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select is a system call and application programming interface (API) in Unix-like and POSIX -compliant operating systems for examining the status of file descriptors of open input/output channels. [ 1 ] The select system call is similar to the poll facility introduced in UNIX System V and later operating systems. However, with the c10k problem , both select and poll have been superseded by the likes of kqueue , epoll , /dev/poll and I/O completion ports . [ 2 ]
One common use of select outside of its stated use of waiting on filehandles is to implement a portable sub-second sleep . This can be achieved by passing NULL for all three fd_set arguments, and the duration of the desired sleep as the timeout argument.
In the C programming language , the select system call is declared in the header file sys/select.h or unistd.h , and has the following syntax:
fd_set type arguments may be manipulated with four utility macros: FD_SET(), FD_CLR(), FD_ZERO() , and FD_ISSET() .
Select returns the total number of bits set in readfds, writefds and errorfds , or zero if the timeout expired, and -1 on error.
The sets of file descriptor used in select are finite in size, depending on the operating system. The newer system call poll provides a more flexible solution. | https://en.wikipedia.org/wiki/Select_(Unix) |
A selectable marker is a gene introduced into cells , especially bacteria or cells in culture , which confers one or more traits suitable for artificial selection . They are a type of reporter gene used in laboratory microbiology , molecular biology , and genetic engineering to indicate the success of a transfection or transformation or other procedure meant to introduce foreign DNA into a cell. Selectable markers are often antibiotic resistance genes: bacteria subjected to a procedure by which exogenous DNA containing an antibiotic resistance gene (usually alongside other genes of interest ) has been introduced are grown on a medium containing an antibiotic , such that only those bacterial cells which have successfully taken up and expressed the introduced genetic material, including the gene which confers antibiotic resistance, can survive and produce colonies . The genes encoding resistance to antibiotics such as ampicillin , chloramphenicol , tetracycline , kanamycin , etc., are all widely used as selectable markers for molecular cloning and other genetic engineering techniques in E. coli .
Selectable markers allow scientists to separate non-recombinant organisms (those which do not contain the selectable marker) from recombinant organisms (those which do); that is, a recombinant DNA molecule such as a plasmid expression vector is introduced into bacterial cells, and some bacteria are successfully transformed while some remain non-transformed. Antibiotics such as ampicillin , at sufficient concentrations, are toxic to most bacteria, which ordinarily lack resistance to them; when cultured on a nutrient medium containing ampicillin, bacteria lacking ampicillin resistance fail to divide and eventually die. The position is later noted on nitrocellulose paper and separated out to move them to a nutrient medium for mass production of the required product. An alternative to a selectable marker is a screenable marker, another type of reporter gene which allows the researcher to distinguish between wanted and unwanted cells or colonies, such as between blue and white colonies in blue–white screening . These wanted or unwanted cells are simply non-transformed cells that were unable to take up the screenable gene during the experiment. [ citation needed ]
For molecular biology research, different types of markers may be used based on the selection sought. These include:
Examples of selectable markers include:
In the future, alternative marker technologies will need to be used more often to, at the least, assuage concerns about their persistence into the final product. It is also possible that markers will be replaced entirely by future techniques which use removable markers, and others which do not use markers at all, instead relying on co-transformation , homologous recombination , and recombinase-mediated excision . [ 6 ] | https://en.wikipedia.org/wiki/Selectable_marker |
Selected area (electron) diffraction (abbreviated as SAD or SAED ) is a crystallographic experimental technique typically performed using a transmission electron microscope (TEM). It is a specific case of electron diffraction used primarily in material science and solid state physics as one of the most common experimental techniques. Especially with appropriate analytical software , SAD patterns (SADP) can be used to determine crystal orientation , measure lattice constants or examine its defects .
In transmission electron microscope, a thin crystalline sample is illuminated by parallel beam of electrons accelerated to energy of hundreds of kiloelectron volts . At these energies samples are transparent for the electrons if the sample is thinned enough (typically less than 100 nm ). Due to the wave–particle duality , the high-energetic electrons behave as matter waves with wavelength of a few thousandths of a nanometer. The relativistic wavelength is given by
where h {\displaystyle h} is the Planck constant , m 0 {\displaystyle m_{0}} is the electron rest mass , e {\displaystyle e} is the elementary charge , c {\displaystyle c} is the speed of light and V {\displaystyle V} is an electric potential accelerating the electrons (also called acceleration voltage ). [ 1 ] For instance the acceleration voltage of 200 kV results in a wavelength of 2.508 pm. [ 2 ]
Since the spacing between atoms in crystals is about a hundred times larger, the electrons are diffracted on the crystal lattice , acting as a diffraction grating . Due to the diffraction, part of the electrons is scattered at particular angles (diffracted beams), while others pass through the sample without changing their direction (transmitted beams). In order to determine the diffraction angles, the electron beam normally incident to the atomic lattice can be seen as a planar wave, which is re-transmitted by each atom as a spherical wave. Due to the constructive interference , the spherical waves from number of diffracted beams under angles θ n {\displaystyle \theta _{n}} given, approximately, by the Bragg condition
where the integer n {\displaystyle n} is an order of diffraction and d {\displaystyle d} is the distance between atoms (if only one row of atoms is assumed as in the illustration aside) or a distance between atomic planes parallel to the beam (in a real 3D atomic structure). For finite samples this equation is only approximately correct.
After being deflected by the microscope's magnetic lens , each set of initially parallel beams intersect in the back focal plane forming the diffraction pattern . The transmitted beams intersect right in the optical axis . The diffracted beams intersect at certain distance from the optical axis (corresponding to interplanar distance of the planes diffracting the beams) and under certain azimuth (corresponding to the orientation of the planes diffracting the beams). This allows to form a pattern of bright spots typical for SAD. [ 3 ]
SAD is called "selected" because it allows the user to select the sample area from which the diffraction pattern will be acquired. For this purpose, there is a selected area aperture located below the sample holder. It is a metallic sheet with several differently sized holes which can be inserted into the beam. The user can select the aperture of appropriate size and position it so that it only allows to pass the portion of beam corresponding to the selected area. Therefore, the resulting diffraction pattern will only reflect the area selected by the aperture. This allows to study small objects such as crystallites in polycrystalline material with a broad parallel beam.
Character of the resulting diffraction image depends on whether the beam is diffracted by one single crystal or by number of differently oriented crystallites for instance in a polycrystalline material. The single-crystalline diffractogram depicts a regular pattern of bright spots. This pattern can be seen as a two-dimensional projection of reciprocal crystal lattice . If there are more contributing crystallites, the diffraction image becomes a superposition of individual crystals' diffraction patterns. Ultimately, this superposition contains diffraction spots of all possible crystallographic plane systems in all possible orientations. For two reasons, these conditions result in a diffractogram of concentric rings:
SAD analysis is widely used in material research for its relative simplicity and high information value. Once the sample is prepared and examined in a modern transmission electron microscope, the device allows for a routine diffraction acquisition in a matter of seconds. If the images are interpreted correctly, they can be used to identify crystal structures, determine their orientations, measure crystal characteristics, examine crystal defects or material textures. The course of analysis depends on whether the diffractogram depicts ring or spot diffraction pattern and on the quantity to be determined.
Software tools based on computer vision algorithms simplifies quantitative analysis. [ 4 ]
If the SAD is taken from one a or a few single crystals, the diffractogram depicts a regular pattern of bright spots. Since the diffraction pattern can be seen as a two-dimensional projection of reciprocal crystal lattice , the pattern can be used to measure lattice constants , specifically the distances and angles between crystallographic planes. The lattice parameters are typically distinctive for various materials and their phases which allows to identify the examined material or at least differentiate between possible candidates.
Even though the SAD-based analyses were not considered quantitative for a long time, computer tools brought accuracy and repeatability allowing to routinely perform accurate measurements of interplanar distances or angles on appropriately calibrated microscopes. Tools such as CrysTBox are capable of automated analysis achieving sub-pixel precision. [ 4 ]
If the sample is tilted against the electron beam, diffraction conditions are satisfied for different set of crystallographic planes yielding different constellation of diffraction spots. This allows to determine the crystal orientation, which can be used for instance to set up the orientation needed for particular experiment, to determine misorientation between adjacent grains or crystal twins . [ 2 ] [ 4 ] Since different sample orientations provide different projections of the reciprocal lattice, they provide an opportunity to reconstruct the three-dimensional information lost in individual projections. A series of diffractograms varying in tilt can be acquired and processed with diffraction tomography analysis in order to reconstruct an unknown crystal structure. [ 5 ]
SAD can also be used to analyze crystal defects such as stacking faults .
If the illuminated area selected by the aperture covers many differently oriented crystallites , their diffraction patterns superimpose forming an image of concentric rings. The ring diffractogram is typical for polycrystalline samples, powders or nanoparticles . Diameter of each ring corresponds to interplanar distance of a plane system present in the sample. Instead of information about individual grains or the sample orientation, this diffractogram provides more of a statistical information for instance about overall crystallinity or texture . Textured materials are characteristic by a non-uniform intensity distribution along the ring circumference despite crystallinity sufficient for generating smooth rings. Ring diffractograms can be also used to discriminate between nanocrystalline and amorphous phases. [ 2 ]
Not all the features depicted in the diffraction image are necessarily wanted. The transmitted beam is often too strong and needs to be shadowed with a beam-stopper in order to protect the camera. The beam-stopper typically shadows part of the useful information as well. Towards the rings center, the background intensity also gradually increases lowering the contrast of diffraction rings. Modern analytical software allows to minimize such unwanted image features and together with other functionalities improves the image readability it helps with image interpretation. [ 4 ]
An SADP is acquired under parallel electron illumination. In the case of convergent beam, a convergent beam electron diffraction (CBED) is achieved. [ 6 ] [ 3 ] The beam used in SAD is broad illuminating a wide sample area. In order to analyze only a specific sample area, the selected area aperture in the image plane is used. This is in contrast with nanodiffraction, where the site-selectivity is achieved using a beam condensed to a narrow probe. [ 3 ] SAD is important in direct imaging for instance when orienting the sample for high resolution microscopy or setting up dark-field imaging conditions.
High-resolution electron microscope images can be transformed into an artificial diffraction pattern using Fourier transform . Then, they can be processed the same way as real diffractograms allowing to determine crystal orientation, measure interplanar angles and distances even with picometric precision. [ 7 ]
SAD is similar to X-ray diffraction , but unique in that areas as small as several hundred nanometres in size can be examined, whereas X-ray diffraction typically samples areas much larger. | https://en.wikipedia.org/wiki/Selected_area_diffraction |
Selected timeline related to orphan wells in Alberta, Canada is a list of events relevant to orphan wells in Alberta, Canada. Orphan wells are inactive oil or gas well sites that have no solvent owner that can be held legally or financially accountable for the decommissioning and reclamation obligations to ensure public safety and to address environmental liabilities. [ 1 ] [ 2 ] [ 3 ]
1910s The province's oldest inactive well has been dormant and unreclaimed since June 30, 1918. [ 4 ]
1920s Some of the legacy sites were in operation in the 1920s or earlier, and have no known operator and no "financial security to cover the cleanup costs." [ 5 ]
Canada's oil production in 1946 was only 21,000 barrels (3,300 m 3 ) of oil per day. By 1956, Alberta was producing 400,000 barrels (64,000 m 3 ) per day. [ 7 ] : 5 [ 9 ]
gas industry, all with the focus of mitigating gas migration and surface casing vent flow issues throughout
not only Alberta, but Canada as a whole
May 1 The OWA's inventory listed "2963 orphan wells for abandonment, 297 orphan facilities for decommissioning, 3781 orphan pipeline segments for abandonment, 3116 orphan sites for reclamation, and 939 orphan reclaimed sites." [ 53 ] [ 54 ]
Reclamation
A
B
C
D
E
G
H
I
J
K
L
M
N
O
P
R
S
T
U
W
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Z | https://en.wikipedia.org/wiki/Selected_timeline_related_to_orphan_wells_in_Alberta |
Selectfluor , a trademark of Air Products and Chemicals , is a reagent in chemistry that is used as a fluorine donor. This compound is a derivative of the nucleophillic base DABCO . It is a colourless salt that tolerates air and even water. It has been commercialized for use for electrophilic fluorination . [ 1 ]
Selectfluor is synthesized by the N - alkylation of diazabicyclo[2.2.2]octane (DABCO) with dichloromethane in a Menshutkin reaction , followed by ion exchange with sodium tetrafluoroborate (replacing the chloride counterion for the tetrafluoroborate). The resulting salt is treated with elemental fluorine and sodium tetrafluoroborate: [ 2 ]
The cation is often depicted with one skewed ethylene ((CH 2 ) 2 ) group. In fact, these pairs of CH 2 groups are eclipsed so that the cation has idealized C 3h symmetry .
Electrophilic fluorinating reagents could in principle operate by electron transfer pathways or an S N 2 attack at fluorine. This distinction has not been decided. [ 2 ] By using a charge-spin separated probe, [ 3 ] it was possible to show that the electrophilic fluorination of stilbenes with Selectfluor proceeds through an SET/fluorine atom transfer mechanism. [ 4 ]
In certain cases Selectfluor can transfer fluorine to alkyl radicals . [ 5 ]
The conventional source of "electrophilic fluorine", i.e. the equivalent to the superelectrophile F + , is gaseous fluorine, which requires specialised equipment for manipulation. Selectfluor reagent is a salt, the use of which requires only routine procedures. Like F 2 , the salt delivers the equivalent of F + . It is mainly used in the synthesis of organofluorine compounds : [ 2 ]
Selectfluor reagent also serves as a strong oxidant, a property that is useful in other reactions in organic chemistry . Oxidation of alcohols and phenols . As applied to electrophilic iodination , Selectfluor reagent activates the I–I bond in I 2 molecule. [ 6 ]
Similar to Selectfluor are N -fluorosulfonimides: [ 7 ] | https://en.wikipedia.org/wiki/Selectfluor |
In computing and user interface engineering , a selection is a list of items on which user operations will take place. The user typically adds items to the list manually, although the computer may create a selection automatically.
Selections are enacted through combinations of key presses on a keyboard , with a precision pointing device ( mouse or touchpad and cursor , stylus ), or by hand on a touchscreen device. The simultaneous selection of a group of items (either a subset of elements in a list, or discontinuous regions in a text) is called a multiple selection .
Context menus will usually include actions related to the objects included in the current selection – the selection provides the "context" for the menu.
A selection method to facilitate the selection of large amounts of text or items in a long list such as files and folders in file managers is range selection , sparing the user from clicking or tapping each item individually.
On desktop file managers such as the Windows Explorer and Nemo file manager , it can be used by clicking on the first item to be selected, then holding the ⇧ shift key while clicking on the last item to be selected. In mobile file managers, it is implemented since early versions of ES File Explorer , where only two listed items (file or directory) need to be highlighted and a button pressed to select all items in-between.
Ideally, the two list items are navigated to with a draggable scroll bar , since it can move through long lists faster.
The user taps or clicks on the first item, drags upward or downward, and waits for the list to scroll to the last desired item, at which the user releases the pointer or finger. The same applies to text. [ 1 ]
Simultaneous editing is a technique in End-user development research to edit all items in a multiple selection. It allows the user to manipulate all the selected items at once through direct manipulation . The technique also appears in data wrangling tools, allowing the user to make the same changes to several records of the same kind in a table.
The term simultaneous editing is also used to describe collaborative editing in collaborative real-time editors .
A column selection is a text selection feature found in text editors which allows the user to select characters in a grid-like fashion, selecting characters in several lines at the same columns. This is usually initiated by pressing the alt key (instead of the shift key, which creates a continuous selection) to select text when dragging . Some applications also enable text to be typed in multiple lines at once using this method as a limited form of simultaneous editing . [ 2 ]
The feature is known by different names in different applications:
The Lapis text editor , [ 9 ] [ 10 ] Mozilla's Firefox developer tools , [ 11 ] and the multi edit [ 12 ] plugin for gedit are examples of the simultaneous editing technique that work on discontinuous regions through direct manipulation. The Lapis editor can also create an automatic multiple selection based on an example item.
This computing article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Selection_(user_interface) |
Selection and amplification binding assay (SAAB) is a molecular biology technique typically used to find the DNA binding site for proteins . [ 1 ] It was developed by T. Keith Blackwell and Harold M. Weintraub in 1990.
SAAB experimental procedure consists of several steps, depending upon the knowledge available about the binding site. A typical SAAB consists of the following steps:
Quox1 is a homeobox gene involved in the regulation of patterns of development ( morphogenesis ) in animals, fungi and plants and was originally isolated from cDNA library of five week quail embryo. It is the only gene in the hox family that has been found to express in both prosencephalon and mesencephalon involved in the differentiation of the central and peripheral nerve cells. The optimal DNA binding site for Quox1 or its mammalian homologs was identified by SAAB in 2004. [ 5 ] The amplified Quox1 DNA fragment obtained from PCR amplification from a human embryo cDNA librarywas digested with EcoRV and XhoI and cloned into the SmaI and XhoI restriction site of the expression vector pGEMEXxBal. The recombinant plasmids were transformed into competent Escherichia coli strain BL21 and Quox1 fusion proteins were isolated by chromatographic techniques.
The radio labeled probe was incubated with 25 pmol of purified Quox1 homeodomain fusion protein in binding buffer for EMSA. The protein bound DNA was detected by autoradiography, and the bands representing protein–DNA complexes were excised from the gel and the eluted DNA were amplified by PCR using primers complementary to the 20 bp nonrandom flanking sequences. After 5 set of the same procedure, the purified DNA was cloned into pMD 18T and sequenced. Finally the sequence CAATC was identified as the consensus binding sequence for Quox1 homeodomain.
By combining the power of random-sequence selection with pooled sequencing, the SAAB imprint assay makes possible simultaneous screening of a large number of binding site mutants. [ 6 ] SAAB also allows the identification of sites with high relative binding affinity since the competition is inherent in the protocol. It can also identify site positions that are neutral or specific bases that can interfere with binding, such as a T at - 4 in the E47 half-site. [ 7 ] We can apply the technique to less affinity binding sequence also, provided to keep high concentration of binding protein at each step of binding. It is also possible to identify the binding site even if both the protein and sequence is not known. [ 1 ] | https://en.wikipedia.org/wiki/Selection_and_amplification_binding_assay |
Selection coefficient , usually denoted by the letter s , is a measure used in population genetics to quantify the relative fitness of a genotype compared to other genotypes. Selection coefficients are central to the quantitative description of evolution, since fitness differences determine the change in genotype frequencies attributable to selection .
The selection coefficient is typically calculated using fitness values. The fitness ( W {\displaystyle W} ) of a genotype is a measure of its reproductive success, often expressed as a fraction of the maximum reproductive success in the population. The formula to calculate the selection coefficient s {\displaystyle s} for a genotype is: s = 1 − W {\displaystyle s=1-W} , where W {\displaystyle W} is the relative fitness of the genotype, ranging between 0 and 1. [ 1 ] [ 2 ]
Suppose we have two genotypes, A A {\displaystyle AA} and A a {\displaystyle Aa} , with relative fitness values of 1 (most fit, standard reference) and 0.8, the selection coefficient ( s {\displaystyle s} ) for A A {\displaystyle AA} is 1 − 1 = 0 {\displaystyle 1-1=0} (no selection against this genotype); the selection coefficient ( s {\displaystyle s} ) for A a {\displaystyle Aa} is 1 − 0.8 = 0.2 {\displaystyle 1-0.8=0.2} (this indicates that the A a {\displaystyle Aa} genotype has 20% reduction in fitness compared to the A A {\displaystyle AA} genotype).
For example, the lactose-tolerant allele spread from very low frequencies to high frequencies in less than 9000 years since farming with an estimated selection coefficient of 0.09-0.19 for a Scandinavian population. Though this selection coefficient might seem like a very small number, over evolutionary time, the favored alleles accumulate in the population and become more and more common, potentially reaching fixation. [ 3 ] | https://en.wikipedia.org/wiki/Selection_coefficient |
A selection limit is a term from animal breeding and quantitative genetics that refers to a cessation of progress even when continued directional selection is being applied to a trait, such as body size . In other words, a breeder or scientist is using selective breeding (artificial selection) and choosing individuals as breeders within a population based on some phenotypic trait or traits. If this is done, then the average value of the population typically evolves across generations in the direction being favored by selection (i.e., for higher or lower values of the trait), but then at some point the population stops evolving. The trait under selection is then said to have reached a limit or plateau at that value.
The existence of limits in artificial selection experiments was discussed in the scientific literature in the 1940s or earlier. [ 1 ] The most obvious possible cause of reaching a limit (or plateau) when a population is under continued directional selection is that all of the additive-genetic variation (see additive genetic effects ) related to that trait gets "used up" or fixed. [ 2 ] For example, if a trait, such as body mass, is under selection to increase, then, over time (i.e., across generations), the alleles (genetic variants) at all loci (most simply, positions on chromosomes) that tend to make individuals larger than average will increase in frequency, while those that tend to make an individual smaller than average will decrease in frequency. Eventually, in principle, the favored alleles at all relevant loci will become the only ones remaining at those loci. In reality, mutation , random genetic drift (especially in small populations), and gene flow from immigrants may stop some loci from becoming fixed for the "good" alleles.
However, other factors may interfere with the realization of genetic gains before loss of genetic variation causes a selection limit. As noted by Lerner and Dempster, [ 1 ] these factors are generally one of two types: 1) negative relations with Darwinian fitness; 2) non-additive gene action and/or genotype-environment interaction (although others are possible [ 3 ] [ 4 ] [ 2 ] [ 5 ] ).
A negative relation with Darwinian fitness is a situation in which an allele that is "good" for the trait under directional selection is "bad" with respect to lifetime reproductive success. For example, an allele that tends to confer larger body size might also lead to infertility , thus reducing the ability of individuals with that allele to produce offspring , limiting further response to selection, and sometimes even leading to extinction of the selected line. [ 6 ]
Non-additive gene action refers to such situations as heterozygote advantage , where heterozygous individuals have higher (or lower) values for a trait (such as body size) than do either of the two homozygotes. In such a case, selection will tend to maintain more than one allele in the population, and a selection limit may be reached while additive-genetic variation (narrow-sense heritability ) remains for the trait under directional selection.
Genotype-environment interaction occurs when the phenotype produced by a particular set of alleles (at one or more loci) confers relatively higher or lower values of a trait depending on the environmental circumstances in which an individual is born or raised, or under which the trait is measured. For instance, somewhat different genes (a term that can refer to alleles or loci) tend to give the highest value of a trait depending on the season . If this occurs, then directional selection will act to favor some genes in winter and others in summer, for example. Again, the result may be that a selection plateau is attained while the population retains some additive-genetic variance for the trait under directional selection. [ citation needed ]
Some traits have a natural physical limit beyond which a trait cannot possibly go. [ 2 ] For example, replicated selection for the building of small thermoregulatory nests in mice reached a limit at or near zero (i.e., none of the provided cotton was being used to make nests). [ 7 ] Similarly, lines of maize selected for low oil or protein content in the kernels reached limits near to zero percent. [ 4 ]
Aside from absolute physical limits, and whatever their cause, limits or plateaus have often been observed in artificial selection experiments with animals, including: bristle number in fruit flies ( Drosophila ); [ 8 ] avoidance behavior in laboratory rats ; [ 9 ] and large body size, [ 4 ] large litter size , [ 10 ] large nest size, [ 7 ] and high voluntary wheel -running behavior in laboratory house mice. [ 11 ]
Experimental approaches to probe the causes of selection are of two general types, quantitative genetic and functional. The former asks general questions about the genetic architecture of the trait when a limit has been attained (e.g., has narrow-sense heritability gone to zero?), whereas the latter attempts to determine what aspect of physiological or other function might have reached a limit or constraint. Experimental studies may involve attempts to break an apparent selection limit. As an example of a genetic approach, two replicate lines of mice at a limit for large nest size were crossed and selection was continued on this new population, resulting in further increase in nest size. [ 12 ] From a functional perspective, in lines of mice at a selection limit for high wheel running, administration of an erythropoietin analog increased the maximal rate of oxygen consumption during forced exercise , but did not increase wheel running, a result suggesting that motivation for exercise may be limiting the behavior, rather than inherent ability to run on wheels. [ 13 ] | https://en.wikipedia.org/wiki/Selection_limits |
In mathematics, a selection principle is a rule asserting
the possibility of obtaining mathematically significant objects by
selecting elements from given sequences of sets . The theory of selection principles studies these principles and their relations to other mathematical properties.
Selection principles mainly describe covering properties, measure- and category-theoretic properties, and local properties in topological spaces , especially function spaces . Often, the
characterization of a mathematical property using a selection
principle is a nontrivial task leading to new insights on the
characterized property.
In 1924, Karl Menger [ 1 ] introduced the following basis property for metric spaces :
Every basis of the topology contains a sequence of sets with vanishing
diameters that covers the space. Soon thereafter, Witold Hurewicz [ 2 ] observed that Menger's basis property is equivalent to the
following selective property: for every sequence of open covers of the space,
one can select finitely many open sets from each cover in the sequence, such that the family of all selected sets covers the space.
Topological spaces having this covering property are called Menger spaces .
Hurewicz's reformulation of Menger's property was the first important topological property described by a selection principle.
Let A {\displaystyle \mathbf {A} } and B {\displaystyle \mathbf {B} } be classes of mathematical objects.
In 1996, Marion Scheepers [ 3 ] introduced the following selection hypotheses,
capturing a large number of classic mathematical properties:
In the case where the classes A {\displaystyle \mathbf {A} } and B {\displaystyle \mathbf {B} } consist of covers of some ambient space, Scheepers also introduced the following selection principle.
Later, Boaz Tsaban identified the prevalence of the following related principle:
The notions thus defined are selection principles . An instantiation of a selection principle, by considering specific classes A {\displaystyle \mathbf {A} } and B {\displaystyle \mathbf {B} } , gives a selection (or: selective) property . However, these terminologies are used interchangeably in the literature.
For a set A ⊂ X {\displaystyle A\subset X} and a family F {\displaystyle {\mathcal {F}}} of subsets of X {\displaystyle X} , the star of A {\displaystyle A} in F {\displaystyle {\mathcal {F}}} is the set St ( A , F ) = ⋃ { F ∈ F : A ∩ F ≠ ∅ } {\displaystyle {\text{St}}(A,{\mathcal {F}})=\bigcup \{F\in {\mathcal {F}}:A\cap F\neq \emptyset \}} .
In 1999, Ljubisa D.R. Kocinac introduced the following star selection principles : [ 4 ]
The star selection principles are special cases of the general selection principles. This can be seen by modifying the definition of the family B {\displaystyle \mathbf {B} } accordingly.
Covering properties form the kernel of the theory of selection principles. Selection properties that are not covering properties are often studied by using implications to and from selective covering properties of related spaces.
Let X {\displaystyle X} be a topological space . An open cover of X {\displaystyle X} is a family of open sets whose union is the entire space X . {\displaystyle X.} For technical reasons, we also request that the entire space X {\displaystyle X} is not a member of the cover. The class of open covers of the space X {\displaystyle X} is denoted by O {\displaystyle \mathbf {O} } . (Formally, O ( X ) {\displaystyle \mathbf {O} (X)} , but usually the space X {\displaystyle X} is fixed in the background.) The above-mentioned property of Menger is, thus, S fin ( O , O ) {\displaystyle {\text{S}}_{\text{fin}}(\mathbf {O} ,\mathbf {O} )} . In 1942, Fritz Rothberger considered Borel's strong measure zero sets, and introduced a topological variation later called Rothberger space (also known as C ″ {\displaystyle ''} space ). In the notation of selections, Rothberger's property is the property S 1 ( O , O ) {\displaystyle {\text{S}}_{1}(\mathbf {O} ,\mathbf {O} )} .
An open cover U {\displaystyle {\mathcal {U}}} of X {\displaystyle X} is point-cofinite if it has infinitely many elements, and every point x ∈ X {\displaystyle x\in X} belongs to all but finitely many sets U ∈ U {\displaystyle U\in {\mathcal {U}}} . (This type of cover was considered by Gerlits and Nagy, in the third item of a certain list in their paper. The list was enumerated by Greek letters, and thus these covers are often called γ {\displaystyle \gamma } -covers .) The class of point-cofinite open covers of X {\displaystyle X} is denoted by Γ {\displaystyle \mathbf {\Gamma } } . A topological space is a Hurewicz space if it satisfies U fin ( O , Γ ) {\displaystyle {\text{U}}_{\text{fin}}(\mathbf {O} ,\mathbf {\Gamma } )} .
An open cover U {\displaystyle {\mathcal {U}}} of X {\displaystyle X} is an ω {\displaystyle \omega } -cover if every finite subset of X {\displaystyle X} is contained in some member of U {\displaystyle {\mathcal {U}}} . The class of ω {\displaystyle \omega } -covers of X {\displaystyle X} is denoted by Ω {\displaystyle \mathbf {\Omega } } . A topological space is a γ-space if it satisfies ( Ω Γ ) {\displaystyle {\binom {\mathbf {\Omega } }{\mathbf {\Gamma } }}} .
By using star selection hypotheses one obtains properties such as star-Menger ( S fin ∗ ( O , O ) {\displaystyle {\text{S}}_{\text{fin}}^{*}(\mathbf {O} ,\mathbf {O} )} ), star-Rothberger ( S 1 ∗ ( O , O ) {\displaystyle {\text{S}}_{1}^{*}(\mathbf {O} ,\mathbf {O} )} ) and star-Hurewicz ( S fin ∗ ( O , Γ ) {\displaystyle {\text{S}}_{\text{fin}}^{*}(\mathbf {O} ,\mathbf {\Gamma } )} ).
There are 36 selection properties of the form Π ( A , B ) {\displaystyle \Pi (\mathbf {A} ,\mathbf {B} )} , for Π ∈ { S 1 , S fin , U fin , ( ) } {\displaystyle \Pi \in \{{\text{S}}_{1},{\text{S}}_{\text{fin}},{\text{U}}_{\text{fin}},{\bigl (}~~{\bigr )}\}} and A , B ∈ { O , Γ , Ω } {\displaystyle \mathbf {A} ,\mathbf {B} \in \{\mathbf {O} ,\mathbf {\Gamma } ,\mathbf {\Omega } \}} . Some of them are trivial (hold for all spaces, or fail for all spaces). Restricting attention to Lindelöf spaces , the diagram below, known as the Scheepers diagram , [ 3 ] [ 5 ] presents nontrivial selection properties of the above form, and every nontrivial selection property is equivalent to one in the diagram. Arrows denote implications.
Selection principles also capture important local properties.
Let Y {\displaystyle Y} be a topological space, and y ∈ Y {\displaystyle y\in Y} . The class of sets A {\displaystyle A} in the space Y {\displaystyle Y} that have the point y {\displaystyle y} in their closure is denoted by Ω y {\displaystyle \mathbf {\Omega _{y}} } . The class Ω y ctbl {\displaystyle \mathbf {\Omega _{y}^{\text{ctbl}}} } consists of the countable elements of the class Ω y {\displaystyle \mathbf {\Omega _{y}} } . The class of sequences in Y {\displaystyle Y} that converge to y {\displaystyle y} is denoted by Γ y {\displaystyle \mathbf {\Gamma _{y}} } .
There are close connections between selection principles and topological games .
Let X {\displaystyle X} be a topological space. The Menger game G fin ( O , O ) {\displaystyle {\text{G}}_{\text{fin}}(\mathbf {O} ,\mathbf {O} )} played on X {\displaystyle X} is a game for two players, Alice and Bob. It has an inning per each natural number n {\displaystyle n} . At the n t h {\displaystyle n^{th}} inning, Alice chooses an open cover U n {\displaystyle {\mathcal {U}}_{n}} of X {\displaystyle X} ,
and Bob chooses a finite subset F n {\displaystyle {\mathcal {F}}_{n}} of U {\displaystyle {\mathcal {U}}} .
If the family ⋃ n = 1 ∞ F n {\displaystyle \bigcup _{n=1}^{\infty }{\mathcal {F}}_{n}} is a cover of the space X {\displaystyle X} , then Bob wins the game. Otherwise, Alice wins.
A strategy for a player is a function determining the move of the player, given the earlier moves of both players. A strategy for a player is a winning strategy if each play where this player sticks to this strategy is won by this player.
Note that among Lindelöf spaces, metrizable is equivalent to regular and second-countable, and so the previous result may alternatively be obtained by considering limited information strategies . [ 8 ] A Markov strategy is one that only uses the most recent move of the opponent and the current round number.
In a similar way, we define games for other selection principles from the given Scheepers Diagram. In all these cases a topological space has a property from the Scheepers Diagram if and only if Alice has no winning strategy in the corresponding game. [ 9 ] But this does not hold in general:
Let K {\displaystyle \mathbf {K} } be the family of k-covers of a space. That is, such that every compact set in the space is covered by some member of the cover.
Francis Jordan demonstrated a space where the selection principle S 1 ( K , O ) {\displaystyle {\text{S}}_{1}(\mathbf {K} ,\mathbf {O} )} holds, but
Alice has a winning strategy for the game G 1 ( K , O ) {\displaystyle {\text{G}}_{1}(\mathbf {K} ,\mathbf {O} )} [ 10 ]
Subsets of the real line R {\displaystyle \mathbb {R} } (with the induced subspace topology ) holding selection principle properties, most notably Menger and Hurewicz spaces, can be characterized by their continuous images in the Baire space N N {\displaystyle \mathbb {N} ^{\mathbb {N} }} . For functions f , g ∈ N N {\displaystyle f,g\in \mathbb {N} ^{\mathbb {N} }} , write f ≤ ∗ g {\displaystyle f\leq ^{*}g} if f ( n ) ≤ g ( n ) {\displaystyle f(n)\leq g(n)} for all but finitely many natural numbers n {\displaystyle n} . Let A {\displaystyle A} be a subset of N N {\displaystyle \mathbb {N} ^{\mathbb {N} }} . The set A {\displaystyle A} is bounded if there is a function g ∈ N N {\displaystyle g\in \mathbb {N} ^{\mathbb {N} }} such that f ≤ ∗ g {\displaystyle f\leq ^{*}g} for all functions f ∈ A {\displaystyle f\in A} . The set A {\displaystyle A} is dominating if for each function f ∈ N N {\displaystyle f\in \mathbb {N} ^{\mathbb {N} }} there is a function g ∈ A {\displaystyle g\in A} such that f ≤ ∗ g {\displaystyle f\leq ^{*}g} .
Let P be a property of spaces. A space X {\displaystyle X} is productively P if, for each space Y {\displaystyle Y} with property P , the product space X × Y {\displaystyle X\times Y} has property P .
Let X {\displaystyle X} be a Tychonoff space , and C ( X ) {\displaystyle C(X)} be the space of continuous functions f : X → R {\displaystyle f\colon X\to \mathbb {R} } with pointwise convergence topology. | https://en.wikipedia.org/wiki/Selection_principle |
In physics and chemistry , a selection rule , or transition rule , formally constrains the possible transitions of a system from one quantum state to another. Selection rules have been derived for electromagnetic transitions in molecules , in atoms , in atomic nuclei , and so on. The selection rules may differ according to the technique used to observe the transition. The selection rule also plays a role in chemical reactions , where some are formally spin-forbidden reactions , that is, reactions where the spin state changes at least once from reactants to products .
In the following, mainly atomic and molecular transitions are considered.
In quantum mechanics the basis for a spectroscopic selection rule is the value of the transition moment integral [ 1 ]
where ψ 1 {\displaystyle \psi _{1}} and ψ 2 {\displaystyle \psi _{2}} are the wave functions of the two states, "state 1" and "state 2", involved in the transition, and μ is the transition moment operator . This integral represents the propagator (and thus the probability) of the transition between states 1 and 2; if the value of this integral is zero then the transition is " forbidden ".
In practice, to determine a selection rule the integral itself does not need to be calculated: It is sufficient to determine the symmetry of the transition moment function ψ 1 ∗ μ ψ 2 . {\displaystyle \psi _{1}^{*}\,\mu \,\psi _{2}.} If the transition moment function is symmetric over all of the totally symmetric representation of the point group to which the atom or molecule belongs, then the integral's value is (in general) not zero and the transition is allowed. Otherwise, the transition is " forbidden ".
The transition moment integral is zero if the transition moment function , ψ 1 ∗ μ ψ 2 , {\displaystyle \psi _{1}^{*}\,\mu \,\psi _{2},} is anti-symmetric or odd , i.e. y ( x ) = − y ( − x ) {\displaystyle y(x)=-y(-x)} holds. The symmetry of the transition moment function is the direct product of the parities of its three components. The symmetry characteristics of each component can be obtained from standard character tables . Rules for obtaining the symmetries of a direct product can be found in texts on character tables. [ 2 ]
The Laporte rule is a selection rule formally stated as follows: In a centrosymmetric environment, transitions between like atomic orbitals such as s – s , p – p , d – d , or f – f , transitions are forbidden. The Laporte rule (law) applies to electric dipole transitions , so the operator has u symmetry (meaning ungerade , odd). [ 3 ] p orbitals also have u symmetry, so the symmetry of the transition moment function is given by the product (formally, the product is taken in the group ) u × u × u , which has u symmetry. The transitions are therefore forbidden. Likewise, d orbitals have g symmetry (meaning gerade , even), so the triple product g × u × g also has u symmetry and the transition is forbidden. [ 4 ]
The wave function of a single electron is the product of a space-dependent wave function and a spin wave function. Spin is directional and can be said to have odd parity . It follows that transitions in which the spin "direction" changes are forbidden. In formal terms, only states with the same total spin quantum number are "spin-allowed". [ 5 ] In crystal field theory , d – d transitions that are spin-forbidden are much weaker than spin-allowed transitions. Both can be observed, in spite of the Laporte rule, because the actual transitions are coupled to vibrations that are anti-symmetric and have the same symmetry as the dipole moment operator. [ 6 ]
In vibrational spectroscopy, transitions are observed between different vibrational states . In a fundamental vibration, the molecule is excited from its ground state ( v = 0) to the first excited state ( v = 1). The symmetry of the ground-state wave function is the same as that of the molecule. It is, therefore, a basis for the totally symmetric representation in the point group of the molecule. It follows that, for a vibrational transition to be allowed, the symmetry of the excited state wave function must be the same as the symmetry of the transition moment operator. [ 7 ]
In infrared spectroscopy , the transition moment operator transforms as either x and/or y and/or z . The excited state wave function must also transform as at least one of these vectors. In Raman spectroscopy , the operator transforms as one of the second-order terms in the right-most column of the character table, below. [ 2 ]
The molecule methane, CH 4 , may be used as an example to illustrate the application of these principles. The molecule is tetrahedral and has T d symmetry. The vibrations of methane span the representations A 1 + E + 2T 2 . [ 8 ] Examination of the character table shows that all four vibrations are Raman-active, but only the T 2 vibrations can be seen in the infrared spectrum. [ 9 ]
In the harmonic approximation , it can be shown that overtones are forbidden in both infrared and Raman spectra. However, when anharmonicity is taken into account, the transitions are weakly allowed. [ 10 ]
In Raman and infrared spectroscopy, the selection rules predict certain vibrational modes to have zero intensities in the Raman and/or the IR. [ 11 ] Displacements from the ideal structure can result in relaxation of the selection rules and appearance of these unexpected phonon modes in the spectra. Therefore, the appearance of new modes in the spectra can be a useful indicator of symmetry breakdown. [ 12 ] [ 13 ]
The selection rule for rotational transitions, derived from the symmetries of the rotational wave functions in a rigid rotor, is Δ J = ±1, where J is a rotational quantum number. [ 14 ]
There are many types of coupled transition such as are observed in vibration–rotation spectra. The excited-state wave function is the product of two wave functions such as vibrational and rotational. The general principle is that the symmetry of the excited state is obtained as the direct product of the symmetries of the component wave functions. [ 15 ] In rovibronic transitions, the excited states involve three wave functions.
The infrared spectrum of hydrogen chloride gas shows rotational fine structure superimposed on the vibrational spectrum. This is typical of the infrared spectra of heteronuclear diatomic molecules. It shows the so-called P and R branches. The Q branch, located at the vibration frequency, is absent. Symmetric top molecules display the Q branch. This follows from the application of selection rules. [ 16 ]
Resonance Raman spectroscopy involves a kind of vibronic coupling. It results in much-increased intensity of fundamental and overtone transitions as the vibrations "steal" intensity from an allowed electronic transition. [ 17 ] In spite of appearances, the selection rules are the same as in Raman spectroscopy. [ 18 ]
In general, electric (charge) radiation or magnetic (current, magnetic moment) radiation can be classified into multipoles E λ (electric) or M λ (magnetic) of order 2 λ , e.g., E1 for electric dipole , E2 for quadrupole , or E3 for octupole. In transitions where the change in angular momentum between the initial and final states makes several multipole radiations possible, usually the lowest-order multipoles are overwhelmingly more likely, and dominate the transition. [ 19 ]
The emitted particle carries away angular momentum, with quantum number λ , which for the photon must be at least 1, since it is a vector particle (i.e., it has J P = 1 − ). Thus, there is no radiation from E0 (electric monopoles) or M0 ( magnetic monopoles , which do not seem to exist).
Since the total angular momentum has to be conserved during the transition, we have that
where ‖ λ ‖ = λ ( λ + 1 ) ℏ , {\displaystyle \|{\boldsymbol {\lambda }}\|={\sqrt {\lambda (\lambda +1)}}\,\hbar ,} and its z projection is given by λ z = μ ℏ ; {\displaystyle \lambda _{z}=\mu \hbar ;} and where J i {\displaystyle \mathbf {J} _{\text{i}}} and J f {\displaystyle \mathbf {J} _{\text{f}}} are, respectively, the initial and final angular momenta of the atom.
The corresponding quantum numbers λ and μ ( z -axis angular momentum) must satisfy
and
Parity is also preserved. For electric multipole transitions
while for magnetic multipoles
Thus, parity does not change for E-even or M-odd multipoles, while it changes for E-odd or M-even multipoles.
These considerations generate different sets of transitions rules depending on the multipole order and type. The expression forbidden transitions is often used, but this does not mean that these transitions cannot occur, only that they are electric-dipole-forbidden . These transitions are perfectly possible; they merely occur at a lower rate. If the rate for an E1 transition is non-zero, the transition is said to be permitted; if it is zero, then M1, E2, etc. transitions can still produce radiation, albeit with much lower transitions rates. The transition rate decreases by a factor of about 1000 from one multipole to the next one, so the lowest multipole transitions are most likely to occur. [ 20 ]
Semi-forbidden transitions (resulting in so-called intercombination lines) are electric dipole (E1) transitions for which the selection rule that the spin does not change is violated. This is a result of the failure of LS coupling .
J = L + S {\displaystyle J=L+S} is the total angular momentum, L {\displaystyle L} is the azimuthal quantum number , S {\displaystyle S} is the spin quantum number , and M J {\displaystyle M_{J}} is the secondary total angular momentum quantum number .
Which transitions are allowed is based on the hydrogen-like atom . The symbol ↮ {\displaystyle \not \leftrightarrow } is used to indicate a forbidden transition.
In hyperfine structure , the total angular momentum of the atom is F = I + J , {\displaystyle F=I+J,} where I {\displaystyle I} is the nuclear spin angular momentum and J {\displaystyle J} is the total angular momentum of the electron(s). Since F = I + J {\displaystyle F=I+J} has a similar mathematical form as J = L + S , {\displaystyle J=L+S,} it obeys a selection rule table similar to the table above.
In surface vibrational spectroscopy , the surface selection rule is applied to identify the peaks observed in vibrational spectra. When a molecule is adsorbed on a substrate, the molecule induces opposite image charges in the substrate. The dipole moment of the molecule and the image charges perpendicular to the surface reinforce each other. In contrast, the dipole moments of the molecule and the image charges parallel to the surface cancel out. Therefore, only molecular vibrational peaks giving rise to a dynamic dipole moment perpendicular to the surface will be observed in the vibrational spectrum. | https://en.wikipedia.org/wiki/Selection_rule |
The selection shadow is a concept involved with the evolutionary theories of aging that states that selection pressures on an individual decrease as an individual ages and passes sexual maturity , resulting in a "shadow" of time where selective fitness is not considered. Over generations, this results in maladaptive mutations that accumulate later in life due to aging being non-adaptive toward reproductive fitness. [ 1 ] The concept was first worked out by J. B. S. Haldane and Peter Medawar in the 1940s, with Medawar creating the first graphical model . [ 1 ]
The model developed by Medawar states that due to the dangerous conditions and pressures from the environment, including predators and diseases, most individuals in the wild die not long after sexual maturity. Therefore, there is a low probability for individuals to survive to an advanced age and suffer the effects related to aging . In conjunction with this, the effects of natural selection decrease as age increases, so that later individual performance is ignored by selection forces. [ 1 ] This results in beneficial mutations not being selected for if they only have a positive result later in life, along with later in life deleterious mutations not being selected against. Due to the fitness of an individual not being affected once it is past its reproductive prime, later mutations and effects are considered to be in the "shadow" of selection. [ 2 ]
This concept would later be adapted into Medawar's 1952 mutation accumulation hypothesis , which was itself expanded upon by George C. Williams in his 1957 antagonistic pleiotropy hypothesis . [ 1 ]
A classical requirement and constraint of the model is that the number of individuals within a population that live to reach senescence must be small in number. If this is not true for a population, then the effects of old age will not be under a selection shadow and instead affect adaptation and evolution of the population as a whole. At the same time, however, this requirement has been challenged by increasing evidence of senescence being more common in wild populations than previously expected, especially among birds and mammals, while the effects of the selection shadow remain present. [ 3 ]
Medawar developed a theoretical model to demonstrate his thought process which explained that most animals will die before aging will be the ultimate cause for death in that animal. This would be from environmental factors such as large storms, drought, and fires, and predation. Medawar wanted to demonstrate this possibility by using test tubes to get his point across. The test tubes would be used to represent a population of species. [ 4 ] If one of these test tubes were to theoretically break, this would represent an individual animal dying. Randomly, test tubes would then be broken in the population to keep his model realistic. The broken test tubes would be replaced with a new one, which represents a new animal being born into the population. [ 4 ] Over time, the model showed that test tubes over a certain age would decline in the population as new test tubes were put in. The overall results in Medawar’s thought model demonstrated an exponential decline in the survivor curve which resulted in the population having a half life. [ 4 ] The amount of older animals, or test tubes in the population would then be harder to maintain and ultimately die. Medawar created this model to ultimately explain what would realistically happen in actual life. [ citation needed ]
Some scientists, however, have criticized the idea of aging being non-adaptive, instead adopting the theory of " death by design ". This theory follows the work of August Weismann , which states that aging specifically evolved as an adaptation , and disagrees with Medawar's model as a perceived oversimplification of the impact older organisms have on evolution. It is also claimed that older organisms have a higher reproductive capacity due to being better fit in order to reach their age, rather than their capacity being equal as in Medawar's calculations. [ 5 ] | https://en.wikipedia.org/wiki/Selection_shadow |
Selective Repeat ARQ or Selective Reject ARQ is a specific instance of the automatic repeat request (ARQ) protocol used to manage sequence numbers and retransmissions in reliable communications .
Selective Repeat is part of the automatic repeat request (ARQ). With selective repeat, the sender sends a number of frames specified by a window size even without the need to wait for individual ACK from the receiver as in Go-Back-N ARQ . The receiver may selectively reject a single frame, which may be retransmitted alone; this contrasts with other forms of ARQ, which must send every frame from that point again. The receiver accepts out-of-order frames and buffers them. The sender individually retransmits frames that have timed out.
It may be used as a protocol for the delivery and acknowledgement of message units, or it may be used as a protocol for the delivery of subdivided message sub-units.
When used as the protocol for the delivery of messages , the sending process continues to send a number of frames specified by a window size even after a frame loss. Unlike Go-Back-N ARQ , the receiving process will continue to accept and acknowledge frames sent after an initial error; this is the general case of the sliding window protocol with both transmit and receive window sizes greater than 1.
The receiver process keeps track of the sequence number of the earliest frame it has not received, and sends that number with every acknowledgement (ACK) it sends. If a frame from the sender does not reach the receiver, the sender continues to send subsequent frames until it has emptied its window . The receiver continues to fill its receiving window with the subsequent frames, replying each time with an ACK containing the sequence number of the earliest missing frame . Once the sender has sent all the frames in its window , it re-sends the frame number given by the ACKs, and then continues where it left off.
The size of the sending and receiving windows must be equal, and half the maximum sequence number (assuming that sequence numbers are numbered from 0 to n −1) to avoid miscommunication in all cases of packets being dropped. To understand this, consider the case when all ACKs are destroyed. If the receiving window is larger than half the maximum sequence number, some, possibly even all, of the packets that are present after timeouts are duplicates that are not recognized as such. The sender moves its window for every packet that is acknowledged. [ 1 ]
When used as the protocol for the delivery of subdivided messages it works somewhat differently. In non-continuous channels where messages may be variable in length, standard ARQ or Hybrid ARQ protocols may treat the message as a single unit. Alternately selective retransmission may be employed in conjunction with the basic ARQ mechanism where the message is first subdivided into sub-blocks (typically of fixed length) in a process called packet segmentation . The original variable length message is thus represented as a concatenation of a variable number of sub-blocks. While in standard ARQ the message as a whole is either acknowledged (ACKed) or negatively acknowledged (NAKed), in ARQ with selective transmission the ACK response would additionally carry a bit flag indicating the identity of each sub-block successfully received. In ARQ with selective retransmission of sub-divided messages each retransmission diminishes in length, needing to only contain the sub-blocks that were linked.
In most channel models with variable length messages, the probability of error-free reception diminishes in inverse proportion with increasing message length. In other words, it's easier to receive a short message than a longer message. Therefore, standard ARQ techniques involving variable length messages have increased difficulty delivering longer messages, as each repeat is the full length. Selective re-transmission applied to variable length messages completely eliminates the difficulty in delivering longer messages, as successfully delivered sub-blocks are retained after each transmission, and the number of outstanding sub-blocks in following transmissions diminishes. Selective Repeat is implemented in UDP transmission.
These examples assume an infinite number of sequence and request numbers. [ 2 ]
There are a few things to keep in mind when choosing a value for N in Selective Repeat ARQ:
The Transmission Control Protocol uses a variant of Go-Back-N ARQ to ensure reliable transmission of data over the Internet Protocol , which does not provide guaranteed delivery of packets; with Selective Acknowledgement (SACK) extension , it may also use Selective Repeat ARQ.
The ITU-T G.hn standard, which provides a way to create a high-speed (up to 1 Gigabit/s) Local area network using existing home wiring ( power lines , phone lines and coaxial cables ), uses Selective Repeat ARQ to ensure reliable transmission over noisy media. G.hn employs packet segmentation to sub-divide messages into smaller units, to increase the probability that each one is received correctly.
The STANAG 5066 Profile for High Frequency (HF) Radio Data Communication uses selective repeat ARQ, with a maximum window size of 128 protocol-data units (PDUs). | https://en.wikipedia.org/wiki/Selective_Repeat_ARQ |
In surface science , selective adsorption is the effect when minima associated with bound-state resonances occur in specular intensity in atom-surface scattering .
In crystal growth , selective adsorption refers to the phenomenon where adsorbing molecules attach preferentially to certain crystal faces.
An example of selective adsorption can be demonstrated in the growth of Rochelle salt crystals. If copper ions are added to solution during the growth process, some crystal faces will slow down as copper apparently becomes a barrier to adsorption. However, by then adding sodium hydroxide to the solution, the preferred crystal faces will change once again. [ 1 ]
Pronounced intensity minima were first observed in 1930 by Theodor Estermann , Otto Frisch , and Otto Stern , [ citation needed ] during a series of gas-surface interaction experiments attempting to demonstrate the wave nature of atoms and molecules. The phenomenon has been explained in 1936 by John Lennard-Jones and Devonshire [ who? ] in terms of resonant transitions to bound surface states. [ citation needed ]
The selective adsorption binding energies can supply information on the gas-surface interaction potentials by yielding the vibrational energy spectrum of the gas atom bound to the surface. Starting from the 1970s, it has been extensively studied, both theoretically and experimentally. Energy levels measured with this technique are available for many systems.
This physical chemistry -related article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Selective_adsorption |
Selective area epitaxy is the local growth of epitaxial layer through a patterned amorphous dielectric mask (typically SiO 2 or Si 3 N 4 ) deposited on a semiconductor substrate. Semiconductor growth conditions are selected to ensure epitaxial growth on the exposed substrate, but not on the dielectric mask. [ 1 ] SAE can be executed in various epitaxial growth methods such as molecular beam epitaxy [ 2 ] (MBE), metalorganic vapour phase epitaxy (MOVPE) [ 1 ] and chemical beam epitaxy (CBE). [ 3 ] By SAE, semiconductor nanostructures such as quantum dots and nanowires can be grown to their designed places. [ 2 ]
The mask used in SAE is usually amorphous dielectric such as SiO2 or SiN4 which is deposited on the semiconductor substrate. The patterns (holes) in the mask are fabricated using standard microfabrication techniques lithography and etching. Variety of lithography and etching techniques can be implemented to SAE mask fabrication. Suitable techniques depend on the pattern feature size and used materials. Electron beam lithography is widely used due to its nanometer resolution. The mask should withstand the high temperature growth conditions of semiconductors in order to limit the growth to the patterned holes in the mask. [ 4 ]
Selectivity in SAE is used to express the growth on the mask. The selectivity of the growth is originated from the property that atoms don't favor sticking to the mask i.e. they have low sticking coefficient . Sticking coefficient can be reduced by the choice of mask material, having lower material flow and having higher growth temperature. High selectivity i.e. no growth on the mask is desired. [ 5 ]
Epitaxial growth mechanism in SAE can be divided in to two parts: Growth before the mask level and growth after the mask level.
Before the mask level, the growth is confined to occur only in the hole in the mask. The growth starts to exceed the crystal of the substrate crystal following the pattern of the mask. The grown semiconductor has the structure of the pattern. This is employed in template assisted selective area epitaxy (TASE), where deep patterns in the mask are used as a template for the whole semiconductor structure and the growth is stopped before the mask level. [ 6 ]
After the mask level, the growth can exceed to any direction, because the mask is no longer limiting the growth direction. The growth continues to the direction which is energetically favorable for crystal to expand in existing growth conditions. The growth is referred as faceted growth, because it is favorable for crystal to form facets. Therefore, in SAE grown semiconductor structures, clear crystalline facets are seen. The growth direction, or more precisely, the growth rates of different crystal facets can be tuned. Growth temperature, V/III ratio, orientation of the pattern and shape of the pattern are properties that affect to the growth rates of facets. By adjusting these properties, the structure of grown semiconductor can be engineered. SAE grown nanowires and epitaxial lateral overgrown structures (ELO) are an example of structures that are engineered by SAE growth conditions. In nanowire growth, the growth rate of lateral facets is suppressed and the structure grows only in vertical direction. [ 4 ] In ELO, the growth is initiated in the mask openings, and after mask level the growth proceeds laterally on the mask, eventually joining the grown semiconductor structures together. The main principle in ELO is to reduce the defects caused by lattice mismatch of the substrate and the grown semiconductor. [ 7 ]
SAE can be achieved in various epitaxial growth techniques, which are listed below. | https://en.wikipedia.org/wiki/Selective_area_epitaxy |
A Selective Availability Anti-spoofing Module ( SAASM ) is used by military Global Positioning System receivers to allow decryption of precision GPS observations , while the accuracy of civilian GPS receivers may be reduced by the United States military through Selective Availability (SA) and anti-spoofing (AS). [ 1 ] However, on May 1, 2000 [ 2 ] it was announced that SA was being discontinued, along with a United States Presidential Directive that no future GPS programs will include it. [ 2 ] Before the advent of L2C , AS was meant to prevent access to dual-frequency observations to civilian users.
SAASM allows satellite authentication, over-the-air rekeying, and contingency recovery. Those features are not available with the similar, but older, PPS-SM (Precise Positioning Service Security Module) system. PPS-SM systems require periodic updates with a classified " Red Key " that may only be transmitted by secure means (such as physically taking the receiver to a secure facility for rekeying or having a trusted courier deliver a paper tape with a new key to the receiver, after which that paper tape must be securely destroyed). SAASM systems can be updated with an encrypted " Black Key " that may be transmitted over unclassified channels. All military receivers newly deployed after the end of September 2006 must use SAASM. [ 1 ]
SAASM does not provide any additional anti-jam capability, however, the higher data (chipping) rate of P(Y) code can provide a higher processing gain which will provide better tracking performance in a jamming environment. Future GPS upgrades, such as M-Code , will provide additional improvements to anti-jam capabilities. [ citation needed ]
SAASM hardware is covered with an anti-tampering coating, to deter analysis of their internal operation. [ citation needed ]
Deployment of the next generation military signal for GPS, called M-code , commenced with the launch of IIR-M and IIF satellites, beginning in 2005. A complete constellation of 18 satellites with M-code capability is planned for 2016. | https://en.wikipedia.org/wiki/Selective_availability_anti-spoofing_module |
Selective breeding (also called artificial selection ) is the process by which humans use animal breeding and plant breeding to selectively develop particular phenotypic traits (characteristics) by choosing which typically animal or plant males and females will sexually reproduce and have offspring together. Domesticated animals are known as breeds , normally bred by a professional breeder , while domesticated plants are known as varieties , cultigens , cultivars , or breeds. [ 1 ] Two purebred animals of different breeds produce a crossbreed , and crossbred plants are called hybrids . Flowers, vegetables and fruit-trees may be bred by amateurs and commercial or non-commercial professionals: major crops are usually the provenance of the professionals.
In animal breeding artificial selection is often combined with techniques such as inbreeding , linebreeding , and outcrossing . In plant breeding , similar methods are used. Charles Darwin discussed how selective breeding had been successful in producing change over time in his 1859 book, On the Origin of Species . Its first chapter discusses selective breeding and domestication of such animals as pigeons , cats , cattle , and dogs . Darwin used artificial selection as an analogy to propose and explain the theory of natural selection but distinguished the latter from the former as a separate process that is non-directed. [ 2 ] [ 3 ] [ 4 ]
The deliberate exploitation of selective breeding to produce desired results has become very common in agriculture and experimental biology.
Selective breeding can be unintentional, for example, resulting from the process of human cultivation; and it may also produce unintended – desirable or undesirable – results. For example, in some grains, an increase in seed size may have resulted from certain ploughing practices rather than from the intentional selection of larger seeds. Most likely, there has been an interdependence between natural and artificial factors that have resulted in plant domestication. [ 5 ]
Selective breeding of both plants and animals has been practiced since prehistory ; key species such as wheat , rice , and dogs have been significantly different from their wild ancestors for millennia, and maize , which required especially large changes from teosinte , its wild form, was selectively bred in Mesoamerica . Selective breeding was practiced by the Romans . [ 6 ] Treatises as much as 2,000 years old give advice on selecting animals for different purposes, and these ancient works cite still older authorities, such as Mago the Carthaginian . [ 7 ] The notion of selective breeding was later expressed by the Persian Muslim polymath Abu Rayhan Biruni in the 11th century. He noted the idea in his book titled India , which included various examples. [ 8 ]
The agriculturist selects his corn, letting grow as much as he requires, and tearing out the remainder. The forester leaves those branches which he perceives to be excellent, whilst he cuts away all others. The bees kill those of their kind who only eat, but do not work in their beehive.
Selective breeding was established as a scientific practice by Robert Bakewell during the British Agricultural Revolution in the 18th century. Arguably, his most important breeding program was with sheep. Using native stock, he was able to quickly select for large, yet fine-boned sheep, with long, lustrous wool. The Lincoln Longwool was improved by Bakewell, and in turn the Lincoln was used to develop the subsequent breed, named the New (or Dishley) Leicester. It was hornless and had a square, meaty body with straight top lines. [ 9 ]
These sheep were exported widely, including to Australia and North America , and have contributed to numerous modern breeds, despite the fact that they fell quickly out of favor as market preferences in meat and textiles changed. Bloodlines of these original New Leicesters survive today as the English Leicester (or Leicester Longwool), which is primarily kept for wool production.
Bakewell was also the first to breed cattle to be used primarily for beef. Previously, cattle were first and foremost kept for pulling ploughs as oxen , [ 10 ] but he crossed long-horned heifers and a Westmoreland bull to eventually create the Dishley Longhorn . As more and more farmers followed his lead, farm animals increased dramatically in size and quality. In 1700, the average weight of a bull sold for slaughter was 370 pounds (168 kg). By 1786, that weight had more than doubled to 840 pounds (381 kg). However, after his death, the Dishley Longhorn was replaced with short-horn versions.
He also bred the Improved Black Cart horse, which later became the Shire horse .
Charles Darwin coined the term 'selective breeding'; he was interested in the process as an illustration of his proposed wider process of natural selection . Darwin noted that many domesticated animals and plants had special properties that were developed by intentional animal and plant breeding from individuals that showed desirable characteristics, and discouraging the breeding of individuals with less desirable characteristics.
Darwin used the term "artificial selection" twice in the 1859 first edition of his work On the Origin of Species , in Chapter IV: Natural Selection, and in Chapter VI: Difficulties on Theory:
Slow though the process of selection may be, if feeble man can do much by his powers of artificial selection, I can see no limit to the amount of change, to the beauty and infinite complexity of the co-adaptations between all organic beings, one with another and with their physical conditions of life, which may be effected in the long course of time by nature's power of selection. [ 11 ]
We are profoundly ignorant of the causes producing slight and unimportant variations; and we are immediately made conscious of this by reflecting on the differences in the breeds of our domesticated animals in different countries,—more especially in the less civilized countries where there has been but little artificial selection. [ 12 ]
Animals with homogeneous appearance, behavior, and other characteristics are known as particular breeds or pure breeds, and they are bred through culling animals with particular traits and selecting for further breeding those with other traits. Purebred animals belong to a single, recognizable breed, and purebreds with recorded lineage are called pedigreed . Crossbreeds are a mix of two purebreds, whereas mixed breeds are a mix of several breeds, often unknown. Animal breeding begins with breeding stock, a group of animals used for the purpose of planned breeding. When individuals are looking to breed animals, they look for certain valuable traits in purebred stock for a certain purpose, or may intend to use some type of crossbreeding to produce a new type of stock with different and presumably superior abilities in a given area of endeavor. For example, to breed chickens, a breeder typically intends to receive eggs, meat, and new, young birds for further reproduction. Thus, the breeder has to study different breeds and types of chickens and analyze what can be expected from a certain set of characteristics before he or she starts breeding them. Therefore, when purchasing initial breeding stock, the breeder seeks a group of birds that will most closely fit the purpose intended.
Purebred breeding aims to establish and maintain stable traits, that animals will pass to the next generation. By "breeding the best to the best," employing a certain degree of inbreeding , considerable culling, and selection for "superior" qualities, one could develop a bloodline superior in certain respects to the original base stock. Such animals can be recorded with a breed registry , the organization that maintains pedigrees and/or stud books . However, single-trait breeding, breeding for only one trait over all others, can be problematic. [ 13 ] In one case mentioned by the animal behaviorist Temple Grandin , roosters bred for fast growth or heavy muscles did not know how to perform typical rooster courtship dances, which alienated the roosters from hens and led the roosters to kill the hens after mating with them. [ 13 ] A Soviet attempt to breed lab rats with higher intelligence led to cases of neurosis severe enough to make the animals incapable of any problem solving unless drugs like phenazepam were used. [ 14 ]
The observable phenomenon of hybrid vigor stands in contrast to the notion of breed purity. However, on the other hand, indiscriminate breeding of crossbred or hybrid animals may also result in degradation of quality. Studies in evolutionary physiology , behavioral genetics , and other areas of organismal biology have also made use of deliberate selective breeding, though longer generation times and greater difficulty in breeding can make these projects challenging in such vertebrates as house mice . [ 15 ] [ 16 ] [ 17 ]
The process of plant breeding has been used for thousands of years, and began with the domestication of wild plants into uniform and predictable agricultural cultigens . These high-yielding varieties have been particularly important in agriculture. As crops improved, humans were able to move from hunter-gatherer style living to a mix of hunter-gatherer and agriculture practices. [ 18 ] Although these higher yielding plants were derived from an extremely primitive version of plant breeding, this form of agriculture was an investment that the people who grew them were planting then could have a more varied diet. This meant that they did not completely stop their hunting and gathering immediately but instead over time transitioned and ultimately favored agriculture. [ 19 ] Originally this was due to humans not wanting to risk using all their time and resources for their crops just to fail. Which was promptly called play farming due to the idea of "farmers" experimenting with agriculture. [ 19 ] In addition, the ability for humans to stay within one place for food and create permanent settlements made the process move along faster. [ 20 ] During this transitional period, crops began to acclimate and evolve with humans encouraging humans to invest further into crops. Over time this reliance on plant breeding has created problems, as highlighted by the book Botany of Desire where Michael Pollan shows the connection between basic human desires through four different plants: apples for sweetness, tulips for beauty, cannabis for intoxication, and potatoes for control. In a form of reciprocal evolution humans have influenced these plants as much as the plants have influenced the people that consume them, is known as coevolution . [ 21 ]
Selective plant breeding is also used in research to produce transgenic animals that breed "true" (i.e., are homozygous ) for artificially inserted or deleted genes. [ 22 ]
Selective breeding in aquaculture holds high potential for the genetic improvement of fish and shellfish for the process of production. Unlike terrestrial livestock, the potential benefits of selective breeding in aquaculture were not realized until recently. This is because high mortality led to the selection of only a few broodstock , causing inbreeding depression, which then forced the use of wild broodstock. This was evident in selective breeding programs for growth rate, which resulted in slow growth and high mortality. [ 23 ]
Control of the reproduction cycle was one of the main reasons as it is a requisite for selective breeding programs. Artificial reproduction was not achieved because of the difficulties in hatching or feeding some farmed species such as eel and yellowtail farming. [ 24 ] A suspected reason associated with the late realization of success in selective breeding programs in aquaculture was the education of the concerned people – researchers, advisory personnel and fish farmers. The education of fish biologists paid less attention to quantitative genetics and breeding plans. [ 25 ]
Another was the failure of documentation of the genetic gains in successive generations. This in turn led to failure in quantifying economic benefits that successful selective breeding programs produce. Documentation of the genetic changes was considered important as they help in fine tuning further selection schemes. [ 23 ]
Aquaculture species are reared for particular traits such as growth rate, survival rate, meat quality, resistance to diseases, age at sexual maturation, fecundity, shell traits like shell size, shell color, etc.
Gjedrem (1979) showed that selection of Atlantic salmon ( Salmo salar ) led to an increase in body weight by 30% per generation. A comparative study on the performance of select Atlantic salmon with wild fish was conducted by AKVAFORSK Genetics Centre in Norway. The traits, for which the selection was done included growth rate, feed consumption, protein retention, energy retention, and feed conversion efficiency. Selected fish had a twice better growth rate, a 40% higher feed intake, and an increased protein and energy retention. This led to an overall 20% better Fed Conversion Efficiency as compared to the wild stock. [ 27 ] Atlantic salmon have also been selected for resistance to bacterial and viral diseases. Selection was done to check resistance to Infectious Pancreatic Necrosis Virus (IPNV). The results showed 66.6% mortality for low-resistant species whereas the high-resistant species showed 29.3% mortality compared to wild species. [ 28 ]
Rainbow trout ( S. gairdneri ) was reported to show large improvements in growth rate after 7–10 generations of selection. [ 29 ] Kincaid et al. (1977) showed that growth gains by 30% could be achieved by selectively breeding rainbow trout for three generations. [ 30 ] A 7% increase in growth was recorded per generation for rainbow trout by Kause et al. (2005). [ 31 ]
In Japan, high resistance to IPNV in rainbow trout has been achieved by selectively breeding the stock. Resistant strains were found to have an average mortality of 4.3% whereas 96.1% mortality was observed in a highly sensitive strain. [ 32 ]
Coho salmon ( Oncorhynchus kisutch ) increase in weight was found to be more than 60% after four generations of selective breeding. [ 33 ] In Chile, Neira et al. (2006) conducted experiments on early spawning dates in coho salmon. After selectively breeding the fish for four generations, spawning dates were 13–15 days earlier. [ 34 ]
Cyprinids
Selective breeding programs for the Common carp ( Cyprinus carpio ) include improvement in growth, shape and resistance to disease. Experiments carried out in the USSR used crossings of broodstocks to increase genetic diversity and then selected the species for traits like growth rate, exterior traits and viability, and/or adaptation to environmental conditions like variations in temperature. Kirpichnikov et al. (1974) [ 35 ] and Babouchkine (1987) [ 36 ] selected carp for fast growth and tolerance to cold, the Ropsha carp. The results showed a 30–40% to 77.4% improvement of cold tolerance but did not provide any data for growth rate. An increase in growth rate was observed in the second generation in Vietnam. [ 37 ] Moav and Wohlfarth (1976) showed positive results when selecting for slower growth for three generations compared to selecting for faster growth. Schaperclaus (1962) showed resistance to the dropsy disease wherein selected lines suffered low mortality (11.5%) compared to unselected (57%). [ 38 ]
Growth was seen to increase by 12–20% in selectively bred Iictalurus punctatus . [ 39 ] More recently, the response of the Channel Catfish to selection for improved growth rate was found to be approximately 80%, that is, an average of 13% per generation.
Selection for live weight of Pacific oysters showed improvements ranging from 0.4% to 25.6% compared to the wild stock. [ 40 ] Sydney-rock oysters ( Saccostrea commercialis ) showed a 4% increase after one generation and a 15% increase after two generations. [ 41 ] [ 42 ] Chilean oysters ( Ostrea chilensis ), selected for improvement in live weight and shell length showed a 10–13% gain in one generation.
