polyacrylic_acid |
Polyaddition A polymerization in which the growth of polymer chains proceeds by addition reactions between molecules of all degrees of polymerization.
Notes:
1. The growth steps are expressed by:
P x +P y → P x+y {x}∈{1,2,…∞};{y}∈{1,2,…∞}
where P x and P y denote chains of degrees of polymerization x and y, respectively.
2. The earlier term 'addition polymerization' embraced both the current concepts of 'polyaddition' and 'chain polymerization', but did not include 'condensative chain polymerization'. [ 1 ]
Polyaddition (or addition polymerisation [ 2 ] [ 3 ] ) is a polymerization reaction that forms polymers via individual independent addition reactions . Polyaddition occurs as a reaction between functional groups on molecules with low degrees of polymerization, such as dimers, trimers and oligomers, to form species of higher molar mass. Only at nearly complete conversions does the polymer form, as in polycondensation and in contrast to chain polymerization . [ 4 ]
A typical polyaddition is the formation of a polyurethane .
This article about polymer science is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Polyaddition |
In differential geometry and mathematical physics (especially string theory ), the Polyakov formula expresses the conformal variation of the zeta functional determinant of a Riemannian manifold . Proposed by Alexander Markovich Polyakov this formula arose in the study of the quantum theory of strings. The corresponding density is local, and therefore is a Riemannian curvature invariant . In particular, whereas the functional determinant itself is prohibitively difficult to work with in general, its conformal variation can be written down explicitly.
This geometry-related article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Polyakov_formula |
In quantum field theory , the Polyakov loop is the thermal analogue of the Wilson loop , acting as an order parameter for confinement in pure gauge theories at nonzero temperatures . In particular, it is a Wilson loop that winds around the compactified Euclidean temporal direction of a thermal quantum field theory . It indicates confinement because its vacuum expectation value must vanish in the confined phase due to its non-invariance under center gauge transformations. This also follows from the fact that the expectation value is related to the free energy of individual quarks , which diverges in this phase. Introduced by Alexander M. Polyakov in 1975, [ 1 ] they can also be used to study the potential between pairs of quarks at nonzero temperatures.
Thermal quantum field theory is formulated in Euclidean spacetime with a compactified imaginary temporal direction of length β {\displaystyle \beta } . This length corresponds to the inverse temperature of the field β ∝ 1 / T {\displaystyle \beta \propto 1/T} . Compactification leads to a special class of topologically nontrivial Wilson loops that wind around the compact direction known as Polyakov loops. [ 2 ] In SU ( N ) {\displaystyle {\text{SU}}(N)} theories a straight Polyakov loop on a spatial coordinate x {\displaystyle {\boldsymbol {x}}} is given by
Φ ( x ) = 1 N tr P exp [ ∫ 0 β d x 4 A 4 ( x , x f ) ] , {\displaystyle \Phi ({\boldsymbol {x}})={\frac {1}{N}}{\text{tr}}\ {\mathcal {P}}\exp {\bigg [}\int _{0}^{\beta }dx_{4}A_{4}({\boldsymbol {x}},x_{f}){\bigg ]},}
where P {\displaystyle {\mathcal {P}}} is the path-ordering operator and A 4 {\displaystyle A_{4}} is the Euclidean temporal component of the gauge field. In lattice field theory this operator is reformulated in terms of temporal link fields U 4 ( m , j ) {\displaystyle U_{4}({\boldsymbol {m}},j)} at a spatial position m {\displaystyle {\boldsymbol {m}}} as [ 3 ]
The continuum limit of the lattice must be taken carefully to ensure that the compact direction has fixed extent. This is done by ensuring that the finite number of temporal lattice points N T {\displaystyle N_{T}} is such that β = N T a {\displaystyle \beta =N_{T}a} is constant as the lattice spacing a {\displaystyle a} goes to zero.
Gauge fields need to satisfy the periodicity condition A μ ( x , x 4 + β ) = A μ ( x , x 4 ) {\displaystyle A_{\mu }({\boldsymbol {x}},x_{4}+\beta )=A_{\mu }({\boldsymbol {x}},x_{4})} in the compactified direction. Meanwhile, gauge transformations only need to satisfy this up to a group center term h {\displaystyle h} as Ω ( x , x 4 + β ) = h Ω ( x , x 4 ) {\displaystyle \Omega ({\boldsymbol {x}},x_{4}+\beta )=h\Omega ({\boldsymbol {x}},x_{4})} . A change of basis can always diagonalize this so that h = z I {\displaystyle h=zI} for a complex number z {\displaystyle z} . The Polyakov loop is topologically nontrivial in the temporal direction so unlike other Wilson loops it transforms as Φ ( x ) → z Φ ( x ) {\displaystyle \Phi ({\boldsymbol {x}})\rightarrow z\Phi ({\boldsymbol {x}})} under these transformations. [ 5 ] Since this makes the loop gauge dependent for z ≠ 1 {\displaystyle z\neq 1} , by Elitzur's theorem non-zero expectation values of ⟨ Φ ⟩ {\displaystyle \langle \Phi \rangle } imply that the center group must be spontaneously broken , implying confinement in pure gauge theory. This makes the Polyakov loop an order parameter for confinement in thermal pure gauge theory, with a confining phase occurring when ⟨ Φ ⟩ = 0 {\displaystyle \langle \Phi \rangle =0} and deconfining phase when ⟨ Φ ⟩ ≠ 0 {\displaystyle \langle \Phi \rangle \neq 0} . [ 6 ] For example, lattice calculations of quantum chromodynamics with infinitely heavy quarks that decouple from the theory shows that the deconfinement phase transition occurs at around a temperature of 270 {\displaystyle 270} MeV. [ 7 ] Meanwhile, in a gauge theory with quarks, these break the center group and so confinement must instead be deduced from the spectrum of asymptotic states, the color neutral hadrons .
For gauge theories that lack a nontrivial group center that could be broken in the confining phase, the Polyakov loop expectation values are nonzero even in this phase. They are however still a good indicator of confinement since they generally experience a sharp jump at the phase transition . This is the case for example in the Higgs model with the exceptional gauge group G 2 {\displaystyle G_{2}} . [ 8 ]
The Nambu–Jona-Lasinio model lacks local color symmetry and thus cannot capture the effects of confinement. However, Polyakov loops can be used to construct the Polyakov-loop-extended Nambu–Jona-Lasinio model which treats both the chiral condensate and the Polyakov loops as classical homogeneous fields that couple to quarks according to the symmetries and symmetry breaking patters of quantum chromodynamics. [ 9 ] [ 10 ] [ 11 ]
The free energy F {\displaystyle F} of N {\displaystyle N} quarks and N ¯ {\displaystyle {\bar {N}}} antiquarks , subtracting out the vacuum energy , is given in terms of the correlation functions of Polyakov loops [ 12 ]
This free energy is another way to see that the Polyakov loop acts as an order parameter for confinement since the free energy of a single quark is given by e − β Δ F = ⟨ Φ ( x ) ⟩ {\displaystyle e^{-\beta \Delta F}=\langle \Phi ({\boldsymbol {x}})\rangle } . [ 13 ] Confinement of quarks means that it would take an infinite amount of energy to create a configuration with a single free quark, therefore its free energy must be infinite and so the Polyakov loop expectation value must vanish in this phase, in agreement with the center symmetry breaking argument.
The formula for the free energy can also be used to calculate the potential between a pair of infinitely massive quarks spatially separated by r = | x 1 − x 2 | {\displaystyle r=|{\boldsymbol {x}}_{1}-{\boldsymbol {x}}_{2}|} . Here the potential V ( r ) {\displaystyle V(r)} is the first term in the free energy, so that the correlation function of two Polyakov loops is
where Δ E {\displaystyle \Delta E} is the energy difference between the potential and the first excited state . In the confining phase the potential is linear V ( r ) = σ r {\displaystyle V(r)=\sigma r} , where the constant of proportionality is known as the string tension. The string tension acquired from the Polyakov loop is always bounded from above by the string tension acquired from the Wilson loop. [ 14 ] | https://en.wikipedia.org/wiki/Polyakov_loop |
Polyalkylimide is a polymer whose structure contains no free monomers . It is used in permanent dermal fillers to treat soft tissue deficits such as facial lipoatrophy , [ 1 ] gluteal atrophy, acne , and scars . [ 2 ]
In plastic and reconstructive surgery it is used for building facial volume in the cheeks , chin , jaw , and lips . Reports of infections [ 3 ] and migration of polyalkylimide in the face [ 4 ] has led Canada to remove it from the market, and the manufacturer of Biolcamid ceasing production. A class action lawsuit was filed against the company.
This article about polymer science is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Polyalkylimide |
Polyaminopropyl biguanide ( PAPB ) is a polymer containing biguanide group connected with a three methylene (propyl) linker. The polymer is a propyl analogue of polyhexamethylene biguanide . The polymer display some antibacterial activity however much lower than PHMB. [ 1 ] As of May 2024, PAPB is not approved as a biocidal active substance under EU regulations. [ 2 ]
In some sources, particularly in lists of cosmetic ingredients ( INCI ), the name polyaminopropyl biguanide is wrongly associated with polyhexamethylene biguanide (PHMB) [ 3 ] [ 4 ] | https://en.wikipedia.org/wiki/Polyaminopropyl_biguanide |
Polyamorphism is the ability of a substance to exist in several different amorphous modifications. It is analogous to the polymorphism of crystalline materials. Many amorphous substances can exist with different amorphous characteristics (e.g. polymers). However, polyamorphism requires two distinct amorphous states with a clear, discontinuous (first-order) phase transition between them. When such a transition occurs between two stable liquid states, a polyamorphic transition may also be referred to as a liquid–liquid phase transition . [ 3 ]
Even though amorphous materials exhibit no long-range periodic atomic ordering, there is still significant and varied local structure at inter-atomic length scales (see structure of liquids and glasses ). Different local structures can produce amorphous phases of the same chemical composition with different physical properties such as density . In several cases sharp transitions have been observed between two different density amorphous states of the same material. Amorphous ice is one important example (see also examples below). [ 4 ] Several of these transitions (including water) are expected to end in a second critical point .
Polyamorphism may apply to all amorphous states, i.e. glasses, other amorphous solids, supercooled liquids, ordinary liquids or fluids. A liquid–liquid transition however, is one that occurs only in the liquid state (red line in the phase diagram, top right). In this article liquid–liquid transitions are defined as transitions between two liquids of the same chemical substance. Elsewhere the term liquid–liquid transition may also refer to the more common transitions between liquid mixtures of different chemical composition.
The stable liquid state unlike most glasses and amorphous solids, is a thermodynamically stable equilibrium state. Thus new liquid–liquid or fluid-fluid transitions in the stable liquid (or fluid) states are more easily analysed than transitions in amorphous solids where arguments are complicated by the non-equilibrium, non-ergodic nature of the amorphous state.
Liquid–liquid transitions were originally considered by Rapoport in 1967 in order to explain high pressure melting curve maxima of some liquid metals. [ 5 ] Rapoport's theory requires the existence of a melting curve maximum in polyamorphic systems.
One physical explanation for polyamorphism is the existence of a double well inter-atomic pair potential (see lower right diagram). It is well known that the ordinary liquid–gas critical point appears when the inter-atomic pair potential contains a minimum. At lower energies (temperatures) particles trapped in this minimum condense into the liquid state. At higher temperatures however, these particles can escape the well and the sharp definition between liquid and gas is lost. Molecular modelling has shown that addition of a second well produces an additional transition between two different liquids (or fluids) with a second critical point. [ 2 ]
Polyamorphism has been experimentally observed or theoretically suggested in silicon , liquid phosphorus , triphenyl phosphate , mannitol , and in some other molecular network -forming substances. [ 6 ]
The most famous case of polyamorphism is amorphous ice . Pressurizing conventional hexagonal ice crystals to about 1.6 GPa at liquid nitrogen temperature (77 K) converts them to the high-density amorphous ice. Upon releasing the pressure, this phase is stable and has density of 1.17 g/cm 3 at 77 K and 1 bar. Consequent warming to 127 K at ambient pressure transforms this phase to a low-density amorphous ice (0.94 g/cm 3 at 1 bar). [ 7 ] Yet, if the high-density amorphous ice is warmed up to 165 K not at low pressures but keeping the 1.6 GPa compression, and then cooled back to 77 K, then another amorphous ice is produced, which has even higher density of 1.25 g/cm 3 at 1 bar. All those amorphous forms have very different vibrational lattice spectra and intermolecular distances. [ 8 ] [ 9 ] A similar abrupt liquid-amorphous phase transition is predicted in liquid silicon when cooled under high pressures. [ 10 ] This observation is based on first principles molecular dynamics computer simulations, and might be expected intuitively since tetrahedral amorphous carbon , silicon, and germanium are known to be structurally analogous to water. [ 11 ]
Yttria - alumina melts are another system reported to exhibit polyamorphism. Observation of a liquid–liquid phase transition in the supercooled liquid has been reported. [ 12 ] Though this is disputed in the literature. [ 13 ] Polyamorphism has also been reported in Yttria-Alumina glasses. Yttria-Alumina melts quenched from about 1900 °C at a rate ~400 °C/s, can form glasses containing a second co-existing phase. This happens for certain Y/Al ratios (about 20–40 mol% Y 2 O 3 ). The two phases have the same average composition but different density, molecular structure and hardness. [ 14 ] However whether the second phase is glassy or crystalline is also debated. [ 15 ] Continuous changes in density were observed upon cooling silicon dioxide or germanium dioxide . Although continuous density changes do not constitute a first order transition, they may be indicative of an underlying abrupt transition.
Polyamorphism has also been observed in organic compounds, such as liquid triphenyl phosphite at temperatures between 210 K and 226 K [ 16 ] [ 17 ] [ 18 ] [ 19 ] and n -butanol at temperatures between 120 K and 140 K. [ 20 ] [ 21 ]
Polyamorphism is also an important area in pharmaceutical science. The amorphous form of a drug typically has much better aqueous solubility (compared to the analogous crystalline form) but the actual local structure in an amorphous pharmaceutical can be different, depending on the method used to form the amorphous phase.
Mannitol is the first pharmaceutical substance featuring polyamorphism. [ 22 ] In addition to the regular amorphous phase, a second amorphous phase can be prepared at room temperature and pressure. This new phase has substantially lower energy, lower density and higher glass transition temperature. Since mannitol is widely used in pharmaceutical tablet formulations, mannitol polyamorphism offers a powerful tool to engineer the property and behavior of tablets. [ 23 ] | https://en.wikipedia.org/wiki/Polyamorphism |
Polyampholytes are polymers that contain both positively charged (cationic) and negatively charged (anionic) functional groups within the same molecule. Their unique structure allows them to exhibit amphoteric behavior, meaning they can interact with a range of substances depending on the surrounding pH, making them useful in applications like drug delivery, water treatment, and biomaterials. [ 1 ]
Polyampholytes can exist as either linear water-soluble polyelectrolytes or as cross-linked structures. Weakly cross-linked polyampholytes swell in water, forming hydrogels . The swelling properties of these hydrogels are highly dependent on the solution pH and its relation to the polyampholyte’s isoelectric point .
The isoelectric point of polyampholytes is the pH at which the polymer exhibits no net charge, balancing its positive and negative charges. This point is important because it dictates the net charge of polyampholyte macromolecules at different pH levels. At a pH less than the isoelectric point, the macromolecules carry a positive charge, while at a pH greater than the isoelectric point, they acquire a negative charge. At pH equal to the isoelectric point, polyampholytes are neutral. Under these conditions, they may show minimal viscosity in solutions or lose solubility and precipitate. [ 2 ]
Proteins are a class of natural polyampholytes, as they contain both positively and negatively charged amino acid residues within their structure. These charges are influenced by the pH of the surrounding environment, which determines the overall charge of the protein. The presence of both acidic (anionic) and basic (cationic) residues allows proteins to interact with various charged species, making them versatile in biological processes.
Gelatin is a well-known example of a protein-derived polyampholyte. It is derived from collagen , a structural protein found in connective tissues, and contains both acidic (anionic) and basic (cationic) amino acid residues, making it capable of exhibiting amphoteric behavior. The unique combination of these charges allows gelatin to interact with a variety of substances, depending on the pH of the surrounding environment.
Synthetic polyampholytes have a range of potential applications. [ 3 ] They can adhere to mucosal surfaces, enhance drug retention and improve bioavailability by adjusting their charge at specific pH's. [ 4 ] In water treatment, polyampholytes act as flocculants. [ 5 ] In biomaterials , they are utilized in tissue engineering, wound dressings , and as scaffolds for cell growth, taking advantage of their biocompatibility and adjustable charge properties. Furthermore, polyampholytes serve as cryoprotectants in cryopreservation, stabilizing biological samples like cells and tissues during freezing by preventing ice crystal formation and reducing cellular damage. [ 6 ] Polyampholytes are potential stealth coatings, creating anti-fouling surfaces that resist biofilm formation. [ 7 ] | https://en.wikipedia.org/wiki/Polyampholytes |
Polyane is a French trademark for insulating and waterproof plastic films used in construction, registered in 1965.
Henri Piat , [ 1 ] a mechanical enthusiast, collaborated with the owner of Société Plastique Soudé , J.-E. Mazuyer and started a new company, Company of Transformation of Synthesis Products, later known as Prosyn . Due to high investment cost, early technical challenges and expensive import licenses, the company struggled. After some time, the company moved to the station Châteaucreux in Saint-Etienne. After a downturn, in 1964, the company Société Nationale des Pétroles d'Aquitaine (SNPA), part of the Rhône-Alpes CRRA Research Centre, [ 2 ] took a co-stake in Prosyn, with Lafarge . The two companies had different applications in mind; Lafarge aimed to improve the waterproofing of its cement bags, while SNPA aimed to develop and promote the use of polyethylene films. As part of these activities, Prosyn moved to Saint-Chamond .
After significant growth in the 60s and 70s, SNPA took over the entire capital of Prosyn and in the 1980s, the Société Nationale des Pétroles d'Aquitaine became the Elf Atochem group . Prosyn was then renamed to Prosyn Polyane, which combined with a marketing campaign led to the brand name, Polyane, in 1965 [ citation needed ] . Products in which Polyane is used include stretch film, garbage bags and printing on plastic film, among other more niche applications.
In 1999, Prosyn Polyane had profitability issues and Elf Atochem aimed to sell off the brand and facilities. A successful sale was made to the Luxembourgian industrial group Verdoso , which also held a majority of the capital of Société de Production de Films Plastiques (SPFP). Prosyn Polyane was integrated into this organization. This re-oriented Prosyn Polyane to focus on the manufacture of film for industry and agriculture.
The company was bought again in October 2001 by the Adelpro group, a specialist in agricultural film, which also held two former competitors: Addem Plastiques (Firminy, Loire) and the former agricultural division of the British group Autobar Flexible France, [ 3 ] [ 4 ] which was then called Deltalène (Sainte Sigolène, Haute-Loire). [ 5 ] Prosyn Polyane was subsequently renamed to Polyane. In July 2003, the group acquired Ribeyron (Sainte Sigolène, Haute-Loire). [ 6 ] [ 7 ]
The court of Saint-Étienne stopped activities of Adelpro, after it continued to struggle. Parts were sold to Ribeyron SAS, [ 8 ] but the court allowed Deltalène and Polyane to fuse to a new company that continued operations under the name Prolène. The company remained under observation by a judicial agent and was ordered to wait for the court's agreement for each of its purchases. It was unable to remove itself from the verge of bankruptcy and under consultation of the court, Plastika Kritis purchased the failing companies. [ 9 ] These were subsequently combined into a new company, Agripolyane , which holds the Polyane brand. The brand has since become widespread and is often used as a generic term to refer to specific types of plastics rather than the brand itself.
The Polyane trademark was registered in 1965. Since then, the company has changed its name from Prosyn Polyane to Agripolyane. It has also diversified its activity to include agriculture, construction, industry and geomembrane . The company wanted to protect its name by registering several dozen trademarks that it has created in its main markets, both nationally and internationally, with organizations specialized in intellectual property such as the INPI (France) or the OHIM (European Union). The company has also developed new products and filed patents to protect its innovations. | https://en.wikipedia.org/wiki/Polyane |
Polyanhydrides are a class of biodegradable polymers characterized by anhydride bonds that connect repeat units of the polymer backbone chain . Their main application is in the medical device and pharmaceutical industry. In vivo , polyanhydrides degrade into non-toxic diacid monomers that can be metabolized and eliminated from the body. Owing to their safe degradation products, polyanhydrides are considered to be biocompatible.
The characteristic anhydride bonds in polyanhydrides are water-labile (the polymer chain breaks apart at the anhydride bond). This results in two carboxylic acid groups which are easily metabolized and biocompatible . Biodegradable polymers , such as polyanhydrides, are capable of releasing physically entrapped or encapsulated drugs by well-defined kinetics and are a growing area of medical research. Polyanhydrides have been investigated as an important material for the short-term release of drugs or bioactive agents. The rapid degradation and limited mechanical properties of polyanhydrides render them ideal as controlled drug delivery devices.
One example, Gliadel [1] , is a device in clinical use for the treatment of brain cancer . This product is made of a polyanhydride wafer containing a chemotherapeutic agent. After removal of a cancerous brain tumor , the wafer is inserted into the brain releasing a chemotherapy agent at a controlled rate proportional to the degradation rate of the polymer . The localized treatment of chemotherapy protects the immune system from high levels of radiation .
Other applications of polyanhydrides include the use of unsaturated polyanhydrides in bone replacement, as well as polyanhydride copolymers as vehicles for vaccine delivery.
There are three main classes of polyanhydrides: aliphatic, unsaturated, and aromatic. These classes are determined by examining their R groups (the chemistry of the molecule between the anhydride bonds).
Aliphatic polyanhydrides consist of R groups containing carbon atoms bonded in straight or branched chains. This class of polymers is characterized by a crystalline structure, melting temperature range of 50–90 °C, and solubility in chlorinated hydrocarbons. They degrade and are eliminated from the body within weeks of being introduced to the bodily environment.
Unsaturated polyanhydrides consist of organic R groups with one or more double bonds (or degrees of unsaturation ). This class of polymers has a highly crystalline structure and is insoluble in common organic solvents.
Aromatic polyanhydrides consist of R groups containing a benzene (aromatic) ring. Properties of this class include a crystalline structure, insolubility in common organic solvents, and melting points greater than 100 °C. They are very hydrophobic and therefore degrade slowly when in the bodily environment. This slow degradation rate makes aromatic polyanhydrides less suitable for drug delivery when used as homopolymers, but they can be copolymerized with the aliphatic class to achieve the desired degradation rate.
Polyanhydrides are synthesized using either melt condensation or solution polymerization. Depending on the synthesis method used,
various characteristics of polyanhydrides can be altered to achieve the desired product. Characterization of polyanhydrides determines the structure, composition, molecular weight, and thermal properties of the molecule. These properties are determined by using various light-scattering and size-exclusion methods.
Polyanhydrides can be easily prepared by using available, low cost resources. The process can be varied to achieve desirable characteristics. Traditionally, polyanhydrides have been prepared by melt condensation polymerization, which results in high molecular weight polymers. Melt condensation polymerization involves reacting dicarboxylic acid monomers with excess acetic anhydride at a high temperature and under a vacuum to form the polymers. Catalysts may be used to achieve higher molecular weights and shorter reaction times. Generally, a one-step synthesis (method involving only one reaction) is used which does not require purification.
There are many other methods used to synthesize polyanhydrides. Some of the other methods include: microwave heating, high-throughput synthesis (synthesis of polymers in parallel), ring opening polymerization (removal of cyclic monomers), interfacial condensation (high temperature reaction of two monomers), dehydrative coupling agents (removing the water group from two carboxyl groups), and solution polymerization (reacting in a solution).
The chemical structure and composition of polyanhydrides can be determined using nuclear magnetic resonance (NMR) spectroscopy . The positions of peaks in proton NMR spectroscopy are determined by the class of polanhydride (aromatic, aliphatic, or unsaturated), and so provide information regarding structural features of the polymer, including whether a copolymer has a random or block-like structure. Molecular weight and degradation rate can also determined by spectroscopically.
Aside from using NMR to determine a polyanhydride’s molecular weight, gel permeation chromatography (GPC), and viscosity measurements may also be used.
Differential scanning calorimetry (DSC) is used to determine the thermal properties of polyanhydrides. Glass transition temperature, melting temperature, and heat of fusion can all be determined by DSC. Crystallinity of a polyanhydride can be determined using DSC, Small angle X-ray scattering (SAXS) , Nuclear magnetic resonance (NMR), and X-ray diffraction .
The erosion and degradation of a polymer describe how the polymer physically loses mass ( degrades ). The two common erosion mechanisms are surface and bulk erosion . Polyanhydrides are surface eroding polymers. Surface eroding polymers do not allow water to penetrate into the material. They erode layer by layer, like a lollipop. The hydrophobic backbone with hydrolytically labile anhydride linkages allows hydrolytic degradation to be controlled by manipulating the polymer composition. This manipulation can occur by adding a hydrophilic group to the polyanhydride to make a copolymer. Polyanhydride copolymers with hydrophilic groups exhibit bulk eroding characteristics. Bulk eroding polymers take in water like a sponge (throughout the material) and erode inside and on the surface of the polymer.
Drug release from bulk eroding polymers is difficult to characterize because the primary mode of release from these polymers is diffusion . Unlike surface eroding polymers, bulk eroding polymers show a very weak relationship between the rate of polymer degradation and the rate of drug release. Therefore, the development of surface eroding polyanhydrides incorporated into the bulk eroding polymers is of increased importance.
Biocompatibility and toxicity of a polymeric material is evaluated by examining systemic toxic responses, local tissue responses, carcinogenic and mutagenic responses, and allergic responses to the material's degradation products. Animal studies are conducted to test the polymer’s effect on each of these negative responses. Polyanhydrides and their degradation products have not been found to cause significant harmful responses and are considered to be biocompatible . | https://en.wikipedia.org/wiki/Polyanhydride |
Polyaniline ( PANI ) is a conducting polymer and organic semiconductor of the semi-flexible rod polymer family. The compound has been of interest since the 1980s because of its electrical conductivity and mechanical properties. Polyaniline is one of the most studied conducting polymers. [ 2 ] [ 3 ]
Polyaniline was discovered in the 19th century by F. Ferdinand Runge (1794–1867), Carl Fritzsche (1808–1871), John Lightfoot (1831–1872), and Henry
Letheby (1816–1876). [ 4 ] Lightfoot studied the oxidation of aniline, which had been isolated only 20 years previously. He developed the first commercially successful route to the dye called Aniline black . [ 5 ] [ 6 ] The first definitive report of polyaniline did not occur until 1862, which included an electrochemical method for the determination of small quantities of aniline. [ 7 ]
From the early 20th century on, occasional reports about the structure of PANI were published.