Bonamia ostrea is a protistan parasite that causes catastrophic losses (nearly 98%) in European flat oyster Ostrea edulis L. This protistan parasite is endemic to three oyster-regions in Europe. Selective breeding programs show that O. edulis susceptibility to the infection differs across oyster strains in Europe. A study carried out by Culloty et al. showed that 'Rossmore' oysters in Cork harbour, Ireland had better resistance compared to other Irish strains. A selective breeding program at Cork harbour uses broodstock from 3– to 4-year-old survivors and is further controlled until a viable percentage reaches market size. [ 43 ] [ 44 ]
Over the years 'Rossmore' oysters have shown to develop lower prevalence of B. ostreae infection and percentage mortality. Ragone Calvo et al. (2003) selectively bred the eastern oyster, Crassostrea virginica , for resistance against co-occurring parasites Haplosporidium nelson (MSX) and Perkinsus marinus (Dermo). They achieved dual resistance to the disease in four generations of selective breeding. The oysters showed higher growth and survival rates and low susceptibility to the infections. At the end of the experiment, artificially selected C. virginica showed a 34–48% higher survival rate. [ 45 ]
Selection for growth in Penaeid shrimps yielded successful results. A selective breeding program for Litopenaeus stylirostris saw an 18% increase in growth after the fourth generation and 21% growth after the fifth generation. [ 46 ] Marsupenaeus japonicas showed a 10.7% increase in growth after the first generation. [ 47 ] Argue et al. (2002) conducted a selective breeding program on the Pacific White Shrimp, Litopenaeus vannamei at The Oceanic Institute, Waimanalo, USA from 1995 to 1998. They reported significant responses to selection compared to the unselected control shrimps. After one generation, a 21% increase was observed in growth and 18.4% increase in survival to TSV. [ 48 ] The Taura Syndrome Virus (TSV) causes mortalities of 70% or more in shrimps. C.I. Oceanos S.A. in Colombia selected the survivors of the disease from infected ponds and used them as parents for the next generation. They achieved satisfying results in two or three generations wherein survival rates approached levels before the outbreak of the disease. [ 49 ] The resulting heavy losses (up to 90%) caused by Infectious hypodermal and haematopoietic necrosis virus (IHHNV) caused a number of shrimp farming industries started to selectively breed shrimps resistant to this disease. Successful outcomes led to development of Super Shrimp, a selected line of L. stylirostris that is resistant to IHHNV infection. Tang et al. (2000) confirmed this by showing no mortalities in IHHNV- challenged Super Shrimp post larvae and juveniles. [ 50 ]
Selective breeding programs for aquatic species provide better outcomes compared to terrestrial livestock. This higher response to selection of aquatic farmed species can be attributed to the following:
Selective breeding in aquaculture provide remarkable economic benefits to the industry, the primary one being that it reduces production costs due to faster turnover rates. When selective breeding is carried out, some characteristics are lost for others that may suit a specific environment or situation. [ 51 ] This is because of faster growth rates, decreased maintenance rates, increased energy and protein retention, and better feed efficiency. [ 23 ] Applying genetic improvement programs to aquaculture species will increase their productivity. Thus allowing them to meet the increasing demands of growing populations. Conversely, selective breeding within aquaculture can create problems within the biodiversity of both stock and wild fish, which can hurt the industry down the road. Although there is great potential to improve aquaculture due to the current lack of domestication, it is essential that the genetic diversity of the fish are preserved through proper genetic management, as we domesticate these species. [ 52 ] It is not uncommon for fish to escape the nets or pens that they are kept in, especially in mass. If these fish are farmed in areas they are not native to they may be able to establish themselves and outcompete native populations of fish, and cause ecological harm as an invasive species. [ 53 ] Furthermore, if they are in areas where the fish being farmed are native too their genetics are selectively bred rather than being wild. These farmed fish could breed with the natives which could be problematic In the sense that they would have been bred for consumption rather than by chance. Resulting in an overall decrease in genetic diversity and rendering local fish populations less fit for survival. [ 53 ] If proper management is not taking place then the economic benefits and the diversity of the fish species will falter. [ 52 ]
Selective breeding is a direct way to determine if a specific trait can evolve in response to selection. A single-generation method of breeding is not as accurate or direct. The process is also more practical and easier to understand than sibling analysis. Selective breeding is better for traits such as physiology and behavior that are hard to measure because it requires fewer individuals to test than single-generation testing.
However, there are disadvantages to this process. This is because a single experiment done in selective breeding cannot be used to assess an entire group of genetic variances, individual experiments must be done for every individual trait. Also, due to the necessity of selective breeding experiments to require maintaining the organisms tested in a lab or greenhouse , it is impractical to use this breeding method on many organisms. Controlled mating instances are difficult to carry out in this case and this is a necessary component of selective breeding. [ 54 ]
Additionally, selective breeding can lead to a variety of issues including reduction of genetic diversity or physical problems. The process of selective breeding can create physical issues for plants or animals such as dogs selectively bred for extremely small sizes dislocating their kneecaps at a much more frequent rate then other dogs. [ 55 ] An example in the plant world is the Lenape potatoes were selectively bred for their disease or pest resistance which was attributed to their high levels of toxic glycoalkaloid solanine which are usually present only in small amounts in potatoes fit for human consumption. [ 56 ] When genetic diversity is lost it can also allow for populations to lack genetic alternatives to adapt to events. This becomes an issue of biodiversity, because attributes are so wide-spread they can result in mass epidemics. As seen in the Southern Corn leaf-blight epidemic of 1970 that wiped out 15% of the United States corn crop due to the wide use of a type of Texan corn strain that was artificially selected due to having sterile pollen to make farming easier. At the same time it was more vulnerable to Southern Corn leaf-blight. [ 57 ] [ 58 ] | https://en.wikipedia.org/wiki/Selective_breeding |
Selective catalytic reduction ( SCR ) means converting nitrogen oxides , also referred to as NO x with the aid of a catalyst into diatomic nitrogen ( N 2 ), and water ( H 2 O ). A reductant , typically anhydrous ammonia ( NH 3 ), aqueous ammonia ( NH 4 OH ), or a urea ( CO(NH 2 ) 2 ) solution, is added to a stream of flue or exhaust gas and is reacted onto a catalyst . As the reaction drives toward completion, nitrogen ( N 2 ), and carbon dioxide ( CO 2 ), in the case of urea use, are produced.
Selective catalytic reduction of NO x using ammonia as the reducing agent was patented in the United States by the Engelhard Corporation in 1957. Development of SCR technology continued in Japan and the US in the early 1960s with research focusing on less expensive and more durable catalyst agents. The first large-scale SCR was installed by the IHI Corporation in 1978. [ 1 ]
Commercial selective catalytic reduction systems are typically found on large utility boilers , industrial boilers , and municipal solid waste boilers and have been shown to lower NO x emissions by 70-95%. [ 1 ] Applications include diesel engines , such as those found on large ships , diesel locomotives , gas turbines , and automobiles .
SCR systems are now the preferred method for meeting Tier 4 Final and EURO 6 diesel emissions standards for heavy trucks, cars and light commercial vehicles. As a result, emissions of NOx, particulates , and hydrocarbons have been lowered by as much as 95% when compared with pre-emissions engines. [ 2 ]
The NO x reduction reaction takes place as the gases pass through the catalyst chamber . [ 3 ] [ 4 ] Before entering the catalyst chamber, ammonia, or other reductant (such as urea ), is injected and mixed with the gases. The intended equations for the reactions using ammonia for a SCR are: [ 4 ]
Several secondary reactions also occur:
With urea, the reactions are:
As with ammonia, several secondary reactions also occur in the presence of sulfur:
The ideal reaction has an optimal temperature range between 630 and 720 K (357 and 447 °C) but can operate as low as 500 K (227 °C) with longer residence times . The minimum effective temperature depends on the fuels, gas constituents, and catalyst. Other possible reductants include cyanuric acid and ammonium sulfate . [ 5 ]
SCR catalysts are made from various porous ceramic materials used as a support , such as titanium oxide , and active catalytic components are usually either oxides (of vanadium , molybdenum and tungsten ), zeolites , or cerium . [ 3 ] [ 4 ]
Base metal catalysts, such as vanadium and tungsten, lack high thermal durability, but are less expensive and operate very well at the temperature ranges most commonly applied in industrial and utility boiler applications. Thermal durability is particularly important for automotive SCR applications that incorporate the use of a diesel particulate filter with forced regeneration. They also have a high catalysing potential to oxidize SO 2 into SO 3 , which can be extremely damaging due to its acidic properties. [ 6 ]
Zeolite catalysts have the potential to operate at substantially higher temperature than base metal catalysts; they can withstand prolonged operation at temperatures of 900 K (627 °C) and transient conditions of up to 1120 K (847 °C). Zeolites also have a lower potential for SO 2 oxidation and thus decrease the related corrosion risks. [ 6 ]
Iron - and copper - exchanged zeolite urea SCRs have been developed with approximately equal performance to that of vanadium-urea SCRs if the fraction of the NO 2 is 20% to 50% of the total NO x . [ 7 ] The two most common catalyst geometries used today are honeycomb catalysts and plate catalysts. The honeycomb form usually consists of an extruded ceramic applied homogeneously throughout the carrier or coated on the substrate. Like the various types of catalysts, their configuration also has advantages and disadvantages. Plate-type catalysts have lower pressure drops and are less susceptible to plugging and fouling than the honeycomb types, but are much larger and more expensive. Honeycomb configurations are smaller than plate types, but have higher pressure drops and plug much more easily. A third type is corrugated , comprising only about 10% of the market in power plant applications. [ 1 ]
Several nitrogen-bearing reductants are used in SCR applications including anhydrous ammonia , aqueous ammonia or dissolved urea . All those three reductants are widely available in large quantities.
Anhydrous ammonia can be stored as a liquid at approximately 10 bar in steel tanks. It is classified as an inhalation hazard , but it can be safely stored and handled if well-developed codes and standards are followed. Its advantage is that it needs no further conversion to operate within a SCR and is typically favoured by large industrial SCR operators. Aqueous ammonia must be first vaporized in order to be used, but it is substantially safer to store and transport than anhydrous ammonia. Urea is the safest to store, but requires conversion to ammonia through thermal decomposition. [ 8 ] At the end of the process, the purified exhaust gasses are sent to the boiler or condenser or other equipment, or discharged into the atmosphere. [ 9 ] [ 1 ]
Most catalysts have finite service life mainly due to the formation of ammonium sulfate and ammonium bisulfate from sulfur compounds when high-sulfur fuels are used, as well as the undesirable catalyst-induced oxidation of SO 2 to SO 3 and H 2 SO 4 . In applications that use exhaust gas boilers, ammonium sulfate and ammonium bisulfate can accumulate on the boiler tubes, inhibiting steam output and increasing exhaust back-pressure. In marine applications, this can increase fresh water requirements as the boiler must be continuously washed to remove the deposits.
Most catalysts on the market have porous structures and a geometries optimized for increasing their specific surface area (a clay planting pot is a good example of what SCR catalyst feels like). This porosity is what gives the catalyst the high surface area needed for reduction of NOx. However, soot, ammonium sulfate, ammonium bisulfate, silica compounds, and other fine particulates can easily clog the pores. Ultrasonic horns and soot blowers can remove most of these contaminants while the unit is online. The unit can also be cleaned by being washed with water or by raising the exhaust temperature.
Of more concern to SCR performance are poisons , which will chemically degrade the catalyst itself or block the catalyst's active sites and render it ineffective at NO x reduction, and in severe cases this can result in the ammonia or urea being oxidized and a subsequent increase in NO x emissions. These poisons are alkali metals , alkaline earth metals , halogens , phosphorus , sulfur , arsenic , antimony , chromium , heavy metals ( copper , cadmium , mercury , thallium , and lead ), and many heavy metal compounds (e.g. oxides and halides).
Catalyst management strategies can include catalyst replacement and catalyst rejuvenation (removal of pluggage and poisons) and regeneration (replenishment of materials). [ 10 ] [ 11 ]
Most SCRs require tuning to properly perform. Part of tuning involves ensuring a proper distribution of ammonia in the gas stream and uniform gas velocity through the catalyst. Without tuning, SCRs can exhibit inefficient NOx reduction along with excessive ammonia slip due to not utilizing the catalyst surface area effectively. Another facet of tuning involves determining the proper ammonia flow for all process conditions. Ammonia flow is in general controlled based on NOx measurements taken from the gas stream or preexisting performance curves from an engine manufacturer (in the case of gas turbines and reciprocating engines ). Typically, all future operating conditions must be known beforehand to properly design and tune an SCR system.
Ammonia slip is an industry term for ammonia passing through the SCR unreacted. This occurs when ammonia is injected in excess, temperatures are too low for ammonia to react, or the catalyst has been poisoned. In applications using both SCR and an alkaline scrubber, the use of high-sulfur fuels also tend to significantly increase ammonia slip, since compounds such as NaOH and Ca(OH) 2 will reduce ammonium sulfate and ammonium bisulfate back into ammonia:
Temperature is SCR's largest limitation. Engines all have a period during start-up where exhaust temperatures are too low, and the catalyst must be pre-heated for the desired NOx reduction to occur when an engine is first started, especially in cold climates.
In power stations , the same basic technology is employed for removal of NO x from the flue gas of boilers used in power generation and industry. In general, the SCR unit is located between the furnace economizer and the air heater, and the ammonia is injected into the catalyst chamber through an ammonia injection grid. As in other SCR applications, the temperature of operation is critical. Ammonia slip (unreacted ammonia) is also an issue with SCR technology used in power plants.
A significant operational difficulty in coal -fired boilers is the binding of the catalyst by fly ash from the fuel combustion . This requires the usage of sootblowers , ultrasonic horns, and careful design of the ductwork and catalyst materials to avoid plugging by the fly ash. SCR catalysts have a typical operational lifetime of about 16,000 – 40,000 hours (1.8 – 4.5 years) in coal-fired power plants, depending on the flue gas composition, and up to 80,000 hours (9 years) in cleaner gas-fired power plants.
Poisons , sulfur compounds , and fly ash can all be removed by installing scrubbers before the SCR system to increase the life of the catalyst , though in most power plants and marine engines, scrubbers are installed after the system to maximize the SCR system's effectiveness.
SCR was applied to trucks by Nissan Diesel Corporation , and the first practical product " Nissan Diesel Quon " was introduced in 2004 in Japan. [ 12 ]
In 2007, the United States Environmental Protection Agency (EPA) enacted requirements to significantly lower harmful exhaust emissions. To achieve this standard, Cummins and other diesel engine manufacturers developed an aftertreatment system that includes the use of a diesel particulate filter (DPF). As the DPF does not function with low-sulfur diesel fuel, diesel engines that conform to 2007 EPA emissions standards require ultra-low sulfur diesel fuel (ULSD) to prevent damage to the DPF. After a brief transition period, ULSD fuel became common at fuel pumps in the United States and Canada. The 2007 EPA regulations were meant to be an interim solution to allow manufacturers time to prepare for the more stringent 2010 EPA regulations, which lowers NOx levels even further. [ 13 ]
Diesel engines manufactured after January 1, 2010 are required to meet lowered NOx standards for the US market.
All of the heavy-duty engine (Class 7-8 trucks) manufacturers except for Navistar International and Caterpillar continuing to manufacture engines after this date have chosen to use SCR. This includes Detroit Diesel (DD13, DD15, and DD16 models), Cummins (ISX, ISL9, and ISB6.7), Paccar , and Volvo / Mack . These engines require the periodic addition of diesel exhaust fluid (DEF, a urea solution) to enable the process. DEF is available in bottles and jugs from most truck stops, and a more recent development is bulk DEF dispensers near diesel fuel pumps. Caterpillar and Navistar had initially chosen to use enhanced exhaust gas recirculation (EEGR) to comply with the Environmental Protection Agency (EPA) standards, but in July 2012 Navistar announced it would be pursuing SCR technology for its engines, except on the MaxxForce 15 which was to be discontinued. Caterpillar ultimately withdrew from the on-highway engine market prior to implementation of these requirements. [ 16 ]
BMW , [ 17 ] [ 18 ] Daimler AG (as BlueTEC ), and Volkswagen have used SCR technology in some of their passenger diesel cars. | https://en.wikipedia.org/wiki/Selective_catalytic_reduction |
In metallurgy , selective leaching , also called dealloying , demetalification , parting and selective corrosion , is a corrosion type in some solid solution alloys , when in suitable conditions a component of the alloys is preferentially leached from the initially homogenous material. The less noble metal is removed from the alloy by a microscopic-scale galvanic corrosion mechanism. The most susceptible alloys are the ones containing metals with high distance between each other in the galvanic series , e.g. copper and zinc in brass. The elements most typically undergoing selective removal are zinc , aluminium , iron , cobalt , chromium , and others.
The most common example is selective leaching of zinc from brass alloys containing more than 15% zinc (dezincification) in the presence of oxygen and moisture, e.g. from brass taps in chlorine -containing water. Dezincification has been studied since the 1860s, [ 1 ] and the mechanism by which it occurs was under extensive examination by the 1960s. It is believed that both copper and zinc gradually dissolve out simultaneously, and copper precipitates back from the solution. The material remaining is a copper-rich sponge with poor mechanical properties, and a color changed from yellow to red. Dezincification can be caused by water containing sulfur , carbon dioxide , and oxygen . Stagnant or low velocity waters tend to promote dezincification.
To combat this, arsenic or tin can be added to brass, or gunmetal can be used instead. Dezincification resistant brass (DZR), also known as Brass C352 is an alloy used to make pipe fittings for use with potable water . Plumbing fittings that are resistant to dezincification are appropriately marked, with the letters "CR" (Corrosion Resistant) or DZR (dezincification resistant) in the UK, and the letters "DR" (dezincification resistant) in Australia.
Graphitic corrosion is selective leaching of iron , from grey cast iron , where iron is removed and graphite grains remain intact. Affected surfaces develop a layer of graphite, rust, and metallurgical impurities that may inhibit further leaching. The effect can be substantially reduced by alloying the cast iron with nickel. [ 2 ]
Dealuminification is a corresponding process for aluminum alloys. Similar effects for different metals are decarburization (removal of carbon from the surface of alloy), decobaltification, denickelification, etc. The prototypical system for dealloying to create nano-porous metals is the np-Au system, which is created by selectively leaching Ag out of an Au-Ag homogenous alloy. [ 3 ]
When an initially homogenous alloy is placed in an acid that can preferentially dissolve one or more components out of the alloy, the remaining component will diffuse and organize into a unique, nano-porous microstructure. The resulting material will have ligaments, formed by the remaining material, surrounded by pores, empty space from which atoms were leached/diffused away.
The way that porosity develops during the dealloying process has been studied computationally to understand the diffusional pathways on an atomistic level. [ 4 ] Firstly, the less noble atoms must be dissolved away from the surface of the alloy. This process is easiest for the lower coordinated atoms, i.e., those bonded to fewer other atoms, usually found as single atoms sitting on the surface ("adatoms"), but it is more difficult for higher coordinated atoms, i.e., those sitting at "steps" or in the bulk of the material. Thus, the slowest step, and that which is most important for determining rate of porosity evolution is the dissolution of these higher coordinated less noble atoms. [ 3 ] Just as the less noble metal is less stable as an adatom on the surface, so is an atom of the more noble metal. Therefore, as dissolution proceeds, any more noble atoms will move to more stable positions, like steps, where its coordination is higher. [ 3 ] This diffusion process is similar to spinodal decomposition . [ 3 ] Eventually, clusters of more noble atoms form this way, and surrounding less noble atoms dissolve away, leaving behind a "bicontinuous structure" and providing a pathway for dissolution to continue deeper into the metal. [ 3 ]
Due to the relatively small sample size achievable with dealloying, the mechanical properties of these materials are often probed using the following techniques: [ 5 ]
A common concept in materials science is that, at ambient conditions, smaller features (like grain size or absolute size) generally lead to stronger materials (see Hall-Petch strengthening, Weibull statistics). However, due to the high-level of porosity in the dealloyed materials, their strengths and stiffnesses are relatively low compared to the bulk counterparts. [ 3 ] The decrease in strength due to porosity can be described with the Gibson-Ashby (GA) relations, [ 3 ] which give the yield strength and Young's modulus of a foam according to the following equations: [ 6 ]
σ y f o a m = C σ σ y b u l k ( ρ ∗ ) n σ {\displaystyle \sigma _{y}^{foam}=C_{\sigma }\sigma _{y}^{bulk}(\rho ^{*})^{n_{\sigma }}}
E f o a m = C E E b u l k ( ρ ∗ ) n E {\displaystyle E^{foam}=C_{E}E^{bulk}(\rho ^{*})^{n_{E}}}
where C σ {\displaystyle C_{\sigma }} and C E {\displaystyle C_{E}} are geometric constants, n σ {\displaystyle n_{\sigma }} and n E {\displaystyle n_{E}} are microstructure dependent exponents, and ρ ∗ = ρ f o a m / ρ b u l k {\displaystyle \rho ^{*}=\rho ^{foam}/\rho ^{bulk}} is the relative density of the foam.
The GA relations can be used to estimate the strength and stiffness of a given dealloyed, porous material, but more extensive study has revealed an additional factor: ligament size. When the ligament diameter is greater than 100 nm, increasing ligament size leads to greater agreement between GA predictions and experimental measurements of yield stress and Young's modulus. [ 7 ] However, when the ligament size is under 100 nm, which is very common in many dealloying processes, there is an addition to the GA strength that looks similar to Hall-Petch strengthening of bulk polycrystalline metals (i.e., the yield stress increases with the inverse square root of grain size). Combining this relationship with the GA relation from before, an expression for the yield stress of dealloyed materials with ligaments smaller than 100 nm can be determined: [ 3 ]
σ y f o a m = C σ A λ − m ( ρ ∗ ) n σ {\displaystyle \sigma _{y}^{foam}=C_{\sigma }A\lambda ^{-m}(\rho ^{*})^{n_{\sigma }}}
where A and m are empirically determined constants, and λ {\displaystyle \lambda } is the ligament size. The λ − m {\displaystyle \lambda ^{-m}} represents the Hall-Petch-like contribution.
There are two theories for why this increase in strength occurs: 1) dislocations are less common in smaller sample volumes, so deformation requires activation of sources (which is a more difficult process), or 2) dislocations pile-up, which strengthens the material. Either way, there would be significant surface and small volume effects in the ligaments <100 nm, which lead to this increase in yield stress. [ 7 ] A relationship between ligament size and Young's modulus has not been studied past the GA relation. [ 3 ]
Occasionally, the metastable nature of these materials means that ligaments in the structure may "pinch off" due to surface diffusion, which decreases the connectivity of the structure, and reduces the strength of the dealloyed material past what would be expected from simply porosity (as predicted by the Gibson-Ashby relations ). [ 8 ]
Because the ligaments of these materials are essentially small metallic samples, they are themselves expected to be quite ductile; although, the entire nano-porous material is often observed to be brittle in tension. [ 3 ] Dislocation behavior is extensive within the ligaments (just as would be expected in a metal): a high density. of partial dislocations, stacking faults and twins have been observed both in simulation and in TEM. [ 3 ] However, the morphology of the ligaments makes bulk dislocation motion very difficult; the limited size of each ligament and complex connectivity within the nano-porous structure means that a dislocation cannot freely travel long distances and thus induce large-scale plasticity. [ 3 ]
Countermeasures involve using alloys not susceptible to grain boundary depletion , using a suitable heat treatment , altering the environment (e.g. lowering oxygen content), and/or use cathodic protection .
Selective leaching can be used to produce powdered materials with extremely high surface area, such as Raney nickel and other heterogeneous catalysts . [ 9 ] Selective leaching can be the pre-final stage of depletion gilding . | https://en.wikipedia.org/wiki/Selective_leaching |
Selective organ targeting ( SORT ) is a novel approach in the field of targeted drug delivery that systematically engineers multiple classes of lipid nanoparticles (LNPs) to enable targeted delivery of therapeutics to specific organs in the body. The SORT molecule alters tissue tropism by adjusting the composition and physical characteristics of the nanoparticle. Adding a permanently cationic lipid, a permanently anionic lipid, or ionizable amino lipid increases delivery to the lung , spleen , and liver , respectively. [ 1 ] [ 2 ] [ 3 ] SORT LNPs utilize SORT molecules to accurately tune and mediate gene delivery and editing, resulting in predictable and manageable protein synthesis from mRNA in particular organ(s), [ 1 ] which can potentially improve the efficacy of drugs while reducing side effects.
LNPs are non-viral synthetic nanoparticles that can carry and deliver different functional molecules to specific tissues . [ 4 ] Traditionally, LNPs are composed of four indispensable lipid components: an ionizable amino lipid that aids in both escaping the endosomes and binding nucleic acids to the particle, an amphipathic phospholipid that promotes fusion with the target cell and endosomes, cholesterol to enhance nanoparticle stability, and a polyethylene glycol lipid that improves colloidal stability and reduces clearance of the particle by the reticuloendothelial system. [ 1 ] [ 2 ]
LNPs have demonstrated safety and effectiveness but are limited to intramuscular and intravenous administration targeting the liver. [ 1 ] [ 5 ] This limitation largely stems from LNPs' resemblance to very-low-density lipoprotein , leading to a propensity for adsorbing apolipoprotein E (ApoE) present in blood plasma . Consequently, LNPs accumulate in the liver by binding to the low-density lipoprotein receptor found in hepatocytes . [ 6 ] SORT LNPs overcome this limitation by augmenting the LNP with an additional component (termed a SORT molecule), allowing delivery to targeted tissues beyond the liver. [ 1 ]
Traditionally, LNPs utilize an optimal balance of ionizable amines and nanoparticle-stabilizing hydrophobicity to deliver functional molecules to cells effectively but are limited to liver hepatocytes. [ 7 ] In the SORT strategy, these nanoparticles are systematically engineered without altering the molar ratio of the core four components in LNPs, ensuring that the ability to encapsulate RNA and escape from endosomes remains intact. [ 2 ] The addition of a SORT molecule alters the biodistribution and redirects the molecules facilitating the uptake in specific organs via endogenous targeting mechanisms of action or by influencing the binding affinity to specific serum proteins. [ 6 ] [ 3 ] [ 8 ]
Tissue tropism is determined by the distinct chemical functional groups present on the surface of the nanoparticle, which alter the physicochemical properties of the LNP. These properties encompass factors such as molarity , percentage added, and various other characteristics. The critical factor that governs tissue tropism is the modulation of the surface's acid dissociation constant (pKa), which corresponds to the pH at which the proportion of charged and uncharged ionizable lipids at the particle's surface is equal and depends on the type of ionizable lipids and charged helper lipids used in the nanoparticle formulation. [ 6 ]
The shift from liver tissues is attributed to the alteration in the surface pKa induced by the addition of an anionic head group, which subsequently reduced the strength of interactions with ApoE. [ 3 ] [ 9 ] Change in surface pKa promotes the adsorption of plasma proteins such as β2- glycoprotein I (β2-GPI) instead of ApoE, resulting in altered protein corona that mediates tissue-specific delivery towards the spleen and lung. [ 6 ] [ 3 ] Adding a cationic quaternary amino lipid, such as 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP), in an increasing molar percentage, was able to shift the distribution progressively from the liver to spleen and then the lung, with a threshold that allowed for exclusive lung delivery. [ 1 ] [ 2 ] Negatively charged SORT lipids allow for direct delivery to the spleen. [ 1 ] [ 2 ]
To prepare self-assembled SORT LNPs, the lipids are mixed in ethanol to create a dissolved lipid mixture solution, ensuring that the initial relative molar ratios of the four fundamental components remain unaltered. [ 2 ] [ 6 ] mRNA is dissolved in citrate buffer separately. To encourage that uniform LNPs are formed, it is necessary to rapidly mix both solutions: the lipid solution containing all lipids and the buffer solution containing mRNA. By employing high-speed mixing, the environmental polarity is enhanced, facilitating the formation of homogenous LNPs. [ 2 ] Mixing methods include pipetting, vortex or microfluidic mixing. [ 2 ] After mixing, characterize SORT LNPs to measure the particle size and encapsulation efficiency and proceed to delivery into the organism. Delivery can be intrathecal , intravenous , intramuscular or through nebulization . [ 10 ]
SORT LNPs can mediate therapeutically relevant protein production levels and safely deliver proteins to specific tissues and even particular cell populations. [ 1 ] The tissue specificity occurs quickly and is not dependent on time. [ 1 ] Further benefits of SORT LNPs include formulation stability and conservation of physiochemical properties over time, including a maintained in vivo efficacy after storage at 4 degrees Celsius. [ 1 ] LNPs, in general, are well tolerated in mice and humans, and no alterations in kidney and liver function or alteration of serum proteins have been found in studies with murine models evaluating in vivo toxicity. [ 1 ] [ 3 ]
SORT has the potential to revolutionize drug delivery by improving the efficacy and pharmacokinetics of drugs while reducing side effects. SORT molecules can reach deep tissues that were previously inaccessible for treatment, enhancing tissue penetration. This holds significant promise in benefiting a wide range of genetic disorders , enabling advancements in protein replacement therapy and gene editing, as this strategy allows for gene editing without local administration. [ 1 ] [ 11 ]
The benefits of targeted delivery of protein products or gene editing machinery to the liver are shown in genetic diseases affecting the liver or in which the altered gene product is produced in the liver, [ 4 ] [ 8 ] [ 12 ] such as tyrosinemia , [ 13 ] and transthyretin amyloidosis , [ 14 ] respectively, and the addition of a SORT molecule has been shown further improve liver-targeting LNP systems further. [ 1 ]
However, the SORT strategy could potentially extrapolate these benefits to other organs. One promising target for gene editing is cystic fibrosis , as a tailored therapy with an effective delivery system could significantly rescue CFTR expression. [ 6 ] [ 8 ] Other possible applications include restoration of gene expression in other organs, such as restoring dystrophin expression in muscle for Duchenne muscular dystrophy . [ 10 ] Targeted approaches for bone marrow and brain tropism are currently in development [ 15 ] [ 16 ] [ 17 ]
One of the most promising applications of SORT is cancer treatment. By targeting the cancerous cells in a specific organ, SORT may be able to deliver drugs or gene therapies directly to the cancerous cells while sparing the healthy cells in other organs. Selectivity for the spleen could also be applicable in treating cancer via chimeric antigen receptor (CAR)-T cell therapy and opens a new path for developing in vivo T-cell targeted mRNA delivery systems able to induce robust and transient CAR expression. [ 2 ]
There are promising applications in the combination of SORT and different delivery methods besides intravenous administration, such as nebulization, intrathecal or intramuscular administration, as these will deliver deliver the SORT molecules directed to targeted organs and further reduce systemic exposure. [ 2 ]
Additionally, SORT technology is applicable to several classes of established four-component LNPs, and various non-lipid nanoparticle components. This broadens the spectrum of its applications and enables the delivery of diverse therapeutics, encompassing not only nucleic acids but also single or multiple proteins, and even entire genome editors. [ 13 ]
At present, the SORT strategy is capable of achieving targeted delivery exclusively to specific organs such as the liver, lungs, and spleen. [ 10 ] [ 13 ] Establishing the SORT LNP formulation is a fine-tuning process, as some concentrations of SORT molecules may aid in delivery to other organs, whereas different concentrations completely select delivery to another organ. [ 2 ] [ 13 ] However, this fine-tuning mechanism is limited as it can also alter the molecule's activity and render it ineffective. [ 2 ] Moreover, it is difficult to accurately predict the biodistribution of LNPs based on their physicochemical parameters, and biodistribution alone cannot predict mRNA-induced protein expression in a specific tissue. [ 6 ] [ 9 ] [ 11 ] There is no indication that a massive accumulation of LNPs in a given tissue will necessarily lead to a high degree of protein expression in the targeted cells [ 6 ] | https://en.wikipedia.org/wiki/Selective_organ_targeting |
Selectivity , also known as circuit breaker discrimination , is the coordination of overcurrent protection devices so that a fault in the installation is cleared by the protection device located immediately upstream of the fault. The purpose of selectivity is to minimize the impact of a failure on the network. Faults in an installation are, for example, overload and short circuit . [ 1 ] [ 2 ]
There are four ways in which selectivity is achieved: [ 3 ] | https://en.wikipedia.org/wiki/Selectivity_(circuit_breakers) |
Selegiline , also known as L -deprenyl and sold under the brand names Eldepryl , Zelapar , and Emsam among others, is a medication which is used in the treatment of Parkinson's disease and major depressive disorder . [ 4 ] [ 6 ] [ 8 ] [ 3 ] It has also been studied and used off-label for a variety of other indications, but has not been formally approved for any other use. [ 23 ] [ 24 ] The medication, in the form licensed for depression, has modest effectiveness for this condition that is similar to that of other antidepressants . [ 24 ] [ 25 ] [ 26 ] Selegiline is provided as a swallowed tablet or capsule [ 4 ] [ 5 ] or an orally disintegrating tablet (ODT) [ 6 ] [ 7 ] for Parkinson's disease and as a patch applied to skin for depression. [ 8 ] [ 9 ]
Side effects of selegiline occurring more often than with placebo include insomnia , dry mouth , dizziness , anxiety , abnormal dreams , and application site reactions (with the patch form), among others. [ 24 ] [ 25 ] [ 27 ] [ 4 ] [ 8 ] At high doses, selegiline has the potential for dangerous food and drug interactions , such as tyramine -related hypertensive crisis (the so-called "cheese reaction") and risk of serotonin syndrome . [ 9 ] [ 28 ] [ 5 ] However, doses within the approved clinical range appear to have little to no risk of these interactions. [ 9 ] [ 28 ] [ 5 ] In addition, the ODT and transdermal patch forms of selegiline have reduced risks of such interactions compared to the conventional oral form. [ 7 ] [ 9 ] Selegiline has no known misuse potential or dependence liability and is not a controlled substance except in Japan . [ 29 ] [ 30 ] [ 31 ] [ 32 ] [ 8 ] [ 33 ]
Selegiline acts as a monoamine oxidase inhibitor (MAOI) and thereby increases levels of monoamine neurotransmitters in the brain . [ 17 ] [ 11 ] [ 28 ] [ 5 ] At typical clinical doses used for Parkinson's disease, selegiline is a selective and irreversible inhibitor of monoamine oxidase B (MAO-B), increasing brain levels of dopamine . [ 17 ] [ 11 ] [ 28 ] [ 5 ] At higher doses, it loses its specificity for MAO-B and also inhibits monoamine oxidase A (MAO-A), which increases serotonin and norepinephrine levels in the brain as well. [ 17 ] [ 11 ] [ 28 ] [ 5 ] In addition to its MAOI activity, selegiline is a catecholaminergic activity enhancer (CAE) and enhances the impulse -mediated release of norepinephrine and dopamine in the brain. [ 34 ] [ 35 ] [ 36 ] [ 37 ] [ 28 ] This action may be mediated by TAAR1 agonism . [ 38 ] [ 39 ] [ 40 ] After administration, selegiline partially metabolizes into levomethamphetamine and levoamphetamine , which act as norepinephrine releasing agents (NRAs) and may contribute to its therapeutic and adverse effects as well. [ 41 ] [ 31 ] [ 42 ] The levels of these metabolites are much lower with the ODT and transdermal patch forms of selegiline. [ 7 ] [ 9 ] Chemically, selegiline is a substituted phenethylamine and amphetamine , [ 43 ] a derivative of methamphetamine , [ 43 ] and the purified levorotatory enantiomer of deprenyl (the racemic mixture of selegiline and D -deprenyl ). [ 44 ] [ 23 ]
Deprenyl was discovered and studied as an antidepressant in the early 1960s by Zoltan Ecseri, József Knoll , and other colleagues at Chinoin Pharmaceutical Company in Hungary . [ 44 ] [ 23 ] Subsequently, selegiline was purified from deprenyl and was studied and developed itself. [ 44 ] Selegiline was first introduced for medical use, to treat Parkinson's disease, in Hungary in 1977. [ 45 ] It was subsequently approved in the United Kingdom in 1982 and in the United States in 1989. [ 45 ] [ 46 ] The ODT was approved for Parkinson's disease in the United States in 2006 and in the European Union in 2010, while the patch was introduced for depression in the United States in 2006. [ 45 ] [ 23 ] Selegiline was the first selective MAO-B inhibitor to be discovered and marketed. [ 13 ] [ 47 ] [ 48 ] In addition to its medical use, there has been interest in selegiline as a potential anti-aging drug and nootropic . [ 49 ] [ 50 ] [ 51 ] However, effects of this sort are controversial and uncertain. [ 49 ] [ 52 ] [ 53 ] [ 54 ] Generic versions of selegiline are available in the case of the conventional oral form, but not in the case of the ODT or transdermal patch forms. [ 55 ] [ 56 ]
In its oral and ODT forms, selegiline is used to treat symptoms of Parkinson's disease (PD). [ 4 ] [ 6 ] It is most often used as an adjunct to medications such as levodopa ( L -DOPA), although it has been used off-label as a monotherapy . [ 57 ] [ 58 ] The rationale for adding selegiline to levodopa is to decrease the required dose of levodopa and thus reduce the motor complications of levodopa therapy . [ 59 ] Selegiline delays the point when levodopa treatment becomes necessary from about 11 months to about 18 months after diagnosis . [ 60 ] There is some evidence that selegiline acts as a neuroprotective and reduces the rate of disease progression , though this is disputed. [ 58 ] [ 59 ] In addition to parkinsonism , selegiline can improve symptoms of depression in people with Parkinson's disease. [ 61 ] [ 62 ] There is evidence that selegiline may be more effective than rasagiline in the treatment of Parkinson's disease. [ 23 ] [ 38 ] [ 63 ] This may be due to pharmacological differences between the drugs, such as the catecholaminergic activity enhancer (CAE) actions of selegiline which rasagiline lacks. [ 23 ] [ 38 ] [ 63 ] [ 35 ]
Selegiline is used as an antidepressant in the treatment of major depressive disorder (MDD). [ 8 ] [ 24 ] Both the oral selegiline and transdermal selegiline patch formulations are used in the treatment of depression. [ 24 ] However, oral selegiline is not approved for depression and is used off-label for this indication, while the transdermal patch is specifically licensed for treatment of depression. [ 4 ] [ 8 ] Both standard clinical doses of oral selegiline (up to 10 mg/day) and higher doses of oral selegiline (e.g., 30 to 60 mg/day) have been used to treat depression, with the lower doses selectively inhibiting MAO-B and the higher doses producing dual inhibition of both MAO-A and MAO-B. [ 9 ] [ 24 ] Unlike oral selegiline, transdermal selegiline bypasses first-pass metabolism , thereby avoiding inhibition of gastrointestinal and hepatic MAO-A and minimizing the risk of food and drug interactions , whilst still allowing for selegiline to reach the brain and inhibit MAO-B. [ 9 ]
A 2023 systematic review and meta-analysis evaluated the effectiveness and safety of selegiline in the treatment of psychiatric disorders including depression. [ 24 ] It included both randomized and non-randomized published clinical studies. [ 24 ] The meta-analysis found that selegiline was more effective than placebo in terms of reduction in depressive symptoms ( SMD Tooltip standardized mean difference = −0.96, k = 10, n = 1,308), response rates for depression improvement ( RR Tooltip risk ratio = 1.61, k = 9, n = 1,238), and response rates for improvement of depression with atypical features ( RR = 2.23, k = 3, n = 136). [ 24 ] Oral selegiline was significantly more effective than the selegiline patch in terms of depressive symptom improvement ( SMD = −1.49, k = 6, n = 282 vs. SMD = −0.27, k = 4, n = 1,026, respectively; p = 0.03). [ 24 ] However, this was largely due to older and less methodologically rigorous trials that were at high risk for bias . [ 24 ] Oral selegiline studies also often employed much higher doses than usual, for instance 20 to 60 mg/day. [ 24 ] The quality of evidence of selegiline for depression was rated as very low overall, very low for oral selegiline, and low to moderate for transdermal selegiline. [ 24 ] For comparison, meta-analyses of other antidepressants for depression have found a mean effect size of about 0.3 (a small effect), [ 26 ] [ 64 ] which is similar to that with transdermal selegiline. [ 24 ]
In two pivotal regulatory clinical trials of 6 to 8 weeks duration, the selegiline transdermal patch decreased scores on depression rating scales (specifically the 17- and 28-item HDRS Tooltip Hamilton Depression Rating Scale ) by 9.0 to 10.9 points, whereas placebo decreased scores by 6.5 to 8.6 points, giving placebo-subtracted differences attributable to selegiline of 2.4 to 2.5 points. [ 8 ] A 2013 quantitative review of the transdermal selegiline patch for depression, which pooled the results of these two trials, found that the placebo-subtracted number needed to treat (NNT) was 11 in terms of depression response (>50% reduction in symptoms) and 9 in terms of remission of depression (score of ≤10 on the MADRS Tooltip Montgomery–Åsberg Depression Rating Scale ). [ 25 ] For comparison, other antidepressants, including fluoxetine , paroxetine , duloxetine , vilazodone , adjunctive aripiprazole , olanzapine/fluoxetine , and extended-release quetiapine , have NNTs ranging from 6 to 8 in terms of depression response and 7 to 14 in terms of depression remission. [ 25 ] On the basis of these results, it was concluded that transdermal selegiline has similar effectiveness to other antidepressants. [ 25 ] [ 65 ] NNTs are measures of effect size and indicate how many individuals would need to be treated in order to encounter one additional outcome of interest. [ 25 ] Lower NNTs are better, and NNTs corresponding to Cohen's d effect sizes have been defined as 2.3 for a large effect (d = 0.8), 3.6 for a medium effect (d = 0.5), and 8.9 for a small effect (d = 0.2). [ 25 ] The effectiveness of transdermal selegiline for depression relative to side effects and discontinuation was considered to be favorable. [ 25 ]
While several large regulatory clinical trials of transdermal selegiline versus placebo for depression have been conducted, there is a lack of trials comparing selegiline to other antidepressants. [ 56 ] [ 65 ] Although multiple doses of transdermal selegiline were assessed, a dose–response relationship for depression was never established. [ 56 ] [ 65 ] Transdermal selegiline has shown similar clinical effectiveness in the treatment of atypical depression relative to typical depression and in the treatment of anxious depression relative to non-anxious depression. [ 56 ] [ 66 ] [ 65 ]
Transdermal selegiline does not cause sexual dysfunction and may improve certain domains of sexual function , for instance sexual interest , maintaining interest during sex, and sexual satisfaction . [ 67 ] These benefits were apparent in women but not in men. [ 67 ] The lack of sexual dysfunction with transdermal selegiline is in contrast to many other antidepressants, such as the selective serotonin reuptake inhibitors (SSRIs) and serotonin–norepinephrine reuptake inhibitors (SNRIs), which are associated with high rates of sexual dysfunction. [ 68 ]
Transdermal selegiline patches have been underutilized in the treatment of depression compared to other antidepressants. [ 56 ] [ 65 ] A variety of factors contributing to this underutilization have been identified. [ 56 ] One major factor is the very high cost of transdermal selegiline, which is often not covered by insurance and frequently proves to be prohibitive. [ 56 ] [ 65 ] Conversely, other widely available antidepressants are much cheaper in comparison. [ 56 ] [ 65 ]
Selegiline is available in the following three pharmaceutical forms : [ 55 ]
The transdermal patch form is also known as the "selegiline transdermal system" or "STS" and is applied once daily. [ 9 ] [ 12 ] [ 27 ] [ 65 ] [ 8 ] They are 20, 30, or 40 cm 2 in size and contain a total of 20, 30, or 40 mg selegiline per patch (so 20 mg/20 cm 2 , 30 mg/30 cm 2 , and 40 mg/40 cm 2 ), respectively. [ 8 ] [ 65 ] The selegiline transdermal patch is a matrix-type adhesive patch with a three-layer structure. [ 8 ] [ 65 ] It is the only approved non-oral MAOI, having reduced dietary restrictions and side effects in comparison to oral MAOIs, and is also the only approved non-oral first-line antidepressant. [ 65 ] The selegiline patch can be useful for those who have difficulty tolerating oral medications. [ 65 ]
Selegiline is contraindicated with serotonergic antidepressants including selective serotonin reuptake inhibitors (SSRIs), serotonin–norepinephrine reuptake inhibitors (SNRIs), and tricyclic antidepressants (TCAs), with serotonergic opioids like meperidine , tramadol , and methadone , with other monoamine oxidase inhibitors (MAOIs) such as linezolid , phenelzine , and tranylcypromine , and with dextromethorphan , St. John's wort , cyclobenzaprine , pentazocine , propoxyphene , and carbamazepine . [ 6 ] [ 8 ] [ 4 ] Combination of selegiline with serotonergic agents may cause serotonin syndrome , while combination of selegiline with adrenergic or sympathomimetic agents like ephedrine or amphetamines may cause hypertensive crisis . [ 6 ] [ 8 ] Long washout periods are required before starting and stopping these medications with discontinuation or initiation of selegiline. [ 6 ] [ 8 ] [ 4 ] [ 65 ]
Consumption of tyramine -rich foods can result in hypertensive crisis with selegiline, also known as the "cheese effect" or "cheese reaction" due to the high amounts of tyramine present in some cheeses. [ 6 ] [ 11 ] [ 47 ] [ 15 ] Examples of other foods that may have high amounts of tyramine and similar substances include yeast products, chicken liver, snails, pickled herring, red wines, some beers, canned figs, broad beans, chocolate, and cream products. [ 15 ]
The preceding drug and food contraindications are dependent on selegiline dose and route, and hence are not necessarily absolute contraindications. [ 4 ] [ 6 ] [ 5 ] [ 7 ] [ 9 ] While high oral doses of selegiline (≥20 mg/day) can cause such interactions, oral doses within the approved clinical range (≤10 mg/day) appear to have little to no risk of these interactions. [ 9 ] [ 28 ] [ 5 ] In addition, the ODT and transdermal forms of selegiline have reduced risks of such interactions compared to the conventional oral form. [ 7 ] [ 9 ]
Selegiline is also contraindicated in children less than 12 years of age and in people with pheochromocytoma , both due to heightened risk of hypertensive crisis. [ 8 ] For all human uses and all forms, selegiline is pregnancy category C, meaning that studies in pregnant animals have shown adverse effects on the fetus but there are no adequate studies in humans. [ 4 ] [ 8 ]
Side effects of the tablet form in conjunction with levodopa include, in decreasing order of frequency, nausea , hallucinations , confusion , depression , loss of balance , insomnia , increased involuntary movements , agitation , slow or irregular heart rate , delusions , hypertension , new or increased angina pectoris , and syncope . [ 4 ] Most of the side effects are due to a high dopamine levels, and can be alleviated by reducing the dose of levodopa. [ 3 ] Selegiline can also cause cardiovascular side effects such as orthostatic hypotension , hypertension , atrial fibrillation , and other types of cardiac arrhythmias . [ 69 ]
The main side effects of the patch form for depression include application-site reactions , insomnia , dry mouth , dizziness , nervousness , and abnormal dreams . [ 8 ] [ 27 ] The selegiline patch carries a black box warning about a possible increased risk of suicide , especially for young people, [ 8 ] as do all antidepressants since 2007. [ 70 ]
Side effects of selegiline that have been identified as occurring significantly more often than with placebo in meta-analyses for psychiatric disorders have included dry mouth ( RR Tooltip Risk ratio = 1.58), insomnia ( RR = 1.61, NNH Tooltip Number needed to harm = 19), and application site reactions with the transdermal form ( RR = 1.81, NNH = 7). [ 24 ] [ 25 ] No significant diarrhea , headache , dizziness , nausea , sexual dysfunction , or weight gain were apparent in these meta-analyses. [ 24 ] [ 25 ]
Selegiline, including in its oral, ODT, and patch forms, has been found to cause hypotension or orthostatic hypotension in some individuals. [ 4 ] [ 6 ] [ 8 ] In a clinical trial, the rate of systolic orthostatic hypotension was 21% versus 9% with placebo and the rate of diastolic orthostatic hypotension was 12% versus 4% with placebo in people with Parkinson's disease taking the ODT form of selegiline. [ 6 ] The risk of hypotension is greater at the start of treatment and in the elderly (3% vs. 0% with placebo). [ 6 ] The rate of hypotension or orthostatic hypotension with the selegiline patch was 2.2% versus 0.5% with placebo in clinical trials of people with depression. [ 27 ] Significant orthostatic blood pressure changes (≥10 mm Hg decrease) occurred in 9.8% versus 6.7% with placebo, but most of these cases were asymptomatic and heart rate was unchanged. [ 27 ] [ 71 ] The rates of other orthostatic hypotension-related side effects in this population were dizziness or vertigo 4.9% versus 3.1% with placebo and fainting 0.5% versus 0.0% with placebo. [ 27 ] It is said that orthostatic hypotension is rarely seen with the selegiline transdermal patch compared to oral MAOIs. [ 56 ] Caution is advised against rapidly rising after sitting or lying, especially after prolonged periods or at the start of treatment, as this can result in fainting. [ 6 ] [ 30 ] [ 71 ] Falls are of particular concern in the elderly. [ 71 ] MAOIs like selegiline may lower blood pressure by increasing dopamine levels and activating dopamine receptors , by increasing levels of the false neurotransmitter octopamine , and/or by other mechanisms. [ 72 ]
Meta-analyses published in the 1990s found that the addition of selegiline to levodopa increased mortality in people with Parkinson's disease. [ 30 ] However, several subsequent meta-analyses with more trials and patients found no increase in mortality with selegiline added to levodopa. [ 30 ] [ 73 ] [ 74 ] If selegiline does increase mortality, it has been theorized that this may be due to cardiovascular side effects, such as its amphetamine-related sympathomimetic effects and its MAO inhibition-related hypotension. [ 75 ] Although selegiline does not seem to increase mortality, it appears to worsen cognition in people with Parkinson's disease over time. [ 76 ] Conversely, rasagiline does not seem to do so and can enhance cognition. [ 76 ]
Rarely, selegiline has been reported to induce or exacerbate impulse control disorders , pathological gambling , hypersexuality , and paraphilias in people with Parkinson's disease. [ 77 ] [ 78 ] [ 79 ] [ 80 ] [ 81 ] [ 82 ] [ 83 ] [ 84 ] However, MAO-B inhibitors like selegiline causing impulse control disorders is uncommon, controversial, and less frequent than with dopamine receptor agonists like pramipexole . [ 77 ] [ 78 ] Impulse control disorders with dopaminergic agents have been linked specifically to activation of dopamine D 3 receptors in the globus pallidus . [ 85 ] [ 86 ] [ 87 ] [ 88 ] Selegiline has also been reported to activate or worsen rapid eye movement (REM) sleep behavior disorder (RBD) in some people with Parkinson's disease. [ 89 ] [ 90 ] [ 91 ]
Selegiline has shown little or no misuse potential in humans or monkeys. [ 29 ] [ 30 ] [ 31 ] [ 92 ] [ 93 ] [ 94 ] Likewise, it has no dependence potential in rodents. [ 32 ] This is in spite of its amphetamine active metabolites , levomethamphetamine and levoamphetamine , and is in contrast to agents like dextroamphetamine and dextromethamphetamine . [ 30 ] [ 31 ] [ 32 ] [ 93 ] [ 94 ] However, selegiline can strongly potentiate the reinforcing effects of exogenous β-phenethylamine by inhibiting its MAO-B -mediated metabolism . [ 31 ] Misuse of the combination of selegiline and β-phenethylamine has been reported. [ 95 ] [ 96 ]
Little information is available about clinically significant selegiline overdose . [ 4 ] The drug has been studied clinically at doses as high as 60 mg/day orally, [ 97 ] [ 24 ] 10 mg/day as an ODT, [ 7 ] and 12 mg/24 hours as a transdermal patch. [ 9 ] In addition, deprenyl (the racemic form) has been clinically studied orally at doses as large as 100 mg/day. [ 17 ] During clinical development of oral selegiline, some individuals who were exposed to doses of 600 mg developed severe hypotension and psychomotor agitation . [ 4 ] [ 6 ] Overdose may result in non-selective inhibition of both MAO-A and MAO-B and may be similar to overdose of other non-selective monoamine oxidase inhibitors (MAOIs) like phenelzine , isocarboxazid , and tranylcypromine . [ 4 ] [ 6 ] Serotonin syndrome , hypertensive crisis , and/or death may occur with overdose. [ 4 ] [ 6 ] [ 8 ] No specific antidote to selegiline overdose is available. [ 8 ]
Both the oral and patch forms of selegiline come with strong warnings against combining it with drugs that could produce serotonin syndrome , such as selective serotonin reuptake inhibitors (SSRIs) and the cough medicine dextromethorphan . [ 4 ] [ 8 ] [ 98 ] Selegiline in combination with the opioid analgesic pethidine is not recommended, as it can lead to severe adverse effects . [ 98 ] Several other synthetic opioids such as tramadol and methadone , as well as various triptans , are also contraindicated due to potential for serotonin syndrome. [ 99 ] [ 100 ]
All three forms of selegiline carry warnings about food restrictions to avoid hypertensive crisis that are associated with MAOIs. [ 4 ] [ 6 ] [ 8 ] The patch form was created in part to overcome food restrictions; clinical trials showed that it was successful. [ 25 ] [ 8 ] Additionally, in post-marketing surveillance from April 2006 to October 2010, only 13 self-reports of possible hypertensive events or hypertension were made out of 29,141 exposures to the drug, and none were accompanied by objective clinical data. [ 25 ] The lowest dose of the patch method of delivery, 6 mg/24 hours, does not require any dietary restrictions . [ 101 ] Higher doses of the patch and oral formulations, whether in combination with the older non-selective MAOIs or in combination with the reversible MAO-A inhibitor (RIMA) moclobemide , require a low-tyramine diet. [ 98 ]
A study found that selegiline in transdermal patch form did not importantly modify the pharmacodynamic effects or pharmacokinetics of the sympathomimetic agents pseudoephedrine and phenylpropanolamine . [ 9 ] [ 102 ] Likewise, oral selegiline at an MAO-B-selective dosage did not appear to modify the pharmacodynamic effects or pharmacokinetics of intravenous methamphetamine in another study. [ 103 ] [ 104 ] Conversely, selegiline, also at MAO-B-selective doses, has been found to reduce the physiological and euphoric subjective effects of cocaine whilst not affecting its pharmacokinetics in some studies but not in others. [ 105 ] [ 106 ] [ 107 ] [ 108 ] [ 109 ] [ 110 ] Cautious safe combination of MAOIs like selegiline with stimulants like lisdexamfetamine has been reported. [ 111 ] [ 112 ] [ 113 ] However, a hypertensive crisis with selegiline and ephedrine has also been reported. [ 4 ] The selegiline drug labels warn about combination of selegiline with indirectly acting sympathomimetic agents, like amphetamines, ephedrine, pseudoephedrine, and phenylpropanolamine, due to the potential risk of hypertensive crisis, and recommend monitoring blood pressure with such combinations. [ 6 ] [ 8 ] The combination of selegiline with certain other medications, like phenylephrine and buspirone , is also warned against for similar reasons. [ 8 ] [ 12 ] [ 114 ] [ 71 ] In the case of phenylephrine, this drug is substantially metabolized by monoamine oxidase, including by both MAO-A and MAO-B . [ 115 ] [ 116 ] Selegiline can interact with exogenous dopamine , which is metabolized by MAO-A and MAO-B, and result in hypertensive crisis as well. [ 117 ] [ 118 ]
Besides norepinephrine releasing agents , selective norepinephrine reuptake inhibitors (NRIs) may be safe in combination with MAOIs like selegiline. [ 119 ] [ 120 ] [ 121 ] Potent NRIs, such as reboxetine , desipramine , protriptyline , and nortriptyline , can reduce or block the pressor effects of tyramine , including in those taking MAOIs. [ 119 ] [ 120 ] [ 121 ] This is by inhibiting the norepinephrine transporter (NET) and preventing entry of tyramine into presynaptic noradrenergic neurons where tyramine induces the release of norepinephrine. [ 119 ] [ 120 ] [ 121 ] As a result, NRIs may reduce the risk of tyramine-related hypertensive crisis in people taking MAOIs. [ 119 ] [ 120 ] [ 121 ] Norepinephrine–dopamine reuptake inhibitors (NDRIs), like methylphenidate and bupropion , are also considered to be safe in combination with MAOIs. [ 122 ] However, initiation at low doses and slow upward dose titration is advisable in the case of both NRIs and NDRIs due to possible potentiation of their effects and side effects by MAOIs. [ 122 ]
Selegiline may potentiate the effects of serotonergic psychedelics that are MAO-B substrates, such as 2C drugs like 2C-B , 2C-I , and 2C-E . [ 123 ] [ 124 ] [ 125 ]
The cytochrome P450 enzymes involved in the metabolism of selegiline have not been fully elucidated. [ 5 ] [ 21 ] CYP2D6 and CYP2C19 metabolizer phenotypes did not significantly affect the pharmacokinetics of selegiline, suggesting that these enzymes are minimally involved in its metabolism and that inhibitors and inducers of these enzymes would not importantly affect its pharmacokinetics. [ 21 ] [ 43 ] [ 126 ] [ 127 ] However, although most pharmacokinetic variables were unaffected, overall exposure to selegiline's metabolite levomethamphetamine was 46% higher in CYP2D6 poor metabolizers compared to extensive metabolizers and exposure to its metabolite desmethylselegiline was 68% higher in CYP2C19 poor metabolizers compared to extensive metabolizers. [ 43 ] [ 126 ] [ 127 ] As with the cases of CYP2D6 and CYP2C19, the strong CYP3A4 and CYP3A5 inhibitor itraconazole has minimal impact on the pharmacokinetics of selegiline, suggesting lack of major involvement of this enzyme as well. [ 21 ] [ 128 ] [ 6 ] On the other hand, the anticonvulsant carbamazepine , which is known to act as a strong inducer of CYP3A enzymes, [ 129 ] has paradoxically been found to increase exposure to selegiline and its metabolites levomethamphetamine and levoamphetamine by approximately 2-fold (with selegiline used as the transdermal patch form). [ 8 ] [ 9 ] One enzyme thought to be majorly involved in the metabolism of selegiline based on in-vitro studies is CYP2B6 . [ 5 ] [ 21 ] [ 9 ] [ 22 ] However, there are no clinical studies of different CYP2B6 metabolizer phenotypes or of CYP2B6 inhibitors or inducers on the pharmacokinetics of selegiline. [ 47 ] In addition to CYP2B6, CYP2A6 may be involved in the metabolism of selegiline to a lesser extent. [ 47 ] [ 130 ]
Birth control pills containing the synthetic estrogen ethinylestradiol and a progestin like gestodene or levonorgestrel have been found to increase peak levels and overall exposure to oral selegiline by 10- to 20-fold. [ 21 ] [ 131 ] [ 132 ] High levels of selegiline can lead to loss of MAO-B selectivity and inhibition of MAO-A as well. [ 21 ] [ 132 ] This increases susceptibility to side effects and interactions of non-selective monoamine oxidase inhibitors (MAOIs), such as tyramine -induced hypertensive crisis and serotonin toxicity when combined with serotonergic medications. [ 21 ] [ 132 ] However, this study had a small sample size of four individuals as well as other methodological limitations. [ 21 ] [ 132 ] The precise mechanism underlying the interaction is unknown, but is likely related to cytochrome P450 inhibition and consequent inhibition of selegiline first-pass metabolism by ethinylestradiol. [ 21 ] In contrast to birth control pills containing ethinylestradiol, menopausal hormone therapy with estradiol and levonorgestrel did not modify peak levels of selegiline and only modestly increased overall exposure (+59%). [ 21 ] [ 131 ] [ 133 ] Hence, menopausal hormone therapy does not pose the same risk of interaction as ethinylestradiol-containing birth control pills when taken together with selegiline. [ 131 ] [ 133 ]
Overall exposure to selegiline with oral selegiline has been found to be 23-fold lower in people taking anticonvulsants known to strongly activate drug-metabolizing enzymes . [ 134 ] The anticonvulsants included phenobarbital , phenytoin , carbamazepine , and amobarbital . [ 134 ] In a previous study however, carbamazepine specifically did not reduce selegiline exposure. [ 8 ] [ 9 ] Phenobarbital and certain other anticonvulsants are known to strongly induce CYP2B6, one of the major enzymes believed to be involved in selegiline metabolism. [ 134 ] As such, it was concluded that strong CYP2B6 induction was most likely responsible for the dramatically reduced exposure to selegiline observed in the study. [ 134 ]
Selegiline has been reported to inhibit several cytochrome P450 enzymes, including CYP2D6, CYP3A4/5, CYP2C19, CYP2B6, and CYP2A6. [ 8 ] [ 135 ] It is a mechanism-based inhibitor (suicide inhibitor) of CYP2B6 and has been said to "potently" or "strongly" inhibit this enzyme in vitro . [ 136 ] [ 135 ] [ 137 ] [ 138 ] It may inhibit the metabolism of bupropion , a major CYP2B6 substrate , into its active metabolite hydroxybupropion . [ 136 ] [ 135 ] [ 137 ] However, a study predicted that inhibition of CYP2B6 by selegiline would non-significantly affect exposure to bupropion. [ 138 ] Selegiline has not been listed or described as a clinically significant CYP2B6 inhibitor by the Food and Drug Administration (FDA) as of 2023. [ 129 ] [ 8 ] One small study observing three patients found that selegiline was safe and well-tolerated in combination with bupropion. [ 137 ] [ 139 ] In addition to CYP2B6 and other cytochrome P450 enzymes, selegiline is a potent mechanism-based inhibitor of CYP2A6 and may increase exposure to nicotine (a major CYP2A6 substrate). [ 140 ] [ 141 ] By inhibiting cytochrome P450 enzymes like CYP2B6 and CYP1A2, selegiline may inhibit its own metabolism and thereby interact with itself. [ 141 ] [ 142 ]
Dopamine antagonists like antipsychotics or metoclopramide , which block dopamine receptors and thereby antagonize the dopaminergic effects of selegiline, could potentially reduce the effectiveness of the medication. [ 6 ] Dopamine-depleting agents like reserpine and tetrabenazine , by reducing dopamine levels, can also oppose the effectiveness of dopaminergic medications like selegiline. [ 143 ]
Selegiline has multiple known mechanisms of action in terms of its pharmacodynamic activity. [ 17 ] [ 11 ] [ 28 ] [ 5 ] It is most notably an irreversible monoamine oxidase (MAO) inhibitor (MAOI). [ 17 ] [ 11 ] [ 28 ] [ 5 ] More specifically, it is a selective inhibitor of monoamine oxidase B (MAO-B) at lower doses (≤10 mg/day) but additionally inhibits monoamine oxidase A (MAO-A) at higher doses (≥20 mg/day). [ 17 ] [ 11 ] [ 28 ] [ 5 ] MAO-B inhibition is thought to result in increased levels of dopamine and β-phenethylamine , whereas MAO-A inhibition results in increased levels of serotonin , norepinephrine , and dopamine. [ 17 ] [ 11 ] [ 28 ] [ 5 ] Selegiline is also a catecholaminergic activity enhancer (CAE) and enhances the action potential -evoked release of norepinephrine and dopamine. [ 34 ] [ 35 ] [ 36 ] [ 37 ] [ 28 ] The CAE activity of selegiline may be mediated by TAAR1 agonism . [ 38 ] [ 39 ] [ 40 ] Both the MAOI activity and CAE activity of selegiline may be involved in its therapeutic effects in the treatment of Parkinson's disease and depression . [ 35 ] [ 23 ] [ 38 ] [ 63 ] [ 144 ] [ 37 ] According to József Knoll and other researchers, selegiline might have dopaminergic neuroprotective effects, might be able to modestly slow the rate of aging -related dopaminergic neurodegeneration , and might thereby have a disease-modifying effect in Parkinson's disease and antiaging effects generally. [ 23 ] [ 145 ] [ 35 ] [ 146 ] [ 147 ] [ 148 ] However, these theoretical effects of selegiline have not been clearly demonstrated in humans as of present and remain to be substantiated. [ 54 ] [ 23 ] [ 149 ] [ 150 ] [ 151 ] Through its active metabolites levomethamphetamine ( L -MA) and levoamphetamine ( L -A), selegiline acts as a weak norepinephrine and/or dopamine releasing agent (NDRA). [ 41 ] [ 31 ] [ 42 ] The clinical significance of this action is unclear, but it may be relevant to the effects and side effects of selegiline, especially at higher doses. [ 41 ] [ 31 ] [ 42 ] Its active metabolite desmethylselegiline (DMS) also has MAOI and CAE activity and likely contributes to its effects as well. [ 41 ] [ 17 ] [ 152 ] [ 153 ] [ 154 ] Levels of selegiline's metabolites are much lower with the ODT and transdermal patch forms of selegiline than with the oral form and this may result in differences in its effects and side effects. [ 7 ] [ 9 ]
Selegiline is available in forms for use by multiple different routes of administration and its pharmacokinetics vary by route. [ 55 ] [ 7 ] [ 9 ] [ 12 ] The bioavailability of the oral form of selegiline is 4 to 10%, [ 5 ] [ 11 ] [ 12 ] of the ODT is 5 to 8 times that of the oral form, [ 13 ] [ 7 ] [ 14 ] and of the transdermal patch is 75%. [ 9 ] The time to peak levels of selegiline with oral administration is about 0.5 to 1.5 hours. [ 5 ] The plasma protein binding of selegiline is 85 to 90%. [ 9 ] [ 8 ] [ 6 ] It is metabolized extensively in the liver by the cytochrome P450 enzyme CYP2B6 among other enzymes . [ 5 ] [ 21 ] [ 9 ] [ 22 ] Metabolites of selegiline include desmethylselegiline (DMS), levomethamphetamine ( L -MA), and levoamphetamine ( L -A). [ 17 ] [ 28 ] [ 5 ] [ 155 ] The oral form of selegiline is subject to strong first-pass metabolism and levels of the metabolites of selegiline are much lower with the ODT and transdermal patch forms than with the oral form. [ 7 ] [ 9 ] The elimination half-lives of selegiline and its metabolites range from 1.2 to 10 hours for selegiline, 2.2 to 9.5 hours for DMS, 14 to 21 hours for levomethamphetamine, and 16 to 18 hours for levoamphetamine. [ 5 ] [ 12 ] [ 7 ] [ 6 ] [ 8 ] Selegiline and its metabolites are eliminated mainly in urine (87% in urine and 15% in feces via oral administration), with its metabolites accounting for virtually all of the eliminated material in the case of the oral form. [ 18 ] [ 19 ] [ 7 ] [ 5 ] [ 20 ] Hepatic impairment and renal impairment have been found to dramatically increase exposure to selegiline. [ 156 ] [ 157 ] [ 134 ] [ 4 ] [ 6 ]
Selegiline is a substituted phenethylamine and amphetamine derivative . [ 43 ] It is also known as ( R )-(–)- N ,α-dimethyl- N -(2-propynyl)phenethylamine, ( R )-(–)- N -methyl- N -2-propynylamphetamine, or N -propargyl- L -methamphetamine. [ 158 ] [ 159 ] [ 160 ] [ 7 ] Selegiline ( L -deprenyl) is the enantiopure levorotatory enantiomer of the racemic mixture deprenyl , whereas D -deprenyl is the dextrorotatory enantiomer. [ 44 ] [ 23 ] Selegiline is a derivative of levomethamphetamine ( L -methamphetamine), the levorotatory enantiomer of the psychostimulant and sympathomimetic agent methamphetamine ( N -methylamphetamine), with a propargyl group attached to the nitrogen atom of the molecule . [ 65 ]
Selegiline is a small-molecule compound , with the molecular formula C 13 H 17 N and a low molecular weight of 187.281 g/mol. [ 158 ] [ 159 ] [ 160 ] [ 4 ] [ 65 ] It has high lipophilicity , with an experimental log P of 2.7 and predicted log P values of 2.9 to 3.1. [ 158 ] [ 159 ] [ 160 ] [ 65 ] Pharmaceutically, selegiline is used almost always as the hydrochloride salt , though the free base form has also been used. [ 4 ] [ 161 ] At room temperature, selegiline hydrochloride is a white to near white crystalline powder . [ 4 ] Selegiline hydrochloride is freely soluble in water , chloroform , and methanol . [ 4 ]
Selegiline is a close analogue of methamphetamine and amphetamine , and in fact produces their levorotatory forms, levomethamphetamine and levoamphetamine , as metabolites . [ 41 ] [ 31 ] Selegiline is structurally similar to the antihypertensive agent pargyline ( N -methyl- N -propargylbenzylamine), an earlier non-selective MAOI of the phenylalkylamine group. [ 162 ] [ 36 ] Besides selegiline and pargyline, another clinically used MAOI of the phenylalkylamine and amphetamine families is the antidepressant tranylcypromine ( trans -2-phenylcyclopropylamine). [ 47 ] Tranylcypromine can be conceptualized as a cyclized amphetamine and has amphetamine-like actions at high doses similarly to selegiline. [ 47 ] [ 163 ] [ 164 ] Another notable analogue of selegiline is 4-fluoroselegiline , a variation of selegiline in which one of the hydrogen atoms of the phenyl ring has been replaced with a fluorine atom. [ 165 ] A large number of other analogues of selegiline derived via structural modification have been synthesized and characterized. [ 166 ] [ 165 ] [ 167 ] [ 168 ]
Rasagiline (( R )- N -propargyl-1-aminoindan) is an analogue of selegiline in which the amphetamine base structure has been replaced with a 1-aminoindan structure and the N -methyl group has been removed. [ 41 ] Like selegiline, it is also a selective MAO-B inhibitor and used to treat Parkinson's disease. [ 41 ] In contrast to selegiline however, rasagiline lacks the amphetamine metabolites and activity of selegiline. [ 41 ] A further derivative of rasagiline, ladostigil ([ N -propargyl-(3 R )-aminoindan-5-yl]- N -propylcarbamate), a dual MAO-B inhibitor and acetylcholinesterase inhibitor , was developed for treatment of Alzheimer's disease and other conditions but was ultimately never introduced for medical use. [ 169 ]
Selegiline can be synthesized by the alkylation of levomethamphetamine using propargyl bromide . [ 47 ] [ 170 ] [ 171 ] [ 172 ] [ 173 ]
Following the discovery in 1952 that the tuberculosis drug iproniazid elevated the mood of people taking it, and the subsequent discovery that the effect was likely due to inhibition of monoamine oxidase (MAO) and elevation of monoamine neurotransmitters in the brain , many people and companies started trying to discover monoamine oxidase inhibitors (MAOIs) to use as antidepressants . [ 11 ] [ 174 ] Deprenyl , the racemic form of selegiline, was synthesized and discovered by Zoltan Ecseri at the Chinoin Pharmaceutical Company (part of Sanofi since 1993) in Budapest, Hungary . [ 11 ] [ 175 ] Chinoin received a patent on the drug in 1962 and the compound was first published in the scientific literature in English in 1965. [ 11 ] [ 176 ] Chinoin researchers had been studying substituted amphetamines since 1960, and decided to try synthesizing amphetamines that acted as MAOIs. [ 15 ] It had been known that methamphetamine was a reversible inhibitor of MAO. [ 15 ] Deprenyl, also known as N -propargyl- N -methylamphetamine, [ 36 ] is closely related to and inspired by pargyline ( N -propargyl- N -methylbenzylamine), another MAOI that had been synthesized earlier. [ 11 ] [ 15 ] [ 177 ] Deprenyl was initially referred to by the chemical name phenylisopropylmethylpropinylamine and the developmental code name E-250 . [ 11 ] [ 15 ] [ 176 ] Work on the biology and effects of E-250 in animals and humans was conducted by a group led by József Knoll at Semmelweis University , which was also in Budapest. [ 11 ]
Deprenyl is a racemic compound (a mixture of two isomers called enantiomers ). [ 11 ] [ 15 ] The racemic form has mild amphetamine-like psychostimulant effects that are diminished compared to those of amphetamine but are still present. [ 15 ] The levorotatory enantiomer has further reduced stimulant effects, and further work, published in 1967, determined that the levorotatory enantiomer was a more potent MAOI than the dextrorotatory enantiomer. [ 11 ] [ 15 ] [ 178 ] [ 179 ] As a result, subsequent work was done with the single enantiomer L -deprenyl. [ 11 ] [ 15 ] [ 178 ] [ 179 ] In 1968, it was discovered by J. P. Johnston that monoamine oxidase exists in multiple forms. [ 11 ] [ 15 ] [ 180 ] In 1971, Knoll showed that selegiline highly selectively inhibits the B-isoform of monoamine oxidase (MAO-B) and proposed that it is unlikely to cause the infamous "cheese effect" ( hypertensive crisis resulting from consuming foods containing tyramine ) that occurs with non-selective MAOIs. [ 11 ] [ 15 ] [ 181 ] The lack of potentiation of tyramine effect by deprenyl had previously been reported in 1966 and 1968 studies, but could not be mechanistically explained until after the existence of multiple forms of MAO was discovered. [ 11 ] [ 15 ] [ 182 ] Selegiline was the first selective MAO-B inhibitor to be discovered [ 13 ] and hence is described as prototypical of these agents. [ 47 ] [ 48 ]
Deprenyl and selegiline were initially studied as antidepressants for treatment of depression . [ 50 ] [ 176 ] Deprenyl was first found to be effective for depression from 1965 to 1967, [ 50 ] [ 183 ] [ 184 ] while selegiline was first found to be effective for depression in 1971 and this was further corroborated in 1980. [ 50 ] [ 185 ] [ 186 ] A 1984 study that combined selegiline with phenylalanine reported remarkably high effectiveness in the treatment of depression similar to that with electroconvulsive therapy (ECT). [ 50 ] [ 187 ] However, selegiline in its original oral form was never further developed or approved for the treatment of depression. [ 50 ]
A few years after the discovery that selegiline was a selective MAO-B inhibitor, two Parkinson's disease researchers based in Vienna, Peter Riederer and Walther Birkmayer, realized that selegiline could be useful in Parkinson's disease. One of their colleagues, Moussa B. H. Youdim , visited Knoll in Budapest and took selegiline from him to Vienna. In 1975, Birkmayer's group published the first paper on the effect of selegiline in Parkinson's disease. [ 178 ] [ 188 ]
Speculation, by József Knoll, that selegiline could be useful as an anti-aging and pro-sexual agent , began in the 1980s. [ 15 ] [ 189 ] [ 190 ] [ 191 ] [ 192 ] The New York Times reported that selegiline was being used non-medically as a "smart drug" by 1992. [ 193 ]
Selegiline was first introduced for clinical use in Hungary in 1977. [ 45 ] It was approved in the oral pill form under the brand name Jumex to treat Parkinson's disease. [ 45 ] The drug was then introduced in the United Kingdom in 1982. [ 45 ] In 1987, Somerset Pharmaceuticals in New Jersey, which had acquired the rights to develop selegiline in the United States , filed a New Drug Application (NDA) with the Food and Drug Administration (FDA) to market the drug for Parkinson's disease in this country. [ 46 ] While the NDA was under review, Somerset was acquired in a joint venture by two generic drug companies, Mylan and Bolan Pharmaceuticals. [ 46 ] Selegiline was approved for Parkinson's disease by the FDA in 1989. [ 46 ]
It had been known since the mid-1960s that high doses of deprenyl had psychostimulant effects. [ 17 ] [ 11 ] [ 176 ] [ 184 ] Selegiline was first shown to metabolize into levomethamphetamine and levoamphetamine in humans in 1978. [ 31 ] [ 194 ] The involvement of these metabolites in the effects and side effects of selegiline has remained controversial and unresolved in the decades afterwards. [ 31 ] [ 41 ] In any case, concerns about these metabolites have contributed to the development of newer MAO-B inhibitors like rasagiline and safinamide that lack such metabolites. [ 41 ] [ 195 ]
The catecholaminergic activity enhancer (CAE) effects of selegiline became well-characterized and distinctly named in 1994. [ 196 ] [ 35 ] [ 28 ] [ 153 ] [ 23 ] [ 37 ] [ 197 ] [ 198 ] [ 199 ] These effects had been observed much earlier, dating back to the 1960s and 1970s, but were not properly distinguished from the other actions of selegiline, like MAO-B inhibition, until the 1990s. [ 35 ] [ 28 ] [ 37 ] [ 196 ] More potent, selective, and/or expansive monoaminergic activity enhancers (MAEs), like phenylpropylaminopentane (PPAP) and benzofuranylpropylaminopentane (BPAP), were derived from selegiline and other compounds and were first described in 1988 and 1999, respectively. [ 36 ] [ 39 ] [ 200 ] [ 50 ] [ 201 ] These drugs had been proposed for potential treatment of psychiatric disorders like depression as well as for Parkinson's disease and Alzheimer's disease , but were never developed or marketed. [ 144 ] [ 37 ] [ 39 ] [ 145 ] [ 50 ]
In the 1990s, J. Alexander Bodkin at McLean Hospital , an affiliate of Harvard Medical School , began a collaboration with Somerset to develop delivery of selegiline via a transdermal patch in order to avoid the well known dietary restrictions of MAOIs . [ 192 ] [ 202 ] [ 203 ] Somerset obtained FDA approval to market the patch for depression in 2006. [ 204 ] Similarly, the orally disintegrating tablet (ODT) form of selegiline, marketed under the brand name Zelapar, was approved for Parkinson's disease in the United States in 2006 and in the European Union in 2010. [ 45 ]
Binding to and agonism of the trace amine-associated receptors (TAARs) as the mechanism responsible for the MAE effects of selegiline and related MAEs like PPAP and BPAP was first suggested in the early 2000s following the discovery of the TAARs. [ 39 ] [ 145 ] [ 40 ] Activation of the TAAR1 as the mechanism of the MAE effects was first clearly substantiated in 2022. [ 205 ] [ 38 ]
Selegiline is the generic name of the drug and its INN Tooltip International Nonproprietary Name , BAN Tooltip British Approved Name , and DCF Tooltip Dénomination Commune Française , while selegiline hydrochloride is the USAN Tooltip United States Adopted Name . [ 206 ] [ 207 ] [ 161 ] The word "selegiline" is pronounced / s ə ˈ l ɛ dʒ ɪ l iː n / ( sə- LEJ -i-leen ) or as "seh-LEH-ji-leen". [ 1 ] [ 2 ] Selegiline is also known as L -deprenyl, L -deprenil, L -deprenalin, L -deprenaline, L -phenylisopropylmethylpropinylamine, and L -E-250. [ 23 ] [ 206 ] [ 207 ] [ 161 ] [ 176 ] It should not be confused with the racemic form, deprenyl (E-250), or with the dextrorotatory enantiomer , D -deprenyl , which are distinct substances. [ 206 ] [ 44 ] [ 23 ]
Major brand names of selegiline include Eldepryl, Jumex, and Movergan (oral tablet and/or capsule), Zelapar (orally disintegrating tablet or ODT), and Emsam (transdermal patch). [ 3 ] [ 161 ] [ 155 ] Selegiline has been marketed under more than 70 brand names worldwide. [ 208 ] [ 3 ] The brand name "Emsam" was derived from the names of two children, Emily and Samuel, of one of the executives at Somerset Pharmaceuticals, the developer of Emsam. [ 65 ] [ 209 ]
Generic forms of oral selegiline are available in the United States . [ 55 ] However, generic forms of the orally disintegrating tablet and the transdermal patch are not available in this country. [ 55 ] [ 56 ] The latter formulations of selegiline are very expensive, and this can be prohibitive to their use. [ 56 ] [ 210 ] There has been poor insurance coverage of the transdermal patch form for depression, with insurance companies often requiring patients to first fail to respond to one or two other antidepressants and to be responsible for larger copayments. [ 56 ] It is expected that generics of the transdermal patch will become available at some point in the future. [ 56 ]
Conventional oral selegiline (brand names Eldepryl, Jumex) is widely marketed throughout the world, including in over 70 countries. [ 3 ] [ 161 ] [ 23 ] [ 208 ] Conversely, the selegiline transdermal patch (brand name Emsam) is only marketed in the United States , while the selegiline orally disintegrating tablet (brand name Zelapar) is marketed in the United States, the United Kingdom , and the European Union . [ 3 ] [ 45 ] [ 23 ]
József Knoll , one of the developers of selegiline, began taking a low 1 mg daily dose of selegiline on January 1, 1989, at the age of 64. [ 147 ] : 92 [ 178 ] He reported in 2012 that this had continued for 22 years uninterrupted. [ 147 ] : 92 Knoll stated that he had become so fascinated with the possible longevity-promoting effects of selegiline that he had decided to start taking it as a self-experiment. [ 147 ] : 92 [ 178 ] Knoll later died in 2018 at the age of 93. [ 211 ]
David Pearce , a British transhumanist philosopher , wrote his self-published book-length internet manifesto The Hedonistic Imperative [ 212 ] six weeks after starting to take selegiline. [ 213 ]
Sam Bankman-Fried , the founder and former CEO of the FTX cryptocurrency exchange , is known to have used selegiline for depression in the form of the Emsam patch for at least 5 to 10 years. [ 214 ] [ 215 ] He is also known to have simultaneously taken Adderall for treatment of attention deficit hyperactivity disorder (ADHD) [ 214 ] [ 215 ] and to have possessed non-pharmaceutical adrafinil , a prodrug of modafinil . [ 216 ]
In Gregg Hurwitz 's novel Out of the Dark , selegiline ( Emsam ) and tyramine -containing food were used to assassinate the president of the United States. [ 217 ]
Selegiline in non-pharmaceutical form is sold on the Internet without a prescription by online vendors for uses such as purported cognitive enhancement (i.e., as a so-called "smart drug" or nootropic ) and anti-aging effects. [ 218 ] [ 153 ] [ 219 ] It is widely available for such purposes, for instance under informal brand names like Dep-Pro, Selepryl, and Cyprenil, which are oral liquid solutions of selegiline at a concentration of 1 mg per drop. [ 153 ] [ 219 ] [ 147 ] : 86
In his 1993 book E for Ecstasy examining the uses of the street drug ecstasy in the United Kingdom , the writer, activist, and ecstasy advocate Nicholas Saunders highlighted test results showing that certain consignments of the drug also contained selegiline. [ 220 ] Consignments of ecstasy known as "Strawberry" contained what Saunders described as a "potentially dangerous combination of ketamine , ephedrine and selegiline," as did a consignment of "Sitting Duck" Ecstasy tablets. [ 221 ]
Selegline is on the World Anti-Doping Agency (WADA)'s list of prohibited substances . [ 222 ] It is classified as a " stimulant " in this list, along with various amphetamines , methylphenidate , adrenergic sympathomimetics , modafinil , and other agents. [ 222 ] A review of the pharmacology of WADA prohibited substances noted that although selegiline is classified as a stimulant in the WADA prohibited substances list and stimulants can enhance physical performance , selegiline was seemingly included in the list not because of any short-term stimulant effects of its own, but rather because it metabolizes into small amounts of levomethamphetamine and levoamphetamine and can produce false positives for amphetamines on drug tests . [ 222 ] In any case, levomethamphetamine and levoamphetamine are catecholamine releasing agents and can produce sympathomimetic and psychostimulant effects with sufficiently high exposure. [ 223 ] [ 224 ] [ 225 ] Such actions may have performance-enhancing effects. [ 222 ]
Selegiline is a prescription drug . [ 4 ] [ 8 ] [ 6 ] It is not specifically a controlled substance in the United States and hence is not an illegal drug . [ 8 ] However, deprenyl and selegiline are controlled substances in Japan . [ 226 ] [ 33 ] They are classified as " Stimulants ", alongside a variety of other amphetamines , under Article 2 of Japan's Narcotics and Psychotropics Control Law . [ 33 ] Selegiline is known to metabolize into small amounts of levoamphetamine and levomethamphetamine but is thought to have little to no misuse potential or dependence liability . [ 29 ] [ 30 ] [ 31 ] [ 32 ] [ 92 ] [ 8 ]
József Knoll and his team are credited with having developed selegiline. Although selegiline's development as a potential treatment for Parkinson's disease , Alzheimer's disease , and depression was headed by other teams, Knoll remained at the forefront of research into the potential longevity enhancing effects of selegiline up until his death in 2018. [ 211 ] [ 227 ] [ 228 ] Knoll published his 2012 book How Selegiline ((–)-Deprenyl) Slows Brain Aging wherein he claims that: [ 147 ] : 90
"In humans, maintenance from sexual maturity on (–)-deprenyl (1mg daily) is, for the time being, the most promising prophylactic treatment to fight against the age related decay of behavioral performances, prolonging life, and preventing or delaying the onset of age-related neurodegenerative diseases such as Parkinson's and Alzheimer's".
The mechanism of selegiline's longevity-promoting effect has been researched by several groups, including Knoll and his associates at Semmelweis University, Budapest. [ 23 ] The drug has been determined to be a catecholaminergic activity enhancer when present in minuscule concentrations far below those at which monoamine oxidase inhibitory activity can be observed, thereby potentiating the release of catecholamine neurotransmitters in response to stimuli. Knoll maintains that micro-doses of selegiline act as a synthetic analogue to a known or unknown trace amine in order to preserve the brain catecholaminergic system, which he perceives as integral to the organism's ability to function in an adaptive, goal-directed and motivated manner during advancing physical age: [ 147 ] : 70, 43
"[...] enhancer regulation in the catecholaminergic brain stem neurons play[s] a key role in controlling the uphill period of life and the transition from adolescence to adulthood. The results of our longevity studies support the hypothesis that quality and duration of life rests upon the inborn efficiency of the catecholaminergic brain machinery, i.e. a high performing, long-living individual has a more active, more slowly deteriorating catecholaminergic system than its low performing, shorter living peer. Thus, a better brain engine allows for a better performance and a longer lifespan."
"Since the catecholaminergic and serotonergic neurons in the brain stem are of key importance in ensuring that the mammalian organism works as a purposeful, motivated, goal-directed entity, it is hard to overestimate the significance of finding safe and efficient means to slow the decay of these systems with passing time. The conclusion that the maintenance on (–)-deprenyl that keeps the catecholaminergic neurons on a higher activity level is a safe and efficient anti-aging therapy follows from the discovery of the enhancer regulation in the catecholaminergic neurons of the brain stem. From the finding that this regulation starts working on a high activity level after weaning and the enhanced activity subsists during the uphill period of life, until sexual hormones dampen the enhancer regulation in the catecholaminergic and serotonergic neurons in the brain stem, and this event signifies the transition from developmental longevity into postdevelopmental longevity, the downhill period of life."
Despite findings by Knoll that selegiline can prolong lifespan in rodents by 35% however, other studies have had conflicting findings and have even found increased mortality with selegiline in rodents. [ 54 ] In humans with Parkinson's disease, selegiline has been associated with cardiovascular and psychiatric complications and has not been found to reduce mortality in long-term studies. [ 54 ] As such, the claimed anti-aging and longevity benefits of selegiline have yet to be substantiated in humans and are controversial and uncertain. [ 54 ] [ 53 ]
Selegiline is considered by some to be a nootropic , otherwise known as a cognitive enhancer or "smart drug", both at clinical and sub-clinical dosages, and has been used off-label and non-medically to improve cognitive performance . [ 49 ] [ 229 ] It is one of the most popular such agents. [ 49 ] Selegiline has been found to have neuroprotective activity against certain neurotoxins and to increase the production of several brain growth factors , such as nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and glial cell line-derived neurotrophic factor (GDNF). [ 23 ] The drug has also been found in animal models to improve learning ability and to help preserve it during ischemia and aging . [ 230 ] [ 231 ] [ 232 ] [ 233 ] Despite claims that selegiline and other claimed nootropics have cogintive-enhancing effects however, these effects are controversial and their benefits versus risks are uncertain. [ 49 ]
Selegiline has been clinically studied in combination with oral L -phenylalanine or β-phenethylamine in the treatment of depression and was reported to be effective. [ 39 ] [ 234 ] [ 187 ] [ 235 ] [ 236 ] L -Phenylalanine is known to be metabolized into β-phenethylamine, selegiline is known to strongly inhibit the metabolism of β-phenethylamine, and β-phenethylamine has been implicated in having psychostimulant -like mood -lifting effects. [ 39 ] [ 17 ] [ 234 ]
A small clinical study found that oral selegiline (10 mg/day) reduced symptoms of social anxiety disorder . [ 12 ] [ 24 ] [ 237 ] The effectiveness was modest, with a reduction in social anxiety scores from baseline of 32% over 6 weeks of treatment. [ 12 ] [ 24 ] [ 237 ] It was seemingly less effective than certain other agents used in the treatment of social anxiety, such as the non-selective MAOI phenelzine (45% symptom reduction) and the benzodiazepine clonazepam (51% symptom reduction), though it was similar to the SSRI sertraline (32% symptom decrease). [ 237 ]
Selegiline has been limitedly studied in the treatment of attention deficit hyperactivity disorder (ADHD) in children, adolescents, and adults. [ 24 ] [ 238 ] [ 239 ] [ 240 ] In a small randomized trial of selegiline for treatment of ADHD in children, there were improvements in attention, hyperactivity, and learning/memory performance but not in impulsivity. [ 241 ] A small clinical randomized trial compared selegiline to methylphenidate, a first line treatment for ADHD, and reported equivalent efficacy as assessed by parent and teacher ratings. [ 242 ] In another small randomized controlled trial of selegiline for the treatment of adult ADHD, a high dose of the medication for 6 weeks was not significantly more effective than placebo in improving symptoms. [ 239 ] [ 243 ] [ 244 ] Selegiline in its transdermal patch form (brand name Emsam) has also been assessed in the treatment of ADHD in children and adolescents in a small open-label pilot study sponsored by the manufacturer in 2003. [ 12 ] [ 245 ] However, there was a high rate of discontinuation and development was not further pursued. [ 12 ] [ 245 ]
Selegiline has been found to produce pro-motivational effects and to reverse motivational deficits in rodents. [ 246 ] [ 247 ] [ 248 ] [ 249 ] In case reports and small clinical studies , selegiline has been reported to improve disorders of diminished motivation like apathy and abulia due to conditions such as traumatic brain injury . [ 246 ] [ 250 ] [ 251 ] [ 252 ] [ 253 ] [ 254 ] In accordance with the preceding findings, selegiline, along with other dopaminergic and activating agents , may be useful in the treatment of disorders of diminished motivation, including apathy, abulia, and akinetic mutism . [ 247 ] [ 255 ] [ 251 ]
Selegiline has been evaluated for smoking cessation both as a monotherapy and in combination with nicotine replacement therapy in five clinical studies. [ 256 ] [ 257 ] [ 24 ] However, it is limitedly or not effective for this use. [ 256 ] [ 257 ] [ 24 ] It was also evaluated for treatment of cocaine dependence in one study, but was similarly not effective. [ 258 ] Studies are mixed on whether selegiline, at MAO-B-selective doses, reduces the effects of cocaine in humans. [ 105 ] [ 106 ] [ 107 ] [ 108 ] [ 109 ] [ 110 ] Selegiline, also at an MAO-B-selective dosage, did not modify or potentiate the pharmacological effects of intravenous methamphetamine in a small clinical study. [ 103 ] [ 104 ]
Selegiline has been assessed for treatment of sexual dysfunction induced by antipsychotics in people with schizophrenia , but was not effective in a single small clinical study. [ 259 ] [ 260 ] It also did not improve sexual function in men with depression, but did improve several domains of sexual function in women with depression. [ 67 ]
Selegiline has been studied as an adjunct to antipsychotics in the treatment of schizophrenia in four clinical studies. [ 24 ] [ 261 ] However, it failed to significantly reduce positive or negative symptoms of schizophrenia in meta-analyses of these studies. [ 24 ] [ 261 ]
Selegiline has been evaluated for the treatment of narcolepsy in three small clinical studies. [ 262 ] [ 263 ] [ 264 ] It was found to be effective in these studies. [ 262 ] [ 263 ] A dosage of 10 mg/day had no effect on symptoms, but 20 to 30 mg/day improved alertness , mood , and somewhat reduced cataplexy , clinical effects that have been described as comparable to the same dosages of amphetamine . [ 263 ] Animal research indicates that the beneficial effects of high doses of selegiline in narcolepsy are likely due to conversion into its active metabolites , levoamphetamine and levomethamphetamine . [ 263 ] [ 264 ] Selegiline has also been evaluated for treatment of hypersomnia (excessive sleeping or sleepiness) in people with myotonic dystrophy , but its effectiveness was very uncertain. [ 265 ] [ 262 ]
Selegiline has been studied in the treatment of periodic limb movement disorder (PLMD) in a single small open-label clinical study. [ 266 ] [ 267 ] [ 268 ] It was reported to be effective as assessed by polysomnography , reducing periodic limb movements during sleep by about 60%. [ 266 ] [ 268 ] Selegiline has not been studied for the related condition restless legs syndrome (RLS) as of 2023. [ 266 ] [ 267 ] The drug has not been studied well enough in PLMD or RLS to be widely used in their treatment. [ 266 ]
Selegiline was studied in the treatment of antipsychotic -induced tardive dyskinesia in one small clinical study, but was ineffective. [ 269 ]
Selegiline has also been used off-label as a palliative treatment for dementia in Alzheimer's disease . [ 58 ] However, its clinical effectiveness is limited or lacking for this use. [ 270 ] [ 271 ] [ 272 ] [ 273 ] It was also ineffective in the treatment of Lewy body dementia . [ 274 ] Selegiline has been used to support motor rehabilitation in stroke recovery , but evidence for this use is inadequate and no recommendation can be made for or against it. [ 275 ]
Selegiline has been studied in patients with disorders of consciousness , such as minimally conscious state , persistent vegetative state , and persistent coma , in a small open-label clinical study. [ 276 ] [ 277 ] It was found to be effective in enhancing arousal and promoting recovery of consciousness in some of these individuals. [ 276 ] [ 277 ]
Selegiline has been reported to protect against the damage caused by the potent dopaminergic and/or noradrenergic neurotoxins 6-hydroxydopamine (6-OHDA), N -(2-chloroethyl)- N -ethyl-2-bromobenzylamine (DSP-4), and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in animals. [ 278 ] [ 17 ] [ 279 ] [ 280 ] [ 281 ] [ 282 ] Conversely, selegiline is ineffective in protecting against the serotonergic and noradrenergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT). [ 17 ] [ 283 ]
Selegiline has also been reported to protect against methylenedioxymethamphetamine (MDMA)-induced serotonergic neurotoxicity in rodents. [ 284 ] [ 285 ] [ 286 ] [ 287 ] [ 288 ] The serotonergic neurotoxicity of MDMA appears to be dependent on release of dopamine and its subsequent metabolism by MAO-B within serotonergic neurons into hydroxyl radicals , which is blocked by MAO-B inhibition. [ 284 ] [ 285 ] Likewise, selegiline prevented the serotonergic neurotoxicity of a combination of methylenedioxyaminoindane (MDAI) and dextroamphetamine . [ 289 ] [ 290 ]
Conversely, selegiline failed to reduce the serotonergic neurotoxicity caused by fenfluramine and either did not affect or potentiated the serotonergic neurotoxicity caused by para -chloroamphetamine (PCA). [ 280 ] [ 291 ] [ 292 ] [ 293 ] In addition, findings are mixed and conflicting on whether selegiline prevents amphetamine - and methamphetamine -induced dopaminergic neurotoxicity in rodents. [ 294 ] [ 295 ] [ 296 ] [ 297 ]
Although MAO-B-selective doses of selegiline protect against MDMA-induced serotonergic neurotoxicity in rodents, combination of amphetamines like MDMA with MAOIs, including selegiline, can produce serious complications, including serotonin syndrome , hypertensive crisis , and death. [ 298 ] [ 299 ]
The original oral formulation of selegiline was developed for the treatment of depression . [ 50 ] However, it ended up being developed and approved for the treatment of Parkinson's disease instead. [ 50 ] [ 45 ] [ 4 ] In any case, oral selegiline has been widely used off-label to treat depression. [ 24 ] The transdermal patch form of selegiline was developed and approved specifically for the treatment of depression. [ 300 ] [ 12 ] [ 9 ] [ 8 ] It was also under development for the treatment of Alzheimer's disease , attention deficit hyperactivity disorder (ADHD), cognition disorders , and Parkinson's disease, but development for these indications was discontinued. [ 300 ] The ODT form of selegiline was developed and licensed exclusively for the treatment of Parkinson's disease. [ 301 ] [ 7 ] [ 6 ]
In veterinary medicine , selegiline is sold under the brand name Anipryl and is manufactured by Zoetis . [ 302 ] It is available in the form of 2, 5, 10, 15, and 30 mg oral tablets for use in animals. [ 302 ] Selegiline is used in dogs to treat canine cognitive dysfunction (CCD) and, at higher doses, to treat pituitary-dependent hyperadrenocorticism (PDH). [ 303 ] [ 304 ]
CCD is a form of dementia that mimics Alzheimer's disease in humans. [ 305 ] Geriatric dogs treated with selegiline show improvements in sleeping pattern , reduced urinary incontinence , and increased activity level , with most showing improvements by one month of treatment. [ 306 ] [ 307 ] Though it is labeled for use in dogs only, selegiline has been used off-label for geriatric cats with cognitive dysfunction . [ 308 ]
PDH is a hormonal disorder and is analogous to pituitary -dependent Cushing's syndrome in humans. [ 302 ] Selegiline's effectiveness in treating PDH has been disputed. [ 303 ] Theoretically, it works by increasing dopamine levels, which downregulates the secretion of adrenocorticotropic hormone (ACTH) from the brain , eventually leading to reduced levels of cortisol . [ 308 ] Some claim that selegiline is only effective at treating PDH caused by lesions in the anterior pituitary (which comprise most canine cases). [ 309 ] The greatest sign of improvement is lessening of PDH-related abdominal distention . [ 306 ]
Side effects in dogs are uncommon, but they include vomiting , diarrhea , diminished hearing , salivation , decreased weight , and behavioral changes such as hyperactivity , listlessness , disorientation , and repetitive motions . [ 304 ] [ 309 ]
Selegiline has been limitedly studied in large animals like horses and its dosage in these animals has not been established. [ 309 ] In preliminary research, a dose of selegiline of 30 mg orally or intravenously in horses had no observable effects on behavior or locomotor activity. [ 309 ]
The doses of selegiline used in animals are described as extremely high relative to those used in humans (which are ~0.1 mg/kg body weight). [ 155 ] | https://en.wikipedia.org/wiki/Selegiline |
A selenenic acid is an organoselenium compound and an oxoacid with the general formula RSeOH, where R ≠ H. It is the first member of the family of organoselenium oxoacids, which also include seleninic acids and selenonic acids , which are RSeO 2 H and RSeO 3 H, respectively. Selenenic acids derived from selenoenzymes are thought to be responsible for the antioxidant activity of these enzymes. This functional group is sometimes called SeO-selenoperoxol.
In contrast to selenonic and seleninic acids, selenenic acids are unstable with respect to a self- condensation reaction to form the corresponding selenoseleninates [ 2 ] or disproportionation into corresponding seleninic acids and diselenides :
Even the very bulky 2,4,6-tri- tert -butylbenzeneselenenic acid disproportionates readily. [ 3 ] A stable selenenic acid was synthesized by burying the SeOH functional group within the cavity of a p - tert -butyl[calix[6]arene macrocycle] . X-ray crystallographic analysis revealed the Se-O bond length to be 1.763 Å. The Se-O absorbs in the IR spectrum at 680–700 cm −1 . [ 4 ] In a stable selenenic acid prepared by oxidizing a highly hindered selenol, BmtSeH, the Se-O bond length was found to be 1.808 Å while the O-Se-C angle was 96.90°. Oxidation of BmtSeOH gave BmtSeO 2 H. [ 5 ]
Selenenic acids are believed to be transient intermediates in a number of redox reactions involving organoselenium compounds . One notable example is the syn -elimination of selenoxides. Selenenic acids are also transient intermediates in the reduction of seleninic acids as well as the oxidation of diselenides. The reasoning for postulating selenenic acids as reactive intermediates is based in part on analogy with their more extensively studied sulfenic acid analogs. [ 6 ]
Selenenic acids derived from selenocysteine are involved in cell signaling and certain enzymatic processes. The best known selenoenzyme, glutathione peroxidase (GPx), catalyzes the reduction of peroxides by glutathione (GSH). The selenenic acid intermediate (E-SeOH) is formed upon oxidation of the catalytically active selenol (E-SeH) by hydrogen peroxide. This selenenic acid derivative of the peroxidase then reacts with a thiol-containing cofactor (GSH) to generate the key intermediate selenenyl sulfide (E-SeSG). This intermediate is subsequently attacked by a second GSH to regenerate the selenol and the glutathione cofactor is released in its oxidized form, GSSG. The catalytic mechanism of GPx, involves selenol (R-SeH), selenenyl sulfide (R1-SeS-R2), and selenenic acid intermediates. [ 7 ]
In the absence of thiols, selenols tend to overoxidize to produce seleninic acids. Many organoselenium compounds (selenenamides, diaryl diselenides) contain "interesting" biological activities. Their activity is attributed to their mimicry of glutathione peroxidase activity. They reduce hydroperoxides that otherwise convert to toxic byproducts and/or reactive oxygen species that can cause further damage to the cell. [ 8 ] | https://en.wikipedia.org/wiki/Selenenic_acid |
Selenium-79 is a radioisotope of selenium present in spent nuclear fuel and the wastes resulting from reprocessing this fuel. It is one of only seven long-lived fission products . Its fission yield is low (about 0.04%), as it is near the lower end of the mass range for fission products . Its half-life has been variously reported as 650,000 years, 65,000 years, 1.13 million years, 480,000 years, 295,000 years, 377,000 years and most recently with best current precision, 327,000 years. [ 1 ] [ 2 ]
79 Se decays to 79 Br by emitting a beta particle with no attendant gamma radiation (i.e., 100% β decay). This complicates its detection and liquid scintillation counting (LSC) is required for measuring it in environmental samples. The low specific activity ( 5.1 × 10 8 Bq/g ) and relatively low energy (151 keV) of its beta particles have been said to limit the radioactive hazards of this isotope. [ 3 ]
Performance assessment calculations for the Belgian deep geological repository estimated 79 Se may be the major contributor to activity release in terms of becquerels (decays per second), "attributable partly to the uncertainties about its migration behaviour in the Boom Clay and partly to its conversion factor in the biosphere ." (p. 169). [ 4 ] However, "calculations for the Belgian safety assessments use a half-life of 65 000 years" (p. 177), much less than the currently estimated half-life, and "the migration parameters ... have been estimated very cautiously for 79 Se." (p. 179)
Neutron absorption cross sections for 79 Se have been estimated at 50 barns for thermal neutrons and 60.9 barns for resonance integral . [ 5 ]
Selenium-80 and selenium-82 have higher fission yields , about 20 times the yield of 79 Se in the case of uranium-235 , 6 times in the case of plutonium-239 or uranium-233 , and 14 times in the case of plutonium-241 . [ 6 ]
Due to redox-disequilibrium , selenium could be very reluctant to abiotic chemical reduction and would be released from the waste (spent fuel or vitrified waste) as selenate ( SeO 2– 4 ), a soluble Se(VI) species, not sorbed onto clay minerals . Without solubility limit and retardation for aqueous selenium, the dose of 79 Se is comparable to that of 129 I. Moreover, selenium is an essential micronutrient as it is present in the catalytic centers in the glutathione peroxidase , an enzyme needed by many organisms for the protection of their cell membrane against oxidative stress damages; therefore, radioactive 79 Se can be easily bioconcentrated in the food web . In the presence of nitrate ( NO – 3 ) released in deep geological clay formations by bituminized waste issued from the spent fuel dissolution step during their reprocessing, even reduced forms of selenium could be easily oxidised and mobilised. [ 7 ] | https://en.wikipedia.org/wiki/Selenium-79 |
Selenium hexasulfide is a chemical compound with the chemical formula Se 2 S 6 . Its molecular structure is an 8-membered ring, consisting of two selenium and six sulfur atoms ( diselenacyclooctasulfane ), analogous to the S 8 ring, an allotrope of sulfur ( cyclooctasulfur or cyclooctasulfane), and other 8-membered rings of selenium sulfides with formula Se n S 8− n . [ 2 ]
There are several isomers depending on the relative placement of the selenium atoms in the ring: 1,2-diselenacyclooctasulfane (with the two Se atoms adjacent), 1,3-diselenacyclooctasulfane , 1,4-diselenacyclooctasulfane , and 1,5-diselenacyclooctasulfane (with the Se atoms opposite). [ 3 ] It is an oxidizing agent.
The 1,2 isomer can be prepared by reaction of chlorosulfanes and dichlorodiselane with potassium iodide in carbon disulfide . The reaction produces also cyclooctaselenium Se 8 and all other eight-member cyclic selenium sulfides, except selenacyclooctasulfane SeS 7 , and several six- and seven-membered rings. [ 2 ]
This inorganic compound –related article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Selenium_hexasulfide |
Selenium is an essential micronutrient for animals, though it is toxic in large doses. In plants, it sometimes occurs in toxic amounts as forage , e.g. locoweed . Selenium is a component of the amino acids selenocysteine and selenomethionine . In humans, selenium is a trace element nutrient that functions as cofactor for glutathione peroxidases and certain forms of thioredoxin reductase . [ 1 ] Selenium-containing proteins are produced from inorganic selenium via the intermediacy of selenophosphate (PSeO 3 3− ).
Selenium is an essential micronutrient in mammals, but is also recognized as toxic in excess. Selenium exerts its biological functions through selenoproteins , which contain the amino acid selenocysteine . Twenty-five selenoproteins are encoded in the human genome . [ 2 ]
The glutathione peroxidase family of enzymes (abbreviated GSH-Px) catalyze reduction of hydrogen peroxide and organic hydroperoxides :
The two H atoms are donated by thiols in a process that begins with oxidation of a selenol side chain in GSH-Px. The organoselenium compound ebselen is a drug used to supplement the action of GSH-Px. It functions as a catalyst for the destruction of hydrogen peroxide. [ 3 ]
A related selenium-containing enzyme in some plants and in animals ( thioredoxin reductase ) generates reduced thioredoxin, a dithiol that serves as an electron source for peroxidases and also the important reducing enzyme ribonucleotide reductase that makes DNA precursors from RNA precursors. [ 4 ]
Selenium also plays a role in the functioning of the thyroid gland. It participates as a cofactor for the three thyroid hormone deiodinases . These enzymes activate and then deactivate various thyroid hormones and their metabolites. [ 5 ] It may inhibit Hashimoto's disease , an auto-immune disease in which the body's own thyroid cells are attacked by the immune system. A reduction of 21% on TPO antibodies was reported with the dietary intake of 0.2 mg of selenium. [ 6 ]
Some microorganisms utilize selenium in formate dehydrogenase . Formate is produced in large amounts in the hepatic (liver cells) mitochondria of embryonic cells and in cancer cells by the folate cycle. [ 7 ]
Formate is reversibly oxidized by the enzyme formate dehydrogenase : [ 8 ]
Thioredoxin reductase uses a cysteine-selenocysteine pair to reduce the disulfide in thioredoxin . The selenocysteine is arranged in an unusual Sec-His-Glu catalytic triad , which tunes its pKa. [ 9 ]
Certain species of plants are considered indicators of high selenium content of the soil, since they require high levels of selenium to thrive. The main selenium indicator plants are Astragalus species (including some locoweeds ), prince's plume ( Stanleya sp.), woody asters ( Xylorhiza sp.), and false goldenweed ( Oonopsis sp.) [ 10 ]
The substance loosely called selenium sulfide (with the approximate formula SeS 2 ) is the active ingredient in some anti-dandruff shampoos. [ 11 ] The selenium compound kills the scalp fungus Malassezia , which causes shedding of dry skin fragments. The ingredient is also used in body lotions to treat Tinea versicolor due to infection by a different species of Malassezia fungus. [ 12 ]
Several clinical trials have assessed the use of selenium supplements in critically ill adults; however, the effectiveness and potential benefits of selenium supplementation in this context is not well understood. [ 13 ]
Selenium may be measured in blood, plasma, serum or urine to monitor excessive environmental or occupational exposure, confirm a diagnosis of poisoning in hospitalized victims or to assist in a forensic investigation in a case of fatal overdosage. Some analytical techniques are capable of distinguishing organic from inorganic forms of the element. Both organic and inorganic forms of selenium are largely converted to monosaccharide conjugates (selenosugars) in the body prior to being eliminated in the urine. Cancer patients receiving daily oral doses of selenothionine may achieve very high plasma and urine selenium concentrations. [ 14 ]
Although selenium is an essential trace element , it is toxic if taken in excess. Exceeding the Tolerable Upper Intake Level of 400 micrograms per day can lead to selenosis. [ 15 ] This 400 microgram ( μg ) Tolerable Upper Intake Level is based primarily on a 1986 study of five Chinese patients who exhibited overt signs of selenosis and a follow-up study on the same five people in 1992. [ 16 ] The 1992 study actually found the maximum safe dietary Se intake to be approximately 800 micrograms per day (15 micrograms per kilogram body weight), but suggested 400 micrograms per day to not only avoid toxicity , but also to avoid creating an imbalance of nutrients in the diet and to account for data from other countries. [ 17 ] In China, people who ingested corn grown in extremely selenium-rich stony coal (carbonaceous shale ) have suffered from selenium toxicity. This coal was shown to have selenium content as high as 9.1%, the highest concentration in coal ever recorded in literature. [ 18 ]
Symptoms of selenosis include a garlic odor on the breath, gastrointestinal disorders, hair loss, sloughing of nails, fatigue, irritability, and neurological damage. Extreme cases of selenosis can result in cirrhosis of the liver, pulmonary edema , and death. [ 19 ] Elemental selenium and most metallic selenides have relatively low toxicities because of their low bioavailability . By contrast, selenates and selenites are very toxic, having an oxidant mode of action similar to that of arsenic trioxide. The chronic toxic dose of selenite for humans is about 2400 to 3000 micrograms of selenium per day for a long time. [ 20 ] Hydrogen selenide is an extremely toxic, corrosive gas. [ 21 ] Selenium also occurs in organic compounds, such as dimethyl selenide, selenomethionine , selenocysteine and methylselenocysteine , all of which have high bioavailability and are toxic in large doses.
Selenium poisoning of water systems may result whenever new agricultural runoff courses through normally dry, undeveloped lands. This process leaches natural soluble selenium compounds (such as selenates) into the water, which may then be concentrated in new "wetlands" as the water evaporates. High selenium levels produced in this fashion have been found to have caused certain congenital disorders in wetland birds. [ 22 ]
In fish and other wildlife, low levels of selenium cause deficiency while high levels cause toxicity. For example, in salmon, the optimal concentration of selenium in the fish tissue (whole body) is about 1 microgram selenium per gram of tissue (dry weight). At levels much below that concentration, young salmon die from selenium deficiency; [ 23 ] much above that level they die from toxic excess. [ 24 ]
Selenium deficiency can occur in patients with severely compromised intestinal function, those undergoing total parenteral nutrition , and [ 25 ] in those of advanced age (over 90). Also, people dependent on food grown from selenium-deficient soil are at risk. Although New Zealand has low levels of selenium in its soil, adverse health effects have not been detected. [ 26 ]
Selenium deficiency as defined by low (<60% of normal) selenoenzyme activity levels in brain and endocrine tissues only occurs when a low selenium status is linked with an additional stress, such as high exposures to mercury [ 27 ] or as a result of increased oxidant stress due to vitamin E deficiency. [ 28 ]
Selenium interacts with other nutrients, such as iodide and vitamin E . The interaction is observed in the etiology of many deficiency diseases in animals, and pure selenium deficiency is rare. The effect of selenium deficiency on health remains uncertain, particularly in relation to Kashin-Beck disease . [ 29 ]
The US Institute of Medicine (IOM) updated Estimated Average Requirements (EARs) and Recommended Dietary Allowances (RDAs) for selenium in 2000. If there is not sufficient information to establish EARs and RDAs, an estimate designated Adequate Intake (AI) is used instead. The current EAR for selenium for people ages 14 and up is 45 μg/day. The RDA is 55 μg/day. RDAs are higher than EARs so as to identify amounts that will cover people with higher-than-average requirements. RDA for pregnancy is 60 μg/day. RDA for lactation is 70 μg/day. For children ages 1–13 years, the RDA increases with age from 20 to 40 μg/day. As for safety, the IOM sets Tolerable upper intake levels (ULs) for vitamins and minerals when evidence is sufficient. In the case of selenium, the UL is 400 μg/day. Collectively the EARs, RDAs, AIs and ULs are referred to as Dietary Reference Intakes (DRIs). [ 30 ]
The European Food Safety Authority (EFSA) refers to the collective set of information as Dietary Reference Values, with Population Reference Intake (PRI) instead of RDA, and Average Requirement instead of EAR. AI and UL are defined the same as in the United States. For women and men ages 15 and older, the AI is set at 70 μg/day. AI for pregnancy is 70 μg/day; for lactation, 85 μg/day. For children ages 1–14 years, the AIs increase with age from 15 to 55 μg/day. These AIs are higher than the U.S. RDAs. [ 31 ] The European Food Safety Authority reviewed the same safety question and set its UL at 300 μg/day, which is lower than the U.S. value. [ 32 ]
In the United States, selenium deficiency is not common. A federal survey of food consumption determined that for women and men over the age of 19, average consumption from foods and beverages was 89 and 125 μg/day, respectively. For women and men of all ages fewer than 3% consumed less than the EAR. [ 33 ]
For US food and dietary supplement labeling purposes, the amount in a serving is expressed as a percent of Daily Value (%DV). For selenium labeling purposes, 100% of the Daily Value was 70 μg, but as of 27 May 2016 it was revised to 55 μg. [ 34 ] [ 35 ] A table of the old and new adult daily values is provided at Reference Daily Intake .
Dietary selenium comes from nuts, cereals , meat , mushrooms , fish , and eggs . Brazil nuts are the richest ordinary dietary source and could cause selenium toxicity if consumed regularly – though the actual concentration of selenium (as with any plant-based food sources, such as another selenium-accumulating "paradise nut" Lecythis , belonging to the same family Lecythidaceae ) is soil-dependent and may vary significantly by geographic location. In descending order of concentration, high levels are also found in kidney , tuna , crab , and lobster . [ 36 ] [ 37 ]
The human body's content of selenium is believed to be in the 13–20 milligram range. [ 38 ]
Selenium has bimodal biological action depending on the concentration. At low nutritional doses, selenium acts as an antioxidant through selenoproteins , scavenging ROS , supporting cell survival and growth; while, at supra-nutritional higher pharmacological doses, selenium acts as a pro-oxidant generating ROS and inducing cell death . In cancer, studies have been conducted mostly on the benefits of selenium intake in reducing the risk of cancer incidence at the nutritional level; however, fewer studies have explored the effects of supra-nutritional or pharmacological doses of selenium on cancer. [ 39 ]
"Although an inverse association between selenium exposure and the risk of some types of cancer was found in some observational studies, this cannot be taken as evidence of a causal relation, and these results should be interpreted with caution... Conflicting results including inverse, null and direct associations have been reported for some cancer types... RCTs assessing the effects of selenium supplementation on cancer risk have yielded inconsistent results... To date, no convincing evidence suggests that selenium supplements can prevent cancer in humans." [ 40 ]
To date, many studies have been conducted on the benefits of selenium intake in reducing the risk of cancer incidence at the nutritional level, indicating that likely selenium functions as an immunostimulator , i.e. reversing the immunosuppression in tumour microenvironment towards antitumour immunity by activating immune cells (e.g. M1 macrophages and CD8+ T-lymphocytes , the elevated number of neutrophils and activated cytotoxic NK cells ) and releasing pro-inflammatory cytokines such as IFNγ and TNFα . [ 39 ]
AIDS appears to involve a slow and progressive decline in levels of selenium in the body. Whether this decline in selenium levels is a direct result of the replication of HIV or related more generally to the overall malabsorption of nutrients by AIDS patients remains debated. Observational studies have found an association between decreased selenium levels and poorer outcomes in patients with HIV, though these studies were mostly done prior to the currently effective treatments with highly active antiretroviral therapy ( HAART ). Currently there is inadequate evidence to recommend routine selenium supplementation for HIV patients, and further research is recommended. [ 41 ]
Selenium supplementation has no effect on overall mortality. [ 42 ]
As with other types of supplementation, there is no good evidence selenium supplementation helps in the treatment of tuberculosis. [ 43 ]
A meta-analysis of four RCTs concluded that there is no support for selenium supplementation for prevention of type 2 diabetes mellitus in Caucasians. [ 44 ]
Abnormally high or low levels of dietary selenium can have an adverse effect on sperm quality, with a consequent lowering of fertility. [ 45 ]
Selenium has a protective effect towards mercury toxicity. Mercury binds to selenium with high affinity, so this metal can inhibit selenium-dependent enzymes. However, increased selenium intake can preserve the enzyme activities, reducing the adverse effects caused by mercury exposure. [ 46 ] [ 47 ]
Selenium is incorporated into several prokaryotic selenoprotein families in bacteria, archaea, and eukaryotes as selenocysteine, [ 48 ] where selenoprotein peroxiredoxins protect bacterial and eukaryotic cells against oxidative damage. Selenoprotein families of GSH-Px and the deiodinases of eukaryotic cells seem to have a bacterial phylogenetic origin. The selenocysteine-containing form occurs in species as diverse as green algae, diatoms, sea urchin, fish and chicken. Selenium enzymes are involved in utilization of the small reducing molecules glutathione and thioredoxin .
Trace elements involved in GSH-Px and superoxide dismutase enzymes activities, i.e. selenium, vanadium , magnesium , copper , and zinc , may have been lacking in some terrestrial mineral-deficient areas. [ 48 ] Marine organisms retained and sometimes expanded their seleno-proteomes, whereas the seleno-proteomes of some terrestrial organisms were reduced or completely lost. These findings suggest that aquatic life supports selenium utilization, whereas terrestrial habitats lead to reduced use of this trace element. [ 49 ] [ 50 ] Marine fishes and vertebrate thyroid glands have the highest concentration of selenium and iodine. From about 500 Mya, freshwater and terrestrial plants slowly optimized the production of "new" endogenous antioxidants such as ascorbic acid (Vitamin C), polyphenols (including flavonoids), tocopherols , etc. A few of these appeared more recently, in the last 50–200 million years, in fruits and flowers of angiosperm plants. In fact, the angiosperms (the dominant type of plant today) and most of their antioxidant pigments evolved during the late Jurassic period.
About 200 Mya, new selenoproteins were developed as mammalian GSH-Px enzymes. [ 51 ] [ 52 ] [ 53 ] [ 54 ] | https://en.wikipedia.org/wiki/Selenium_in_biology |
Selenium trioxide is the inorganic compound with the formula Se O 3 . It is white, hygroscopic solid. It is also an oxidizing agent and a Lewis acid . It is of academic interest as a precursor to Se(VI) compounds. [ 4 ]
Selenium trioxide is difficult to prepare because it is unstable with respect to the dioxide :
It has been generated in a number of ways despite the fact that the dioxide does not combust under normal conditions. [ 4 ] One method entails dehydration of anhydrous selenic acid with phosphorus pentoxide at 150–160 °C. Another method is the reaction of liquid sulfur trioxide with potassium selenate .
In its chemistry SeO 3 generally resembles sulfur trioxide , SO 3 , rather than tellurium trioxide , TeO 3 . [ 4 ] The substance reacts explosively with oxidizable organic compounds . [ 5 ]
At 120 °C SeO 3 reacts with selenium dioxide to form the Se(VI)-Se(IV) compound diselenium pentaoxide: [ 6 ]
It reacts with selenium tetrafluoride to form selenoyl fluoride , the selenium analogue of sulfuryl fluoride
As with SO 3 adducts are formed with Lewis bases such as pyridine , dioxane and ether . [ 4 ]
With lithium oxide and sodium oxide it reacts to form salts of Se VI O 5 4− and Se VI O 6 6− : [ 7 ] With Li 2 O, it gives Li 4 SeO 5 , containing the trigonal pyramidal anion Se VI O 5 4− with equatorial bonds, 170.6–171.9 pm; and longer axial Se−O bonds of 179.5 pm. With Na 2 O it gives Na 4 SeO 5 , containing the square pyramidal Se VI O 5 4− , with Se−O bond lengths ranging from range 172.9 → 181.5 pm, and Na 12 (SeO 4 ) 3 (SeO 6 ), containing octahedral Se VI O 6 6− . Se VI O 6 6− is the conjugate base of the unknown orthoselenic acid (Se(OH) 6 ).
In the solid phase SeO 3 consists of cyclic tetramers, with an 8 membered (Se−O) 4 ring. Selenium atoms are 4-coordinate, bond lengths being Se−O bridging are 175 pm and 181 pm, non-bridging 156 and 154 pm. [ 7 ]
SeO 3 in the gas phase consists of tetramers and monomeric SeO 3 which is trigonal planar with an Se−O bond length of 168.78 pm. [ 8 ] | https://en.wikipedia.org/wiki/Selenium_trioxide |
Selenium yeast is a feed additive for livestock, used to increase the selenium content in their fodder . It is a form of selenium currently approved for human consumption in the EU and Britain. [ 1 ] Inorganic forms of selenium are used in feeds (namely sodium selenate and sodium selenite , which appear to work in roughly the same manner). Since these products can be patented, producers can demand premium prices. [ 2 ] It is produced by fermenting Saccharomyces cerevisiae (baker's yeast) in a selenium-rich media. [ 3 ]
There is considerable variability in products described as Se-yeast and the selenium compounds found within. Many manufacturers and products on the market are simply mixtures of largely inorganic selenium and some yeast. [ 4 ] Selenium is found in different forms based upon the food in which it is found. For instance, the form found in mustard and garlic is different from the form found in wheat or corn. In some products, the added selenium is structurally substituted for sulfur in the amino acid methionine , thus forming an organic chemical called selenomethionine via the same pathways and enzymes. Owing to its similarity to sulfur-containing methionine, selenomethionine is mistaken for an amino acid by the yeast anabolism and incorporated in its proteins. It has been claimed that selenomethionine makes a better source of dietary selenium in animal nutrition, since it is an organic chemical compound sometimes found in some common crops such as wheat. [ 3 ] [ 5 ]
Large amounts of selenium are toxic ; however, it is physiologically necessary for animals in extremely small amounts. Many other uncharacterized selenium-containing organic chemicals are also produced by a method similar to that of selenomethionine; some have recently been characterized but remain relatively unknown, such as S -seleno-methyl-glutathione and glutathione- S -selenoglutathione. [ 3 ] [ 4 ] Due to this, the European Union has questioned the safety and potential toxicity of this food supplement for humans, and it may not be used as an additive after 2002.
G.N Schrauzer , who has written two papers about selenomethionine, claims it should be an essential amino acid , and that the product is completely safe. [ 3 ] The European Food Safety Authority does allow the use of selenomethionine as a feed additive for animals. [ 6 ] Because organic forms of selenium appear to be excreted from the body slower than inorganic forms, products enriched with organic selenium might detrimentally bioaccumulate in the body. Because selenium-enriched foods contain much more selenium than natural foods, selenium toxicity is a potential problem, and such foods must be treated with caution. [ 4 ] The EU allows up to 300 micrograms of selenium per day, but one long-term study of selenium supplementation showed no evidence of toxicity at a dose as high as 800 micrograms per day. [ 7 ]
An organic selenium-containing chemical found in selenium yeast has been shown to differ in bioavailability and metabolism compared with common inorganic forms of dietary selenium. [ 8 ] Dietary supplementation using selenium yeast is ineffective in the production of antioxidants in bovine milk compared to inorganic selenium (sodium selenate). [ 9 ] One study examined if increased selenium in the diet of mutant mice (via a selenium yeast product) caused a higher production of selenium-containing enzymes which have an antioxidant effect. The effect was modest. [ 10 ]
Selenium supplementation in yeast form has been shown to increase pig selenium-containing antioxidant enzymes, [ 11 ] broiler growth and meat quality, [ 12 ] [ 13 ] the shelf life of turkey and rooster semen, [ 14 ] [ 15 ] [ 16 ] and possibly cattle fertility. [ 17 ]
Selenium supplementation in animal feeds may be profitable for agribusinesses. It may be possible to market selenium-fortified foods to consumers as functional foods , such as selenium-enriched eggs, meat, [ 18 ] [ 19 ] [ 20 ] [ 21 ] or milk. [ 9 ]
A patented cultivar of yeast ( Saccharomyces cerevisiae 'CNCM I-3060') marketed as Sel-Plex® has been approved for use in animal fodder :
Total selenium in selenium yeast can be reliably determined using open acid digestion to extract selenium from the yeast matrix followed by flame atomic absorption spectrometry. [ 24 ] Determination of the selenium species selenomethionine can be achieved via proteolytic digestion of selenium yeast followed by high-performance liquid chromatography with inductively coupled plasma mass spectrometry. [ 25 ] [ 26 ] [ 27 ]
Nutritional muscular dystrophy | https://en.wikipedia.org/wiki/Selenium_yeast |
The selenographic coordinate system is used to refer to locations on the surface of Earth 's moon . Any position on the lunar surface can be referenced by specifying two numerical values, which are comparable to the latitude and longitude of Earth. The longitude gives the position east or west of the Moon's prime meridian, which is the line passing from the lunar north pole through the point on the lunar surface directly facing Earth to the lunar south pole . (See also Earth's prime meridian .) This can be thought of as the midpoint of the visible Moon as seen from the Earth. The latitude gives the position north or south of the lunar equator . Both of these coordinates are given in degrees .
Astronomers defined the fundamental location in the selenographic coordinate system by the small, bowl-shaped satellite crater ' Mösting A ' [ citation needed ] . The coordinates of this crater are defined as:
Later, the coordinate system has become more precisely defined due to the Lunar Laser Ranging Experiment .
Anything past 90°E or 90°W would not be seen from Earth, except for libration , which makes 59% of the Moon visible .
Longitude on the Moon is measured both east and west from its prime meridian . When no direction is specified, east is positive and west is negative.
Roughly speaking, the Moon's prime meridian lies near the center of the Moon's disc as seen from Earth. For precise applications, many coordinate systems have been defined for the Moon, each with a slightly different prime meridian. The IAU recommends the "mean Earth/polar axis" system, [ 1 ] in which the prime meridian is the average direction (from the Moon's center) of the Earth's center. [ 2 ]
The selenographic colongitude is the longitude of the morning terminator on the Moon , as measured in degrees westward from the prime meridian. The morning terminator forms a half-circle across the Moon where the Sun is just starting to rise. As the Moon continues in its orbit, this line advances in longitude. The value of the selenographic colongitude increases from 0° to 359° in the direction of the advancing terminator.
Sunrise occurs at the prime meridian when the Lunar phase reaches First Quarter, after one fourth of a lunar day . At this location the selenographic colongitude at sunrise is defined as 0°. Thus, by the time of the Full Moon the colongitude increases to 90°; at Last Quarter it is 180°, and at the New Moon the colongitude reaches 270°. Note that the Moon is nearly invisible from the Earth at New Moon phase except during a solar eclipse .
The low angle of incidence of arriving sunlight tends to pick out features by the sharp shadows they cast, thus the area near the terminator is usually the most favorable for viewing or photographing lunar features through a telescope . The observer will need to know the location of the terminator to plan observations of selected features. The selenographic colongitude is useful for this purpose.
The selenographic longitude of the evening terminator is equal to the colongitude plus 180°. [ 3 ] | https://en.wikipedia.org/wiki/Selenographic_coordinate_system |
Selenols are organic compounds that contain the functional group with the connectivity C − Se − H . Selenols are sometimes also called selenomercaptans and selenothiols . Selenols are one of the principal classes of organoselenium compounds . [ 1 ] A well-known selenol is the amino acid selenocysteine .
Selenols are structurally similar to thiols , but the C−Se bond is about 8% longer at 196 pm . The C−Se−H angle approaches 90°. The bonding involves almost pure p-orbitals on Se, hence the near 90 angles. The Se−H bond energy is weaker than the S −H bond, consequently selenols are easily oxidized and serve as H-atom donors . The Se-H bond is weaker than the S−H bond as reflected in their respective bond dissociation energy (BDE). For C 6 H 5 Se−H , the BDE is 326 kJ/mol , while for C 6 H 5 S−H , the BDE is 368 kJ/mol. [ 2 ]
Selenols are about 1000 times stronger acids than thiols: the p K a of CH 3 SeH is 5.2 vs 8.3 for CH 3 SH . Deprotonation affords the selenolate anion , RSe − , most examples of which are highly nucleophilic and rapidly oxidized by air. [ 3 ]
The boiling points of selenols tend to be slightly greater than for thiols. This difference can be attributed to the increased importance of stronger van der Waals bonding for larger atoms. Volatile selenols have highly offensive odors.
Selenols have few commercial applications, being limited by the toxicity of selenium as well as the sensitivity of the Se−H bond. Their conjugate bases , the selenolates, also have limited applications in organic synthesis .
Selenols are important in certain biological processes. Three enzymes found in mammals contain selenols at their active sites: glutathione peroxidase , iodothyronine deiodinase , and thioredoxin reductase . The selenols in these proteins are part of the essential amino acid selenocysteine . [ 3 ] The selenols function as reducing agents to give selenenic acid derivative ( RSe−OH ), which in turn are re-reduced by thiol-containing enzymes. Methaneselenol (commonly named "methylselenol") ( CH 3 SeH ), which can be produced in vitro by incubating selenomethionine with a bacterial methionine gamma-lyase (METase) enzyme , by biological methylation of selenide ion or in vivo by reduction of methaneseleninic acid ( CH 3 −Se(=O)−OH ), has been invoked to explain the anticancer activity of certain organoselenium compounds. [ 4 ] [ 5 ] [ 6 ] Precursors of methaneselenol are under active investigation in cancer prevention and therapy. In these studies, methaneselenol is found to be more biologically active than ethaneselenol ( CH 3 CH 2 SeH ) or 2-propaneselenol ( (CH 3 ) 2 CH(SeH) ). [ 7 ]
Selenols are usually prepared by the reaction of organolithium reagents or Grignard reagents with elemental Se. [ 8 ] For example, benzeneselenol is generated by the reaction of phenylmagnesium bromide with selenium followed by acidification : [ 9 ]
Another preparative route to selenols involves the alkylation of selenourea , followed by hydrolysis . Selenols are often generated by reduction of diselenides followed by protonation of the resulting selenolate:
Dimethyl diselenide can be easily reduced to methaneselenol within cells. [ 10 ]
Selenols are easily oxidized to diselenides, compounds containing an Se−Se bond. For example, treatment of benzeneselenol with bromine gives diphenyl diselenide .
In the presence of base, selenols are readily alkylated to give selenides. This relationship is illustrated by the methylation of methaneselenol to give dimethylselenide .
Organoselenium compounds (or any selenium compound) are cumulative poisons despite the fact that trace amounts of Se are required for health. [ 11 ] | https://en.wikipedia.org/wiki/Selenol |
A selenonic acid is an organoselenium compound containing the −SeO 3 H functional group . The formula of selenonic acids is R−Se(=O) 2 −OH , where R is organyl group . Selenonic acids are the selenium analogs of sulfonic acids . [ 1 ] Examples of the acid are rare. Benzeneselenonic acid PhSeO 3 H (where Ph stands for phenyl ) is a white solid. It can be prepared by the oxidation of benzeneselenol . [ 2 ]
This organic chemistry article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Selenonic_acid |
In chemistry , a selenosulfide refers to distinct classes of inorganic and organic compounds containing sulfur and selenium. The organic derivatives contain Se-S bonds, whereas the inorganic derivatives are more variable.
These species are classified as both organosulfur and organoselenium compounds. They are hybrids of organic disulfides and organic diselenides .
Selenosulfides have been prepared by the reaction of selenyl halides with thiols: [ 2 ]
The equilibrium between diselenides and disulfides lies on the left:
Because of the facility of this equilibrium, many of the best characterized examples of selenosulfides are cyclic, whereby S-Se bonds are stabilized intramolecularly. One example is the 1,8-selenosulfide of naphthalene . [ 3 ] The selenium-sulfur bond length is about 220 picometers, the average of a typical S-S and Se-Se bond.
Selenosulfide groups can be found in almost all living organisms as part of various peroxidase enzymes, such as glutathione peroxidase and thioredoxin reductase . They are formed by the oxidative coupling of selenocysteine and cysteine residues. [ 2 ] This reaction is powered by the decomposition of cellular peroxides, which can be highly damaging and a source of oxidative stress . Selenocysteine has a lower reduction potential than cysteine, making it very suitable for proteins that are involved in antioxidant activity. [ 4 ]
Selenosulfides have been identified in some species of Allium [ 1 ] and in roasted coffee. [ 5 ] The mammalian version of the protein thioredoxin reductase contains a selenocysteine residue which forms a thioselenide (analogous to a disulfide ) upon oxidation. [ 6 ]
Some inorganic selenide sulfide compounds are also known. Simplest is the material selenium sulfide , which has medicinal properties. It adopt the diverse structures of elemental sulfur but with some S atoms replaced by Se.
Other inorganic selenide sulfide compounds occur as minerals and as pigments. One example is antimony selenosulfide.
The pigment cadmium red consists of cadmium sulfoselenide. It is a solid solution of cadmium sulfide , which is yellow, and cadmium selenide , which is dark brown. It is used as an artist's pigment. [ 7 ] Unlike the organic selenosulfides and unlike selenide sulfide itself, no S-Se bond exists in CdS 1−x Se x or in Sb 2 S 3−x Se x . | https://en.wikipedia.org/wiki/Selenosulfide |
Selenoxide elimination (also called α-selenation) [ 1 ] is a method for the chemical synthesis of alkenes from selenoxides . It is most commonly used to synthesize α,β-unsaturated carbonyl compounds from the corresponding saturated analogues. [ 2 ] It is mechanistically related to the Cope reaction .
After the development of sulfoxide elimination as an effective method for generating carbon –carbon double bonds , [ 3 ] it was discovered that selenoxides undergo a similar process, albeit much more rapidly. Most selenoxides decompose to the corresponding alkenes at temperatures between −50 and 40 °C. Evidence suggests that the elimination is syn ; however, epimerization at both carbon and selenium (both of which are stereogenic ) may occur during the reaction. As selenoxides can be readily prepared from nucleophilic carbonyl derivatives ( enols and enolates ), [ 4 ] selenoxide elimination has grown into a general method for the preparation of α,β-unsaturated carbonyl compounds.
(1)
Elimination of selenoxides takes place through an intramolecular syn elimination pathway. The carbon– hydrogen and carbon– selenium bonds are co-planar in the transition state. [ 5 ]
(2)
The reaction is highly trans -selective when acyclic α-phenylseleno carbonyl compounds are employed. Formation of conjugated double bonds is favored. Endocyclic double bonds tend to predominate over exocyclic ones, unless no syn hydrogen is available in the ring. Selenium in these reactions is almost always stereogenic, and the effect of epimerization at selenium (which is acid - catalyzed and occurs readily) on the elimination reaction is nearly unknown. In one example, separation and warming of selenoxides 1 and 2 revealed that 2 decomposes at 0 °C, while 1 , which presumably has more difficulty accessing the necessary syn conformation for elimination, is stable to 5 °C. [ 6 ]
(3)
Kinetic isotope effect studies have found a ratio of pre-exponential factors of A H / A D of 0.092 for sulfoxide elimination reactions, indicating that quantum tunneling plays an important role in the hydrogen transfer process. [ 7 ] [ 8 ]
α-Selanylation of carbonyl compounds can be accomplished with electrophilic or nucleophilic selanylating reagents. Usually, simple phenylseleno compounds are used in elimination reactions; although 2-nitrophenylselenides react more quickly, they are more expensive to prepare, and phenylselenides typically react in minutes. Electrophilic selanylating reagents can be used in conjunction with enols , enolates , or enol ethers . Phenylselanating reagents include:
The most common oxidizing agent employed is hydrogen peroxide (H 2 O 2 ). [ 9 ] It is sometimes used in excess, to overcome catalytic decomposition of H 2 O 2 by selenium; however, undesired oxidation of starting material has been observed under these conditions. Oxidation of products (via the Baeyer-Villiger reaction, for instance) has also been observed. [ 10 ]
(4)
For substrates whose product olefins are sensitive to oxidation, meta-Chloroperoxybenzoic acid (mCPBA) can be employed as an oxidant. It oxidizes selenides below the temperature at which they decompose to alkenes; thus, all oxidant is consumed before elimination begins. Buffering with an amine base is necessary before warming to avoid acid-mediated side reactions. [ 11 ]
(5)
Ozone , which gives only dioxygen as a byproduct after oxidation, is used to oxidize selenides when special conditions are required for thermolysis or extreme care is necessary during workup. Quinones can be synthesized from the corresponding cyclic unsaturated carbonyl compounds using this method. [ 12 ]
(6)
α-Phenylseleno aldehydes , which are usually prepared from the corresponding enol ethers , are usually oxidized with mCPBA or ozone, as hydrogen peroxide causes over-oxidation. α-Phenylseleno ketones can be prepared by kinetically controlled enolate formation and trapping with an electrophilic selanylating reagent such as benzeneselenyl chloride. A second deprotonation, forming a selenium-substituted enolate, allows alkylation or hydroxyalkylation of these substrates. [ 13 ]
(7)
Base-sensitive substrates may be selanylated under acid-catalyzed conditions (as enols) using benzeneselenyl chloride. Hydrochloric acid generated during the selanylation of transient enol catalyzes tautomerization . [ 14 ]
(8)
The seleno- Pummerer reaction is a significant side reaction that may occur under conditions when acid is present. [ 15 ] Protonation of the selenoxide intermediate, followed by elimination of hydroxide and hydrolysis , leads to α-dicarbonyl compounds. The reaction is not a problem for more electron-rich carbonyls—generally, fewer side reactions are observed in eliminations of esters and amides . [ 15 ]
(9)
A second significant side reaction in reactions of ketones and aldehydes is selanylation of the intermediate selenoxide. This process leads to elimination products retaining a carbon-selenium bond, [ 16 ] and is more difficult to prevent than the seleno-Pummerer reaction. Tertiary selenoxides, which are unable to undergo enolization, do not react further with selenium electrophiles.
(10)
Analogous sulfoxide eliminations are generally harder to implement than selenoxide eliminations. Formation of the carbon– sulfur bond is usually accomplished with highly reactive sulfenyl chlorides , which must be prepared for immediate use. However, sulfoxides are more stable than the corresponding selenoxides, and elimination is usually carried out as a distinct operation. This allows thermolysis conditions to be optimized (although the high temperatures required may cause other thermal processes). In addition, sulfoxides may be carried through multiple synthetic steps before elimination is carried out. [ 17 ]
(11)
The combination of silyl enol ethers with palladium(II) acetate (Pd(OAc) 2 ), the Saegusa oxidation , gives enones . However, the reaction requires stoichiometric amounts of Pd(OAc) 2 and thus is not amenable to large-scale synthesis. [ 18 ] Catalytic variants have been developed. [ 19 ]
(12)
For β-dicarbonyl compounds, DDQ can be used as an oxidizing agent in the synthesis of enediones. Additionally, some specialized systems give better yields upon DDQ oxidation. [ 20 ]
(13) | https://en.wikipedia.org/wiki/Selenoxide_elimination |
Selexol is the trade name for an acid gas removal solvent that can separate acid gases such as hydrogen sulfide and carbon dioxide from feed gas streams such as synthesis gas produced by gasification of coal, coke, or heavy hydrocarbon oils. [ 1 ] [ 2 ] By doing so, the feed gas is made more suitable (less sour ) for combustion and/or further processing. It is made up of dimethyl ethers of polyethylene glycol. [ 3 ]
In the Selexol process (now licensed by UOP LLC ), the Selexol solvent dissolves (absorbs) the acid gases from the feed gas at relatively high pressure, usually 300 to 2000 psia (2.07 to 13.8 MPa). The rich solvent containing the acid gases is then let down in pressure and/or steam stripped to release and recover the acid gases. The Selexol process can operate selectively to recover hydrogen sulfide and carbon dioxide as separate streams, so that the hydrogen sulfide can be sent to either a Claus unit for conversion to elemental sulfur or to a wet sulfuric acid process unit for conversion to sulfuric acid while, at the same time, the carbon dioxide can be sequestered or used for enhanced oil recovery . The Selexol process is similar to the Rectisol process, which uses refrigerated methanol as the solvent. The Selexol solvent is a mixture of the dimethyl ethers of polyethylene glycol .
Selexol is a physical solvent, unlike amine based acid gas removal solvents that rely on a chemical reaction with the acid gases. Since no chemical reactions are involved, Selexol usually requires less energy than the amine based processes. However, at feed gas pressures below about 300 psia (2.07 MPa), the Selexol solvent capacity (in amount of acid gas absorbed per volume of solvent) is reduced and the amine based processes will usually be superior. | https://en.wikipedia.org/wiki/Selexol |
Self-adaptive mechanisms , sometimes simply called adaptive mechanisms, in engineering , are underactuated mechanisms that can adapt to their environment. One of the most well-known example of this type of mechanisms are underactuated fingers, grippers, and robotic hands . Contrary to standard underactuated mechanisms where the motion is governed by the dynamics of the system, the motion of self-adaptive mechanisms is generally constrained by compliant elements cleverly located in the mechanisms.
Underactuated mechanisms have a lower number of actuators than the number of degrees of freedom (DOF) . In a two-dimensional plane , a mechanism can have up to three DOF (two translations, one rotation), and in three-dimensional Euclidean space , up to six (three translations, three rotations). In the case of self-adaptive mechanisms, the lack of actuators is compensated by passive elements that constrain the motion of the system. Springs are a good example of such elements, but other can be used depending on the type of mechanisms.