Polymerized from the inexpensive aniline , polyaniline can be found in one of three idealized oxidation states: [ 8 ]
In the figure, x equals half the degree of polymerization (DP). Leucoemeraldine with n = 1, m = 0 is the fully reduced state. Pernigraniline is the fully oxidized state (n = 0, m = 1) with imine links instead of amine links. Studies have shown that most forms of polyaniline are one of the three states or physical mixtures of these components. The emeraldine (n = m = 0.5) form of polyaniline, often referred to as emeraldine base (EB), is neutral, if doped (protonated) it is called emeraldine salt (ES), with the imine nitrogens protonated by an acid. Protonation helps to delocalize the otherwise trapped diiminoquinone-diaminobenzene state. Emeraldine base is regarded as the most useful form of polyaniline due to its high stability at room temperature and the fact that, upon doping with acid, the resulting emeraldine salt form of polyaniline is highly electrically conducting. [ 6 ] Leucoemeraldine and pernigraniline are poor conductors, even when doped with an acid.
The colour change associated with polyaniline in different oxidation states can be used in sensors and electrochromic devices . [ 9 ] Polyaniline sensors typically exploit changes in electrical conductivity between the different oxidation states or doping levels. [ 10 ] Treatment of emeraldine with acids increases the electrical conductivity by up to ten orders of magnitude. Undoped polyaniline has a conductivity of 6.28 × 10 −9 S/m, whereas conductivities of 4.60 × 10 −5 S/m can be achieved by doping to 4% HBr. [ 11 ] The same material can be prepared by oxidation of leucoemeraldine.
Although the synthetic methods to produce polyaniline are quite simple, the mechanism of polymerization is probably complex. The formation of leucoemeraldine can be described as follows, where [O] is a generic oxidant: [ 12 ]
A common oxidant is ammonium persulfate in 1 M hydrochloric acid (other acids can be used). The polymer precipitates as an unstable dispersion with micrometer-scale particulates.
(Per)nigraniline is prepared by oxidation of the emeraldine base with a peracid : [ 13 ]
Aniline can also be electrochemically polymerised directly onto conductive surfaces without the use of a chemical oxidant. [ 14 ] [ 15 ]
The synthesis of polyaniline nanostructures is facile. [ 16 ]
Using surfactant dopants, the polyaniline can be made dispersible and hence useful for practical applications. Bulk synthesis of polyaniline nanofibers has been researched extensively. [ 17 ]
A multi-stage model for the formation of emeraldine base is proposed. In the first stage of the reaction the pernigraniline PS salt oxidation state is formed. In the second stage pernigraniline is reduced to the emeraldine salt as aniline monomer gets oxidized to the radical cation . [ 8 ] In the third stage this radical cation couples with ES salt. This process can be followed by light scattering analysis which allows the determination of the absolute molar mass . According to one study in the first step a DP of 265 is reached with the DP of the final polymer at 319. Approximately 19% of the final polymer is made up of the aniline radical cation which is formed during the reaction. [ 18 ]
Polyaniline is typically produced in the form of long-chain polymer aggregates, surfactant (or dopant) stabilized nanoparticle dispersions, or stabilizer-free nanofiber dispersions depending on the supplier and synthetic route. Surfactant or dopant stabilized polyaniline dispersions have been available for commercial sale since the late 1990s. [ 19 ]
The major applications are printed circuit board manufacturing: final finishes, used in millions of m 2 every year, antistatic and ESD coatings , and corrosion protection. [ 5 ] [ 19 ] Polyaniline and its derivatives are also used as the precursor for the production of N-doped carbon materials through high-temperature heat treatment. [ 20 ] Printed emeraldine polyaniline-based sensors have also gained much attention for widespread applications where devices are typically fabricated via screen, inkjet [ 21 ] or aerosol jet [ 22 ] printing. | https://en.wikipedia.org/wiki/Polyaniline |
Polyaspartic ester chemistry was first introduced in the early 1990s making it a relatively new technology. [ 1 ] [ 2 ] [ 3 ] The patents were issued to Bayer in Germany and Miles Corporation in the United States . It utilizes the aza- Michael addition reaction. [ 4 ] [ 5 ] These products are then used in coatings , adhesives , sealants and elastomers . [ 6 ] Pure polyurea reacts extremely quickly making them almost unusable without plural component spray equipment. Polyaspartic technology utilizes a partially blocked amine to react more slowly with the isocyanates and thus produce a modified polyurea . The amine / diamine or even triamine functional coreactant for aliphatic polyisocyanate is typically reacted with a maleate. Polyaspartic esters (PAE) [ 7 ] initially found use in conventional solvent -borne two-component polyurethane coatings.
To manufacture a polyaspartic ester, an amine is reacted with dialkyl maleate by the aza- Michael reaction . [ 8 ] Diethyl maleate is the usual maleate used. This converts the primary amines to secondary amines and also introduces bulky groups to the molecule which causes steric hindrance , slowing the reaction down. As the resulting aspartic molecule is now much bigger, less of the isocyanate is needed on a weight for weight basis. The isocyanate is often the most expensive part of the system especially if an aliphatic isocyanate oligomer is used and so may result in an overall lower system cost per applied film thickness. Isocyanates are known pulmonary sensitizers and hence oligomeric forms are often used with polyaspartic technology as these are much less volatile.
Eventually, the advantages of using polyaspartic esters as the main component of the co-reactant for reaction with an aliphatic polyisocyanate in low to zero volatile organic compound (VOC) coatings were realized. [ 9 ] The rate of reaction of polyaspartic esters can be manipulated, thus extending the pot life and controlling the cure rate of aliphatic coatings. This allows formulators to create high solids coatings systems which are user-friendly with longer working times and still maintain a fast-cure. [ 10 ] Traditional aliphatic polyurea formulations required high-pressure, temperature-controlled plural component spray systems to be applied due to fast initial reaction rates. Aliphatic polyaspartics can be formulated with slower reaction rates to accommodate batch-mixing and application by roller-applied methods or spray-applied through conventional single components paint sprayers without the use of solvent. As with aliphatic polyurethane or acrylic coatings, polyaspartic coatings made with aliphatic isocyanates and derivatives are UV and light stable and have a low yellowing tendency. When coating concrete, polyaspartics can be installed in both clear and pigmented form. Additionally, broadcast media such as quartz and/or vinyl paint chips can be incorporating, as well as metallic pigments. [ 11 ]
Once the aspartic ester is formed, it is basically a sterically hindered diamine and thus in polymer science terms is a Chain extender rather than a chain terminator . Chain extenders ( f = 2) and cross linkers ( f ≥ 3) are low molecular weight amine terminated compounds that play an important role in polyurea compounds, coatings , elastomers and adhesives . However, the isocyanate component is often an oligomer that is trifunctional and so the crosslinking comes from that part of the cured polymer. [ 12 ]
Major producers of polyaspartic esters and polyaspartic coatings are: [ 13 ] [ 14 ] | https://en.wikipedia.org/wiki/Polyaspartic_esters |
A polyatomic ion (also known as a molecular ion ) is a covalent bonded set of two or more atoms , or of a metal complex , that can be considered to behave as a single unit and that usually has a net charge that is not zero, [ 1 ] or in special case of zwitterion wear spatially separated charges where the net charge may be variable depending on acidity conditions. The term molecule may or may not be used to refer to a polyatomic ion, depending on the definition used. The prefix poly- carries the meaning "many" in Greek, but even ions of two atoms are commonly described as polyatomic. [ 2 ]
In older literature, a polyatomic ion may instead be referred to as a radical (or less commonly, as a radical group ). [ citation needed ] In contemporary usage, the term radical refers to various free radicals , which are species that have an unpaired electron and need not be charged. [ 3 ]
A simple example of a polyatomic ion is the hydroxide ion, which consists of one oxygen atom and one hydrogen atom, jointly carrying a net charge of −1 ; its chemical formula is O H − . In contrast, an ammonium ion consists of one nitrogen atom and four hydrogen atoms, with a charge of +1; its chemical formula is N H + 4 .
Polyatomic ions often are useful in the context of acid–base chemistry and in the formation of salts .
Often, a polyatomic ion can be considered as the conjugate acid or base of a neutral molecule . For example, the conjugate base of sulfuric acid (H 2 SO 4 ) is the polyatomic hydrogen sulfate anion ( HSO − 4 ). The removal of another hydrogen ion produces the sulfate anion ( SO 2− 4 ).
There are several patterns that can be used for learning the nomenclature of polyatomic anions. First, when the prefix bi is added to a name, a hydrogen is added to the ion's formula and its charge is increased by 1, the latter being a consequence of the hydrogen ion's +1 charge. An alternative to the bi- prefix is to use the word hydrogen in its place: the anion derived from H + . For example, let us consider the carbonate( CO 2− 3 ) ion:
which is called either bicarbonate or hydrogen carbonate. The process that forms these ions is called protonation .
Most of the common polyatomic anions are oxyanions , conjugate bases of oxyacids (acids derived from the oxides of non-metallic elements ). For example, the sulfate anion, S O 2− 4 , is derived from H 2 SO 4 , which can be regarded as SO 3 + H 2 O .
The second rule is based on the oxidation state of the central atom in the ion, which in practice is often (but not always) directly related to the number of oxygen atoms in the ion, following the pattern shown below. The following table shows the chlorine oxyanion family:
As the number of oxygen atoms bound to chlorine increases, the chlorine's oxidation number becomes more positive. This gives rise to the following common pattern: first, the -ate ion is considered to be the base name; adding a per- prefix adds an oxygen, while changing the -ate suffix to -ite will reduce the oxygens by one, and keeping the suffix -ite and adding the prefix hypo- reduces the number of oxygens by one more, all without changing the charge. The naming pattern follows within many different oxyanion series based on a standard root for that particular series. The -ite has one less oxygen than the -ate , but different -ate anions might have different numbers of oxygen atoms.
These rules do not work with all polyatomic anions, but they do apply to several of the more common ones. The following table shows how these prefixes are used for some of these common anion groups.
Some oxo-anions can dimerize with loss of an oxygen atom. The prefix pyro is used, as the reaction that forms these types of chemicals often involves heating to form these types of structures. [ 4 ] The prefix pyro is also denoted by the prefix di- . For example, dichromate ion is a dimer.
The following tables give additional examples of commonly encountered polyatomic ions. Only a few representatives are given, as the number of polyatomic ions encountered in practice is very large.
Many polyatomic molecules can carry spatially separated charges, forming zwitterions or, in general, polycharged polyatomic ions. A typical example are amino acids , which carry both charged amino and carboxyl groups. These charges can influence the chemical [ 6 ] and physical properties of substances. [ 7 ]
Polyatomic ion structure may influence thin film growth. [ 8 ] Analyses of polyatomic ion composition is key point in mass-spectrometry. [ 9 ] [ 10 ] [ 11 ] | https://en.wikipedia.org/wiki/Polyatomic_ion |
Polybasic may refer to: | https://en.wikipedia.org/wiki/Polybasic |
Polybius is an urban legend about a lost video game. According to the legend, a new game appeared in arcades around Portland, Oregon , in 1981. The gameplay was supposedly psychoactive , abstract, and dangerous. Children who played the arcade game were said to suffer from amnesia, seizures, night terrors , and hallucinations. Despite these adverse effects, the arcade cabinet was described as so addictive that players returned to Polybius repeatedly until they went insane, died, or vanished. The lack of any surviving Polybius cabinets is explained by men in black who were said to record data on the players before removing all the arcade machines. [ 2 ]
There is no evidence that Polybius ever existed. The earliest known print reference to the game is the September 2003 issue of GamePro . The earliest online reference to Polybius is the coinop.org entry, dated to 1998. There is no evidence that the supposed publisher, Sinneslöschen , existed. Snopes has called it a modern-day version of the early '80s urban legends about "men in black" recording the high-score initials saved in arcade machines.
This urban legend has persisted in video game journalism and has inspired video games with the same name.
There is no evidence that the game ever existed. [ 2 ] The first online mention of Polybius is a coinop.org article dated 1998. It includes a purported screenshot of the title screen allegedly from a ROM image file created from one of the original arcade machines. On May 16, 2009, an update to the page promised to bring further information after a trip to Kyiv, Ukraine , to investigate. As of 2025, no further information has been added to the page. [ 1 ] [ 3 ] Researchers have found evidence that the entry was first uploaded a few years after 1998, making even the upload date fictitious. [ 4 ] [ 5 ]
The earliest known printed mention of Polybius is in the September 2003 issue of GamePro . [ 2 ] The feature story "Secrets and Lies" declared the game's existence to be "inconclusive". [ 6 ] The article spread the legend to a broader audience, but GamePro' s writer only heard about the legend when the owner of coinop.org contacted him. [ 4 ] Interest was further kindled in 2006 when a post appeared on the coinop.org forums by an individual presenting himself as "Steven Roach" who claimed to have set up the publishing company Sinneslöschen . [ 7 ] Based on the timeline of publications, Inverse concluded that the game "was almost certainly invented" to promote coinop.org. [ 8 ]
There is no record of the game's purported publisher, Sinneslöschen . Author Brian Dunning describes the word as "not-quite-idiomatic German" meaning "sense delete" or " sensory deprivation ". In German it would be pronounced [ˈzɪnəslœʃn̩] . It is derived from the German words Sinne ("senses") and löschen ("to extinguish" or "to delete"), though the way they are combined is unusual; Sinnlöschen would be a more standard German construction. [ 2 ]
Polybius has been described as "almost too good a name for a spooky video game." [ 8 ] It is named after the classical Greek historian Polybius , born in Arcadia and known for his assertion that historians should never report what they cannot verify through interviews with eyewitnesses. [ 3 ] [ 9 ] He was also known for his work on puzzles and cryptography, such as the Polybius square . [ 8 ]
According to the legend, a new video game appeared in suburban arcades around Portland, Oregon , in 1981. [ 10 ] Released during the golden age of arcade video games , it was housed in a simple black cabinet . [ 11 ] [ 12 ] The game was abstract and geometric, but highly addictive. [ 13 ] People who claim to remember playing the game recall different and contradictory gameplay experiences. [ 14 ] Players supposedly suffered from seizures , amnesia , insomnia , night terrors , and hallucinations . [ 15 ] Addicted to the new game, children formed lines around the cabinet and fought each other for access to the machine. [ 2 ] They continued to play, becoming increasingly unwell, until after just a month all of the Polybius machines were taken away. [ 10 ]
It is claimed that the machines were visited by men in black , who collected some form of unknown data but left the coins inside. [ 2 ] It was said that the game could turn itself on autonomously. [ 16 ] Polybius was suspected to be not a product, but an experiment to test responses to the game's psychoactive stimuli. [ 1 ] As the legend comes to an end, some of the players went insane, some took their own lives, and after a month of experimentation, the game vanished without a trace. [ 4 ]
The alleged original Polybius arcade game is generally considered a hoax . [ 2 ] Fact-checking website Snopes.com calls the game a modern-day version of 1980s rumors about " men in black " recording the high-score initials from arcade machines. [ 17 ] No newspapers or gaming magazines from the 1980s mention Polybius . [ 18 ] Aside from the mockup cabinets and games inspired by the myth, no authentic cabinets or ROM dumps have ever been documented. [ 3 ]
Internet writer Patrick Kellogg believes that players claiming to remember having played or seen Polybius may be recalling the video game Cube Quest . It was released in arcades in 1983 as a shooting game played from laserdisc . Kellogg describes its visuals as "revolutionary" and far ahead of typical games of the time. He states that frequent breakdowns are typical of laserdisc games, so this one was often removed from arcades. [ 19 ] University of Arizona professor Judd Ruggill has also proposed Tempest as an influence. [ 20 ] Tempest is an abstract and geometric game that requires intense focus and was inspired by the nightmares of its creator. [ 16 ]
While some people have claimed to remember earlier mentions on Usenet , no archived Usenet posts discuss Polybius . However, the Publius Enigma , an unsolved puzzle connected to Pink Floyd 's 1994 album The Division Bell , was discussed on Usenet in the early '90s. These early recollections of the game may be false memories , recalling instead the similarly named Publius Enigma. [ 8 ] [ 21 ]
Ben Silverman of Yahoo! Games remarked, "Unfortunately, there is no evidence that the game ever existed, no less turned its users into babbling lunatics ... Still, Polybius has enjoyed cult-like status as a throwback to a more technologically paranoid era." [ 15 ] Ripley's Believe It or Not! called Polybius "the most dangerous video game to never exist". [ 22 ] Portland historian Joe Streckert says of the game, "What H.P. Lovecraft’s Necronomicon is for books, Polybius is for videogames." [ 10 ] Portland Monthly calls it "one of Portland's craziest urban legends", comparing it to the CIA 's MKUltra mind control program of the 1950s-1970s. [ 23 ]
The story of Polybius may be rooted in several real but unrelated events. [ 4 ] Author Brian Dunning believes the Polybius legend grew out of a mixture of 1980s influences. [ 2 ] For example, two players fell ill in a Portland arcade on the same day in 1981. [ 2 ] Teenage Portland resident Michael Lopez developed a migraine headache after playing Tempest and left the arcade. He was found unconscious on a stranger's lawn after blacking out. [ 4 ] During a filmed attempt to break the Asteroids world record at the same arcade where Lopez became ill, another Portland teen played for 28 hours until suffering from stomach cramps. [ 24 ] Ten days later, the Federal Bureau of Investigation raided several Portland arcades in the area. The lead-up to the raid involved FBI agents monitoring arcade cabinets for evidence of tampering and recording high scores. [ 2 ] The FBI made 52 Portland arcade arrests in 1981, and 25 arrests at a single arcade a year later. [ 25 ] Some arcade machines had been converted to gambling machines by arcade owners. [ 7 ]
The presence of men in black or government agents prowling arcades has precursors beyond Oregon. [ 2 ] Arcades were broadly seen both as targeting children and as sites of vice, such as gambling, smoking, theft, and drug use. [ 26 ] Government agencies targeted video game arcades with anti-gambling and anti-drug programs. [ 27 ] Major arcade developer Bally Manufacturing was repeatedly investigated for connections to illegal gambling. [ 26 ] In 1983, one Massachusetts town banned all arcade video games due to fears of arcades run by criminals with the police chief stating, "I don’t want my children supervised by those kind of people." [ 27 ] To catch drug sales occurring in arcades, the FBI rigged classic arcade games such as Tempest and Galaxian with surveillance equipment. [ 8 ] During the 1990s, before the spread of the Polybius story, the FBI launched its " Winners Don't Use Drugs " campaign, which flashed full-screen public service announcements from the FBI on arcade machines when no person was playing the game. [ 28 ] [ 29 ]
Atari 's testing procedures for new games may have also contributed to men in black rumors. Atari, then a major arcade game developer, would covertly place small numbers of incomplete new video games into public arcades. Atari's employees swapped in test versions of these unfinished games and observed players' responses in the arcades. To prevent their competitors from rushing copycat games to market, Atari did not disclose the true nature of these field tests to any arcade patrons. [ 30 ]
Many adults in the 1980s expressed concern about the new video games children spent so much time on. [ 8 ] Early video games were the subject of fears and anxieties about their possible dangers. A 1980 New York Times article commented on their potential for addiction, and the National Safety Council claimed that video games promoted violence . [ 27 ] There are nine recorded cases of arcade games triggering epileptic seizures in the 1980s. [ 2 ] In 1982, a teenager died while playing Berzerk in Calumet City, Illinois . His heart failure was not triggered by the game but led to rumors of a cursed arcade game. [ 31 ] [ 32 ]
Although contemporary magazines and newspapers do not mention Polybius , [ 18 ] similar legends and rumors circulated in the late '70s and early '80s. Trust was declining in government institutions, and rumors spread of arcade machines designed to hypnotize, brainwash, or recruit players. The unusual machines and the affected players were then said to disappear. These rumors influenced science fiction such as the film The Last Starfighter , in which a teenage gamer is monitored and recruited by aliens. [ 30 ] [ 2 ] The novel Arcade by Brian Maxxe was less popular than Starfighter but featured a fictional game very similar to Polybius . Arcade had a more sinister plot that Starfighter , based around government mind control experiments done on children in arcades. [ 4 ] [ 33 ]
In 2007, freeware developers and arcade constructors Rogue Synapse published a free downloadable game titled Polybius for Windows at sinnesloschen.com. Its design is partly based on a contested description of the Polybius arcade machine posted on a forum by an individual named Steven Roach who claimed to have worked on the original. [ 34 ] To complete the illusion, Rogue Synapse's owner Dr. Estil Vance founded a Texas-based corporation bearing the name Sinnesloschen (without umlaut) in 2007. [ 35 ] He transferred to it the "Rogue Synapse" trademark [ 36 ] and a newly registered trademark on "Polybius". [ 37 ] Its website says that it is an "attempt to recreate the Polybius game as it might have existed in 1981". [ 38 ]
In 2016, Llamasoft announced Polybius for the PlayStation 4 with PlayStation VR support, [ 39 ] released on the PlayStation store on Tuesday, May 9, 2017. [ 40 ] In early marketing, its co-author Jeff Minter claimed to have been permitted to play the original Polybius arcade machine in a warehouse in Basingstoke , England. [ 41 ] He later acknowledged that his game was inspired by the urban legend but does not attempt to reproduce its alleged gameplay. [ 42 ]
It has a central cameo as the "main attraction" in the Nine Inch Nails music video " Less Than ". [ 43 ]
Polybius has cameos in many TV series, such as The Goldbergs (2013), The Simpsons (2006), Dimension 404 (2017), and Smiling Friends (2020). The Loki (2021) cameo gained its own acclaim on social media, including that the game seems catastrophically integral to the multiverse, and is a key example of Loki interplaying conspiracy with reality. [ 3 ] For Paper Girls (2022), CBR reported that the Polybius cameo conferred the series with 1980s science fiction credentials, and differentiated it from Stranger Things (2016). [ 44 ]
The Polybius Conspiracy is a seven-part podcast published in 2017, adapted from a canceled feature film project. [ 23 ] [ 45 ] | https://en.wikipedia.org/wiki/Polybius_(urban_legend) |
Polybrominated biphenyls ( PBBs ), also called brominated biphenyls or polybromobiphenyls , are a group of manufactured chemicals that consist of polyhalogenated derivatives of a biphenyl core. Their chlorine analogs are the PCBs . While once widely used commercially, PBBs are now controlled substances under the Restriction of Hazardous Substances Directive , which limits their use in electrical and electronic products sold in the EU.
PBBs usually exist as colorless to off-white solids. PBBs soften at 72 degrees Celsius and decompose above 300 degrees Celsius. They have low vapor pressure , are very soluble in benzene and toluene , and insoluble in water . They are degraded by UV light .
PBBs are used as flame retardants of the brominated flame retardant group. They are added to plastics used in products such as home electrical appliances , textiles , plastic foams, laptop cabinets, etc. to make them difficult to burn.
A number of substituted PBBs found application in medicine, namely bromophene and bromofenofos .
Exposure to the coplanar stereoisomer 3,3',4,4',5,5'-hexabromobiphenyl (but not the non-coplanar stereoisomer) in genetically susceptible mice is known to cause immunotoxicity and disorders related to the central nervous system, and even at doses as low as 2.5 mg/kg, excess neonatal fatalities are observed ( LD 50 is from 5–10 mg/kg). [ 1 ] The mechanism of toxicity is cellular oxidative stress by aryl hydrocarbon receptor activation. [ 1 ]
There is evidence that pre- and post-natal exposure to PBB in girls leads to menarche at an earlier age. [ 2 ]
Early studies on the effects of PBBs on human beings concerned the people in Michigan , United States who consumed PBB-contaminated animal products (see history of PBBs below). [ 3 ] A study of 327 girls aged 5–24 years in Michigan found those who were exposed in utero to PBBs at or above a level of 7 ppb found had an earlier age at menarche compared to a case-control group. [ 4 ] Michigan dairy farmers exposed to PBBs had significant immune system abnormalities including reduced numbers of circulating blood lymphocytes, increases in lymphocytes with no detectable surface markers, and reduced functional response to specific test antigens. [ 5 ] Some residents complained of nausea , abdominal pain , loss of appetite , joint pain and lethargy , though it could not be clearly established that PBBs were the cause of these health problems.
Workers who were exposed to PBB during PBB production suffered hypothyroidism , [ 6 ] although no deterioration in memory performance was found in PBB-exposed workers in tests conducted several years after final PBB exposure, and there was also no correlation of performance with PBB concentration. [ 7 ]
There is stronger evidence that PBBs may have caused skin problems, such as acne , in consumers of the contaminated food. Some workers exposed to PBBs by breathing and skin contact for days to months also developed acne.
Studies in animals exposed to large amounts of PBBs for a short period or to smaller amounts over a longer period show that PBBs can cause weight loss , skin disorders , nervous and immune systems effects, as well as effects on the liver , kidneys , and thyroid gland .
It is not known for certain if PBBs could cause cancer in human beings, but it has been observed that they can lead to cancer in lab mice exposed to very high concentrations of PBBs. Based on such animal tests , the United States Department of Health and Human Services has determined that PBBs may reasonably be anticipated to be carcinogens . The International Agency for Research on Cancer also suggests that PBBs are possibly carcinogenic to humans.