One of the earliest example of self-adaptive mechanism is the flapping wing proposed by Leonardo da Vinci in the Codex Atlanticus . [ 1 ]
The first commonly known underactuated finger was the Soft-Gripper designed by Shigeo Hirose in the late 1970s. [ 2 ] The most common type of transmission mechanisms used in self-adaptive hands are linkages and tendons. [ 3 ]
Underactuated fingers and hands are usually analyzed with respect to their kinetostatics (negligible kinetic energy, static analysis of a mechanism in motion) rather than the dynamics of the system, as the kinetic energy of these systems is generally negligible compared to the potential energy stored into the passive elements. The forces applied by each phalanx of an underactuated finger can be computed with the following expression:
F = J − T T ∗ T t {\displaystyle \mathbf {F} =\mathbf {J} ^{-T}\mathbf {T} ^{*T}\mathbf {t} }
where F is the vector made of the forces applied, J is the Jacobian matrix of the finger, T * is the transmission matrix, and t is the torque vector made (actuator and passive elements). [ 4 ]
A self-adaptive robotic hand, SARAH (Self-Adaptive Robot Auxiliary Hand), was designed and built to be part of the Dextre ’s toolbox. Dextre is a robotic telemanipulator that resides at the end of CANADARM-2 on the International Space Station . [ 5 ] The Yale OpenHand is an example of open source self-adaptive mechanisms that can be found online. [ 6 ] Some companies are also selling self-adaptive hands for industrial purposes. [ 7 ] Prosthetics is another application for self-adaptive hands. One known example is the SPRING (Self-Adaptive Prosthesis for Restoring Natural Grasping) hand. [ 8 ]
Self-adaptive mechanisms can be used for other applications, such as walking robots . [ 9 ] [ 10 ]
Compliant mechanisms are another example of self-adaptive mechanisms, where the passive elements and the transmission mechanism are a single monolithic block. [ 11 ] | https://en.wikipedia.org/wiki/Self-adaptive_mechanisms |
In mathematics , an element of a *-algebra is called self-adjoint if it is the same as its adjoint (i.e. a = a ∗ {\displaystyle a=a^{*}} ).
Let A {\displaystyle {\mathcal {A}}} be a *-algebra. An element a ∈ A {\displaystyle a\in {\mathcal {A}}} is called self-adjoint if a = a ∗ {\displaystyle a=a^{*}} . [ 1 ]
The set of self-adjoint elements is referred to as A s a {\displaystyle {\mathcal {A}}_{sa}} .
A subset B ⊆ A {\displaystyle {\mathcal {B}}\subseteq {\mathcal {A}}} that is closed under the involution *, i.e. B = B ∗ {\displaystyle {\mathcal {B}}={\mathcal {B}}^{*}} , is called self-adjoint. [ 2 ]
A special case of particular importance is the case where A {\displaystyle {\mathcal {A}}} is a complete normed *-algebra , that satisfies the C*-identity ( ‖ a ∗ a ‖ = ‖ a ‖ 2 ∀ a ∈ A {\displaystyle \left\|a^{*}a\right\|=\left\|a\right\|^{2}\ \forall a\in {\mathcal {A}}} ), which is called a C*-algebra .
Especially in the older literature on *-algebras and C*-algebras, such elements are often called hermitian. [ 1 ] Because of that the notations A h {\displaystyle {\mathcal {A}}_{h}} , A H {\displaystyle {\mathcal {A}}_{H}} or H ( A ) {\displaystyle H({\mathcal {A}})} for the set of self-adjoint elements are also sometimes used, even in the more recent literature.
Let A {\displaystyle {\mathcal {A}}} be a *-algebra. Then:
Let A {\displaystyle {\mathcal {A}}} be a *-algebra. Then:
Let A {\displaystyle {\mathcal {A}}} be a C*-algebra and a ∈ A s a {\displaystyle a\in {\mathcal {A}}_{sa}} . Then: | https://en.wikipedia.org/wiki/Self-adjoint |
Self-assembled monolayers ( SAM ) are assemblies of organic molecules that form spontaneously on surfaces by adsorption and organize themselves into more or less distinct domains (head group, chain/backbone, and tail/end group). [ 1 ] [ 2 ] In some cases, molecules that form the monolayer do not interact strongly with the substrate. This is the case for porphyrins on HOPG [ 3 ] and two-dimensional supramolecular networks [ 4 ] of PTCDA on gold . [ 5 ] In other cases, the head group has a strong affinity for the substrate and anchors the molecule. [ 6 ] Such an SAM consisting of a head group, chain (labeled "tail"), and functional end group is depicted in Figure 1. Common head groups include thiols , silanes , and phosphonates .
SAMs are created by the chemisorption of head groups onto a substrate from either the vapor or liquid phase [ 7 ] [ 8 ] followed by a slower organization of "tail groups". [ 9 ] Initially, at small molecular density on the surface, adsorbate molecules form either a disordered mass of molecules or an ordered two-dimensional "lying down phase". [ 7 ] At higher molecular coverage, adsorbates can begin to form three-dimensional crystalline or semicrystalline structures on the substrate surface over a period of minutes to hours. [ 10 ] The head groups assemble on the substrate, while the tail groups assemble far from the substrate. Areas of close-packed molecules nucleate and grow until the surface of the substrate is covered in a single monolayer.
Adsorbate molecules adsorb readily because they lower the surface free-energy of the substrate [ 1 ] and are stable due to the strong chemisorption of the head groups. These bonds create monolayers that are more stable than the physisorbed bonds of Langmuir–Blodgett films . [ 11 ] [ 12 ] For example, the trichlorosilane head group of an FDTS molecule reacts with a hydroxyl group on a substrate to form a very stable covalent bond [R-Si-O-substrate] with an energy of 452 kJ/mol. [ citation needed ] Thiol-metal bonds are on the order of 100 kJ/mol, making them fairly stable in a variety of temperatures, solvents, and potentials. [ 10 ] Monolayers pack tightly due to van der Waals interactions , [ 1 ] [ 12 ] thereby reducing their own free energy. [ 1 ] The adsorption can be described by the Langmuir adsorption isotherm if lateral interactions are neglected. If they cannot be neglected, the adsorption is better described by the Frumkin isotherm. [ 10 ]
Selecting the type of head group depends on the application of the SAM. [ 1 ] Typically, head groups are connected to a molecular chain in which the terminal end can be functionalized (i.e. adding –OH, –NH2, –COOH, or –SH groups) to vary the wetting and interfacial properties. [ 11 ] [ 13 ] An appropriate substrate is chosen to react with the head group. Substrates can be planar surfaces, such as silicon and metals, or curved surfaces, such as nanoparticles. Alkanethiols are the most commonly used molecules for SAMs. Alkanethiols are molecules with an alkyl chain, (C-C)ⁿ chain, as the back bone, a tail group, and a S-H head group. Other types of interesting molecules include aromatic thiols, of interest in molecular electronics, in which the alkane chain is (partly) replaced by aromatic rings. An example is the dithiol 1,4-Benzenedimethanethiol (SHCH 2 C 6 H 4 CH 2 SH)). Interest in such dithiols stems from the possibility of linking the two sulfur ends to metallic contacts, which was first used in molecular conduction measurements. [ 14 ] Thiols are frequently used on noble metal substrates because of the strong affinity of sulfur for these metals. The sulfur gold interaction is semi-covalent and has a strength of approximately 45 kcal/mol. In addition, gold is an inert and biocompatible material that is easy to acquire. It is also easy to pattern via lithography, a useful feature for applications in nanoelectromechanical systems (NEMS). [ 1 ] Additionally, it can withstand harsh chemical cleaning treatments. [ 10 ] Recently other chalcogenide SAMs: selenides and tellurides have attracted attention [ 15 ] [ 16 ] in a search for different bonding characteristics to substrates affecting the SAM characteristics and which could be of interest in some applications such as molecular electronics. Silanes are generally used on nonmetallic oxide surfaces; [ 1 ] however monolayers formed from covalent bonds between silicon and carbon or oxygen cannot be considered self assembled because they do not form reversibly. Self-assembled monolayers of thiolates on noble metals are a special case because the metal-metal bonds become reversible after the formation of the thiolate-metal complex. [ 17 ] This reversibility is what gives rise to vacancy islands and it is why SAMs of alkanethiolates can be thermally desorbed and undergo exchange with free thiols. [ 18 ]
Metal substrates for use in SAMs can be produced through physical vapor deposition techniques, electrodeposition or electroless deposition. [ 1 ] Thiol or selenium SAMs produced by adsorption from solution are typically made by immersing a substrate into a dilute solution of alkane thiol in ethanol, though many different solvents can be used [ 1 ] besides use of pure liquids. [ 16 ] While SAMs are often allowed to form over 12 to 72 hours at room temperature, [ 10 ] [ 19 ] SAMs of alkanethiolates form within minutes. [ 20 ] [ 21 ] Special attention is essential in some cases, such as that of dithiol SAMs to avoid problems due to oxidation or photoinduced processes, which can affect terminal groups and lead to disorder and multilayer formation. [ 22 ] [ 23 ] In this case appropriate choice of solvents, their degassing by inert gasses and preparation in the absence of light is crucial [ 22 ] [ 23 ] and allows formation of "standing up" SAMs with free –SH groups. Self-assembled monolayers can also be adsorbed from the vapor phase. [ 8 ] [ 24 ] In some cases when obtaining an ordered assembly is difficult or when different density phases need to be obtained substitutional self-assembly is used. Here one first forms the SAM of a given type of molecules, which give rise to ordered assembly and then a second assembly phase is performed (e.g. by immersion into a different solution). This method has also been used to give information on relative binding strengths of SAMs with different head groups and more generally on self-assembly characteristics. [ 18 ] [ 25 ]
The thicknesses of SAMs can be measured using ellipsometry and X-ray photoelectron spectroscopy (XPS) , which also give information on interfacial properties. [ 22 ] [ 26 ] The order in the SAM and orientation of molecules can be probed by Near Edge Xray Absorption Fine Structure (NEXAFS) and Fourier Transform Infrared Spectroscopy in Reflection Absorption Infrared Spectroscopy (RAIRS) [ 20 ] [ 23 ] studies. Numerous other spectroscopic techniques are used [ 8 ] such as Second-harmonic generation (SHG), Sum-frequency generation (SFG), Surface-enhanced Raman scattering (SERS), as well as [ 27 ] High-resolution electron energy loss spectroscopy (HREELS) . The structures of SAMs are commonly determined using scanning probe microscopy techniques such as atomic force microscopy (AFM) and scanning tunneling microscopy (STM). STM has been able to help understand the mechanisms of SAM formation as well as determine the important structural features that lend SAMs their integrity as surface-stable entities. In particular STM can image the shape, spatial distribution, terminal groups and their packing structure. AFM offers an equally powerful tool without the requirement of the SAM being conducting or semi-conducting. AFM has been used to determine chemical functionality, conductance, magnetic properties, surface charge, and frictional forces of SAMs. [ 28 ] The scanning vibrating electrode technique (SVET) is a further scanning probe microscopy which has been used to characterize SAMs, with defect free SAMs showing homogeneous activity in SVET. [ 29 ] More recently, however, diffractive methods have also been used. [ 1 ] The structure can be used to characterize the kinetics and defects found on the monolayer surface. These techniques have also shown physical differences between SAMs with planar substrates and nanoparticle substrates.
An alternative characterisation instrument for measuring the self-assembly in real time is dual polarisation interferometry where the refractive index, thickness, mass and birefringence of the self assembled layer are quantified at high resolution. Another method that can be used to measure the self-assembly in real-time is Quartz Crystal Microbalance with Dissipation monitoring technology where the mass and viscoelastic properties of the adlayer are quantified. Contact angle measurements can be used to determine the surface free-energy which reflects the average composition of the surface of the SAM and can be used to probe the kinetics and thermodynamics of the formation of SAMs. [ 20 ] [ 21 ] The kinetics of adsorption and temperature induced desorption as well as information on structure can also be obtained in real time by ion scattering techniques such as low energy ion scattering (LEIS) and time of flight direct recoil spectroscopy (TOFDRS) . [ 24 ]
Defects due to both external and intrinsic factors may appear. External factors include the cleanliness of the substrate, method of preparation, and purity of the adsorbates. [ 1 ] [ 10 ] SAMs intrinsically form defects due to the thermodynamics of formation, e.g. thiol SAMs on gold typically exhibit etch pits (monatomic vacancy islands) likely due to extraction of adatoms from the substrate and formation of adatom-adsorbate moieties. Recently, a new type of fluorosurfactants have found that can form nearly perfect monolayer on gold substrate due to the increase of mobility of gold surface atoms. [ 30 ] [ 31 ] [ 32 ]
The structure of SAMs is also dependent on the curvature of the substrate. SAMs on nanoparticles, including colloids and nanocrystals, "stabilize the reactive surface of the particle and present organic functional groups at the particle-solvent interface". [ 1 ] These organic functional groups are useful for applications, such as immunoassays or sensors , that are dependent on chemical composition of the surface. [ 1 ]
There is evidence that SAM formation occurs in two steps: an initial fast step of adsorption and a second slower step of monolayer organization. Adsorption occurs at the liquid–liquid, liquid–vapor, and liquid-solid interfaces. The transport of molecules to the surface occurs due to a combination of diffusion and convective transport. According to the Langmuir or Avrami kinetic model the rate of deposition onto the surface is proportional to the free space of the surface. [ 7 ]
Where θ is the proportional amount of area deposited and k is the rate constant. Although this model is robust it is only used for approximations because it fails to take into account intermediate processes. [ 7 ] Dual polarisation interferometry being a real time technique with ~10 Hz resolution can measure the kinetics of monolayer self-assembly directly.
Once the molecules are at the surface the self-organization occurs in three phases: [ 7 ]
The phase transitions in which a SAM forms depends on the temperature of the environment relative to the triple point temperature, the temperature in which the tip of the low-density phase intersects with the intermediate-phase region. At temperatures below the triple point the growth goes from phase 1 to phase 2 where many islands form with the final SAM structure, but are surrounded by random molecules. Similar to nucleation in metals, as these islands grow larger they intersect forming boundaries until they end up in phase 3, as seen below. [ 7 ]
At temperatures above the triple point the growth is more complex and can take two paths. In the first path the heads of the SAM organize to their near final locations with the tail groups loosely formed on top. Then as they transit to phase 3, the tail groups become ordered and straighten out. In the second path the molecules start in a lying down position along the surface. These then form into islands of ordered SAMs, where they grow into phase 3, as seen below. [ 7 ]
The nature in which the tail groups organize themselves into a straight ordered monolayer is dependent on the inter-molecular attraction, or van der Waals forces , between the tail groups. To minimize the free energy of the organic layer the molecules adopt conformations that allow high degree of Van der Waals forces with some hydrogen bonding. The small size of the SAM molecules are important here because Van der Waals forces arise from the dipoles of molecules and are thus much weaker than the surrounding surface forces at larger scales. The assembly process begins with a small group of molecules, usually two, getting close enough that the Van der Waals forces overcome the surrounding force. The forces between the molecules orient them so they are in their straight, optimal, configuration. Then as other molecules come close by they interact with these already organized molecules in the same fashion and become a part of the conformed group. When this occurs across a large area the molecules support each other into forming their SAM shape seen in Figure 1. The orientation of the molecules can be described with two parameters: α and β. α is the angle of tilt of the backbone from the surface normal. In typical applications α varies from 0 to 60 degrees depending on the substrate and type of SAM molecule. β is the angle of rotation along the long axis of tee molecule. β is usually between 30 and 40 degrees. [ 1 ] In some cases existence of kinetic traps hindering the final ordered orientation has been pointed out. [ 8 ] Thus in case of dithiols formation of a "lying down" phase [ 8 ] was considered an impediment to formation of "standing up" phase, however various recent studies indicate this is not the case. [ 22 ] [ 23 ]
Many of the SAM properties, such as thickness, are determined in the first few minutes. However, it may take hours for defects to be eliminated via annealing and for final SAM properties to be determined. [ 7 ] [ 10 ] The exact kinetics of SAM formation depends on the adsorbate, solvent and substrate properties. In general, however, the kinetics are dependent on both preparations conditions and material properties of the solvent, adsorbate and substrate. [ 7 ] Specifically, kinetics for adsorption from a liquid solution are dependent on: [ 1 ]
The final structure of the SAM is also dependent on the chain length and the structure of both the adsorbate and the substrate. Steric hindrance and metal substrate properties, for example, can affect the packing density of the film, [ 1 ] [ 10 ] while chain length affects SAM thickness. [ 12 ] Longer chain length also increases the thermodynamic stability. [ 1 ]
This first strategy involves locally depositing self-assembled monolayers on the surface only where the nanostructure will later be located. This strategy is advantageous because it involves high throughput methods that generally involve fewer steps than the other two strategies. The major techniques that use this strategy are: [ 33 ]
The locally remove strategy begins with covering the entire surface with a SAM. Then individual SAM molecules are removed from locations where the deposition of nanostructures is not desired. The result is the same as in the locally attract strategy, the difference being in the way this is achieved. The major techniques that use this strategy are: [ 33 ]
The final strategy focuses not on the deposition or removal of SAMS, but the modification of terminal groups. In the first case the terminal group can be modified to remove functionality so that SAM molecule will be inert. In the same regards the terminal group can be modified to add functionality [ 35 ] so it can accept different materials or have different properties than the original SAM terminal group. The major techniques that use this strategy are: [ 33 ]
SAMs are an inexpensive and versatile surface coating for applications including control of wetting and adhesion, [ 36 ] chemical resistance, bio compatibility, sensitization, and molecular recognition for sensors [ 37 ] and nano fabrication. [ 7 ] Areas of application for SAMs include biology, electrochemistry and electronics, nanoelectromechanical systems (NEMS) and microelectromechanical systems (MEMS), and everyday household goods. SAMs can serve as models for studying membrane properties of cells and organelles and cell attachment on surfaces. [ 1 ] SAMs can also be used to modify the surface properties of electrodes for electrochemistry, general electronics, and various NEMS and MEMS. [ 1 ] For example, the properties of SAMs can be used to control electron transfer in electrochemistry. [ 38 ] They can serve to protect metals from harsh chemicals and etchants. SAMs can also reduce sticking of NEMS and MEMS components in humid environments. In the same way, SAMs can alter the properties of glass. A common household product, Rain-X , utilizes SAMs to create a hydrophobic monolayer on car windshields to keep them clear of rain. Another application is an anti-adhesion coating on nanoimprint lithography (NIL) tools and stamps. One can also coat injection molding tools for polymer replication with a Perfluordecyltrichlorosilane SAM. [ 39 ]
Thin film SAMs can also be placed on nanostructures . In this way they functionalize the nanostructure . This is advantageous because the nanostructure can now selectively attach itself to other molecules or SAMs. This technique is useful in biosensors or other MEMS devices that need to separate one type of molecule from its environment. One example is the use of magnetic nanoparticles to remove a fungus from a blood stream. The nanoparticle is coated with a SAM that binds to the fungus. As the contaminated blood is filtered through a MEMS device the magnetic nanoparticles are inserted into the blood where they bind to the fungus and are then magnetically driven out of the blood stream into a nearby laminar waste stream. [ 40 ]
Photolithographic methods are useful in patterning SAMs. [ 41 ] SAMs are also useful in depositing nanostructures , because each adsorbate molecule can be tailored to attract two different materials. Current techniques utilize the head to attract to a surface, like a plate of gold. The terminal group is then modified to attract a specific material like a particular nanoparticle , wire, ribbon, or other nanostructure . In this way, wherever the SAM is patterned to a surface there will be nanostructures attached to the tail groups. One example is the use of two types of SAMs to align single wall carbon nanotubes , SWNTs. Dip pen nanolithography was used to pattern a 16-mercaptohexadecanoic acid (MHA)SAM and the rest of the surface was passivated with 1-octadecanethiol (ODT) SAM. The polar solvent that is carrying the SWNTs is attracted to the hydrophilic MHA; as the solvent evaporates, the SWNTs are close enough to the MHA SAM to attach to it due to Van der Waals forces . The nanotubes thus line up with the MHA-ODT boundary. Using this technique Chad Mirkin , Schatz and their co-workers were able to make complex two-dimensional shapes, a representation of a shape created is shown to the right. [ 33 ] [ 42 ] Another application of patterned SAMs is the functionalization of biosensors . The tail groups can be modified so they have an affinity for cells , proteins , or molecules . The SAM can then be placed onto a biosensor so that binding of these molecules can be detected. The ability to pattern these SAMs allows them to be placed in configurations that increase sensitivity and do not damage or interfere with other components of the biosensor . [ 28 ]
There has been considerable interest in use of SAMs for new materials e.g. via formation of two- or three-dimensional metal organic superlattices by assembly of SAM capped nanoparticles [ 43 ] or layer by layer SAM-nanoparticle arrays using dithiols. [ 44 ] A detailed review on this subject using dithiols is given by Hamoudi and Esaulov [ 45 ] | https://en.wikipedia.org/wiki/Self-assembled_monolayer |
Self-assembling peptides are a category of peptides which undergo spontaneous assembling into ordered nanostructures . Originally described in 1993, [ 1 ] these designer peptides have attracted interest in the field of nanotechnology for their potential for application in areas such as biomedical nanotechnology, [ 2 ] tissue cell culturing, [ 3 ] [ 4 ] molecular electronics , and more. [ 5 ]
Effectively self-assembling peptides act as building blocks for various material and device applications. The essence of this technology is to replicate what nature does: to use molecular recognition processes to form ordered assemblies of building blocks capable of conducting biochemical activities.
Peptides can serve as sturdy building blocks for a wide range of materials as they can be designed to combine with a range of other building blocks such as lipids , sugars, nucleic acids , metallic nanocrystals, and so on; this gives the peptides an edge over carbon nanotubes , which are another popular nanomaterial, as the carbon structure is unreactive. They also exhibit biocompatibility and molecular recognition; the latter is particularly useful as it enables specific selectivity for building ordered nanostructures. Additionally, peptides have superb resistance to extreme temperature, detergents, and denaturants . [ 6 ]
The ability of peptides to perform self-assembly allows them to be used as fabrication tools, which will continue to grow as a fundamental part of nanomaterials production. [ 7 ] The self-assembling of peptides is facilitated through the molecules' structural and chemical compatibility with each other. The structures formed demonstrate physical and chemical stability. [ 6 ]
An advantage to using self-assembling peptides to build nanostructures in a bottom-up approach is that specific features can be incorporated; the peptides can be modified to serve specific functions. This approach means the final structures are made from the self-integration of small, simple building blocks. This approach is needed for nanoscale structure, as the top-down method of miniaturizing devices using sophisticated lithography and etching techniques has reached a physical limit. Moreover, the top-down approach applies mainly to silicon-based technology and cannot be used for biological developments.
The peptide structure is organized hierarchically into four levels. The primary structure of a peptide is the sequence of the amino acids of the peptide chain. Amino acids are monomer molecules that carry a carboxyl and an amine functional group ; a spectrum of other chemical groups are attached to different amino acids, such as thiols and alcohols . This facilitates the wide range of chemical interactions and, therefore, molecular recognitions that peptides are capable of; for designer self-assembling peptides, both natural and non-natural amino acids are used. They link together in a controlled manner to form short peptides, which link to form long polypeptide chains.
Along these chains, the alternating amine (NH) and carbonyl (CO) groups are highly polar, and they readily form hydrogen bonds with each other. These hydrogen bonds bind peptide chains together to give rise to secondary structures. Stable secondary structures include the alpha-helices and beta-sheets. Unstable secondary structures are random loops, turns, and coils that are formed. The secondary structure that is formed depends on the primary structure; different sequences of the amino acids exhibit different preferences.
Secondary structures usually fold, with a variety of loops and turns, into a tertiary structure . What differentiates the secondary structure from the tertiary structure is that the latter includes non-covalent interactions. The quaternary structure combines two or more different chains of polypeptide to form what is known as a protein sub-unit.
The self-assembly process of the peptide chains includes dynamic—reassembly, which occurs repeatedly in a self-healing manner. [ 8 ] The types of interactions that facilitate the reassembly of peptide structures include van der Waals forces , ionic bonds , hydrogen bonds, and hydrophobic forces. [ 8 ] These forces also facilitate the molecular recognition function that the peptides encompass. These interactions work on the basis of preference dependent on energy properties and specificity.
A range of different nanostructures can be formed. Nanotubes are defined as elongated nano-objects with definite inner holes. [ 9 ] [ 10 ] [ 11 ] [ 12 ] Nanofibrils are solid on the inside, as opposed to the hollow nanotubes.
Peptide synthesis can be easily conducted by the established method of solid-phase chemistry in gram or kilogram quantities. The d-isomer conformation can be used for peptide synthesis.
Nanostructures can be made by dissolving dipeptides in 1,1,1,3,3,3- hexafluoro-2-propanol at 100 mg/ml and then diluting it with water for a concentration of less than 2 mg/ml. [ 11 ] Multiwall nanotubes with diameters of 80–300 nm, made of dipeptides from the diphenylalanine motif of Alzheimer's β-amyloid peptide are made by this method. If a thiol is introduced into the diphenylalanine then nano-spheres can be formed instead; nanospheres with diameters of 10–100 nm can also be made this way, from a diphenylglycine peptide. [ 11 ]
Atomic force microscopy can measure the mechanical properties of nanotubes. [ 9 ] [ 10 ] [ 13 ] [ 11 ] Scanning-electron and atomic-forces microscopy are used to examine Lego peptide nanofiber structures. [ 7 ]
Dynamic light scattering studies show structures of surfactant peptides. [ 7 ] Surfactant peptides have been studied using a quick-freeze/deep–etch sample preparation method which minimizes effects on the structure. The sample nanostructures are flash frozen at −196 °C and can be studied three-dimensionally, using Transmission electron microscopy . [ 7 ]
Using computer technology , a molecular model of peptides and their interactions can be built and studied.
Specific tests can be performed on certain peptides: for example, a fluorescent emission test could be applied to amyloid fibrils by using the dye Thioflavin T, which binds specifically to the peptide and emits blue fluorescence when excited. [ 6 ]
The simplest peptide building blocks are dipeptides. Nanotubes formed from dipeptides are the widest among peptide nanotubes. An example of a dipeptide that has been studied is a peptide from the diphenylalanine motif of the Alzheimer's β-amyloid peptide. [ 11 ]
Dipeptides have also been shown to self-assemble into hydrogels , another form of nanostructures, when connected to the protecting group Fluorenylmethyloxycarbonyl chloride . Experiments focusing on the dipeptide Fmoc-Diphenylalanine have been conducted that have explored the mechanism in which Fmoc-diphenylalanine self-assembles into hydrogels via π-π interlocked β-sheets . [ 14 ] Phenylalanine has an aromatic ring, a crucial part of the molecule due to its high electron-density, which favors self-assembly where the rings stack and enable the assembly to occur.
These peptides are approximately 5 nm in size and have 16 amino acids. [ 8 ] The class of Lego peptides has the unique characteristics of having two distinct surfaces being either hydrophobic or hydrophilic, similar to the pegs and holes of Lego blocks. [ 7 ] The hydrophobic side promotes self-assembly in water and the hydrophilic side has a regular arrangement of charged amino-acid residues, which in turn brings about a defined pattern of ionic bonds. [ 7 ] The arrangement of the residues can be classified according to the order of the charges; Modulus I has a charge pattern of + − + − + − , modulus II + + − − + + − − , and modulus III + + + − − − + + + , and so on. [ 7 ] The peptides self-assemble into nanofibers approximately 10 nm long in the presence of alkaline cations or an addition of peptide solution. [ 7 ] The fibers form ionic interactions with each other to form checkerboard-like matrices, which develop into a scaffold hydrogel with a high water content of larger than 99.5–99.9% [ 8 ] and pores of 10–200 nm in diameter. [ 7 ] These hydrogels allow neurite outgrowth and therefore can be used as scaffolds for tissue engineering. [ 15 ]
Surfactant–like peptides that undergo self-assembly in water to form nanotubes and nanovesicles have been designed using natural lipids as guides. [ 7 ] [ 9 ] [ 10 ] [ 13 ] This class of peptides has a hydrophilic head (with one or two charged amino acids such as aspartic or glutamic acids, or lysine or histidine acids) with a hydrophobic tail (with 4 or more hydrophobic amino acids such as alanine, valine, or leucine). The peptide monomers are about 2-3 nm long and consist of seven or eight amino acids; the peptide length can be adjusted by adding or removing acids. [ 16 ]
In water, surfactant peptides undergo self-assembling to form well-ordered nanotubes and nanovesicles of 30–50 nm through intermolecular hydrogen bonds and the packing of the hydrophobic tails in between the residues, [ 7 ] like micelle formation. Transmission electron microscopy examination on quick-frozen samples of surfactant-peptide structures showed helical open-ended nanotubes. The samples also showed dynamic behaviours and some vesicle "buds" sprouting out of the peptide nanotubes. [ 7 ]
This class of peptides undergoes self-assembling on a surface and form monolayers just few nanometers thick. [ 7 ] These types of molecular "paint" or "carpet" peptides are able to form cell patterns, interacting with or trapping other molecules onto the surface. [ 7 ] This class of peptides consists of three segments: the head is a ligand part, which has functional groups attached for recognition by other molecules or cell surface receptors ; the middle segment is a "linker", allows the head to interact at a distance away from the surface [ 7 ] and which also controls the flexibility and the rigidity of the peptide structure; [ 7 ] and, at the other end of the linker, a surface anchor where a chemical group on the peptide forms a covalent bond with a particular surface. [ 7 ] This class of peptides has the unique property of being able to change molecular structure dramatically. [ 7 ] This property is best illustrated using an example. The DAR16-IV peptide, has 16 amino acids and forms a 5 nm β-sheet structure at ambient temperatures; a swift change in structure occurs at high temperature or a change in pH when a 2.5 nm α-helix forms. [ 7 ]
Extensive research has been performed on nanotubes formed by stacking cyclic peptides with an even number of alternating D and L amino acids. [ 11 ] These nanotubes are the narrowest formed by peptides. The stacking occurs through intermolecular hydrogen bonding, and the end product is a cylindrical structure with the amino acid side chains of the peptide defining the properties of the outer surface of the tube [ 11 ] and the peptide backbone determining the properties of the inner surface of the tube. [ 11 ] Polymers can also be covalently attached to the peptides, in which case a polymer shell around the nanotube can be formed. By applying peptide design, the inner diameter, which is completely uniform, can be specified; the outer surface properties can also be affected by peptide design. Therefore, these cyclic nanotubes can form in a range of different environments. [ 11 ]
One should evaluate the properties (mechanical, electronic, optical, magnetic, etc.) of the material that has been chosen and indicate what the major differences would be if the same material were not at nanoscale. Nanotubes formed from dipeptides are stable under extreme conditions. Dry nanotubes do not degrade until 200 °C; nanotubes display exceptional chemical stability at a range of pH and in the presence of organic solvents. This is a marked difference from natural biological systems, which are often unstable and sensitive to temperature and chemical conditions.
Indentation-atomic-force-microscopy experiments showed that dry nanotubes on mica have an average stiffness of 160 N/m and a high Young's modulus of 19–27 GPa. [ 11 ] Although they are less stiff than carbon and non-carbon nanotubes , with these values these nanotubes are amongst some of the stiffest known biological materials. [ 11 ] The mechanisms which facilitates the mechanical stiffness has been suggested to be the intermolecular hydrogen bonds and rigid aromatic side chains on the peptides. [ 11 ] Apart from those made by cyclic peptides, the nanotubes' inner and outer surface properties have not yet been successfully independently modified. [ 11 ] Hence, it presents a limitation that the inner and outer tube surfaces are identical.
Molecular assembly mostly occurs through weak non-covalent bonds, which include: hydrogen bonds, ionic bonds, van der Waals interactions, and hydrophobic interactions .
Carbon nanotubes (CNTs) are another type of nanomaterial that have attracted much interest for their potential to serve as building blocks for bottom-up applications. They have excellent mechanical, electrical, and thermal properties and can be fabricated to a wide range of nanoscale diameters, making them attractive and appropriate for the development of electronic and mechanical devices. [ 17 ] They demonstrate metal-like properties and can act as remarkable conductors.
However, there are several areas where peptides have advantages over CNTs. One advantage is that peptides have almost limitless chemical functionality compared with the very limited chemical interactions that CNTs can perform due to their non-reactiveness. [ 17 ] Furthermore, CNTs exhibits strong hydrophobicity which results in a tendency to clump in aqueous solutions [ 17 ] and therefore have limited solubility; their electrical properties are also affected by humidity, and the presence of oxygen, N 2 O, and NH 3 . [ 11 ]
It is also difficult to produce CNTs with uniform properties and this poses serious drawbacks as the reproducibility of precise structural properties is a key concern for commercial purposes. Lastly, CNTs are expensive, with prices in the range of hundreds of dollars per gram, rendering most applications commercially unviable. [ 17 ]
The appeal of designer peptides is that they are structurally simple and are simple and affordable to produce a large scale. [ 7 ]
Peptide scaffolds formed from LEGO peptides have been used extensively for 3D cell culturing as they closely resemble the porosity and the structure of extra-cellular matrices. [ 3 ] These scaffolds have also been used in cell proliferation and differentiation into desired cell types. [ 7 ] Experimentations with rat neurons demonstrated the usefulness of LEGO peptides in cell culturing. Rat neurons that were attached to the peptides projected functional axons that followed the contours of the peptide scaffolds. [ 7 ]
By examining the behaviours of the molecular 'switch' peptides, more information about interactions between proteins and, more significantly, the pathogenesis of some protein conformational diseases can be obtained. These diseases include scrapie, kuru, Huntington's, Parkinson's and Alzheimer's. [ 7 ]
Self-assembling and surfactant peptides can be used as targeting delivery systems for genes, [ 18 ] drugs [ 19 ] and RNAi. [ 20 ] [ 21 ] Research has already shown that cationic dipeptides NH 2 -Phe-Phe-NH 2 nanovesicles, which are about 100 nm in diameter, can be absorbed into cells through endocytosis and deliver oligonucleotides into the cell; [ 11 ] this is one example of how peptide nanostructure can in used in gene and drug delivery . It is also envisaged that water-soluble molecules and biological molecules would be able to be delivered to cells in this way. [ 11 ] Self-assembling LEGO peptides can form biologically compatible scaffolds for tissue repair and engineering, [ 17 ] which should be of great potential, as a large number of diseases cannot be cured by small molecule drugs; a cell-based therapy approach is needed and peptides could potentially play a huge role in this. [ 17 ] Cyclic peptide nanotubes formed from self-assembly can act as ion channels , which form pores through the cell membrane and cause cellular osmotic collapse. Peptide can be designed to preferentially form on bacterial cell membranes and thus these tubes can perform as antibacterial and cytotoxin agents. [ 11 ] [ 17 ]
Molecular 'switch' peptides can be made into nanoswitches when an electronic component is incorporated. [ 7 ] Metal nanocrystals can be covalently linked to the peptides to make them electronically responsive; research is currently being conducted on how to develop electronically controlled molecules and molecular 'machines' using such molecular 'switches'. [ 7 ] Peptide nanofibers can also be used as growth templates for a range of inorganic materials, such as silver, gold, platinum, cobalt, nickel, and various semiconducting materials. [ 6 ] Electrons transferring aromatic moieties can also be attached to the side chains of peptides to form conducting nanostructures that can transfer electrons in a certain direction. [ 17 ] Metal and semiconductor binding peptides have been used for the fabrication of nanowires. [ 6 ] Peptides self-assemble into hollow nanotubes to act as casting molds; metal ions that migrate inside the tube undergo reduction to metallic form. The peptide 'mold' can then be enzymatically destroyed to produce a metal nanowire of about 20 nm diameter. [ 17 ] This has been done making gold nanowires and this application is especially significant because nanowires at this scale cannot be made by lithography. Researchers have also successfully developed multi-layer nanocables with a silver core nanowire, a peptide insulation layer, and a gold outer coat. [ 11 ] This is done by reducing AgNO 3 inside nanotubes, and then bounding a layer of thiol -containing peptides with gold particles attached. [ 11 ] This layer acts as a nucleation site during the next step, where a process of electroless deposition layers a coating of gold on the nanotubes to form metal-insulator-metal trilayer coaxial nanocables. [ 11 ] Peptide nanotubes are able to produce nanowires of uniform size, and this is particularly useful in the nano-electric applications as electrical and magnetic properties are sensitive to size. [ 11 ]
Nanotubes' exceptional mechanical strength and stability makes them excellent materials for application in this area.
Nanotubes have also been used in developing electrochemical biosensing platforms and have proved to have great potential. Dipeptide nanotubes deposited on graphite electrodes improved electrode sensitivity; thiol-modified nanotubes deposited on gold with a coating of enzymes improved sensitivity and reproducibility for the detection of glucose and ethanol, as well as a shortened detection time, large current density , and improved stability. [ 11 ] Nanotubes have also been successfully coated with proteins, nanocrystals, and metalloporphyrin through hydrogen bonding, and these coated tubes have great potential as chemical sensors. [ 11 ]
Designed peptides with a known structure that would self-assemble into a regular growth template would enable the self-assembly of nanoscale electronic circuits and devices. However, one issue that has yet to be resolved is the ability to control the positioning of the nanostructures. This positioning relative to substrates, to each other, and to other functional components is crucial. Although progress has been made in this domain, more work has to be completed before this control can be established. [ 11 ]
Molecular carpet/paint peptides can be used in diverse industries. They can be used as 'nano-organizers' for non-biological materials, or could be used to study cell-cell communications and behavior. [ 7 ] It has also been found that the catalytic abilities of the lipase enzyme is greatly improved when encapsulated in a peptide nanotube. [ 11 ] After incubation in a nanotube for a week, the catalytic activities of the enzyme is improved by 33%, compared with free-standing lipases at room temperature ; at 65 °C the improvement rises to 70%. It is suggested that the enhanced ability is due to a conformational change to an enzymatically active structure. [ 11 ]
Although well ordered nanostructures have already been successfully formed from self-assembling peptides, their potential will not be fully fulfilled until useful functionality is incorporated into the structures.
Moreover, so far most of the peptide structures formed are in one or two dimensions. In contrast, in nature, most biological structures are in three dimensions. [ 17 ] Criticism has come because there is a lack of theoretical knowledge about the self-assembling behaviours of peptides. Further knowledge could prove to be very useful in facilitating rational designs and precise control of the peptide assemblies.
Lastly, although an extensive amount of work is being conducted on developing self-assembling peptide-related applications, issues such as commercial viability and processability have not been paid the same amount of attention. Yet these issues must be assessed if further useful applications are to be realized. | https://en.wikipedia.org/wiki/Self-assembling_peptide |
Self-assembly is a process in which a disordered system of pre-existing components forms an organized structure or pattern as a consequence of specific, local interactions among the components themselves, without external direction. When the constitutive components are molecules, the process is termed molecular self-assembly .
Self-assembly can be classified as either static or dynamic. In static self-assembly, the ordered state forms as a system approaches equilibrium , reducing its free energy . However, in dynamic self-assembly, patterns of pre-existing components organized by specific local interactions are not commonly described as "self-assembled" by scientists in the associated disciplines. These structures are better described as " self-organized ", although these terms are often used interchangeably.
Self-assembly in the classic sense can be defined as the spontaneous and reversible organization of molecular units into ordered structures by non-covalent interactions . The first property of a self-assembled system that this definition suggests is the spontaneity of the self-assembly process: the interactions responsible for the formation of the self-assembled system act on a strictly local level—in other words, the nanostructure builds itself .
Although self-assembly typically occurs between weakly-interacting species, this organization may be transferred into strongly-bound covalent systems. An example for this may be observed in the self-assembly of polyoxometalates . Evidence suggests that such molecules assemble via a dense-phase type mechanism whereby small oxometalate ions first assemble non-covalently in solution, followed by a condensation reaction that covalently binds the assembled units. [ 4 ] This process can be aided by the introduction of templating agents to control the formed species. [ 5 ] In such a way, highly organized covalent molecules may be formed in a specific manner.
Self-assembled nano-structure is an object that appears as a result of ordering and aggregation of individual nano-scale objects guided by some physical principle.
A particularly counter-intuitive example of a physical principle that can drive self-assembly is entropy maximization. Though entropy is conventionally associated with disorder , under suitable conditions [ 6 ] entropy can drive nano-scale objects to self-assemble into target structures in a controllable way. [ 7 ]
Another important class of self-assembly is field-directed assembly. An example of this is the phenomenon of electrostatic trapping. In this case an electric field is applied between two metallic nano-electrodes. The particles present in the environment are polarized by the applied electric field. Because of dipole interaction with the electric field gradient the particles are attracted to the gap between the electrodes. [ 8 ] Generalizations of this type approach involving different types of fields, e.g., using magnetic fields, using capillary interactions for particles trapped at interfaces, elastic interactions for particles suspended in liquid crystals have also been reported.
Regardless of the mechanism driving self-assembly, people take self-assembly approaches to materials synthesis to avoid the problem of having to construct materials one building block at a time. Avoiding one-at-a-time approaches is important because the amount of time required to place building blocks into a target structure is prohibitively difficult for structures that have macroscopic size.
Once materials of macroscopic size can be self-assembled, those materials can find use in many applications. For example, nano-structures such as nano-vacuum gaps are used for storing energy [ 9 ] and nuclear energy conversion. [ 10 ] Self-assembled tunable materials are promising candidates for large surface area electrodes in batteries and organic photovoltaic cells, as well as for microfluidic sensors and filters. [ 11 ]
At this point, one may argue that any chemical reaction driving atoms and molecules to assemble into larger structures, such as precipitation , could fall into the category of self-assembly. However, there are at least three distinctive features that make self-assembly a distinct concept.
First, the self-assembled structure must have a higher order than the isolated components, be it a shape or a particular task that the self-assembled entity may perform. This is generally not true in chemical reactions , where an ordered state may proceed towards a disordered state depending on thermodynamic parameters.
The second important aspect of self-assembly is the predominant role of weak interactions (e.g. Van der Waals , capillary , π − π {\displaystyle \pi -\pi } , hydrogen bonds , or entropic forces ) compared to more "traditional" covalent, ionic , or metallic bonds . These weak interactions are important in materials synthesis for two reasons.
First, weak interactions take a prominent place in materials, especially in biological systems. For instance, they determine the physical properties of liquids, the solubility of solids, and the organization of molecules in biological membranes. [ 12 ]
Second, in addition to the strength of the interactions, interactions with varying degrees of specificity can control self-assembly. Self-assembly that is mediated by DNA pairing interactions constitutes the interactions of the highest specificity that have been used to drive self-assembly. [ 13 ] At the other extreme, the least specific interactions are possibly those provided by emergent forces that arise from entropy maximization . [ 6 ]
The third distinctive feature of self-assembly is that the building blocks are not only atoms and molecules, but span a wide range of nano- and mesoscopic structures, with different chemical compositions, functionalities, [ 14 ] and shapes. [ 15 ] [ 16 ] Research into possible three-dimensional shapes of self-assembling micrites examines Platonic solids (regular polyhedral). The term 'micrite' was created by DARPA to refer to sub-millimeter sized microrobots , whose self-organizing abilities may be compared with those of slime mold . [ 17 ] [ 18 ] Recent examples of novel building blocks include polyhedra and patchy particles . [ 14 ] Examples also included microparticles with complex geometries, such as hemispherical, [ 19 ] dimer, [ 20 ] discs, [ 21 ] rods, molecules, as well as multimers. These nanoscale building blocks can in turn be synthesized through conventional chemical routes or by other self-assembly strategies such as directional entropic forces . More recently, inverse design approaches have appeared where it is possible to fix a target self-assembled behavior, and determine an appropriate building block that will realize that behavior. [ 7 ]
Self-assembly in microscopic systems usually starts from diffusion, followed by the nucleation of seeds, subsequent growth of the seeds, and ends at Ostwald ripening . The thermodynamic driving free energy can be either enthalpic or entropic or both. [ 6 ] In either the enthalpic or entropic case, self-assembly proceeds through the formation and breaking of bonds, [ 22 ] possibly with non-traditional forms of mediation.
The kinetics of the self-assembly process is usually related to diffusion , for which the absorption/adsorption rate often follows a Langmuir adsorption model which in the diffusion controlled concentration (relatively diluted solution) can be estimated by the Fick's laws of diffusion . The desorption rate is determined by the bond strength of the surface molecules/atoms with a thermal activation energy barrier. The growth rate is the competition between these two processes.
Important examples of self-assembly in materials science include the formation of molecular crystals , colloids , lipid bilayers , phase-separated polymers , and self-assembled monolayers . [ 23 ] [ 24 ] The folding of polypeptide chains into proteins and the folding of nucleic acids into their functional forms are examples of self-assembled biological structures. Recently, the three-dimensional macroporous structure was prepared via self-assembly of diphenylalanine derivative under cryoconditions, the obtained material can find the application in the field of regenerative medicine or drug delivery system. [ 25 ] P. Chen et al. demonstrated a microscale self-assembly method using the air-liquid interface established by Faraday wave as a template. This self-assembly method can be used for generation of diverse sets of symmetrical and periodic patterns from microscale materials such as hydrogels , cells, and cell spheroids. [ 26 ] Yasuga et al. demonstrated how fluid interfacial energy drives the emergence of three-dimensional periodic structures in micropillar scaffolds. [ 27 ] Myllymäki et al. demonstrated the formation of micelles, that undergo a change in morphology to fibers and eventually to spheres, all controlled by solvent change. [ 28 ]
Self-assembly extends the scope of chemistry aiming at synthesizing products with order and functionality properties, extending chemical bonds to weak interactions and encompassing the self-assembly of nanoscale building blocks at all length scales. [ 29 ] In covalent synthesis and polymerization, the scientist links atoms together in any desired conformation, which does not necessarily have to be the energetically most favoured position; self-assembling molecules, on the other hand, adopt a structure at the thermodynamic minimum, finding the best combination of interactions between subunits but not forming covalent bonds between them. In self-assembling structures, the scientist must predict this minimum, not merely place the atoms in the location desired.
Another characteristic common to nearly all self-assembled systems is their thermodynamic stability . For self-assembly to take place without intervention of external forces, the process must lead to a lower Gibbs free energy , thus self-assembled structures are thermodynamically more stable than the single, unassembled components. A direct consequence is the general tendency of self-assembled structures to be relatively free of defects. An example is the formation of two-dimensional superlattices composed of an orderly arrangement of micrometre-sized polymethylmethacrylate (PMMA) spheres, starting from a solution containing the microspheres, in which the solvent is allowed to evaporate slowly in suitable conditions. In this case, the driving force is capillary interaction, which originates from the deformation of the surface of a liquid caused by the presence of floating or submerged particles. [ 30 ]
These two properties—weak interactions and thermodynamic stability—can be recalled to rationalise another property often found in self-assembled systems: the sensitivity to perturbations exerted by the external environment. These are small fluctuations that alter thermodynamic variables that might lead to marked changes in the structure and even compromise it, either during or after self-assembly. The weak nature of interactions is responsible for the flexibility of the architecture and allows for rearrangements of the structure in the direction determined by thermodynamics. If fluctuations bring the thermodynamic variables back to the starting condition, the structure is likely to go back to its initial configuration. This leads us to identify one more property of self-assembly, which is generally not observed in materials synthesized by other techniques: reversibility .
Self-assembly is a process which is easily influenced by external parameters. This feature can make synthesis rather complex because of the need to control many free parameters. Yet self-assembly has the advantage that a large variety of shapes and functions on many length scales can be obtained. [ 31 ]
The fundamental condition needed for nanoscale building blocks to self-assemble into an ordered structure is the simultaneous presence of long-range repulsive and short-range attractive forces. [ 32 ]
By choosing precursors with suitable physicochemical properties, it is possible to exert a fine control on the formation processes that produce complex structures. Clearly, the most important tool when it comes to designing a synthesis strategy for a material, is the knowledge of the chemistry of the building units. For example, it was demonstrated that it was possible to use diblock copolymers with different block reactivities in order to selectively embed maghemite nanoparticles and generate periodic materials with potential use as waveguides . [ 33 ]
In 2008 it was proposed that every self-assembly process presents a co-assembly, which makes the former term a misnomer. This thesis is built on the concept of mutual ordering of the self-assembling system and its environment. [ 34 ]
The most common examples of self-assembly at the macroscopic scale can be seen at interfaces between gases and liquids, where molecules can be confined at the nanoscale in the vertical direction and spread over long distances laterally. Examples of self-assembly at gas-liquid interfaces include breath-figures , self-assembled monolayers , droplet clusters , and Langmuir–Blodgett films , while crystallization of fullerene whiskers is an example of macroscopic self-assembly in between two liquids. [ 35 ] [ 36 ] Another remarkable example of macroscopic self-assembly is the formation of thin quasicrystals at an air-liquid interface, which can be built up not only by inorganic, but also by organic molecular units. [ 37 ] [ 38 ] Furthermore, it was reported that Fmoc protected L-DOPA amino acid (Fmoc-DOPA) [ 39 ] [ 40 ] can present a minimal supramolecular polymer model, displaying a spontaneous structural transition from meta-stable spheres to fibrillar assemblies to gel-like material and finally to single crystals. [ 41 ]
Self-assembly processes can also be observed in systems of macroscopic building blocks. These building blocks can be externally propelled [ 42 ] or self-propelled. [ 43 ] Since the 1950s, scientists have built self-assembly systems exhibiting centimeter-sized components ranging from passive mechanical parts to mobile robots. [ 44 ] For systems at this scale, the component design can be precisely controlled. For some systems, the components' interaction preferences are programmable. The self-assembly processes can be easily monitored and analyzed by the components themselves or by external observers. [ 45 ]
In April 2014, a 3D printed plastic was combined with a "smart material" that self-assembles in water, [ 46 ] resulting in " 4D printing ". [ 47 ]
People regularly use the terms " self-organization " and "self-assembly" interchangeably. As complex system science becomes more popular though, there is a higher need to clearly distinguish the differences between the two mechanisms to understand their significance in physical and biological systems. Both processes explain how collective order develops from "dynamic small-scale interactions". [ 48 ] Self-organization is a non-equilibrium process where self-assembly is a spontaneous process that leads toward equilibrium. Self-assembly requires components to remain essentially unchanged throughout the process. Besides the thermodynamic difference between the two, there is also a difference in formation. The first difference is what "encodes the global order of the whole" in self-assembly whereas in self-organization this initial encoding is not necessary. Another slight contrast refers to the minimum number of units needed to make an order. Self-organization appears to have a minimum number of units whereas self-assembly does not. The concepts may have particular application in connection with natural selection . [ 49 ] Eventually, these patterns may form one theory of pattern formation in nature. [ 50 ] | https://en.wikipedia.org/wiki/Self-assembly |
A self-averaging physical property of a disordered system is one that can be described by averaging over a sufficiently large sample. The concept was introduced by Ilya Mikhailovich Lifshitz .
Frequently in physics one comes across situations where quenched randomness plays an important role. Any physical property X of such a system, would require an averaging over all disorder realisations. The system can be completely described by the average [ X ] where [...] denotes averaging over realisations (“averaging over samples”) provided the relative variance R X = V X / [ X ] 2 → 0 as N →∞, where V X = [ X 2 ] − [ X ] 2 and N denotes the size of the realisation. In such a scenario a single large system is sufficient to represent the whole ensemble. Such quantities are called self-averaging. Away from criticality, when the larger lattice is built from smaller blocks, then due to the additivity property of an extensive quantity , the central limit theorem guarantees that R X ~ N −1 thereby ensuring self-averaging. On the other hand, at the critical point, the question whether X {\displaystyle X} is self-averaging or not becomes nontrivial, due to long range correlations .
At the pure critical point randomness is classified as relevant if, by the standard definition of relevance, it leads to a change in the critical behaviour (i.e., the critical exponents) of the pure system. It has been shown by recent renormalization group and numerical studies that self-averaging property is lost if randomness or disorder is relevant. [ 1 ] Most importantly as N → ∞, R X at the critical point approaches a constant. Such systems are called non self-averaging. Thus unlike the self-averaging scenario, numerical simulations cannot lead to an improved picture in larger lattices (large N), even if the critical point is exactly known. In summary, various types of self-averaging can be indexed with the help of the asymptotic size dependence of a quantity like R X . If R X falls off to zero with size, it is self-averaging whereas if R X approaches a constant as N → ∞, the system is non-self-averaging.
There is a further classification of self-averaging systems as strong and weak. If the exhibited behavior is R X ~ N −1 as suggested by the central limit theorem, mentioned earlier, the system is said to be strongly self-averaging. Some systems shows a slower power law decay R X ~ N − z with 0 < z < 1. Such systems are classified weakly self-averaging. The known critical exponents of the system determine the exponent z .
It must also be added that relevant randomness does not necessarily imply non self-averaging, especially in a mean-field scenario. [ 2 ] The RG arguments mentioned above need to be extended to situations with sharp limit of T c distribution and long range interactions. | https://en.wikipedia.org/wiki/Self-averaging |
A column can buckle due to its own weight with no other direct forces acting on it, in a failure mode called self-buckling . In conventional column buckling problems, the self-weight is often neglected since it is assumed to be small when compared to the applied axial loads . However, when the weight of the column is significant compared to its buckling strength, it is important to take self-buckling into account.
Elastic buckling of a "heavy" column i.e., column buckling under its own weight , was first investigated by Greenhill in 1881 . [ 1 ] He found that a free-standing, vertical column, with density ρ {\displaystyle \rho } , Young's modulus E {\displaystyle E} , and cross-sectional area A {\displaystyle A} , will buckle under its own weight if its height exceeds a certain critical value:
where g {\displaystyle g} is the acceleration due to gravity , and I {\displaystyle I} is the second moment of area of the beam cross section.
One interesting example for the use of the equation was suggested by Greenhill in his paper. He estimated the maximal height of a pine tree , and found it cannot grow over 300 feet (90 m) tall if limited to a trunk diameter of 20 inches. This length sets the maximum height for trees on earth if we assume the trees to be prismatic and the branches are neglected.
Suppose a uniform column fixed in a vertical direction at its lowest point, and carried to a height l {\displaystyle l} , in which the vertical position becomes unstable and flexure begins. There is a body force q {\displaystyle q} per unit length q = ρ g A {\displaystyle q=\rho gA} , where A {\displaystyle A} is the cross-sectional area of the column, g {\displaystyle g} is the acceleration due to gravity and ρ {\displaystyle \rho } is its mass density.
The column is slightly curved under its own weight, so the curve w ( x ) {\displaystyle w(x)} describes the deflection of the beam in the y {\displaystyle y} direction at some position x {\displaystyle x} . Looking at any point on the column, we can write the moment equilibrium:
where the right-hand side of the equation is the moment of the weight of BP about P.
According to Euler–Bernoulli beam theory :
Where E {\displaystyle E} is the Young's modulus of elasticity of the substance, I {\displaystyle I} is the second moment of area.
Therefore, the differential equation of the central line of BP is: {\displaystyle } {\displaystyle }
Differentiating with respect to x, we get
We get that the governing equation is the third order linear differential equation with a variable coefficient. The way to solve the problem is to use new variables n , z , k {\displaystyle n,z,k} and r {\displaystyle r} :
Then, the equation transforms to the Bessel equation
The solution of the transformed equation is z = A J 1 3 ( k r ) + B J − 1 3 ( k r ) {\displaystyle z=AJ_{\frac {1}{3}}(kr)+BJ_{-{\frac {1}{3}}}(kr)}
Where J n {\displaystyle J_{n}} is the Bessel function of the first kind. Then, the solution of the original equation is:
Now, we will use the boundary conditions :
From the second boundary condition, we get that the critical length in which a vertical column will buckle under its own weight is:
Using j 1 3 , 1 ≈ 1.86635 {\displaystyle j_{{\frac {1}{3}},1}\approx 1.86635} , the first zero of the Bessel function of the first kind of order − 1 3 {\displaystyle -{\frac {1}{3}}} , l max {\displaystyle l_{\text{max}}} can be approximated to:
The column under its own weight was considered by Euler in three famous papers (1778a, 1778b, 1778c). [ 2 ] [ 3 ] [ 4 ] In his first paper, Euler (1778a) concluded that the column simply supported under its own weight would never lose its stability. In his second paper on this topic Euler (1778b) described his previous result as paradoxical and suspicious (see Panovko and Gubanova (1965); Nicolai, (1955); [ 5 ] Todhunter and Pierson (1866) [ 6 ] on this topic). In the next, third in series, paper, Euler (1778c) found that he had made a conceptual mistake and the “infinite buckling load” conclusion was proved to be wrong. Unfortunately, however, he made a numerical mistake and instead of the first eigenvalue, he calculated a second one. Correct solutions were derived by Dinnik (1912), [ 7 ] 132 years later, as well as Willers (1941), [ 8 ] Engelhardt (1954) [ 9 ] and Frich-Fay (1966). [ 10 ] Numerical solution with arbitrary accuracy was given by Eisenberger (1991). [ 11 ]
222 years after Euler's mistake in 1778, Elishakoff [ 12 ] [ 13 ] revisited this problem and derived closed-form solutions for self-buckling problems. [ 14 ] | https://en.wikipedia.org/wiki/Self-buckling |
A self-cleaning floor is a surface that has the ability to clean itself without external action. This is usually accomplished through automated pods set to dispense water regularly, but can also be accomplished through the use of materials that naturally reduce pathogens. Such floors are designed to stay hygienic with little maintenance, and are most often installed in places that require constant maintenance and cleaning, such as hospitals and washrooms. In Europe and a few African countries, many ceramic tiles and sanitary ware manufacturers have products on the market with self-cleaning features. [ 1 ]
The Grabo Silver Knight resilient floor covering is the first photocatalytic self-disinfecting surface. Its self-disinfecting property is accomplished through the use of Nano-Silver and Nano-TiO 2 particles. This product is targeted towards the health sector, aiding in preventing the spread of infection in hospitals. [ 2 ] [ 3 ]
The Cozy Floor is a self-supported, self-cleaning hot water heated floor system, designed to eliminate the use of bedding under calves. [ 4 ]
The SunWash self-cleaning floor coatings provide a washable finish and are developed to withstand high traffic and humid conditions in food processing facilities. [ 5 ]
Mechline Developments’ Sani-Floor consists of a suction pump and automatic waste lifting. The system is intended for use in any food production area to take care of spills hygienically and safely. [ 6 ]
Many public toilets, such as the Sanisette , have utilized self-cleaning floor mechanisms.
The Smart-Floor is a concept designed by Svetozar Belogrozdev of Swansea Metropolitan University . It is a self-cleaning floor design that is intended to prevent dust and dirt from settling. Regulated vacuums cycle through the floor constantly while intelligent pressure sensors detect if there is anything in the room and automatically regulate the vacuum going through the tiles. The Smart-Floor is one of the six UK designs that won the top 100 designs in Electrolux Design Lab 2013. [ 7 ]
Another approach to self-cleaning floors involves the use of central vacuum systems beneath micro-perforated raised floor tiles . These systems create a small negative pressure to automatically collect all dust and dirt. | https://en.wikipedia.org/wiki/Self-cleaning_floor |
Self-cleaning glass is a specific type of glass with a surface that keeps itself free of dirt and grime.
The field of self-cleaning coatings on glass is divided into two categories: hydrophobic and hydrophilic .
These two types of coating both clean themselves through the action of water, the former by rolling droplets and the latter by sheeting water that carries away dirt.
Hydrophilic coatings based on titania (titanium dioxide), however, have an additional property: they can chemically break down absorbed dirt in sunlight.
The requirements for a self-cleaning hydrophobic surface are a very high static water contact angle θ, the condition often quoted is θ>160°, and a very low roll-off angle, i.e. the minimum inclination angle necessary for a droplet to roll off the surface. [ 1 ]
Several techniques are known for the patterning of hydrophobic surfaces through the use of moulded polymers and waxes , by physical processing methods such as ion etching and compression of polymer beads, and by chemical methods such as plasma-chemical roughening, which can all result in ultra-hydrophobic coatings. [ 2 ] While these surfaces are effective self-cleaners, they suffer from a number of drawbacks which have so far prevented widespread application. Batch processing a hydrophobic material is a costly and time-consuming technique, and the coatings produced are usually hazy, precluding applications on lenses and windows, and fragile materials.
The second class of self-cleaning surfaces are hydrophilic surfaces which do not rely solely on the flow of water to wash away dirt. These coatings chemically break down dirt when exposed to light, a process known as photocatalysis . Despite the commercialization of a hydrophilic self-cleaning coating in a number of products, the field is far from mature; investigations into the fundamental mechanisms of self-cleaning and characterizations of new coatings are regularly published in the primary literature.
The first self-cleaning glass was based on a thin film titania coating. [ 3 ] The film can be applied by spin coating of organo-titanate chelated precursor (for example titanium iso-tetrapropoxide chelated by acetylacetone), followed by heat treatment at elevated temperatures to burn the organic residues and to form the anatase phase. In that case, sodium might diffuse from the glass into the nascent titanium dioxide, causing a degradation in the hydrophilic/catalytic effect [ 4 ] unless preventive measures are taken. The glass cleans itself in two stages. The " photocatalytic " stage of the process breaks down the organic dirt on the glass using ultraviolet light and makes the glass superhydrophilic (normally glass is hydrophobic ). During the following "superhydrophilic" stage, rain washes away the dirt, leaving almost no streaks, because water spreads evenly on superhydrophilic surfaces. [ 5 ]
In 2001, Pilkington Glass announced the development of the first self-cleaning windows, Pilkington Activ™, and in the following months several other major glass companies released similar products. As a result, glazing is perhaps the largest commercial application of self-cleaning coatings to date. All of these windows are coated with a thin transparent layer of titanium dioxide . This coating acts to clean the window in two stages, using two distinct properties: photocatalysis and hydrophilicity . In sunlight, photocatalysis causes the coating to chemically break down organic dirt adsorbed onto the window. When the glass is wet by rain or other water, hydrophilicity reduces contact angles to very low values, causing the water to form a thin layer rather than droplets, and this layer washes dirt away.
Titanium dioxide has become the material of choice for self-cleaning windows, and hydrophilic self-cleaning surfaces in general, because of its favorable physical and chemical properties. [ citation needed ] Not only is titanium dioxide highly efficient at photocatalysing dirt in sunlight and reaching the superhydrophilic state, it is also non-toxic, chemically inert in the absence of light, inexpensive, relatively easy to handle and deposit into thin films and is an established household chemical that is used as a pigment in cosmetics and paint and as a food additive. [ 6 ]
The metastable anatase phase is generally considered to be the most photocatalytic among the polymorphic structures of titanium, possibly as the result of a typically higher specific surface area. [ 7 ] Moreover, ultraviolet irradiation creates surface oxygen vacancies at bridging sites, resulting in the conversion of relevant Ti 4+ sites to Ti 3+ sites which are favourable for dissociative water adsorption. [ 8 ] These defects presumably influence the affinity to chemisorbed water of their surrounding sites, forming hydrophilic domains, whereas the rest of the surface remains oleophilic. Hydrophilic domains are areas where dissociative water is adsorbed, associated with oxygen vacancies that are preferentially photogenerated along the [001] direction of the (110) plane; the same direction in which oxygen bridging sites align. [ 9 ]
Other possible application areas are computer monitors and PDA screens, where fingerprints are undesirable. [ 10 ]
Titanium dioxide–based glass cannot decompose thick non-transparent deposits, such as paint or silicone , waterstop fingerprints or bleeding after weathering, or stucco dust produced during construction. [ 11 ]
Since 2001 the TC24 "Coatings on Glass" committee International Commission on Glass has been trying to set up test methods for evaluation of photocatalytic self-cleaning coatings on glass. [ 12 ] | https://en.wikipedia.org/wiki/Self-cleaning_glass |
A self-cleaning or pyrolytic oven is an oven which uses high temperature (approximately 932 °F (500 °C)) to burn off leftovers from baking using pyrolysis , which uses no chemical agents. The oven can be powered by domestic (non-commercial) electricity or gas.
Self-cleaning pyrolytic ovens reduce food soiling to ash with exposure to temperature around 932 °F (500 °C). The oven walls are coated with heat- and acid-resistant porcelain enamel .
A self-cleaning oven is designed to stay locked until the high temperature process is completed. To prevent possible burn injuries, a mechanical interlock is used to keep the oven door locked and closed during and immediately after the high-temperature cleaning cycle, which lasts approximately three hours. Usually, the door can be opened after the temperature cools to approximately 600 °F (316 °C). [ 1 ]
Self-cleaning ovens usually have more insulation than standard ovens to reduce the possibility of fire . The insulation also reduces the amount of energy needed for normal cooking. [ 2 ]
Self-cleaning ovens are considered more convenient and time-saving, and therefore more cost-effective. However, because of the high temperatures, they produce smoke and require a high amount of energy. A typical 150-minute cycle will require more than 3 kWh of electricity. [ 3 ] According to most professionals, the triggering of smoke alarms can be avoided by regular usage of the self-cleaning program. [ 4 ]
Catalytic "continuous clean" ovens rely on high-metal porous enamels to catalyze the reduction of oils to ash at normal cooking temperatures. The walls of catalytic self-cleaning ovens are coated with materials acting as oxidation catalysts , usually in the form of catalyst particles in a binder matrix. Cerium(IV) oxide is one of the common materials used. Other possibilities are copper , vanadium , bismuth , molybdenum , manganese , iron , nickel , tin , niobium , chromium , tungsten , rhenium , platinum , cobalt , and their oxides, either alone or in mixtures. Highly active coatings typically contain a copper oxide , manganese oxide or cobalt oxide , and copper and manganese oxides are often used together. The binder may be a fluoropolymer or an enamel frit . [ 5 ] In the 1990s, SRI International performed a study for Whirlpool Corporation , and changed the composition and application of the porcelain enamel surface found in ovens to one with low ionic content, and a film that makes fat into water-soluble esters. [ 6 ]
Another alternative to self cleaning ovens is steam cleaning ovens. It uses water with lower temperatures to clean the oven. [ citation needed ] | https://en.wikipedia.org/wiki/Self-cleaning_oven |
Self-cleaning surfaces are a class of materials with the inherent ability to remove any debris or bacteria from their surfaces in a variety of ways. The self-cleaning functionality of these surfaces are commonly inspired by natural phenomena observed in lotus leaves, gecko feet, and water striders to name a few. The majority of self-cleaning surfaces can be placed into three categories:
The first instance of a self-cleaning surface was created in 1995. [ 1 ] Paz et al. created a transparent titanium dioxide (TiO 2 ) film that was used to coat glass and provide the ability for the glass to self-clean. The first commercial application of this self-cleaning surface, Pilkington Activ, was developed by Pilkington glass in 2001. This product implements a two-stage cleaning process. The first stage consists of photocatalysis of any fouling matter on the glass. This stage is followed by the glass becoming superhydrophilic and allowing water to wash away the catalyzed debris on the surface of the glass. Since the creation of self-cleaning glass, titanium dioxide has also been used to create self-cleaning nanoparticles that can be incorporated into other material surfaces to allow them to self-clean. [ 2 ]
The ability of a surface to self-clean commonly depends on the hydrophobicity or hydrophilicity of the surface. Whether cleaning aqueous or organic matter from a surface, water plays an important role in the self-cleaning process. Specifically, the contact angle of water on the surface is an important characteristic that helps determine the ability of a surface to self-clean. This angle is affected by the roughness of the surface and the following models have been developed to describe the "stickiness" or wettability of a self-cleaning surface.
Young and colleagues proposed Young's model of wetting that relates the contact angle of a water droplet on a flat surface to the surface energies of the water, the surface, and the surrounding air. This model is typically an oversimplification of a water droplet on an ideally flat surface. This model has been expanded upon to consider surface roughness as a factor in predicting water contact angle on a surface. Young's model is described by the following equation:
c o s ( θ 0 ) = ( γ S A − γ S L γ L A ) {\displaystyle cos(\theta _{0})=\left({\frac {\gamma _{SA}-\gamma _{SL}}{\gamma _{LA}}}\right)} [ 3 ]
θ 0 {\displaystyle \theta _{0}} = Contact angle of water on the surface
γ S A {\displaystyle \gamma _{SA}} = Surface energy of the surface-air interface
γ S L {\displaystyle \gamma _{SL}} = Surface energy of surface-liquid interface
γ L A {\displaystyle \gamma _{LA}} = Surface energy of liquid-air interface
When a water droplet is on a surface that is not flat and the surface topographical features lead to a surface area that is larger than that of a perfectly flat version of the same surface, the Wenzel model is a more accurate predictor of the wettability of this surface. Wenzel's model is described by the following equation:
c o s ( θ ) = R f c o s ( θ 0 ) {\displaystyle cos(\theta )=R_{f}cos(\theta _{0})} [ 3 ] [ 4 ]
θ {\displaystyle \theta } = Contact angle of water predicted by Wenzel's model
R f {\displaystyle R_{f}} = Ratio of surface area of rough surface to the surface area of a flat projection of the same surface
For more complex systems that are representative of water-surface interactions in nature, the Cassie-Baxter model is used. This model takes into consideration the fact that a water droplet may trap air between itself and the surface that it is on. The Cassie-Baxter model is described by the following equation:
c o s ( θ C B ) = R f c o s ( θ 0 ) − f L A ( R f c o s ( θ 0 ) + 1 ) {\displaystyle cos(\theta _{CB})=R_{f}cos(\theta _{0})-f_{LA}(R_{f}cos(\theta _{0})+1)} [ 3 ] [ 5 ]
θ C B {\displaystyle \theta _{CB}} = Contact angle of water predicted by Cassie-Baxter's model
f L A {\displaystyle f_{LA}} = Liquid-air fraction, the fraction of the liquid droplet that is in contact with air
Control over surface wettability is a critical aspect of self-cleaning surfaces. Both superhydrophobic and superhydrophilic surfaces have been used as self-cleaning materials.
Superhydrophobic surfaces can be created in a number of different ways including plasma or ion etching, crystal growth on a material surface, and nanolithography to name a few. [ 6 ] All of these processes create nano-topographical features which imbue a surface with superhydrophobicity. The ultimate goal in developing superhydrophobic surfaces is to recreate the self-cleaning properties of the Lotus Leaf that has the inherent ability to repel all water in nature. The basis for superhydrophobic self-cleaning is the ability of these surfaces to prevent water from spreading out when in contact with the surface. This is reflected in a water contact angle nearing 180 degrees. Superhydrophobic self-cleaning surfaces also have low sliding angles which allows for water that is collected on the surface to easily be removed, commonly by gravity. While superhydrophobic surfaces are great for removing any water-based debris, these surfaces likely will not be able to clean away other types of fouling matter such as oil.
Superhydrophilicity allows for surfaces to clean away a wide variety of dirt or debris. This mechanism is very different than the aforementioned superhydrophobic surfaces. For superhydrophilic self-cleaning surfaces, cleaning occurs because water on the surface is able to spread out to a great degree (extremely low water contact angle) to get between any fouling debris and the surface to wash away the debris.
One of the most commonly used self-cleaning products, titanium dioxide , utilizes a unique self-cleaning mechanism that combines an initial photocatalytic step and subsequent superhydrophilicity. A titanium dioxide coating, typically on glass windows, when exposed to UV light, will generate free electrons that will interact with oxygen and water in the air to create free radicals. These free radicals will in turn breakdown any fouling organic matter deposited on the surface of the glass. Titanium dioxide also changes the normally hydrophobic glass to a superhydrophilic surface. Thus, when rainfall occurs, instead of water beading up on the window surface and instantly falling down the glass, rain drops will rapidly spread out on the hydrophilic surface. The water will then move down the surface of the window, as a film rather than a droplet, essentially acting like a squeegee to remove surface debris.
Heating of the surfaces via passing current through a conductive transparent film has been shown to repel and remove contamination. It has been used in inkjet printers to reduced ink contamination on sensor windows. [ 7 ]
Cleaning surfaces in environments without water has been a challenge. Electric curtain devices were designed to remove particles by creating electric fields on the surface and carrying away particles due to their charged nature. It has been used in solar panels as well as 3D printers. [ 8 ]
The lotus flower has been known as a symbol of purity in some Asian cultures. [ 9 ] The lotus leaves (Nelumbo nucifera) are water-repellent and poorly adhesive which keep them free from contamination or pollution even being immersed in dirty water. This ability, called self-cleaning, shields the plant from dirt and pathogens and plays a vital role in providing resistance towards invading microbes. Indeed, numerous spores and conidia of pathogenic life forms, mainly fungi, need water for germination and taint leaves within the first signs of water. [ 10 ] It had been a curiosity how lotus flower could remain clean even in muddy water, until German botanists, Barthlott and Neinhuis, introduced the unique dual structure of leaves with the help of a scanning electron microscopy (SEM). [ 11 ] [ 12 ] Papillose epidermis cells carpet the exterior of a plant, particularly the leaf. These cells generate papillae or microasperties which make surface very rough. On top of microscale roughness, the papillae surface is superimposed with nanoscale asperities consisting of three-dimensional (3-D) hydrophobic hydrocarbons: epicuticular waxes. Basically, the plant cuticle is a composite material composed of a network of cutin and low surface energy waxes, designed at different hierarchical levels. [ 13 ] [ 14 ] [ 15 ] The various leveled surface of lotus leaves is made out of convex cells (looks like bumps) and a much smaller layer of waxy tubules. [ 16 ] The water beads on plant leaves rest on the apex of the nanofeatures since air is enclosed in the valley of convex cells which minimizes the contact area of water droplet. Hence, the lotus leaves represent remarkable superhydrophobicity. Static contact angle and contact angle hysteresis of the lotus leaf are determined around 164° and 3°, respectively. [ 17 ] With small tilting angles, water droplets on the leaf roll off and take any dirt or contaminant along, leading to self-cleaning. [ 18 ] The ability of drops to form and roll off, depends not only on hydrophobicity, but also on contact angle hysteresis .
In plants world, the lotus leaf is not the only example of natural superhydrophobic surfaces. For instance, taro (Colocasia esculenta) leaves were found to exhibit self-cleaning behavior, too. [ 19 ] They have a binary roughness built up by averagely 10 μm elliptical protrusions in diameter and nano-sized pins. India canna (Cannageneralis bailey) leaves and the rice leaves (whatever the kind of rice) also represent superhydrophobicity, arising from the hierarchical surface morphology. [ 20 ]
The Nepenthes carnivorous pitcher, widespread in a lot of countries such as India, Indonesia, Malaysia and Australia, possesses a superhydrophilic surface, on which wetting angle approaches to zero to create uniform water film. Therefore, it increases the slipperiness of the surface and the prey slides off from its rims (peristome). [ 21 ] Surface topography of Nepenthes rim demonstrates multiple scale radial ridges. The second order ridges are quite small in size and generated by straight rows of overlapping epidermidis cells. The surface of epidermidis cells are smooth and wax-free. The absence of wax crystals and microscopic roughness enhance the hydrophilicity and capillary forces, in doing so, water can swiftly wet the surface of rim. [ 22 ]
Butterfly wings possess not only ultra-hydrophobic trait but also directional adhesive characteristics. If the water bead is along the radial outward (RO) direction from the body’s central axis, it rolls off and cleans the dirt away, leading to self-cleaning. On the other hand, if droplets stand against the opposite direction, they are pinned at the surface, leading adhesion and securing the flight stability of the butterfly by preventing deposit of dirt on the wings near the center of the body. SEM micrographs of wings exhibit hierarchy along the RO direction, arising from aligned microgrooves, covered by fine lamella-stacking nanostripes. [ 23 ]
Water striders ( Gerris remigis ) , most commonly called Jesus bugs, have an extraordinary ability that lets them walk on the water. In a fashion similar to superhydrophobic plants, their legs are highly water repellent due to their hierarchical morphology. They are built up with hydrophobic waxy microhairs, microsetae, and each hair is covered with nanogrooves. As a result, air is entrapped between micro- and nanohairs, which repels water. [ 24 ] Feng et al. measured how deep the leg can dip into water and the contact angle of the leg. They found the contact angle at least 168° and the maximum depth reported 4.38 ± 0.02 mm. [ 25 ]
Gecko feet are the most famous reversible adhesion mechanism in nature. The anti-fouling ability of feet allows geckos to run on dusty ceilings and corners without the accumulation of dirt on their feet. In 2000, Autumn et al. revealed the origin of gecko’s strong adhesion by investigating the surface features of the toes under electron microscope. [ 26 ] They observed a hierarchical morphology of each foot which is composed of millions of small hair called setae. Moreover, each setae is composed of a smaller hair, and each hair is tailed with a flat spatula and these spatulae are bonded by the van der Waals forces. This surface feature, regardless of the surface type (hydrophobic, hydrophilic, dry, wet, rough etc.), enables geckos to stick the surface. In addition to strong adhesion, the gecko foot has a unique self-cleaning property which does not require water as the lotus leaf. [ 27 ]
Shark skin is another example of antifouling, self-cleaning and low adhesion surfaces. This hydrophobic surface allows sharks to maneuvers fast in water. Shark skin is composed of periodically arranged diamond-shape dermal denticles, superimposed with triangular riblets. [ 28 ]
To fabricate synthetic self-cleaning surfaces, there are a variety of methods [ 10 ] used to obtain the desired nanotopography and then characterize surface nanostructure and wettability.
Templating utilizes a mold to add nanostructure to a polymer. [ 29 ] Molds can come from a variety of sources including natural sources, such as the lotus leaf, due to their self-cleaning properties.
Nanocasting is a method based on soft lithography which uses elastomeric molds to make nano-structured surfaces. For example, polydimethylsiloxane (PDMS) was cast over the lotus leaf and used to make a negative PDMS template. PDMS was then coated with an anti-stick monolayer of trimethylchlorosilane and used to make a positive PDMS template from the first. As the natural lotus leaf structure enables pronounced self-cleaning ability, this templating technique was able to replicate the nanostructure, resulting in a surface wettability similar to the lotus leaf. [ 30 ] Further, the ease of this methodology enables translation to mass replication of nano-structured surfaces.
Imprint nanolithography also utilizes templates, pressing a hard mold into a polymer above the polymer glass transition temperature (Tg). Thus, the driving forces for this type of fabrication are heat and high pressure. [ 29 ] Porous templates consisting of aluminum with anodized aluminum oxide (a hard mold) were used to imprint polystyrene. To achieve this, the polystyrene was heated well above its Tg to 130 degrees Celsius and pressed against the template. The template was then removed by dissolving the aluminum and producing either nanoemboss or nanofiber surfaces. Increasing the aspect ratio of the nanofibers disrupted the uniform hexagonal pattern and caused the fibers to form bundles. Ultimately, the longest nanofibers resulted in the greatest surface roughness, which significantly decreased surface wettability. [ 31 ]
Similar to imprint nanolithography, capillary nanolithography employs a patterned elastomeric mold. However, instead of utilizing high pressure, when the temperature is raised above the Tg, capillary forces enable the polymer to fill the voids within the mold. Suh and Jon used molds made from poly(urethane acrylate) (PUA). These were placed on spin coated, water-soluble polymer, polyethylene glycol (PEG), which was raised above PEG's Tg. This study found that the addition of nanotopography increased the contact angle, and this increase was dependent on the height of the nanotopography. [ 32 ] Often, this technique produces a meniscus on the tip of the protruding nanostructures, characteristic of capillary action. [ 33 ] The mold can later be dissolved away. [ 29 ] Combinatorial lithography approaches are also used. One study used capillarity to fill PDMS molds with PUA, first partially curing the polymer resin with UV light. After microstructures were formed, pressure was applied to fabricate nanostructures, and UV curing was used again. This study is a good example of the use of hierarchical structures to increase surface hydrophobicity. [ 34 ]
Photolithography and X-ray lithography have been used to etch substrates, often silicon. [ 35 ] A resist, or photosensitive material, is coated onto a substrate. A mask is applied above the resist that often consists of gold or other compounds that absorb X-rays. The region exposed to light either becomes soluble in a photoresist developer (e.g. radical species) or insoluble in a photoresist developer (e.g. crosslinked species), ultimately resulting in a patterned surface. X-ray sources are beneficial over UV-visible light sources as the shorter wavelengths enable production of smaller features.
Plasma treatment of surfaces is essentially a dry etching of the surface. This is achieved by filling a chamber with gas, such as oxygen, fluorine, or chlorine, and accelerating ions species from an ion source through plasma. The ion acceleration towards the surface forms deep grooves within the surface. In addition to the topography, plasma treatment can also provide surface functionalization by using different gases to deposit different elements on surfaces. [ 29 ] Surface roughness is dependent on the duration of plasma etching. [ 36 ]
Generally, chemical deposition uses liquid or vapor phases to deposit inorganic materials or halides onto surfaces as thin films. [ 37 ] Reagents are supplied in the appropriate stoichiometric amounts to react on the surface. Types of chemical deposition include chemical vapor deposition , chemical bath deposition , and electrochemical deposition. These methodologies produce thin crystalline nanostructures. [ 29 ] For example, brucite-type cobalt hydroxide crystalline surfaces were produced by chemical bath deposition and coated with lauric acid. These surfaces had individual nanofiber tips with diameters of 6.5 nm, ultimately resulting in a contact angle as high as 178 degrees. [ 38 ]
SEM is used to examine morphology of fabricated surfaces, enabling the comparison of natural surfaces [ 18 ] with synthetic surfaces. The size of nanotopography can be measured. [ 38 ] [ 35 ] To prepare samples for SEM, surfaces are often sputter coated using platinum, gold/palladium, or silver, which reduces sample damage and charging and improves edge resolution.
As described above, contact angle is used to characterize surface wettability. A droplet of solvent, typically water for hydrophobic surfaces, is placed perpendicular to the surface. The droplet is imaged and the angle between the solid/liquid and liquid/vapor interfaces is measured. Samples are considered to be superhydrophobic when the contact angle is greater than 150 degrees. [ 9 ] Refer to section on Wenzel and Cassie-Baxter models for information on the different behaviors of droplets on topographical surfaces. For drops to roll effectively on a superhydrophobic surface, Contact angle hysteresis is an important consideration. Low levels of contact angle hysteresis will enhance the self-cleaning effect of a superhydrophobic surface.
Atomic-force microscopy is used to study the local roughness and mechanical properties of a surface. AFM is also used to characterize adhesion and friction properties for micro- and nano-patterned superhydrophobic surfaces. Results can be used to fit a curve to the surface topography and determine the radius of curvature of nanostructures. [ 39 ]
Biomimicry is the imitation, or mimicry, of biological systems, models, or structures, in synthetic areas. Oftentimes, biological materials can produce structures, that have properties and qualities far exceeding what synthetic materials can achieve. Biomimicry is being used to create comparable properties in synthetic materials, particularly in wettability and self-cleaning abilities of self-cleaning surfaces.
There are several biological surfaces that have superhydrophobic properties far superior to any synthetic materials: lotus leaves, rice leaves, cicadia wings, and butterfly wings.
Researchers have been using carbon nanotubes (CNTs) to mimic the papillae of lotus leaves . CNT nanoforests can be made using chemical vapor deposition techniques. [ 40 ] CNT’s can be applied on a surface to modify its water contact angle. Lau et al. created vertical CNT forests with a polytetrafluroethylene (PTFE) coating that was both stable and superhydrophobic with an advancing and receding contact angle of 170° and 160°. [ 41 ] Jung and Bhushan have created a superhydrophobic surface by spray coating CNTs with an epoxy resin. [ 42 ] The spacing and alignment of the CNTs have been shown to impact the degree of hydrophobicity a surface has. Sun et al., have found that CNTs aligned vertically with a medium spacing display the best hydrophobic properties. [ 43 ] Small and large spacing shows increased drop spreading, while horizontal orientation may even display hydrophilic properties.
Glass silica beads in an epoxy resin, [ 44 ] and the electrochemical deposition of gold into dendritic structures [ 43 ] has also created synthetic biomimetic surfaces similar to lotus leaves.
Carbon nanotubes have also been used to create surfaces similar to rice leaves. [ 43 ] Similar to the lotus leaf, a hierarchical structure provides the hydrophobicity of rice leaf. [ 40 ] Unlike the lotus leaf, rice leaves have an anisotropic structure. [ 45 ] When CNT’s are made to mimic rice leaf papillae patterns, the contact angle to differ along the CNT direction or perpendicular. Sun et al. observed anisotropic dewetting of this CNT film. [ 43 ] They then hypothesized and tested a three-dimensional anisotropic CNT array, which in fact exhibited anisotropic dewetting depending on the CNT spacing. [ 46 ]
Cicadia wings have a surface of hexagonally close packed nanopillars that have been shown to have self-cleaning properties. [ 40 ] Similarly templated nanopatterned silica arrays have been shown to have hydrophobic, anti-reflective, and self-cleaning properties. [ 40 ] [ 47 ] These silica arrays begin as non-close packed monolayers, and are patterned in a series of etching steps involving chlorine and oxygen reactive ion etching, and a hydrofluoric acid wash. [ 40 ] These properties have implicated that this surface pattern may prove to be useful in solar cell applications. [ 40 ] Biomimetic materials based on the cicadia wing have also been made from polytetrafluoroethylene films with carbon/epoxy supports treated with argon and oxygen ion beams. [ 48 ] A nanoimprint patterned surface based on the cicadia wings has been made by electrochemically templating and aluminum sheet with alumina oxide, and using this template to pattern a polymer surface. [ 31 ]
Butterfly wings also exhibit anisotropic self-cleaning, superhydrophobic properties. The butterfly wings exhibit anisotropy on a one dimensional level, compared to the other biological materials, which exhibited the anisotropy on a two dimensional level. [ 40 ] Butterfly wings are composed of overlapping layers of scales, that have the best self-cleaning properties in the radial directions. [ 40 ] This anisotropic interface my prove important for fluid controllable interfaces. [ 40 ] Alumina layers patterned from the original butterfly wing have been used to mimic the structure and properties of the butterfly wings. [ 49 ] Additionally, butterfly wing mimetic structures have used to fabricate anatase titania photoanodes. [ 50 ] Butterfly wing structures have also been made using layer-by-layer sol-gel-based deposition [ 51 ] and soft lithography molding. [ 28 ]
Gecko feet are hydrophobic, but that is not the only property that assists in their self-cleaning nature. Estrada and Lin created polypropylene, polyethylene, and polycaprolactone nanofibers using a porous template. [ 52 ] These nanofiber geometries were shown to be self-cleaning in fiber dimensions of 5, 0.6, and 0.2 microns. [ 52 ] However, a hydrophobic surface alone does not explain the perpetually clean toe pad of the gecko, even in dry environments, where water is not available for self-cleaning. This resulting fouling is a common problem for reversible adhesives modeled after the gecko toe pad. Digital hyperextension, or a movement of the toe with each gecko step, contributes to the self-cleaning. [ 53 ] A surface or system that mimics this dynamic self-cleaning process has yet to be developed.
Snail shell is an aragonite-protein composite, with a hierarchical groove structure. [ 40 ] The regular roughness of the structure creates a hydrophilic structure, a thin layer of water trapped on the surface, that doesn’t allow oil to attach to the snail shell, thereby keeping the shell clean. These surface properties of snail shell have inspired the use of similar surface patterns on ceramic tiles and ceramic structures by the INAX corporation, which applies these techniques to kitchens and bathrooms. [ 40 ]
Fish scales are calcium phosphate composites coated with a mucus layer. [ 40 ] Fish scale properties have been mimicked by polyacrylamide hydrogels, which are both hydrophilic and mimic the mucus’ retention of water. [ 40 ] Additionally, fish scales have been used as a template for a casting technique, and as a model for a lithography and chemical etching techniques on silicon wafers that exhibited oleophobic contact angles of oil in water of 163° and 175°, respectively. [ 40 ] [ 54 ]
Molded and laser-ablated shark skin replicas have been fabricated, and shown to be oleophobic in water. The molded replicas use a negative made of polyvinylsiloxane dental wax and the positive replica was made of epoxy. [ 55 ] These replicas have also shown that the structure of shark skin reduces the fluid drag caused by turbulent flow. The fluid dynamic properties of sharkskin have been mimicked in swimsuit, nautical, and aerospace applications. [ 40 ]
Wong et al. developed a surface inspired by the system on the pitcher plant. [ 56 ] This surface, named “slippery liquid-infused porous surfaces” (SLIPS) is a micro- or nano-porous substrate, with a lubricating liquid locked in place. For the system to work, the lubricating liquid must fully wet the substrate, the solid must be preferentially wetted by the lubricating substrate when compared to the repelling substrate, and the lubricating and encroaching liquid must be immiscible. Although the concept of SLIPS was biomimetic of the pitcher plant, it is not superhydrophilic with a contact angle of 116°, though it does repel blood and oil. [ 56 ] | https://en.wikipedia.org/wiki/Self-cleaning_surfaces |
Self-complementary adeno-associated virus ( scAAV ) is a viral vector engineered from the naturally occurring adeno-associated virus (AAV) to be used as a tool for gene therapy . [ 1 ] Use of recombinant AAV (rAAV) has been successful in clinical trials addressing a variety of diseases. [ 2 ] This lab-made progeny of rAAV is termed "self-complementary" because the coding region has been designed to form an intra-molecular double-stranded DNA template. A rate-limiting step for the standard AAV genome involves the second-strand synthesis since the typical AAV genome is a single-stranded DNA template. [ 3 ] [ 4 ] However, this is not the case for scAAV genomes. Upon infection, rather than waiting for cell mediated synthesis of the second strand, the two complementary halves of scAAV will associate to form one double stranded DNA (dsDNA) unit that is ready for immediate replication and transcription . The caveat of this construct is that instead of the full coding capacity found in rAAV (4.7–6 kb ) [ 5 ] scAAV can only hold about half of that amount (≈2.4kb). [ 6 ]
In gene therapy application utilizing rAAV, the virus transduces the cell with a single stranded DNA (ssDNA) flanked by two inverted terminal repeats (ITRs). These ITRs form hairpins at the end of the sequence to serve as primers to initiate synthesis of the second strand before subsequent steps of infection can begin. The second strand synthesis is considered to be one of several blocks to efficient infection. [ 7 ] Additional advantages of scAAV include increased and prolonged transgene expression in vitro and in vivo , as well as "higher in vivo DNA stability and more effective circularization." [ 8 ]
scAAV is an attractive vector for use in gene therapy for many reasons. Its parent vector, AAV, is already being used in clinical trials. [ 9 ] Due to a variety of scAAV serotypes available, scientists can choose a serotype which has properties desirable for their therapy. Selecting only a subset of cells improves specificity and lowers the risk of being inhibited by the immune system . Different scAAV and AAV serotypes can efficiently transfect a variety of cellular targets. [ 10 ] [ 11 ] Like all vector-based approaches to gene therapy, one obstacle in translating therapies from pre-clinical trials into a human clinical application will be the production of large quantities of highly concentrated virus. [ 12 ] One disadvantage that scAAV faces is that due to robust gene expression, transgene products delivered via scAAV elicit a stronger immune response than those same transgenes delivered via a single-stranded AAV vector. [ 13 ]
Like AAV, scAAV is a member of the family Parvoviridae , commonly known as parvoviruses . These viruses are nonenveloped , single-strand DNA (ssDNA) viruses. Within Parvoviridae, scAAV further belongs to the Dependovirus genus, characteristically defined by an inability to replicate on their own. In nature, these viruses depend on another virus to provide replication machinery; adeno-associated virus can only replicate during an active infection of adenovirus or some types of herpesvirus . In lab use, this obstacle is overcome by addition of the helper plasmids , which exogenously expresses replication genes which AAV itself lacks. [ 14 ]
As a dependovirus, scAAV remains in a latent state within the cell until the cell experiences certain permissive conditions. These can include presence of a helper virus infection (such as adenovirus) or other toxic events such as exposure to UV light or carcinogens . [ 14 ] Because the endogenous rep ORF has been replaced with transgene, exogenously provided rep genes encode the proteins required for genome replication and other viral life cycle components. The ITRs located 5' and 3' of the viral genome serve as the origin of replication. [ 15 ]
Like the rep ORF, scAAV's cap ORF has been replaced by transgene and therefore is provided exogenously in a lab environment. The genes encoded in this ORF build capsid proteins and are responsible (along with intracellular processing) for conveying target specificity. Rep proteins participate in the integration of the genome into preformed capsids. [ 15 ] Despite the fact that scAAV is designed to form dsDNA upon infection, the two complementary strands are not packaged in a double stranded manner. Parvoviruses package their viral genome such that the ssDNA bases come in contact with the amino acids on the inside of the viral capsid. Thus the sequence of scAAV is likely unwound by a virally encoded DNA helicase before being packaged into viral protein capsid. [ 7 ] | https://en.wikipedia.org/wiki/Self-complementary_adeno-associated_virus |
The self-consistency principle was established by Rolf Hagedorn in 1965 to explain the thermodynamics of fireballs in high energy physics collisions. A thermodynamical approach to the high energy collisions first proposed by E. Fermi . [ 1 ]
The partition function of the fireballs can be written in two forms, one in terms of its density of states, σ ( E ) {\displaystyle \sigma (E)} , and the other in terms of its mass spectrum, ρ ( m ) {\displaystyle \rho (m)} .
The self-consistency principle says that both forms must be asymptotically equivalent for energies or masses sufficiently high (asymptotic limit). Also, the density of states and the mass spectrum must be asymptotically equivalent in the sense of the weak constraint proposed by Hagedorn [ 2 ] as
These two conditions are known as the self-consistency principle or bootstrap-idea . After a long mathematical analysis Hagedorn was able to prove that there is in fact ρ ( m ) {\displaystyle \textstyle \rho (m)} and σ ( E ) {\displaystyle \textstyle \sigma (E)} satisfying the above conditions, resulting in
and
with a {\displaystyle \textstyle a} and α {\displaystyle \textstyle \alpha } related by
Then the asymptotic partition function is given by
where a singularity is clearly observed for β {\displaystyle \beta } → β o {\displaystyle \beta _{o}} . This singularity determines the limiting temperature T o = 1 / β o {\displaystyle \textstyle T_{o}=1/\beta _{o}} in Hagedorn's theory, which is also known as Hagedorn temperature .
Hagedorn was able not only to give a simple explanation for the thermodynamical aspect of high energy particle production, but also worked out a formula for the hadronic mass spectrum and predicted the limiting temperature for hot hadronic systems.
After some time this limiting temperature was shown by N. Cabibbo and G. Parisi to be related to a phase transition , [ 3 ] which characterizes by the deconfinement of quarks at high energies. The mass spectrum was further analyzed by Steven Frautschi . [ 4 ]
The Hagedorn theory was able to describe correctly the experimental data from collision with center-of-mass energies up to approximately 10 GeV, but above this region it failed. In 2000 I. Bediaga , E. M. F. Curado and J. M. de Miranda [ 5 ] proposed a phenomenological generalization of Hagedorn's theory by replacing the exponential function that appears in the partition function by the q-exponential function from the Tsallis non-extensive statistics. With this modification the generalized theory was able again to describe the extended experimental data.
In 2012 A. Deppman proposed a non-extensive self-consistent thermodynamical theory [ 6 ] that includes the self-consistency principle and the non-extensive statistics. This theory gives as result the same formula proposed by Bediaga et al., which describes correctly the high energy data, but also new formulas for the mass spectrum and density of states of fireball. It also predicts a new limiting temperature and a limiting entropic index. | https://en.wikipedia.org/wiki/Self-consistency_principle_in_high_energy_physics |
The self-consistent mean field (SCMF) method is an adaptation of mean field theory used in protein structure prediction to determine the optimal amino acid side chain packing given a fixed protein backbone . [1] It is faster but less accurate than dead-end elimination and is generally used in situations where the protein of interest is too large for the problem to be tractable by DEE. [2]
Like dead-end elimination, the SCMF method explores conformational space by discretizing the dihedral angles of each side chain into a set of rotamers for each position in the protein sequence. The method iteratively develops a probabilistic description of the relative population of each possible rotamer at each position, and the probability of a given structure is defined as a function of the probabilities of its individual rotamer components.
The basic requirements for an effective SCMF implementation are:
The process is generally initialized with a uniform probability distribution over the rotamers—that is, if there are p {\displaystyle p} rotamers at the k t h {\displaystyle kth} position in the protein, then the probability of any individual rotamer r k A {\displaystyle r_{k}^{A}} is 1 / p {\displaystyle 1/p} . The conversion between energies and probabilities is generally accomplished via the Boltzmann distribution , which introduces a temperature factor (thus making the method amenable to simulated annealing ). Lower temperatures increase the likelihood of converging to a single solution, rather than to a small subpopulation of solutions.
The energy of an individual rotamer r k {\displaystyle r_{k}} is dependent on the "mean-field" energy of the other positions—that is, at every other position, each rotamer's energy contribution is proportional to its probability. For a protein of length N {\displaystyle N} with p {\displaystyle p} rotamers per residue, the energy at the current iteration is described by the following expression. Note that for clarity, the mean-field energy at iteration i {\displaystyle i} is denoted by M i {\displaystyle M_{i}} , whereas the precomputed energies are denoted by E {\displaystyle E} , and the probability of a given rotamer is denoted by P i ( r k A ) {\displaystyle P_{i}(r_{k}^{A})} .
These mean-field energies are used to update the probabilities through the Boltzmann law:
where k {\displaystyle k} is the Boltzmann constant and T {\displaystyle T} is the temperature factor.
Although computing the system energy is not required in carrying out the SCMF method, it is useful to know the overall energies of the converged results. The system energy M s y s {\displaystyle M_{sys}} consists of two sums:
where the addends are defined as:
Perfect convergence for the SCMF method would result in a probability of 1 for exactly one rotamer at each position k {\displaystyle k} in the protein, and a probability of zero for all other rotamers at each position. Convergence to a unique solution requires probabilities close to 1 for exactly one rotamer at each position. In practice, especially when higher temperatures are used, the algorithm instead identifies a small number of high-probability rotamers at each position, allowing the resulting conformations' relative energies to then be enumerated (based on the precomputed energies, not on those derived from the mean-field approximation). One way to improve convergence is to run again at a lower temperature using the probabilities calculated from a previous higher-temperature run.
Unlike dead-end elimination, SCMF is not guaranteed to converge on the optimal solution. However, it is deterministic (as in, it will converge to the same solution every time given the same initial conditions), unlike alternatives that rely on Monte Carlo analysis. By comparison to DEE, which is guaranteed to find the optimal solution, SCMF is faster but less accurate overall; it is significantly better at identifying correct side chain conformations in the protein's core than it is on identifying correct surface conformations [3] . Geometric packing constraints are less restrictive on the surface and thus provide fewer boundaries to the conformational search. | https://en.wikipedia.org/wiki/Self-consistent_mean_field_(biology) |
In computing , self-contained system (SCS) is a software architecture approach that focuses on a separation of the functionality into many independent systems, making the complete logical system a collaboration of many smaller software systems. [ 1 ]
SCSs have certain characteristics:
Implementations [ 2 ] create larger systems using this approach – in particular web applications. There are many case studies [ 3 ] and further links available. [ 4 ]
While self-contained systems are similar to microservices there are differences: A system will usually contain fewer SCS than microservices. Also microservices can communicate with other microservices – even synchronously. SCS prefer no communication or asynchronous communication. Microservices might also have a separate UI unlike the SCS that include a UI. [ 5 ]
Self-contained systems and vertical slice architecture have similarities but also decisive differences. Both approaches divide a system into smaller, manageable units. Vertical slices are cut on the basis of features , Self-contained systems along the boundaries of functional domains . They also differ in the strictness with which they attempt to encapsulate and isolate specialized logic. [ 6 ]
There are quite a few known usages of SCS – e.g. at Otto, [ 7 ] Galeria Kaufhof , [ 8 ] and Kühne+Nagel. [ 9 ]
This computing article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Self-contained_system_(software) |
Self-dissimilarity is a measure of complexity defined in a series of papers by David Wolpert and William G. Macready . [ 1 ] [ 2 ] The degrees of self-dissimilarity between the patterns of a system observed at various scales (e.g. the average matter density of a physical body for volumes at different orders of magnitude ) constitute a complexity "signature" of that system.
This mathematics -related article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Self-dissimilarity |
A self-extracting archive ( SFX or SEA ) is a computer executable program which combines compressed data in an archive file with machine-executable code to extract the information. Running on a compatible operating system, it does not need a suitable extractor in the target computer to extract the data. [ citation needed ] The executable part of the file is known as a decompressor stub .
Self-extracting files are used to share compressed files with a party that may not have the software needed to decompress a regular archive. Users can also use self-extracting archives to distribute their own software. For example, the WinRAR installation program is made using the graphical GUI RAR self-extracting module Default.sfx. [ citation needed ]
Self-extracting archives contain an executable file module, which is used to run uncompressed files from compressed files. The latter does not require an external program to decompress the contents of the self-extracting file and can run the operation itself. However, file archivers like WinRAR can still treat a self-extracting file as if it were any other type of compressed file. By using a file archiver, users can view or decompress self-extracting files they received without running executable code (for example, if they are concerned about viruses). [ citation needed ]
A self-extracting archive is extracted and stored on a disk when executed under an operating system that supports it. Many embedded self-extractors support a number of command-line arguments, such as specifying the target location or selecting only specific files. [ citation needed ]
Unlike self-extracting archives, non-self-extracting archives only contain archived files and must be extracted with a program that is compatible with them. While some formats of self-extracting archives cannot be extracted under another operating system, non-self-extracting ones can usually still be opened using a suitable extractor. This tool will disregard the executable part of the file and extract only the archive resource. The self-extracting executable may need to be renamed to contain a file extension associated with the corresponding packer; archive file formats known to support this include ARJ [ 1 ] and ZIP . [ 2 ] [ 3 ] Typically, self-extracting files for Microsoft operating systems such as DOS and Windows have a .exe extension, just like any other executable file.
For example, an archive may be called "somefiles.zip—it", which can be opened under any operating system by a suitable archive manager that supports both the file format and compression algorithm used. [ 2 ] It may also be converted into somefiles.exe, which will self-extract under Microsoft Windows . It will not self-extract under Linux but can be opened with a suitable archive manager. [ citation needed ] Files that are not recognized as archives by archive managers due to their executable extension can be renamed into .zip. [ 3 ] This works for ZIP archives due to the way the ZIP header is defined, but not necessarily for other less flexible archive formats.
There are several functionally equivalent but incompatible archive file formats, such as ZIP, RAR, 7z , and others. [ citation needed ] Many programs can handle multiple types of archives, whereas others can create, extract, or modify only one type. [ citation needed ] Additionally, there is a distinction between the file format and the compression algorithm. A single file format, such as 7z, can support multiple different compression algorithms, including LZMA , LZMA2 , PPMd , and BZip2 . [ citation needed ] Decompression utilities must be able to handle both the file format and the algorithm used when expanding self-extracting or standard archives. Depending on the options used to create a self-extracting archive, the executable code placed at the beginning may vary. When comparing a LZMA 7z archive to a LZMA2 7z archive, for example, the decompression routines will differ. [ citation needed ]
Several programs can create self-extracting archives. Among the Windows archivers are WinZip , WinRAR , 7-Zip , WinUHA, KGB Archiver , Make SFX, the built-in IExpress wizard, and others, including experimental ones. [ citation needed ] Macintosh users can choose among StuffIt , The Unarchiver , or 7z X as their archivers. There are also programs that create self-extracting archives on Unix as shell scripts , which utilize programs like tar and gzip (which must be present in the destination system). [ citation needed ] Others (like 7-Zip or RAR ) can create self-extracting archives as regular executables in ELF format. [ citation needed ] One of the early examples of self-extracting archives is the Unix shar archive, which combined a number of text files into a shell script that recreated their original content after being executed. [ citation needed ]
It is possible to archive both data and executable files with self-extracting archives. It must be distinguished from executable compression , where the executable file only contains a single executable, and running the file does not result in the uncompressed file being stored on disk but in its code being executed in memory after decompression. [ citation needed ]
As long as the underlying compression algorithm and format allow it, self-extracting archives can also be encrypted for security. [ citation needed ] It is important to note, however, that in many cases, the file and directory names are not included in the encryption and can be viewed by anyone without a key or password. If a person can guess part of the contents of the files from their names or context alone, an attacker may be able to break the encryption on the entire archive with a short amount of computing power and time. | https://en.wikipedia.org/wiki/Self-extracting_archive |
Self-framing metal buildings are a form of pre-engineered building which utilizes roll formed roof and wall panel diaphragms as significant parts of the structural supporting system. Additional structural elements may include mill or cold-formed elements to stiffen the diaphragm perimeters, transfer forces between diaphragms and provide appropriate. As with most pre-engineered buildings, each building will be supplied with all necessary component parts to form a complete building system.
Regardless of project site location, buildings must be designed in accordance with appropriate engineering due diligence. Buildings should be designed for all applicable loads including the following:
Engineered structural design must comply with the applicable sections of the latest edition of the "Specification for Structural Steel Buildings" of the American Institute of Steel Construction (AISC) and the "Specification for the Design of Cold Formed Steel Structural Members" of the American Iron and Steel Institute (AISI).
Many areas of the United States require the use of state or local building codes which may differ from the "International Building Code". Building codes such as the "International Building Code" and Uniform Building Code (UBC) are markedly different from each other and are often revised at the local level.
Self-framing buildings are within the scope of the National Building Code of Canada (NBCC) as adopted and modified by each Province and Territory. For steel structures, NBCC references CAN/CSA S16 Design of Steel Structures and CAN/CSA S136 North American Specification for the Design of Cold-Formed Steel Structural Members.
Manufacturers are required to be certified in accordance with CSA A660 Certification of Manufacturers of Steel Building Systems. Among other requirements, the manufacturer must supply drawings and documents sealed by a professional engineer licensed in the province or territory of the project site. A Certificate of Design and Manufacturing Conformance, duly completed by an engineer knowledgeable with the design and manufacturing, must be provided to the owner and submitted to the Authority Having Jurisdiction (AHJ) with the permit application.
The building code requires documents to be adequate to allow a review of the structural competence of the building (e.g. NBCC Part 4). The Authority Having Jurisdiction (AHJ) will usually require drawings expressing architectural aspects (e.g. Parts 3, 5 and 11). Due to the limited complexity and size of self-framed buildings, the manufacturer's drawings are frequently accepted for this purpose but the owner should be aware that this may not always be the case.
Exterior roof panels are usually a single continuous length from eave to ridge line for gable style buildings or from low eave to high eave on single slope or shed style buildings. Many manufacturers provide minimum 24 gauge (nominal: 0.0239 inch; 0.61 mm) thick sheet steel in self-framing roof designs.
Exterior wall panels are usually a single continuous length from the base channel to the eave of the building except where interrupted by wall openings. Many manufacturers provide minimum 24 gauge (nominal: 0.0239 inch; 0.61 mm) thick sheet steel in self-framing wall designs.
Diaphragm or racking strength of the wall and roof systems are dependent on issues such as the manufacturer's panel lap seam assembly and should be qualified by full-scale testing. Openings reduce the local structural capacity of the wall or roof assembly and should be considered in the original structural design. the manufacturer may provide guidance for limited field modifications for additional openings.
Building width: 3 m (10' +/-) to 10 m (32' +/-) is common. Width is primarily limited by the capability of the roof panel to support the applied gravity loads (e.g. self-weight, snow) and wind uplift loads. In taller buildings, the wall panel may be a limiting factor to width due to buckling of the unsupported wall panel length.
Building length: 3 m (10' +/-) to 10 m (32' +/-) is common. Length is primarily limited by the ability of the load path to transfer loads to a vertical brace system (e.g. gable endwall). Building length can be extended with added discrete brace systems (e.g. roof level horizontal brace, portal frame, diagonal brace, interior partition shear wall).
Building height: 2.5 m (8' +/-) to 7.5 m (24' +/-) is common. Height is primarily limited by the capability of the wall panel to support the wind load. Height may be limited in narrow buildings due to shear capacity limit in the gable endwalls.
Many manufacturers publish tables relating loads to building dimensions and limitations (e.g. ratio of partial height panels to full height panels where wall openings are required).
Typically, self-framed buildings will be shipped to site in knocked-down condition with all parts and hardware. Smaller self-framed buildings may be fully assembled at the manufacturer's facility and transported to site.
The project architect, sometimes called the Architect of Record, is typically responsible for aspects such as aesthetic, dimensional, occupant comfort and fire safety. When a pre-engineered building is selected for a project, the architect accepts conditions inherent in the manufacturer's product offerings for aspects such as materials, colours, structural form, dimensional modularity, etc. Despite the existence of the manufacturer's standard assembly details, the Architect remains responsible to ensure that the manufacturer's product and assembly is consistent with the building code requirements (e.g. continuity of air/vapour retarders, insulation, rain screen; size and location of exits; fire rated assemblies) and occupant or owner expectations.
Many jurisdictions recognize the distinction between the project engineer, sometimes called the Engineer of Record, and the manufacturer's employee or subcontract engineer, sometimes called a specialty engineer. The principal differences between these two entities on a project are the limits of commercial obligation, professional responsibility and liability.
The structural Engineer of Record is responsible to specify the design parameters for the project (e.g. materials, loads, design standards, service limits) and to ensure that the element and assembly designs by others are consistent in the global context of the finished building.
The specialty engineer is responsible to design only those elements which the manufacturer is commercially obligated to supply (e.g. by contract) and to communicate the assembly procedures, design assumptions and responses, to the extent that the design relies on or affects work by others, to the Engineer of Record – usually described in the manufacturer's erection drawings and assembly manuals. The manufacturer produces an engineered product but does not typically provide engineering services to the project.
In the context described, the Architect and Engineer of Record are the designers of the building and bear ultimate responsibility for the performance of the completed work. A buyer should be aware of the project professional distinctions when developing the project plan. | https://en.wikipedia.org/wiki/Self-framing_metal_buildings |
Self-healing concrete is characterized as the capability of concrete to fix its cracks on its own autogenously or autonomously. It not only seals the cracks but also partially or entirely recovers the mechanical properties of the structural elements. This kind of concrete is also known as self-repairing concrete. Because concrete has a poor tensile strength compared to other building materials , it often develops cracks in the surface. These cracks reduce the durability of the concrete because they facilitate the flow of liquids and gases that may contain harmful compounds. If microcracks expand and reach the reinforcement, not only will the concrete itself be susceptible to attack, but so will the reinforcement steel bars . [ 1 ] [ 2 ] Therefore, it is essential to limit the crack's width and repair it as quickly as feasible. Self-healing concrete would not only make the material more sustainable, but it would also contribute to an increase in the service life of concrete structures and make the material more durable and environmentally friendly. [ 3 ] [ 4 ]
Self-healing is an old and well-known phenomenon for concrete, given that it contains innate autogenous healing characteristics. Cracks may heal over time due to continued hydration of clinker minerals or carbonation of calcium hydroxide . [ 5 ] Autogenous healing is difficult to control since it can only heal small cracks and is only effective when water is present. This limitation makes it tough to use. On the other hand, concrete may be altered to provide self-healing capabilities for cracks. There are many solutions for improving autogenous healing by adding the admixtures, such as mineral additions, crystalline admixtures, and superabsorbent polymers . [ 6 ] Further, concrete can be modified to built-in autonomous self-healing techniques. The capsule -based self-healing, the vascular self-healing, and the microbiological self-healing are the most common types of autonomous self-healing techniques. [ 7 ]
The ancient Romans used a type of lime mortar that has been found to be self-healing. [ 8 ] The stratlingite crystals form along the interfacial zones of Roman concrete , binding the aggregate and mortar together and this process continued even after 2000 years and it was discovered by the geologist Marie Jackson and her colleagues in 2014. [ 9 ] [ 8 ] In the early 1990s, Carolyn M. Dry created the first modern contemporary self-healing approach by developing a configuration that facilitates the release of repair chemicals from fibers embedded in a cementitious matrix. [ 10 ] Since then, research community has developed various techniques to incorporate self-healing properties in the concrete. Among the other self-healing materials, in recent years, self-healing concrete research works are growing exponentially because of government-funded consortiums like SARCOS COST Action , RM4L , ReSHEALience , and SMARTINCS . The worldwide market for self-healing concrete is anticipated to grow at a CAGR of 36.8% during the forecast period, with revenue increasing from US$34.10 billion in 2021 to US$562.97 billion in 2030. Rising investment in large-scale infrastructure projects and rising collaboration among governments of different nations to engage in infrastructure projects for long-term goals are factors driving market expansion. [ 11 ]
Autogenous healing of cementitious materials influences crack self-closure and, subsequently, durability and physical-mechanical performance of composites. It is considered to be one of the main reasons for substantial life extension of ancient structures and buildings. [ 12 ] Autogenous self-healing in cement-based composites was first noticed by the French Academy of Science in 1836, when cracks in pipes, water-retaining structures, etc., self-healed. [ 13 ] Significant theoretical and experimental research in the 1900s demonstrated that autogenous self-healing processes are mostly linked to physical, mechanical, and chemical processes inside the cementitious matrix are shown in the scheme. [ 14 ] During the so-called "surface-controlled crystal development" that occurs when cracking is induced, calcium ions are immediately accessible from the fracture faces, and crystal growth is accelerated. After an initial layer of calcite is formed on the crack walls and the surrounding concrete matrix becomes less rich in calcium ions, the transition to the so-called "diffusion-controlled crystal growth" occurs, which means that the Ca 2+ ions must diffuse through the concrete, and the CaCO 3 layer in order to reach the crack surface and ensure the precipitation of the healing products. Clearly, the second phase is much slower than the first. In the case of composite cement, including pozzolanic additions, a portion of the calcium hydroxide , which has been identified as a primary source of Ca 2+ ions, is used in the particular pozzolanic reaction for CSH formation. This will result in a delayed and weaker precipitation of calcium carbonate . Other minor mechanisms depicted in the scheme include the swelling of hydrated cement paste along the crack walls due to water absorption by calcium silicate hydrates and mechanical crack blocking by means of debris and fine concrete particles, direct results of the cracking process or as a result of impurities in the water entering the crack. [ 6 ] Autogenous healing mechanisms are only effective for small cracks, although there is a wide range of maximum widths for healable cracks: 10–100 μm, sometimes up to 200 μm but less than 300 μm, only in the presence of water. [ 6 ] They are challenging to control and forecast because to their usually scattered outcomes and dependence on a number of factors and variables. 1) the age and composition of the concrete itself, 2) the presence of water, and 3) the thickness and form of the concrete fracture are the most influential elements. [ 6 ]
When crack widths are constrained, autogenous healing is more successful. [ 6 ] The presence of water is also a significant element. The stimulation of continuous hydration or crystallization promotes self-healing as well. Therefore, methods that restrict crack width, provide water, or boost hydration or crystallization will be categorized as promoting or enhancing autogenous healing. [ 6 ]
Most research on the effects of mineral addition on self-healing has been conducted on blast-furnace slag and fly ash . Continuous hydration promotes autogenous healing because major sections of these additions remain unhydrated even at older ages. The pozzolanic reaction , which is specific to siliceous and/or aluminous additions ( fly ash , blast-furnace slag , silica fume , calcined clay, etc.) in composite cement, can strengthen the continuous hydration of cement grains in terms of long-term CSH development and, as a result, a certain degree of autogenous self-healing. [ 6 ]
The phrase "crystalline admixtures" is a label that does not necessarily indicate functionality or molecular structure since it is derived from commercially accessible goods whose components are often not specified. Practically, commercial crystalline admixtures may be distinguished from supplementary cementitious materials (SCMs) by their dosage, generally 1% by cement weight for crystalline admixtures and more than 5% for SCMs. Crystalline admixtures (CA) are categorized as a unique type of permeability -reducing admixtures. The category of permeability reducing admixtures includes a diverse variety of materials, which may also be referred to by the general term "crystalline admixtures." Furthermore, most commercial products include proprietary ingredients, and their formulations are kept secret. However, in general, CAs are extremely hydrophilic products created by "active chemicals" that are often blended with cement and sand. In the presence of water, they react, creating water-insoluble pore/crack-blocking precipitates that improve CSH density and resistance to water penetration. [ 15 ] CAs have been demonstrated to enhance the mechanical qualities of concrete when used at 3%, 5%, and 7% of the cement content when exposed to moisture. However, the percentages mentioned above may be fairly high for an addition.
Superabsorbent polymers are natural, or synthetic 3D cross-linked homopolymers or copolymers with a high fluid absorption capacity. The swelling capacity varies according to the monomers ' type and the cross-linking density and may reach 1000 g g-1. The maximal swelling results from a balance between osmotic pressure , which is related to the presence of electrically charged groups, and the elastic retractive forces of the polymer matrix. Furthermore, since osmotic pressure is related to the concentration of ions in the aqueous solution, the ionic strength of the swollen medium substantially influences absorption behavior. Aside from the several application areas (e.g., sanitary and biomedical sector, agricultural sector) where SAPs are currently used, more and more research is focusing on the use of SAPs in mortar/concrete. To limit self-desiccation shrinkage during hardening, SAPs were added as an internal curing agent in cementitious systems with a low water-to-binder ratio. Aside from reducing autogenous shrinkage, SAPs may be added to cementitious materials to improve freeze-thaw resistance and induce self-sealing and self-healing properties. In terms of the latter, the inclusion of SAPs serves many purposes. [ 6 ]
First and foremost, SAPs, which absorb mixing water during concrete mixing and shrink when the matrix hardens, leave behind macropores . [ 16 ] These macropores operate as weak matrix sites, attracting and encouraging multiple cracking. Both actions promote crack closure by allowing cracks to cross SAP macropores and generate narrower cracks. However, these macropores may be accountable for strength loss, but not always, since SAPs can also operate as an internal healing agent and drive more hydration, as previously mentioned. It all relies on the kind of SAP utilized, particle size and shape, the number of SAPs, the w/c ratio of the mix, the addition of water to compensate for the loss in workability, and the mixing technique, among other things. [ 17 ]
Carbon Nanotube Reinforced Concrete (CNT-RC) can heal after being subjected to fires and high temperatures. Research by Szeląg investigated the healing ability of CNT-RC after being subjected to high temperatures. [ 18 ] The study found that the addition of CNTs to cement paste improved the thermal stability of the material and allowed for it to maintain its mechanical properties at elevated temperatures up to 800 °C. Additionally, after the material was exposed to high temperatures and subsequently cooled, it still maintained its healing ability and was able to repair any cracks that formed during the thermal loading process. [ 18 ]
Autonomous self-healing depends on integrating atypical engineering modifications in the matrix to give a self-healing function. Encapsulation has long been the favored method for delivering healing agents directly to the cracks, allowing in-place repair. In encapsulating healing compounds, there are two approaches: discrete and continuous. The key distinction is the mechanism utilized to store the healing agent, which determines the extent of damage that may be treated, the repeatability of healing, and the recovery rate for each strategy. However, several elements must be addressed in the design of an encapsulated-based self-healing system, from capsule system creation through integration, mechanical characterization, triggering, and healing assessment. [ 6 ]
Microencapsulation (diameter < 1 mm) remains a popular technology for manufacturing autonomous self-healing components for cementitious systems, inspired by the pioneering study of White et al. Microcapsules were directly incorporated into the matrix and upon crack development, and releasing the core in the crack volume. [ 19 ] The discharged substance would then react with a distributed catalyst in the matrix to heal the crack. On several occasions, the proof of concept for microcapsule-based healing in concrete has been proven. Recent capsule research has continued to emphasize the usage of adhesive two-component systems necessitating the simultaneous embedding of a catalyst into the matrix for activation and hardening. Wang et al. advised a ratio of 0.5 catalyst to microcapsules, although others have suggested a ratio of 1.3 catalyst to microcapsules to guarantee activation of the encapsulated epoxy . [ 20 ] [ 21 ] However, the long-term stability of reacted organic healing agents in the extremely alkaline cementitious matrix and their long-term functioning remain uncertain. Emerging research, however, promotes compatibility and bonding with the mineral substrate of the cementitious matrix, moving toward a capsule that may provide such healing products; these include encapsulated bacterial spores and mineral cargos such as colloidal silica and sodium silicate . The former may increase carbonate precipitation, while the latter can convert calcium hydroxide to a more desirable CSH gel. [ 22 ] [ 23 ]
Dry conducted one of the early researches using macroencapsulation, proposing polypropylene and glass fibers with a mono- or multicomponent methyl methacrylate core for healing concrete cracks. [ 24 ] The selection of the fibers was prompted by the combination of mechanical strengthening, crack sealing, and a cost-effective encapsulating technique. Moreover, this method was favored over implanted microcapsules because it gave the benefit of retaining a higher quantity of the healing agent and the possibility of many healings. The ultimate objective was to avoid adhesive breakdown over time. The release of the healing agent was triggered by the creation of cracks, which led to the destruction of the implanted brittle fibers.
Lower processing temperatures and the ability to integrate extrusion, filling, and sealing stages make polymeric capsules potentially simpler to manufacture. In the case of cylindrical capsules, the diameters range from 0.8 to 5 mm so that the attractive capillary force of the crack and the gravitational force on the fluid mass is sufficient to overcome the capillary resistive force of the cylindrical capsules and the negative pressure forces resulting from the sealed ends. In other words, the crack width of the matrix should be less than the capsules' inner diameter. [ 25 ]
The concept of vascular healing in concrete utilizes a biomimetic approach to self-healing. The human cardiovascular system , which conducts blood throughout the body, and the plant vascular tissue system, which transports food, water, and minerals via xylem and phloem networks, are examples of vascular network systems. Similarly, vascular networks in concrete may transport liquid healing chemicals to damaged areas. Theoretically, there is no limit to the quantity of damaged material that may be fixed when this healing substance is provided from an external source. Early work by Dry included embedding long, thin glass channels in concrete. [ 26 ] This self-healing mechanism was eventually scaled up and used on a sample bridge deck. [ 27 ] The difficulty of casting concrete with these very fragile materials was one obstacle preventing this technique's widespread use.
The significant advantage of the vascular technique over the encapsulation method is that the healing agent may be administered continuously. [ 28 ] Indeed, different healing agents may be used at different periods to heal different kinds of concrete damage. Additionally, the healing agent may be delivered under pressure to guarantee that it reaches the desired damage zones, similar to the notion of injecting epoxy for fixing concrete fractures. In concrete, several types of vascular networks have been implemented. The simplest version consists of a 1D channel, both ends of which are accessible from the concrete surface. Complex two- and three-dimensional channel networks have been developed in concrete to give various and alternative routes for the transfer of healing agents to damaged areas. Using multiflow junction nodes inside the network, these complex shapes also have been utilized. [ 29 ]
The formation of calcium carbonate as a byproduct of microbial activity is an additional method for "engineering" the self-healing ability of concrete. It holds the potential for active and long-lasting crack repair while also being a potentially ecologically beneficial technique. Calcium carbonate (CaCO 3 ), often known as limestone , has an effective bonding capability and is compatible with current concrete formulations. As a result of the carbonation of existing calcium hydroxide (portlandite) minerals, calcium carbonate may be included in the concrete mix design or chemically created inside the concrete matrix. Limestone generated inside the matrix of concrete may result in the densification of the matrix by pore filling and can help to self-heal crack, reducing its (water) permeability and resulting in the recovery of lost strength. If circumstances are favorable, most bacteria can precipitate CaCO 3 from the solution. [ 30 ] However, the carbonatogenesity of bacteria following distinct metabolic routes for the precipitation of bacterial CaCO 3 varies. Additionally, many extrinsic variables influence the precipitation efficiency and cause the same bacterial strain to produce varying amounts of carbonate. It is probable that in a wet-dry environment, healing happens more quickly. [ 31 ] In addition, the regulation of crack width is crucial for achieving quicker and more effective healing through biological activity. [ 32 ] | https://en.wikipedia.org/wiki/Self-healing_concrete |
Self-healing hydrogels are a specialized type of polymer hydrogel . A hydrogel is a macromolecular polymer gel constructed of a network of crosslinked polymer chains. Hydrogels are synthesized from hydrophilic monomers by either chain or step growth, along with a functional crosslinker to promote network formation. A net-like structure along with void imperfections enhance the hydrogel's ability to absorb large amounts of water via hydrogen bonding . As a result, hydrogels, self-healing alike, develop characteristic firm yet elastic mechanical properties. Self-healing refers to the spontaneous formation of new bonds when old bonds are broken within a material. The structure of the hydrogel along with electrostatic attraction forces drive new bond formation through reconstructive covalent dangling side chain or non-covalent hydrogen bonding. These flesh-like properties have motivated the research and development of self-healing hydrogels in fields such as reconstructive tissue engineering as scaffolding, as well as use in passive and preventive applications.
A variety of different polymerization methods may be utilized for the synthesis of the polymer chains that make up hydrogels. Their properties depend on how these chains are crosslinked.
Crosslinking is the process of joining two or more polymer chains. Both chemical and physical crosslinking exists. In addition, both natural polymers such as proteins or synthetic polymers with a high affinity for water may be used as starting materials when selecting a hydrogel. [ 1 ] Different crosslinking methods can be implemented for the design of a hydrogel. By definition, a crosslinked polymer gel is a macromolecule that solvent will not dissolve. Due to the polymeric domains created by crosslinking in the gel microstructure, hydrogels are not homogenous within the selected solvent system. The following sections summarize the chemical and physical methods by which hydrogels are crosslinked. [ 1 ]
Other chain growth methods include anionic and cationic polymerization. Both anionic and cationic methods suffer from extreme sensitivity toward aqueous environments and therefore, are not used in the synthesis of polymeric hydrogels.
Hydrogen bonding is a strong intermolecular force that forms a special type of dipole-dipole attraction. Hydrogen bonds form when a hydrogen atom bonded to a strongly electronegative atom is around another electronegative atom with a lone pair of electrons. [ 3 ] Hydrogen bonds are stronger than normal dipole-dipole interactions and dispersion forces but they remain weaker than covalent and ionic bonds. In hydrogels, structure and stability of water molecules are highly affected by the bonds. The polar groups in the polymer strongly bind water molecules and form hydrogen bonds which also cause hydrophobic effects to occur. [ 4 ] These hydrophobic effects can be exploited to design physically crosslinked hydrogels that exhibit self healing abilities. The hydrophobic effects combined with the hydrophilic effects within the hydrogel structure can be balanced through dangling side chains that mediates the hydrogen bonding that occurs between two separate hydrogel pieces or across a ruptured hydrogel.
A dangling side chain is a hydrocarbon chain side chains that branch off of the backbone of the polymer. Attached to the side chain are polar functional groups. The side chains "dangle" across the surface of the hydrogel, allowing it to interact with other functional groups and form new bonds. [ 5 ] The ideal side chain would be long and flexible so it could reach across the surface to react, but short enough to minimize steric hindrance and collapse from the hydrophobic effect. [ 5 ] The side chains need to keep both the hydrophobic and hydrophilic effects in balance. In a study performed by the University of California San Diego to compare healing ability, hydrogels of varying side chain lengths with similar crosslinking contents were compared and the results showed that healing ability of the hydrogels depends nonmonotonically on the side chain length. [ 5 ] With shorter side chain lengths, there is limited reach of the carboxyl group which decreases the mediation of the hydrogen bonds across the interface. As the chain increases in length, the reach of the carboxyl group becomes more flexible and the hydrogen bonds can mediated. However, when a side chain length is too long, the interruption between the interaction of the carboxyl and amide groups that help to mediate the hydrogen bonds. It can also accumulate and collapse the hydrogel and prevent the healing from occurring.
Most self-healing hydrogels rely on electrostatic attraction to spontaneously create new bonds. [ 3 ] [ 4 ] [ 5 ] The electrostatic attraction can be masked using protonation of the polar functional groups. When the pH is raised the polar functional groups become deprotonated, freeing the polar functional group to react.
Since the hydrogels rely on electrostatic attraction for self-healing, the process can be affected by electrostatic screening. The effects of a change in salinity can be modeled using the Gouy-Chapman-Stern theory Double Layer .
To calculate the Gouy–Chapmanm potential, the salinity factor must be calculated. The expression given for the salinity factor is as follows:
These effects become important when considering the application of self-healing hydrogels to the medical field. They will be affected by the pH and salinity of blood.
These effects also come into play during synthesis when trying to add large hydrophobes to a hydrophilic polymer backbone. A research group from the Istanbul Technical University has shown that large hydrophobes can be added by adding an electrolyte in a sufficient amount. During synthesis, the hydrophobes were held in micelles before attaching to the polymer backbone. [ 6 ] By increasing the salinity of the solution, the micelles were able to grow and encompass more hydrophobes. If there are more hydrophobes in a micelle, then the solubility of the hydrophobe increases. The increase in the solubility lead to an increase in the formation of hydrogels with large hydrophobes. [ 6 ]
The surface tension (γ) of a material is directly related to its intramolecular and intermolecular forces . The stronger the force, the greater the surface tension. This can be modeled by an equation:
Where Δ vap U is the energy of vaporization, N A is the Avogadro constant, and a 2 is the surface area per molecule. This equation also implies that the energy of vaporization affects surface tension. It is known that the stronger the force, the higher the energy of vaporization. Surface tension can then be used to calculate surface energy (u σ ). An equation describing this property is:
where T is temperature and the system is at constant pressure and area. Specifically for hydrogels, the free surface energy can be predicted using the Flory–Huggins free energy function for the hydrogels. [ 7 ]
For hydrogels, surface tension plays a role in several additional characteristics including swelling ratio and stabilization.
Hydrogels have the remarkable ability to swell in water and aqueous solvents. During the process of swelling, surface instability can occur. This instability depends on the thickness of the hydrogel layers and the surface tension. [ 7 ] A higher surface tension stabilizes the flat surface of the hydrogel, which is the outer-most layer. The swelling ratio of the flat layer can be calculated using the following equation derived from the Flory–Huggins theory of free surface energy in hydrogels:
where λ h is the swelling ratio, μ is the chemical potential, p is pressure, k B is the Boltzmann constant, and χ and N v are unitless hydrogel constants.
As swelling increases, mechanical properties generally suffer.
The surface deformation of hydrogels is important because it can result in self-induced cracking. Each hydrogel has a characteristic wavelength of instability ( λ ) that depends on elastocapillary length. This length is calculated by dividing the surface tension ( γ ) by the elasticity ( μ ) of the hydrogel. The greater the wavelength of instability, the greater the elastocapillary length of instability, which makes a material more prone to cracking. [ 8 ] The characteristic wavelength of instability can be modeled by:
where H is the thickness of the hydrogel.
Some hydrogels are able to respond to stimuli and their surrounding environments. Examples of these stimuli include light, temperature, pH, and electrical fields. [ citation needed ] Hydrogels that are temperature sensitive are known as thermogels. Thermo-responsive hydrogels undergo reversible, thermally induced phase transition upon reaching either the upper or lower critical solution temperature. By definition, a crosslinked polymer gel is a macromolecule that cannot dissolve. Due to the polymeric domains created by crosslinking, in the gel microstructure, hydrogels are not homogenous within the solvent system in which they are placed into. Swelling of the network, however, does occur in the presence of a proper solvent. Voids in the microstructure of the gel where crosslinking agent or monomer has aggregated during polymerization can cause solvent to diffuse into or out of the hydrogel. The microstructure of hydrogel therefore are not constant, and imperfections occur where water from outside of the gel can accumulate these voids. This process is temperature dependent, and solvent behavior depends on whether the solvent-gel system has reached, or surpassed, the critical solution temperature (LCST). The LCST defines a boundary between which a gel or polymer chain will separate solvent into one or two phases. The spinodial and binodial regions of a polymer-solvent phase diagram represent the energetic favorability of the hydrogel becoming miscible in solution or separating into two phases.
Self-healing hydrogels encompass a wide range of applications. With a high biocompatibility, hydrogels are useful for a number of medical applications. Areas where active research is currently being conducted include:
Hydrogels are created from crosslinked polymers that are water-insoluble. Polymer hydrogels absorb significant amounts of aqueous solutions, and therefore have a high water content. This high water content makes hydrogel more similar to living body tissues than any other material for tissue regeneration. [ 10 ] Additionally, polymer scaffolds using self-healing hydrogels are structurally similar to the extracellular matrices of many of the tissues. Scaffolds act as three-dimensional artificial templates in which the tissue targeted for reconstruction is cultured to grow onto. The high porosity of hydrogels allows for the diffusion of cells during migration, as well as the transfer of nutrients and waste products away from cellular membranes. Scaffolds are subject to harsh processing conditions during tissue culturing. [ 11 ] These include mechanical stimulation to promote cellular growth, a process which places stress on the scaffold structure. This stress may lead to localized rupturing of the scaffold which is detrimental to the reconstruction process. [ 12 ] In a self-healing hydrogel scaffold, ruptured scaffolds have the ability for localized self-repair of their damaged three-dimensional structure. [ 13 ]
Current research is exploring the effectiveness of using various types of hydrogel scaffolds for tissue engineering and regeneration including synthetic hydrogels, biological hydrogels, and biohybrid hydrogels.
In 2019, researchers Biplab Sarkar and Vivek Kumar of the New Jersey Institute of Technology developed a self-assembling peptide hydrogel that has proven successful in increasing blood vessel regrowth and neuron survival in rats affected by Traumatic Brain Injuries (TBI). [ 14 ] By adapting the hydrogel to closely resemble brain tissue and injecting it into the injured areas of the brain, the researchers’ studies have shown improved mobility and cognition after only a week of treatment. If trials continued to prove successful, this peptide hydrogel may be approved for human trials and eventual widespread use in the medical community as a treatment for TBIs. This hydrogel also has the potential to be adapted to other forms of tissue in the human body, and promote regeneration and recovery from other injuries.
Polyethylene glycol(PEG) polymers are synthetic materials that can be crosslinked to form hydrogels. PEG hydrogels are not toxic to the body, do not elicit an immune response, and have been approved by the US Food and Drug Administration for clinical use. The surfaces of PEG polymers are easily modified with peptide sequences that can attract cells for adhesion and could therefore be used for tissue regeneration. [ 15 ]
Poly (2-hydroxyethyl methacrylate) (PHEMA) hydrogels can be combined with rosette nanotubes (RNTs). RNTs can emulate skin structures such as collagen and keratin and self-assemble when injected into the body. This type of hydrogel is being explored for use in skin regeneration and has shown promising results such as fibroblast and keratinocyte proliferation. Both of these cell types are crucial for the production of skin components. [ 16 ]
Biological hydrogels are derived from preexisting components of body tissues such as collagen, hyaluronic acid (HA), or fibrin . Collagen, HA, and fibrin are components that occur naturally in the extracellular matrix of mammals. Collagen is the main structural component in tissues and it already contains cell-signaling domains that can promote cell growth. In order to mechanically enhance collagen into a hydrogel, it must be chemically crosslinked, crosslinked using UV light or temperature, or mixed with other polymers. Collagen hydrogels would be nontoxic and biocompatible. [ 15 ]
Hybrid hydrogels combine synthetic and biological materials and take advantage of the best properties of each. Synthetic polymers are easily customizable and can be tailored for specific functions such as biocompatibility. Biological polymers such as peptides also have adventitious properties such as specificity of binding and high affinity for certain cells and molecules. A hybrid of these two polymer types allows for the creation of hydrogels with novel properties. An example of a hybrid hydrogel would include a synthetically created polymer with several peptide domains. [ 17 ]
Peptide-based self-healing hydrogels may be selectively grown onto nanofiber material which can then incorporated into the desired reconstructive tissue target. [ 18 ] The hydrogel framework is then chemically modified to promote cell adhesion to the nanofiber peptide scaffold. Because the growth of the extracellular matrix scaffold is pH dependent, the materials selected must be factored for pH response when selecting the scaffolding material.
The swelling and bioadhesion of hydrogels can be controlled based on the fluid environment they are introduced to in the body. [ 10 ] These properties make them excellent for use as controlled drug delivery devices. Where the hydrogel adheres in the body will be determined by its chemistry and reactions with the surrounding tissues. If introduced by mouth, the hydrogel could adhere to anywhere in the gastrointestinal tract including the mouth, the stomach, the small intestine, or the colon. Adhesion in a specifically targeted region will cause for a localized drug delivery and an increased concentration of the drug taken up by the tissues. [ 10 ]
Smart hydrogels are sensitive to stimuli such as changes in temperature or pH. Changes in the environment alter the swelling properties of the hydrogels and can cause them to increase or decrease the release of the drug impregnated into the fibers. [ 10 ] An example of this would be hydrogels that release insulin in the presence of high glucose levels in the bloodstream. [ 19 ] These glucose sensitive hydrogels are modified with the enzyme glucose oxidase . In the presence of glucose, the glucose oxidase will catalyze a reaction that ends in increased levels of H + . These H + ions raise the pH of the surrounding environment and could therefore cause a change in a smart hydrogel that would initiate the release of insulin.
Although research is currently focusing on the bioengineering aspect of self-healing hydrogels, several non-medical applications do exist, including:
Dangling type side chain self-healing hydrogels are activated by changes in the relative acidity of solution they are in. Depending on user specified application, side chains may be selectively used in self-healing hydrogels as pH indicators. If a specified functional group chain end with a low pKa , such as a carboxylic acid, is subject to a neutral pH conditions, water will deprotonate the acidic chain end, activating the chain ends. Crosslinking or what is known as self-healing will begin, causing two or more separated hydrogels to fuse into one.
Research into the use self-healing hydrogels has revealed an effective method for mitigating acid spills through the ability to selectively crosslink under acidic conditions. In a testing done by the University of California San Diego, various surfaces were coated with self healing hydrogels and then mechanically damaged with 300 micrometer wide cracks with the coatings healing the crack within seconds upon exposure of low pH buffers. [ 5 ] The hydrogels also can adhere to various plastics due to hydrophobic interactions. Both findings suggest the use of these hydrogels as a sealant for vessels containing corrosive acids. No commercial applications currently exist for implementation of this technology.
Drying of hydrogels under controlled circumstances may yield xerogels and aerogels . A xerogel is a solid that retains significant porosity (15-50%) with a very small pore size (1–10 nm). In an aerogel, the porosity is somewhat higher and the pores are more than an order of magnitude larger, resulting in an ultra-low-density material with a low thermal conductivity and an almost translucent, smoke-like appearance. [ citation needed ] | https://en.wikipedia.org/wiki/Self-healing_hydrogels |
Self-healing materials are artificial or synthetically created substances that have the built-in ability to automatically repair damages to themselves without any external diagnosis of the problem or human intervention. Generally, materials will degrade over time due to fatigue , environmental conditions, or damage incurred during operation. Cracks and other types of damage on a microscopic level have been shown to change thermal , electrical , and acoustical properties of materials, and the propagation of cracks can lead to eventual failure of the material. In general, cracks are hard to detect at an early stage, and manual intervention is required for periodic inspections and repairs. In contrast, self-healing materials counter degradation through the initiation of a repair mechanism that responds to the micro-damage. [ 1 ] : 1–2 Some self-healing materials are classed as smart structures, and can adapt to various environmental conditions according to their sensing and actuation properties. [ 1 ] : 145
Although the most common types of self-healing materials are polymers or elastomers , self-healing covers all classes of materials, including metals , ceramics , and cementitious materials . Healing mechanisms vary from an instrinsic repair of the material to the addition of a repair agent contained in a microscopic vessel. For a material to be strictly defined as autonomously self-healing, it is necessary that the healing process occurs without human intervention. Self-healing polymers may, however, activate in response to an external stimulus (light, temperature change, etc.) to initiate the healing processes.
A material that can intrinsically correct damage caused by normal usage could prevent costs incurred by material failure and lower costs of a number of different industrial processes through longer part lifetime, and reduction of inefficiency caused by degradation over time. [ 2 ]
The ancient Romans used a form of lime mortar that has been found to have self-healing properties. [ 3 ] By 2014, geologist Marie Jackson and her colleagues had recreated the type of mortar used in Trajan's Market and other Roman structures such as the Pantheon and the Colosseum and studied its response to cracking. [ 4 ] The Romans mixed a particular type of volcanic ash called Pozzolane Rosse , from the Alban Hills volcano, with quicklime and water . They used it to bind together decimeter-sized chunks of tuff , an aggregate of volcanic rock. [ 3 ] As a result of pozzolanic activity as the material cured, the lime interacted with other chemicals in the mix and was replaced by crystals of a calcium aluminosilicate mineral called strätlingite . Crystals of platey strätlingite grow in the cementitious matrix of the material including the interfacial zones where cracks would tend to develop. This ongoing crystal formation holds together the mortar and the coarse aggregate, countering crack formation and resulting in a material that has lasted for 1,900 years. [ 5 ] [ 6 ]
Related processes in concrete have been studied microscopically since the 19th century.
Self healing materials only emerged as a widely recognized field of study in the 21st century. The first international conference on self-healing materials was held in 2007. [ 7 ] The field of self-healing materials is related to biomimetic materials as well as to other novel materials and surfaces with the embedded capacity for self-organization, such as the self-lubricating and self-cleaning materials. [ 8 ]
Plants and animals have the capacity to seal and heal wounds. In all plants and animals examined, firstly a self-sealing phase and secondly a self-healing phase can be identified. In plants, the rapid self-sealing prevents the plants from desiccation and from infection by pathogenic germs. This gives time for the subsequent self-healing of the injury which in addition to wound closure also results in the (partly) restoration of mechanical properties of the plant organ. Based on a variety of self-sealing and self-healing processes in plants, different functional principles were transferred into bio-inspired self-repairing materials. [ 9 ] [ 10 ] [ 11 ] The connecting link between the biological model and the technical application is an abstraction describing the underlying functional principle of the biological model which can be for example an analytical model [ 12 ] or a numerical model. In cases where mainly physical-chemical processes are involved a transfer is especially promising.