Before the 1970s, PBBs were widely used commercially as a flame retardant . Michigan Chemical Corporation (MCC) in St. Louis, Michigan , which was then owned by Velsicol Chemical Corporation , was a major producer of the FireMaster range of PBB-based flame retardants. FireMaster BP-6 (a yellow-brown powder) is a mixture of many different PBB congeners with 2,2',4,4',5,5'-hexabromobiphenyl and 2,2',3,4,4',5,5'-heptabromobiphenyl being significant constituents by mass (60-80% and 12-25%, respectively). [ 8 ] [ 9 ] FireMaster FF-1 (a white powder) is FireMaster BP-6 with the addition of 2% calcium silicate as an anti-caking agent. [ 9 ] Mixed bromochlorobiphenyls and polybrominated naphthalenes, as well as lower brominated compounds formed by incomplete bromination, have also been found as minor constituents of FireMaster products. [ 10 ]
In summer 1973, several thousand pounds of FireMaster BP-6 were accidentally mixed with livestock feed that was distributed to farms in Michigan , United States because the MCC plant also produced a feed precursor ingredient, magnesium oxide , which was sold to the feed manufacturer. [ 11 ] Some 1.5 million chickens , 30,000 cattle , 5,900 pigs , and 1,470 sheep then consumed this feed, became contaminated with PBBs and the carcasses were disposed of in landfill sites throughout the state. [ 12 ] In 1976, the Michigan Department of Community Health established a PBB registry to gather and analyze data on exposed residents. It now resides at the Rollins School of Public Health at Emory University , [ 13 ] [ 14 ] and is maintained by epidemiologist Michele Marcus. [ 15 ]
Michigan Farmer magazine staff members Richard Lehnert and Bonnie Pollard broke the news of the contamination. The magazine continued coverage of the issue until the eventual bankruptcy proceedings of the farm cooperative responsible for the accidental contamination and subsequent distribution of the feed. [ 16 ] These events were also portrayed in the 1981 documentary Cattlegate by Jeff Jackson, the true-fiction film Bitter Harvest starring Ron Howard , and in the book The Poisoning of Michigan by Joyce Egginton. A 1978 episode of Lou Grant ("Slaughter") portrays a similar, but fictionalized account. One year elapsed before the animals were culled. [ 11 ]
This incident is cited amongst a handful of other noxious substances as the driver for Gerald Ford 's half-hearted approval in 1976 of the Toxic Substances Control Act , which "remains one of the most controversial regulatory bills ever passed". [ 17 ] [ better source needed ]
A study was undertaken on 4,545 people to determine the effects of PBBs on human beings . These include three exposure groups – all people who lived on the quarantined farms, people who received food from these farms and workers (and their families) engaged in PBB manufacture – as well as 725 people with low-level PBB exposure. [ 18 ]
All were queried concerning 17 symptoms and conditions possibly related to PBBs. Venous blood was drawn and analyzed for PBB by gas chromatography . Mean serum PBB levels were found to be 26.9 ppb by weight (26.9 μg/kg) in farm residents, 17.1 in recipients, 43.0 ppb in workers, and 3.4 ppb in the low exposure group. No associations could be established between serum PBB levels and symptom prevalence rates. [ 18 ]
To evaluate peripheral lymphocyte function, T and B cell quantitation and in vitro responses to three nonspecific mitogens were studied in 34 persons with highest PBB levels (mean: 787 ppb), and in 56 with low levels (mean: 2.8 ppb). No statistically significant differences in lymphocyte function were noted. [ 18 ]
However, as of 2015 these studies are still ongoing, and 40 years later adverse reproductive-system effects (as measured by the Apgar score of the newborns) continue to be found in the grandchildren of those who consumed tainted farm products. [ 19 ] [ 20 ]
Noting the possible hazards on the environment, however, PBBs were listed as one of six controlled substances under the Restriction of Hazardous Substances Directive (RoHS), which was enacted into European Law in February 2003. RoHS legislation lists PBBs as a "restricted substance" group. Other countries followed suit, resulting most recently in restriction dates instituted in China on March 1, 2007, and South Korea on July 1, 2007. | https://en.wikipedia.org/wiki/Polybrominated_biphenyl |
Polybrominated diphenyl ethers or PBDEs , are a class of organobromine compounds that are used as flame retardants . Like other brominated flame retardants , PBDEs have been used in a wide array of products, including building materials, electronics, furnishings, motor vehicles, airplanes, plastics, polyurethane foams, [ 1 ] and textiles. They are structurally akin to polychlorinated diphenyl ethers (PCDEs), polychlorinated biphenyls (PCBs) and other polyhalogenated compounds , consisting of two halogenated aromatic rings. PBDEs are classified according to the average number of bromine atoms in the molecule. The life-saving benefits of fire retardants led to their popularization. Standards for mass transit vehicles continues to increase as of 2021. [ 2 ] : 1–26
Because of their toxicity and persistence, all commercially relevant PBDEs have been marked for elimination under the Stockholm Convention , a treaty to control and phase out major persistent organic pollutants (POPs). [ 3 ] [ 4 ]
The family of PBDEs consists of 209 possible substances, which are called congeners (PBDE = C 12 H (10− x ) Br x O ( x = 1, 2, ..., 10 = m + n )). The number of isomers for mono-, di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, and decabromodiphenyl ethers are 3, 12, 24, 42, 46, 42, 24, 12, 3 and 1, respectively. [ 5 ]
Lower-brominated PBDEs with 1–4 bromine atoms per molecule are regarded as more dangerous because they more efficiently bioaccumulate . They have been known to affect thyroid hormone levels, and studies have linked them to reproductive and neurological risks at certain concentrations or higher. [ 6 ] Higher-brominated PBDEs are less acutely dangerous but biotically and photochemically debrominate to lower-brominated congeners. [ 7 ]
PBDEs were produced commercially via the bromination of diphenyl ether , [ 8 ] with three technical-grade mixtures being sold, varying by degree of bromination.
In the United States, PBDEs were marketed with the trade names DE-60F, DE-61, DE-62, and DE-71 applied to pentaBDE mixtures, DE-79 applied to octaBDE mixtures, and DE 83R and Saytex 102E applied to decaBDE mixtures. The available commercial PBDE products were not single compounds or even single congeners but rather mixtures of congeners.
Technical pentaBDE predominantly contained pentabromo derivatives (50–62%); however, the mixture also contained tetrabromides (24–38%) and hexabromides (4–8%), as well as traces of the tribromides (0–1%). Technical octaBDE was a mixture of homologs: hexa-, hepta-, octa-, nona-, and decabromides. Technical decaBDE was 97% decabromide, with small amounts of octa- and nonabromides. [ 7 ]
Commercial production of PBDEs began in the 1970s, [ 9 ] and continued until the early 2010s. Cumulative global production is estimated to have been 175 kt for pentaBDE, 130 kt for octaBDE, and 1600 kt for decaBDE. [ 10 ]
Polybrominated diphenyl ethers (PBDEs) can be released into the environment where they are used or produced, possibly entering air, water, soil or the human digestive system when consumed, inhaled or via the skin. [ 11 ] [ 12 ] Despite the banning and phase out of several forms of PBDEs, many consumer products still contain them in the 21st century, and represent potential exposure sources, including furniture and other consumer products containing polyurethane foam , appliances, pipes, plastics, and old electronic equipment. [ 11 ] [ 13 ]
Generally, governments have determined that PBDEs are not harmful to human health in the exposure amounts assessed. [ 11 ] [ 12 ] [ 14 ] Ingestion of house dust accounts for 80–90% of total PBDE exposure, while the remaining exposure occurs from food ingestion. [ 11 ] [ 12 ] PBDE-contaminated foods, particularly those high in fat content, such as fatty meats or fish, are possible sources of exposure. [ 11 ] In breastfeeding infants, breast milk may be an exposure source because PBDEs can be present in the mother and her milk. [ 11 ] Various other food items may contain PBDEs, including meat, meat products, dairy products, and seafood. [ 11 ] [ 14 ]
PBDEs have not been detected beyond trace levels in water. [ 11 ] In the environment, soils and sediments are the major deposits for PBDEs. [ 11 ] PBDEs can enter soil from discarded products, such as in landfills. As biosolids (sewage) may contain PBDEs, exposure from soils or farmlands that have been fertilized with biosolids may occur. [ 11 ] Wildlife may have exposure by consuming foods containing PBDEs, whereas organisms that live in sediments may be contaminated by PBDEs. [ 12 ]
PBDEs and their metabolites are excreted mainly in the feces and some in the urine. [ 11 ] Owing to their deposition in body fat stores, some PBDEs remain in the body for many years, and may enter the bodies of unborn babies via the placenta . [ 11 ]
Nothing certain has been established about the effects of PBDEs on human health. [ 11 ] [ 12 ] [ 14 ] Most information regarding toxicity of PBDEs and their metabolites is from early-stage animal studies. [ 14 ] Evidence for PBDE-mediated effects from human studies in systems other than the developing nervous system, such as in cancer development, is inconclusive or non-existent. [ 11 ] Particularly for the potential effects of PBDEs on the developing fetus, research has focused on the health status of mothers and gestational age of the infant. [ 15 ]
Increasing environmental concentrations and changing distributions of PBDEs in sediments of the Clyde River Estuary in Scotland, UK have been assessed. [ 16 ] Analysis of six sediment cores each of 1 m depth from Glasgow city to Greenock revealed that total concentrations increased toward the river bed surface (0–10 cm). Amounts of PBDE ranged from 1 to 2,645 µg/kg (dry wt. sediment) with a mean of 287 µg/kg (dry wt. sediment). [ 16 ] Down-core PBDE congener profiles showed that higher concentrations were due to elevated levels of BDE-209. The majority of the sediment records clearly showed a change from mainly lower molecular weight BDEs 47,99, 183, 153 at lower depths to BDE-209 near the surface, a change in congener and homologue group patterns that corresponds to the restrictions of penta- and octaBDE commercial mixtures under EU law in 2004–2006.
While biodegradation is not considered the main pathway for PBDEs, photolysis and pyrolysis can be of interest in studies of transformation of PBDEs. [ 17 ] [ 18 ]
In August 2003, the State of California outlawed the sale of penta- and octaBDE and products containing them, effective 1 January 2008. [ 19 ] PBDEs are ubiquitous in the environment, and, according to the EPA, exposure may pose health risks. According to U.S. EPA's Integrated Risk Information System, evidence indicates that PBDEs may possess liver toxicity, thyroid toxicity, and neurodevelopmental toxicity. [ 20 ] [ 21 ] In June 2008, the U.S. EPA set a safe daily exposure level ranging from 0.1 to 7 μg/kg body weight per day for the four most common PBDE congeners. [ 20 ] [ 21 ] [ 22 ] [ 23 ] In April 2007, the legislature of the state of Washington passed a bill banning the use of PBDEs. [ 24 ] The State of Maine Department of Environmental Protection has restrictions on PBDEs, and in 2008, the legislature passed a bill phasing out the use of decaBDE. [ 25 ]
The U.S. importers and manufacturers of PBDEs withdrew pentaBDE and octaBDE from sale in 2004, and decaBDE from sale by the end of 2013. [ 7 ] : 10 [ 26 ] In November 2024, the EPA added decaBDE to the Toxic Substances Control Act to prohibit release into water during manufacturing, processing or distribution in commerce of decaBDE and decaBDE-containing products, and a phase-out of processing and distribution of wire and cable insulation containing decaDBE for nuclear power facilities. [ 27 ]
Since 2012, PBDEs are among chemicals prohibited from manufacture, use, sale, offer for sale or import, as regulated in the Prohibition of Certain Toxic Substances Regulations of the 1999 Canadian Environmental Protection Act. [ 28 ] PBDEs are regarded in the regulation as "toxic to the environment and/or human health, are generally persistent, bioaccumulative, and/or inherently toxic." [ 28 ]
The European Union decided to ban the use of two classes of flame retardants, in particular, PBDEs and polybrominated biphenyls (PBBs) in electric and electronic devices. [ 13 ] This ban was formalised in the RoHS Directive, and an upper limit of 1 g/kg for the sum of PBBs and PBDEs was set. In February 2009, the Institute for Reference Materials and Measurements released two certified reference materials to help analytical laboratories better detect these two classes of flame retardants. The reference materials were custom-made to contain all relevant PBDEs and PBBs at levels close to the legal limit. [ 13 ]
At an international level, in May 2009 the Parties of the Stockholm Convention on Persistent Organic Pollutants (POPs) decided to list commercial pentaBDE and commercial octaBDE as POP substances. This listing is due to the properties of hexaBDE and heptaBDE, which are components of commercial octaBDE, and to the properties of tetraBDE and pentaBDE, which are the main components of commercial pentaBDE. [ 29 ] In 2017, it was decided to also list decaBDE. [ 4 ] [ 30 ]
Major decaBDE producers have switched to manufacturing decabromodiphenyl ethane , a structurally related compound that has also come under suspicion as an environmental pollutant. [ 2 ] : 21 Non-halogenated alternatives also exist. [ 2 ] [ 26 ] | https://en.wikipedia.org/wiki/Polybrominated_diphenyl_ethers |
Polybutadiene [butadiene rubber, BR] is a synthetic rubber . It offers high elasticity, high resistance to wear, good strength even without fillers , and excellent abrasion resistance when filled and vulcanized. "Polybutadiene" is a collective name for homopolymers formed from the polymerization of the monomer 1,3-butadiene . The IUPAC refers to polybutadiene as "poly(buta-1,3-diene)". Historically, an early generation of synthetic polybutadiene rubber produced in Germany by Bayer using sodium as a catalyst was known as "Buna rubber". Polybutadiene is typically crosslinked with sulphur, however, it has also been shown that it can be UV cured when bis-benzophenone additives are incorporated into the formulation. [ 1 ]
Polybutadiene rubber (BR) accounted for about 28% of total global consumption of synthetic rubbers in 2020, whereas styrene-butadiene rubber (SBR) was by far the most important grade (S-SBR 12%, E-SBR 27% of the entire synthetic rubber market). It is mainly used in the manufacture of tires , which consumes about 70% of the production. Another 25% is used as an additive to improve the toughness (impact resistance) of plastics such as polystyrene and acrylonitrile butadiene styrene (ABS). [ 2 ] Polybutadiene is also used to manufacture golf balls , various elastic objects and to coat or encapsulate electronic assemblies, offering high electrical resistivity . [ 3 ]
The Russian chemist Sergei Vasilyevich Lebedev was the first to polymerize butadiene in 1910. [ 4 ] [ 5 ] In 1926 he invented a process for manufacturing butadiene from ethanol, and in 1928, developed a method for producing polybutadiene using sodium as a catalyst .
The government of the Soviet Union strove to use polybutadiene as an alternative to natural rubber and built the first pilot plant in 1930, [ 6 ] using ethanol produced from potatoes. The experiment was a success and in 1936 the Soviet Union built the world's first polybutadiene plant in which the butadiene was obtained from petroleum. By 1940, the Soviet Union was by far the largest producer of polybutadiene with 50,000 tons per year. [ 7 ]
Following Lebedev's work, other industrialized countries such as Germany and the United States developed polybutadiene and SBR as an alternative to natural rubber .
In the mid-1950s there were major advances in the field of catalysts that led to the development of an improved versions of polybutadiene. The leading manufacturers of tires and some petrochemical companies began to build polybutadiene plants on all inhabited continents; the boom lasted until the 1973 oil crisis . Since then, the growth rate of the production has been more modest, focused mainly in the Far East .
In Germany, scientists from Bayer (at the time a part of the conglomerate IG Farben ) reproduced Lebedev's processes of producing polybutadiene by using sodium as a catalyst. For this, they used the trade name Buna, derived from Bu for butadiene , Na for sodium (natrium in Latin, Natrium in German). [ 6 ] They discovered that the addition of styrene to the process resulted in better properties, and thus opted for this route. They had invented styrene-butadiene , which was named Buna-S ( S for styrene ). [ 8 ] [ 9 ]
Although the Goodrich Corporation had successfully developed a process for producing polybutadiene in 1939, [ 10 ] the U.S. federal government opted for the use of Buna-S to develop its synthetic-rubber industry after its entry into the World War II, [ 6 ] using patents of IG Farben obtained via Standard Oil . Because of this, there was little industrial production of polybutadiene in America during this time.
After the war, the production of synthetic rubber was in decline due to the decrease in demand when natural rubber was readily available again. However, interest was renewed in the mid-1950s after the discovery of the Ziegler–Natta catalyst . [ 11 ] This method proved to be much better for tire manufacturing than the old sodium polybutadiene. The following year, Firestone Tire and Rubber Company was first to produce low cis polybutadiene using butyllithium as a catalyst.
The relatively high production costs were a hindrance to commercial development until 1960 when production on a commercial scale began. [ 11 ] Tire manufacturers like Goodyear Tire and Rubber Company [ 12 ] and Goodrich were the first to produce plants for high cis polybutadiene, this was followed by oil companies like Shell and chemical manufacturers such as Bayer.
Initially, with plants built in the United States and France, Firestone enjoyed a monopoly on low cis polybutadiene, licensing it to plants in Japan and the United Kingdom. In 1965, the Japanese JSR Corporation developed its own low cis process and began licensing it during the next decade.
The 1973 oil crisis marked a halt to the growth of synthetic rubber production; the expansion of existing plants almost ceased for a few years. Since then, the construction of new plants has mainly taken place in industrializing countries in the Far East (such as South Korea, Taiwan, Thailand, and China), while Western countries have chosen to increase the capacity of existing plants.
In 1987, Bayer started to use neodymium-based catalysts to catalyze polybutadiene. Soon thereafter other manufacturers deployed related technologies such as EniChem (1993) and Petroflex (2002).
In the early 2000s, the synthetic rubber industry was once again hit by one of its periodic crises. The world's largest producer of polybutadiene, Bayer, went through major restructuring as it was troubled by financial losses; between 2002 and 2005 it closed its cobalt-polybutadiene plants in Sarnia (Canada) and Marl (Germany), [ 13 ] transferring their production to neodymium plants in Port Jérôme (France) and Orange (USA). [ 14 ] During the same time, the synthetic rubber business was transferred from Bayer to Lanxess , a company founded in 2004 when Bayer spun off its chemicals operations and parts of its polymer activities. [ 15 ]
1,3-Butadiene is an organic compound that is a simple conjugated diene hydrocarbon (dienes have two carbon-carbon double bonds ). Polybutadiene forms by linking many 1,3-butadiene monomers to make a much longer polymer chain molecule. In terms of the connectivity of the polymer chain, butadiene can polymerize in three different ways, called cis , trans and vinyl . The cis and trans forms arise by connecting the butadiene molecules end-to-end, so-called 1,4-polymerisation. The properties of the resulting isomeric forms of polybutadiene differ. For example, "high cis"-polybutadiene has a high elasticity and is very popular, whereas the so-called "high trans " is a plastic crystal with few useful applications. The vinyl content of polybutadiene is typically no more than a few percent. In addition to these three kinds of connectivity, polybutadienes differ in terms of their branching and molecular weights.
The trans double bonds formed during polymerization allow the polymer chain to stay rather straight, allowing sections of polymer chains to align to form microcrystalline regions in the material. The cis double bonds cause a bend in the polymer chain, preventing polymer chains from aligning to form crystalline regions, which results in larger regions of amorphous polymer. It has been found that a substantial percentage of cis double bond configurations in the polymer will result in a material with flexible elastomer (rubber-like) qualities. In free radical polymerization, both cis and trans double bonds will form in percentages that depend on temperature. The catalysts influence the cis vs trans ratio.
The catalyst used in the production significantly affects the type of polybutadiene product.
This type is characterized by a high proportion of cis (typically over 92%) [ 17 ] and a small proportion of vinyl (less than 4%). It is manufactured using Ziegler–Natta catalysts based on transition metals . [ 18 ] Depending on the metal used, the properties vary slightly. [ 16 ]
Using cobalt gives branched molecules , resulting in a low viscosity material that is easy to use, but its mechanical strength is relatively low. Neodymium gives the most linear structure (and therefore higher mechanical strength) and a higher percentage of 98% cis . [ 19 ] Other less-used catalysts include nickel and titanium. [ 16 ]
Using an alkyllithium (e.g. butyllithium ) as the catalyst produces a polybutadiene called "low cis " which typically contains 36% cis , 54% trans and 10% vinyl. [ 18 ]
Despite its high liquid-glass transition, low cis polybutadiene is used in tire manufacturing and is blended with other tire polymers, also it can be advantageously used as an additive in plastics due to its low contents of gels. [ 20 ]
In 1980, researchers from the Japanese company, Zeon , discovered that high-vinyl polybutadiene (over 70%), despite having a high liquid-glass transition, could be advantageously used in combination with high cis in tires. [ 21 ] This material is produced with an alkyllithium catalyst.
Polybutadiene can be produced with more than 90% trans using catalysts similar to those of high cis : neodymium, lanthanum , nickel. This material is a plastic crystal (i.e. not an elastomer) which melts at about 80 °C. It was formerly used for the outer layer of golf balls. Today it is only used industrially, but companies like Ube are investigating other possible applications. [ 22 ]
The use of metallocene catalysts to polymerize butadiene is being explored by Japanese researchers. [ 23 ] The benefits seem to be a higher degree of control both in the distribution of molecular mass and the proportion of cis/trans/vinyl. As of 2006, no manufacturer produces "metallocene polybutadiene" on a commercial basis.
1,3-butadiene is normally copolymerized with other types of monomers such as styrene and acrylonitrile to form rubbers or plastics with various qualities. The most common form is styrene-butadiene copolymer, which is a commodity material for car tires . It is also used in block copolymers and tough thermoplastics such as ABS plastic . This way a copolymer material can be made with good stiffness , hardness , and toughness .
Because the chains have a double bond in each and every repeat unit , the material is sensitive to ozone cracking .
The annual production of polybutadiene was 2.0 million tons in 2003. [ 18 ] This makes it the second most produced synthetic rubber by volume, behind the styrene-butadiene rubber (SBR). [ 16 ] [ 24 ]
The production processes of high cis polybutadiene and low cis used to be quite different and were carried out in separate plants. Lately, the trend has changed to use a single plant to produce as many different types of rubber as possible, including, low cis polybutadiene, high cis (with neodymium used as a catalyst) and SBR.
Polybutadiene rubber is seldom used alone, but is instead mixed with other rubbers. Polybutadiene is difficult to band in a two roll mixing mill. Instead, a thin sheet of polybutadiene may be prepared and kept separate. Then, after proper mastication of natural rubber, the polybutadiene rubber may be added to the two roll mixing mill. A similar practice may be adopted, for example, if polybutadiene is to be mixed with Styrene Butadiene Rubber (SBR). *Polybutadiene rubber may be added with Styrene as an impact modifier. High dosages may affect clarity of Styrene.
In an internal mixer, natural rubber and/or styrene-butadiene rubber may be placed first, followed by polybutadiene.
The plasticity of polybutadiene is not reduced by excessive mastication.
The annual production of polybutadiene is 2.1 million tons (2000). This makes it the second most produced synthetic rubber by volume, behind styrene-butadiene rubber (SBR). [ 25 ]
Polybutadiene is largely used in various parts of automobile tires; the manufacture of tires consumes about 70% of the world production of polybutadiene, [ 19 ] [ 20 ] with a majority of it being high cis . The polybutadiene is used primarily in the sidewall of truck tires, this helps to improve fatigue to failure life due to the continuous flexing during run. As a result, tires will not blow out in extreme service conditions. It is also used in the tread portion of giant truck tires to improve the abrasion, i.e. less wearing, and to run the tire comparatively cool, since the internal heat comes out quickly. Both parts are formed by extrusion . [ 26 ]
Its main competitors in this application are styrene-butadiene rubber (SBR) and natural rubber. Polybutadiene has the advantage compared to SBR in its lower liquid-glass transition temperature, which gives it a high resistance to wear and a low rolling resistance. [ 19 ] [ 27 ] This gives the tires a long life and low fuel consumption. However, the lower transition temperature also lowers the friction on wet surfaces, which is why polybutadiene almost always is used in combination with any of the other two elastomers. [ 16 ] [ 28 ] About 1 kg of polybutadiene is used per tire in automobiles, and 3.3 kg in utility vehicles. [ 29 ]
About 25% of the produced polybutadiene is used to improve the mechanical properties of plastics, in particular of high-impact polystyrene (HIPS) and to a lesser extent acrylonitrile butadiene styrene (ABS). [ 20 ] [ 30 ] The addition of between 4 and 12% polybutadiene to polystyrene transforms it from a fragile and delicate material to a ductile and resistant one.
The quality of the process is more important in the use in plastics than in tires, especially when it comes to color and content of gels which have to be as low as possible. In addition, the products need to meet a list of health requirements due to its use in the food industry.
Most golf balls are made of an elastic core of polybutadiene surrounded by a layer of a harder material. Polybutadiene is preferred to other elastomers due to its high resilience. [ 31 ]
The core of the balls are formed by compression molding with chemical reactions. First, polybutadiene is mixed with additives, then extruded, pressed using a calender and cut into pieces which are placed in a mold. The mold is subjected to high pressure and high temperature for about 30 minutes, enough time to vulcanize the material.
The golf ball production consumes about 20,000 tonnes of polybutadiene per year (1999). [ 20 ]
Media related to Polybutadiene at Wikimedia Commons | https://en.wikipedia.org/wiki/Polybutadiene |
Polybutene is an organic polymer made from a mixture of 1-butene , 2-butene , and isobutylene . Ethylene steam cracker C4s are also used as supplemental feed for polybutene. It is similar to polyisobutylene (PIB), which is produced from essentially pure isobutylene made in a C4 complex of a major refinery. The presence of isomers other than isobutylene can have several effects including: 1) lower reactivity due to steric hindrance at the terminal carbon in, e.g., manufacture of polyisobutenyl succinic anhydride (PIBSA) dispersant manufacture; 2) the molecular weight—viscosity relationships of the two materials may also be somewhat different. [ 1 ]
Industrial product applications include sealants, adhesives, extenders for putties used for sealing roofs and windows, coatings, polymer modification, tackified polyethylene films, personal care, polybutene emulsions. Hydrogenated polybutenes are used in a wide variety of cosmetic preparations, such as lipstick and lip gloss . It is used in adhesives owing to its tackiness . Polybutene finds a niche use in bird and squirrel repellents and is ubiquitous as the active agent in mouse and insect "sticky traps". [ 2 ]
An important physical property is that higher molecular weight grades thermally degrade to lower-molecular weight polybutenes; those evaporate as well as degrade to butene monomers which can also evaporate. [ 3 ] This depolymerization mechanism which allows clean and complete volatization is in contrast to mineral oils which leave gum and sludge or thermoplastics which melt and spread. The property is very valuable for a variety of applications. For smoke inhibition in two stroke engine fuels, the lubricant can degrade at temperatures below the combustion temperature. For electrical lubricants and carriers which might be subject to overheating or fires, polybutene does not result in increased insulation (accelerating the overheating) or conductive carbon deposits. | https://en.wikipedia.org/wiki/Polybutene |
Polycaprolactone ( PCL ) is a synthetic, semi-crystalline, biodegradable polyester with a melting point of about 60 °C and a glass transition temperature of about −60 °C. [ 2 ] [ 3 ] The most common use of polycaprolactone is in the production of speciality polyurethanes . Polycaprolactones impart good resistance to water, oil, solvent and chlorine to the polyurethane produced.