There is evidence in the academic literature [ 13 ] of these biomimetic design approaches being used in the development of self-healing systems for polymer composites. [ 14 ] The DIW [ clarification needed ] structure from above can be used to essentially mimic the structure of skin. Toohey et al. did this with an epoxy substrate containing a grid of microchannels containing dicyclopentadiene (DCPD), and incorporated Grubbs' catalyst to the surface. This showed partial recovery of toughness after fracture, and could be repeated several times because of the ability to replenish the channels after use. The process is not repeatable forever, because the polymer in the crack plane from previous healings would build up over time. [ 15 ] Inspired by rapid self-sealing processes in the twining liana Aristolochia macrophylla and related species (pipevines) a biomimetic PU-foam coating for pneumatic structures was developed. [ 16 ] With respect to low coating weight and thickness of the foam layer maximum repair efficiencies of 99.9% and more have been obtained. [ 17 ] [ 18 ] [ 19 ] Other role models are latex bearing plants as the weeping fig (Ficus benjamina), the rubber tree (Hevea brasiliensis) and spurges (Euphorbia spp.), in which the coagulation of latex is involved in the sealing of lesions. [ 20 ] [ 21 ] [ 22 ] Different self-sealing strategies for elastomeric materials were developed showing significant mechanical restoration after a macroscopic lesion. [ 23 ] [ 24 ]
In the last century, polymers became a base material in everyday life for products like plastics, rubbers, films, fibres or paints. This huge demand has forced to extend their reliability and maximum lifetime, and a new design class of polymeric materials that are able to restore their functionality after damage or fatigue was envisaged. These polymer materials can be divided into two different groups based on the approach to the self-healing mechanism: intrinsic or extrinsic. [ 25 ] [ 26 ] Autonomous self-healing polymers follow a three-step process very similar to that of a biological response. In the event of damage, the first response is triggering or actuation, which happens almost immediately after damage is sustained. The second response is transport of materials to the affected area, which also happens very quickly. The third response is the chemical repair process. This process differs depending on the type of healing mechanism that is in place (e.g., polymerization , entanglement, reversible cross-linking). These materials can be classified according to three mechanisms (capsule-based, vascular-based, and intrinsic), which can be correlated chronologically through four generations. [ 27 ] While similar in some ways, these mechanisms differ in the ways that response is hidden or prevented until actual damage is sustained.
From a molecular perspective, traditional polymers yield to mechanical stress through cleavage of sigma bonds . [ 28 ] While newer polymers can yield in other ways, traditional polymers typically yield through homolytic or heterolytic bond cleavage . The factors that determine how a polymer will yield include: type of stress, chemical properties inherent to the polymer, level and type of solvation , and temperature. [ 28 ] From a macromolecular perspective, stress induced damage at the molecular level leads to larger scale damage called microcracks. [ 29 ] A microcrack is formed where neighboring polymer chains have been damaged in close proximity, ultimately leading to the weakening of the fiber as a whole. [ 29 ]
Polymers have been observed to undergo homolytic bond cleavage through the use of radical reporters such as DPPH (2,2-diphenyl-1-picrylhydrazyl) and PMNB (pentamethylnitrosobenzene.) When a bond is cleaved homolytically, two radical species are formed that can recombine to repair damage or can initiate other homolytic cleavages which can in turn lead to more damage. [ 28 ]
Polymers have also been observed to undergo heterolytic bond cleavage through isotope labeling experiments. When a bond is cleaved heterolytically, cationic and anionic species are formed that can in turn recombine to repair damage, can be quenched by solvent, or can react destructively with nearby polymers. [ 28 ]
Certain polymers yield to mechanical stress in an atypical, reversible manner. [ 30 ] Diels-Alder -based polymers undergo a reversible cycloaddition , where mechanical stress cleaves two sigma bonds in a retro Diels-Alder reaction. This stress results in additional pi-bonded electrons as opposed to radical or charged moieties. [ 2 ]
Supramolecular polymers are composed of monomers that interact non-covalently . [ 31 ] Common interactions include hydrogen bonds , [ 32 ] metal coordination , and van der Waals forces . [ 31 ] Mechanical stress in supramolecular polymers causes the disruption of these specific non-covalent interactions, leading to monomer separation and polymer breakdown.
In intrinsic systems, the material is inherently able to restore its integrity. While extrinsic approaches are generally autonomous, intrinsic systems often require an external trigger for the healing to take place (such as thermo-mechanical, electrical, photo-stimuli, etc.). It is possible to distinguish among 5 main intrinsic self-healing strategies. The first one is based on reversible reactions, and the most widely used reaction scheme is based on Diels-Alder (DA) and retro-Diels-Alder (rDA) reactions. [ 33 ] Another strategy achieves the self-healing in thermoset matrices by incorporating meltable thermoplastic additives. A temperature trigger allows the redispertion of thermoplastic additives into cracks, giving rise to mechanical interlocking. [ 34 ] Polymer interlockings based on dynamic supramolecular bonds or ionomers represent a third and fourth scheme. The involved supramolecular interactions and ionomeric clusters are generally reversible and act as reversible cross-links, thus can equip polymers with self-healing ability. [ 35 ] [ 36 ] Finally, an alternative method for achieving intrinsic self-healing is based on molecular diffusion. [ 37 ]
Reversible systems are polymeric systems that can revert to the initial state whether it is monomeric , oligomeric , or non-cross-linked. Since the polymer is stable under normal condition, the reversible process usually requires an external stimulus for it to occur. For a reversible healing polymer, if the material is damaged by means such as heating and reverted to its constituents, it can be repaired or "healed" to its polymer form by applying the original condition used to polymerize it.
Among the examples of reversible healing polymers, the Diels-Alder (DA) reaction and its retro- Diels-Alder (RDA) analogue seems to be very promising due to its thermal reversibility. In general, the monomer containing the functional groups such as furan or maleimide form two carbon-carbon bonds in a specific manner and construct the polymer through DA reaction. This polymer, upon heating, breaks down to its original monomeric units via RDA reaction and then reforms the polymer upon cooling or through any other conditions that were initially used to make the polymer. During the last few decades, two types of reversible polymers have been studied: (i) polymers where the pendant groups, such as furan or maleimide groups, cross-link through successive DA coupling reactions; (ii) polymers where the multifunctional monomers link to each other through successive DA coupling reactions. [ 30 ]
In this type of polymer , the polymer forms through the cross linking of the pendant groups from the linear thermoplastics . For example, Saegusa et al. have shown the reversible cross-linking of modified poly( N -acetylethyleneimine)s containing either maleimide or furancarbonyl pendant moideties. The reaction is shown in Scheme 3. They mixed the two complementary polymers to make a highly cross-linked material through DA reaction of furan and maleimide units at room temperature, as the cross-linked polymer is more thermodynamically stable than the individual starting materials. However, upon heating the polymer to 80 °C for two hours in a polar solvent , two monomers were regenerated via RDA reaction, indicating the breaking of polymers . [ 38 ] This was possible because the heating energy provided enough energy to go over the energy barrier and results in the two monomers . Cooling the two starting monomers , or damaged polymer , to room temperature for 7 days healed and reformed the polymer.
The reversible DA/RDA reaction is not limited to furan-meleimides based polymers as it is shown by the work of Schiraldi et al. They have shown the reversible cross-linking of polymers bearing pendent anthracene group with maleimides. However, the reversible reaction occurred only partially upon heating to 250 °C due to the competing decomposition reaction. [ 39 ]
In these systems, the DA reaction takes place in the backbone itself to construct the polymer, not as a link. For polymerization and healing processes of a DA-step-growth furan - maleimide based polymer (3M4F) were demonstrated by subjecting it to heating/cooling cycles. Tris-maleimide (3M) and tetra-furan (4F) formed a polymer through DA reaction and, when heated to 120 °C, de-polymerized through RDA reaction, resulting in the starting materials. Subsequent heating to 90–120 °C and cooling to room temperature healed the polymer, partially restoring its mechanical properties through intervention. [ 33 ] [ 40 ] The reaction is shown in Scheme 4.
The thiol-based polymers have disulfide bonds that can be reversibly cross-linked through oxidation and reduction . Under reducing condition, the disulfide (SS) bridges in the polymer breaks and results in monomers, however, under oxidizing condition, the thiols (SH) of each monomer forms the disulfide bond , cross-linking the starting materials to form the polymer. Chujo et al. have shown the thiol -based reversible cross-linked polymer using poly( N -acetylethyleneimine). (Scheme 5) [ 41 ]
A soft poly(urea-urethane) network uses the metathesis reaction in aromatic disulphides to provide room-temperature self-healing properties, without the need for external catalysts. This chemical reaction is naturally able to create covalent bonds at room temperature, allowing the polymer to autonomously heal without an external source of energy. Left to rest at room temperature, the material mended itself with 80 percent efficiency after only two hours and 97 percent after 24 hours. [ citation needed ] In 2014 a polyurea elastomer -based material was shown to be self-healing, melding together after being cut in half, without the addition of catalysts or other chemicals. The material also include inexpensive commercially available compounds. The elastomer molecules were tweaked, making the bonds between them longer. The resulting molecules are easier to pull apart from one another and better able to rebond at room temperature with almost the same strength. The rebonding can be repeated. Stretchy, self-healing paints and other coatings recently took a step closer to common use, thanks to research being conducted at the University of Illinois. Scientists there have used "off-the-shelf" components to create a polymer that melds back together after being cut in half, without the addition of catalysts or other chemicals. [ 42 ] [ 43 ]
The urea-urethane polymers however have glassy transition temperatures below 273 K therefore at room temperature they are gels and their tensile strength is low. [ 43 ] To optimize the tensile strength the reversible bonding energy, or the polymer length must be increased to increase the degree of covalent or mechanical interlocking respectively. However, increase polymer length inhibits mobility and thereby impairs the ability for polymers to re-reversibly bond. Thus at each polymer length an optimal reversible bonding energy exists. [ 44 ]
Vitrimers are a subset of polymers that bridge the gap between thermoplastics and thermosets. [ 45 ] [ 46 ] Their dependence on dissociative and associative exchange within dynamic covalent adaptable networks allows for a variety of chemical systems to be accessed that allow for the synthesis of mechanically robust materials with the ability to be reprocessed many times while maintaining their structural properties and mechanical strength. [ 47 ] The self-healing aspect of these materials is due to the bond exchange of crosslinked species as a response to applied external stimuli, such as heat. Dissociative exchange is the process by which crosslinks are broken prior to recombination of crosslinking species, thereby recovering the crosslink density after exchange. [ 48 ] Examples of dissociative exchange include reversible pericyclic reactions, nucleophilic transalkylation, and aminal transamination. Associative exchange involves the substitution reaction with an existing crosslink and the retention of crosslinks throughout exchange. [ 48 ] Examples of associative exchange include transesterification, transamination of vinylogous urethanes, [ 49 ] imine exchange, [ 50 ] and transamination of diketoneamines. [ 48 ] Vitrimers possessing nanoscale morphology are being studied, through the use of block copolymer vitrimers in comparison to statistical copolymer analogues, to understand the effects of self-assembly on exchange rates, viscoelastic properties, and reprocessability. [ 51 ] Other than recycling, vitrimer materials show promise for applications in medicine, for example self-healable bioepoxy, [ 52 ] and applications in self-healing electronic screens. [ 53 ] While these polymeric systems are still in their infancy they serve to produce commercially relevant, recyclable materials in the coming future as long as more work is done to tailor these chemical systems to commercially relevant monomers and polymers, as well as develop better mechanical testing and understanding of material properties throughout the lifetime of these materials (i.e. post reprocess cycles).
Copolymers with van der Waals force
If perturbation of van der Waals forces upon mechanical damage is energetically unfavourable, interdigitated alternating or random copolymer motifs will self-heal to an energetically more favourable state without external intervention. This self-healing behavior occurs within a relatively narrow compositional range depended on a viscoelastic response that energetically favours self-recovery upon chain separation, owing to ‘key-and-lock’ associations of the neighbouring chains. In essence, van der Waals forces stabilize neighbouring copolymers, which is reflected in enhanced cohesive-energy density (CED) values. Urban etc. illustrates how induced dipole interactions for alternating or random poly(methyl methacrylate-alt-ran-n-butyl acrylate) (p(MMA-alt-ran-nBA)) copolymers owing to directional van der Waals forces may enhance the CED at equilibrium (CEDeq) of entangled and side-by-side copolymer chains.
[ 54 ] [ 55 ] [ 56 ]
In extrinsic systems, the healing chemistries are separated from the surrounding polymer in microcapsules or vascular networks which, after material damage/cracking, release their content into the crack plane, reacting and allowing the restoration of material functionalities. [ 57 ] These systems can be further subdivided in several categories. While capsule-based polymers sequester the healing agents in little capsules that only release the agents if they are ruptured, vascular self-healing materials sequester the healing agent in capillary type hollow channels that can be interconnected one dimensionally, two dimensionally, or three dimensionally. After one of these capillaries is damaged, the network can be refilled by an outside source or another channel that was not damaged. Intrinsic self-healing materials do not have a sequestered healing agent but instead have a latent self-healing functionality that is triggered by damage or by an outside stimulus. [ 57 ] Extrinsic self-healing materials can achieve healing efficiencies over 100% even when the damage is large. [ 58 ]
Capsule-based systems have in common that healing agents are encapsulated into suitable microstructures that rupture upon crack formation and lead to a follow-up process in order to restore the materials' properties. If the walls of the capsule are created too thick, they may not fracture when the crack approaches, but if they are too thin, they may rupture prematurely. [ 59 ] In order for this process to happen at room temperature , and for the reactants to remain in a monomeric state within the capsule, a catalyst is also imbedded into the thermoset. The catalyst lowers the energy barrier of the reaction and allows the monomer to polymerize without the addition of heat. The capsules around the monomer are important to maintain separation until the crack facilitates the reaction. [ 30 ] [ 60 ] In the capsule-catalyst system, the encapsulated healing agent is released into the polymer matrix and reacts with the catalyst, already present in the matrix. [ 61 ] There are many challenges in designing this type of material. First, the reactivity of the catalyst must be maintained even after it is enclosed in wax. Additionally, the monomer must flow at a sufficient rate (have low enough viscosity ) to cover the entire crack before it is polymerized, or full healing capacity will not be reached. Finally, the catalyst must quickly dissolve into the monomer in order to react efficiently and prevent the crack from spreading further. [ 60 ]
This process has been demonstrated with dicyclopentadiene (DCPD) and Grubbs' catalyst (benzylidene-bis(tricyclohexylphosphine)dichlororuthenium). Both DCPD and Grubbs' catalyst are imbedded in an epoxy resin . The monomer on its own is relatively unreactive and polymerization does not take place. When a microcrack reaches both the capsule containing DCPD and the catalyst , the monomer is released from the core–shell microcapsule and comes in contact with exposed catalyst, upon which the monomer undergoes ring opening metathesis polymerization (ROMP). [ 60 ] The metathesis reaction of the monomer involves the severance of the two double bonds in favor of new bonds. The presence of the catalyst allows for the energy barrier (energy of activation) to be lowered, and the polymerization reaction can proceed at room temperature. [ 62 ] The resulting polymer allows the epoxy composite material to regain 67% of its former strength.
Grubbs' catalyst is a good choice for this type of system because it is insensitive to air and water, thus robust enough to maintain reactivity within the material. Using a live catalyst is important to promote multiple healing actions. [ 63 ] The major drawback is the cost. It was shown that using more of the catalyst corresponded directly to higher degree of healing. Ruthenium is quite costly, which makes it impractical for commercial applications.
In contrast, in multicapsule systems both the catalyst and the healing agent are encapsulated in different capsules. [ 64 ] In a third system, called latent functionality, a healing agent is encapsulated, that can react with the polymerizer component that is present in the matrix in the form of residual reactive functionalities. [ 60 ] In the last approach (phase separation), either the healing agent or the polymerizer is phase-separated in the matrix material. [ 65 ]
The same strategies can be applied in 1D, 2D and 3D vascular based systems. [ 66 ] [ 67 ] [ 15 ]
For the first method, fragile glass capillaries or fibers are imbedded within a composite material . (Note: this is already a commonly used practice for strengthening materials. See Fiber-reinforced plastic .) [ 68 ] The resulting porous network is filled with monomer . When damage occurs in the material from regular use, the tubes also crack and the monomer is released into the cracks. Other tubes containing a hardening agent also crack and mix with the monomer , causing the crack to be healed. [ 63 ] There are many things to take into account when introducing hollow tubes into a crystalline structure . First to consider is that the created channels may compromise the load bearing ability of the material due to the removal of load bearing material. [ 69 ] Also, the channel diameter, degree of branching, location of branch points, and channel orientation are some of the main things to consider when building up microchannels within a material. Materials that don't need to withstand much mechanical strain , but want self-healing properties, can introduce more microchannels than materials that are meant to be load bearing. [ 69 ] There are two types of hollow tubes: discrete channels, and interconnected channels. [ 69 ]
Discrete channels can be built independently of building the material and are placed in an array throughout the material. [ 69 ] When creating these microchannels, one major factor to take into account is that the closer the tubes are together, the lower the strength will be, but the more efficient the recovery will be. [ 69 ] A sandwich structure is a type of discrete channels that consists of tubes in the center of the material, and heals outwards from the middle. [ 70 ] The stiffness of sandwich structures is high, making it an attractive option for pressurized chambers. [ 70 ] For the most part in sandwich structures, the strength of the material is maintained as compared to vascular networks. Also, material shows almost full recovery from damage. [ 70 ]
Interconnected networks are more efficient than discrete channels, but are harder and more expensive to create. [ 69 ] The most basic way to create these channels is to apply basic machining principles to create micro scale channel grooves. These techniques yield channels from 600 to 700 micrometers. [ 69 ] This technique works great on the two-dimensional plane, but when trying to create a three-dimensional network, they are limited. [ 69 ]
The Direct Ink Writing (DIW) technique is a controlled extrusion of viscoelastic inks to create three-dimensional interconnected networks. [ 69 ] It works by first setting organic ink in a defined pattern. Then the structure is infiltrated with a material like an epoxy . This epoxy is then solidified , and the ink can be sucked out with a modest vacuum, creating the hollow tubes. [ 69 ]
Through dissolving a linear polymer inside a solid three-dimensional epoxy matrix, so that they are miscible to each other, the linear polymer becomes mobile at a certain temperature [ 71 ] When carbon nanotubes are also incorporated into epoxy material, and a direct current is run through the tubes, a significant shift in sensing curve indicates permanent damage to the polymer , thus ‘sensing’ a crack. [ 72 ] When the carbon nanotubes sense a crack within the structure , they can be used as thermal transports to heat up the matrix so the linear polymers can diffuse to fill the cracks in the epoxy matrix. Thus healing the material. [ 71 ]
A different approach was suggested by Prof. J. Aizenberg from Harvard University, who suggested to use Slippery Liquid-Infused Porous Surfaces (SLIPS), a porous material inspired by the carnivorous pitcher plant and filled with a lubricating liquid immiscible with both water and oil. [ 73 ] SLIPS possess self-healing and self-lubricating properties as well as icephobicity and were successfully used for many purposes.
Organic threads (such as polylactide filament for example) are stitched through laminate layers of fiber reinforced polymer, which are then boiled and vacuumed out of the material after curing of the polymer, leaving behind empty channels than can be filled with healing agents. [ 74 ]
Methods for the implementation of self-healing functionality into filled composites and fibre reinforced polymers (FRPs) are almost exclusively based on extrinsic systems and thus can be broadly classified into two approaches; discrete capsule-based systems and continuous vascular systems. In contrast to non-filled polymers, the success of an intrinsic approach based on bond reversibility has yet to be proven in FRPs.
To date, self-healing of FRPs has mostly been applied to simple structures such as flat plates and panels. There is however a somewhat limited application of self-healing in flat panels, as access to the panel surface is relatively simple and repair methods are very well established in industry. Instead, there has been a strong focus on implementing self-healing in more complex and industrially relevant structures such as T-Joints [ 75 ] [ 76 ] and Aircraft Fuselages. [ 77 ]
The creation of a capsule-based system was first reported by White et al. in 2001, [ 59 ] and this approach has since been adapted by a number of authors for introduction into fibre reinforced materials. [ 78 ] [ 79 ] [ 80 ] This method relies on the release of an encapsulated healing agent into the damage zone, and is generally a once off process as the functionality of the encapsulated healing agent cannot be restored. Even so, implemented systems are able to restore material integrity to almost 100% and remain stable over the material lifetime.
A vascular or fibre-based approach may be more appropriate for self-healing impact damage in fibre-reinforced polymer composite materials.
In this method, a network of hollow channels known as vascules, similar to the blood vessels within human tissue, are placed within the structure and used for the introduction of a healing agent. During a damage event cracks propagate through the material and into the vascules causing them to be cleaved open. A liquid resin is then passed through the vascules and into the damage plane, allowing the cracks to be repaired. Vascular systems have a number of advantages over microcapsule based systems, such as the ability to continuously deliver large volumes of repair agents and the potential to be used for repeated healing. The hollow channels themselves can also be used for additional functionality, such as thermal management and structural health monitoring. [ 81 ] A number of methods have been proposed for the introduction of these vascules, including the use of hollow glass fibres (HGFs), [ 82 ] [ 83 ] 3D printing, [ 15 ] a "lost wax" process [ 84 ] [ 85 ] and a solid preform route. [ 86 ]
Coatings allow the retention and improvement of bulk properties of a material. They can provide protection for a substrate from environmental exposure. Thus, when damage occurs (often in the form of microcracks), environmental elements like water and oxygen can diffuse through the coating and may cause material damage or failure. Microcracking in coatings can result in mechanical degradation or delamination of the coating, or in electrical failure in fibre-reinforced composites and microelectronics, respectively. As the damage is on such a small scale, repair, if possible, is often difficult and costly. Therefore, a coating that can automatically heal itself (“self-healing coating”) could prove beneficial by automatic recovering properties (such as mechanical, electrical and aesthetic properties), and thus extending the lifetime of the coating. The majority of the approaches that are described in literature regarding self-healing materials can be applied to make “self-healing” coatings, including microencapsulation [ 87 ] [ 59 ] and the introduction of reversible physical bonds such as hydrogen bonding, [ 88 ] ionomers [ 89 ] [ 90 ] and chemical bonds (Diels-Alder chemistry). [ 91 ] Microencapsulation is the most common method to develop self-healing coatings. The capsule approach originally described by White et al., using microencapsulated dicyclopentadiene (DCPD) monomer and Grubbs' catalyst to self-heal epoxy polymer [ 59 ] was later adapted to epoxy adhesive films that are commonly used in the aerospace and automotive industries for bonding metallic and composite substrates. [ 92 ] Recently, microencapsulated liquid suspensions of metal or carbon black were used to restore electrical conductivity in a multilayer microelectronic device and battery electrodes respectively; [ 93 ] [ 94 ] however the use of microencapsulation for restoration of electrical properties in coatings is limited.
Liquid metal microdroplets have also been suspended within silicone elastomer to create stretchable electrical conductors that maintain electrical conductivity when damaged, mimicking the resilience of soft biological tissue. [ 95 ] The most common application of this technique is proven in polymer coatings for corrosion protection. Corrosion protection of metallic materials is of significant importance on an economical and ecological scale. To prove the effectiveness of microcapsules in polymer coatings for corrosion protection, researchers have encapsulated a number of materials. These materials include isocyanates [ 96 ] [ 97 ] monomers such as DCPD [ 61 ] [ 79 ] GMA [ 98 ] epoxy resin, [ 99 ] linseed oil [ 100 ] [ 101 ] and tung oil., [ 102 ] and drugs.
By using the aforementioned materials for self healing in coatings, it was proven that microencapsulation effectively protects the metal against corrosion and extends the lifetime of a coating.
Coatings in high temperature applications may be designed to exhibit self-healing performance through the formation of a glass. In such situations, such as high emissivity coatings , the viscosity of the glass formed determines the self healing ability of the coating, which may compete with defect formation due to oxidation or ablation . [ 103 ] Silicate glass based self-healing materials are of particular value in thermal barrier coatings and towards space applications such as heat shields. Composite materials based on Molybdenum disilicide are the subject of various studies towards enhancing their glass-based self healing performance in coating applications. [ 104 ]
Cementitious materials have existed since the Roman era. These materials have a natural ability to self-heal, which was first reported by the French Academy of Science in 1836. [ 105 ] This ability can be improved by the integration of chemical and biochemical strategies.
Autogenous healing is the natural ability of cementitious materials to repair cracks. This ability is principally attributed to further hydration of unhydrated cement particles and carbonation of dissolved calcium hydroxide. [ 105 ] Cementitious materials in fresh-water systems can autogenously heal cracks up to 0.2 mm over a period of 7 weeks. [ 106 ]
In order to promote autogenous healing and to close wider cracks, superabsorbent polymers can be added to a cementitious mixture. [ 107 ] [ 108 ] Addition of 1 m% of selected superabsorbent polymer versus cement to a cementitious material, stimulated further hydration with nearly 40% in comparison with a traditional cementitious material, if 1 h water contact per day was allowed. [ 109 ]
Self-healing of cementitious materials can be achieved through the reaction of certain chemical agents. Two main strategies exist for housing these agents, namely capsules and vascular tubes. These capsules and vascular tubes, once ruptured, release these agents and heal the crack damage. Studies have mainly focused on improving the quality of these housings and encapsulated materials in this field. [ 110 ]
According to a 1996 study by H. L. Erlich in Chemical Geology journal, the self-healing ability of concrete has been improved by the incorporation of bacteria, which can induce calcium carbonate precipitation through their metabolic activity. [ 111 ] These precipitates can build up and form an effective seal against crack related water ingress. At the First International Conference on Self Healing Materials held in April, 2007 in The Netherlands, Henk M. Jonkers and Erik Schlangen presented their research in which they had successfully used the "alkaliphilic spore-forming bacteria" as a "self-healing agent in concrete". [ 112 ] [ 113 ] They were the first to incorporate bacteria within cement paste for the development of self-healing concrete. [ 114 ] It was found that the bacteria directly added to the paste only remained viable for 4 months. Later studies saw Jonkers use expanded clay particles [ 115 ] and Van Tittlelboom use glass tubes, [ 116 ] to protect the bacteria inside the concrete. Other strategies to protect the bacteria have also since been reported. [ 117 ]
Generally, ceramics are superior in strength to metals at high temperatures, however, they are brittle and sensitive to flaws, and this brings into question their integrity and reliability as structural materials. [ 118 ] M n + 1 AX n {\displaystyle {\ce {M_{{\mathit {n}}+1}AX_{\mathit {n}}}}} phase ceramics, also known as MAX Phases , can autonomously heal crack damage by an intrinsic healing mechanism. Micro cracks caused by wear or thermal stress are filled with oxides formed from the MAX phase constituents, commonly the A-element, during high temperature exposure to air. [ 119 ] Crack gap filling was first demonstrated for Ti 3 AlC 2 by oxidation at 1200 °C in air. [ 120 ] Ti 2 AlC and Cr 2 AlC have also demonstrated said ability, and more ternary carbides and nitrides are expected to be able to autonomously self-heal. [ 121 ] The process is repeatable up to the point of element depletion, distinguishing MAX phases from other self-healing materials that require external healing agents (extrinsic healing) for single crack gap filling. Depending on the filling-oxide, improvement of the initial properties such as local strength can be achieved. [ 122 ] On the other hand, mullite, alumina and zirconia do not have the ability to heal intrinsically, but could be endowed with self-healing capabilities by embedding second phase components into the matrix. Upon cracking, these particles are exposed to oxygen, and in the presence of heat, they react to form new materials which fill the crack gap under volume expansion. [ 123 ] This concept has been proven using SiC to heal cracks in an Alumina matrix, [ 124 ] and further studies have investigated the high temperature strength, [ 125 ] and the static and cyclic fatigue strength of the healed part. [ 126 ] The strength and bonding between the matrix and the healing agent are of prime importance and thus govern the selection of the healing particles.
When exposed for long times to high temperatures and moderate stresses, metals exhibit premature and low-ductility creep fracture, arising from the formation and growth of cavities. Those defects coalesce into cracks which ultimately cause macroscopic failure. Self-healing of early stage damage is thus a promising new approach to extend the lifetime of the metallic components. In metals, self-healing is intrinsically more difficult to achieve than in most other material classes, due to their high melting point and, as a result, low atom mobility. Generally, defects in the metals are healed by the formation of precipitates at the defect sites that immobilize further crack growth.
Improved creep and fatigue properties have been reported for underaged aluminium alloys compared to the peak hardening Al alloys, which is due to the heterogeneous precipitation at the crack tip and its plastic zone. [ 127 ] The first attempts to heal creep damage in steels were focused on the dynamic precipitation of either Cu or BN at the creep-cavity surface. [ 128 ] [ 129 ] Cu precipitation has only a weak preference for deformation-induced defects as a large fraction of spherical Cu precipitates is simultaneously formed with the matrix. [ 130 ] [ 131 ] Recently, gold atoms were recognized as a highly efficient healing agents in Fe-based alloys. A defect-induced mechanism is indicated for the Au precipitation, i.e. the Au solute remains dissolved until defects are formed. [ 132 ] Autonomous repair of high-temperature creep damage was reported by alloying with a small amount of Au. Healing agents selectively precipitate at the free surface of a creep cavity, resulting in pore filling. For the lower stress levels up to 80% filling of the creep cavities with Au precipitates is achieved [ 133 ] resulting in a substantial increase in creep life time. Work to translate the concept of creep damage healing in simple binary or ternary model systems to real multicomponent creep steels is ongoing.
In 2023 the Sandia National Laboratories reported the finding of self-healing of fatigue cracks in metal [ 134 ] [ 135 ] and reported that the observations seems to confirm a 2013 study predicting the effect. [ 136 ]
Hydrogels are soft solids consisting of a three dimensional network of natural or synthetic polymers with a high water content. Hydrogels based on non-covalent interactions or dynamic covalent chemistry can exhibit self-healing properties after cutting or breaking. [ 137 ] Hydrogels that can fully fluidize followed by self-healing are of particular interest in biomedical engineering for the development of injectable hydrogels for tissue regeneration or 3D bioprinting inks. [ 138 ]
Recently, several classes of organic dyes were discovered that self-heal after photo-degradation when doped in PMMA and other polymer matrices. [ 139 ] This is also known as reversible photo-degradation . It was shown that, unlike common process like molecular diffusion, [ 140 ] the mechanism is caused by dye-polymer interaction. [ 141 ]
It has recently been shown that micrometer-sized defects in a pristine layer of ice heal spontaneously within a matter of several hours. The generated curvature by any defect causes a local increased vapor pressure and therefore enhances the volatility of the surface molecules. Hence, the mobility of the upper layer of water molecules increases significantly. The main mechanism, that dominates this healing effect is therefore sublimation from, and condensation onto the surface. [ 142 ] This opposes earlier work that describes sintering of ice spheres by surface diffusion. [ 143 ]
Self-healing epoxies can be incorporated onto metals in order to prevent corrosion.
A substrate metal showed major degradation and rust formation after 72 hours of exposure. But after being coated with the self-healing epoxy, there was no visible damage under SEM after 72 hours of the same exposure. [ 144 ]
Numerous methodologies for the assessment of self-healing capabilities have been developed for each material class (Table 1).
Hence, when self-healing is assessed, different parameters need to be considered: type of stimulus (if any), healing time, maximum amount of healing cycles the material can tolerate, and degree of recovery, all whilst considering the material's virgin properties. [ 145 ] [ 146 ] [ 88 ] This typically takes account of relevant physical parameters such as tensile modulus, elongation at break, fatigue-resistance, barrier properties, colour and transparency.
The self-healing ability of a given material generally refers to the recovery of a specific property relative to the virgin material, designated as the self-healing efficiency. The self-healing efficiency can be quantified by comparing the respective experimental value obtained for the undamaged virgin sample ( f virgin ) with the healed sample ( f healed ) (eq. 1 ) [ 147 ]
In a variation of this definition that is relevant to extrinsic self-healing materials, the healing efficiency takes into consideration the modification of properties caused by introducing the healing agent. Accordingly, the healed sample property is compared to that of an undamaged control equipped with self-healing agent f non-healed (equation 2 ).
For a certain property Pi of a specific material, an optimal self-healing mechanism and process is characterized by the full restoration of the respective material property after a suitable, normalized damaging process. For a material where 3 different properties are assessed, it should be determined 3 efficiencies given as ƞ 1 ( P 1 ), ƞ 2 ( P 2 ) and ƞ 3 ( P 3 ).
The final average efficiency based on a number n of properties for a self-healing material is accordingly determined as the harmonic mean given by equation 3 . The harmonic mean is more appropriate than the traditional arithmetic mean, as it is less sensitive to large outliers.
At least two companies are attempting to bring the newer applications of self-healing materials to the market. Arkema , a leading chemicals company, announced in 2009 the beginning of industrial production of self-healing elastomers. [ 148 ] As of 2012, Autonomic Materials Inc., had raised over three million US dollars. [ 149 ] [ 150 ] | https://en.wikipedia.org/wiki/Self-healing_material |
Self-interference cancellation ( SIC ) is a signal processing technique that enables a radio transceiver to simultaneously transmit and receive on a single channel, a pair of partially-overlapping channels, or any pair of channels in the same frequency band. When used to allow simultaneous transmission and reception on the same frequency, sometimes referred to as “in-band full-duplex” or “simultaneous transmit and receive,” SIC effectively doubles spectral efficiency . SIC also enables devices and platforms containing two radios that use the same frequency band to operate both radios simultaneously.
Self-interference cancellation has applications in mobile networks , the unlicensed bands, cable TV , mesh networks , the military, and public safety.
In-band full-duplex has advantages over conventional duplexing schemes. A frequency division duplexing (FDD) system transmits and receives at the same time by using two (usually widely separated) channels in the same frequency band. In-band full-duplex performs the same function using half of the spectrum resources. A time division duplexing (TDD) system operates half-duplex on a single channel, creating the illusion of full-duplex communication by rapidly switching back-and-forth between transmit and receive. In-band full-duplex radios achieve twice the throughput using the same spectrum resources. [ 1 ]
A radio transceiver cannot cancel out its own transmit signal based solely on knowledge of what information is being sent and how the transmit signal is constructed. The signal that the receiver sees is not entirely predictable. The signal that appears at the receiver is subject to varying delays. It consists of a combination of leakage (the signal traveling directly from the transmitter to the receiver) and local reflections. In addition, transmitter components (such as mixers and power amplifiers) introduce non-linearities that generate harmonics and noise. These distortions must be sampled at the output of the transmitter. Finally, the self-interference cancellation solution must detect and compensate for real-time changes caused by temperature variations, mechanical vibrations, and the motion of things in the environment. [ 2 ]
The transmit signal can be cancelled out at the receiver by creating an accurate model of the signal and using it to generate a new signal that when combined with the signal arriving at the receiver leaves only the desired receive signal. The precise amount of cancellation required will vary depending on the power of the transmit signal that is the source of the self-interference and the signal-to-noise ratio (SNR) that the link is expected to handle in half-duplex mode. A typical figure for Wi-Fi and cellular applications is 110 dB of signal cancellation, though some applications require greater cancellation.
Cancelling a local transmit signal requires a combination of analog and digital electronics. The strength of the transmit signal can be modestly reduced before it reaches the receiver by using a circulator (if a shared antenna is used) or antenna isolation techniques (such as cross polarization) if separate antennas are used. The analog canceller is most effective at handling strong signals with a short delay spread. A digital canceller is most effective at handling weak signals with delays greater than 1,000 nanoseconds. The analog canceller should contribute at least 60 dB of cancellation. The digital canceller must process both linear and non-linear signal components, producing about 50 dB of cancellation. Both the analog and digital cancellers consist of a number of “taps” composed of attenuators, phase shifters, and delay elements. The cost, size, and complexity of the SIC solution is primarily determined by the analog stage. Also essential are the tuning algorithms that enable the canceller to adapt to rapid changes. Cancellation algorithms typically need to adapt at the rate of once every few hundred microseconds to keep up with changes in the environment. [ 3 ] [ 4 ]
SIC can also be employed to reduce or eliminate adjacent channel interference. This allows a device containing two radios (such as a Wi-Fi access Point with two 5 GHz radios) to use any pair of channels regardless of separation. Adjacent channel interference consists of two main components. The signal on the transmit frequency, known as the blocker, may be so strong that it desensitizes a receiver listening on an adjacent channel. A strong, local transmitter also produces noise that spills over onto the adjacent channel. SIC may be used to reduce both the blocker and the noise that might otherwise prevent use of an adjacent channel.
Transmitting and receiving on exactly the same frequency at exactly the same time has multiple purposes. In-band full duplex can potentially double spectral efficiency. It permits true full duplex operation where only a single frequency is available. And it enables “listen while talking” operation (see cognitive radio, below).
Though most small cells are expected to be fed using fiber optic cable, running fiber isn't always practical. Reuse of the frequencies used by a small cell to communicate with users (“access”) for communication between the small cell and the network (“backhaul”) will be part of the 3GPP's 5G standards. When implemented using SIC, the local backhaul radio's transmit signal is cancelled out at the small cell's receiver, and the small cell's transmit signal is cancelled out at the local backhaul radio's receiver. No changes are required to the users’ devices or the remote backhaul radio. The use of SIC in this applications has been successfully field-tested by Telecom Italia Mobile and Deutsche Telekom . [ 5 ] [ 6 ]
SIC enables satellite repeaters to extend coverage to indoor, urban canyon, and other locations by reusing the same frequencies. This type of repeater is essentially two radios connected back-to-back. One radio faces the satellite, while the other radio faces the area not in direct coverage. The two radios relay the signals (rather than store-and-forward data bits) and must be isolated from each other to prevent feedback. The satellite-facing radio listens to the satellite and must be isolated from the transmitter repeating the signal. Likewise, the indoor-facing radio listens for indoor users and must be isolated from the transmitter repeating their signals to the satellite. SIC may be used to cancel out each radio's transmit signal at the other radio's receiver.
Cable networks have traditionally allocated most of their capacity to downstream transmissions. The recent growth in user-generated content calls for more upstream capacity. Cable Labs developed the Full Duplex DOCSIS 3.1 standard to enable symmetrical service at speeds up to 10 Gbit/s in each direction. In DOCSIS 3.1, different frequencies are allocated for upstream and downstream transmissions, separated by a guard band. Full Duplex DOCSIS establishes a new band allowing a mix of upstream and downstream channels on adjacent channels. The headend must support simultaneous transmission and reception across the full duplex band, which requires SIC technology. The cable modems are not required to transmit and receive on the same channels simultaneously, but they are required to use different combinations of upstream and downstream channels as instructed by the headend. [ 7 ]
Mesh networks are used to extend coverage (to cover entire homes) and for ad-hoc networking (emergency communication). Wireless mesh networks use a mesh topology to provide the desired coverage. The data travels from one node to another until it reaches its destination. In mesh networks using a single frequency, the data is typically store-and-forwarded, with each hop adding a delay. SIC can enable wireless mesh nodes to reuse frequencies so that the data is retransmitted (relayed) as it is received. In mesh networks using multiple frequencies, such as whole-home Wi-Fi networks using “tri-band” routers, SIC can enable greater flexibility in channel selection. Tri-band routers have one 2.4 GHz and one 5 GHz radio to communicate with client devices, and a second 5 GHz radio that is used exclusively for internode communication. Most tri-band routers use the same pair of 80 MHz channels (at opposite ends of the 5 GHz band) to minimize interference. SIC can allow tri-band routers to use any of the six 80-MHz channels in the 5 GHz band for coordination both within networks and between neighboring networks.
The military frequently requires multiple, high power radios on the same air, land, or sea platform for tactical communication. These radios must be reliable even in the face of interference and enemy jamming. SIC enables multiple radios to operate on the same platform at the same time. SIC also has potential applications in military and vehicular radar, allowing radar systems to transmit and receive continuously rather than constantly switching between transmit and receive, yielding higher resolution. These new capabilities have been recognized as a potential 'superpower' for armed forces that may bring about a paradigm shift in tactical communications and electronic warfare. [ 8 ] [ 9 ]
National regulatory agencies, such as the Federal Communications Commission in the U.S., often address the need for more spectrum resources by permitting sharing of underutilized spectrum. For instance, billions of Wi-Fi and Bluetooth devices compete for access to the ISM bands . Smartphones, Wi-Fi routers, and smart home hubs frequently support Wi-Fi, Bluetooth, and other wireless technologies in the same device. SIC technology enables these devices to operate two radios in the same band at the same time. Spectrum sharing is a topic of great interest to the mobile phone industry as it begins to deploy 5G systems.
Radios that dynamically select idle channels to make more efficient use of finite spectrum resources are the subject of considerable research. Traditional spectrum sharing schemes rely on Listen-before-talk protocols. However, when two or more radios choose to transmit on the same channel at the same time there is a collision. Collisions take time to detect and resolve. SIC enables listen-while-talking, ensuring immediate detection and faster resolution of collisions. [ 10 ]
Y. Hua, Y. Ma, A. Gholian, Y. Li, A. Cirik, P. Liang, “Radio Self-Interference Cancellation by Transmit Beamforming, All-Analog Cancellation and Blind Digital Tuning,” Signal Processing, Vol. 108, pp. 322–340, 2015. | https://en.wikipedia.org/wiki/Self-interference_cancellation |
The self-ionization of water (also autoionization of water , autoprotolysis of water , autodissociation of water , or simply dissociation of water) is an ionization reaction in pure water or in an aqueous solution , in which a water molecule, H 2 O, deprotonates (loses the nucleus of one of its hydrogen atoms) to become a hydroxide ion, OH − . The hydrogen nucleus, H + , immediately protonates another water molecule to form a hydronium cation, H 3 O + . It is an example of autoprotolysis , and exemplifies the amphoteric nature of water.
The self-ionization of water was first proposed in 1884 by Svante Arrhenius as part of the theory of ionic dissociation which he proposed to explain the conductivity of electrolytes including water. Arrhenius wrote the self-ionization as H 2 O ↽ − − ⇀ H + + OH − {\displaystyle {\ce {H2O <=> H+ + OH-}}} . At that time, nothing was yet known of atomic structure or subatomic particles, so he had no reason to consider the formation of an H + {\displaystyle {\ce {H+}}} ion from a hydrogen atom on electrolysis as any less likely than, say, the formation of a Na + {\displaystyle {\ce {Na+}}} ion from a sodium atom.
In 1923 Johannes Nicolaus Brønsted and Martin Lowry proposed that the self-ionization of water actually involves two water molecules: H 2 O + H 2 O ↽ − − ⇀ H 3 O + + OH − {\displaystyle {\ce {H2O + H2O <=> H3O+ + OH-}}} . By this time the electron and the nucleus had been discovered and Rutherford had shown that a nucleus is very much smaller than an atom. This would include a bare ion H + {\displaystyle {\ce {H+}}} which would correspond to a proton with zero electrons. Brønsted and Lowry proposed that this ion does not exist free in solution, but always attaches itself to a water (or other solvent) molecule to form the hydronium ion H 3 O + {\displaystyle {\ce {H3O+}}} (or other protonated solvent).
Later spectroscopic evidence has shown that many protons are actually hydrated by more than one water molecule. The most descriptive notation for the hydrated ion is H + ( aq ) {\displaystyle {\ce {H+(aq)}}} , where aq (for aqueous) indicates an indefinite or variable number of water molecules. However the notations H + {\displaystyle {\ce {H+}}} and H 3 O + {\displaystyle {\ce {H3O+}}} are still also used extensively because of their historical importance. This article mostly represents the hydrated proton as H 3 O + {\displaystyle {\ce {H3O+}}} , corresponding to hydration by a single water molecule.
Chemically pure water has an electrical conductivity of 0.055 μ S /cm. According to the theories of Svante Arrhenius , this must be due to the presence of ions . The ions are produced by the water self-ionization reaction, which applies to pure water and any aqueous solution:
Expressed with chemical activities a , instead of concentrations, the thermodynamic equilibrium constant for the water ionization reaction is:
which is numerically equal to the more traditional thermodynamic equilibrium constant written as:
under the assumption that the sum of the chemical potentials of H + and H 3 O + is formally equal to twice the chemical potential of H 2 O at the same temperature and pressure. [ 1 ]
Because most acid–base solutions are typically very dilute, the activity of water is generally approximated as being equal to unity, which allows the ionic product of water to be expressed as: [ 2 ]
In dilute aqueous solutions, the activities of solutes (dissolved species such as ions) are approximately equal to their concentrations. Thus, the ionization constant , dissociation constant , self-ionization constant , water ion-product constant or ionic product of water, symbolized by K w , may be given by:
where [H 3 O + ] is the molarity ( molar concentration ) [ 3 ] of hydrogen cation or hydronium ion , and [OH − ] is the concentration of hydroxide ion. When the equilibrium constant is written as a product of concentrations (as opposed to activities) it is necessary to make corrections to the value of K w {\displaystyle K_{\rm {w}}} depending on ionic strength and other factors (see below). [ 4 ]
At 24.87 °C and zero ionic strength , K w is equal to 1.0 × 10 −14 . Note that as with all equilibrium constants, the result is dimensionless because the concentration is in fact a concentration relative to the standard state , which for H + and OH − are both defined to be 1 molal (= 1 mol/kg) when molality is used or 1 molar (= 1 mol/L) when molar concentration is used. For many practical purposes, the molality (mol solute/kg water) and molar (mol solute/L solution) concentrations can be considered as nearly equal at ambient temperature and pressure if the solution density remains close to one ( i.e. , sufficiently diluted solutions and negligible effect of temperature changes). The main advantage of the molal concentration unit (mol/kg water) is to result in stable and robust concentration values which are independent of the solution density and volume changes (density depending on the water salinity ( ionic strength ), temperature and pressure); therefore, molality is the preferred unit used in thermodynamic calculations or in precise or less-usual conditions, e.g., for seawater with a density significantly different from that of pure water, [ 3 ] or at elevated temperatures, like those prevailing in thermal power plants.
We can also define p K w ≡ {\displaystyle \equiv } −log 10 K w (which is approximately 14 at 25 °C). This is analogous to the notations pH and p K a for an acid dissociation constant , where the symbol p denotes a cologarithm . The logarithmic form of the equilibrium constant equation is p K w = pH + pOH.
The dependence of the water ionization on temperature and pressure has been investigated thoroughly. [ 5 ] The value of p K w decreases as temperature increases from the melting point of ice to a minimum at c. 250 °C, after which it increases up to the critical point of water c. 374 °C. It decreases with increasing pressure
With electrolyte solutions, the value of p K w is dependent on ionic strength of the electrolyte. Values for sodium chloride are typical for a 1:1 electrolyte. With 1:2 electrolytes, MX 2 , p K w decreases with increasing ionic strength. [ 8 ]
The value of K w is usually of interest in the liquid phase . Example values for superheated steam (gas) and supercritical water fluid are given in the table.
Heavy water , D 2 O, self-ionizes less than normal water, H 2 O;
This is due to the equilibrium isotope effect , a quantum mechanical effect attributed to oxygen forming a slightly stronger bond to deuterium because the larger mass of deuterium results in a lower zero-point energy .
Expressed with activities a , instead of concentrations, the thermodynamic equilibrium constant for the heavy water ionization reaction is:
Assuming the activity of the D 2 O to be 1, and assuming that the activities of the D 3 O + and OD − are closely approximated by their concentrations
The following table compares the values of p K w for H 2 O and D 2 O. [ 9 ]
In water–heavy water mixtures equilibria several species are involved: H 2 O, HDO, D 2 O, H 3 O + , D 3 O + , H 2 DO + , HD 2 O + , HO − , DO − .
The rate of reaction for the ionization reaction
depends on the activation energy , Δ E ‡ . According to the Boltzmann distribution the proportion of water molecules that have sufficient energy, due to thermal population, is given by
where k is the Boltzmann constant . Thus some dissociation can occur because sufficient thermal energy is available. The following sequence of events has been proposed on the basis of electric field fluctuations in liquid water. [ 10 ] Random fluctuations in molecular motions occasionally (about once every 10 hours per water molecule [ 11 ] ) produce an electric field strong enough to break an oxygen–hydrogen bond , resulting in a hydroxide (OH − ) and hydronium ion (H 3 O + ); the hydrogen nucleus of the hydronium ion travels along water molecules by the Grotthuss mechanism and a change in the hydrogen bond network in the solvent isolates the two ions, which are stabilized by solvation. Within 1 picosecond , however, a second reorganization of the hydrogen bond network allows rapid proton transfer down the electric potential difference and subsequent recombination of the ions. This timescale is consistent with the time it takes for hydrogen bonds to reorientate themselves in water. [ 12 ] [ 13 ] [ 14 ]
The inverse recombination reaction
is among the fastest chemical reactions known, with a reaction rate constant of 1.3 × 10 11 M −1 s −1 at room temperature. Such a rapid rate is characteristic of a diffusion-controlled reaction , in which the rate is limited by the speed of molecular diffusion . [ 15 ]
Water molecules dissociate into equal amounts of H 3 O + and OH − , so their concentrations are almost exactly 1.00 × 10 −7 mol dm −3 at 25 °C and 0.1 MPa. A solution in which the H 3 O + and OH − concentrations equal each other is considered a neutral solution. In general, the pH of the neutral point is numerically equal to 1 / 2 p K w .
Pure water is neutral, but most water samples contain impurities. If an impurity is an acid or base , this will affect the concentrations of hydronium ion and hydroxide ion. Water samples that are exposed to air will absorb some carbon dioxide to form carbonic acid (H 2 CO 3 ) and the concentration of H 3 O + will increase due to the reaction H 2 CO 3 + H 2 O = HCO 3 − + H 3 O + . The concentration of OH − will decrease in such a way that the product [H 3 O + ][OH − ] remains constant for fixed temperature and pressure. Thus these water samples will be slightly acidic. If a pH of exactly 7.0 is required, it must be maintained with an appropriate buffer solution . | https://en.wikipedia.org/wiki/Self-ionization_of_water |
A self-making bed (also known as a smart bed ) is designed to automatically rearrange the bedding on a bed and prepare itself for use.
In 2008, inventor Enrico Berruti featured his self-making bed, dubbed "Selfy", at The International Exhibition of Inventions in Geneva, Switzerland. The bed makes itself by stretching and smoothing the sheets over the mattress by using metal rails that connect to the bed sheets alongside the bed. [ 1 ]
In 2017, the company Smartduvet released a fabric to make the bed through a network of air chambers. This is a breathable layer that is made of lightweight material. When activated it inflates the sheet's air chamber, placing the duvet and sheets back in position. Using an app, the user can preset a different bed-making time for each day of the week. It does not replace the existing bed and is non-permanent so it can be used with existing duvet and duvet coverbedding. [ 2 ] [ 3 ]
This article about furniture or furnishing is a stub . You can help Wikipedia by expanding it .
This technology-related article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Self-making_bed |
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Self-organization , also called spontaneous order in the social sciences , is a process where some form of overall order arises from local interactions between parts of an initially disordered system . The process can be spontaneous when sufficient energy is available, not needing control by any external agent. It is often triggered by seemingly random fluctuations , amplified by positive feedback . The resulting organization is wholly decentralized, distributed over all the components of the system. As such, the organization is typically robust and able to survive or self-repair substantial perturbation . Chaos theory discusses self-organization in terms of islands of predictability in a sea of chaotic unpredictability.
Self-organization occurs in many physical , chemical , biological , robotic , and cognitive systems. Examples of self-organization include crystallization , thermal convection of fluids, chemical oscillation , animal swarming , neural circuits , and black markets .
Self-organization is realized [ 2 ] in the physics of non-equilibrium processes , and in chemical reactions , where it is often characterized as self-assembly . The concept has proven useful in biology, from the molecular to the ecosystem level. [ 3 ] Cited examples of self-organizing behavior also appear in the literature of many other disciplines, both in the natural sciences and in the social sciences (such as economics or anthropology ). Self-organization has also been observed in mathematical systems such as cellular automata . [ 4 ] Self-organization is an example of the related concept of emergence . [ 5 ]
Self-organization relies on four basic ingredients: [ 6 ]
The cybernetician William Ross Ashby formulated the original principle of self-organization in 1947. [ 7 ] [ 8 ] It states that any deterministic dynamic system automatically evolves towards a state of equilibrium that can be described in terms of an attractor in a basin of surrounding states. Once there, the further evolution of the system is constrained to remain in the attractor. This constraint implies a form of mutual dependency or coordination between its constituent components or subsystems. In Ashby's terms, each subsystem has adapted to the environment formed by all other subsystems. [ 7 ]
The cybernetician Heinz von Foerster formulated the principle of " order from noise " in 1960. [ 9 ] It notes that self-organization is facilitated by random perturbations ("noise") that let the system explore a variety of states in its state space. This increases the chance that the system will arrive into the basin of a "strong" or "deep" attractor, from which it then quickly enters the attractor itself. The biophysicist Henri Atlan developed this concept by proposing the principle of " complexity from noise" [ 10 ] [ 11 ] ( French : le principe de complexité par le bruit ) [ 12 ] first in the 1972 book L'organisation biologique et la théorie de l'information and then in the 1979 book Entre le cristal et la fumée . The physicist and chemist Ilya Prigogine formulated a similar principle as "order through fluctuations" [ 13 ] or "order out of chaos". [ 14 ] It is applied in the method of simulated annealing for problem solving and machine learning . [ 15 ]
The idea that the dynamics of a system can lead to an increase in its organization has a long history. The ancient atomists such as Democritus and Lucretius believed that a designing intelligence is unnecessary to create order in nature, arguing that given enough time and space and matter, order emerges by itself. [ 16 ]
The philosopher René Descartes presents self-organization hypothetically in the fifth part of his 1637 Discourse on Method . He elaborated on the idea in his unpublished work The World . [ a ]
Immanuel Kant used the term "self-organizing" in his 1790 Critique of Judgment , where he argued that teleology is a meaningful concept only if there exists such an entity whose parts or "organs" are simultaneously ends and means. Such a system of organs must be able to behave as if it has a mind of its own, that is, it is capable of governing itself. [ 17 ]
In such a natural product as this every part is thought as owing its presence to the agency of all the remaining parts, and also as existing for the sake of the others and of the whole, that is as an instrument, or organ... The part must be an organ producing the other parts—each, consequently, reciprocally producing the others... Only under these conditions and upon these terms can such a product be an organized and self-organized being, and, as such, be called a physical end . [ 17 ]
Sadi Carnot (1796–1832) and Rudolf Clausius (1822–1888) discovered the second law of thermodynamics in the 19th century. It states that total entropy , sometimes understood as disorder, will always increase over time in an isolated system . This means that a system cannot spontaneously increase its order without an external relationship that decreases order elsewhere in the system (e.g. through consuming the low-entropy energy of a battery and diffusing high-entropy heat). [ 18 ] [ 19 ]
18th-century thinkers had sought to understand the "universal laws of form" to explain the observed forms of living organisms. This idea became associated with Lamarckism and fell into disrepute until the early 20th century, when D'Arcy Wentworth Thompson (1860–1948) attempted to revive it. [ 20 ]
The psychiatrist and engineer W. Ross Ashby introduced the term "self-organizing" to contemporary science in 1947. [ 7 ] It was taken up by the cyberneticians Heinz von Foerster , Gordon Pask , Stafford Beer ; and von Foerster organized a conference on "The Principles of Self-Organization" at the University of Illinois' Allerton Park in June, 1960 which led to a series of conferences on Self-Organizing Systems. [ 21 ] Norbert Wiener took up the idea in the second edition of his Cybernetics: or Control and Communication in the Animal and the Machine (1961).
Self-organization was associated [ by whom? ] with general systems theory in the 1960s, but did not become commonplace in the scientific literature until physicists Hermann Haken et al. and complex systems researchers adopted it in a greater picture from cosmology Erich Jantsch , [ clarification needed ] chemistry with dissipative system , biology and sociology as autopoiesis to system thinking in the following 1980s ( Santa Fe Institute ) and 1990s ( complex adaptive system ), until our days with the disruptive emerging technologies profounded by a rhizomatic network theory . [ 22 ] [ original research? ]
Around 2008–2009, a concept of guided self-organization started to take shape. This approach aims to regulate self-organization for specific purposes, so that a dynamical system may reach specific attractors or outcomes. The regulation constrains a self-organizing process within a complex system by restricting local interactions between the system components, rather than following an explicit control mechanism or a global design blueprint. The desired outcomes, such as increases in the resultant internal structure and/or functionality, are achieved by combining task-independent global objectives with task-dependent constraints on local interactions. [ 23 ] [ 24 ]
The many self-organizing phenomena in physics include phase transitions and spontaneous symmetry breaking such as spontaneous magnetization and crystal growth in classical physics , and the laser , [ 25 ] superconductivity and Bose–Einstein condensation in quantum physics . Self-organization is found in self-organized criticality in dynamical systems , in tribology , in spin foam systems, and in loop quantum gravity , [ 26 ] in plasma , [ 27 ] in river basins and deltas, in dendritic solidification (snow flakes), in capillary imbibition [ 28 ] and in turbulent structure. [ 3 ] [ 4 ]
Self-organization in chemistry includes drying-induced self-assembly, [ 30 ] molecular self-assembly , [ 31 ] reaction–diffusion systems and oscillating reactions , [ 32 ] autocatalytic networks, liquid crystals , [ 33 ] grid complexes , colloidal crystals , self-assembled monolayers , [ 34 ] [ 35 ] micelles , microphase separation of block copolymers , and Langmuir–Blodgett films . [ 36 ]
Self-organization in biology [ 37 ] can be observed in spontaneous folding of proteins and other biomacromolecules, self-assembly of lipid bilayer membranes, pattern formation and morphogenesis in developmental biology , the coordination of human movement, eusocial behavior in insects ( bees , ants , termites ) [ 38 ] and mammals , and flocking behavior in birds and fish. [ 39 ]
The mathematical biologist Stuart Kauffman and other structuralists have suggested that self-organization may play roles alongside natural selection in three areas of evolutionary biology , namely population dynamics , molecular evolution , and morphogenesis . However, this does not take into account the essential role of energy in driving biochemical reactions in cells. The systems of reactions in any cell are self-catalyzing , but not simply self-organizing, as they are thermodynamically open systems relying on a continuous input of energy. [ 40 ] [ 41 ] Self-organization is not an alternative to natural selection, but it constrains what evolution can do and provides mechanisms such as the self-assembly of membranes which evolution then exploits. [ 42 ]
The evolution of order in living systems and the generation of order in certain non-living systems was proposed to obey a common fundamental principal called “the Darwinian dynamic” [ 43 ] that was formulated by first considering how microscopic order is generated in simple non-biological systems that are far from thermodynamic equilibrium . Consideration was then extended to short, replicating RNA molecules assumed to be similar to the earliest forms of life in the RNA world . It was shown that the underlying order-generating processes of self-organization in the non-biological systems and in replicating RNA are basically similar.
In his 1995 conference paper "Cosmology as a problem in critical phenomena" Lee Smolin said that several cosmological objects or phenomena, such as spiral galaxies , galaxy formation processes in general, early structure formation , quantum gravity and the large scale structure of the universe might be the result of or have involved certain degree of self-organization. [ 44 ] He argues that self-organized systems are often critical systems , with structure spreading out in space and time over every available scale, as shown for example by Per Bak and his collaborators. Therefore, because the distribution of matter in the universe is more or less scale invariant over many orders of magnitude, ideas and strategies developed in the study of self-organized systems could be helpful in tackling certain unsolved problems in cosmology and astrophysics .
Phenomena from mathematics and computer science such as cellular automata , random graphs , and some instances of evolutionary computation and artificial life exhibit features of self-organization. In swarm robotics , self-organization is used to produce emergent behavior. In particular the theory of random graphs has been used as a justification for self-organization as a general principle of complex systems. In the field of multi-agent systems , understanding how to engineer systems that are capable of presenting self-organized behavior is an active research area. [ 45 ] Optimization algorithms can be considered self-organizing because they aim to find the optimal solution to a problem. If the solution is considered as a state of the iterative system, the optimal solution is the selected, converged structure of the system. [ 46 ] [ 47 ] Self-organizing networks include small-world networks [ 48 ] self-stabilization [ 49 ] and scale-free networks . These emerge from bottom-up interactions, unlike top-down hierarchical networks within organizations, which are not self-organizing. [ 50 ] Cloud computing systems have been argued to be inherently self-organizing, [ 51 ] but while they have some autonomy, they are not self-managing as they do not have the goal of reducing their own complexity. [ 52 ] [ 53 ]
Norbert Wiener regarded the automatic serial identification of a black box and its subsequent reproduction as self-organization in cybernetics . [ 54 ] The importance of phase locking or the "attraction of frequencies", as he called it, is discussed in the 2nd edition of his Cybernetics: Or Control and Communication in the Animal and the Machine . [ 55 ] K. Eric Drexler sees self-replication as a key step in nano and universal assembly . By contrast, the four concurrently connected galvanometers of W. Ross Ashby 's Homeostat hunt , when perturbed, to converge on one of many possible stable states. [ 56 ] Ashby used his state counting measure of variety [ 57 ] to describe stable states and produced the " Good Regulator " [ 58 ] theorem which requires internal models for self-organized endurance and stability (e.g. Nyquist stability criterion ). Warren McCulloch proposed "Redundancy of Potential Command" [ 59 ] as characteristic of the organization of the brain and human nervous system and the necessary condition for self-organization. Heinz von Foerster proposed Redundancy, R =1 − H / H max , where H is entropy . [ 60 ] [ 61 ] In essence this states that unused potential communication bandwidth is a measure of self-organization.
In the 1970s Stafford Beer considered self-organization necessary for autonomy in persisting and living systems. He applied his viable system model to management. It consists of five parts: the monitoring of performance of the survival processes (1), their management by recursive application of regulation (2), homeostatic operational control (3) and development (4) which produce maintenance of identity (5) under environmental perturbation. Focus is prioritized by an alerting "algedonic loop" feedback: a sensitivity to both pain and pleasure produced from under-performance or over-performance relative to a standard capability. [ 62 ]
In the 1990s Gordon Pask argued that von Foerster's H and Hmax were not independent, but interacted via countably infinite recursive concurrent spin processes [ 63 ] which he called concepts. His strict definition of concept "a procedure to bring about a relation" [ 64 ] permitted his theorem "Like concepts repel, unlike concepts attract" [ 65 ] to state a general spin-based principle of self-organization. His edict, an exclusion principle, "There are No Doppelgangers " means no two concepts can be the same. After sufficient time, all concepts attract and coalesce as pink noise . The theory applies to all organizationally closed or homeostatic processes that produce enduring and coherent products which evolve, learn and adapt. [ 66 ] [ 63 ]
The self-organizing behavior of social animals and the self-organization of simple mathematical structures both suggest that self-organization should be expected in human society . Tell-tale signs of self-organization are usually statistical properties shared with self-organizing physical systems. Examples such as critical mass , herd behavior , groupthink and others, abound in sociology , economics , behavioral finance and anthropology . [ 67 ] Spontaneous order can be influenced by arousal . [ 68 ]
In social theory, the concept of self-referentiality has been introduced as a sociological application of self-organization theory by Niklas Luhmann (1984). For Luhmann the elements of a social system are self-producing communications, i.e. a communication produces further communications and hence a social system can reproduce itself as long as there is dynamic communication. For Luhmann, human beings are sensors in the environment of the system. Luhmann developed an evolutionary theory of society and its subsystems, using functional analyses and systems theory. [ 69 ]
The market economy is sometimes said to be self-organizing. Paul Krugman has written on the role that market self-organization plays in the business cycle in his book The Self Organizing Economy . [ 70 ] Friedrich Hayek coined the term catallaxy [ 71 ] to describe a "self-organizing system of voluntary co-operation", in regards to the spontaneous order of the free market economy. Neo-classical economists hold that imposing central planning usually makes the self-organized economic system less efficient. On the other end of the spectrum, economists consider that market failures are so significant that self-organization produces bad results and that the state should direct production and pricing. Most economists adopt an intermediate position and recommend a mixture of market economy and command economy characteristics (sometimes called a mixed economy ). When applied to economics, the concept of self-organization can quickly become ideologically imbued. [ 72 ] [ 73 ]
Enabling others to "learn how to learn" [ 74 ] is often taken to mean instructing them [ 75 ] how to submit to being taught. Self-organized learning (SOL) [ 76 ] [ 77 ] [ 78 ] denies that "the expert knows best" or that there is ever "the one best method", [ 79 ] [ 80 ] [ 81 ] insisting instead on "the construction of personally significant, relevant and viable meaning" [ 82 ] to be tested experientially by the learner. [ 83 ] This may be collaborative, and more rewarding personally. [ 84 ] [ 85 ] It is seen as a lifelong process, not limited to specific learning environments (home, school, university) or under the control of authorities such as parents and professors. [ 86 ] It needs to be tested, and intermittently revised, through the personal experience of the learner. [ 87 ] It need not be restricted by either consciousness or language. [ 88 ] Fritjof Capra argued that it is poorly recognized within psychology and education. [ 89 ] It may be related to cybernetics as it involves a negative feedback control loop, [ 64 ] or to systems theory . [ 90 ] It can be conducted as a learning conversation or dialog between learners or within one person. [ 91 ] [ 92 ]
The self-organizing behavior of drivers in traffic flow determines almost all the spatiotemporal behavior of traffic, such as traffic breakdown at a highway bottleneck, highway capacity, and the emergence of moving traffic jams. These self-organizing effects are explained by Boris Kerner 's three-phase traffic theory . [ 93 ]
Order appears spontaneously in the evolution of language as individual and population behavior interacts with biological evolution. [ 94 ]
Self-organized funding allocation ( SOFA ) is a method of distributing funding for scientific research . In this system, each researcher is allocated an equal amount of funding, and is required to anonymously allocate a fraction of their funds to the research of others. Proponents of SOFA argue that it would result in similar distribution of funding as the present grant system, but with less overhead. [ 95 ] In 2016, a test pilot of SOFA began in the Netherlands. [ 96 ]
Heinz Pagels , in a 1985 review of Ilya Prigogine and Isabelle Stengers 's book Order Out of Chaos in Physics Today , appeals to authority: [ 97 ]
Most scientists would agree with the critical view expressed in Problems of Biological Physics (Springer Verlag, 1981) by the biophysicist L. A. Blumenfeld, when he wrote: "The meaningful macroscopic ordering of biological structure does not arise due to the increase of certain parameters or a system above their critical values. These structures are built according to program-like complicated architectural structures, the meaningful information created during many billions of years of chemical and biological evolution being used." Life is a consequence of microscopic, not macroscopic, organization.
Of course, Blumenfeld does not answer the further question of how those program-like structures emerge in the first place. His explanation leads directly to infinite regress .
In short, they [Prigogine and Stengers] maintain that time irreversibility is not derived from a time-independent microworld, but is itself fundamental. The virtue of their idea is that it resolves what they perceive as a "clash of doctrines" about the nature of time in physics . Most physicists would agree that there is neither empirical evidence to support their view, nor is there a mathematical necessity for it. There is no "clash of doctrines." Only Prigogine and a few colleagues hold to these speculations which, in spite of their efforts, continue to live in the twilight zone of scientific credibility.
In theology , Thomas Aquinas (1225–1274) in his Summa Theologica assumes a teleological created universe in rejecting the idea that something can be a self-sufficient cause of its own organization: [ 98 ]
Since nature works for a determinate end under the direction of a higher agent, whatever is done by nature must needs be traced back to God, as to its first cause. So also whatever is done voluntarily must also be traced back to some higher cause other than human reason or will, since these can change or fail; for all things that are changeable and capable of defect must be traced back to an immovable and self-necessary first principle, as was shown in the body of the Article. | https://en.wikipedia.org/wiki/Self-organization |
Collective intelligence Collective action Self-organized criticality Herd mentality Phase transition Agent-based modelling Synchronization Ant colony optimization Particle swarm optimization Swarm behaviour
Social network analysis Small-world networks Centrality Motifs Graph theory Scaling Robustness Systems biology Dynamic networks
Evolutionary computation Genetic algorithms Genetic programming Artificial life Machine learning Evolutionary developmental biology Artificial intelligence Evolutionary robotics
Reaction–diffusion systems Partial differential equations Dissipative structures Percolation Cellular automata Spatial ecology Self-replication
Conversation theory Entropy Feedback Goal-oriented Homeostasis Information theory Operationalization Second-order cybernetics Self-reference System dynamics Systems science Systems thinking Sensemaking Variety
Ordinary differential equations Phase space Attractors Population dynamics Chaos Multistability Bifurcation
Rational choice theory Bounded rationality
Self-organized criticality ( SOC ) is a property of dynamical systems that have a critical point as an attractor . Their macroscopic behavior thus displays the spatial or temporal scale-invariance characteristic of the critical point of a phase transition , but without the need to tune control parameters to a precise value, because the system, effectively, tunes itself as it evolves towards criticality.
The concept was put forward by Per Bak , Chao Tang and Kurt Wiesenfeld ("BTW") in a paper [ 1 ] following an earlier paper [ 2 ] by Jonathan Katz published in 1987 in Physical Review Letters , and is considered to be one of the mechanisms by which complexity [ 3 ] arises in nature. Its concepts have been applied across fields as diverse as geophysics , [ 4 ] [ 5 ] [ 6 ] physical cosmology , evolutionary biology and ecology , bio-inspired computing and optimization (mathematics) , economics , quantum gravity , sociology , solar physics , plasma physics , neurobiology [ 7 ] [ 8 ] [ 9 ] [ 10 ] [ 11 ] and others.
SOC is typically observed in slowly driven non-equilibrium systems with many degrees of freedom and strongly nonlinear dynamics. Many individual examples have been identified since BTW's original paper, but to date there is no known set of general characteristics that guarantee a system will display SOC.
Self-organized criticality is one of a number of important discoveries made in statistical physics and related fields over the latter half of the 20th century, discoveries which relate particularly to the study of complexity in nature. For example, the study of cellular automata , from the early discoveries of Stanislaw Ulam and John von Neumann through to John Conway 's Game of Life and the extensive work of Stephen Wolfram , made it clear that complexity could be generated as an emergent feature of extended systems with simple local interactions. Over a similar period of time, Benoît Mandelbrot 's large body of work on fractals showed that much complexity in nature could be described by certain ubiquitous mathematical laws, while the extensive study of phase transitions carried out in the 1960s and 1970s showed how scale invariant phenomena such as fractals and power laws emerged at the critical point between phases.
The term self-organized criticality was first introduced in Bak , Tang and Wiesenfeld 's 1987 paper, which clearly linked together those factors: a simple cellular automaton was shown to produce several characteristic features observed in natural complexity ( fractal geometry, pink (1/f) noise and power laws ) in a way that could be linked to critical-point phenomena . Crucially, however, the paper emphasized that the complexity observed emerged in a robust manner that did not depend on finely tuned details of the system: variable parameters in the model could be changed widely without affecting the emergence of critical behavior: hence, self-organized criticality. Thus, the key result of BTW's paper was its discovery of a mechanism by which the emergence of complexity from simple local interactions could be spontaneous —and therefore plausible as a source of natural complexity—rather than something that was only possible in artificial situations in which control parameters are tuned to precise critical values. An alternative view is that SOC appears when the criticality is linked to a value of zero of the control parameters. [ 12 ]
Despite the considerable interest and research output generated from the SOC hypothesis, there remains no general agreement with regards to its mechanisms in abstract mathematical form. Bak Tang and Wiesenfeld based their hypothesis on the behavior of their sandpile model. [ 1 ]
In chronological order of development:
Early theoretical work included the development of a variety of alternative SOC-generating dynamics distinct from the BTW model, attempts to prove model properties analytically (including calculating the critical exponents [ 17 ] [ 18 ] ), and examination of the conditions necessary for SOC to emerge. One of the important issues for the latter investigation was whether conservation of energy was required in the local dynamical exchanges of models: the answer in general is no, but with (minor) reservations, as some exchange dynamics (such as those of BTW) do require local conservation at least on average [ clarification needed ] .