This polymer is often used as an additive for resins to improve their processing characteristics and their end use properties (e.g., impact resistance ). Being compatible with a range of other materials, PCL can be mixed with starch to lower its cost and increase biodegradability or it can be added as a polymeric plasticizer to polyvinyl chloride (PVC).
Polycaprolactone is also used for splinting, modeling, and as a feedstock for prototyping systems such as fused filament fabrication 3D printers .
PCL is prepared by ring opening polymerization of ε-caprolactone using a catalyst such as stannous octoate . A wide range of catalysts can be used for the ring opening polymerization of caprolactone. [ 4 ]
PCL is degraded by hydrolysis of its ester linkages in physiological conditions (such as in the human body) and has therefore received a great deal of attention for use as an implantable biomaterial . In particular it is especially interesting for the preparation of long term implantable devices, owing to its degradation which is even slower than that of polylactide .
PCL has been widely used in long-term implants and controlled drug release applications. However, when it comes to tissue engineering, PCL suffers from some shortcomings such as slow degradation rate, poor mechanical properties, and low cell adhesion. The incorporation of calcium phosphate-based ceramics and bioactive glasses into PCL has yielded a class of hybrid biomaterials with remarkably improved mechanical properties, controllable degradation rates, and enhanced bioactivity that are suitable for bone tissue engineering. [ 5 ] PCL–Hydroxyapatite composite scaffolds for bone tissue engineering can mimic the composition and morphology of the bone mineral phase and can be 3D printed into intricate designs. [ 6 ]
PCL has been approved by the Food and Drug Administration (FDA) in specific applications used in the human body as (for example) a drug delivery device, suture , or adhesion barrier . [ 7 ] PCL is used in the rapidly growing field of human esthetics following the recent introduction of a PCL-based microsphere dermal filler belonging to the collagen stimulator class (Ellansé). [ 8 ]
Through the stimulation of collagen production, PCL-based products are able to reduce facial ageing signs such as volume loss and contour laxity, providing an immediate and long-lasting natural effect. [ 8 ] [ 9 ] It is being investigated as a scaffold for tissue repair by tissue engineering , GBR membrane . It has been used as the hydrophobic block of amphiphilic synthetic block copolymers used to form the vesicle membrane of polymersomes .
A variety of drugs have been encapsulated within PCL beads for controlled release and targeted drug delivery . [ 10 ]
In dentistry (as the composite named Resilon), it is used as a component of "night guards" (dental splints) and in root canal filling. It performs like gutta-percha , has similar handling properties, and for re-treatment purposes may be softened with heat, or dissolved with solvents like chloroform. Similar to gutta-percha, there are master cones in all ISO sizes and accessory cones in different sizes and taper available. The major difference between the polycaprolactone-based root canal filling material (Resilon and Real Seal) and gutta-percha is that the PCL-based material is biodegradable, [ 11 ] whereas gutta-percha is not. There is a lack of consensus in the expert dental community as to whether a biodegradable root canal filling material, such as Resilon or Real Seal is desirable.
PCL also has many applications in the hobbyist market where it is known as Re-Form, Polydoh, Plastimake, NiftyFix, Protoplastic, InstaMorph, Polymorph, Shapelock, ReMoldables, Plastdude, TechTack, or Friendly Plastic. It has physical properties of a very tough, nylon-like plastic that softens to a putty-like consistency at only 60 °C, easily achieved by immersing in hot water. [ 12 ] PCL's specific heat and conductivity are low enough that it is not hard to handle by hand at this temperature. This makes it ideal for small-scale modeling, part fabrication, repair of plastic objects, and rapid prototyping where heat resistance is not needed. Though softened PCL readily sticks to many other plastics when at higher temperature, if the surface is cooled, the stickiness can be minimized while still leaving the mass pliable.
Bacillota and Pseudomonadota can degrade PCL. [ 13 ] Penicillium sp. strain 26-1 can degrade high density PCL; though not as quickly as thermotolerant Aspergillus sp. strain ST-01. Species of Clostridium can degrade PCL under anaerobic conditions. | https://en.wikipedia.org/wiki/Polycaprolactone |
Polycarbonyl , (also known as polymeric-CO , p-CO or poly-CO ) is a solid, metastable , and explosive polymer of carbon monoxide . [ 1 ] The polymer is produced by exposing carbon monoxide to high pressures. The structure of the solid appears amorphous , but may include a zigzag of equally-spaced CO groups . [ 2 ]
Poly-CO can be produced at pressures of 5.2 GPa; it is amorphous and yellow to dark red in color. [ 3 ] Polymerisation is catalysed by blue light at slightly lower pressures in the δ-phase of solid CO. [ 4 ] Another white, crystalline phase can be made at higher temperatures at 6 or 7 GPa. [ 1 ]
R. J. Mills discovered this solid, which was first produced in a tungsten carbide anvil in 1947. Originally this was thought to be polymeric carbon suboxide , but the formation does not yield any gas byproduct such as carbon dioxide. [ 5 ] The yield of the solid can be up to 95%. [ 6 ]
The polymer is stable above about 80 K. Below this temperature the ε form of solid molecular CO is formed instead. When the pressure is released the polymer remains stable at atmospheric pressure. The solid dissolves in water, alcohol and acetone. [ 5 ] When exposed to the atmosphere it is hygroscopic, becomes gluey, and changes colour, becoming darker. [ 6 ] The reaction with water produces carboxylic groups. [ 7 ] [ 8 ]
The solid stores a high energy. It can decompose explosively forming glassy carbon and carbon dioxide . [ 6 ] The energy density stored can be up to 8 kJ/g. During the decomposition the temperature can be 2500 K. [ 6 ] The density is 1.65 g/cm 3 , however most of the solid produced is porous, so the true density is likely to be higher. [ 6 ]
Infrared spectroscopy shows bands at 650, 1210, 1440, 1650 and 1760 cm −1 . The 1760 band is likely to be due to the -C-(C=O)-C- structure. [ 4 ] The 1600 is due to vibration of a C=C double bond. [ 6 ]
The solid is electrically insulating with an electronic gap energy of 1.9 eV. [ 4 ]
Nuclear magnetic resonance for the material made from 13 CO shows sharp resonance at 223 ppm due to ester or lactone attached carbon, and 151 ppm due to C=C double bonds. There is also broad resonance at 109 and 189 ppm. Over time of a few days, the 223 ppm peak reduces and all the other features increase in strength. [ 6 ]
Ideas of the structure include a zigzag chain of CO pointing in opposite directions, or five atom rings connected by CO and C−C bonds. The rings are lactones of tetronic acid : −C:−(C=O)−(C−O − )−(C=O)−O− . Interconnections between the rings are zigzags of CO. [ 4 ]
Other ideas of the structure of the solid, include graphitic carbon with carbon dioxide under pressure, and a polymer with this C 3 O 2 monomer: −(C=O)−O−(C − )=C< . Yet other ideas are that the solid is the same as the polymer of carbon suboxide with oxalic anhydride . [ 9 ] | https://en.wikipedia.org/wiki/Polycarbonyl |
Polycarboxylates are organic compounds with several carboxylic acid groups. Butane-1,2,3,4-tetracarboxylate is one example. Often, polycarboxylate refers to linear polymers with a high molecular mass (M r ≤ 100 000) and with many carboxylate groups. They are polymers of acrylic acid or copolymers of acrylic acid and maleic acid . The polymer is used as the sodium salt (see: sodium polyacrylate ). [ 1 ]
Polycarboxylates are used as builders in detergents. [ 2 ] Their high chelating power, even at low concentrations, lowers deposits on the laundry and inhibits the growth of calcite crystals.
As applied to concrete, PCE-based superplasticizers can reduce water content by up to 40% at low dosages of around 0.12% to 0.22% without compromising fluidity. Polycarboxylate superplasticizers accelerate the early strength development of concrete and mortar. They also help with slump retention: polycarboxylate superplasticizer ensures concrete consistency and workability. With a higher slump rating, concrete can be more fluid and pumpable, especially for projects involving long-distance pumping and bulk construction. [ 3 ]
Polycarboxylates are poorly biodegradable but have a low ecotoxicity . In the sewage treatment plant , the polymer remains largely in the sludge and is separated from the wastewater .
Polyamino acids like polyaspartic acid and polyglutamic acid have better biodegradability but lower chelating performance than polyacrylates. They are also less stable towards heat and alkali. Since they contain nitrogen, they can contribute to eutrophication . [ 4 ] | https://en.wikipedia.org/wiki/Polycarboxylates |
A polycatenane is a chemical substance that, like polymers , is chemically constituted by a large number of units. [ 2 ] These units are made up of concatenated rings into a chain-like structure.
It consists of mechanically linked catenane [ 3 ] structures, via topological Hopf links , resulting in a higher dimensionality than the repeating unit. [ 4 ] [ 5 ] They are a class of catenanes where the number of macrocycles is greater than two and as catenanes they belong to the big family of mechanically interlocked molecular architectures (MIMAs) . [ 6 ] [ 7 ] [ 8 ]
The characteristic feature of a polycatenane compound, that distinguishes it from other polymers, is the presence of mechanical bonds in addition to covalent bonds. [ 7 ] [ 9 ] [ 10 ] The rings in this chain-like structure can be separated only when high energy is provided to break at least a covalent bond of the macrocycle. [ n ]-Catenanes (for large n), which consist solely of the mechanically interlocked cyclic components, can be viewed as “optimized” polycatenanes. The main difference between poly-[ 2 ]-catenanes and poly-[ n ]-catenanes is the repeating unit, as a monomer is for the polymer. [ 11 ] [ 1 ] [ 12 ] In the first case the monomer is made of two interlocked rings that repeat continuously in the final polycatenane, while in the latter case there is only one ring that repeat the interlocking process for a large number of times. If the rings of the polycatenane are all of the same type, it can be defined as a homocatenane while if the subunits are different it is defined as heterocatenane.
As a chain, the degree of motion of these structures is very high, greater than the one of a usual polymer, because the rings possess a reciprocal rotational, elongational and rocking motion. [ 1 ] This flexibility is retained even if the macrocycles themselves are very rigid units, because the mobility is given by the ability of the rings to move with respect to each other. This mobility influences the final properties of the material (mechanical, rheological and thermal), and provides a dynamic behavior. [ 13 ]
Depending on the location of the catenane structures in the polymer chain, the polycatenanes can be divided into main-chain polycatenanes and side-chain polycatenanes. [ 2 ] [ 7 ] [ 14 ]
Main-chain polycatenanes are linear catenanes in which the rings are interlocked in a large number of units. They can also be a series of oligomers linked physically even if not interlocked together. The stability of the structure is not only given by mechanical bonds but also hydrogen bonds and π-π interactions between the rings. [ 2 ]
On the other hand, the side-chain polycatenanes are polycatenanes with ramifications where more oligomers are connected on the same cycle with respect to the main backbone. This type of catenane is synthesised, functionalising the macrocycles so that there is directionality with the possibility to control the ramification. [ 2 ]
There are other types of polycatenanes like the ones based on cyclic polymers, where the macrocyles are interlocked to the cyclic polymers, or the polycatenane networks, where catenanes are interlocked into a net. [ 15 ] [ 16 ]
The basic unit of the polycatenane can differ from the relatively simple organic macrocycle. When organic and inorganic building blocks come together, they can form coordination cages (or macromolecular cages ) that can interlock with one another to form a polycatenane structure. [ 17 ] The mechanism is still unexplored but generally the subunits self-assemble into a 0D cage and, in a concerted process, interlock together into a linear or more intricate catenane structure. [ 18 ] [ 19 ] [ 20 ] [ 21 ] [ 22 ] [ 23 ] Sometimes the catenated cages structure is more stable with respect to the monomeric cage state, and it can be formed passing through a favored reaction intermediate. [ 24 ] The synthesis can follow a statistical or a directed routes, forming more or less product, but there are some cases when post-synthetic modifications can increase the product yields. [ 25 ] [ 26 ] Catenated cages can be applied in a wide range of application due to the high presence of voids. [ 27 ] [ 28 ] [ 29 ]
The synthesis of polycatenanes is considered a challenging task with most of the reported examples being in the solution state and very few in the solid state. [ 30 ] The formation of poly-[ 2 ]-catenanes can be achieved by polymerisation of functionalised [ 2 ]-catenanes. [ 31 ] Also the synthesis of [ 3 ]-catenanes, [ 5 ]-catenanes, [ 6 ]-catenanes and [ 7 ]-catenanes is reported in many articles. [ 32 ] [ 33 ] The synthesis of poly-[ n ]-catenanes has instead some practical issues. [ 34 ] [ 35 ] To this purpose, molecular dynamic simulation is very used as a tool for the design of the optimal synthetic path toward the desired product by predicting the final topology. [ 36 ] [ 13 ]
There are two main synthetic routes: the statistical approach and the template-directed approach. [ 37 ]
The statistical approach is based on a stochastic methodology. [ 38 ] [ 39 ] When the reactants are together, there is a probability that they will fit together first and then close on top of each other in a process of cyclisation. The catenation of two rings into a catenane is already complex, thus, as expected, the interlocking of multiple cycles into a polycatenane is statistically improbable. Being an unfavored entropically process the product is obtained in very small amounts. Also, the cyclisation process requires high dilutions, but the elongation of the chain is favored at high concentrations, making the synthesis even more difficult. [ 38 ] [ 39 ]
The template-directed approach is based on the host-guest interactions that can direct the cyclisation of pre-organised linear unit upon the existing macrocycle. [ 37 ] [ 31 ] These interactions can be hydrogen bonds, π-π interactions, hydrophobic interactions or metal ions coordinations. In this way the synthesis can be enthalpy-driven, obtaining quantitative results. [ 37 ] [ 31 ]
The yield and selectivity are restrained by the kinetic or thermodynamic control of the reaction. [ 40 ]
Generally the kinetic control induces the formation of a product after short reaction times because it is favoued by irreversible reactions (or equilibrium reaction moved very much toward the formation of the products). [ 25 ] The thermodynamic product is obtained for longer reaction times for reversible processes. [ 41 ] In this case the units have the time to rearrange themselves toward the most stable state, in a sort of error-checking process. This is obtained by breaking covalent and coordination bonds and forming the most stable ones. [ 42 ]
Given that polycatenanes are a relatively recent field of study, the properties of these materials are not yet fully explored and understood. [ 43 ] The type of bonds that characterize the whole structure (covalent, non-covalent or mechanical bonds), the degree of mobility of the chain, the interactions between different chains and the fraction of voids of the catenanes are all factors that contribute to the final properties. As they can be strictly related to the family of metal-organic frameworks , the catenanes share all the potential applications of this class of compounds. Among these, there are applications in biomedicine, catalysis, as conducting bridges or in electronic devices, sensing or in recent fields like molecular machines. [ 44 ] [ 45 ] [ 46 ] [ 47 ] [ 48 ] [ 49 ] [ 50 ] | https://en.wikipedia.org/wiki/Polycatenane |
Polychlorinated biphenyls ( PCBs ) are organochlorine compounds with the formula C 12 H 10− x Cl x ; they were once widely used in the manufacture of carbonless copy paper, as heat transfer fluids , and as dielectric and coolant fluids for electrical equipment. [ 2 ] They are highly toxic and carcinogenic chemical compounds, formerly used in industrial and consumer electronic products, whose production was banned internationally by the Stockholm Convention on Persistent Organic Pollutants in 2001.
Because of their longevity, PCBs are still widely in use, even though their manufacture has declined drastically since the 1960s, when a host of problems were identified. [ 3 ] With the discovery of PCBs' environmental toxicity , and classification as persistent organic pollutants , their production was banned for most uses by United States federal law on January 1, 1978.
The International Agency for Research on Cancer (IARC) rendered PCBs as definite carcinogens in humans. According to the U.S. Environmental Protection Agency (EPA), PCBs cause cancer in animals and are probable human carcinogens. [ 4 ] Moreover, because of their use as a coolant in electric transformers, PCBs still persist in built environments. [ 5 ] [ 6 ]
Some PCBs share a structural similarity and toxic mode of action with dioxins . [ 7 ] Other toxic effects such as endocrine disruption (notably blocking of thyroid system functioning) and neurotoxicity are known. [ 8 ] The bromine analogues of PCBs are polybrominated biphenyls (PBBs), which have analogous applications and environmental concerns.
An estimated 1.2 million tons have been produced globally. [ 9 ] Though the US EPA enforced the federal ban as of 1978, PCBs continued to create health problems in later years through their continued presence in soil and sediment, and from products which were made before 1979. [ 10 ] In 1988, Japanese scientists Tanabe et al. estimated 370,000 tons were in the environment globally, and 780,000 tons were present in products, landfills, and dumps or kept in storage. [ 9 ]
The compounds are pale-yellow viscous liquids . They are hydrophobic, with low water solubilities : 0.0027–0.42 ng /L for Aroclors brand , [ 11 ] [ page needed ] but they have high solubilities in most organic solvents , oils, and fats . They have low vapor pressures at room temperature. They have dielectric constants of 2.5–2.7, [ 12 ] very high thermal conductivity , [ 11 ] [ page needed ] and high flash points (from 170 to 380 °C). [ 11 ] [ page needed ]
The density varies from 1.182 to 1.566 g/cm 3 . [ 11 ] [ page needed ] Other physical and chemical properties vary widely across the class. As the degree of chlorination increases, melting point and lipophilicity increase, and vapour pressure and water solubility decrease. [ 11 ] [ page needed ]
PCBs do not easily break down or degrade, which made them attractive for industries. PCB mixtures are resistant to acids, bases, oxidation, hydrolysis, and temperature change. [ 13 ] They can generate extremely toxic dibenzodioxins and dibenzofurans through partial oxidation. Intentional degradation as a treatment of unwanted PCBs generally requires high heat or catalysis (see Methods of destruction below).
PCBs readily penetrate skin , PVC (polyvinyl chloride), and latex (natural rubber). [ 14 ] PCB-resistant materials include Viton , polyethylene , polyvinyl acetate (PVA), polytetrafluoroethylene (PTFE), butyl rubber , nitrile rubber , and Neoprene . [ 14 ]
PCBs are derived from biphenyl , which has the formula C 12 H 10 , sometimes written (C 6 H 5 ) 2 . In PCBs, some of the hydrogen atoms in biphenyl are replaced by chlorine atoms. There are 209 different chemical compounds in which one to ten chlorine atoms can replace hydrogen atoms. PCBs are typically used as mixtures of compounds and are given the single identifying CAS number 1336-36-3 . About 130 different individual PCBs are found in commercial PCB products. [ 11 ] : 2
Toxic effects vary depending on the specific PCB. In terms of their structure and toxicity, PCBs fall into two distinct categories, referred to as coplanar or non- ortho -substituted arene substitution patterns and noncoplanar or ortho -substituted congeners .
Di- ortho -substituted, non-coplanar PCBs interfere with intracellular signal transduction dependent on calcium which may lead to neurotoxicity . [ 19 ] ortho -PCBs can disrupt thyroid hormone transport by binding to transthyretin . [ 20 ]
Commercial PCB mixtures were marketed under the following names: [ 21 ] [ 22 ]
Brazil
Czech Republic and Slovakia
France
Germany
Italy
Japan
Former USSR
United Kingdom
United States
The only North American producer, Monsanto Company , marketed PCBs under the trade name Aroclor from 1930 to 1977. These were sold under trade names followed by a four-digit number. In general, the first two digits refer to the product series as designated by Monsanto (e.g. 1200 or 1100 series); the second two numbers indicate the percentage of chlorine by mass in the mixture. Thus, Aroclor 1260 is a 1200 series product and contains 60% chlorine by mass. It is a myth that the first two digits referred to the number of carbon atoms; the number of carbon atoms do not change in PCBs. The 1100 series was a crude PCB material which was distilled to create the 1200 series PCB product. [ 24 ]
The exception to the naming system is Aroclor 1016 which was produced by distilling 1242 to remove the highly chlorinated congeners to make a more biodegradable product. "1016" was given to this product during Monsanto's research stage for tracking purposes but the name stuck after it was commercialized.
Different Aroclors were used at different times and for different applications. In electrical equipment manufacturing in the US, Aroclor 1260 and Aroclor 1254 were the main mixtures used before 1950; Aroclor 1242 was the main mixture used in the 1950s and 1960s until it was phased out in 1971 and replaced by Aroclor 1016. [ 11 ] [ page needed ]
One estimate (2006) suggested that 1 million tonnes of PCBs had been produced. 40% of this material was thought to remain in use. [ 2 ] Another estimate put the total global production of PCBs on the order of 1.5 million tonnes. The United States was the single largest producer with over 600,000 tonnes produced between 1930 and 1977. The European region follows with nearly 450,000 tonnes through 1984. It is unlikely that a full inventory of global PCB production will ever be accurately tallied, as there were factories in Poland, East Germany, and Austria that produced unknown amounts of PCBs. As of 2002 [update] , there were still 21,500 tons of PCBs stored in the easternmost regions of Slovakia. [ 25 ]
Although deliberate production of PCBs is banned by international treaty, significant amounts of PCBs are still being "inadvertently" produced. Research suggests that 45,000 tons of 'by-product' PCBs are legally produced per year in the US as part of certain chemical and product formulations. [ 26 ] [ 27 ]
Commercial production of PCBs was banned in the United States in 1979, with the passage of the Toxic Substances Control Act (TSCA). [ 28 ]
The utility of PCBs is based largely on their chemical stability, including low flammability and high dielectric constant. In an electric arc, PCBs generate incombustible gases.
Use of PCBs is commonly divided into closed and open applications. [ 2 ] Examples of closed applications include coolants and insulating fluids ( transformer oil ) for transformers and capacitors , such as those used in old fluorescent light ballasts, [ 29 ] and hydraulic fluids considered a semi-closed application. In contrast, the major open application of PCBs was in carbonless copy ("NCR") paper , which even presently results in paper contamination. [ 30 ]
Other open applications were lubricating and cutting oils, and as plasticizers in paints and cements, stabilizing additives in flexible PVC coatings of electrical cables and electronic components, pesticide extenders, reactive flame retardants and sealants for caulking , adhesives , wood floor finishes, such as Fabulon and other products of Halowax in the U.S., [ 31 ] de-dusting agents, waterproofing compounds, casting agents. [ 11 ] It was also used as a plasticizer in paints and especially "coal tars" that were used widely to coat water tanks, bridges and other infrastructure pieces.
Modern sources include pigments , which may be used in inks for paper or plastic products. [ 32 ] PCBs are also still found in old equipment like capacitors, ballasts, X-ray machine, and other e-waste. [ 33 ]
PCBs have entered the environment through both use and disposal. The environmental fate of PCBs is complex and global in scale. [ 11 ]
Because of their low vapour pressure , PCBs accumulate primarily in the hydrosphere , despite their hydrophobicity , in the organic fraction of soil , and in organisms including the human body. [ 34 ] The hydrosphere is the main reservoir. The immense volume of water in the oceans is still capable of dissolving a significant quantity of PCBs. [ 35 ]
As the pressure of ocean water increases with depth, PCBs become heavier than water and sink to the deepest ocean trenches where they are concentrated. [ 36 ]
A small volume of PCBs has been detected throughout the Earth's atmosphere. The atmosphere serves as the primary route for global transport of PCBs, particularly for those congeners with one to four chlorine atoms. [ 37 ]
In the atmosphere, PCBs may be degraded by hydroxyl radicals , or directly by photolysis of carbon–chlorine bonds (even if this is a less important process). [ 38 ]
Atmospheric concentrations of PCBs tend to be lowest in rural areas, where they are typically in the picogram per cubic meter range, higher in suburban and urban areas, and highest in city centres, where they can reach 1 ng /m 3 or more. [ 39 ] In Milwaukee , an atmospheric concentration of 1.9 ng/m 3 has been measured, and this source alone was estimated to account for 120 kg/year of PCBs entering Lake Michigan . [ 40 ] In 2008, concentrations as high as 35 ng /m 3 , 10 times higher than the EPA guideline limit of 3.4 ng/m 3 , have been documented inside some houses in the U.S. [ 31 ]
Volatilization of PCBs in soil was thought to be the primary source of PCBs in the atmosphere, but research suggests ventilation of PCB-contaminated indoor air from buildings is the primary source of PCB contamination in the atmosphere. [ 41 ]
In the biosphere , PCBs can be degraded by the sun , bacteria or eukaryotes , but the speed of the reaction depends on both the number and the disposition of chlorine atoms in the molecule: less substituted, meta - or para -substituted PCBs undergo biodegradation faster than more substituted congeners. [ citation needed ]
In bacteria, PCBs may be dechlorinated through reductive dechlorination , or oxidized by dioxygenase enzyme. [ citation needed ] In eukaryotes, PCBs may be oxidized by the cytochrome P450 enzyme. [ 42 ]
Like many lipophilic toxins, PCBs undergo biomagnification and bioaccumulation primarily due to the fact that they are easily retained within organisms. [ 43 ] [ 44 ]
Plastic pollution, specifically microplastics , are a major contributor of PCBs into the biosphere and especially into marine environments. PCBs concentrate in marine environments because freshwater systems, like rivers, act as a bridge for plastic pollution to be transported from terrestrial environments into marine environments. [ 45 ] It has been estimated that 88–95% of marine plastic is exported into the ocean by just 10 major rivers. [ 43 ]
An organism can accumulate PCBs by consuming other organisms that have previously ingested PCBs from terrestrial, freshwater, or marine environments. The concentration of PCBs within an organism will increase over their lifetime; this process is called bioaccumulation. PCB concentrations within an organism also change depending upon which trophic level they occupy. When an organism occupies a high trophic level, like orcas or humans , they will accumulate more PCBs than an organism that occupies a low trophic level, like phytoplankton . If enough organisms with a trophic level are killed due to the accumulation of toxins, like PCB, a trophic cascade can occur.
PCBs can cause harm to human health or even death when eaten. [ 46 ] PCBs can be transported by birds from aquatic sources onto land via feces and carcasses. [ 47 ]
PCBs undergo xenobiotic biotransformation, a mechanism used to make lipophilic toxins more polar and more easily excreted from the body. [ 48 ] The biotransformation is dependent on the number of chlorine atoms present, along with their position on the rings. Phase I reactions occur by adding an oxygen to either of the benzene rings by Cytochrome P450 . [ 49 ] The type of P450 present also determines where the oxygen will be added; phenobarbital (PB)-induced P450s catalyze oxygenation to the meta-para positions of PCBs while 3-methylcholanthrene (3MC)-induced P450s add oxygens to the ortho – meta positions. [ 50 ] PCBs containing ortho – meta and meta – para protons can be metabolized by either enzyme, making them the most likely to leave the organism. However, some metabolites of PCBs containing ortho – meta protons have increased steric hindrance from the oxygen, causing increased stability and an increased chance of accumulation. [ 51 ]
Metabolism is also dependent on the species of organism; different organisms have slightly different P450 enzymes that metabolize certain PCBs better than others. Looking at the PCB metabolism in the liver of four sea turtle species (green, olive ridley , loggerhead and hawksbill), green and hawksbill sea turtles have noticeably higher hydroxylation rates of PCB 52 than olive ridley or loggerhead sea turtles. This is because the green and hawksbill sea turtles have higher P450 2-like protein expression. This protein adds three hydroxyl groups to PCB 52, making it more polar and water-soluble. P450 3-like protein expression that is thought to be linked to PCB 77 metabolism, something that was not measured in this study. [ 48 ]
Temperature plays a key role in the ecology, physiology and metabolism of aquatic species. The rate of PCB metabolism was temperature dependent in yellow perch ( Perca flavescens ). In fall and winter, only 11 out of 72 introduced PCB congeners were excreted and had halflives of more than 1,000 days. During spring and summer when the average daily water temperature was above 20 °C, persistent PCBs had halflives of 67 days. The main excretion processes were fecal egestion, growth dilution and loss across respiratory surfaces. The excretion rate of PCBs matched with the perch's natural bioenergetics , where most of their consumption, respiration and growth rates occur during the late spring and summer. Since the perch is performing more functions in the warmer months, it naturally has a faster metabolism and has less PCB accumulation. However, multiple cold-water periods mixed with toxic PCBs with coplanar chlorine molecules can be detrimental to perch health. [ 52 ]
Enantiomers of chiral compounds have similar chemical and physical properties, but can be metabolized by the body differently. This was looked at in bowhead whales ( Balaena mysticetus ) for two main reasons: they are large animals with slow metabolisms (meaning PCBs will accumulate in fatty tissue) and few studies have measured chiral PCBs in cetaceans. They found that the average PCB concentrations in the blubber were approximately four times higher than the liver; however, this result is most likely age- and sex-dependent. As reproductively active females transferred PCBs and other poisonous substances to the fetus, the PCB concentrations in the blubber were significantly lower than males of the same body length (less than 13 meters). [ 53 ]
The toxicity of PCBs varies considerably among congeners . The coplanar PCBs, known as nonortho PCBs because they are not substituted at the ring positions ortho to (next to) the other ring, (such as PCBs 77, 126 and 169), tend to have dioxin -like properties, and generally are among the most toxic congeners. Because PCBs are almost invariably found in complex mixtures, the concept of toxic equivalency factors ( TEFs ) has been developed to facilitate risk assessment and regulation, where more toxic PCB congeners are assigned higher TEF values on a scale from 0 to 1. One of the most toxic compounds known, 2,3,7,8-tetrachlorodibenzo[p]dioxin , a PCDD , is assigned a TEF of 1. [ 54 ] In June 2020, State Impact of Pennsylvania stated that "In 1979, the EPA banned the use of PCBs, but they still exist in some products produced before 1979. They persist in the environment because they bind to sediments and soils. High exposure to PCBs can cause birth defects, developmental delays, and liver changes." [ 10 ]
In general, people are exposed to PCBs overwhelmingly through food, much less so by breathing contaminated air, and least by skin contact. Once exposed, some PCBs may change to other chemicals inside the body. These chemicals or unchanged PCBs can be excreted in feces or may remain in a person's body for years, with half lives estimated at 10–15 years. [ 55 ] PCBs collect in body fat and milk fat. [ 56 ] PCBs biomagnify up the food web and are present in fish and overflow of contaminated aquifers. [ 57 ] Human infants are exposed to PCBs through breast milk or by intrauterine exposure through transplacental transfer of PCBs [ 56 ] and are at the top of the food chain. [ 58 ] : 249ff
Workers recycling old equipment in the electronics recycling industry can also be exposed to PCBs. [ 33 ]
The most commonly observed health effects in people exposed to extremely high levels of PCBs are skin conditions, such as chloracne and rashes , but these were known to be symptoms of acute systemic poisoning dating back to 1922. Studies in workers exposed to PCBs have shown changes in blood and urine that may indicate liver damage. In Japan in 1968, 280 kg of PCB-contaminated rice bran oil was used as chicken feed, resulting in a mass poisoning, known as Yushō disease , in over 1800 people. [ 59 ] Common symptoms included dermal and ocular lesions , irregular menstrual cycles and lowered immune responses . [ 59 ] [ 60 ] [ 61 ] Other symptoms included fatigue, headaches, coughs, and unusual skin sores. [ 62 ] Additionally, in children, there were reports of poor cognitive development. [ 59 ] Women exposed to PCBs before or during pregnancy can give birth to children with lowered cognitive ability, compromised immune systems, and motor control problems. [ 63 ] [ 56 ] [ 64 ]
There is evidence that crash dieters that have been exposed to PCBs have an elevated risk of health complications. Stored PCBs in the adipose tissue become mobilized into the blood when individuals begin to crash diet. [ 65 ]
PCBs have shown toxic and mutagenic effects by interfering with hormones in the body. PCBs, depending on the specific congener, have been shown to both inhibit and imitate estradiol , the main sex hormone in females. Imitation of the estrogen compound can feed estrogen-dependent breast cancer cells, and possibly cause other cancers, such as uterine or cervical . Inhibition of estradiol can lead to serious developmental problems for both males and females, including sexual, skeletal, and mental development issues. [ citation needed ] [ 66 ] In a cross-sectional study, PCBs were found to be negatively associated with testosterone levels in adolescent boys. [ 67 ]
High PCB levels in adults have been shown to result in reduced levels of the thyroid hormone triiodothyronine , which affects almost every physiological process in the body, including growth and development, metabolism, body temperature, and heart rate. It also resulted in reduced immunity and increased thyroid disorders. [ 55 ] [ 68 ] [ unreliable medical source? ]
Animals that eat PCB-contaminated food, even for short periods of time, suffer liver damage and may die. In 1968 in Japan, 400,000 birds died after eating poultry feed that was contaminated with PCBs. [ 69 ] Animals that ingest smaller amounts of PCBs in food over several weeks or months develop various health effects, including anemia ; acne-like skin conditions ( chloracne ); liver, stomach , and thyroid gland injuries (including hepatocarcinoma), [ citation needed ] and thymocyte apoptosis . [ 55 ] Other effects of PCBs in animals include changes in the immune system , behavioral alterations, and impaired reproduction. [ 55 ] PCBs that have dioxin -like activity are known to cause a variety of teratogenic effects in animals. Exposure to PCBs causes hearing loss and symptoms similar to hypothyroidism in rats. [ 70 ]
In 2013, the International Agency for Research on Cancer (IARC) classified dioxin-like PCBs as human carcinogens . [ 71 ] According to the U.S. EPA, PCBs have been shown to cause cancer in animals and evidence supports a cancer-causing effect in humans. [ 4 ] Per the EPA, studies have found increases in malignant melanoma and rare liver cancers in PCB workers. [ 4 ]
In 2013, the IARC determined that the evidence for PCBs causing non-Hodgkin lymphoma is "limited" and "not consistent". [ 71 ] In contrast an association between elevated blood levels of PCBs and non-Hodgkin lymphoma had been previously accepted. [ 72 ] PCBs may play a role in the development of cancers of the immune system because some tests of laboratory animals subjected to very high doses of PCBs have shown effects on the animals' immune system, and some studies of human populations have reported an association between environmental levels of PCBs and immune response. [ 4 ]
In the early 1990s, Monsanto faced several lawsuits over harm caused by PCBs from workers at companies such as Westinghouse that bought PCBs from Monsanto and used them to build electrical equipment. [ 73 ] Monsanto and its customers, such as Westinghouse and GE, also faced litigation from third parties, such as workers at scrap yards that bought used electrical equipment and broke them down to reclaim valuable metals. [ 74 ] [ 75 ] Monsanto settled some of these cases and won the others, on the grounds that it had clearly told its customers that PCBs were dangerous chemicals and that protective procedures needed to be implemented. [ 76 ]
In 2003, Monsanto and Solutia Inc. , a Monsanto corporate spin-off, reached a $700 million settlement with the residents of West Anniston, Alabama , who had been affected by the manufacturing and dumping of PCBs. [ 77 ] [ 78 ] In a trial lasting six weeks, the jury found that "Monsanto had engaged in outrageous behavior, and held the corporations and its corporate successors liable on all six counts it considered – including negligence, nuisance, wantonness and suppression of the truth." [ 79 ] [ unreliable source? ]
In 2014, the Los Angeles Superior Court found that Monsanto was not liable for cancers claimed to be from PCBs permeating the food supply of three plaintiffs who had developed non-Hodgkin's lymphoma. After a four-week trial, the jury found that Monsanto's production and sale of PCBs between 1935 and 1977 were not substantial causes of the cancer. [ 80 ]
In 2015, the cities of Spokane, San Diego, and San Jose initiated lawsuits against Monsanto to recover cleanup costs for PCB contaminated sites, alleging that Monsanto continued to sell PCBs without adequate warnings after they knew of their toxicity. Monsanto issued a media statement concerning the San Diego case, claiming that improper use or disposal by third-parties, of a lawfully sold product, was not the company's responsibility. [ 81 ] [ 82 ] [ 83 ]
In July 2015, a St Louis county court in Missouri found that Monsanto, Solutia, Pharmacia and Pfizer were not liable for a series of deaths and injuries caused by PCBs manufactured by Monsanto Chemical Company until 1977. The trial took nearly a month and the jury took a day of deliberations to return a verdict against the plaintiffs from throughout the USA. [ 84 ] [ 85 ] Similar cases are ongoing. "The evidence simply doesn't support the assertion that the historic use of PCB products was the cause of the plaintiffs' harms. We are confident that the jury will conclude, as two other juries have found in similar cases, that the former Monsanto Company is not responsible for the alleged injuries," a Monsanto statement said. [ 86 ]
In May 2016, a Missouri state jury ordered Monsanto to pay $46.5 million to three plaintiffs whose exposure to PCB caused non-Hodgkin lymphoma. [ 87 ] [ 88 ]
In December 2016, the state of Washington filed suit in King County. The state sought damages and clean up costs related to PCBs. [ 89 ] [ 90 ] In March 2018 Ohio Attorney General Mike DeWine also filed a lawsuit against Monsanto over health issues posed by PCBs. [ 91 ]
On November 21, 2019, a federal judge denied a bid by Monsanto to dismiss a lawsuit filed by LA County calling the company to clean up cancer-causing PCBs from Los Angeles County waterways and storm sewer pipelines. [ 92 ] The lawsuit calls for Monsanto to pay for cleanup of PCBs from dozens of waterways, including the LA River, San Gabriel River and the Dominguez Watershed. [ 92 ]
In June 2020, Bayer agreed to pay $650 million to settle local lawsuits related to Monsanto's pollution of public waters in various areas of the United States with PCBs. [ 10 ]
In 2023, over 90 Vermont school districts joined a lawsuit against Monsanto alleging that PCBs created by the company were used in the construction of their schools. The Vermont Attorney General 's office also filed its own lawsuit against Monsanto related to the use of its PCBs. [ 93 ]
In 1865, the first "PCB-like" chemical was discovered, and was found to be a byproduct of coal tar . Years later in 1876, German chemist Oscar Döbner (Doebner) synthesized the first PCB in a laboratory. [ 94 ] Since then, large amounts of PCBs were released into the environment, to the extent that there are even measurable amounts of PCBs in feathers of birds currently held in museums before the production of PCBs peaked. [ 95 ]
In 1935, Monsanto Chemical Company (later Solutia Inc ) took over commercial production of PCBs from Swann Chemical Company which had begun in 1929. PCBs, originally termed "chlorinated diphenyls", were commercially produced as mixtures of isomers at different degrees of chlorination. The electric industry used PCBs as a non-flammable replacement for mineral oil to cool and insulate industrial transformers and capacitors. PCBs were also commonly used as heat stabilizer in cables and electronic components to enhance the heat and fire resistance of PVC. [ 96 ]
In the 1930s, the toxicity associated with PCBs and other chlorinated hydrocarbons, including polychlorinated naphthalenes , was recognized because of a variety of industrial incidents. [ 97 ] Between 1936 and 1937, there were several medical cases and papers released on the possible link between PCBs and its detrimental health effects. In 1936, a U.S. Public health Service official described the wife and child of a worker from the Monsanto Industrial Chemical Company who exhibited blackheads and pustules on their skin. The official attributed these symptoms to contact with the worker's clothing after he returned from work. In 1937, a conference about the hazards was organized at Harvard School of Public Health , and a number of publications referring to the toxicity of various chlorinated hydrocarbons were published before 1940. [ 98 ]
In 1947, Robert Brown reminded chemists that Arochlors were "objectionably toxic": "Thus the maximum permissible concentration for an 8-hr. day is 1 mg. per cu.m. [1.0 mg/m 3 ] of air. They also produce a serious and disfiguring dermatitis ". [ 99 ]
In 1954, Kanegafuchi Chemical Co. Ltd. ( Kaneka Corporation ) first produced PCBs, and continued until 1972. [ 11 ] [ page needed ]
Through the 1960s Monsanto Chemical Company knew increasingly more about PCBs' harmful effects on humans and the environment, per internal leaked documents released in 2002, yet PCB manufacture and use continued with few restraints until the 1970s. [ 100 ]
In 1966, PCBs were determined by Swedish chemist Sören Jensen to be an environmental contaminant. [ 101 ] Jensen, according to a 1994 article in Sierra , named chemicals PCBs, which previously, had simply been called "phenols" or referred to by various trade names, such as Aroclor, Kanechlor, Pyrenol, Chlorinol and others. In 1972, PCB production plants existed in Austria, West Germany , France, the UK, Italy, Japan, Spain, the USSR and the US. [ 11 ] [ page needed ]
In the early 1970s, Ward B. Stone of the New York State Department of Environmental Conservation (NYSDEC) first published his findings that PCBs were leaking from transformers and had contaminated the soil at the bottom of utility poles.
There have been allegations that Industrial Bio-Test Laboratories engaged in data falsification in testing relating to PCBs. [ 102 ] [ 103 ] [ 104 ] [ 105 ] In 2003, Monsanto and Solutia Inc. , a Monsanto corporate spinoff, reached a US$700 million settlement with the residents of West Anniston , Alabama, who had been affected by the manufacturing and dumping of PCBs. [ 77 ] [ 78 ] In a trial lasting six weeks, the jury found that "Monsanto had engaged in outrageous behavior, and held the corporations and its corporate successors liable on all six counts it considered – including negligence, nuisance, wantonness and suppression of the truth." [ 79 ]
Existing products containing PCBs which are "totally enclosed uses" such as insulating fluids in transformers and capacitors, vacuum pump fluids, and hydraulic fluid , are allowed to remain in use in the US. [ 106 ] The public, legal, and scientific concerns about PCBs arose from research indicating they are likely carcinogens having the potential to adversely impact the environment and, therefore, undesirable as commercial products. Despite active research spanning five decades, extensive regulatory actions, and an effective ban on their production since the 1970s, PCBs still persist in the environment and remain a focus of attention. [ 11 ] [ page needed ]
In 1999, the Dioxin Affair occurred when 50 kg of PCB transformer oils were added to a stock of recycled fat used for the production of 500 tonnes of animal feed, eventually affecting around 2,500 farms in several countries. [ 107 ] [ 108 ] The name Dioxin Affair was coined from early misdiagnosis of dioxins as the primary contaminants, when in fact they turned out to be a relatively small part of the contamination caused by thermal reactions of PCBs. The PCB congener pattern suggested the contamination was from a mixture of Aroclor 1260 and 1254. Over 9 million chickens, and 60,000 pigs were destroyed because of the contamination. The extent of human health effects has been debated, in part because of the use of differing risk assessment methods. One group predicted increased cancer rates, and increased rates of neurological problems in those exposed as neonates. A second study suggested carcinogenic effects were unlikely and that the primary risk would be associated with developmental effects due to exposure in pregnancy and neonates. [ 108 ] Two businessmen who knowingly sold the contaminated feed ingredient received two-year suspended sentences for their role in the crisis. [ 109 ]
The Italian company Caffaro, located in Brescia , specialized in producing PCBs from 1938 to 1984, following the acquisition of the exclusive rights to use the patent in Italy from Monsanto. [ 110 ] The pollution resulting from this factory and the case of Anniston, in the US, are the largest known cases in the world of PCB contamination in water and soil, in terms of the amount of toxic substance dispersed, size of the area contaminated, number of people involved and duration of production.
The values reported by the local health authority (ASL) of Brescia since 1999 are 5,000 times above the limits set by Ministerial Decree 471/1999 (levels for residential areas, 0.001 mg/kg). As a result of this and other investigations, in June 2001, a complaint of an environmental disaster was presented to the Public Prosecutor's Office of Brescia. Research on the adult population of Brescia showed that residents of some urban areas, former workers of the plant, and consumers of contaminated food, have PCB levels in their bodies that are in many cases 10–20 times higher than reference values in comparable general populations. [ 111 ] [ medical citation needed ] PCBs entered the human food supply by animals grazing on contaminated pastures near the factory, especially in local veal mostly eaten by farmers' families. [ 112 ] The exposed population showed an elevated risk of Non-Hodgkin lymphoma , but not for other specific cancers. [ 113 ]
In 1968, a mixture of dioxins and PCBs got into rice bran oil produced in northern Kyushu . Contaminated cooking oil sickened more than 1,860 people. The symptoms were called Yushō disease . [ 59 ]
In Okinawa, high levels of PCB contamination in soil on Kadena Air Base were reported in 1987 at thousands of parts per million, some of the highest levels found in any pollution site in the world. [ 114 ]
In December 2008, a number of Irish news sources reported testing had revealed "extremely high" levels of dioxins, by toxic equivalent , in pork products, ranging from 80 to 200 times the EU's upper safe limit of 1.5 pg WHO-TEQ DFP /μg i.e. 0.12 to 0.3 parts per billion. [ 115 ] [ 116 ]
Brendan Smith , the Minister for Agriculture, Fisheries and Food, stated the pork contamination was caused by PCB-contaminated feed that was used on 9 of Ireland's 400 pig farms, and only one feed supplier was involved. [ 115 ] [ 117 ] Smith added that 38 beef farms also used the same contaminated feed, but those farms were quickly isolated and no contaminated beef entered the food chain. [ 118 ] While the contamination was limited to just 9 pig farms, the Irish government requested the immediate withdrawal and disposal of all pork-containing products produced in Ireland and purchased since September 1, 2008. This request for withdrawal of pork products was confirmed in a press release by the Food Safety Authority of Ireland on December 6. [ 119 ]
It is thought that the incident resulted from the contamination of fuel oil used in a drying burner at a single feed processor, with PCBs. The resulting combustion produced a highly toxic mixture of PCBs, dioxins and furans , which was included in the feed produced and subsequently fed to a large number of pigs. [ 120 ]
In Kenya, a number of cases have been reported in the 2010s of thieves selling transformer oil , stolen from electric transformers , to the operators of roadside food stalls for use in deep frying . When used for frying, it is reported that transformer oil lasts much longer than regular cooking oil . The downside of this misuse of the transformer oil is the threat to the health of the consumers, due to the presence of PCBs. [ 121 ]
The chemical plant Chemko in Strážske (east Slovakia ) was an important producer of polychlorinated biphenyls for the former communist bloc ( Comecon ) until 1984. Chemko contaminated a large part of east Slovakia, especially the sediments of the Laborec river and reservoir Zemplínska šírava . [ 122 ] [ 123 ]
Between 1962 and 1983, the Iskra Kondenzatorji company in Semič ( White Carniola , Southeast Slovenia ) manufactured capacitors using PCBs. Due to the wastewater and improperly disposed waste products, the area (including the Krupa and Lahinja rivers) became highly contaminated with PCBs. The pollution was discovered in 1983, when the Krupa river was meant to become a water supply source. The area was sanitized then, but the soil and water are still highly polluted. Traces of PCBs were found in food (eggs, cow milk, walnuts) and Krupa is still the most PCB-polluted river in the world.
Several cetacean species have very high mean blubber PCB concentrations likely to cause population declines and suppress population recovery. Striped dolphins , bottlenose dolphins and orcas were found to have mean levels that markedly exceeded all known marine mammal PCB toxicity thresholds. The western Mediterranean Sea and the south-west Iberian Peninsula were identified as "hotspots". [ 124 ]
Monsanto manufactured PCBs at its chemical plant in Newport , South Wales, until the mid- to late-1970s. During this period, waste matter, including PCBs, from the Newport site was dumped at a disused quarry near Groes-faen , west of Cardiff , and Penhros landfill site [ 125 ] from where it continues to be released in waste water discharges. [ 126 ]
Monsanto was the only company that manufactured PCBs in the US. Its production was entirely halted in 1977. (Kimbrough, 1987, 1995) [ 9 ] On November 25, 2020, U.S. District Judge Fernando M. Olguin rejected a proposed $650 million settlement from Bayer, the company which acquired Monsanto in 2018, and allowed Monsanto-related lawsuits involving PCB to proceed. [ 127 ]
PCBs originating from Monsanto Chemical Company in Anniston , Alabama, were dumped into Snow Creek, which then spread to Choccolocco Creek , then Logan Martin Lake . [ 128 ] In the early 2000s, class action lawsuits were settled by local land owners, including those on Logan Martin Lake, and Lay Reservoir (downstream on the Coosa River ), for the PCB pollution. Donald Stewart, former Senator from Alabama, first learned of the concerns of hundreds of west Anniston residents after representing a church which had been approached about selling its property by Monsanto. Stewart went on to be the pioneer and lead attorney in the first and majority of cases against Monsanto and focused on residents in the immediate area known to be most polluted. Other attorneys later joined in to file suits for those outside the main immediate area around the plant; one of these was the late Johnnie Cochran .
In 2007, the highest pollution levels remained concentrated in Snow and Choccolocco Creeks. [ 129 ] Concentrations in fish have declined and continue to decline over time; sediment disturbance, however, can resuspend the PCBs from the sediment back into the water column and food web.
San Francisco Bay has been contaminated by PCBs, "a legacy of PCBs spread widely across the land surface of the watershed, mixed deep into the sediment of the Bay, and contaminating the Bay food web". [ 130 ] Levels of PCBs in fish and shellfish exceed thresholds for safe consumption. Signs around the Bay warn anglers of which species to avoid. State water quality regulators set a Total Maximum Daily Load for PCBs require city and county governments around the Bay to implement control measures to limit PCBs in urban runoff. [ 131 ] An important part of the second, revised version of this permit was the requirement for municipalities to install green infrastructure with a goal of reducing pollutant levels in stormwater. [ 132 ]
In New Haven , the decommissioned English Station has a high concentration of PCB contamination due to the chemicals used in the running of the plant. This, along with asbestos contamination, has made cleaning and demolishing the abandoned site extremely difficult. The PCB contamination has spread to the soil, and to the river, where locals will sometimes fish unaware of the danger. [ 133 ] [ 134 ] [ 135 ]
In 1976, environmentalists found PCBs in the sludge at Waukegan Harbor, the southwest end of Lake Michigan . They were able to trace the source of the PCBs back to the Outboard Marine Corporation that was producing boat motors next to the harbor. By 1982, the Outboard Marine Corporation was court-ordered to release quantitative data referring to their PCB waste released. The data stated that from 1954 they released 100,000 tons of PCB into the environment, and that the sludge contained PCBs in concentrations as high as 50%. [ 136 ] [ 137 ] [ self-published source? ]
In 1989, during construction near the Zilwaukee bridge, workers uncovered an uncharted landfill containing PCB-contaminated waste which cost $100,000 to clean up. [ 138 ]
Much of the Great Lakes area were still heavily polluted with PCBs in 1988, despite extensive remediation work. [ 139 ]
From the late 1950s through 1977, Westinghouse Electric used PCBs in the manufacture of capacitors in its Bloomington , Indiana, plant. Reject capacitors were hauled and dumped in area salvage yards and landfills, including Bennett's Dump, Neal's Landfill and Lemon Lane Landfill. [ 140 ] Workers also dumped PCB oil down factory drains, which contaminated the city sewage treatment plant. [ 141 ] The City of Bloomington gave away the sludge to area farmers and gardeners, creating anywhere from 200 to 2,000 sites, which remain unaddressed.