It has been argued that the energy released in the BTW "sandpile" model should actually generate 1/f 2 noise rather than 1/f noise. [ 19 ] This claim was based on untested scaling assumptions, and a more rigorous analysis showed that sandpile models
generally produce 1/f a spectra, with a<2. [ 20 ] However, the dynamics of the accumulated stress does exhibit the 1/f noise in the BTW model. [ 21 ] Other simulation models were proposed later that could also produce true 1/f noise. [ 22 ]
In addition to the nonconservative theoretical model mentioned above [ clarification needed ] , other theoretical models for SOC have been based upon information theory , [ 23 ] mean field theory , [ 24 ] the convergence of random variables , [ 25 ] and cluster formation. [ 26 ] A continuous model of self-organised criticality is proposed by using tropical geometry . [ 27 ]
Key theoretical issues yet to be resolved include the calculation of the possible universality classes of SOC behavior and the question of whether it is possible to derive a general rule for determining if an arbitrary algorithm displays SOC.
SOC has become established as a strong candidate for explaining a number of natural phenomena, including:
Despite the numerous applications of SOC to understanding natural phenomena, the universality of SOC theory has been questioned. For example, experiments with real piles of rice revealed their dynamics to be far more sensitive to parameters than originally predicted. [ 37 ] [ 1 ] Furthermore, it has been argued that 1/f scaling in EEG recordings are inconsistent with critical states, [ 38 ] and whether SOC is a fundamental property of neural systems remains an open and controversial topic. [ 39 ]
It has been found that the avalanches from an SOC process make effective patterns in a random search for optimal solutions on graphs. [ 40 ] An example of such an optimization problem is graph coloring . The SOC process apparently helps the optimization from getting stuck in a local optimum without the use of any annealing scheme, as suggested by previous work on extremal optimization . | https://en.wikipedia.org/wiki/Self-organized_criticality |
In applied physics , the concept of controlling self-organized criticality refers to the control of processes by which a self-organized system dissipates energy . The objective of the control is to reduce the probability of occurrence of and size of energy dissipation bursts, often called avalanches , of self-organized systems. Dissipation of energy in a self-organized critical system into a lower energy state can be costly for society, since it depends on avalanches of all sizes usually following a kind of power law distribution and large avalanches can be damaging and disruptive. [ 1 ] [ 2 ] [ 3 ]
Several strategies have been proposed to deal with the issue of controlling self-organized criticality:
There are several events that arise in nature or society and that these ideas of control may help to avoid: [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 6 ] [ 7 ] [ 8 ]
The failure cascades in electrical transmission and financial sectors occur because economic forces that push for efficiency cause these systems to operate near a critical point, where avalanches of indeterminate size become possible. Financial investments that are vulnerable to this kind of failure may exhibit a Taleb distribution . | https://en.wikipedia.org/wiki/Self-organized_criticality_control |
A self-organizing network ( SON ) is an automation technology designed to make the planning, configuration, management, optimization and healing of mobile radio access networks simpler and faster. SON functionality and behavior has been defined and specified in generally accepted mobile industry recommendations produced by organizations such as 3GPP (3rd Generation Partnership Project) and the NGMN (Next Generation Mobile Networks).
SON has been codified within 3GPP Release 8 and subsequent specifications in a series of standards including 36.902, [ 1 ] as well as public white papers outlining use cases from the NGMN. [ 2 ] The first technology making use of SON features will be Long Term Evolution (LTE), but the technology has also been retro-fitted to older radio access technologies such as Universal Mobile Telecommunications System (UMTS). The LTE specification inherently supports SON features like Automatic Neighbor Relation (ANR) detection, which is the 3GPP LTE Rel. 8 flagship feature. [ 3 ]
Newly added base stations should be self-configured in line with a "plug-and-play" paradigm while all operational base stations will regularly self-optimize parameters and algorithmic behavior in response to observed network performance and radio conditions. Furthermore, self-healing mechanisms can be triggered to temporarily compensate for a detected equipment outage, while awaiting a more permanent solution.
Self-organizing networks are commonly divided into three major architectural types.
In this type of SON (D-SON), functions are distributed among the network elements at the edge of the network, typically the ENodeB elements. This implies a certain degree of localization of functionality and is normally supplied by the network equipment vendor manufacturing the radio cell.
In centralized SON (C-SON), function is more typically concentrated closer to higher-order network nodes or the network OSS , to allow a broader overview of more edge elements and coordination of e.g. load across a wide geographic area. Due to the need to inter-work with cells supplied by different equipment vendors, C-SON systems are more typically supplied by 3rd parties.
Hybrid SON is a mix of centralized and distributed SON, combining elements of each in a hybrid solution.
Self-organizing network functionalities are commonly divided into three major sub-functional groups, each containing a wide range of decomposed use cases.
Self-configuration strives towards the "plug-and-play" paradigm in the way that new base stations shall automatically be configured and integrated into the network. This means both connectivity establishment, and download of configuration parameters are software. Self-configuration is typically supplied as part of the software delivery with each radio cell by equipment vendors. When a new base station is introduced into the network and powered on, it gets immediately recognized and registered by the network. The neighboring base stations then automatically adjust their technical parameters (such as emission power, antenna tilt, etc.) in order to provide the required coverage and capacity, and, in the same time, avoid the interference.
Every base station contains hundreds of configuration parameters that control various aspects of the cell site. Each of these can be altered to change network behaviour, based on observations of both the base station itself and measurements at the mobile station or handset. One of the first SON features establishes neighbour relations automatically (ANR) while others optimise random access parameters or mobility robustness in terms of handover oscillations. A very illustrative use case is the automatic switch-off of a percent of base stations during the night hours. The neighbouring base station would then re-configure their parameters in order to keep the entire area covered by the signal. In case of a sudden growth in connectivity demand for any reason, the "sleeping" base stations "wake up" almost instantaneously. This mechanism leads to significant energy savings for operators.
When some nodes in the network become inoperative, self-healing mechanisms aim at reducing the impacts from the failure, for example by adjusting parameters and algorithms in adjacent cells so that other nodes can support the users that were supported by the failing node. In legacy networks, the failing base stations are at times hard to identify and a significant amount of time and resources is required to fix it. This function of SON permits to spot such a failing base stations immediately in order to take further measures, and ensure no or insignificant degradation of service for the users.
It is a proactive approach of a system for defending itself from the penetration of any unauthorised user in the system and from any active or passive attack. The main objectives of self-protection are to make the security of the system unbreakable and also make the data confidential and secure.
Self-organizing Networks features are being introduced gradually with the arrival of new 4G systems in radio access networks, allowing for the impact of potential ‘teething troubles’ to be limited and gradually increasing confidence. Self-optimization mechanisms in mobile radio access networks can be seen to have some similarities to automated trading algorithms in financial markets. SON has also been retrofitted to existing 3G networks to help reduce cost and improve service reliability.
The Mobile World Congress trade conference in 2009 saw the first major announcements of SON functionality for LTE mobile networks. First deployments occurred in Japan and USA during 2009/10. [ 4 ]
Among other benefits, SON deployments have enabled mobile operators to decrease network roll-out times, reduce dropped calls, improve throughput, lessen congestion and achieve other operational efficiencies including energy and cost savings.
C. Brunner, D. Flore: Generation of Pathloss and Interference Maps as SON Enabler in Deployed UMTS Networks. In: Proceedings of IEEE Vehicular Technology Conf. (VTC Spring '09). Barcelona, Spain, April 2009 | https://en.wikipedia.org/wiki/Self-organizing_network |
Self-oscillation is the generation and maintenance of a periodic motion by a source of power that lacks any corresponding periodicity. The oscillator itself controls the phase with which the external power acts on it. Self-oscillators are therefore distinct from forced and parametric resonators , in which the power that sustains the motion must be modulated externally.
In linear systems , self-oscillation appears as an instability associated with a negative damping term, which causes small perturbations to grow exponentially in amplitude. This negative damping is due to a positive feedback between the oscillation and the modulation of the external source of power. The amplitude and waveform of steady self-oscillations are determined by the nonlinear characteristics of the system .
Self-oscillations are important in physics, engineering, biology, and economics.
The study of self-oscillators dates back to the early 1830s, with the work of Robert Willis and George Biddell Airy on the mechanism by which the vocal cords produce the human voice. [ 1 ] Another instance of self-oscillation, associated with the unstable operation of centrifugal governors , was studied mathematically by James Clerk Maxwell in 1867. [ 2 ] In the second edition of his treatise on The Theory of Sound , published in 1896, Lord Rayleigh considered various instances of mechanical and acoustic self-oscillations (which he called "maintained vibration") and offered a simple mathematical model for them. [ 1 ]
Interest in the subject of self-oscillation was also stimulated by the work of Heinrich Hertz , starting in 1887, in which he used a spark-gap transmitter to generate radio waves that he showed correspond to electrical oscillations with frequencies of hundreds of millions of cycles per second. Hertz's work led to the development of wireless telegraphy . The first detailed theoretical work on such electrical self-oscillation was carried out by Henri Poincaré in the early 20th century. [ 3 ]
The term "self-oscillation" (also translated as "auto-oscillation") was coined by the Soviet physicist Aleksandr Andronov , who studied them in the context of the mathematical theory of the structural stability of dynamical systems . [ 1 ] Other important work on the subject, both theoretical and experimental, was due to André Blondel , Balthasar van der Pol , Alfred-Marie Liénard , and Philippe Le Corbeiller in the 20th century. [ 1 ]
The same phenomenon is sometimes labelled as "maintained", "sustained", "self-exciting", "self-induced", "spontaneous", or "autonomous" oscillation. Unwanted self-oscillations are known in the mechanical engineering literature as hunting , and in electronics as parasitic oscillations . [ 1 ]
Self-oscillation is manifested as a linear instability of a dynamical system's static equilibrium . Two mathematical tests that can be used to diagnose such an instability are the Routh–Hurwitz and Nyquist criteria. The amplitude of the oscillation of an unstable system grows exponentially with time (i.e., small oscillations are negatively damped), until nonlinearities become important and limit the amplitude. This can produce a steady and sustained oscillation. In some cases, self-oscillation can be seen as resulting from a time lag in a closed loop system, which makes the change in variable x t dependent on the variable x t-1 evaluated at an earlier time. [ 1 ]
Simple mathematical models of self-oscillators involve negative linear damping and positive non-linear damping terms, leading to a Hopf bifurcation and the appearance of limit cycles . [ 1 ] The van der Pol oscillator is one such model that has been used extensively in the mathematical literature.
Hunting oscillation in railway wheels and shimmy in automotive tires can cause an uncomfortable wobbling effect, which in extreme cases can derail trains and cause cars to lose grip.
Early central heating thermostats were guilty of self-exciting oscillation because they responded too quickly. The problem was overcome by hysteresis , i.e., making them switch state only when the temperature varied from the target by a specified minimum amount.
Self-exciting oscillation occurred in early automatic transmission designs when the vehicle was traveling at a speed which was between the ideal speeds of 2 gears. In these situations the transmission system would switch almost continuously between the 2 gears, which was both annoying and hard on the transmission. Such behavior is now inhibited by introducing hysteresis into the system.
There are many examples of self-exciting oscillation caused by delayed course corrections, ranging from light aircraft in a strong wind to erratic steering of road vehicles by a driver who is inexperienced or drunk.
If an induction motor is connected to a capacitor and the shaft turns above synchronous speed, it operates as a self-excited induction generator.
Many early radio systems tuned their transmitter circuit, so the system automatically created radio waves of the desired frequency. This design has given way to designs that use a separate oscillator to provide a signal that is then amplified to the desired power.
For example, a reduction in population of an herbivore species because of predation , this makes the populations of predators of that species decline, the reduced level of predation allows the herbivore population to increase, this allows the predator population to increase, etc. Closed loops of time-lagged differential equations are a sufficient explanation for such cycles - in this case the delays are caused mainly by the breeding cycles of the species involved.
. | https://en.wikipedia.org/wiki/Self-oscillation |
Self-perpetuation , the capability of something to cause itself to continue to exist , is one of the main characteristics of life . Organisms ' capability of reproduction leads to self-perpetuation of the species, if not to the individual. Populations self-perpetuate and grow. Entire ecosystems show homeostasis , and thus perpetuate themselves. [ 1 ] [ 2 ] The slow modifying effect of succession and similar shifts in the composition of the system can, however, not be neglected in the long run. [ 3 ] Overall, life's object's capabilities of self-perpetuation are always accompanied by evolution , a perfect steady state of the biological system is never reached. Sexual reproduction is also a form of imperfect self-replication and thus imperfect self-perpetuation because of recombination and mutation . Organisms are not like self-replicating machine but amass random modifications from generation to generation. The property of self-perpetuation in the strict sense thus only applies to life itself.
In a social context, self-perpetuation is tied to reflexivity and (usually) positive feedback loops:
To overcome strong prior beliefs , strong evidence to the contrary is needed. If a person is predisposed to choosing a certain action, the advice from an advisor who sets a low threshold for recommending the alternative action is not of much use. The preference for like-minded advisors who supply coarse information implies that the advice a person receives is likely to
reinforce his existing priors . This effect can lead to polarisation of opinion and the emergence of self-serving beliefs. The learning process is prolonged and the induced short run bias can become perpetual if information is costly
Depending on the time scope or the context, self-perpetuation either depends on self-sustainability , or is equivalent to it. While we may talk about the self-sustainability of an ecosystem, this depends amongst other factor on the self-perpetuation of its constituting species.
In computer science, self-reproducing programs constitute an incomplete metaphor for self-perpetuation. A better analogue can be seen in computer viruses which are actually able to self-reproduce - given a suitable computing environment. | https://en.wikipedia.org/wiki/Self-perpetuation |
A self-powered dynamic system [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 6 ] [ 7 ] [ excessive citations ] is defined as a dynamic system powered by its own excessive kinetic energy , renewable energy or a combination of both. The particular area of work is the concept of fully or partially self-powered dynamic systems requiring zero or reduced external energy inputs. The exploited technologies are particularly associated with self-powered sensors , regenerative actuators , human powered devices, and dynamic systems powered by renewable resources (e.g. solar-powered airships [ 8 ] [ 9 ] ) as self-sustained systems. Various strategies can be employed to improve the design of a self-powered system and among them adopting a bio-inspired design is investigated to demonstrate the advantage of biomimetics in improving power density.
The concept of "self-powered dynamic systems" in the figure is described as follows.
I. Input power (e.g. fuel energy powering a vehicle engine or propulsion system), or input excitation (e.g. vibration excitation to a structure) to the system. The source of this input energy can be of renewable energy source (e.g. solar power to a dynamic system).
II. The kinetic energy in the direction of motion of a dynamic system is only recovered if the system is stationary (e.g. a bridge structure), or the recoverable energy is negligible in comparison with the power required for motion (e.g. a low powered sensor).
III. The movement of the dynamic system perpendicular to the desired direction of the motion is usually the wasted kinetic energy in the system (e.g. the vertical motion of an automobile suspension is wasted to heat energy in the shock absorbers, or vibration of an aircraft wing is converted into heat energy through structural damping).
IV. The vertical movement of the dynamic system is a source of recoverable kinetic energy.
V. The recoverable kinetic energy can be converted to electrical energy through an energy conversion mechanism such as an electromagnetic scheme (e.g. replacing the viscous damper of a car shock absorber with regenerative actuator), piezoelectric (e.g. embedding piezoelectric material in aircraft wings), or electrostatic (e.g. vibration of a micro cantilever in a MEMS sensor).
VI. The recovered electrical power can be stored or used as a power source.
VII. The recovered electrical energy can power subsystems of the dynamic system such as sensors and actuators.
VIII. The recovered electrical power can be realized as an input to the dynamic system itself.
Such self-powered schemes are particularly beneficial in development of self-powered sensors [ 10 ] and self-powered actuators [ 11 ] by employing energy harvesting techniques, [ 12 ] [ 13 ] [ 14 ] where kinetic energy is converted to electrical energy through piezoelectric, electromagnetic or electrostatic electromechanical mechanisms. [ 15 ] Developing a self-powered sensor eliminates the use of an external source of power such as a battery and therefore can be considered as a self-sustained system. A self-sustained system does not required maintenance (e.g. replacing the battery of the sensor at the end of the battery life). This is particularly beneficial in remote sensing and applications in hostile or inaccessible environments. | https://en.wikipedia.org/wiki/Self-powered_dynamic_systems |
Self-preservation is a behavior or set of behaviors that ensures the survival of an organism. [ 1 ] It is thought to be universal among all living organisms. For sentient organisms, pain and fear are integral parts of this mechanism. Pain motivates the individual to withdraw from damaging situations, to protect a damaged body part while it heals, and to avoid similar experiences in the future. [ 2 ] Most pain resolves promptly once the painful stimulus is removed and the body has healed, but sometimes pain persists despite removal of the stimulus and apparent healing of the body; and sometimes pain arises in the absence of any detectable stimulus, damage or disease. [ 3 ] Fear causes the organism to seek safety and may cause a release of adrenaline , [ 4 ] [ 5 ] which has the effect of increased strength and heightened senses such as hearing, smell, and sight. Self-preservation may also be interpreted figuratively, in regard to the coping mechanisms one needs to prevent emotional trauma from distorting the mind (see Defence mechanisms ).
Even the most simple of living organisms (for example, the single-celled bacteria) are typically under intense selective pressure to evolve a response that would help avoid a damaging environment, if such an environment exists. Organisms also evolve while adapting – even thriving – in a benign environment (for example, a marine sponge modifies its structure in response to current changes, in order to better absorb and process nutrients). Self-preservation is therefore an almost universal hallmark of life. However, when introduced to a novel threat, many species will have a self-preservation response either too specialised, or not specialised enough, to cope with that particular threat. [ citation needed ] An example is the dodo , which evolved in the absence of natural predators and hence lacked an appropriate, general self-preservation response to heavy predation by humans and rats, showing no fear of them.
Self-preservation is essentially the process of an organism preventing itself from being harmed or killed and is considered a basic instinct in most organisms. [ 6 ] Most call it a "survival instinct". Self-preservation is thought to be tied to an organism's reproductive fitness and can be more or less present according to perceived reproduction potential. [ 7 ] If perceived reproductive potential is low enough, self-destructive behavior (i.e., the opposite) is not uncommon in social species. [ 8 ] Self-preservation is also thought by some to be the basis of rational and logical thought and behavior. [ 9 ]
An organism's fitness is measured by its ability to pass on its genes . The most straightforward way to accomplish this is to survive to a reproductive age, mate, and then have offspring. These offspring will hold at least a portion of their parent's genes, up to all of the parent's genes in asexual organisms. But in order for this to happen, an organism must first survive long enough to reproduce, and this would mainly consist of adopting selfish behaviors that would allow organisms to maximize their own chances for survival.
Animals in a social group (of kin) often work cooperatively in order to survive, but when one member perceives itself as a burden for an extended period of time, it may commit self-destructive behavior. [ 7 ] This allows its relatives to have a better chance at survival, and if enough close relatives survive, then its genes get indirectly passed on. [ 7 ] This behavior works in the exact opposite direction of the survival instinct and could be considered a highly altruistic behavior evolved from a cooperative group. Self-destructive behavior is not the same as risk-taking behavior (see below in Social implications), although risk-taking behavior could turn into destructive behavior.
The desire for self-preservation has led to countless laws and regulations surrounding a culture of safety in society. [ 10 ] Seat belt laws, speed limits, texting regulations, and the " stranger danger " campaign are examples of societal guides and regulations to enhance survival, and these laws are heavily influenced by the pursuit of self-preservation.
Self-preservation urges animals to collect energy and resources required to prolong life as well as resources that increase chances of survival. Basic needs are available to most humans (roughly 7 out of 8 people), [ 11 ] and usually rather cheaply. The instinct that drives humans to gather resources now drives them to over-consumption or to patterns of collection and possession that essentially make hoarding resources the priority. [ 12 ]
Self-preservation is not just limited to individual organisms; this can be scaled up or down to other levels of life. Narula and Young [ 13 ] indicate that cardiac myocytes have an acute sense of self-preservation. They are able to duck, dart, and dodge foreign substances that may harm the cell. In addition, when a myocardiac arrest – a heart attack – occurs, it is actually the cardiac myocytes entering a state of hibernation in an attempt to wait out a lack of resources. [ 13 ] While this is ultimately deadly to the organism, it prolongs the cell's survival as long as possible for hopeful resuscitation. [ 13 ]
When scaled in the opposite direction, Hughes-Jones [ 14 ] makes the argument that "social groups that fight each other are self‐sustaining, self‐replicating wholes containing interdependent parts" indicating that the group as a whole can have self-preservation with the individuals acting as the cells.
He makes an analogy between the survival practices such as hygiene and the ritual nature within small human groups or the nations that engage in religious warfare with the complex survival mechanisms of multi-cellular organisms that evolved from the cooperative association of single cell organisms in order to better protect themselves. [ clarification needed ] | https://en.wikipedia.org/wiki/Self-preservation |
Self-propagating high-temperature synthesis (SHS) is a method for producing both inorganic and organic compounds by exothermic combustion reactions in solids of different nature. [ 1 ] Reactions can occur between a solid reactant coupled with either a gas, liquid, or other solid. If the reactants, intermediates, and products are all solids, it is known as a solid flame. [ 2 ] If the reaction occurs between a solid reactant and a gas phase reactant, it is called infiltration combustion. Since the process occurs at high temperatures, the method is ideally suited for the production of refractory materials including powders, metallic alloys, or ceramics.
The modern SHS process was reported and patented in 1971, [ 3 ] [ 4 ] although some SHS-like processes were known previously.
Self-propagating high-temperature synthesis is a green synthesis technique that is highly energy efficient, using little if any toxic solvents. There have been environmental analysis conducted to show that SHS has a lesser environmental impact than traditional solution-phase processing techniques. [ 5 ] The technique uses less energy for production of materials, and the energy cost savings increase as synthesis batch sizes increase.
SHS is not a suitable technique for production of nanoparticles. Typically, the high-temperature nature of the process leads to particle sintering during and after the reaction. The high-temperatures generated during synthesis also lead to problems with energy dissipation and suitable reaction vessels, however, some systems use this excess heat to drive other plant-processes.
In its usual format, SHS is conducted starting from finely powdered reactants that are intimately mixed. In some cases, the reagents are finely powdered whereas in other cases, they are sintered to minimize their surface area and prevent uninitiated exothermic reactions, which can be dangerous. [ 6 ] In other cases, the particles are mechanically activated through techniques such as high energy ball milling (e.g. in a planetary mill), which results in nanocomposite particles that contain both reactants within individual chemical cells. [ 7 ] [ 8 ] After reactant preparation, synthesis is initiated by point-heating of a small part (usually the top) of the sample. Once started, a wave of exothermic reaction sweeps through the remaining material. SHS has also been conducted with thin films, liquids, gases, powder–liquid systems, gas suspensions, layered systems, gas-gas systems, and others. Reactions have been conducted in a vacuum and under both inert or reactive gases. The temperature of the reaction can be moderated by the addition of inert salt that absorbs heat in the process of melting or evaporation, such as sodium chloride , or by adding "chemical oven"—a highly exothermic mixture—to decrease the ratio of cooling. [ 9 ]
The reaction of alkali metal chalcogenides (S, Se, Te) and pnictides (N, P, As) with other metal halides produce the corresponding metal chalcogenides and pnictides. [ 10 ] The synthesis of gallium nitride from gallium triiodide and lithium nitride is illustrative:
The process is so exothermic (ΔH = -515 kJ/mol) that the LiI evaporates, leaving a residue of GaN. With GaCl 3 in place of GaI 3 , the reaction is so exothermic that the product GaN decomposes. Thus, the selection of the metal halide affects the success of the method.
Other compounds prepared by this method include metal dichalcogenides such as MoS 2 . The reaction is conducted in a stainless steel reactor with excess Na 2 S. [ 6 ]
Self-propagating high-temperature synthesis can also be conducted in an artificial high gravity environment to control the phase composition of products. [ 11 ]
SHS has been used to vitrify various nuclear waste streams including ashes from incineration, spent inorganic ion exchangers such as clinoptilolite and contaminated soils. [ 12 ]
Due to the solid-state nature of SHS processes, it is possible to measure reaction kinetics in-situ using a variety of experimental techniques, including electrothermal explosion, differential thermal analysis , combustion velocity approaches, among others. [ 13 ] There have been a variety of systems studied, including intermetallic, thermite, carbides, and others. Using SHS, it was shown that the particle size has a significant effect on the reaction kinetics. [ 14 ] It was further shown that these effects are related to the relationship between the surface area/volume ratio of the particles, and that the kinetics can be controlled via high-energy ball-milling. [ 15 ] Depending on the morphology of the reactants, it is possible to initiate a SHS reaction where a liquid phase occurs prior to phase formation or to directly result in solid-phase products without any melt. [ 16 ] | https://en.wikipedia.org/wiki/Self-propagating_high-temperature_synthesis |
A self-propelled modular transporter or sometimes self-propelled modular trailer (SPMT) is a platform heavy hauler with a large array of wheels which is an upgraded version of a hydraulic modular trailer . SPMTs are used for transporting massive objects, such as large bridge sections, [ 1 ] oil refining equipment, cranes, motors, spacecraft and other objects that are too big or heavy for trucks . Ballast tractors can however provide traction and braking for the SPMTs on inclines and descents.
SPMTs are used in many industry sectors worldwide such as the construction and oil industries, in the shipyard and offshore industry, for road transportation, on plant construction sites and even for moving oil platforms . [ 2 ] They have begun to be used to replace bridge spans in the United States , Europe , Asia [ 3 ] and more recently Canada .
A typical SPMT has a grid of computer-controlled axles, usually 2 axles across and 4–8 axles along. [ 4 ] When two (or more) axles are placed in series, this is called an axle line. All axles are individually controllable, in order to evenly distribute weight and to steer accurately. Each axle can swivel through 270°, with some manufacturers offering up to a full 360° of motion. The axles are coordinated by the control system to allow the SPMT to turn, move sideways or even rotate in place. Some SPMTs allow the axles to telescope independently of each other so that the load can be kept flat and evenly distributed while moving over uneven terrain. Each axle can also contain a hydrostatic drive unit.
A hydraulic power pack can be attached to the SPMT to provide power for steering, suspension and drive functions. This power pack is driven by an internal combustion engine. A single power pack can drive a string of SPMTs. As SPMTs often carry the world's heaviest loads on wheeled vehicles, they are very slow, often moving at under one mile per hour (1.6 km/h) while fully loaded. Some SPMTs are controlled by a worker with a hand-held control panel, while others have a driver cabin. Multiple SPMTs can be linked (lengthwise and side-by-side) to transport massive building-sized objects. The linked SPMTs can be controlled from a single control panel. [ 5 ]
The first modular self-propelled trailers were built in the 1970s. In the early 1980s, heavy haulage company Mammoet [ 6 ] refined the concept into the form seen today. [ 7 ] They set the width of the modules at 2.44 m, so the modules would fit on an ISO container flatrack . They also added 360° steering. [ 8 ] They commissioned Scheuerle [ 9 ] to develop and build the first units. Deliveries started in 1983. The two companies defined the standard units: a 4-axle SPMT, a 6-axle SPMT and a hydraulic power pack. Over the years, new types of modules were added to this system [ 4 ] to accommodate a range of payloads.
In 2016 ESTA (the European Association of Abnormal Load Transport and Mobile Cranes) published the first SPMT best practice guide [ 10 ] to help address the problem of trailers occasionally tipping over, which happened even when the operating rules and stability calculations had been precisely followed.
Some shipbuilding companies have started to use SPMT instead of gantry cranes for carrying ship sections. This has reduced the cost of transporting huge loads by millions of dollars. [ 11 ]
In 2022 Mammoet and Scheuerle developed and employed world's first electric SPMT. This was done with the help of an Electric power pack unit (EPPU) which replaced the gas powered PPU. The ESPMTs help to reduce carbon footprint of the companies and also the haulage industry. These electric modules are safer and quieter when compared with the diesel modules, which can benefit operations which are held in mines and energy plants. [ 12 ]
ESTA has plans to develop European Trailer Operator's License (ETOL) for SPMT operators, this idea is backed by top company operating in heavy haulage sector like Goldhofer and Tii Group. There will be training and practice to obtain this specific license which the SPMT operators have to complete before handling these heavy machines on public roads, but this will improve the safety standards of the industry. [ 13 ]
Executing the salvage operation of the sunken ferry MV Sewol in the East China Sea in 2017, the company ALE used SPMTs equivalent to a 600-axle line and a load weight of 17,000 t (18,739 short tons; 16,732 long tons), exceeding two world records. [ 14 ] [ 15 ]
In December 2022 Shell plc a London based oil company ordered decommission of their 20,300ton FPSO Curlew ship when it reached the end of its operational life. This operation was assigned to AF Offshore Decom, a decommissioning specialist company based in Oslo which partnered with Mammoet of Utrecht to load-in and set-down the structure in Norway with the help of 748 SPMT axle line. This claimed to break two world records, one for the heaviest SPMT movement and another for most SPMT axle lines used for transportation. [ 16 ]
In February 2023 Sinotrans Heavy-Lift a China based heavy transport company moved a hotel building 500 meters in Sanya , Hainan using 254 axle lines of Scheuerle SPMT with the help of 15 power packs. This was claimed to be the world's heaviest building transportation ever. The building in the subject was almost 300ft long, 115 ft wide, 65 ft high and weighed 7,500 tons. The relocation was done to comply with the environmental regulations of the state. [ 17 ]
In December 2023 China Shipping Vastwin Project Logistic a China based logistics company a subsidiary of China based multinational company COSCO Shipping moved five number of buildings at the Ningxia Saishang Jiangnan Museum located in Ningxia based in Northern China . The relocation was done to adhere with the environmental regulations. The buildings in subject were 11,450 tonnes in total of five with the main building weighing 10,000 tonnes, 43 mtr high, 36.9 mtr long and 31.5 mtr wide which was moved on 300 lines of SPMT and ten powerpacks. This resulted in breaking three records of most heights, heaviest building transportation over the longest distance. [ 18 ] | https://en.wikipedia.org/wiki/Self-propelled_modular_transporter |
Self-propelled particles (SPP), also referred to as self-driven particles, are terms used by physicists to describe autonomous agents , which convert energy from the environment into directed or persistent random walk . Natural systems which have inspired the study and design of these particles include walking, swimming or flying animals. Other biological systems include bacteria, cells, algae and other micro-organisms. Generally, self-propelled particles often refer to artificial systems such as robots or specifically designed particles such as swimming Janus colloids , [ 2 ] bimetallic nanorods, nanomotors and walking grains. In the case of directed propulsion, which is driven by a chemical gradient, this is referred to as chemotaxis , observed in biological systems, e.g. bacteria quorum sensing and ant pheromone detection, and in synthetic systems, e.g. enzyme molecule chemotaxis [ 3 ] and enzyme powered hard and soft particles.
Self-propelled particles interact with each other, which can lead to the emergence of collective behaviours. These collective behaviours mimic the self-organization observed with the flocking of birds, the swarming of bugs, the formation of sheep herds, etc.
To understand the ubiquity of such phenomena, physicists have developed a number of self-propelled particles models. These models predict that self-propelled particles share certain properties at the group level, regardless of the type of animals (or artificial particles) in the swarm. [ 1 ] It has become a challenge in theoretical physics to find minimal statistical models that capture these behaviours. [ 4 ] [ 5 ] [ 6 ]
Most animals can be seen as SPP: they find energy in their food and exhibit various locomotion strategies, from flying to crawling. The most prominent examples of collective behaviours in these systems are fish schools, birds flocks, sheep herds, human crowds. At a smaller scale, cells and bacteria can also be treated as SPP. These biological systems can propel themselves based on the presence of chemoattractants. At even smaller scale, molecular motors transform ATP energy into directional motion. Recent work has shown that enzyme molecules will also propel themselves. [ 7 ] Further, it has been shown that they will preferentially move towards a region of higher substrate concentration, [ 8 ] [ 9 ] a phenomenon that has been developed into a purification technique to isolate live enzymes. [ 10 ] Additionally, microparticles, vesicles, and even macroscale sheets can become self-propelled when they are functionalized with enzymes. [ 11 ] The catalytic reactions of the enzymes direct the particles or vesicles based on corresponding substrate gradients. [ 12 ] [ 13 ] [ 14 ]
There is a distinction between wet and dry systems. In the first case the particles "swim" in a surrounding fluid; in the second case the particles "walk" on a substrate.
Active colloidal particles, dubbed nanomotors , are the prototypical example of wet SPP. Janus particles are colloidal particles with two different sides, having different physical or chemical properties. [ 15 ] This symmetry breaking allows, by properly tuning the environment (typically the surrounding solution), for the motion of the Janus particle. For instance, the two sides of the Janus particle can induce a local gradient of, temperature, electric field, or concentration of chemical species. [ 16 ] [ 17 ] This induces motion of the Janus particle along the gradient through, respectively, thermophoresis , electrophoresis or diffusiophoresis . [ 17 ] Because the Janus particles consume energy from their environment (catalysis of chemical reactions, light absorption, etc.), the resulting motion constitutes an irreversible process and the particles are out of equilibrium.
Walking grains are a typical realization of dry SPP: The grains are milli-metric disks sitting on a vertically vibrating plate, which serves as the source of energy and momentum. The disks have two different contacts ("feet") with the plate, a hard needle-like foot in the front and a large soft rubber foot in the back. When shaken, the disks move in a preferential direction defined by the polar (head-tail) symmetry of the contacts. This together with the vibrational noise result in a persistent random walk. [ 27 ]
Symmetry breaking is a necessary condition for SPPs, as there must be a preferential direction for moving. However, the symmetry breaking may not come solely from the structure itself but from its interaction with electromagnetic fields, in particular when taken into account retardation effects. This can be used for the phototactic motion of even highly symmetrical nanoparticles. [ 28 ] [ 29 ] In 2021, it was experimentally shown that completely symmetric particles (spherical microswimmers in this case) experience a net thermophoretic force when illuminated from a given direction. [ 30 ] For self-propelled enzyme molecules, symmetry breaking can also arise from diffusion and kinetic asymmetry. [ 31 ] [ 32 ]
In 2020, researchers from the University of Leicester reported a hitherto unrecognised state of self-propelled particles — which they called a "swirlonic state". The swirlonic state consists of "swirlons", formed by groups of self-propelled particles orbiting a common centre of mass. These quasi-particles demonstrate a surprising behaviour: In response to an external load they move with a constant velocity proportional to the applied force, just as objects in viscous media. Swirlons attract each other and coalesce forming a larger, joint swirlon. The coalescence is an extremely slow, decelerating process, resulting in a rarified state of immobile quasi-particles. In addition to the swirlonic state, gaseous, liquid and solid states were observed, depending on the inter-particle and self-driving forces. In contrast to molecular systems, liquid and gaseous states of self-propelled particles do not coexist. [ 33 ] [ 34 ]
Typical collective motion generally includes the formation of self-assembled structures, such as clusters and organized assemblies. [ 35 ]
The prominent and most spectacular emergent large scale behaviour observed in assemblies of SPP is directed collective motion . In that case all particles move in the same direction. On top of that, spatial structures can emerge such as bands, vortices, asters, moving clusters.
Another class of large scale behaviour, which does not imply directed motion is either the spontaneous formation of clusters or the separation in a gas-like and a liquid-like phase, an unexpected phenomenon when the SPP have purely repulsive interaction. This phase separation has been called Motility Induced Phase Separation (MIPS).
The modeling of SPP was introduced in 1995 by Tamás Vicsek et al. [ 36 ] as a special case of the Boids model introduced in 1986 by Reynolds . [ 37 ] In that case the SPP are point particles, which move with a constant speed. and adopt (at each time increment) the average direction of motion of the other particles in their local neighborhood up to some added noise. [ 38 ] [ 39 ]
Simulations demonstrate that a suitable "nearest neighbour rule" eventually results in all the particles swarming together or moving in the same direction. This emerges, even though there is no centralised coordination, and even though the neighbours for each particle constantly change over time (see the interactive simulation in the box on the right). [ 36 ]
Since then a number of models have been proposed, ranging from the simple active Brownian particle to detailed and specialized models aiming at describing specific systems and situations. Among the important ingredients in these models, one can list
One can also include effective influences of the surrounding; for instance the nominal velocity of the SPP can be set to depend on the local density, in order to take into account crowding effects.
Self-propelled particles can also be modeled using on-lattice models, which offer the advantage of being simple and efficient to simulate, and in some cases, may be easier to analyze mathematically. [ 41 ] On-lattice models such as BIO-LGCA models have been used to study physical aspects of self-propelled particle systems (such as phase transitions and pattern-forming potential [ 42 ] ) as well as specific questions related to real active matter systems (for example, identifying the underlying biological processes involved in tumor invasion [ 43 ] ).
Young desert locusts are solitary and wingless nymphs . If food is short they can gather together and start occupying neighbouring areas, recruiting more locusts. Eventually they can become a marching army extending over many kilometres. [ 44 ] This can be the prelude to the development of the vast flying adult locust swarms which devastate vegetation on a continental scale. [ 45 ]
One of the key predictions of the SPP model is that as the population density of a group increases, an abrupt transition occurs from individuals moving in relatively disordered and independent ways within the group to the group moving as a highly aligned whole. [ 46 ] Thus, in the case of young desert locusts, a trigger point should occur which turns disorganised and dispersed locusts into a coordinated marching army. When the critical population density is reached, the insects should start marching together in a stable way and in the same direction.
In 2006, a group of researchers examined how this model held up in the laboratory. Locusts were placed in a circular arena, and their movements were tracked with computer software. At low densities, below 18 locusts per square metre, the locusts mill about in a disordered way. At intermediate densities, they start falling into line and marching together, punctuated by abrupt but coordinated changes in direction. However, when densities reached a critical value at about 74 locusts/m 2 , the locusts ceased making rapid and spontaneous changes in direction, and instead marched steadily in the same direction for the full eight hours of the experiment.
This confirmed the behaviour predicted by the SPP models. [ 1 ]
In the field, according to the Food and Agriculture Organization of the United Nations , the average density of marching bands is 50 locusts/m 2 (50 million locusts/km 2 ), with a typical range from 20 to 120 locusts/m 2 . [ 45 ] : 29 The research findings discussed above demonstrate the dynamic instability that is present at the lower locust densities typical in the field, where marching groups randomly switch direction without any external perturbation. Understanding this phenomenon, together with the switch to fully coordinated marching at higher densities, is essential if the swarming of desert locusts is to be controlled. [ 1 ]
Swarming animals, such as ants, bees, fish and birds, are often observed suddenly switching from one state to another. For example, birds abruptly switch from a flying state to a landing state, or fish switch from schooling in one direction to schooling in another direction. Such state switches can occur with astonishing speed and synchronicity, as though all the members in the group made a unanimous decision at the same moment. Phenomena like these have long puzzled researchers. [ 48 ]
In 2010, Bhattacharya and Vicsek used an SPP model to analyse what is happening here. As a paradigm, they considered how flying birds arrive at a collective decision to make a sudden and synchronised change to land. The birds, such as the starlings in the image on the right, have no decision-making leader, yet the flock know exactly how to land in a unified way. The need for the group to land overrides deviating intentions by individual birds. The particle model found that the collective shift to landing depends on perturbations that apply to the individual birds, such as where the birds are in the flock. [ 47 ] It is behaviour that can be compared with the way that sand avalanches, if it is piled up, before the point at which symmetric and carefully placed grains would avalanche, because the fluctuations become increasingly non-linear. [ 49 ]
"Our main motivation was to better understand something which is puzzling and out there in nature, especially in cases involving the stopping or starting of a collective behavioural pattern in a group of people or animals ... We propose a simple model for a system whose members have the tendency to follow the others both in space and in their state of mind concerning a decision about stopping an activity. This is a very general model, which can be applied to similar situations." [ 47 ] The model could also be applied to a swarm of unmanned drones , to initiate the desired motion in a crowd of people, or to interpreting group patterns when stock market shares are bought or sold. [ 50 ]
SPP models have been applied in many other areas, such as schooling fish , [ 51 ] robotic swarms , [ 52 ] molecular motors , [ 53 ] the development of human stampedes [ 54 ] and the evolution of human trails in urban green spaces. [ 55 ] SPP in Stokes flow , such as Janus particles , are often modeled by the squirmer model. [ 56 ] | https://en.wikipedia.org/wiki/Self-propelled_particles |
Self-propulsion is the autonomous displacement of nano-, micro- and macroscopic natural and artificial objects, containing their own means of motion. [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 6 ] [ 7 ] Self-propulsion is driven mainly by interfacial phenomena . [ 8 ] Various mechanisms of self-propelling have been introduced and investigated, which exploited phoretic effects , [ 9 ] gradient surfaces, breaking the wetting symmetry of a droplet on a surface, [ 10 ] [ 11 ] the Leidenfrost effect , [ 12 ] [ 13 ] [ 14 ] the self-generated hydrodynamic and chemical fields originating from the geometrical confinements, [ 15 ] and soluto- and thermo-capillary Marangoni flows . [ 16 ] [ 17 ] [ 1 ] Self-propelled system demonstrate a potential as micro-fluidics devices [ 18 ] and micro-mixers. [ 19 ] Self-propelled liquid marbles have been demonstrated. [ 14 ] | https://en.wikipedia.org/wiki/Self-propulsion |
Self-protein refers to all proteins endogenously produced by DNA -level transcription and translation within an organism of interest. This does not include proteins synthesized due to viral infection , but may include those synthesized by commensal bacteria within the intestines . Proteins that are not created within the body of the organism of interest, but nevertheless enter through the bloodstream , a breach in the skin , or a mucous membrane , may be designated as “non-self” and subsequently targeted and attacked by the immune system . Tolerance to self-protein is crucial for overall wellbeing; when the body erroneously identifies self-proteins as “non-self”, the subsequent immune response against endogenous proteins may lead to the development of an autoimmune disease . [ 1 ] [ 2 ]
Of note, the list provided above is not exhaustive; the list does not mention all possible proteins targeted by the provided autoimmune diseases.
Autoimmune responses and diseases are primarily instigated by T lymphocytes that are incorrectly screened for reactivity to self-protein during cell development. [ citation needed ]
During T-cell development, early T-cell progenitors first move via chemokine gradients from the bone marrow into the thymus , where T-cell receptors are randomly rearranged at the gene level to allow for T-cell receptor generation. [ 7 ] These T-cells have the potential to bind to anything, including self-proteins. [ citation needed ]
The immune system must differentiate the T-cells that have receptors capable of binding to self versus non-self proteins; T-cells that can bind to self-proteins must be destroyed to prevent development of an autoimmune disorder. In a process known as “ Central Tolerance ”, T-cells are exposed to cortical epithelial cells that express a variety of different major histocompatibility complexes (MHC) of both class 1 and class 2 , which have the ability to bind to T-cell receptors of CD8+ cytotoxic T-cells , and CD4+ helper T-cells , respectively. The T-cells that display affinity for these MHC are positively selected to continue to the second stage of development, while those that cannot bind to MHC undergo apoptosis . [ 8 ] In the second stage, immature T-cells are exposed to a variety of macrophages , dendritic cells , and medullary epithelial cells that express self-protein on MHC class 1 and class 2 . These epithelial cells also express the transcription factor labelled autoimmune regulator (AIRE) – this crucial transcription factor allows the medullary epithelial cells of the thymus to express proteins would normally be present in peripheral tissue rather than in an epithelial cell, such as insulin -like peptides, myelin -like peptides, and more. [ 9 ] As these epithelial cells now present a large variety of self-proteins that could be encountered across the body, the immature T-cells are tested for affinity to self-protein and self-MHC. If any T-cell has strong affinity for self-protein and self-MHC, the cell undergoes apoptosis to prevent autoimmune function. [ 8 ] T-cells that display low/medium affinity are allowed to leave the thymus and circulate throughout the body to react to novel non-self antigen. In this manner, the body attempts to systematically destroy T-cells that could lead to autoimmunity. [ citation needed ] | https://en.wikipedia.org/wiki/Self-protein |
A self-replicating machine is a type of autonomous robot that is capable of reproducing itself autonomously using raw materials found in the environment, thus exhibiting self-replication in a way analogous to that found in nature . [ 1 ] [ 2 ] [ 3 ] The concept of self-replicating machines has been advanced and examined by Homer Jacobson , Edward F. Moore , Freeman Dyson , John von Neumann , Konrad Zuse [ 4 ] [ 5 ] and in more recent times by K. Eric Drexler in his book on nanotechnology , Engines of Creation (coining the term clanking replicator for such machines) and by Robert Freitas and Ralph Merkle in their review Kinematic Self-Replicating Machines [ 6 ] which provided the first comprehensive analysis of the entire replicator design space. The future development of such technology is an integral part of several plans involving the mining of moons and asteroid belts for ore and other materials, the creation of lunar factories, and even the construction of solar power satellites in space. The von Neumann probe [ 7 ] is one theoretical example of such a machine. Von Neumann also worked on what he called the universal constructor , a self-replicating machine that would be able to evolve and which he formalized in a cellular automata environment. Notably, Von Neumann's Self-Reproducing Automata scheme posited that open-ended evolution requires inherited information to be copied and passed to offspring separately from the self-replicating machine, an insight that preceded the discovery of the structure of the DNA molecule by Watson and Crick and how it is separately translated and replicated in the cell. [ 8 ] [ 9 ]
A self-replicating machine is an artificial self-replicating system that relies on conventional large-scale technology and automation. The concept, first proposed by Von Neumann no later than the 1940s, has attracted a range of different approaches involving various types of technology. Certain idiosyncratic terms are occasionally found in the literature. For example, the term clanking replicator was once used by Drexler [ 10 ] to distinguish macroscale replicating systems from the microscopic nanorobots or " assemblers " that nanotechnology may make possible, but the term is informal and is rarely used by others in popular or technical discussions. Replicators have also been called "von Neumann machines" after John von Neumann, who first rigorously studied the idea. However, the term "von Neumann machine" is less specific and also refers to a completely unrelated computer architecture that von Neumann proposed and so its use is discouraged where accuracy is important. [ 6 ] Von Neumann used the term universal constructor to describe such self-replicating machines.
Historians of machine tools , even before the numerical control era, sometimes figuratively said that machine tools were a unique class of machines because they have the ability to "reproduce themselves" [ 11 ] by copying all of their parts. Implicit in these discussions is that a human would direct the cutting processes (later planning and programming the machines), and would then assemble the parts. The same is true for RepRaps , which are another class of machines sometimes mentioned in reference to such non-autonomous "self-replication". Such discussions refer to collections of machine tools, and such collections have an ability to reproduce their own parts which is finite and low for one machine, and ascends to nearly 100% with collections of only about a dozen similarly made, but uniquely functioning machines, establishing what authors Frietas and Merkle refer to as matter or material closure. Energy closure is the next most difficult dimension to close, and control the most difficult, noting that there are no other dimensions to the problem. In contrast, machines that are truly autonomously self-replicating (like biological machines ) are the main subject discussed here, and would have closure in each of the three dimensions.
The general concept of artificial machines capable of producing copies of themselves dates back at least several hundred years. An early reference is an anecdote regarding the philosopher René Descartes , who suggested to Queen Christina of Sweden that the human body could be regarded as a machine; she responded by pointing to a clock and ordering "see to it that it reproduces offspring." [ 12 ] Several other variations on this anecdotal response also exist. Samuel Butler proposed in his 1872 novel Erewhon that machines were already capable of reproducing themselves but it was man who made them do so, [ 13 ] and added that "machines which reproduce machinery do not reproduce machines after their own kind" . [ 14 ] In George Eliot's 1879 book Impressions of Theophrastus Such , a series of essays that she wrote in the character of a fictional scholar named Theophrastus, the essay "Shadows of the Coming Race" speculated about self-replicating machines, with Theophrastus asking "how do I know that they may not be ultimately made to carry, or may not in themselves evolve, conditions of self-supply, self-repair, and reproduction". [ 15 ]
In 1802 William Paley formulated the first known teleological argument depicting machines producing other machines, [ 16 ] suggesting that the question of who originally made a watch was rendered moot if it were demonstrated that the watch was able to manufacture a copy of itself. [ 17 ] Scientific study of self-reproducing machines was anticipated by John Bernal as early as 1929 [ 18 ] and by mathematicians such as Stephen Kleene who began developing recursion theory in the 1930s. [ 19 ] Much of this latter work was motivated by interest in information processing and algorithms rather than physical implementation of such a system, however. In the course of the 1950s, suggestions of several increasingly simple mechanical systems capable of self-reproduction were made — notably by Lionel Penrose . [ 20 ] [ 21 ]
A detailed conceptual proposal for a self-replicating machine was first put forward by mathematician John von Neumann in lectures delivered in 1948 and 1949, when he proposed a kinematic model of self-reproducing automata as a thought experiment . [ 22 ] [ 23 ] Von Neumann's concept of a physical self-replicating machine was dealt with only abstractly, with the hypothetical machine using a "sea" or stockroom of spare parts as its source of raw materials. The machine had a program stored on a memory tape that instructed it to retrieve parts from this "sea" using a manipulator, assemble them into a copy of itself, and then transfer the contents of its memory tape into the new duplicate. The machine was envisioned as consisting of as few as eight different types of components: four logic elements for sending and receiving stimuli and four mechanical elements for providing structural support and mobility. Although qualitatively sound, von Neumann was evidently dissatisfied with this self-replicating machine model due to the difficulty of analyzing it with mathematical precision. He went on to instead develop an even more abstract model self-replicator based on cellular automata . [ 24 ] His original kinematic concept remained obscure until it was popularized in a 1955 issue of Scientific American . [ 25 ]
Von Neumann's goal for his self-reproducing automata theory , as specified in his lectures at the University of Illinois in 1949, [ 22 ] was to design a machine whose complexity could grow automatically akin to biological organisms under natural selection . He asked what is the threshold of complexity that must be crossed for machines to be able to evolve. [ 8 ] His answer was to design an abstract machine which, when run, would replicate itself. Notably, his design implies that open-ended evolution requires inherited information to be copied and passed to offspring separately from the self-replicating machine, an insight that preceded the discovery of the structure of the DNA molecule by Watson and Crick and how it is separately translated and replicated in the cell. [ 8 ] [ 9 ]
In 1956 mathematician Edward F. Moore proposed the first known suggestion for a practical real-world self-replicating machine, also published in Scientific American . [ 26 ] [ 27 ] Moore's "artificial living plants" were proposed as machines able to use air, water and soil as sources of raw materials and to draw its energy from sunlight via a solar battery or a steam engine . He chose the seashore as an initial habitat for such machines, giving them easy access to the chemicals in seawater, and suggested that later generations of the machine could be designed to float freely on the ocean's surface as self-replicating factory barges or to be placed in barren desert terrain that was otherwise useless for industrial purposes. The self-replicators would be "harvested" for their component parts, to be used by humanity in other non-replicating machines.
The next major development of the concept of self-replicating machines was a series of thought experiments proposed by physicist Freeman Dyson in his 1970 Vanuxem Lecture. [ 28 ] [ 29 ] He proposed three large-scale applications of machine replicators. First was to send a self-replicating system to Saturn 's moon Enceladus , which in addition to producing copies of itself would also be programmed to manufacture and launch solar sail -propelled cargo spacecraft. These spacecraft would carry blocks of Enceladean ice to Mars , where they would be used to terraform the planet . His second proposal was a solar-powered factory system designed for a terrestrial desert environment, and his third was an "industrial development kit" based on this replicator that could be sold to developing countries to provide them with as much industrial capacity as desired. When Dyson revised and reprinted his lecture in 1979 he added proposals for a modified version of Moore's seagoing artificial living plants that was designed to distill and store fresh water for human use [ 30 ] and the " Astrochicken ."
In 1980, inspired by a 1979 "New Directions Workshop" held at Wood's Hole, NASA conducted a joint summer study with ASEE entitled Advanced Automation for Space Missions to produce a detailed proposal for self-replicating factories to develop lunar resources without requiring additional launches or human workers on-site. The study was conducted at Santa Clara University and ran from June 23 to August 29, with the final report published in 1982. [ 31 ] The proposed system would have been capable of exponentially increasing productive capacity and the design could be modified to build self-replicating probes to explore the galaxy.
The reference design included small computer-controlled electric carts running on rails inside the factory, mobile "paving machines" that used large parabolic mirrors to focus sunlight on lunar regolith to melt and sinter it into a hard surface suitable for building on, and robotic front-end loaders for strip mining . Raw lunar regolith would be refined by a variety of techniques, primarily hydrofluoric acid leaching . Large transports with a variety of manipulator arms and tools were proposed as the constructors that would put together new factories from parts and assemblies produced by its parent.
Power would be provided by a "canopy" of solar cells supported on pillars. The other machinery would be placed under the canopy.
A " casting robot " would use sculpting tools and templates to make plaster molds . Plaster was selected because the molds are easy to make, can make precise parts with good surface finishes, and the plaster can be easily recycled afterward using an oven to bake the water back out. The robot would then cast most of the parts either from nonconductive molten rock ( basalt ) or purified metals. A carbon dioxide laser cutting and welding system was also included.
A more speculative, more complex microchip fabricator was specified to produce the computer and electronic systems, but the designers also said that it might prove practical to ship the chips from Earth as if they were "vitamins."
A 2004 study supported by NASA's Institute for Advanced Concepts took this idea further. [ 32 ] Some experts are beginning to consider self-replicating machines for asteroid mining .
Much of the design study was concerned with a simple, flexible chemical system for processing the ores, and the differences between the ratio of elements needed by the replicator, and the ratios available in lunar regolith . The element that most limited the growth rate was chlorine , needed to process regolith for aluminium . Chlorine is very rare in lunar regolith.
In 1995, inspired by Dyson's 1970 suggestion of seeding uninhabited deserts on Earth with self-replicating machines for industrial development, Klaus Lackner and Christopher Wendt developed a more detailed outline for such a system. [ 33 ] [ 34 ] [ 35 ] They proposed a colony of cooperating mobile robots 10–30 cm in size running on a grid of electrified ceramic tracks around stationary manufacturing equipment and fields of solar cells. Their proposal didn't include a complete analysis of the system's material requirements, but described a novel method for extracting the ten most common chemical elements found in raw desert topsoil (Na, Fe, Mg, Si, Ca, Ti, Al, C, O 2 and H 2 ) using a high-temperature carbothermic process. This proposal was popularized in Discover magazine , featuring solar-powered desalination equipment used to irrigate the desert in which the system was based. [ 36 ] They named their machines "Auxons", from the Greek word auxein which means "to grow".
In the spirit of the 1980 "Advanced Automation for Space Missions" study, the NASA Institute for Advanced Concepts began several studies of self-replicating system design in 2002 and 2003. Four phase I grants were awarded:
In 2012, NASA researchers Metzger , Muscatello, Mueller, and Mantovani argued for a so-called "bootstrapping approach" to start self-replicating factories in space. [ 43 ] They developed this concept on the basis of In Situ Resource Utilization (ISRU) technologies that NASA has been developing to "live off the land" on the Moon or Mars. Their modeling showed that in just 20 to 40 years this industry could become self-sufficient then grow to large size, enabling greater exploration in space as well as providing benefits back to Earth. In 2014, Thomas Kalil of the White House Office of Science and Technology Policy published on the White House blog an interview with Metzger on bootstrapping solar system civilization through self-replicating space industry. [ 44 ] Kalil requested the public submit ideas for how "the Administration, the private sector, philanthropists, the research community, and storytellers can further these goals." Kalil connected this concept to what former NASA Chief technologist Mason Peck has dubbed "Massless Exploration", the ability to make everything in space so that you do not need to launch it from Earth. Peck has said, "...all the mass we need to explore the solar system is already in space. It's just in the wrong shape." [ 45 ] In 2016, Metzger argued that fully self-replicating industry can be started over several decades by astronauts at a lunar outpost for a total cost (outpost plus starting the industry) of about a third of the space budgets of the International Space Station partner nations, and that this industry would solve Earth's energy and environmental problems in addition to providing massless exploration. [ 46 ]
In 2011, a team of scientists at New York University created a structure called 'BTX' (bent triple helix) based around three double helix molecules, each made from a short strand of DNA. Treating each group of three double-helices as a code letter, they can (in principle) build up self-replicating structures that encode large quantities of information. [ 47 ] [ 48 ]
In 2001, Jarle Breivik at University of Oslo created a system of magnetic building blocks, which in response to temperature fluctuations, spontaneously form self-replicating polymers. [ 49 ]
In 1968, Zellig Harris wrote that "the metalanguage is in the language," [ 50 ] suggesting that self-replication is part of language. In 1977 Niklaus Wirth formalized this proposition by publishing a self-replicating deterministic context-free grammar . [ 51 ] Adding to it probabilities, Bertrand du Castel published in 2015 a self-replicating stochastic grammar and presented a mapping of that grammar to neural networks , thereby presenting a model for a self-replicating neural circuit. [ 52 ]
November 29, 2021 a team at Harvard Wyss Institute built the first living robots that can reproduce. [ 53 ]
The idea of an automated spacecraft capable of constructing copies of itself was first proposed in scientific literature in 1974 by Michael A. Arbib , [ 54 ] [ 55 ] but the concept had appeared earlier in science fiction such as the 1967 novel Berserker by Fred Saberhagen or the 1950 novellette trilogy The Voyage of the Space Beagle by A. E. van Vogt . The first quantitative engineering analysis of a self-replicating spacecraft was published in 1980 by Robert Freitas , [ 56 ] in which the non-replicating Project Daedalus design was modified to include all subsystems necessary for self-replication. The design's strategy was to use the probe to deliver a "seed" factory with a mass of about 443 tons to a distant site, have the seed factory replicate many copies of itself there to increase its total manufacturing capacity, and then use the resulting automated industrial complex to construct more probes with a single seed factory on board each.
As the use of industrial automation has expanded over time, some factories have begun to approach a semblance of self-sufficiency that is suggestive of self-replicating machines. [ 57 ] However, such factories are unlikely to achieve "full closure" [ 58 ] until the cost and flexibility of automated machinery comes close to that of human labour and the manufacture of spare parts and other components locally becomes more economical than transporting them from elsewhere. As Samuel Butler has pointed out in Erewhon , replication of partially closed universal machine tool factories is already possible. Since safety is a primary goal of all legislative consideration of regulation of such development, future development efforts may be limited to systems which lack either control, matter, or energy closure. Fully capable machine replicators are most useful for developing resources in dangerous environments which are not easily reached by existing transportation systems (such as outer space ).
An artificial replicator can be considered to be a form of artificial life . Depending on its design, it might be subject to evolution over an extended period of time. [ 59 ] However, with robust error correction , and the possibility of external intervention, the common science fiction scenario of robotic life run amok will remain extremely unlikely for the foreseeable future. [ 60 ]
Authors who have used self-replicating machine in works of fiction include: Philip K. Dick , [ 2 ] Arthur C. Clarke , [ 2 ] Karel Čapek : ( R.U.R. : Rossum’s Universal Robots (1920)), [ 2 ] [ 1 ] John Sladek ( The Reproductive System ), [ 2 ] Samuel Butler ( Erewhon ), [ 2 ] Dennis E. Taylor [ 61 ] and E. M. Forster ( The Machine Stops (1909)). [ 1 ]
The power source might be solar or possibly radioisotope based given that new liquid based compounds can generate substantial power from radioactive decay. | https://en.wikipedia.org/wiki/Self-replicating_machine |
Self-service is a system whereby customers acquire (or serve) themselves goods or services, paying for the items at a point-of-sale , as opposed to a shop assistant or clerk acquiring goods or providing services in addition to taking payment. Common examples include ATMs , coin-operated laundrettes , self-service checkouts , self-service petrol stations , and buffet restaurants. [ 1 ] [ 2 ] [ 3 ]
Before the 20th century many businesses such as grocery stores had clerks or assistants who would serve customers individually, taking required items from the shelves, before adding up the total at the till. Some products such as ham, cheese, and bacon were sliced to order, while dry goods such as flour would be weighed out from large barrels. [ 4 ] [ 5 ]
On September 6th 1916 the first Piggly Wiggly opened in Memphis, Tennessee by Clarence Saunders , the world's first self-service grocery store. Customers would pick up a wicker basket upon entering the store, and then walk through the store placing items they intended to purchase in their baskets. As the duties of the shop clerks were reduced to stocking shelves with goods and taking payment at the tills, a "small army of clerks" was no longer necessary, allowing for cost reductions to be passed on to the consumer. [ 4 ] In 1937, Saunders start opening Keedoozle stores, a further development of his idea of automated grocery stores. [ 6 ] By the 1950s about 80% of the grocery trade in America was on a self-service basis. [ 5 ]
In the United Kingdom , trials with self-service stores began in the Second World War , with the first permanent self-service store, a co-op , opened in 1948, Tesco likewise opened its first self-service store in St Albans later in the same year. The reduction in the number of staff needed to operate such a store, and the increased speed at which customers could be served, helped to mitigate problems created by the labour shortages in the war. The concept caught on quickly, with Sainsbury's , Waitrose , Morrisons and Marks & Spencer adopting self-service models in the 1950s, and one sixth of all co-op grocery stores being self-service by 1957. [ 5 ]
In 2020, Amazon Fresh (a subsidiary of Amazon ) opened its first till-less store . Some of these stores use "grab and go" systems where surveillance cameras and other technology tracks what each customer takes and places back, whereas most use "dash carts" which use touchscreens, barcode scanners, cameras, and various sensors to track items placed into and removed from the cart. Payment is done by scanning a QR code from their Amazon app, connecting the purchase to their Amazon account and allowing it to be billed through the payment method linked to their account. [ 7 ] [ 8 ] [ 9 ]
In 1930 the Hoosier Petroleum Co. attempted to trial self-serve fuelling, but was prevented from doing so as it was considered a fire hazard. [ 10 ]
In 1947, Frank Urich opened the first self-service gasoline station in Los Angeles , California . It was an unbranded station with rows of self-service pumps and roller-skating attendants who would collect money and reset dispensers. The pumps used mechanical computers to track how much fuel was dispensed, and were manually reset between each customer. A few other unbranded stations using this model were created, but the idea didn't catch on with major retailers at the time. [ 10 ]
In 1964, Herb Timms showcased an invention to John Roscoe that would allow for an attendant inside the store to dispense gasoline at the pumps. This remote fuelling system quickly took off, with three of Roscoe's twelve stores employing it and averaging 4,500 gallons in sales per week. [ 10 ]
In 1961 Britain's first self-service petrol station opened in Southwark , London . [ 11 ] In 1968, the use of "unattended fuelling" was permitted in the City of London , with BP announcing plans to open self-service units within the city. [ 10 ]
By the mid-1980s, credit card readers were integrated into pump dispensers, allowing for " pay-at-the-pump " transactions. [ 10 ]
In 1998, Japan abolished the Special Petroleum Law, allowing for self-service petrol stations, although at least one attendant is still required to keep watch over customers to ensure safety. [ 12 ] [ 13 ]
In the 21st century, self-service gas stations are the norm across the US, and New Jersey is the only state "where drivers are not allowed to pump their own gasoline." [ 14 ]
In 1960, Armenian-American inventor Luther Simjian invented an automated deposit machine (accepting coins, cash and cheques) although it did not have cash dispensing features. [ 15 ] His US patent was first filed on 30 June 1960 and granted on 26 February 1963. [ 16 ] The roll-out of this machine, called Bankograph, was delayed by a couple of years, due in part to Simjian's Reflectone Electronics Inc. being acquired by Universal Match Corporation. [ 17 ] The New York Times wrote in 1998 that it was his most famous invention and "the basis for the now-ubiquitous A.T.M., from which he never made a penny." [ 18 ] His device did not see widespread adoption however.
In Europe, in 1967, three independent efforts to create ATMs entered use simultaneously, the Swedish Bankomat, and in the UK the Barclaycash and Chubb MD2. [ 19 ] In 1968 a joint effort between IBM and Swedish banks began testing a networked cashpoint, with Lloyds Bank soon following, deploying networked devices in 1973. [ 19 ]
The first vending machine was described in a work by Hero of Alexandria in the 1st century AD . The machine accepted a coin which when deposited fell upon a pan attached t o a lever. The lever opened a valve which let wine or holy water to flow out. The pan continued to tilt with the weight of the coin until it fell off, at which point a counterweight snapped the lever back up and turned off the valve after a predetermined amount of liquid was dispensed. [ 20 ] [ 21 ]
Coin-operated machines that dispensed tobacco were being operated as early as 1615 in the taverns of England. The machines were portable and made of brass . [ 22 ] An English bookseller, Richard Carlile , devised a newspaper dispensing machine for the dissemination of banned works in 1822. Simon Denham was awarded British Patent no. 706 for his stamp dispensing machine in 1867, the first fully automatic vending machine. [ 23 ]
Vending machines are considerably popular in Japan. There are more than 5.5 million machines installed throughout the nation, and Japan holds the highest ratio of machines per person for any country with one machine for every twenty-three people. [ 24 ] [ 25 ] [ 26 ]
Starting in the 19th century, supper , a lighter post- dinner evening meal began to sometimes be served as (and so called) a 'buffet', particularly at larger events such as grand balls. Likewise large cooked English breakfasts were often served this way. The term came from the French sideboard which the food was traditionally placed on, before becoming applied to the self-service format of food.
The all-you-can-eat restaurant was introduced in Las Vegas by Herbert "Herb" Cobb McDonald in 1946.
Selfsourcing is the internal development and support of IT systems by knowledge workers with minimal contribution from IT specialists, and has been described as essentially outsourcing development effort to the end user. [ 27 ] At times they use in-house Data warehouse systems, which often run on mainframes. [ 28 ]
Various terms have been used to describe end user self service, when someone who is not a professional programmer programs, codes, scripts, writes macros, and in other ways uses a computer in a user-directed data processing accomplishment, such as End user computing and End user development .
In the 1990s, Windows versions of mainframe packages were already available. [ 29 ]
When desktop personal computers became nearly as widely distributed as having a work phone, in companies having a data processing department, the PC was often unlinked to the corporate mainframe, and data was keyed in from printouts. Software was for do-it-yourself/selfsourcing, including spreadsheets , programs written in DOS-BASIC or, somewhat later, dBASE . Use of spreadsheets, the most popular End-user development tool, [ 30 ] [ 31 ] was estimated in 2005 to done by 13 million American employees. [ 30 ]
Some data became siloed [ 32 ] Once terminal emulation arrived, more data was available, and it was more current. Techniques such as Screen scraping and FTP reduced rekeying. Mainframe products such as FOCUS were ported to the PC, and business intelligence (BI) software became more widespread.
Companies large enough to have mainframes and use BI, having departments with analysts and other specialists, have people doing this work full-time. Selfsourcing, in such situations, [ 33 ] is taking people away from their main job (such as designing ads, creating surveys, planning advertising campaigns); pairs of people, one from an analysis group and another from a "user" group, is the way the company wants to operate. Selfsourcing is not viewed as an improvement.
Data warehouse was an earlier term in this space. [ 34 ]
It is crucial for the system's purposes and goals to be aligned with that of the organizational goals. [ 35 ] Developing a system that contradicts organizational goals will most likely lead to a reduction in sales and customer retention . As well, due to the large amount of time it may take for development, it is important allocate your time efficiently as time is valuable.
Knowledge workers must also determine what kind of external support they will require. In-house IT specialists can be a valuable commodity and are often included in the planning process.
It is important to document how the system works, to ensure that if the developing knowledge workers move on others can use it and even attempt to make needed updates. [ 36 ]
Knowledge workers are often exactly aware of their immediate needs, and can avoid formalizations and time needed for project cost/benefit analysis and delays due to chargebacks or need for managerial/supervisory signoffs.
Additional benefits are:
Some knowledge workers involved in selfsourcing do not have experience or expertise with IT tools, resulting in:
Although departmental computing has decades of history, [ 29 ] one-person-show situations either suffer from inability to interact with a helpdesk [ 40 ] or fail to benefit from wheels already invented. [ 41 ]
Among the basic examples of various categories are: | https://en.wikipedia.org/wiki/Self-service |
In the study of partial differential equations , particularly in fluid dynamics , a self-similar solution is a form of solution which is similar to itself if the independent and dependent variables are appropriately scaled. Self-similar solutions appear whenever the problem lacks a characteristic length or time scale (for example, the Blasius boundary layer of an infinite plate, but not of a finite-length plate). These include, for example, the Blasius boundary layer or the Sedov–Taylor shell . [ 1 ] [ 2 ]
A powerful tool in physics is the concept of dimensional analysis and scaling laws. By examining the physical effects present in a system, we may estimate their size and hence which, for example, might be neglected. In some cases, the system may not have a fixed natural length or time scale, while the solution depends on space or time. It is then necessary to construct a scale using space or time and the other dimensional quantities present—such as the viscosity ν {\displaystyle \nu } . These constructs are not 'guessed' but are derived immediately from the scaling of the governing equations.
The normal self-similar solution is also referred to as a self-similar solution of the first kind , since another type of self-similar exists for finite-sized problems, which cannot be derived from dimensional analysis , known as a self-similar solution of the second kind .
The early identification of self-similar solutions of the second kind can be found in problems of imploding shock waves ( Guderley–Landau–Stanyukovich problem ), analyzed by G. Guderley (1942) and Lev Landau and K. P. Stanyukovich (1944), [ 3 ] and propagation of shock waves by a short impulse, analysed by Carl Friedrich von Weizsäcker [ 4 ] and Yakov Borisovich Zel'dovich (1956), who also classified it as the second kind for the first time. [ 5 ] An independent study about the same field was published by Leonid Ivanovich Sedov in 1959. [ 6 ] A complete description was made in 1972 by Grigory Barenblatt and Yakov Borisovich Zel'dovich . [ 7 ] The self-similar solution of the second kind also appears in different contexts such as in boundary-layer problems subjected to small perturbations, [ 8 ] as was identified by Keith Stewartson , [ 9 ] Paul A. Libby and Herbert Fox. [ 10 ] Moffatt eddies are also a self-similar solution of the second kind.