Over 1,000 tons of PCBs were estimated to have been dumped in Monroe and Owen counties. [ citation needed ] Although federal and state authorities have been working on the sites' environmental remediation , many areas remain contaminated. Concerns have been raised regarding the removal of PCBs from the karst limestone topography, and regarding the possible disposal options. To date, the Westinghouse Bloomington PCB Superfund site case does not have a Remedial Investigation/Feasibility Study (RI/FS) and Record of Decision (ROD), although Westinghouse signed a US Department of Justice Consent Decree in 1985. [ 140 ] The 1985 consent decree required Westinghouse to construct an incinerator that would incinerate PCB-contaminated materials. Because of public opposition to the incinerator, however, the State of Indiana passed a number of laws that delayed and blocked its construction. The parties to the consent decree began to explore alternative remedies in 1994 for six of the main PCB contaminated sites in the consent decree. Hundreds of sites remain unaddressed as of 2014. Monroe County will never be PCB-free, as noted in a 2014 Indiana University program about the local contamination. [ 140 ]
On February 15, 2008, Monroe County approved a plan to clean up the three remaining contaminated sites in the City of Bloomington, at a cost of $ 9.6 million to CBS Corp. , the successor of Westinghouse. In 1999, Viacom bought CBS, so they are current responsible party for the PCB sites. [ 142 ]
Pittsfield , in western Massachusetts, was home to the General Electric (GE) transformer, capacitor, and electrical generating equipment divisions . The electrical generating division built and repaired equipment that was used to power the electrical utility grid throughout the nation. PCB-contaminated oil routinely migrated from GE's 254-acre (1.03 km 2 ) industrial plant located in the very center of the city to the surrounding groundwater, nearby Silver Lake, and to the Housatonic River , which flows through Massachusetts, Connecticut, and down to Long Island Sound. [ 143 ] PCB-containing solid material was widely used as fill, including oxbows of the Housatonic River. Fish and waterfowl which live in and around the river contain significant levels of PCBs and are not safe to eat. [ 144 ] EPA designated the Pittsfield plant and several miles of the river as a Superfund site in 1997, and ordered GE to remediate the site. EPA and GE began a cleanup of the area in 1999. [ 143 ]
New Bedford Harbor , which is a listed Superfund site, [ 145 ] contained some of the highest sediment concentrations of PCBs in the marine environment. [ 146 ] Cleanup of the area began in 1994 and is mostly complete as of 2020. [ 145 ]
Investigations into historic waste dumping in the Bliss Corner neighborhood have revealed the existence of PCBs, among other hazardous materials, buried in soil and waste material. [ 147 ]
In 1982, Martha C. Rose Chemical Inc. began processing and disposing of materials contaminated with PCBs in Holden, Missouri , a small rural community about 40 miles (64 km) east of Kansas City. From 1982 until 1986, nearly 750 companies, including General Motors Corp., Commonwealth Edison, Illinois Power Co. and West Texas Utilities, sent millions of pounds of PCB contaminated materials to Holden for disposal. [ 148 ] Instead, according to prosecutors, the company began storing the contaminated materials while falsifying its reports to the EPA to show they had been removed. After investigators learned of the deception, Rose Chemical was closed and filed for bankruptcy. The site had become the nation's largest waste site for the chemical PCB. [ 149 ] In the four years the company was operational, the EPA inspected it four times and assessed $206,000 in fines but managed to collect only $50,000. [ 150 ]
After the plant closed the state environmental agency found PCB contamination in streams near the plant and in the city's sewage treatment sludge. A 100,000 square-foot warehouse and unknown amounts of contaminated soil and water around the site had to be cleaned up. Most of the surface debris, including close to 13 million pounds of contaminated equipment, carcasses and tanks of contaminated oil, had to be removed. [ 151 ] Walter C. Carolan, owner of Rose Chemical, and five others pleaded guilty in 1989 to committing fraud or falsifying documents. Carolan and two other executives served sentences of less than 18 months; the others received fines and were placed on probation. Cleanup costs at the site are estimated at $35 million. [ 151 ]
Two launch facilities at Malmstrom Air Force Base showed PCB levels higher than the thresholds recommended by the Environmental Protection Agency when extensive sampling began of active U.S. intercontinental ballistic missile bases to address specific cancer concerns in 2023. [ 152 ]
Pollution of the Hudson River is largely due to dumping of PCBs by General Electric from 1947 to 1977. GE dumped an estimated 1.3 million pounds of PCBs into the Hudson River during these years. The PCBs came from the company's two capacitor manufacturing plants at Hudson Falls and Fort Edward , New York. This pollution caused a range of harmful effects to wildlife and people who eat fish from the river or drink the water. [ 153 ] In 1984, EPA declared a 200-mile (320 km) stretch of the river, from Hudson Falls to New York City, to be a Superfund site requiring cleanup. [ 154 ] Extensive remediation actions on the river began in the 1970s with the implementation of wastewater discharge permits and consequent control or reduction of wastewater discharges, and sediment removal operations, which have continued into the 21st century. [ 155 ]
Love Canal is a neighborhood in Niagara Falls , New York, that was heavily contaminated with toxic waste including PCBs. [ 156 ] Eighteen Mile Creek in Lockport , New York, is an EPA Superfund site for PCBs contamination. [ 157 ]
PCB pollution at the State Office Building in Binghamton was responsible for what is now considered to be the first indoor environmental disaster in the United States. [ 158 ] In 1981, a transformer explosion in the basement spewed PCBs throughout the entire 18-story building. [ 159 ] The contamination was so severe that cleanup efforts kept the building closed for 13 years. [ 160 ] [ 161 ]
One of the largest deliberate PCB spills in American history occurred in the summer of 1978 when 31,000 gallons (117 m^3) of PCB-contaminated oil were illegally sprayed by the Ward PCB Transformer Company in 3-foot (0.91 m) swaths along the roadsides of some 240 miles (390 km) of North Carolina highway shoulders in 14 counties and at the Fort Bragg Army Base. The crime, known as " the midnight dumpings ", occurred over nearly two weeks, as drivers of a black-painted tanker truck drove down one side of rural Piedmont highways spraying PCB-laden waste and then up the other side the following night. [ 162 ]
Under Governor James B. Hunt , Jr., state officials then erected large, yellow warning signs along the contaminated highways that read: "CAUTION: PCB Chemical Spills Along Highway Shoulders". The illegal dumping is believed to have been motivated by the passing of the Toxic Substances Control Act (TSCA), which became effective on August 2, 1978, and increased the expense of chemical waste disposal.
Within a couple of weeks of the crime, Robert Burns and his sons, Timothy and Randall, were arrested for dumping the PCBs along the roadsides. Burns was a business partner of Robert "Buck" Ward Jr., of the Ward PCB Transformer Company, in Raleigh. Burns and sons pleaded guilty to state and Federal criminal charges; Burns received a three to five-year prison sentence. Ward was acquitted of state charges in the dumping, but was sentenced to 18 months prison time for violation of TSCA. [ 162 ]
Cleanup and disposal of the roadside PCBs generated controversy, as the Governor's plan to pick up the roadside PCBs and to bury them in a landfill in rural Warren County were strongly opposed in 1982 by local residents. [ 162 ]
In October 2013, at the request of the South Carolina Department of Health and Environmental Control (SCDHEC), the City of Charlotte , North Carolina, decided to stop applying sewage sludge to land while authorities investigated the source of PCB contamination. [ 163 ] In February 2014, the City of Charlotte admitted PCBs have entered their sewage treatment centers as well. [ 164 ]
After the 2013 SCDHEC had issued emergency regulations, [ 165 ] the City of Charlotte discovered high levels of PCBs entering its sewage waste water treatment plants, where sewage is converted to sewage sludge. [ 164 ] The city at first denied it had a problem, then admitted an "event" occurred in February 2014, and in April that the problem had occurred much earlier. [ 163 ] [ 166 ] The city stated that its very first test with a newly changed test method revealed very high PCB levels in its sewage sludge farm field fertilizer. Because of the widespread use of the contaminated sludge, SCDHEC subsequently issued PCB fish advisories for nearly all streams and rivers bordering farm fields that had been applied with city waste. [ 167 ]
The Clyde cancer cluster (also known as the Sandusky County cancer cluster) is a childhood cancer cluster that has affected many families in Clyde , Ohio, and surrounding areas. PCBs were found in soil in a public park within the area of the cancer cluster. [ 168 ]
In Akron , Ohio, soil was contaminated and noxious PCB-laden fumes had been put into the air by an electrical transformer deconstruction operation from the 1930s to the 1960s. [ 169 ]
From 1955 until 1977, the Sangamo Weston plant in Pickens, South Carolina, used PCBs to manufacture capacitors, and dumped 400,000 pounds of PCB contaminated wastewater into the Twelve Mile Creek. In 1990, the EPA declared the 228 acres (0.92 km 2 ) site of the capacitor plant, its landfills and the polluted watershed, which stretches nearly 1,000 acres (4.0 km 2 ) downstream to Lake Hartwell as a Superfund site. Two dams on the Twelve Mile Creek are to be removed and on Feb. 22, 2011 the first of two dams began to be dismantled. Some contaminated sediment is being removed from the site and hauled away, while other sediment is pumped into a series of settling ponds. [ 170 ] [ 171 ]
In 2013, the state environmental regulators issued a rare emergency order, banning all sewage sludge from being land applied or deposited on landfills, as it contained very high levels of PCBs. The problem had not been discovered until thousands of acres of farm land in the state had been contaminated by the hazardous sludge. A criminal investigation to determine the perpetrator of this crime was launched. [ 172 ]
As of 2015, several bodies of water in the state of Washington were contaminated with PCBs, including the Columbia River , the Duwamish River , Green Lake , Lake Washington , the Okanogan River , Puget Sound , the Spokane River , the Walla Walla River , the Wenatchee River , and the Yakima River . [ 173 ] A study by Washington State published in 2011 found that the two largest sources of PCB flow into the Spokane River were City of Spokane stormwater (44%) and municipal and industrial discharges (20%). [ 174 ]
PCBs entered the environment through paint, hydraulic fluids, sealants, inks and have been found in river sediment and wildlife. Spokane utilities will spend $300 million to prevent PCBs from entering the river in anticipation of a 2017 federal deadline to do so. [ 175 ] In August 2015 Spokane joined other U.S. cities like San Diego and San Jose , California, and Westport , Massachusetts, in seeking damages from Monsanto. [ 176 ]
From 1954 until 1971, the Fox River in Appleton, Wisconsin , had PCBs deposited into it from Appleton Paper/NCR, P.H. Gladfelter, Georgia-Pacific and other notable local paper manufacturing facilities. The Wisconsin DNR estimates that after wastewater treatment the PCB discharges to the Fox River due to production losses ranged from 81,000 kg to 138,000 kg. (178,572 lbs. to 304,235 lbs). The production of Carbon Copy Paper and its byproducts led to the discharge into the river. Fox River clean up is ongoing. [ 177 ]
Polychlorinated biphenyls have been discovered in organisms living in the Mariana Trench in the Pacific Ocean . Levels were as high as 1,900 nanograms per gram of amphipod tissue in the organisms analyzed. [ 178 ]
In 1972 the Japanese government banned the production, use, and import of PCBs. [ 11 ] [ page needed ]
In 1973, the use of PCBs in "open" or " dissipative " sources (such as plasticisers in paints and cements, casting agents, fire retardant fabric treatments and heat stabilizing additives for PVC electrical insulation, adhesives , paints and waterproofing, railroad ties ) was banned in Sweden.
In 1981, the UK banned closed uses of PCBs in new equipment, and nearly all UK PCB synthesis ceased; closed uses in existing equipment containing in excess of 5 litres of PCBs were not stopped until December 2000. [ 179 ]
In 1976, concern over the toxicity and persistence (chemical stability) of PCBs in the environment led the United States Congress to ban their domestic production, effective January 1, 1978, pursuant to the Toxic Substances Control Act . [ 180 ] [ 181 ] To implement the law, EPA banned new manufacturing of PCBs, but issued regulations that allowed for their continued use in electrical equipment for economic reasons. [ 182 ] EPA began issuing regulations for PCB usage and disposal in 1979. [ 183 ] The agency has issued guidance publications for safe removal and disposal of PCBs from existing equipment. [ 184 ]
EPA defined the "maximum contaminant level goal" for public water systems as zero, but because of the limitations of water treatment technologies, a level of 0.5 parts per billion is the actual regulated level ( maximum contaminant level ). [ 185 ]
PCBs were technically attractive because of their inertness, which includes their resistance to combustion. Nonetheless, they can be effectively destroyed by incineration at 1000 °C. When combusted at lower temperatures, they convert in part to more hazardous unintentional persistent organic pollutants, including polychlorinated dibenzofurans and dibenzo-p-dioxins. When conducted properly, the combustion products are water, carbon dioxide, and hydrogen chloride . In some cases, the PCBs are combusted as a solution in kerosene. PCBs have also been destroyed by pyrolysis in the presence of alkali metal carbonates . [ 2 ]
Thermal desorption is highly effective at removing PCBs from soil. [ 186 ]
PCBs are fairly chemically unreactive, this property being attractive for its application as an inert material. They resist oxidation . [ self-published source? ] [ citation needed ] Many chemical compounds are available to destroy or reduce the PCBs. Commonly, PCBs are degraded by basic mixtures of glycols , which displace some or all chloride. Also effective are reductants such as sodium or sodium naphthalene . [ 2 ] Vitamin B12 has also shown promise. [ 187 ]
The use of microorganisms to degrade PCBs from contaminated sites, relying on multiple microorganisms' co-metabolism, is known as bioremediation of polychlorinated biphenyl . Some micro-organisms degrade PCBs by reducing the C-Cl bonds. Microbial dechlorination tends to be rather slow-acting in comparison to other methods. Enzymes extracted from microbes can show PCB activity. In 2005, Shewanella oneidensis biodegraded a high percentage of PCBs in soil samples. [ 188 ] A low voltage current can stimulate the microbial degradation of PCBs. [ 189 ]
There is research showing that some ligninolytic fungi can degrade PCBs. [ 190 ]
The remediation, or removal, of PCBs from estuarian and coastal river sediments is quite difficult due to the overlying water column and the potential for resuspension of contaminants during the removal process. The most common method of PCB extraction from sediments is to dredge an area and dispose of the sediments in a landfill. This method is troubling for a number of reasons, namely that it has a risk of resuspension of the chemicals as the sediments are disturbed, and this method can be very damaging to ecosystems. [ 191 ]
A potential cost effective, low risk remediation technique is bioremediation. Bioremediation involves the use of biota to remediate sediments. Phytoremediation, the use of plants to remediate soils, has been found to be effective for a broad range of contaminants such as mercury PCB and PAHs in terrestrial soils. [ 192 ] A promising study conducted in New Bedford Harbor found that Ulva rigida , a type of seaweed common throughout the world, is effective at removing PCB from sediments. [ 193 ] During a typical bloom in New Bedford Harbor, U. rigida forms a thick mat that lies on top of and in contact with the sediment. This allows for U. rigida to uptake large amounts of PCB from the sediment with concentrations of PCB in U. rigida reaching 1580 μg kg −1 within 24 hours of the bloom. Live tissue tended to take up higher concentrations of PCB than dead tissue, but this is not to say that dead tissue did not still take up large amounts of PCB as well.
For a complete list of the 209 PCB congeners, see PCB congener list . Note that biphenyl, while not technically a PCB congener because of its lack of chlorine substituents, is still typically included in the literature. | https://en.wikipedia.org/wiki/Polychlorinated_biphenyl |
Polychlorinated dibenzodioxins ( PCDDs ), or simply dioxins , are a group of long-lived polyhalogenated organic compounds that are primarily anthropogenic, and contribute toxic, persistent organic pollution in the environment. [ 1 ]
They are commonly but inaccurately referred to as dioxins for simplicity, because every PCDD molecule contains a dibenzo-1,4-dioxin skeletal structure, with 1,4-dioxin as the central ring . Members of the PCDD family bioaccumulate in humans and wildlife because of their lipophilic properties, and may cause developmental disturbances and cancer .
Because dioxins can persist in the environment for more than 100 years, the majority of PCDD pollution today is not the result of recent emissions, but the cumulative result of synthetic processes undertaken since the beginning of the 20th century, including organochloride -related manufacturing, incineration of chlorine-containing substances such as polyvinyl chloride (PVC), and chlorine bleaching of paper. [ 2 ] [ 3 ] Forest fires and volcanic eruptions have also been cited as an airborne source, although their contribution to the current levels of PCDD accumulation are minor in comparison. [ 4 ] [ 5 ] Incidents of dioxin poisoning resulting from industrial emissions and accidents were first recorded as early as the mid 19th century during the Industrial Revolution . [ 6 ]
The word "dioxins" may also refer to other similarly acting chlorinated compounds (see Dioxins and dioxin-like compounds ).
The structure of dibenzo-1,4-dioxin consists of two benzene rings joined by two oxygen bridges. This makes the compound an aromatic di ether . The name dioxin formally refers to the central dioxygenated ring, which is stabilized by the two flanking benzene rings .
In PCDDs, chlorine atoms are attached to this structure at any of 8 different places on the molecule, at positions 1–4 and 6–9. There are 75 different PCDD congeners (that is, related dioxin compounds). [ 7 ]
The toxicity of PCDDs depends on the number and positions of the chlorine atoms. Congeners that have chlorine in the 2, 3, 7, and 8 positions have been found to be significantly toxic. In fact, 7 congeners have chlorine atoms in the relevant positions which were considered toxic by the World Health Organization toxic equivalent (WHO-TEQ) scheme. [ 8 ]
Low concentrations of dioxins existed in nature prior to industrialization as a result of natural combustion and geological processes. [ 9 ] [ 10 ] Dioxins were first unintentionally produced as by-products from 1848 onwards as Leblanc process plants started operating in Germany. [ 6 ] The first intentional synthesis of chlorinated dibenzodioxin was in 1872. Today, concentrations of dioxins are found in all humans, with higher levels commonly found in persons living in more industrialized countries. The most toxic dioxin, 2,3,7,8-tetrachlorodibenzodioxin (TCDD), became well known as a contaminant of Agent Orange , a herbicide used in the Malayan Emergency and the Vietnam War . [ 11 ] Later, dioxins were found in Times Beach , Missouri [ 12 ] and Love Canal , New York [ 13 ] and Seveso , Italy . [ 14 ] More recently, dioxins have been in the news with the poisoning of President Viktor Yushchenko of Ukraine in 2004, [ 15 ] the Naples Mozzarella Crisis, [ 16 ] the 2008 Irish pork crisis , and the German feed incident of 2010. [ 17 ]
The United States Environmental Protection Agency inventory of sources of dioxin-like compounds is possibly the most comprehensive review of the sources and releases of dioxins, [ 18 ] but other countries now have substantial research as well.
Occupational exposure is an issue for some in the chemical industries , historically for those making chlorophenols or chlorophenoxy acid herbicides or in the application of chemicals, notably herbicides. In many developed nations there are now emissions regulations which have dramatically decreased the emissions [ 18 ] and thus alleviated some concerns, although the lack of continuous sampling of dioxin emissions causes concern about the understatement of emissions. In Belgium , through the introduction of a process called AMESA , continuous sampling showed that periodic sampling understated emissions by a factor of 30 to 50 times. Few facilities have continuous sampling.
Dioxins are produced in small concentrations when organic material is burned in the presence of chlorine , whether the chlorine is present as chloride ions or as organochlorine compounds , so they are widely produced in many contexts. According to the most recent US EPA data, the major sources of dioxins are broadly in the following types: [ 18 ]
When first carried out in 1987, the original US EPA inventory of dioxin sources revealed that incineration represented more than 80% of known dioxin sources. As a result, US EPA implemented new emissions requirements. These regulations succeeded in reducing dioxin stack emissions from incinerators. Incineration of municipal solid waste , medical waste , sewage sludge , and hazardous waste together now produce less than 3% of all dioxin emissions. Since 1987, however, backyard barrel burning has showed almost no decrease, and is now the largest source of dioxin emissions, producing about one third of the total output. [ 18 ]
In incineration, dioxins can also reform or form de novo in the atmosphere above the stack as the exhaust gases cool through a temperature window of 600 to 200 °C. The most common method of reducing the quantity of dioxins reforming or forming de novo is through rapid (30 millisecond) quenching of the exhaust gases through that 400 °C window. [ 19 ] Incinerator emissions of dioxins have been reduced by over 90% as a result of new emissions control requirements. Incineration in developed countries is now a very minor [ citation needed ] contributor to dioxin emissions.
Dioxins are also generated in reactions that do not involve burning — such as chlorine bleaching fibers for paper or textiles, and in the manufacture of chlorinated phenols, particularly when reaction temperature is not well controlled. [ 20 ] Compounds involved include the wood preservative pentachlorophenol , and also herbicides such as 2,4-dichlorophenoxyacetic acid (or 2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T). Higher levels of chlorination require higher reaction temperatures and greater dioxin production. Dioxins may also be formed during the photochemical breakdown of the common antimicrobial compound triclosan . [ 21 ]
Tolerable daily, monthly or annual intakes have been set by the World Health Organization and a number of governments. Dioxins enter the general population almost exclusively from ingestion of food, specifically through the consumption of fish, meat, and dairy products since dioxins are fat-soluble and readily climb the food chain . [ 22 ] [ 23 ]
Children are passed substantial body burdens by their mothers, and breastfeeding increases the child's body burden. [ 24 ] Dioxin exposure can also occur from contact with Pentachlorophenol (Penta) treated lumber as Pentachlorophenol often contains dioxins as a contaminant. Children's daily intakes during breast feeding are often many times above the intakes of adults based on body weight. This is why the WHO consultation group assessed the tolerable intake so as to prevent a woman from accumulating harmful body burdens before her first pregnancy. [ 25 ] Breast fed children usually still have higher dioxin body burdens than non breast fed children. The WHO still recommends breast feeding for its other benefits. [ 26 ] In many countries dioxins in breast milk have decreased by even 90% during the two last decades. [ 27 ]
Dioxins are present in cigarette smoke . [ 28 ] Dioxin in cigarette smoke was noted as "understudied" by the US EPA in its "Re-Evaluating Dioxin" (1995). In that same document, the US EPA acknowledged that dioxin in cigarettes is "anthropogenic" (man-made, "not likely in nature").
Dioxins are absorbed primarily through dietary intake of fat, as this is where they accumulate in animals and humans. In humans, the highly chlorinated dioxins are stored in fatty tissues and are neither readily metabolized nor excreted. The estimated elimination half-life for highly chlorinated dioxins (4–8 chlorine atoms) in humans ranges from 4.9 to 13.1 years. [ 29 ]
The persistence of a particular dioxin congener in an animal is thought to be a consequence of its structure. Dioxins with no lateral (2, 3, 7, and 8) chlorines, which thus contain hydrogen atoms on adjacent pairs of carbons, can more readily be oxidized by cytochromes P450 . [ 30 ] The oxidized dioxins can then be more readily excreted rather than stored for a long time. [ citation needed ]
2,3,7,8-Tetrachlorodibenzodioxin (TCDD) is considered the most toxic of the congeners (for the mechanism of action, see 2,3,7,8-Tetrachlorodibenzodioxin and Aryl hydrocarbon receptor ). Other dioxin congeners including PCDFs and PCBs with dioxin-like toxicity, are given a toxicity rating from 0 to 1, where TCDD = 1 (see Dioxins and dioxin-like compounds ). This toxicity rating is called the Toxic Equivalence Factor concept, or TEF . TEFs are consensus values and, because of the strong species dependence for toxicity, are listed separately for mammals, fish, and birds. TEFs for mammalian species are generally applicable to human risk calculations. The TEFs have been developed from detailed assessment of literature data to facilitate both risk assessment and regulatory control. [ 8 ] Many other compounds may also have dioxin-like properties, particularly non-ortho PCBs , one of which has a TEF as high as 0.1.
The total dioxin toxic equivalence (TEQ) value expresses the toxicity as if the mixture were pure TCDD. The TEQ approach and current TEFs have been adopted internationally as the most appropriate way to estimate the potential health risks of mixture of dioxins. Recent data suggest that this type of simple scaling factor may not be the most appropriate treatment for complex mixtures of dioxins; both transfer from the source and absorption and elimination vary among different congeners, and the TEF value is not able to accurately reflect this. [ 31 ]
Dioxins and other persistent organic pollutants (POPs) are subject to the Stockholm Convention . The treaty obliges signatories to take measures to eliminate where possible, and minimize where not possible to eliminate, all sources of dioxin.
Dioxins build up primarily in fatty tissues over time ( bioaccumulation ), so even small exposures may eventually reach dangerous levels. In 1994, the US EPA reported that dioxins are a probable carcinogen , but noted that non-cancer effects (reproduction and sexual development, immune system) may pose a greater threat to human health. TCDD , the most toxic of the dibenzodioxins, is classified as a Group 1 carcinogen by the International Agency for Research on Cancer (IARC). TCDD has a half-life of approximately 8 years in humans, although at high concentrations, the elimination rate is enhanced by metabolism. [ 32 ] The health effects of dioxins are mediated by their action on a cellular receptor, the aryl hydrocarbon receptor (AhR). [ 33 ]
Exposure to high levels of dioxins in humans causes a severe form of persistent acne , known as chloracne . [ 34 ] High occupational or accidental levels of exposures to dioxins have been shown by epidemiological studies to lead to an increased risk of tumors at all sites. [ 35 ] Other effects in humans (at high dose levels) may include:
Recent studies have shown that high exposure to dioxins changes the ratio of male to female births among a population such that more females are born than males. [ 43 ]
Dioxins accumulate in food chains in a fashion similar to other chlorinated compounds ( bioaccumulation ). This means that even small concentrations in contaminated water can be concentrated up a food chain to dangerous levels because of the long biological half life and low water solubility of dioxins.
While it has been difficult to establish specific health effects in humans due to the lack of controlled dose experiments, studies in animals have shown that dioxin causes a wide variety of toxic effects. [ 44 ] In particular, TCDD has been shown to be teratogenic , mutagenic , carcinogenic , immunotoxic , and hepatotoxic . Furthermore, alterations in multiple endocrine and growth factor systems have been reported. The most sensitive effects, observed in multiple species, appear to be developmental, including effects on the developing immune , nervous , and reproductive systems . [ 45 ] The most sensitive effects are caused at body burdens relatively close to those reported in humans.
Among the animals for which TCDD toxicity has been studied, there is strong evidence for the following effects:
The LD50 of dioxin also varies wildly between species with the most notable disparity being between the ostensibly similar species of hamster and guinea pig. The oral LD50 for guinea pigs is as low as 0.5 to 2 μg/kg body weight, whereas the oral LD50 for hamsters can be as high as 1 to 5 mg/kg body weight, a difference of as much as thousandfold or more, and even among rat strains there may be thousandfold differences. [ 44 ]
Agent Orange was the code name for one of the herbicides and defoliants the U.S. military used as part of its herbicidal warfare program, Operation Ranch Hand , during the Vietnam War from 1961 to 1971. It was a mixture of 2,4,5-T and 2,4-D . The 2,4,5-T used was contaminated with 2,3,7,8-tetrachlorodibenzodioxin (TCDD), an extremely toxic dioxin compound .
During the Vietnam war, between 1962 and 1971, the United States military sprayed 20,000,000 U.S. gallons (76,000,000 L) of chemical herbicides and defoliants in Vietnam, eastern Laos and parts of Cambodia, as part of Operation Ranch Hand. [ 58 ]
By 1971, 12% of the total area of South Vietnam had been sprayed with defoliating chemicals, which were often applied at rates that were 13 times as high as the legal USDA limit. [ 59 ] In South Vietnam alone, an estimated 10 million hectares of agricultural land were ultimately destroyed. [ 60 ] In some areas, TCDD concentrations in soil and water were hundreds of times greater than the levels considered safe by the U.S. Environmental Protection Agency . [ 61 ] [ 62 ]
According to Vietnamese Ministry of Foreign Affairs, 4.8 million Vietnamese people were exposed to Agent Orange, resulting in 400,000 people being killed or maimed, and 500,000 children born with birth defects . [ 63 ] The Red Cross of Vietnam estimates that up to 1 million people are disabled or have health problems due to Agent Orange contamination. [ 64 ] The United States government has challenged these figures as being unreliable and unrealistically high. [ 65 ] [ 66 ]
The analyses used to determine these compounds' relative toxicity share common elements that differ from methods used for more traditional analytical determinations. The preferred methods for dioxins and related analyses use high resolution gas chromatography/mass spectrometry (HRGC/HRMS). Concentrations are determined by measuring the ratio of the analyte to the appropriate isotopically labeled internal standard. [ 91 ]
Also novel bio-assays like DR CALUX are nowadays used in identification of dioxins and dioxin-like compounds. The advantage in respect to HRGC/HRMS is that it is able to scan many samples at lower costs. Also it is able to detect all compounds that interact with the Ah-receptor which is responsible for carcinogenic effects. [ 92 ] | https://en.wikipedia.org/wiki/Polychlorinated_dibenzodioxins |
Polychlorinated dibenzofurans ( PCDFs ) are a family of organic compounds with one or several of the hydrogens in the dibenzofuran structure replaced by chlorines. For example, 2,3,7,8-tetrachlorodibenzofuran (TCDF) has chlorine atoms substituted for each of the hydrogens on the number 2, 3, 7, and 8 carbons (see structure in the upper left corner of the second image). Polychlorinated dibenzofurans with chlorines at least in positions 2,3,7 and 8 are much more toxic than the parent compound dibenzofuran, with properties and chemical structures similar to polychlorinated dibenzodioxins . These groups together are often inaccurately called dioxins . They are known developmental toxicants, and suspected human carcinogens . PCDFs tend to co-occur with polychlorinated dibenzodioxins (PCDDs). PCDFs can be formed by pyrolysis or incineration at temperatures below 1200 °C of chlorine containing products, such as PVC , PCBs , and other organochlorides , or of non-chlorine containing products in the presence of chlorine donors. [ 1 ] Dibenzofurans are known persistent organic pollutants (POP), classified among the dirty dozen in the Stockholm Convention on Persistent Organic Pollutants .
Occupational exposure to PCDFs may occur through inhalation and contact with the skin, although intake even in workers at waste incineration plants is not particularly high. [ 2 ] [ 3 ] For general population the most important source is food of animal origin like with other dioxin-like compounds. [ 4 ] [ 5 ] [ 6 ] The most relevant congener is 2,3,4,7,8-pentachlorodibenzofuran (2,3,4,7,8-PCDF) which is more toxic and based on relative toxicity more prevalent than other PCDFs. [ 4 ] | https://en.wikipedia.org/wiki/Polychlorinated_dibenzofurans |
Polychlorinated naphthalene ( PCN ) are the products obtained upon treatment of naphthalene with chlorine . The generic chemical formula is C 10 H 8−(m+n) Cl (m+n) . Commercial PCNs are mixtures of up to 75 components and byproducts. [ 1 ] The material is an oil or a waxy solid, depending on the degree of chlorination. PCNs were once used in insulating coatings for electrical wires, as well as other applications, but their use has been largely phased out. [ 2 ]
There are 75 different PCN congeners .
PCNs started to be produced for high-volume uses around 1910 in both Europe and the United States. [ 2 ] [ 3 ] In Europe the largest volume products were called Nibren waxes, made in Germany by Bayer . Other European PCN tradenames included Seekay (UK, from ICI ), Clonacire (France), Cerifal (Italy) and Woskol (Poland). In the United States, the largest volume PCN products were called Halowax, from a New York company of the same name that was later owned by Union Carbide and then taken over by Koppers of Pittsburgh, PA, now Beazer East. Although trace amounts of PCNs may be released by natural processes such as wildfires, their industrial uses increased the apparent rates of accumulation in the environment by factors of 10,000 or more. [ 4 ]
After about 20 years of commercial production, health hazards began to be reported in workers exposed to PCNs: chloracne , [ 1 ] severe skin rashes [ 5 ] and liver disease that led to deaths of workers. [ 6 ] [ 7 ] A conference about the hazards was organized at Harvard School of Public Health in 1937, and several more publications dealing with PCN hazards appeared before 1940. [ 8 ] PCNs containing three or more chlorines per molecule have typically been found more hazardous than those with fewer, [ 9 ] but as the maximum of eight is approached, hazards appear to decrease. [ 10 ]
There was a lag of about 40 years between disclosure of PCN hazards and government regulation. In the U.S. exposure to PCNs was drastically reduced after 1976, following enactment of the Toxic Substances Control Act . Major equipment manufacturers banned PCNs in their products, and major PCN producers discontinued operations. By 1983 worldwide PCN production had almost halted except for small amounts used in testing and research. Until recent years duPont produced a synthetic rubber, Neoprene FB, made in Northern Ireland using pentachloronaphthalene. [ 11 ]
Increased cancer risks have been suspected but so far not shown. Current concerns about PCNs include their release as byproducts of waste incineration. [ 12 ]
In 2013, the 9th meeting of the Persistent Organic Pollutants Review Committee, established under the Stockholm Convention on Persistent Organic Pollutants proposed di-,tri-,tetra-,penta-,hexa-, hepta- and octa-chlorinated naphthalenes, for listing in Annexes A and C to that Convention. [ 13 ]
While some PCNs can be broken down by sunlight and, at slow rates, by certain microorganisms, many PCNs persist in the environment. After more than 80 years of use and total production of several hundred thousand tons, PCN residues are widespread. [ 10 ] | https://en.wikipedia.org/wiki/Polychlorinated_naphthalene |
Polychloro phenoxy phenols (polychlorinated phenoxy phenols, PCPPs) are a group of organic polyhalogenated compounds . Among them include triclosan and predioxin which can degrade to produce certains types of dioxins and furans . [ 1 ] Notably, however, the particular dioxin formed by degradation of triclosan, 2,8-DCDD, [ 2 ] was found to be non-toxic in fish embryos. [ 3 ]
This article about an organic halide is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Polychloro_phenoxy_phenol |
Polychotomous key refers to the number of alternatives which a decision point may have in a non-temporal hierarchy of independent variables. The number of alternatives are equivalent to the root or nth root of a mathematical or logical variable. [ citation needed ] Decision points or independent variables with two states have a binary root that is referred to as a dichotomous key whereas, the term polychotomous key refers to roots which are greater than one or unitary and usually greater than two or binary. Polychotomous keys are used in troubleshooting to build troubleshooting charts and in classification/identification schemes with characteristics that have more than one attribute and the order of characteristics is not inherently based on the progression of time.
This logic -related article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Polychotomous_key |
Polychromatic symmetry is a colour symmetry which interchanges three or more colours in a symmetrical pattern. It is a natural extension of dichromatic symmetry . The coloured symmetry groups are derived by adding to the position coordinates ( x and y in two dimensions, x , y and z in three dimensions) an extra coordinate, k , which takes three or more possible values (colours). [ 1 ]
An example of an application of polychromatic symmetry is crystals of substances containing molecules or ions in triplet states, that is with an electronic spin of magnitude 1, should sometimes have structures in which the spins of these groups have projections of + 1, 0 and -1 onto local magnetic fields. If these three cases are present with equal frequency in an orderly array, then the magnetic space group of such a crystal should be three-coloured. [ 2 ] [ 3 ]
The group p3 has three different rotation centres of order three (120°), but no reflections or glide reflections.
There are two distinct ways of colouring the p3 pattern with three colours: p3 [3] 1 and p3 [3] 2 where the figure in square brackets indicates the number of colours, and the subscript distinguishes between multiple cases of coloured patterns. [ 5 ]
Taking a single motif in the pattern p3 [3] 1 it has a symmetry operation 3', consisting of a rotation by 120° and a cyclical permutation of the three colours white, green and red as shown in the animation.
This pattern p3 [3] 1 has the same colour symmetry as M. C. Escher's Hexagonal tessellation with animals: study of regular division of the plane with reptiles (1939). Escher reused the design in his 1943 lithograph Reptiles and it was also used as the cover art of Mott the Hoople’s debut album .
Initial research by Wittke and Garrido (1959) [ 7 ] and by Niggli and Wondratschek (1960) [ 8 ] identified the relation between the colour groups of an object and the subgroups of the object's geometric symmetry group . In 1961 van der Waerden and Burckhardt [ 9 ] built on the earlier work by showing that colour groups can be defined as follows: in a colour group of a pattern (or object) each of its geometric symmetry operations s is associated with a permutation σ of the k colours in such a way that all the pairs ( s , σ ) form a group. Senechal showed that the permutations are determined by the subgroups of the geometric symmetry group G of the uncoloured pattern. [ 10 ] When each symmetry operation in G is associated with a unique colour permutation the pattern is said to be perfectly coloured. [ 11 ] [ 12 ]
The Waerden-Burckhardt theory defines a k -colour group G ( H ) as being determined by a subgroup H of index k in the symmetry group G . [ 13 ] If the subgroup H is a normal subgroup then the quotient group G / H permutes all the colours. [ 14 ]
Both of the 3-colour p3 patterns, the unique 4-, 6-, 7-colour p3 patterns, one of the three 9-colour p3 patterns, and one of the four 12-colour p3 patterns are illustrated in the Example section above. | https://en.wikipedia.org/wiki/Polychromatic_symmetry |
Polyclonal antibodies (pAbs) are antibodies that are secreted by different B cell lineages within the body (whereas monoclonal antibodies come from a single cell lineage). They are a collection of immunoglobulin molecules that react against a specific antigen , each identifying a different epitope .
The general procedure to produce polyclonal antibodies is as follows:
An antigen/adjuvant conjugate is injected into an animal of choice to initiate an amplified immune response. After a series of injections over a specific length of time, the animal is expected to have created antibodies against the conjugate. Blood is then extracted from the animal and then purified to obtain the antibody of interest.
Inoculation is performed on a suitable mammal , such as a mouse, rabbit or goat. Larger mammals are often preferred as the amount of serum that can be collected is greater. An antigen is injected into the mammal. This induces the B- lymphocytes to produce IgG immunoglobulins specific for the antigen. This polyclonal IgG is purified from the mammal's serum .
By contrast, monoclonal antibodies are derived from a single cell line.
Many methodologies exist for polyclonal antibody production in laboratory animals. Institutional guidelines governing animal use and procedures relating to these methodologies are generally oriented around humane considerations and appropriate conduct for adjuvant (agents which modify the effect of other agents while having few if any direct effects when given by themselves) use. This includes adjuvant selection, routes and sites of administration, injection volumes per site and number of sites per animal. Institutional policies generally include allowable volumes of blood per collection and safety precautions including appropriate restraint and sedation or anesthesia of animals for injury prevention to animals or personnel.
The primary goal of antibody production in laboratory animals is to obtain high titer , high affinity antisera for use in experimentation or diagnostic tests. Adjuvants are used to improve or enhance an immune response to antigens. Most adjuvants provide for an injection site, antigen depot which allows for a slow release of antigen into draining lymph nodes.
Many adjuvants also contain or act directly as:
Such antigens by themselves are generally poor immunogens. Most complex protein antigens induce multiple B-cell clones during the immune response, thus, the response is polyclonal. Immune responses to non-protein antigens are generally poorly or enhanced by adjuvants and there is no system memory.
Antibodies are currently also being produced from isolation of human B-lymphocytes to produce specific recombinant monoclonal antibody mixtures. The biotechnology company, Symphogen , develops this type of antibodies for therapeutic applications. They are the first research company to reach phase two trials with the monoclonal antibody mixtures that mimic the diversity of the polyclonal antibody drugs. This production prevents viral and prion transmission and this is the simple process.
Animals frequently used for polyclonal antibody production include chickens, goats, guinea pigs, hamsters, horses, mice, rats, and sheep. However, the rabbit is the most commonly used laboratory animal for this purpose. Animal selection should be based upon:
Goats or horses are generally used when large quantities of antisera are required. Many investigators favor chickens because of their phylogenetic distance from mammals. Chickens transfer high quantities of IgY (IgG) into the egg yolk and harvesting antibodies from eggs eliminates the need for the invasive bleeding procedure. One week's eggs can contain 10 times more antibodies than the volume of rabbit blood obtained from one weekly bleeding. However, there are some disadvantages when using certain chicken derived antibodies in immunoassays. Chicken IgY does not fix mammalian complement component C1 and it does not perform as a precipitating antibody using standard solutions.
Although mice are used most frequently for monoclonal antibody production, their small size usually prevents their use for sufficient quantities of polyclonal, serum antibodies. However, polyclonal antibodies in mice can be collected from ascites fluid using any one of a number of ascites producing methodologies.
When using rabbits, young adult animals (2.5–3.0 kg or 5.5–6.5 lb) should be used for primary immunization because of the vigorous antibody response. Immune function peaks at puberty and primary responses to new antigens decline with age. Female rabbits are generally preferred because they are more docile and are reported to mount a more vigorous immune response than males. At least two animals per antigen should be used when using outbred animals. This principle reduces potential total failure resulting from non-responsiveness to antigens of individual animals.
The size, extent of aggregation and relative nativity of protein antigens can all dramatically affect the quality and quantity of antibody produced. Small polypeptides (< 10 kDa ) and non-protein antigens generally need to be conjugated or crosslinked to larger, immunogenic, carrier proteins to increase immunogenicity and provide T cell epitopes. Generally, the larger the immunogenic protein the better. Larger proteins, even in smaller amounts, usually result in better engagement of antigen presenting antigen processing cells for a satisfactory immune response. Injection of soluble, non-aggregated proteins has a higher probability of inducing tolerance rather than a satisfactory antibody response.
Keyhole limpet hemocyanin (KLH) and bovine serum albumin are two widely used carrier proteins. Poly-L-lysine has also been used successfully as a backbone for peptides. Although the use of Poly-L-lysine reduces or eliminates production of antibodies to foreign proteins, it may result in failure of peptide-induced antibody production. Recently, liposomes have also been successfully used for delivery of small peptides and this technique is an alternative to delivery with oily emulsion adjuvants.
Selection of antigen quantity for immunization varies with the properties of the antigen and the adjuvant selected. In general, microgram to milligram quantities of protein in adjuvant are necessary to elicit high titer antibodies. Antigen dosage is generally species, rather than body weight, associated. The so-called "window" of immunogenicity in each species is broad but too much or too little antigen can induce tolerance, suppression or immune deviation towards cellular immunity rather than a satisfactory humoral response. Optimal and usual protein antigen levels for immunizing specific species have been reported in the following ranges:
Optimal "priming" doses are reported to be at the low end of each range.
The affinity of serum antibodies increases with time (months) after injection of antigen-adjuvant mixtures and as antigen in the system decreases. Widely used antigen dosages for "booster" or secondary immunizations are usually one half to equal the priming dosages. Antigens should be free of preparative byproducts and chemicals such as polyacrylamide gel, SDS, urea, endotoxin, particulate matter and extremes of pH.
When a peptide is being used to generate the antibody, it is extremely important to design the antigens properly. There are several resources that can aid in the design as well as companies that offer this service. Expasy has aggregated a set of public tools under its ProtScale page that require some degree of user knowledge to navigate. For a more simple peptide scoring tool there is a Antigen Profiler tool available that will enable you to score individual peptide sequences based upon a relation epitope mapping database of previous immunogens used to generate antibodies. Finally, as a general rule peptides should follow some basic criteria.
When examining peptides for synthesis and immunization, it is recommended that certain residues and sequences be avoided due to potential synthesis problems. This includes some of the more common characteristics:
Investigators should also consider the status of nativity of protein antigens when used as immunogens and reaction with antibodies produced. Antibodies to native proteins react best with native proteins and antibodies to denatured proteins react best with denatured proteins. If elicited antibodies are to be used on membrane blots (proteins subjected to denaturing conditions) then antibodies should be made against denatured proteins. On the other hand, if antibodies are to be used to react with a native protein or block a protein active site, then antibodies should be made against the native protein. Adjuvants can often alter the nativity of the protein. Generally, absorbed protein antigens in a preformed oil-in-water emulsion adjuvant, retain greater native protein structure than those in water-in-oil emulsions.
Antigens should always be prepared using techniques that ensure that they are free of microbial contamination. Most protein antigen preparations can be sterilized by passage through a 0.22 μm filter. Septic abscesses often occur at inoculation sites of animals when contaminated preparations are used. This can result in failure of immunization against the targeted antigen.
There are many commercially available immunologic adjuvants . Selection of specific adjuvants or types varies depending upon whether they are to be used for research and antibody production or in vaccine development. Adjuvants for vaccine use only need to produce protective antibodies and good systemic memory while those for antiserum production need to rapidly induce high titer, high avidity antibodies. No single adjuvant is ideal for all purposes and all have advantages and disadvantages. Adjuvant use generally is accompanied by undesirable side effects of varying severity and duration. Research on new adjuvants focuses on substances which have minimal toxicity while retaining maximum immunostimulation. Investigators should always be aware of potential pain and distress associated with adjuvant use in laboratory animals.
The most frequently used adjuvants for antibody production are Freund's, Alum, the Ribi Adjuvant System and Titermax.
Digoxin Immune Fab is the antigen binding fragment of polyclonal antibodies raised to Digitalis derivative as a hapten bound to a protein and is used for the reversal of life-threatening digoxin or digitoxin toxicity. [ 1 ] [ 2 ] [ 3 ]
Rho(D) immune globulin is made from pooled human plasma provided by Rh-negative donors with antibodies to the D antigen. It is used to provide passive immune binding of antigen, preventing a maternal active immune response which could potentially result in hemolytic disease of the newborn . [ 4 ] [ 5 ]
Rozrolimupab is the anti- RhD recombinant human polyclonal antibody composed of 25 unique IgG1 antibodies and is used for the treatment of immune thrombocytopenia purpura and prevention of isoimmunization in Rh-negative pregnant women.
REGN-COV2 ( Regeneron Pharmaceuticals ) – potential treatment for people with COVID-19 and to prevent SARS-CoV-2 coronavirus infection.
The use of polyclonal antibodies (pAbs) over monoclonal antibodies has its advantages. The technical skills needed to produce polyclonal antibodies is not as demanding. They're inexpensive to make and can be generated fairly quickly, taking up to several months to produce. PAbs are heterogeneous, which allows them to bind to a wide range of antigen epitopes. Because PAbs are produced from a large number of B cell clones, they're more likely to successfully bind to a specific antigen. PAbs remain stable in different environments, such as a change in pH or salt concentration, which allows them to be more applicable in certain procedures. Additionally, depending on the amount needed, PAbs can be made in large quantities in relation to the size of the animal used. [ 6 ] [ 7 ] [ 8 ]
There are drawbacks using polyclonal antibodies over monoclonal antibodies. This is mostly on the reproducibility aspect. [ 9 ] pAbs are generated by immune response to antigen. Individual animal or human will generate different antibodies against the same antigen. In the case of multiple exposure of the same antigen, the pAbs mixture will be different over time. This so called 'batch-to-batch' variability is one of the biggest concerns in using pAbs.
Despite the drawbacks, polyclonal antibodies' broad epitope recognition [ 10 ] and higher sensitivity [ 11 ] are key across different applications. Scientists use mixtures of monoclonal antibodies (mAbs) to mimic pAbs. The recombinant production of mAbs solved the variability and reproducibility concerns in pAbs. The mixture of many mAbs will mimic pAbs' broader epitope recognition and higher sensitivity. [ 12 ] Recent technology advancements allowed the direct sequencing of polyclonal antibody proteins from human and immunized animals. [ 13 ] [ 14 ] Those antibody proteins can then be recombinantly produced to mimic the original pAbs generated by the immune system. | https://en.wikipedia.org/wiki/Polyclonal_antibodies |
X chromosome inactivation (XCI) is the phenomenon that has been selected during the evolution to balance X-linked gene dosage between XX females and XY males. [ 1 ]
XCI is usually divided in two phases, the establishment phase when gene silencing is reversible, and maintenance phase when gene silencing becomes irreversible. [ 2 ] During the establishment phase of X Chromosome Inactivation (XCI) , Xist RNA , the master regulator of this process, is monoallelically upregulated [ 3 ] and it spreads in cis along the future inactive X (Xi), relocates to the nuclear periphery. [ 4 ] [ 5 ] [ 6 ] and recruits repressive chromatin-remodelling complexes [ 7 ] Among these, Xist recruits proteins of the Polycomb repressive complexes . [ 8 ] [ 9 ] Whether Xist directly recruits Polycomb repressive complex 2 (PRC2) to the chromatin [ 10 ] or this recruitment is the consequence of Xist-mediated changes on the chromatin has been object of intense debate. [ 11 ]
Some studies showed that PRC2 components are not associated with Xist RNA or do not interact functionally. [ 12 ] [ 13 ] [ 14 ] [ 15 ] However another study has shown by means of mass spectrometry analysis, [ 16 ] that two subunits of PRC2 may interact with Xist, although these proteins are also found in other complexes and are not unique components of the PRC2 complex.
PRC2 binds the A-repeat (RepA) of Xist RNA directly and with very high affinity (dissociation constants of 10-100 nanomolar), [ 17 ] [ 18 ] supporting Xist-mediated recruitment of PRC2 to the X chromosome. However it is not clear whether such interactions occurs in vivo under physiological conditions. [ 19 ] Failure to turn up PRC2 proteins in function screens may be due to cells not being able to survive or compete without PRC2 or incomplete screens. Two super resolution microscopy analyses have presented different views from each other. One showed that Xist and PRC2 are spatially separated, [ 20 ] while another showed that Xist and PRC2 are tightly linked. [ 21 ] It is possible that several mechanisms recruit PRC2 in parallel, including direct Xist-mediated recruitment, adaptor proteins, chromatin changes, RNA pol II exclusion, or PRC1 recruitment. [ 22 ] [ 23 ] For instance, PRC2 recruitment is linked to PRC1-mediated H2A119 ubiquitination in differentiating embryonic stem cells (ESCs). [ 24 ] [ 25 ] [ 26 ] where PRC1 recruitment is mediated by hnrnpK and Xist repB. [ 25 ] [ 26 ] In fully differentiated cells, PRC2 recruitment seems to be dependent on Xist RepA. [ 26 ] It is possible that alternative and complementary pathways such as phase separation [ 27 ] [ 28 ] work to establish PRC2 recruitment on the X in different experimental systems and during different stages of development. See also work from the Tartaglia lab ( https://en.wikipedia.org/wiki/Gian_Gaetano_Tartaglia ) | https://en.wikipedia.org/wiki/Polycomb_recruitment_in_X_chromosome_inactivation |
A Polycyclic aromatic hydrocarbon ( PAH ) is any member of a class of organic compounds that is composed of multiple fused aromatic rings . Most are produced by the incomplete combustion of organic matter — by engine exhaust fumes, tobacco, incinerators, in roasted meats and cereals, [ 1 ] or when biomass burns at lower temperatures as in forest fires . [ 2 ] [ 3 ] The simplest representative is naphthalene , having two aromatic rings, and the three-ring compounds anthracene and phenanthrene . PAHs are uncharged, non-polar and planar. Many are colorless. Many of them are also found in fossil fuel deposits such as coal and in petroleum . Exposure to PAHs can lead to different types of cancer , to fetal development complications, and to cardiovascular issues. [ medical citation needed ]
Polycyclic aromatic hydrocarbons are discussed as possible starting materials for abiotic syntheses of materials required by the earliest forms of life . [ 4 ] [ 5 ]
The terms polyaromatic hydrocarbon , [ 6 ] or polynuclear aromatic hydrocarbon [ 7 ] (abbreviated as PNA) are also used for this concept. [ 8 ]
By definition, polycyclic aromatic hydrocarbons have multiple aromatic rings, precluding benzene from being considered a PAH. Sources such as the US EPA and CDC consider naphthalene to be the simplest PAH. [ 9 ] Most authors exclude compounds that include heteroatoms in the rings, or carry substituents . [ 10 ]
A polyaromatic hydrocarbon may have rings of various sizes, including some that are not aromatic. Those that have only six-membered rings are said to be alternant . [ 11 ]
The following are examples of PAHs that vary in the number and arrangement of their rings:
Most PAHs, like naphthalene, anthracene, and coronene, are planar. This geometry is a consequence of the fact that the σ-bonds that result from the merger of sp 2 hybrid orbitals of adjacent carbons lie on the same plane as the carbon atom. Those compounds are achiral , since the plane of the molecule is a symmetry plane.
In rare cases, PAHs are not planar. In some cases, the non-planarity may be forced by the topology of the molecule and the stiffness (in length and angle) of the carbon-carbon bonds. For example, unlike coronene , corannulene adopts a bowl shape in order to reduce the bond stress. The two possible configurations, concave and convex, are separated by a relatively low energy barrier (about 11 kcal / mol ). [ 12 ]
In theory, there are 51 structural isomers of coronene that have six fused benzene rings in a cyclic sequence, with two edge carbons shared between successive rings. All of them must be non-planar and have considerable higher bonding energy (computed to be at least 130 kcal/mol) than coronene; as of 2002, none of them had been synthesized. [ 13 ]
Other PAHs that might seem to be planar, considering only the carbon skeleton, may be distorted by repulsion or steric hindrance between the hydrogen atoms in their periphery. Benzo[c]phenanthrene, with four rings fused in a "C" shape, has a slight helical distortion due to repulsion between the closest pair of hydrogen atoms in the two extremal rings. [ 14 ] This effect also causes distortion of picene. [ 15 ]
Adding another benzene ring to form dibenzo[c,g]phenanthrene creates steric hindrance between the two extreme hydrogen atoms. [ 16 ] Adding two more rings on the same sense yields heptahelicene in which the two extreme rings overlap. [ 17 ] These non-planar forms are chiral, and their enantiomers can be isolated. [ 18 ]
The benzenoid hydrocarbons have been defined as condensed polycyclic unsaturated fully-conjugated hydrocarbons whose molecules are essentially planar with all rings six-membered. Full conjugation means that all carbon atoms and carbon-carbon bonds must have the sp 2 structure of benzene. This class is largely a subset of the alternant PAHs, but is considered to include unstable or hypothetical compounds like triangulene or heptacene . [ 18 ]
As of 2012, over 300 benzenoid hydrocarbons had been isolated and characterized. [ 18 ]
The aromaticity varies for PAHs. According to Clar's rule , [ 19 ] the resonance structure of a PAH that has the largest number of disjoint aromatic pi sextets —i.e. benzene -like moieties—is the most important for the characterization of the properties of that PAH. [ 20 ]
For example, phenanthrene has two Clar structures: one with just one aromatic sextet (the middle ring), and the other with two (the first and third rings). The latter case is therefore the more characteristic electronic nature of the two. Therefore, in this molecule the outer rings have greater aromatic character whereas the central ring is less aromatic and therefore more reactive. [ citation needed ] In contrast, in anthracene the resonance structures have one sextet each, which can be at any of the three rings, and the aromaticity spreads out more evenly across the whole molecule. [ citation needed ] This difference in number of sextets is reflected in the differing ultraviolet–visible spectra of these two isomers, as higher Clar pi-sextets are associated with larger HOMO-LUMO gaps; [ 21 ] the highest-wavelength absorbance of phenanthrene is at 293 nm, while anthracene is at 374 nm. [ 22 ] Three Clar structures with two sextets each are present in the four-ring chrysene structure: one having sextets in the first and third rings, one in the second and fourth rings, and one in the first and fourth rings. [ citation needed ] Superposition of these structures reveals that the aromaticity in the outer rings is greater (each has a sextet in two of the three Clar structures) compared to the inner rings (each has a sextet in only one of the three).
PAHs are nonpolar and lipophilic . Larger PAHs are generally insoluble in water, although some smaller PAHs are soluble. [ 23 ] [ 24 ] The larger members are also poorly soluble in organic solvents and in lipids . The larger members, e.g. perylene, are strongly colored. [ 18 ]
Polycyclic aromatic compounds characteristically yield radicals and anions upon treatment with alkali metals. The large PAH form dianions as well. [ 25 ] The redox potential correlates with the size of the PAH.
The dominant sources of PAHs in the environment are from human activity: wood-burning and combustion of other biofuels such as dung or crop residues contribute more than half of annual global PAH emissions, particularly due to biofuel use in India and China. [ 28 ] [ 29 ] As of 2004, industrial processes and the extraction and use of fossil fuels made up slightly more than one quarter of global PAH emissions, dominating outputs in industrial countries such as the United States. [ 28 ]
A year-long sampling campaign in Athens, Greece found a third (31%) of PAH urban air pollution to be caused by wood-burning, like diesel and oil (33%) and gasoline (29%). It also found that wood-burning is responsible for nearly half (43%) of annual PAH cancer-risk ( carcinogenic potential) compared to the other sources and that wintertime PAH levels were 7 times higher than in other seasons, especially if atmospheric dispersion is low. [ 30 ] [ 31 ]
Lower-temperature combustion, such as tobacco smoking or wood-burning , tends to generate low molecular weight PAHs, whereas high-temperature industrial processes typically generate PAHs with higher molecular weights. [ 32 ] Incense is also a source. [ 33 ]
PAHs are typically found as complex mixtures. [ 34 ] [ 32 ]
PAHs may result from the incomplete combustion of organic matter in natural wildfires . [ 29 ] [ 28 ] Substantially higher outdoor air, soil, and water concentrations of PAHs have been measured in Asia, Africa, and Latin America than in Europe, Australia, the U.S., and Canada. [ 28 ] [ relevant? ]
Polycyclic aromatic hydrocarbons are primarily found in natural sources such as bitumen . [ 35 ] [ 36 ]
PAHs can also be produced geologically when organic sediments are chemically transformed into fossil fuels such as oil and coal . [ 34 ] The rare minerals idrialite , curtisite , and carpathite consist almost entirely of PAHs that originated from such sediments, that were extracted, processed, separated, and deposited by very hot fluids. [ 37 ] [ 15 ] [ 38 ] High levels of such PAHs have been detected in the Cretaceous-Tertiary (K-T) boundary , more than 100 times the level in adjacent layers. The spike was attributed to massive fires that consumed about 20% of the terrestrial above-ground biomass in a very short time. [ 39 ]
PAHs are prevalent in the interstellar medium (ISM) of galaxies in both the nearby and distant Universe and make up a dominant emission mechanism in the mid-infrared wavelength range, containing as much as 10% of the total integrated infrared luminosity of galaxies. [ 40 ] PAHs generally trace regions of cold molecular gas, which are optimum environments for the formation of stars. [ 40 ]
NASA's Spitzer Space Telescope and James Webb Space Telescope include instruments for obtaining both images and spectra of light emitted by PAHs associated with star formation . These images can trace the surface of star-forming clouds in our own galaxy or identify star forming galaxies in the distant universe. [ 41 ] In June 2013, PAHs were detected in the upper atmosphere of Titan , the largest moon of the planet Saturn . [ 42 ]
Volcanic eruptions may emit PAHs. [ 34 ]
Certain PAHs such as perylene can also be generated in anaerobic sediments from existing organic material, although it remains undetermined whether abiotic or microbial processes drive their production. [ 43 ] [ 44 ] [ 45 ]
Most PAHs are insoluble in water, which limits their mobility in the environment, although PAHs sorb to fine-grained organic-rich sediments . [ 46 ] [ 47 ] [ 48 ] [ 49 ] Aqueous solubility of PAHs decreases approximately logarithmically as molecular mass increases. [ 50 ]
Two-ringed PAHs, and to a lesser extent three-ringed PAHs, dissolve in water, making them more available for biological uptake and degradation . [ 49 ] [ 50 ] [ 51 ] Further, two- to four-ringed PAHs volatilize sufficiently to appear in the atmosphere predominantly in gaseous form, although the physical state of four-ring PAHs can depend on temperature. [ 52 ] [ 53 ] In contrast, compounds with five or more rings have low solubility in water and low volatility; they are therefore predominantly in solid state , bound to particulate air pollution , soils , or sediments . [ 49 ] In solid state, these compounds are less accessible for biological uptake or degradation, increasing their persistence in the environment. [ 50 ] [ 54 ]
Human exposure varies across the globe and depends on factors such as smoking rates, fuel types in cooking, and pollution controls on power plants, industrial processes, and vehicles. [ 34 ] [ 28 ] [ 55 ] Developed countries with stricter air and water pollution controls, cleaner sources of cooking (i.e., gas and electricity vs. coal or biofuels), and prohibitions of public smoking tend to have lower levels of PAH exposure, while developing and undeveloped countries tend to have higher levels. [ 34 ] [ 28 ] [ 55 ] Surgical smoke plumes have been proven to contain PAHs in several independent research studies. [ 56 ]
Burning solid fuels such as coal and biofuels in the home for cooking and heating is a dominant global source of PAH emissions that in developing countries leads to high levels of exposure to indoor particulate air pollution containing PAHs, particularly for women and children who spend more time in the home or cooking. [ 28 ] [ 57 ]
Emissions from vehicles such as cars and trucks can be a substantial outdoor source of PAHs in particulate air pollution. [ 34 ] [ 28 ] Geographically, major roadways are thus sources of PAHs, which may distribute in the atmosphere or deposit nearby. [ 58 ] Catalytic converters are estimated to reduce PAH emissions from gasoline-fired vehicles by 25-fold. [ 34 ]
People can also be occupationally exposed during work that involves fossil fuels or their derivatives, wood-burning, carbon electrodes , or exposure to diesel exhaust . [ 59 ] [ 60 ] Industrial activity that can produce and distribute PAHs includes aluminum , iron , and steel manufacturing; coal gasification , tar distillation, shale oil extraction ; production of coke , creosote , carbon black , and calcium carbide ; road paving and asphalt manufacturing; rubber tire production; manufacturing or use of metal working fluids; and activity of coal or natural gas power stations . [ 34 ] [ 59 ] [ 60 ]
In industrial countries, people who smoke tobacco products, or who are exposed to second-hand smoke , are among the most highly exposed groups; tobacco smoke contributes to 90% of indoor PAH levels in the homes of smokers. [ 55 ] For the general population in developed countries, the diet is otherwise the dominant source of PAH exposure, particularly from smoking or grilling meat or consuming PAHs deposited on plant foods, especially broad-leafed vegetables, during growth. [ 61 ] Exposure also occurs through drinking alcohol aged in charred barrels, flavored with peat smoke, or made with roasted grains. [ 62 ] PAHs are typically at low concentrations in drinking water. [ 55 ]
PAHs typically disperse from urban and suburban non-point sources through road runoff , sewage , and atmospheric circulation and subsequent deposition of particulate air pollution. [ 63 ] [ 64 ] Soil and river sediment near industrial sites such as creosote manufacturing facilities can be highly contaminated with PAHs. [ 34 ] Oil spills , creosote, coal mining dust, and other fossil fuel sources can also distribute PAHs in the environment. [ 34 ] [ 65 ]
Two- and three-ringed PAHs can disperse widely while dissolved in water or as gases in the atmosphere, while PAHs with higher molecular weights can disperse locally or regionally adhered to particulate matter that is suspended in air or water until the particles land or settle out of the water column . [ 34 ] PAHs have a strong affinity for organic carbon , and thus highly organic sediments in rivers , lakes , and the ocean can be a substantial sink for PAHs. [ 58 ]
Algae and some invertebrates such as protozoans , mollusks , and many polychaetes have limited ability to metabolize PAHs and bioaccumulate disproportionate concentrations of PAHs in their tissues; however, PAH metabolism can vary substantially across invertebrate species. [ 64 ] [ 66 ] Most vertebrates metabolize and excrete PAHs relatively rapidly. [ 64 ] Tissue concentrations of PAHs do not increase ( biomagnify ) from the lowest to highest levels of food chains. [ 64 ]
PAHs transform slowly to a wide range of degradation products. Biological degradation by microbes is a dominant form of PAH transformation in the environment. [ 54 ] [ 67 ] Soil-consuming invertebrates such as earthworms speed PAH degradation, either through direct metabolism or by improving the conditions for microbial transformations. [ 67 ] Abiotic degradation in the atmosphere and the top layers of surface waters can produce nitrogenated, halogenated, hydroxylated, and oxygenated PAHs; some of these compounds can be more toxic, water-soluble, and mobile than their parent PAHs. [ 64 ] [ 68 ] [ 69 ]
The British Geological Survey reported the amount and distribution of PAH compounds including parent and alkylated forms in urban soils at 76 locations in Greater London . [ 70 ] The study showed that parent (16 PAH) content ranged from 4 to 67 mg/kg (dry soil weight) and an average PAH concentration of 18 mg/kg (dry soil weight) whereas the total PAH content (33 PAH) ranged from 6 to 88 mg/kg and fluoranthene and pyrene were generally the most abundant PAHs. [ 70 ] Benzo[ a ]pyrene (B a P), the most toxic of the parent PAHs, is widely considered a key marker PAH for environmental assessments; [ 71 ] the normal background concentration of B a P in the London urban sites was 6.9 mg/kg (dry soil weight). [ 70 ] London soils contained more stable four- to six-ringed PAHs which were indicative of combustion and pyrolytic sources, such as coal and oil burning and traffic-sourced particulates. However, the overall distribution also suggested that the PAHs in London soils had undergone weathering and been modified by a variety of pre-and post-depositional processes such as volatilization and microbial biodegradation .
Managed burning of moorland vegetation in the UK has been shown to generate PAHs which become incorporated into the peat surface. [ 72 ] Burning of moorland vegetation such as heather initially generates high amounts of two- and three-ringed PAHs relative to four- to six-ringed PAHs in surface sediments, however, this pattern is reversed as the lower molecular weight PAHs are attenuated by biotic decay and photodegradation . [ 72 ] Evaluation of the PAH distributions using statistical methods such as principal component analyses (PCA) enabled the study to link the source (burnt moorland) to pathway (suspended stream sediment) to the depositional sink (reservoir bed). [ 72 ]
Concentrations of PAHs in river and estuarine sediments vary according to a variety of factors including proximity to municipal and industrial discharge points, wind direction and distance from major urban roadways, as well as tidal regime which controls the diluting effect of generally cleaner marine sediments relative to freshwater discharge. [ 63 ] [ 73 ] [ 74 ] Consequently, the concentrations of pollutants in estuaries tends to decrease at the river mouth. [ 75 ] Understanding of sediment hosted PAHs in estuaries is important for the protection of commercial fisheries (such as mussels ) and general environmental habitat conservation because PAHs can impact the health of suspension and sediment feeding organism. [ 76 ] River-estuary surface sediments in the UK tend to have a lower PAH content than sediments buried 10–60 cm from the surface reflecting lower present day industrial activity combined with improvement in environmental legislation of PAH. [ 74 ] Typical PAH concentrations in UK estuaries range from about 19 to 16,163 µg/kg (dry sediment weight) in the River Clyde and 626 to 3,766 µg/kg in the River Mersey . [ 74 ] [ 77 ] In general estuarine sediments with a higher natural total organic carbon content (TOC) tend to accumulate PAHs due to high sorption capacity of organic matter. [ 77 ] A similar correspondence between PAHs and TOC has also been observed in the sediments of tropical mangroves located on the coast of southern China. [ 78 ]
Cancer is a primary human health risk of exposure to PAHs. [ 79 ] Exposure to PAHs has also been linked with cardiovascular disease and poor fetal development.
PAHs have been linked to skin , lung , bladder , liver , and stomach cancers in well-established animal model studies. [ 79 ] Specific compounds classified by various agencies as possible or probable human carcinogens are identified in the section " Regulation and Oversight " below.
Historically, PAHs contributed substantially to our understanding of adverse health effects from exposures to environmental contaminants , including chemical carcinogenesis . [ 80 ] In 1775, Percivall Pott , a surgeon at St. Bartholomew's Hospital in London, observed that scrotal cancer was unusually common in chimney sweepers and proposed the cause as occupational exposure to soot . [ 81 ] A century later, Richard von Volkmann reported increased skin cancers in workers of the coal tar industry of Germany, and by the early 1900s increased rates of cancer from exposure to soot and coal tar was widely accepted. In 1915, Yamigawa and Ichicawa were the first to experimentally produce cancers, specifically of the skin, by topically applying coal tar to rabbit ears. [ 81 ]
In 1922, Ernest Kennaway determined that the carcinogenic component of coal tar mixtures was an organic compound consisting of only carbon and hydrogen. This component was later linked to a characteristic fluorescent pattern that was similar but not identical to benz[ a ]anthracene , a PAH that was subsequently demonstrated to cause tumors . [ 81 ] Cook, Hewett and Hieger then linked the specific spectroscopic fluorescent profile of benzo[ a ]pyrene to that of the carcinogenic component of coal tar, [ 81 ] the first time that a specific compound from an environmental mixture (coal tar) was demonstrated to be carcinogenic.
In the 1930s and later, epidemiologists from Japan, the UK, and the US, including Richard Doll and various others, reported greater rates of death from lung cancer following occupational exposure to PAH-rich environments among workers in coke ovens and coal carbonization and gasification processes. [ 82 ]
The structure of a PAH influences whether and how the individual compound is carcinogenic. [ 79 ] [ 83 ] Some carcinogenic PAHs are genotoxic and induce mutations that initiate cancer; others are not genotoxic and instead affect cancer promotion or progression. [ 83 ] [ 84 ]
PAHs that affect cancer initiation are typically first chemically modified by enzymes into metabolites that react with DNA, leading to mutations. When the DNA sequence is altered in genes that regulate cell replication , cancer can result. Mutagenic PAHs, such as benzo[ a ]pyrene, usually have four or more aromatic rings as well as a "bay region", a structural pocket that increases reactivity of the molecule to the metabolizing enzymes. [ 85 ] Mutagenic metabolites of PAHs include diol epoxides, quinones , and radical PAH cations . [ 85 ] [ 86 ] [ 87 ] These metabolites can bind to DNA at specific sites, forming bulky complexes called DNA adducts that can be stable or unstable. [ 81 ] [ 88 ] Stable adducts may lead to DNA replication errors, while unstable adducts react with the DNA strand, removing a purine base (either adenine or guanine ). [ 88 ] Such mutations, if they are not repaired, can transform genes encoding for normal cell signaling proteins into cancer-causing oncogenes . [ 83 ] Quinones can also repeatedly generate reactive oxygen species that may independently damage DNA. [ 85 ]
Enzymes in the cytochrome family ( CYP1A1 , CYP1A2 , CYP1B1 ) metabolize PAHs to diol epoxides. [ 89 ] PAH exposure can increase production of the cytochrome enzymes, allowing the enzymes to convert PAHs into mutagenic diol epoxides at greater rates. [ 89 ] In this pathway, PAH molecules bind to the aryl hydrocarbon receptor (AhR) and activate it as a transcription factor that increases production of the cytochrome enzymes. The activity of these enzymes may at times conversely protect against PAH toxicity, which is not yet well understood. [ 89 ]
Low molecular weight PAHs, with two to four aromatic hydrocarbon rings, are more potent as co-carcinogens during the promotional stage of cancer. In this stage, an initiated cell (a cell that has retained a carcinogenic mutation in a key gene related to cell replication) is removed from growth-suppressing signals from its neighboring cells and begins to clonally replicate. [ 90 ] Low-molecular-weight PAHs that have bay or bay-like regions can dysregulate gap junction channels, interfering with intercellular communication, and also affect mitogen-activated protein kinases that activate transcription factors involved in cell proliferation. [ 90 ] Closure of gap junction protein channels is a normal precursor to cell division. Excessive closure of these channels after exposure to PAHs results in removing a cell from the normal growth-regulating signals imposed by its local community of cells, thus allowing initiated cancerous cells to replicate. These PAHs do not need to be enzymatically metabolized first. Low molecular weight PAHs are prevalent in the environment, thus posing a significant risk to human health at the promotional phases of cancer.
Adult exposure to PAHs has been linked to cardiovascular disease . [ 91 ] PAHs are among the complex suite of contaminants in tobacco smoke and particulate air pollution and may contribute to cardiovascular disease resulting from such exposures. [ 92 ]
In laboratory experiments, animals exposed to certain PAHs have shown increased development of plaques ( atherogenesis ) within arteries. [ 93 ] Potential mechanisms for the pathogenesis and development of atherosclerotic plaques may be similar to the mechanisms involved in the carcinogenic and mutagenic properties of PAHs. [ 93 ] A leading hypothesis is that PAHs may activate the cytochrome enzyme CYP1B1 in vascular smooth muscle cells. This enzyme then metabolically processes the PAHs to quinone metabolites that bind to DNA in reactive adducts that remove purine bases. The resulting mutations may contribute to unregulated growth of vascular smooth muscle cells or to their migration to the inside of the artery, which are steps in plaque formation. [ 92 ] [ 93 ] These quinone metabolites also generate reactive oxygen species that may alter the activity of genes that affect plaque formation. [ 93 ]
Oxidative stress following PAH exposure could also result in cardiovascular disease by causing inflammation , which has been recognized as an important factor in the development of atherosclerosis and cardiovascular disease. [ 94 ] [ 95 ] Biomarkers of exposure to PAHs in humans have been associated with inflammatory biomarkers that are recognized as important predictors of cardiovascular disease, suggesting that oxidative stress resulting from exposure to PAHs may be a mechanism of cardiovascular disease in humans. [ 96 ]
Multiple epidemiological studies of people living in Europe, the United States, and China have linked in utero exposure to PAHs, through air pollution or parental occupational exposure, with poor fetal growth, reduced immune function, and poorer neurological development, including lower IQ . [ 97 ] [ 98 ] [ 99 ] [ 100 ]
Some governmental bodies, including the European Union as well as NIOSH and the United States Environmental Protection Agency (EPA), regulate concentrations of PAHs in air, water, and soil. [ 101 ] The European Commission has restricted concentrations of 8 carcinogenic PAHs in consumer products that contact the skin or mouth. [ 102 ]
Priority polycyclic aromatic hydrocarbons identified by the US EPA, the US Agency for Toxic Substances and Disease Registry (ATSDR), and the European Food Safety Authority (EFSA) due to their carcinogenicity or genotoxicity and/or ability to be monitored are the following: [ 103 ] [ 104 ] [ 105 ]
A spectral database exists [ 4 ] for tracking polycyclic aromatic hydrocarbons (PAHs) in the universe . [ 106 ] Detection of PAHs in materials is often done using gas chromatography-mass spectrometry or liquid chromatography with ultraviolet-visible or fluorescence spectroscopic methods or by using rapid test PAH indicator strips. [ 107 ] Structures of PAHs have been analyzed using infrared spectroscopy. [ 108 ]
PAHs possess very characteristic UV absorbance spectra . These often possess many absorbance bands and are unique for each ring structure. Thus, for a set of isomers , each isomer has a different UV absorbance spectrum than the others. This is particularly useful in the identification of PAHs. Most PAHs are also fluorescent , emitting characteristic wavelengths of light when they are excited (when the molecules absorb light). The extended pi-electron electronic structures of PAHs lead to these spectra, as well as to certain large PAHs also exhibiting semi-conducting and other behaviors.
PAHs may be abundant in the universe. [ 5 ] [ 109 ] [ 110 ] [ 111 ] They seem to have been formed as early as a couple of billion years after the Big Bang , and are associated with new stars and exoplanets . [ 4 ] More than 20% of the carbon in the universe may be associated with PAHs. [ 4 ] PAHs are considered possible starting material for the earliest forms of life . [ 4 ] [ 5 ] Light emitted by the Red Rectangle nebula possesses spectral signatures that suggest the presence of anthracene and pyrene . [ 112 ] [ 113 ] This report was considered a controversial hypothesis that as nebulae of the same type as the Red Rectangle approach the ends of their lives, convection currents cause carbon and hydrogen in the nebulae's cores to get caught in stellar winds, and radiate outward. As they cool, the atoms supposedly bond to each other in various ways and eventually form particles of a million or more atoms. Adolf Witt and his team inferred [ 112 ] that PAHs—which may have been vital in the formation of early life on Earth —can only originate in nebulae. [ 113 ]
PAHs, subjected to interstellar medium (ISM) conditions, are transformed, through hydrogenation , oxygenation , and hydroxylation , to more complex organic compounds —"a step along the path toward amino acids and nucleotides , the raw materials of proteins and DNA , respectively". [ 115 ] [ 116 ] Further, as a result of these transformations, the PAHs lose their spectroscopic signature which could be one of the reasons "for the lack of PAH detection in interstellar ice grains , particularly the outer regions of cold, dense clouds or the upper molecular layers of protoplanetary disks ." [ 115 ] [ 116 ]
Low-temperature chemical pathways from simple organic compounds to complex PAHs are of interest. Such chemical pathways may help explain the presence of PAHs in the low-temperature atmosphere of Saturn 's moon Titan , and may be significant pathways, in terms of the PAH world hypothesis , in producing precursors to biochemicals related to life as we know it. [ 117 ] [ 118 ] | https://en.wikipedia.org/wiki/Polycyclic_aromatic_hydrocarbon |
Polydactyly-myopia syndrome , also known as Czeizel-Brooser syndrome , is a very rare genetic disorder which is characterized by post-axial polydactyly on all 4 limbs and progressive myopia . [ 1 ] Additional symptoms include bilateral congenital inguinal hernia and undescended testes. [ 2 ] It has only been described in nine members of a 4-generation Hungarian family in the year 1986. [ 3 ] [ 4 ] [ 5 ] [ 6 ] This disorder is inherited in an autosomal dominant manner. [ 7 ] [ 8 ]
This genetic disorder article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Polydactyly-myopia_syndrome |
Polydicyclopentadiene ( PDCPD ) is a polymer material which is formed through ring-opening metathesis polymerization [ 2 ] (ROMP) of dicyclopentadiene (DCPD). PDCPD exhibits high crosslinking , which grants its properties, such as high impact resistance , good chemical corrosion resistance, and high heat deflection temperature . PDCPD is frequently used in the automotive industry to make body panels, bumpers, and other components for trucks, buses, tractors, and construction equipment. PDCPD is being investigated for the creation of porous materials for tissue engineering or gas storage applications, as well as for self-healing polymers . [ 3 ]
Polymerization can be achieved through the use of different transition metal catalysts as ruthenium, molybdenum, tungsten, and titanium, as well as under metal-free conditions through photoredox catalysis . The exact structure of the PDCPD polymer depends upon the reaction conditions used for the polymerization. While the crosslinked polymer may arise from the metathesis of both alkenes in the parent monomer, it has been suggested that much polymerization conditions result in only the strained norbornene ring in the monomer undergoing olefin metathesis while subsequent crosslinking steps result from thermal condensation of the remaining olefins in the linear polymer. [ 3 ] Several new catalytic systems for the synthesis of linear PDCPD have been run successfully [ 4 ] using tungsten hexachloride , tungsten(VI) oxytetrachloride , and organosilicon compounds.
The reacting system is formulated to maximize the speed of the reaction, and in this system, two components must be mixed in a ration of equal volume. Both components contain mainly DCPD with some additional additives. The catalyst system is divided into two parts, each part going into a separate component. When both components are mixed, the complete catalyst system is recombined and becomes active. This is an important difference from other reaction injection molding (RIM) systems, such as polyurethane , since the reaction is not stoichiometric . The 1:1 volume ratio for DCPD molding is not critical since this is not a combination of two different chemical elements to form a specific matrix . However, significant changes in ratio will slow down the system's reactivity because fewer active reaction nuclei are being formed.
DCPD resins are transformed using high pressure RIM equipment as used in the polyurethane industry, with some small changes to be considered. The most important change is that the resin can never be in contact with air or moisture, which requires a nitrogen blanket in the tanks. The tools or molds are closed tools and are being clamped using a hydraulic press . Because the resins shrink approximately 6% in volume during reaction, these presses (also called clamping units) do not have to handle high pressures, such as for sheet molding compound (SMC) or expanding polyurethane.
Most tooling for PDCPD is made from aluminium . Flat parts can be made from machined aluminum while deeper 3D-shaped parts are often made as cast aluminium tools. It is important to take volumetric shrinkage into account, and gaskets must be used around all cavities.
The liquid resin has a relative density of 0.97 and reacts into a solid with a relative density of 1.03, which makes up a volumetric shrinkage of 6%. Since most parts are panels, most of the shrinkage will happen on the Z-axis — causing a change in thickness. This makes the parts self-demolding as they do not have a good contact with the core side (which is the back side) of the tool.
A reacting system is always governed by temperature - in any form. This means that the temperature of the liquid components has a strong influence on the reactivity. To ensure that one side has the required surface finish, the temperature on that side needs to be higher than on the core side. Both tool-halves are therefore tempered at a different temperature with typical values of 60 °C and 80 °C.
Typical cycle times for molding parts range between 4 and 6 minutes.
PDCPD has several useful properties:
PDCPD does not contain any fiber reinforcement, although a fiber reinforced version has been in development. PDCPD allows the thickness to vary throughout a part, to incorporate ribs, and to overmold inserts for an uncomplicated assembly of the parts. PDCPD cannot be painted in mass and needs to be painted after molding.
Since PDCPD is still a new material, the number of applications is quite limited. The major applications is in body panels, mainly for tractors , construction equipment , trucks and buses . In the industrial applications, the main usage is components for chlor-alkali production (e.g. cell covers for electrolyzers ). It is used in other applications where impact resistance in combination with rigidity, 3D design and/or corrosion resistance are required.
PDCPD is not recyclable. In July 2020, researchers reported the development of a technique to produce a degradable version of this tough thermoset plastic , which may also apply to other plastics, that are not included among the 75% of plastics that are recyclable . [ 6 ] [ 7 ] | https://en.wikipedia.org/wiki/Polydicyclopentadiene |
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