A simple example is a semi-infinite domain bounded by a rigid wall and filled with viscous fluid. [ 11 ] At time t = 0 {\displaystyle t=0} the wall is made to move with constant speed U {\displaystyle U} in a fixed direction (for definiteness, say the x {\displaystyle x} direction and consider only the x − y {\displaystyle x-y} plane), one can see that there is no distinguished length scale given in the problem. This is known as the Rayleigh problem . The boundary conditions of no-slip is u ( y = 0 ) = U {\displaystyle u{(y\!=\!0)}=U}
Also, the condition that the plate has no effect on the fluid at infinity is enforced as u ( y → ∞ ) = 0. {\displaystyle u{(y\!\to \!\infty )}=0.}
Now, from the Navier-Stokes equations ρ ( ∂ u → ∂ t + u → ⋅ ∇ u → ) = − ∇ p + μ ∇ 2 u → {\displaystyle \rho \left({\dfrac {\partial {\vec {u}}}{\partial t}}+{\vec {u}}\cdot \nabla {\vec {u}}\right)=-\nabla p+\mu \nabla ^{2}{\vec {u}}} one can observe that this flow will be rectilinear , with gradients in the y {\displaystyle y} direction and flow in the x {\displaystyle x} direction, and that the pressure term will have no tangential component so that ∂ p ∂ y = 0 {\displaystyle {\dfrac {\partial p}{\partial y}}=0} . The x {\displaystyle x} component of the Navier-Stokes equations then becomes ∂ u → ∂ t = ν ∂ y 2 u → {\displaystyle {\dfrac {\partial {\vec {u}}}{\partial t}}=\nu \partial _{y}^{2}{\vec {u}}} and the scaling arguments can be applied to show that U t ∼ ν U y 2 {\displaystyle {\frac {U}{t}}\sim \nu {\frac {U}{y^{2}}}} which gives the scaling of the y {\displaystyle y} co-ordinate as y ∼ ( ν t ) 1 / 2 {\displaystyle y\sim (\nu t)^{1/2}} .
This allows one to pose a self-similar ansatz such that, with f {\displaystyle f} and η {\displaystyle \eta } dimensionless, u = U f ( η ≡ y ( ν t ) 1 / 2 ) {\displaystyle u=Uf\left(\eta \equiv {\dfrac {y}{(\nu t)^{1/2}}}\right)}
The above contains all the relevant physics and the next step is to solve the equations, which for many cases will include numerical methods. This equation is − η f ′ / 2 = f ″ {\displaystyle -\eta f'/2=f''} with solution satisfying the boundary conditions that f = 1 − erf ( η / 2 ) or u = U ( 1 − erf ( y / ( 4 ν t ) 1 / 2 ) ) {\displaystyle f=1-\operatorname {erf} (\eta /2)\quad {\text{ or }}\quad u=U\left(1-\operatorname {erf} \left(y/(4\nu t)^{1/2}\right)\right)} which is a self-similar solution of the first kind.
In transient heat transfer applications, such as impingement heating on a ship deck during missile launches and the sizing of thermal protection systems , self-similar solutions can be found for semi-infinite solids. [ 12 ] [ 13 ] The governing equation when heat conduction is the primary heat transfer mechanism is the one-dimensional energy equation: ρ c p ∂ T ∂ t = ∂ ∂ x ( k ∂ T ∂ x ) {\displaystyle \rho c_{p}{\frac {\partial T}{\partial t}}={\frac {\partial }{\partial x}}\left(k{\frac {\partial T}{\partial x}}\right)} where ρ {\displaystyle \rho } is the material's density , c p {\displaystyle c_{p}} is the material's specific heat capacity , k {\displaystyle k} is the material's thermal conductivity . In the case when the material is assumed to be homogeneous and its properties constant, the energy equation is reduced to the heat equation : ∂ T ∂ t = α ∂ 2 T ∂ x 2 , α = k ρ c p {\displaystyle {\frac {\partial T}{\partial t}}=\alpha {\frac {\partial ^{2}T}{\partial x^{2}}},\quad \alpha ={\frac {k}{\rho c_{p}}}} with α {\displaystyle \alpha } being the thermal diffusivity . By introducing the similarity variable η = x / t {\displaystyle \eta =x/{\sqrt {t}}} and assuming that T ( t , x ) = f ( η ) {\displaystyle T(t,x)=f(\eta )} , the PDE can be transformed into the ODE: f ″ ( η ) + 1 2 α η f ′ ( η ) = 0 {\displaystyle f''(\eta )+{\frac {1}{2\alpha }}\eta f'(\eta )=0} If a simple model of thermal protection system sizing is assumed, where decomposition, pyrolysis gas flow, and surface recession are ignored, with the initial temperature T ( 0 , x ) = f ( ∞ ) = T i {\displaystyle T(0,x)=f(\infty )=T_{i}} and a constant surface temperature T ( t , 0 ) = f ( 0 ) = T s {\displaystyle T(t,0)=f(0)=T_{s}} , then the ODE can be solved for the temperature at a depth x {\displaystyle x} and time t {\displaystyle t} : [ 13 ] T ( t , x ) = erf ( x 2 α t ) ( T i − T s ) + T s {\displaystyle T(t,x)={\text{erf}}\left({\frac {x}{2{\sqrt {\alpha t}}}}\right)\left(T_{i}-T_{s}\right)+T_{s}} where erf ( ⋅ ) {\displaystyle {\text{erf}}(\cdot )} is the error function . | https://en.wikipedia.org/wiki/Self-similar_solution |
A self-tiling tile set , or setiset , of order n is a set of n shapes or pieces, usually planar, each of which can be tiled with smaller replicas of the complete set of n shapes. That is, the n shapes can be assembled in n different ways so as to create larger copies of themselves, where the increase in scale is the same in each case. Figure 1 shows an example for n = 4 using distinctly shaped decominoes . The concept can be extended to include pieces of higher dimension. The name setisets was coined by Lee Sallows in 2012, [ 1 ] [ 2 ] but the problem of finding such sets for n = 4 was asked decades previously by C. Dudley Langford, and examples for polyaboloes (discovered by Martin Gardner , Wade E. Philpott and others) and polyominoes (discovered by Maurice J. Povah) were previously published by Gardner. [ 3 ]
From the above definition it follows that a setiset composed of n identical pieces is the same thing as a 'self-replicating tile' or rep-tile , of which setisets are therefore a generalization. [ 4 ] Setisets using n distinct shapes, such as Figure 1, are called perfect . Figure 2 shows an example for n = 4 which is imperfect because two of the component shapes are the same.
The shapes employed in a setiset need not be connected regions. Disjoint pieces composed of two or more separated islands are also permitted. Such pieces are described as disconnected , or weakly-connected (when islands join only at a point), as seen in the setiset shown in Figure 3.
The fewest pieces in a setiset is two. Figure 4 encapsulates an infinite family of order 2 setisets each composed of two triangles, P and Q . As shown, the latter can be hinged together to produce a compound triangle that has the same shape as P or Q , depending upon whether the hinge is fully open or fully closed. This unusual specimen thus provides an example of a hinged dissection .
The properties of setisets mean that their pieces form substitution tilings , or tessellations in which the prototiles can be dissected or combined so as to yield smaller or larger duplicates of themselves. Clearly, the twin actions of forming still larger and larger copies (known as inflation), or still smaller and smaller dissections (deflation), can be repeated indefinitely. In this way, setisets can produce non-periodic tilings. However, none of the non-periodic tilings thus far discovered qualify as aperiodic , because the prototiles can always be rearranged so as to yield a periodic tiling. Figure 5 shows the first two stages of inflation of an order 4 set leading to a non-periodic tiling.
Besides self-tiling tile sets, which can be interpreted as loops of length 1, there exist longer loops, or closed chains of sets, in which every set tiles its successor. [ 5 ] Figure 6 shows a pair of mutually tiling sets of decominoes , in other words, a loop of length 2. Sallows and Schotel did an exhaustive search of order 4 sets that are composed of octominoes . In addition to seven ordinary setisets (i.e., loops of length 1) they found a bewildering variety of loops of every length up to a maximum of 14. The total number of loops identified was nearly one and a half million. More research in this area remains to be done, but it seems safe to suppose that other shapes may also entail loops. [ 6 ]
To date, two methods have been used for producing setisets. In the case of sets composed of shapes such as polyominoes , which entail integral piece sizes, a brute force search by computer is possible, so long as n , the number of pieces involved, is not prohibitive. It is easily shown that n must then be a perfect square . [ 4 ] Figures 1,2,3,5 and 6 are all examples found by this method.
Alternatively, there exists a method whereby multiple copies of a rep-tile can be dissected in certain ways so as to yield shapes that create setisets. Figures 7 and 8 show setisets produced by this means, in which each piece is the union of 2 and 3 rep-tiles, respectively. In Figure 8 can be seen how the 9 pieces above together tile the 3 rep-tile shapes below, while each of the 9 pieces is itself formed by the union of 3 such rep-tile shapes. Hence each shape can be tiled with smaller duplicates of the entire set of 9. [ 4 ] | https://en.wikipedia.org/wiki/Self-tiling_tile_set |
Self-verifying theories are consistent first-order systems of arithmetic , much weaker than Peano arithmetic , that are capable of proving their own consistency . Dan Willard was the first to investigate their properties, and he has described a family of such systems. According to Gödel's incompleteness theorem , these systems cannot contain the theory of Peano arithmetic nor its weak fragment Robinson arithmetic ; nonetheless, they can contain strong theorems.
In outline, the key to Willard's construction of his system is to formalise enough of the Gödel machinery to talk about provability internally without being able to formalise diagonalisation . Diagonalisation depends upon being able to prove that multiplication is a total function (and in the earlier versions of the result, addition also). Addition and multiplication are not function symbols of Willard's language; instead, subtraction and division are, with the addition and multiplication predicates being defined in terms of these. Here, one cannot prove the Π 2 0 {\displaystyle \Pi _{2}^{0}} sentence expressing totality of multiplication: ( ∀ x , y ) ( ∃ z ) m u l t i p l y ( x , y , z ) . {\displaystyle (\forall x,y)\ (\exists z)\ {\rm {multiply}}(x,y,z).} where m u l t i p l y {\displaystyle {\rm {multiply}}} is the three-place predicate which stands for z / y = x . {\displaystyle z/y=x.} When the operations are expressed in this way, provability of a given sentence can be encoded as an arithmetic sentence describing termination of an analytic tableau . Provability of consistency can then simply be added as an axiom. The resulting system can be proven consistent by means of a relative consistency argument with respect to ordinary arithmetic.
One can further add any true Π 1 0 {\displaystyle \Pi _{1}^{0}} sentence of arithmetic to the theory while still retaining consistency of the theory.
This logic -related article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Self-verifying_theories |
Selig Hecht (February 8, 1892 – September 18, 1947) was an American physiologist who studied photochemistry in photoreceptor cells .
Hecht was born into a Jewish family in Glogau , [ 1 ] then in the German Empire (now Głogów, Poland), the son of Mandel Hecht and Mirel Mresse. The family migrated to the United States in 1898, settling in New York City . [ 2 ]
In June 1917 Hecht received his Ph.D. and married Celia Huebschmann. Their daughter Maressa was born in 1924. [ 3 ] He became professor of biophysics at Columbia University in 1928.
Hecht began his study into light sensitivity with clams ( Mya arenaria ) and insects. His specialty was photochemistry, the kinetics of the reactions initiated by light in the receptors. He made contributions to the knowledge of dark adaptation , visual acuity , brightness discrimination, color vision , and the mechanism of the visual threshold.
He spent time as a post-doctoral researcher with the group of Edward Charles Cyril Baly at the University of Liverpool, UK. Baly was a pioneer in the application of the technique of spectroscopy in chemistry, and Hecht took this further by applying it to biological molecules. [ 4 ] : 409
Hecht's responsibility in showing the protein character of rhodopsin was recounted by historians of protein science: [ 5 ]
Identification of visual purple as a protein of high molecular weight ...[came] from the work of Selig Hecht at Columbia University in New York, begun in 1937. Ultracentrifugation was one of methods he used for characterization and this produced an added dividend, demonstrating that the complex absorption of the 'pigment' (suggesting the possibility of many components) segmented in toto with the protein. By this time the carotenoid prosthetic group had been discovered as the source of colour by George Wald and Hecht pointed out that this meant that the protein had to be a conjugated protein , with the chromophore firmly attached.
According to biographer Pirenne, [ 6 ] Hecht was a "brilliant lecturer and expositor." Pirenne continues,
In 1941, The Optical Society of America awarded him the Frederic Ives Medal , the Society's highest honor.
When World War II ended with the use of atomic weapons which had been developed in secret by the Manhattan Project , Hecht was concerned that the American public was uninformed about the development of this new source of energy. He wrote a book Explaining the Atom (1947), to educate the public. He wrote,
In a 1947 review in the New York Times , Stephen Wheeler wrote that it was "by all odds the best book on atomic energy so far to be published for the ordinary reader." [ 9 ] Similarly, James J. Jelinek wrote that it was an "invaluable contribution to the layman." He credits Hecht with "conveying to the layman the intellectual drama" of the development. Jelinek asserts that the book is "profoundly provocative in its political and sociological implications." [ 10 ]
After Hecht died, a second edition was issued in 1959 by Eugene Rabinowitch . Both editions were recommended by George Gamow . [ 11 ] | https://en.wikipedia.org/wiki/Selig_Hecht |
Selinexor sold under the brand name Xpovio among others, is a selective inhibitor of nuclear export used as an anti-cancer medication . It works by blocking the action of exportin 1 [ 6 ] and thus blocking the transport of several proteins involved in cancer-cell growth from the cell nucleus to the cytoplasm , which ultimately arrests the cell cycle and leads to apoptosis . [ 8 ] It is the first drug with this mechanism of action . [ 9 ] [ 10 ]
The most common side effects include nausea (feeling sick), vomiting, decreased appetite, weight loss, diarrhea, tiredness, thrombocytopenia (low blood-platelet counts), anaemia (low red-blood cell counts), low levels of white blood cells and hyponatraemia (low blood sodium levels). [ 6 ]
Selinexor was granted accelerated approval by the U.S. Food and Drug Administration (FDA) in July 2019, for use in combination with the corticosteroid dexamethasone for the treatment of adults with relapsed refractory multiple myeloma (RRMM) who have received at least four prior therapies and whose disease is resistant to several other forms of treatment, including at least two proteasome inhibitors, at least two immunomodulatory agents, and an anti-CD38 monoclonal antibody. [ 11 ] In December 2020, selinexor was approved by the FDA in combination with bortezomib and dexamethasone for the treatment of adults with multiple myeloma who have received at least one prior therapy. [ 12 ] In clinical trials, it was associated with a high incidence of severe side effects, including low platelet counts and low blood sodium levels . [ 10 ] [ 13 ] [ 14 ]
The U.S. Food and Drug Administration (FDA) considers it to be a first-in-class medication . [ 15 ] Selinexor was approved for medical use in the European Union in March 2021. [ 6 ]
Selinexor is approved in combination with bortezomib and dexamethasone for the treatment of adults with multiple myeloma who have received at least one prior therapy. [ 12 ] Selinexor is also approved for use in combination with the steroid dexamethasone in people with relapsed or refractory multiple myeloma who have received at least four prior therapies and whose disease is refractory to at least two proteosome inhibitors, at least two immunomodulatory agents, and an anti-CD38 monoclonal antibody (so-called "quad-refractory" or "penta-refractory" myeloma), [ 16 ] for whom no other treatment options are available. [ 10 ] [ 13 ] It is the first drug to be approved for this indication. [ 17 ]
In June 2020, the U.S. Food and Drug Administration (FDA) approved an additional indication for selinexor to treat adults with relapsed or refractory diffuse large B-cell lymphoma (DLBCL), not otherwise specified, including DLBCL arising from follicular lymphoma, after at least two lines of systemic therapy. [ 18 ]
In the European Union, selinexor is indicated in combination with dexamethasone for the treatment of multiple myeloma in adults who have received at least four prior therapies and whose disease is refractory to at least two proteasome inhibitors, two immunomodulatory agents and an anti-CD38 monoclonal antibody, and who have demonstrated disease progression on the last therapy. [ 6 ]
In the clinical study (the BOSTON study) used to support FDA approval in patients with multiple myeloma after at least one prior therapy (once-weekly selinexor in combination with once-weekly bortezomib and dexamethasone),the most common adverse reactions were cytopenias, along with gastrointestinal and constitutional symptoms and were consistent with those previously reported from other selinexor studies. Most adverse reactions were manageable with dose modifications and/or standard supportive care. The most common non-hematologic adverse reactions were fatigue (59%), nausea (50%), decreased appetite (35%), and diarrhea (32%) and were mostly Grade 1 and 2 events. The most common Grade 3 and 4 adverse reactions were thrombocytopenia (43%), lymphopenia (38%), fatigue (28%) and anemia (17%). [ 19 ]
The most common adverse reactions (incidence ≥20%) in people with diffuse large B-cell lymphoma (DLBCL), excluding laboratory abnormalities, were fatigue, nausea, diarrhea, appetite decrease, weight decrease, constipation, vomiting, and pyrexia. [ 18 ] Grade 3-4 laboratory abnormalities in ≥15% were thrombocytopenia, lymphopenia, neutropenia, anemia, and hyponatremia. [ 18 ] Serious adverse reactions occurred in 46% of people, most often from infection. [ 18 ] Thrombocytopenia was the leading cause of dose modifications. [ 18 ] Gastrointestinal toxicity developed in 80% of people and any grade hyponatremia developed in 61%. [ 18 ] Central neurological adverse reactions occurred in 25% of people, including dizziness and mental status changes. [ 18 ]
The prescribing information provides warnings and precautions for thrombocytopenia, neutropenia, gastrointestinal toxicity, hyponatremia, serious infection, neurological toxicity, and embryo-fetal toxicity. [ 18 ] [ 5 ]
Like other selective inhibitors of nuclear export (SINEs), selinexor works by binding to exportin 1 (also known as XPO1 or CRM1). XPO1 is a karyopherin which performs nuclear transport of several proteins, including tumor suppressors , oncogenes , and proteins involved in governing cell growth, from the cell nucleus to the cytoplasm ; it is often overexpressed and its function misregulated in several types of cancer. [ 8 ] By inhibiting the XPO1 protein, SINEs lead to a buildup of tumor suppressors in the nucleus of malignant cells and reduce levels of oncogene products which drive cell proliferation. This ultimately leads to cell cycle arrest and death of cancer cells by apoptosis . [ 8 ] [ 9 ] [ 5 ] In vitro , this effect appeared to spare normal (non-malignant) cells. [ 8 ] [ 20 ]
Inhibiting XPO1 affects many different cells in the body which may explain the incidence of adverse reactions to selinexor. [ 9 ] Thrombocytopenia, for example, is a mechanistic and dose-dependent effect, occurring because selinexor causes a buildup of the transcription factor STAT3 in the nucleus of hematopoietic stem cells , preventing their differentiation into mature megakaryocytes (platelet-producing cells) and thus slowing production of new platelets. [ 9 ]
Selinexor is a fully synthetic small-molecule compound, developed by means of a structure-based drug design process known as induced-fit docking . It binds to a cysteine residue in the nuclear export signal groove of exportin 1. Although this bond is covalent , it is slowly reversible . [ 8 ]
Selinexor was developed by Karyopharm Therapeutics, a pharmaceutical company focused on the development of drugs that target nuclear transport . It was approved in the United States in July 2019, [ 21 ] [ 11 ] [ 22 ] on the basis of a single-arm Phase IIb clinical trial. The FDA decided to grant accelerated approval despite a previous recommendation from an FDA Advisory Committee Panel which had voted 8–5 to delay approving the drug until the results from an ongoing Phase III study were known. [ 10 ]
Selinexor in combination with dexamethasone was granted accelerated approval and was granted orphan drug designation. [ 11 ] The FDA granted the approval of Xpovio to Karyopharm Therapeutics. [ 11 ]
In June 2020, the U.S. Food and Drug Administration (FDA) approved an additional indication for selinexor to treat adults with relapsed or refractory diffuse large B-cell lymphoma (DLBCL), not otherwise specified, including DLBCL arising from follicular lymphoma, after at least two lines of systemic therapy. [ 18 ]
Approval was based on SADAL (KCP-330-009; NCT02227251), a multicenter, single-arm, open-label trial in participants with DLBCL after two to five systemic regimens. [ 18 ] Participants received selinexor 60 mg orally on days one and three of each week. [ 18 ]
In December 2020, the FDA expanded selinexor's approved indication to include its combination with bortezomib and dexamethasone for the treatment of adults with multiple myeloma who have received at least one prior therapy.
On 28 January 2021, the Committee for Medicinal Products for Human Use (CHMP) of the European Medicines Agency (EMA) adopted a positive opinion, recommending the granting of a conditional marketing authorization for the medicinal product Nexpovio intended for the treatment of relapsed and refractory multiple myeloma. [ 23 ] The applicant for this medicinal product is Karyopharm Europe GmbH. [ 23 ] Selinexor was approved for medical use in the European Union in March 2021. [ 6 ]
Under the codename KPT-330, selinexor was tested in several preclinical animal models of cancer, including pancreatic cancer , breast cancer , non-small-cell lung cancer , lymphomas , and acute and chronic leukemias . [ 24 ] In humans, early clinical trials (phase I) have been conducted in non-Hodgkin lymphoma , blast crisis , and a wide range of advanced or refractory solid tumors, including colon cancer , head and neck cancer , melanoma , ovarian cancer , and prostate cancer . [ 24 ] Compassionate use in patients with acute myeloid leukemia has also been reported. [ 24 ]
The pivotal clinical trial which served to support approval of selinexor for people with relapsed/refractory multiple myeloma was an open-label study of 122 patients known as the STORM trial. [ 5 ] In all of the enrolled patients, patients had been treated with a median of seven prior treatment regimens including conventional chemotherapy , targeted therapy with bortezomib , carfilzomib , lenalidomide , pomalidomide , and a monoclonal antibody ( daratumumab or isatuximab ); [ 16 ] nearly all had also undergone hematopoietic stem cell transplantation but had disease that continued to progress. [ 5 ] The overall response rate was 26%, including two stringent complete responses; 39% of patients had a minimal response or better. The median duration of response was 4.4 months, median progression-free survival was 3.7 months, and median overall survival was 8.6 months. [ 25 ]
As of 2019, phase I/II and III trials are ongoing, [ 10 ] [ 24 ] [ 26 ] including the use of selinexor in other cancers and in combinations with other drugs used for multiple myeloma. [ 9 ]
In November 2020, results from the multi-center, Phase III, randomized study (NCT03110562) which evaluated 402 participants with relapsed or refractory multiple myeloma who had received one to three prior lines of therapy were published in The Lancet. [ 27 ] The study was designed to compare the efficacy, safety and certain health-related quality of life parameters of once-weekly selinexor in combination with once-weekly Velcade® (bortezomib) plus low-dose dexamethasone (SVd) versus twice-weekly Velcade® plus low-dose dexamethasone (Vd). The primary endpoint of the study was progression-free survival (PFS) and key secondary endpoints included overall response rate (ORR), rate of peripheral neuropathy, and others. Additionally, the BOSTON study allowed for patients on the Vd control arm to crossover to the SVd arm following objective (quantitative) progression of disease verified by an Independent Review Committee (IRC). The BOSTON study was conducted at over 150 clinical sites internationally.
Although the study had one of the highest proportions of patients with high-risk cytogenetics (~50%) as compared with other Velcade-based studies in previously treated myeloma, the median PFS in the SVd arm was 13.93 months compared to 9.46 months in the Vd arm, representing a 4.47 month (47%) increase in median PFS (hazard ratio[HR]=0.70; p=0.0075). The SVd group also demonstrated a significantly greater ORR compared to the Vd group (76.4% vs. 62.3%, p=0.0012). Patients who had received only one prior line of therapy also demonstrated a higher ORR on the SVd arm as compared to the Vd arm (80.8% vs. 65.7%, p=0.0082). Importantly, SVd therapy compared to Vd therapy showed consistent PFS benefit and higher ORR across several important subgroups. [ 27 ]
In 2020, selinexor underwent a clinical trial for treatment of COVID-19 . [ 28 ] In this phase 2 randomized placebo-controlled single-blind trial named XPORT-CoV-1001 with a total of 190 participants with severe COVID-19, treatment with selinexor resulted in higher mortality (16% vs. 9%) and more serious adverse events (23% vs. 16%) than placebo. [ 29 ] | https://en.wikipedia.org/wiki/Selinexor |
Seliwanoff’s test is a chemical test which distinguishes between aldose and ketose sugars . If the sugar contains a ketone group , it is a ketose. If a sugar contains an aldehyde group, it is an aldose. This test relies on the principle that, when heated, ketoses are more rapidly dehydrated than aldoses. It is named after Theodor Seliwanoff , the chemist who devised the test. When added to a solution containing ketoses, a red color is formed rapidly indicating a positive test. When added to a solution containing aldoses, a slower forming light pink is observed instead.
The reagents consist of resorcinol and concentrated hydrochloric acid :
Fructose and sucrose are two common sugars which give a positive test. Sucrose gives a positive test as it is a disaccharide consisting of fructose and glucose.
Generally, 6M HCl is used to run this test. Ketoses are dehydrated faster and give stronger colors. Aldoses react very slowly and give faint colors. | https://en.wikipedia.org/wiki/Seliwanoff's_test |
In chemistry, a selone (also known as a selenoketone ) is the structural analog of a ketone where selenium replaces oxygen . Selenium-77 is one of the isotopes of selenium that is stable and naturally occurring , so selenoketone-containing chemicals can be analyzed by nuclear magnetic resonance spectroscopy (NMR). Selones can be used as chiral derivatizing agents for 77 Se-NMR. [ 1 ] Chiral oxazolidineselones can be used for stereoselective control of aldol reactions , analogous to the Evans aldol reaction that uses oxazolidinones , which allows 77 Se-NMR to be used to determine the diastereomeric ratio of the aldol product. [ 2 ]
Selenobenzophenone reversibly dimerizes. It is known to undergo cycloaddition with 1,3-dienes in a reaction similar to the Diels-Alder reaction. [ 3 ]
This organic chemistry article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Selone |
Selters is a German brand of natural mineral water sourced from wells in the area of Selters in Hesse , at the Taunus mountains.
The water has been known since the Bronze Age and famous as a natural soda water because of its high concentration of sodium bicarbonate , "soda".
The Selters water also contains raised levels of calcium , chloride , magnesium , sulfate and potassium ions . The water is naturally carbonated , over 250 mg/L, but sold in both sparkling and still versions.
The name and the water of Selters are the prototype of seltzer , a generic term for soda water in the United States.
The Romans used the wells and may have given the origin of the current name Selters, either from Latin : aqua saltare (water jump) or Latin : saltrissa (salt rising), but both possibilities present linguistic uncertainties. The same name is also used in some other places in Germany with mineral wells. The wells are mentioned already in 772 in documents at the nearby monasteries in Fulda and Lorsch . In the 16th century under the rule of Johann von der Leyen , the water from the wells gained international fame. [ 1 ]
Selters has been popular as a spa resort , and the water has been used for health effects, as well as for its taste. The water has been exported in large quantities for many centuries; in 1787 J. F. Westrumb reported that over a million Selters bottles were exported all around the world. [ 2 ] The Selters bottle was common during the 17th to 19th centuries, made of stoneware, not to confuse with the modern " seltzer bottle " i.e. a soda siphon.
Artificial "selters waters" with added minerals have been created to make competition, thus helping to establish the fame of the original water as an international reference of soda water , e.g. by Torbern Bergman , who made thorough analyses of mineral waters and in 1775 presented how to make carbonated water to mimic genuine mineral waters. [ 3 ]
The production at the famous main well in Niederselters was terminated in 1999, but production continues at a well in the nearby (25 km away) village of Selters-Löhnberg , in commercial use for almost 200 years, as well as at the competing one in Oberselters . There is a Selters water museum in Niederselters.
The words "Selters", "Selterwasser" or "Selter" have become synonyms for all kinds and brands of mineral water primarily in north and eastern Germany. | https://en.wikipedia.org/wiki/Selters |
Semantic P2P networks are a new type of P2P network . It combines the advantages of unstructured P2P networks and structural P2P networks, and avoids their disadvantages.
In Semantic P2P networks, nodes are classified as DNS -like domain names with semantic meanings such as Alice @Brittney.popular.music. Semantic P2P networks contains prerequisite virtual tree topology and net-like topology formed by cached nodes. Semantic P2P networks keep the semantic meanings of nodes and their contents. The nodes within semantic P2P networks can communicate each other by various languages. Semantic P2P network can execute complicated queries by SQL -like language.
There are similarities between semantic P2P systems and software agents . P2P means that entities exchange information directly without a mediator. Semantic is a concept to add meaning to information. Peer are usually autonomous systems as well as agents. Agents follow a goal, though. Such goal attainment requires a knowledge base and rules and strategies. That's the major difference between software agents and semantic peers. The latter lacks that kind of intelligence.
This computing article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Semantic_P2P_networks |
Semantic audio is the extraction of meaning from audio signals . The field of semantic audio is primarily based around the analysis of audio to create some meaningful metadata, which can then be used in a variety of different ways.
Semantic analysis of audio is performed to reveal some deeper understanding of an audio signal. This typically results in high-level metadata descriptors such as musical chords and tempo, or the identification of the individual speaking, to facilitate content-based management of audio recordings. In recent years, the growth of automatic data analysis techniques has grown considerably,
With the development of applications that use this semantic information to support the user in identifying, organising, and exploring audio signals, and interacting with them. These applications include music information retrieval, semantic web technologies, audio production, sound reproduction, education, and gaming. Semantic technology involves some kind of understanding of the meaning of the information it deals with and to this end may incorporate machine learning, digital signal processing, speech processing, source separation, perceptual models of hearing, musicological knowledge, metadata, and ontologies.
Aside from audio retrieval and recommendation technologies, the semantics of audio signals are also becoming increasingly important, for instance, in object-based audio coding, as well as intelligent audio editing, and processing. Recent product releases already demonstrate this to a great extent, however, more innovative functionalities relying on semantic audio analysis and management are imminent. These functionalities may utilise, for instance, (informed) audio source separation, speaker segmentation and identification, structural music segmentation, or social and Semantic Web technologies, including ontologies and linked open data.
Speech recognition is an important semantic audio application. But for speech, other semantic operations include language identification , speaker identification or gender identification. For more general audio or music, it includes identifying a piece of music (e.g. Shazam (music app) ) or a movie soundtrack.
Areas of research in semantic audio include the ability to label an audio waveform with where the harmonies change and what they are and where material is repeated and what instruments are playing.
The Semantic Web provides a powerful framework for the expression and reuse of structured data. The use and storage of semantic audio descriptors in the semantic web framework, allows for a much greater reach and unifying standard for storing and managing associated semantic audio metadata. A number of ontologies have been developed for storing and managing audio on the semantic web, including the (Music Ontology) [1] , the (Studio Ontology) [2] , and the (Audio Feature ontology) [3]
Semantic hearing has been proposed for headsets to allow users to select what sounds they want to hear in their environment, based on their semantic description. [ 1 ] This noise-canceling headphone technology use real-time neural networks to let users opt back in to certain sounds they’d like to hear, such as babies crying, birds tweeting, or alarms ringing. [ 2 ] This type of capability on headphone and earbuds could provide users with a degree of control over the sounds that are around them. This could benefit people who require focused listening for their job, such as health-care, military, and engineering professionals, or for factory or construction workers as well as for designing intelligent hearing aids. [ 2 ] | https://en.wikipedia.org/wiki/Semantic_audio |
A semantic broker is a computer service that automatically provides semantic mapper services. A semantic broker is frequently part of a semantic middleware system that leverage semantic equivalence statements. To qualify as a semantic broker product a system must be able to automatically extract data from a message and use semantic equivalence statements to transform this into another namespace.
This software article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Semantic_broker |
In computer programming , a logic error is a bug in a program that causes it to operate incorrectly, but not to terminate abnormally (or crash ). [ 1 ] A logic error produces unintended or undesired output or other behaviour, although it may not immediately be recognized as such.
Logic errors occur in both compiled and interpreted languages. Unlike a program with a syntax error , a program with a logic error is a valid program in the language, though it does not behave as intended. Often the only clue to the existence of logic errors is the production of wrong solutions, though static analysis may sometimes spot them.
One of the ways to find this type of error is to put out the program's variables to a file or on the screen in order to determine the error's location in code. Although this will not work in all cases, for example when calling the wrong subroutine , it is the easiest way to find the problem if the program uses the incorrect results of a bad mathematical calculation .
This example function in C to calculate the average of two numbers contains a logic error. It is missing parentheses in the calculation, so it compiles and runs but does not give the expected answer due to operator precedence (division is evaluated before addition).
This computer-programming -related article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Semantic_error |
A semantic theory of truth is a theory of truth in the philosophy of language which holds that truth is a property of sentences. [ 1 ]
The semantic conception of truth, which is related in different ways to both the correspondence and deflationary conceptions, is due to work by Polish logician Alfred Tarski . Tarski, in "On the Concept of Truth in Formal Languages" (1935), attempted to formulate a new theory of truth in order to resolve the liar paradox . In the course of this he made several metamathematical discoveries, most notably Tarski's undefinability theorem using the same formal technique Kurt Gödel used in his incompleteness theorems . Roughly, this states that a truth-predicate satisfying Convention T for the sentences of a given language cannot be defined within that language.
To formulate linguistic theories [ 2 ] without semantic paradoxes such as the liar paradox , it is generally necessary to distinguish the language that one is talking about (the object language ) from the language that one is using to do the talking (the metalanguage ). In the following, quoted text is use of the object language, while unquoted text is use of the metalanguage; a quoted sentence (such as " P ") is always the metalanguage's name for a sentence, such that this name is simply the sentence P rendered in the object language. In this way, the metalanguage can be used to talk about the object language; Tarski's theory of truth ( Alfred Tarski 1935) demanded that the object language be contained in the metalanguage.
Tarski's material adequacy condition , also known as Convention T , holds that any viable theory of truth must entail, for every sentence " P ", a sentence of the following form (known as "form (T)"):
(1) "P" is true if, and only if , P.
For example,
(2) 'snow is white' is true if and only if snow is white.
These sentences (1 and 2, etc.) have come to be called the "T-sentences". The reason they look trivial is that the object language and the metalanguage are both English; here is an example where the object language is German and the metalanguage is English:
(3) 'Schnee ist weiß' is true if and only if snow is white.
It is important to note that as Tarski originally formulated it, this theory applies only to formal languages , cf. also semantics of first-order logic . He gave a number of reasons for not extending his theory to natural languages , including the problem that there is no systematic way of deciding whether a given sentence of a natural language is well-formed, and that a natural language is closed (that is, it can describe the semantic characteristics of its own elements). But Tarski's approach was extended by Davidson into an approach to theories of meaning for natural languages, which involves treating "truth" as a primitive, rather than a defined, concept. (See truth-conditional semantics .)
Tarski developed the theory to give an inductive definition of truth as follows. (See T-schema )
For a language L containing ¬ ("not"), ∧ ("and"), ∨ ("or"), ∀ ("for all"), and ∃ ("there exists"), Tarski's inductive definition of truth looks like this:
These explain how the truth conditions of complex sentences (built up from connectives and quantifiers ) can be reduced to the truth conditions of their constituents . The simplest constituents are atomic sentences . A contemporary semantic definition of truth would define truth for the atomic sentences as follows:
Tarski himself defined truth for atomic sentences in a variant way that does not use any technical terms from semantics, such as the "expressed by" above. This is because he wanted to define these semantic terms in the context of truth. Therefore it would be circular to use one of them in the definition of truth itself. Tarski's semantic conception of truth plays an important role in modern logic and also in contemporary philosophy of language . It is a rather controversial point whether Tarski's semantic theory should be counted either as a correspondence theory or as a deflationary theory . [ 3 ]
Kripke's theory of truth ( Saul Kripke 1975) is based on partial logic (a logic of partially defined truth predicates instead of Tarski's logic of totally defined truth predicates) with the strong Kleene evaluation scheme . [ 4 ] | https://en.wikipedia.org/wiki/Semantic_theory_of_truth |
The Semantics of Business Vocabulary and Business Rules ( SBVR ) is an adopted standard of the Object Management Group (OMG) intended to be the basis for formal and detailed natural language declarative description of a complex entity, such as a business. SBVR is intended to formalize complex compliance rules, such as operational rules for an enterprise, security policy, standard compliance, or regulatory compliance rules. Such formal vocabularies and rules can be interpreted and used by computer systems. SBVR is an integral part of the OMG's model-driven architecture (MDA).
The SBVR standard defines the vocabulary and rules for documenting the semantics of business vocabularies, business facts, and business rules; as well as an XMI schema for the interchange of business vocabularies and business rules among organizations and between software tools.
SBVR allows the production of business vocabularies and rules; vocabulary plus rules constitute a shared domain model with the same expressive power of standard ontological languages. SBVR allows multilingual development, since it is based on separation between
symbols and their meaning. SBVR enables making business rules accessible to software tools, including tools that support the business experts in creating, finding, validating, and managing business rules, and tools that support the information technology experts in converting business rules into implementation rules for automated systems.
SBVR uses OMG's Meta-Object Facility (MOF) to provide interchange capabilities MOF/XMI mapping rules, enable
generating MOF-compliant models and define an XML schema. SBVR proposes Structured English as one of possibly many notations that can map to the SBVR Metamodel.
SBVR and Knowledge Discovery Metamodel (KDM) are designed as two parts of a unique OMG Technology Stack for software analytics related to existing software systems. KDM defines an ontology related to software artifacts and thus provides an initial formalization of the information related to a software system. SBVR can be further used to formalize complex compliance rules related to the software.
Business rules represent the primary means by which an organization can direct its business, defining the operative way to reach its objectives and perform its actions.
A rule-based approach to managing business and the information used by that business is a way of identifying and articulating the rules which define the structure and control the operation of an enterprise [ 1 ] it represents a new way to think about enterprise and its rules, in order to enable a complete business representation made by and for business people. Business rules can play an important role in defining business semantics: they can influence or guide behaviours and support policies, responding to environmental situations and events. Semantics of Business Vocabulary and Business Rules (SBVR) is the OMG implementation of the business rules approach .
In June 2003 OMG issued the Business Semantics of Business Rule (BSBR) Request For Proposal, [ 2 ] in order to create a standard to allow business people to define the policies and rules by which they run their business in their own language, in terms of the things they deal with in the business, and to capture those rules in a way that is clear, unambiguous and readily translatable into other representations. [ 2 ] The SBVR proposal was developed by the Business Rules Team, a consortium organized in August 2003 to respond to the BSBR RFP. [ 3 ] [ 4 ]
In September 2005, The Business Modeling and Integration Task Force and the Architecture Board of the Object Management Group approved the proposal Semantics of Business Vocabulary and Business Rules (SBVR) to become a final adopted specification in response to the RFP. Later SBVR proposal was ratified by the Domain Technical Committee (DTC), approved of the OMG Board of Directors, and SBVR finalization task force was launched to convert the proposal into ISO/OMG standard format and perform final editing prior to release as an OMG formal specification.
In January 2008, the finalization phase was completed and the Semantics of Business Vocabulary and Business Rules (SBVR), Version 1.0 formal specification was released and is publicly available [ 5 ] at the Catalog of OMG Business Strategy, Business Rules and Business Process Management Specifications web page.
SBVR is a landmark for the OMG, the first OMG specification to incorporate the formal use of natural language in modeling and the first to provide explicitly a model of formal logic. Based on a fusion of linguistics, logic, and computer science, and two years in preparation, SBVR provides a way to capture specifications in natural language and represent them in formal logic so they can be machine-processed.
Methodologies used in software development are typically applied only when a problem is already formulated and well described. The actual difficulty lies in the previous step, that is describing problems and expected functionalities. Stakeholders involved in software development can express their ideas using a language very close to them, but they usually are not able to formalize these concepts in a clear and unambiguous way. This implies a large effort in order to interpret and understand real meanings and concepts hidden among stakeholders' words. Special constraints on syntax or predefined linguistic structures can be used in order to overcome this problem, enabling natural language to well represent and formally define problems and requirements.
The main purpose of natural language modelling is hence to make natural language suitable for conceptual modelling. The focus is on semantic aspects and shared meanings, while syntax is thought in a perspective based on formal logic mapping.
Conceptualization and representation play fundamental roles in thinking, communicating, and modeling. There is a triad of
1) concepts in our minds,
2) real-world things conceptualized by concepts, and
3) representations of concepts that we can use to think and communicate about the concept and its corresponding real-world things.
(Note that real-world things include both concrete things and representations of those concrete things as records and processes in operational information systems.)
A conceptual model is a formal structure representing a possible world, comprising a conceptual schema and a set of facts that instantiate the conceptual schema. The conceptual schema is a combination of concepts and facts of what is possible, necessary, permissible, and obligatory in each possible world. The set of facts instantiates the conceptual schema by assertion to describe one possible world. A rule is a fact that asserts either a logical necessity or an obligation. Obligations are not necessarily satisfied by the facts; necessities are always satisfied. [ 6 ]
SBVR contains a vocabulary for conceptual modeling and captures expressions based on this vocabulary as formal logic structures. The SBVR vocabulary allows one to formally specify representations of concepts, definitions, instances, and rules of any knowledge domain in natural language, including tabular forms. These features make SBVR well suited for describing business domains and requirements for business processes and information systems to implement business models.
People communicate facts, that is the fact is the unit of communication. The fact-oriented approach enables multidimensional categorization. [ 7 ]
Conceptual formalization describes a business domain, and is composed of 1) a conceptual schema (fact structure) and 2) a population of ground facts. A business domain ( universe of discourse ) comprises those aspects of the business that are of interest.
The schema declares:
A fact is a proposition taken to be true by the business. Population facts are restricted to elementary and existential facts.
Constraints can be static or dynamic:
e.g. a person’s marital status may change from single to married, but not from divorced to single
Derivation of facts.
Rules play a very important role in defining business semantics: they can influence or guide behaviours and support policies, responding to environmental situations and events. This means that rules represent the primary means by which an organization can direct its business, defining the operative way to reach its objectives and perform its actions.
The rule-based approach aims to address two different kinds of users:
The essence of the rule-based conceptual formalizations is that rules build on facts, and facts build on concepts as expressed by terms . [ 8 ]
This mantra is memorable, but a simplification since in SBVR: Meaning is separate from expression; Fact Types (Verb Concepts) are built on Noun Concepts; Noun Concepts are represented by Terms; and Fact Types are represented by Fact Symbols (verb phrases).
Rule statements are expressed using either alethic modality or deontic modality and require elements of modal logic as formalization.
SBVR Structural Business Rules use two alethic modal operators :
SBVR Operative Business Rules use two deontic modal operators :
Structural business rules (static constraints) are treated as alethic necessities by default, where each state of the fact model corresponds to a possible world. Pragmatically, the rule is understood to apply to all future states of the fact model, until the rule is revoked or changed. For the model theory, the necessity operator is omitted from the formula. Instead, the rule is merely tagged as a necessity. For compliance with Common Logic , such formulae can be treated as irregular expressions, with the necessity modal operator treated as an uninterpreted symbol.
If the rule includes exactly one deontic operator, e.g. O (obligation), and this is at the front, then the rule may be formalized as Op, where p is a first-order formula that is tagged as obligatory. In SBVR, this tag is assigned the informal semantics: it ought to be the case that p (for all future states of the fact model, until the constraint is revoked or changed). From a model-theoretic perspective, a model is an interpretation where each non-deontic formula evaluates to true, and the model is classified as: a permitted model if the p in each deontic formula (of the form Op) evaluates to true, otherwise the model is a forbidden model (though still a model). This approach removes any need to assign a truth value to expressions of the form Op.
SBVR is for modeling in natural language. Based on linguistics and formal logic, SBVR provides a way to represent statements in controlled natural languages as logic structures called semantic formulations. SBVR is intended for expressing business vocabulary and business rules, and for specifying business requirements for information systems in natural language. SBVR models are
declarative, not imperative or procedural. SBVR has the greatest expressivity of any OMG modeling language. The logics supported by SBVR are typed first order predicate logic with equality, restricted higher order logic (Henkin semantics), restricted deontic and alethic modal logic, set theory with bag comprehension, and mathematics. SBVR also includes projections, to support definitions and answers to queries, and questions, for formulating queries. Interpretation of SBVR semantic formulations is based on model theory. SBVR has a MOF model, so models can be structurally linked at the level of individual facts with other MDA models based on MOF.
SBVR is aligned with Common Logic – published by ISO as ISO/IEC 24707:2007.
SBVR captures business facts and business rules that may be expressed either informally or formally. Business rule expressions are formal only if they are expressed purely in terms of: fact types in the pre-declared schema for the business domain, certain logical/ mathematical operators, quantifiers etc. Formal rules are transformed into a logical formulation that is used for exchange with other rules-based software tools. Informal rules may be exchanged as un-interpreted comments. An approach to automatically generate SBVR business rules from natural language specification is presented in. [ 9 ]
SBVR specification defines a metamodel and allows to instance it, in order to create different vocabularies and to define the related business rules; it is also possible to complete these models with data suitable to describe a specific organization. the SBVR approach provides means (i.e. mapping rules) to translate natural language artifacts into MOF-compliant artifacts; this allows to exploit all the advantages related to MOF (repository facilities, interchangeability, tools, ...).
Several MDA-related OMG works in progress are expected to incorporate SBVR, including:
The Ontology Definition Metamodel (ODM) has been made compatible with SBVR, primarily by aligning the logic grounding of the ISO Common Logic specification (CL) referenced by ODM with the SBVR Logical Formulation of Semantics vocabulary. CL itself was modified specifically so it potentially can include the modal sentence requirements of SBVR. ODM provides a bridge to link SBVR to the Web Ontology Language for Services (OWL-S), Resource Description Framework Schema (RDFS), Unified Modeling Language (UML), Topic Map (TM), Entity Relationship Modeling (ER), Description Logic (DL), and CL.
Other programs outside the OMG are adopting SBVR. The Digital Business Ecosystem (DBE), an integrated project of the European Commission Framework Programme 6, has adopted SBVR as the basis for its Business Modeling Language. [ citation needed ] The World Wide Web Consortium (W3C) is assessing SBVR for use in the Semantic Web, through the bridge provided by ODM. [ citation needed ] SBVR will extend the capability of MDA in all these areas. | https://en.wikipedia.org/wiki/Semantics_of_Business_Vocabulary_and_Business_Rules |
In logic , the semantics of logic or formal semantics is the study of the meaning and interpretation of formal languages , formal systems , and (idealizations of) natural languages . This field seeks to provide precise mathematical models that capture the pre-theoretic notions of truth , validity , and logical consequence . While logical syntax concerns the formal rules for constructing well-formed expressions, logical semantics establishes frameworks for determining when these expressions are true and what follows from them.
The development of formal semantics has led to several influential approaches, including model-theoretic semantics (pioneered by Alfred Tarski ), proof-theoretic semantics (associated with Gerhard Gentzen and Michael Dummett ), possible worlds semantics (developed by Saul Kripke and others for modal logic and related systems), algebraic semantics (connecting logic to abstract algebra ), and game semantics (interpreting logical validity through game-theoretic concepts). These diverse approaches reflect different philosophical perspectives on the nature of meaning and truth in logical systems.
The truth conditions of various sentences we may encounter in arguments will depend upon their meaning, and so logicians cannot completely avoid the need to provide some treatment of the meaning of these sentences. The semantics of logic refers to the approaches that logicians have introduced to understand and determine that part of meaning in which they are interested; the logician traditionally is not interested in the sentence as uttered but in the proposition , an idealised sentence suitable for logical manipulation. [ citation needed ]
Until the advent of modern logic, Aristotle 's Organon , especially De Interpretatione , provided the basis for understanding the significance of logic. The introduction of quantification , needed to solve the problem of multiple generality , rendered impossible the kind of subject–predicate analysis that governed Aristotle's account, although there is a renewed interest in term logic , attempting to find calculi in the spirit of Aristotle's syllogisms , but with the generality of modern logics based on the quantifier.
The main modern approaches to semantics for formal languages are the following: | https://en.wikipedia.org/wiki/Semantics_of_logic |
Semantides (or semantophoretic molecules ) are biological macromolecules that carry genetic information or a transcript thereof. Three different categories of semantides are distinguished: primary, secondary and tertiary. Primary Semantides are genes , which consist of DNA . Secondary semantides are chains of messenger RNA , which are transcribed from DNA. Tertiary semantides are polypeptides , which are translated from messenger RNA. [ 1 ] In eukaryotic organisms , primary semantides may consist of nuclear , mitochondrial or plastid DNA. [ 2 ] Not all primary semantides ultimately form tertiary semantides. Some primary semantides are not transcribed into mRNA ( non-coding DNA ) and some secondary semantides are not translated into polypeptides ( non-coding RNA ). The complexity of semantides varies greatly. For tertiary semantides, large globular polypeptide chains are most complex while structural proteins , consisting of repeating simple sequences, are least complex. The term semantide and related terms were coined by Linus Pauling and Emile Zuckerkandl . [ 1 ] Although semantides are the major type of data used in modern phylogenetics, the term itself is not commonly used.
DNA or RNA that differs in base sequence, but translate into identical polypeptide chains are referred to as being isosemantic. [ 1 ]
Molecules that are synthesized by enzymes (tertiary semantides) are referred to as episemantic molecules. Episemantic molecules have a larger variety in types than semantides, which only consist of three types (DNA, RNA or polypeptides). Not all polypeptides are tertiary semantides. Some, mainly small polypeptides, can also be episemantic molecules. [ 1 ]
Molecules that are not produced by an organism are referred to as asemantic molecules, because they do not contain any genetic information. Asementic molecules may be changed into episemantic molecules by anabolic processes . Asemantic molecules may also become semantic molecules when they integrate into a genome. Certain viruses and episomes have this ability. [ 1 ]
When referring to a molecule as being semantic, episemantic or asemantic, then this only applies to a specific organism. A semantic molecule for one organism may be asemantic for another organism.
Semantides are used as phylogenetic information for studying the evolutionary history of organisms. Primary semantides are also used in comparative biodiversity analyses. However, since extracellular DNA can persist for some time, these types of analysis cannot discern active from inactive and or dead organisms. [ 3 ] [ 4 ]
The extent to which biological macromolecules are informative for studying evolutionary history differs. The more complex a molecule, the more informative it contains for phylogenetics. Primary and secondary semantides contain the most information. In tertiary semantides, some information is lost, because many amino acids are coded for by more than one codon . [ 1 ] [ 5 ]
Episemantic molecules (e.g. carotenoids ) are also informative for phylogenetics. However, the distributions of these molecules do not correlate perfectly with phylogenies based on semantides. [ 6 ] Therefore, independent confirmation is often still needed. [ 1 ] The more enzymes involved in a synthesis pathway , the more unlikely that such pathways have evolved separately. Therefore, for episemantic molecules, molecules that are synthesized from the least complex asemantic molecules are the most informative in phylogenetics. However, different pathways may synthesize similar or even identical molecules. For example, in animals, plants and other eukaryotes, different pathways have been found for vitamin C synthesis. [ 7 ] Therefore, certain molecules should not be used for studying phylogenetic relationships. [ 1 ]
Although asemantic molecules could indicate some quantitative or qualitative features of a group of organisms, they are considered to be unreliable and uninformative for phylogenetics. [ 1 ]
Analyses using different semantides may yield conflicting phylogenies. However, if the phylogenies are congruent, then there is more support for the evolutionary relationship. By analyzing larger sequences (e.g. complete mitochondrial genome sequences), phylogenies can be constructed, which are more resolved and have more support. [ 8 ]
Semantides often used in studies are common to most organisms and are known to only change slowly over time. Examples of these macromolecules are: | https://en.wikipedia.org/wiki/Semantide |
In computer networking , Semaphore Flag Signaling System (SFSS) is a humorous proposal to carry Internet Protocol (IP) traffic by semaphores . Semaphore Flag Signaling System was initially described in RFC 4824, [ 1 ] an April Fools' Day RFC issued by the Internet Engineering Task Force edited by J. Hofmueller, et al. and released on April Fools' Day 2007. It is one of several April 1 RFCs .
A reference implementation of IP over SFS has been done by the authors of the RFC within the project "Talking the Fish".
An email was transmitted using SMTP over Semaphore Flag Signals. [ 2 ] | https://en.wikipedia.org/wiki/Semaphore_Flag_Signaling_System |
Semelparity and iteroparity are two contrasting reproductive strategies available to living organisms. A species is considered semelparous if it is characterized by a single reproductive episode before death, and iteroparous if it is characterized by multiple reproductive cycles over the course of its lifetime. Iteroparity can be further divided into continuous iteroparity (primates, including humans and chimpanzees) and seasonal iteroparity (birds, dogs, etc.) Some botanists use the parallel terms monocarpy and polycarpy . (See also plietesials .)
In truly semelparous species, death after reproduction is part of an overall strategy that includes putting all available resources into maximizing reproduction, at the expense of future life (see § Trade-offs ). In any iteroparous population there will be some individuals who happen to die after their first and before any second reproductive episode, but unless this is part of a syndrome of programmed death after reproduction, this would not be called "semelparity".
This distinction is also related to the difference between annual and perennial plants: An annual is a plant that completes its life cycle in a single season, and is usually semelparous. Perennials live for more than one season and are usually (but not always) iteroparous. [ 1 ]
Semelparity and iteroparity are not, strictly speaking, alternative strategies, but extremes along a continuum of possible modes of reproduction. Many organisms considered to be semelparous can, under certain conditions, separate their single bout of reproduction into two or more episodes. [ 2 ] [ 3 ]
The word "semelparity" was coined by evolutionary biologist Lamont Cole, [ 4 ] and comes from the Latin semel ('once, a single time') and pario ('to beget'). This differs from iteroparity in that iteroparous species are able to have multiple reproductive cycles and therefore can mate more than once in their lifetime. Semelparity is also known as "big bang" reproduction, since the single reproductive event of semelparous organisms is usually large as well as fatal. [ 5 ] A classic example of a semelparous organism is (most) Pacific salmon ( Oncorhynchus spp.), which live for many years in the ocean before swimming to the freshwater stream of its birth, spawning, and dying. Other semelparous animals include many insects, including some species of butterflies, cicadas, and mayflies , many arachnids , and some molluscs such as some species of squid and octopus .
Semelparity also occurs in smelt and capelin , but other than bony fish it is a very rare strategy in vertebrates. In amphibians , it is known only among some Hyla frogs including the gladiator frog ; [ 6 ] [ full citation needed ] in reptiles only a few lizards such as Labord's chameleon of southwestern Madagascar , [ 7 ] Sceloporus bicanthalis of the high mountains of Mexico, [ 8 ] [ full citation needed ] and some species of Ichnotropis from dry savanna areas of Africa. [ 9 ] Among mammals, it exists only in a few didelphid and dasyurid marsupials. [ 10 ] Annual plants, including all grain crops and most domestic vegetables, are semelparous. Long-lived semelparous plants include century plant ( agave ), Lobelia telekii , and some species of bamboo . [ 11 ]
This form of lifestyle is consistent with r-selected strategies as many offspring are produced and there is low parental input, as one or both parents die after mating. All of the male's energy is diverting into mating and the immune system is repressed. High levels of corticosteroids are sustained over long periods of time. This triggers immune and inflammatory system failure and gastrointestinal hemorrhage , which eventually leads to death. [ 12 ]
The term iteroparity comes from the Latin itero , to repeat, and pario , to beget. An example of an iteroparous organism is a human—humans are biologically capable of having offspring many times over the course of their lives.
Iteroparous vertebrates include all birds, most reptiles, virtually all mammals, and most fish. Among invertebrates, most mollusca and many insects (for example, mosquitoes and cockroaches) are iteroparous. Most perennial plants are iteroparous.
It is a biological precept that within its lifetime an organism has a limited amount of energy/resources available to it, and must always partition it among various functions such as collecting food and finding a mate. Of relevance here is the trade-off between fecundity , growth, and survivorship in its life history strategy. These trade-offs come into play in the evolution of iteroparity and semelparity. It has been repeatedly demonstrated that semelparous species produce more offspring in their single fatal reproductive episode than do closely related iteroparous species in any one of theirs. However, the opportunity to reproduce more than once in a lifetime, and possibly with greater care for the development of offspring produced, can offset this strictly numerical benefit.
One class of models that tries to explain the differential evolution of semelparity and iteroparity examines the shape of the trade-off between offspring produced and offspring forgone. In economic terms, offspring produced is equivalent to a benefit function, while offspring forgone is comparable to a cost function. The reproductive effort of an organism—the proportion of energy that it puts into reproducing, as opposed to growth or survivorship—occurs at the point where the distance between offspring produced and offspring forgone is the greatest. [ 13 ]
In some situations, the marginal cost of offspring produced decreases over time (each additional offspring is less "expensive" than the average of all previous offspring) and the marginal cost of offspring forgone increases. In these cases, the organism only devotes a portion of its resources to reproduction and uses the rest for growth and survivorship so that it can reproduce again in the future. [ 14 ]
In other situations, the marginal cost of offspring produced increases while the marginal cost of offspring forgone decreases. When this is the case, it is favorable for the organism to reproduce a single time. The individual devotes all of its resources to that one episode of reproduction, then dies as it has not reserved enough resources to meet its own ongoing survival needs.
Empirical, quantitative support for this mathematical model is limited.
A second set of models examines the possibility that iteroparity is a hedge against unpredictable juvenile survivorship (avoiding putting all one's eggs in one basket). Again, mathematical models have not found empirical support from real-world systems. In fact, many semelparous species live in habitats characterized by high (not low) environmental unpredictability, such as deserts and early successional habitats.
The models that have the strongest support from living systems are demographic. In Lamont Cole's classic 1954 paper, he came to the conclusion that:
For an annual species, the absolute gain in intrinsic population growth which could be achieved by changing to the perennial reproductive habit would be exactly equivalent to adding one individual to the average litter size. [ 15 ]
For example, imagine two species—an iteroparous species that has annual litters averaging three offspring each, and a semelparous species that has one litter of four, and then dies. These two species have the same rate of population growth, which suggests that even a tiny fecundity advantage of one additional offspring would favor the evolution of semelparity. This is known as Cole's paradox.
In his analysis, Cole assumed that there was no mortality of individuals of the iteroparous species, even seedlings. Twenty years later, Charnov and Schaffer [ 16 ] showed that reasonable differences in adult and juvenile mortality yield much more reasonable costs of semelparity, essentially solving Cole's paradox. An even more general demographic model was produced by Young. [ 17 ]
These demographic models have been more successful than the other models when tested with real-world systems. It has been shown that semelparous species have higher expected adult mortality, making it more economical to put all reproductive effort into the first (and therefore final) reproductive episode. [ 18 ] [ 19 ]
Semelparous species of Dasyuridae are typically small and carnivorous, with the exception of the northern quoll ( Dasyurus hallucatus ), which is large. Species with this reproductive strategy include members of the genus Antechinus , Phascogale tapoatafa and Phascogale culura . The males of all three groups exhibit similar characteristics that classify them as semelparous: First, all of the males of each species disappear immediately after the mating season. Also, males that are captured and isolated from others live for 2 to 3 years. [ 20 ] If these captured males are allowed to mate, they die immediately after the mating season, like those in the wild. Their behaviour also changes drastically before and after the mating season. Before mating, males are extremely aggressive and will fight with other males if placed close together. Males that are captured before they are allowed to mate remain aggressive through the winter months. After the mating season, if allowed to mate, males become extremely lethargic and never regain their aggressiveness even if they survive to the next mating season. [ 20 ] Other changes that occur post-mating include fur degradation and testicular degeneration. During adolescence, male fur is thick and becomes dull and thin after mating, but regains its original condition if the individual manages to survive past the mating season. The fur on the scrotum completely falls off and does not grow back, even if the male survives months after the first mating season. As the marsupial ages, its testicles grow until they reach a peak size and weight at the beginning of the mating season. After the individual mates, the weight and size of the testes and scrotum decrease. They remain small and do not produce spermatozoa later in life, if maintained in a laboratory. [ 20 ] The 1966 Woolley study on Antechinus spp. noticed that males were only able to be maintained past mating in the laboratory, and no senile males were found in the wild, suggesting that all males die shortly after mating. [ 20 ]
Studies on Antechinus stuartii reveal that male mortality is highly correlated to stress and andrenocortical activity. The study measured the corticosteroid concentration in males in the wild, males injected with cortisol, males injected with saline, and females in the wild. While both males and females exhibit high levels of corticosteroid concentration in the wild, this proves fatal only to males due to females having a higher maximum high affinity corticosteroid binding capacity (MCBC). [ 21 ] Thus, free corticosteroid in the plasma of male A. stuartii rises sharply, while it remains constant in females. High levels of free corticosteroid, resulting from mating in wild males and injected cortisol in laboratory males, resulted in stomach ulcers , gastrointestinal bleeding , and liver abscesses , all of which increased mortality. These side-effects were not found in the males that were injected with saline, [ 21 ] strengthening the hypothesis that high, free corticosteroids result in higher mortality in male dasyurids. A similar study on Phascogale calura showed that similar endocrine system changes happen in P. calura as A. stuartii . [ 22 ] This supports stress-induced mortality as a characteristic of small dasyurid semelparity.
Dasyurus hallucatus , the northern quoll , is a large dasyurid and exhibits increased male mortality after the mating season. Unlike smaller dasyurids, male die-off in D. hallucatus is not due to endocrine system changes, and there was no observed spermatogenic failure after the mating season ended. [ 12 ] If male D. hallucatus survive past their first mating season, they may be able to engage in a second mating season. While the individuals in a 2001 study mostly died from vehicles or predation, researchers found evidence of physiological degradation in males, similar to the physiological degradation in small dasyurids. This includes fur loss, parasite infestations, and weight loss. As the mating period went on, males became increasingly anemic , but the anemia was not due to ulceration or gastrointestinal bleeding. [ 12 ] Lack of elevated cortisol levels during mating periods in D. hallucatus means that there is no current universal explanation for the mechanism behind increased male mortality in Dasyuridae. Post-reproductive senescence has also been proposed as an explanation. [ 23 ]
The grey slender mouse opossum exhibits a semelparous reproductive strategy in both males and females. Males disappear from their endemic area after the reproductive season (February–May). Males found months later (June–August) are of lighter body weight and the molar teeth are less worn down, suggesting these males belong to a different generation. There is a drop off in the female population, but during the months of July and August, evidence of a gap between generations like the male gap. There is also lower body weight and less molar wear observed in females found after August. This is further supported by the evidence that females that reproduce are not observed the following year. [ 24 ] [ full citation needed ] This species has been compared to a related species, Marmosa robinsoni , in order to answer what would happen if a female that has reproduced were to survive to the next mating season. M. robinsoni has a monoestrus reproductive cycle, like M. incanus , and females are no longer fertile after 17 months so it is unlikely that females that survive past the drop off in female populations would be able to reproduce a second time. [ 24 ]
Gracilinanus microtarsus , or the Brazilian gracile opossum, is considered to be partially semelparous because male mortality increases significantly after the mating season, but some males survive to mate again in the next reproductive cycle. The males also exhibit similar physiological degradation, demonstrated in Antechinus and other semelparous marsupials , such as fur loss and increase of infection from parasites. [ 25 ]
Highly elevated cortisol levels mediate the post-spawning death of semelparous Oncorhynchus Pacific salmon by causing tissue degeneration, suppressing the immune system, and impairing various homeostatic mechanisms. [ 26 ] After swimming for such a long distance, salmon expend all of their energy on reproduction. One of the key factors in salmon rapid senescence is that these fish do not feed during reproduction so body weight is extremely reduced. [ 27 ] In addition to physiological degradation, Pacific salmon become more lethargic as mating goes on, which makes some individuals more susceptible to predation because they have less energy to avoid predators. [ 28 ] This also increases mortality rates of adults post-mating.
Traditionally, semelparity was usually defined within the time-frame of a year. Critics of this criterion note that this scale is inappropriate in discussing patterns of insect reproduction because many insects breed more than once within one annual period, but generation times of less than one year. Under the traditional definition, insects are considered semelparous as a consequence of time scale rather than the distribution of reproductive effort over their adult life span. [ 29 ] In order to resolve this inconsistency, Fritz, Stamp & Halverson (1982) define semelparous insects as "insects that lay a single clutch of eggs in their lifetime and deposit them at one place are clearly semelparous or 'big bang' reproducers. Their entire reproductive effort is committed at one time and they die shortly after oviposition ". [ 29 ] Semelparous insects are found in Lepidoptera , Ephemeroptera , Dermaptera , Plecoptera , Strepsiptera , Trichoptera , and Hemiptera .
Females of certain families of Lepidoptera, like the spongy moth of family Erebidae , have reduced mobility or are wingless ( apterous ), so they disperse in the larval stage as opposed to in the adult stage. In iteroparous insects, dispersal mainly occurs in the adult stage. All semelparous Lepidopterans share similar characteristics: larvae only feed in restricted periods of the year because of the nutritional state of their host plants (as a result, they are univoltine ), initial food supply is predictably abundant, and larval host plants are abundant and adjacent. [ 29 ] Death most commonly occurs by starvation. In the case of the spongy moth, adults do not possess an active digestive system and cannot feed, but can drink moisture. Mating occurs fairly rapidly after adults emerge from their pupal form and, without a way to digest food, the adult moths die after about a week. [ 30 ]
The evolution for semelparity in both sexes has occurred many times in plants, invertebrates, and fish. It is rare in mammals because mammals have obligate maternal care due to internal fertilization and incubation of offspring and nursing young after birth, which requires high maternal survival rate after fertilization and offspring weaning . Also, female mammals have relatively low reproductive rates compared to invertebrates or fish because they invest a lot of energy in maternal care. However, male reproductive rate is much less constrained in mammals because only females bear young. A male that dies after one mating season can still produce a large number of offspring if he invests all of his energy in mating with many females. [ 31 ]
Scientists have hypothesized that natural selection has allowed semelparity to evolve in Dasyuridae and Didelphidae because of certain ecological constraints. Female mammals ancestral to these groups may have shortened their mating period to coincide with peak prey abundance. Because this window is so small, the females of these species exhibit a reproduction pattern where the estrous of all females occurs simultaneously. Selection would then favor aggressive males due to increased competition between males for access to females. Since the mating period is so short, it is more beneficial for males to expend all their energy on mating, even more so if they are unlikely to survive to the next mating season. [ 32 ]
Reproduction is costly for anadromous salmonids , because their life history requires transition from saltwater to freshwater streams, and long migrations, which can be physiologically taxing. The transition between cold oceanic water to warm freshwater and steep elevation changes in Northern Pacific rivers could explain the evolution of semelparity because it would be extremely difficult to return to the ocean. A noticeable difference between semelparous fish and iteroparous salmonids is that egg size varies between the two types of reproductive strategies. Studies show that egg size is also affected by migration and body size. Egg number, however, shows little variation between semelparous and iteroparous populations or between resident and anadromous populations for females of the same body size. [ 33 ] The current hypothesis behind this reason is that iteroparous species reduce the size of their eggs in order to improve the mother's chances of survival, since she invests less energy in gamete formation. Semelparous species do not expect to live past one mating season, so females invest a lot more energy in gamete formation resulting in large eggs. Anadromous salmonids may also have evolved semelparity to boost the nutrition density of the spawning grounds . The most productive Pacific salmon spawning grounds contain the most carcasses of spawned adults. The dead bodies of the adult salmon decompose and provide nitrogen and phosphorus for algae to grow in the nutrient-poor water. Zooplankton then feed on the algae, and newly hatched salmon feed on the zooplankton. [ 34 ]
An interesting trait has evolved in semelparous insects, especially in those that have evolved from parasitic ancestors, like in all subsocial and eusocial aculeate Hymenoptera . This is because larvae are morphologically specialized for development within a host's innards and thus are entirely helpless outside of that environment. Females would need to invest a lot of energy in protecting their eggs and hatched offspring. They do this through such behaviours as egg guarding. Mothers that actively defend offspring, for example, risk injury or death by doing so. [ 35 ] This is not beneficial in an iteroparous species because the female risks dying and not reaching her full reproductive potential by not being able to reproduce in all reproductive periods in her lifetime. Since semelparous insects only live for one reproductive cycle, they can afford to expend energy on maternal care because those offspring are her only offspring. An iteroparous insect does not need to expend energy on the eggs of one mating period because it is likely that she will mate again. There is ongoing research in maternal care in semelparous insects from lineages not descended from parasites to further understand this relationship between semelparity and maternal care. | https://en.wikipedia.org/wiki/Semelparity_and_iteroparity |
The Semenov Institute of Chemical Physics [ 1 ] [ 2 ] of Russian Academy of Sciences (RAS) (now known as the Semenov ICP in Moscow Russian : Институт химической физики им. Н. Н. Семёнова, Москва, учреждение Российской академии наук ), was established in 1931 under the direction of Professor Nikolay Semyonov , Nobel Laureate in Chemistry (1956) on the basis of the Physico-Chemical Sector of the Leningrad Physical Technical Institute ( Ioffe Institute ). The staff of the Institute includes about 450 researchers. The Institute has been situated in Moscow [ 3 ] since 1943. It is affiliated with the Moscow State University and has chairs at the Moscow Institute of Physics and Technology .
The Institute's aim has been defined as "the implementation of physical theories and methods into chemistry, chemical industry and some other branches of national economy". Its main areas of scientific inquiry are the kinetics and mechanism of chemical reactions, catalysis of chemical reactions, theory and dynamics of elementary processes, physics and chemistry of solids, structure and properties of polymers and composites, fundamentals of polymerization processes, physics and chemistry of combustion, shock waves and detonation.
In 1956, a branch of the Institute of Chemical Physics was established in Chernogolovka ; this was reorganized in 1997 as the independent Institute of Problems of Chemical Physics (IPCP RAS). [ 4 ] Another department of the Semenov Institute was reorganized in 1994 as the N. M. Emanuel Institute of Biochemical Physics in Moscow [ 5 ] [ 6 ] of the Russian Academy of Sciences.
This article about a chemistry organization is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Semenov_Institute_of_Chemical_Physics |
The Semi-Conductor Laboratory (SCL) , Mohali is a research institute under the Ministry of Electronics and Information Technology , Government of India . SCL was formerly under the Department of Space . SCL's aims include research and development (R&D) in the field of semiconductor technology. [ 1 ] Semiconductors manufactured by SCL have been used in the Mars Orbiter Mission . [ 2 ] The Indian Government is attempting to modernize SCL and upgrade its facilities. [ 3 ]
The Semi-Conductor Laboratory (SCL) is a public sector undertaking (PSU) of the Government of India , and was founded as the Semiconductor Complex Limited, in Mohali , Punjab . [ 4 ] SCL was formed to develop India's ambitions for establishing a semiconductor manufacturing industry. [ 5 ] In 1976, the Cabinet of India approved the formation of SCL, and the company began production in 1984. [ 4 ] [ 5 ] At first, Navi Mumbai was the leading contender for where SCL would be established, however, then Prime Minister Indira Gandhi eventually selected Mohali as the location for where SCL would be set up. [ 5 ] Then Chief Minister of Punjab , Zail Singh offered 51- acres of land to SCL for a token one rupee cost. [ 4 ] When SCL initially began production in 1984, the company had entered into a technical collaboration with American Microsystems and started the production of 5 micron complementary metal-oxide semiconductor (CMOS) technology. [ 6 ]
In 1989, a fire broke out and destroyed SCL's facility in Mohali. [ 7 ] The facility restarted production in 1997, however, the fire halted the growth of India's semiconductor industry. [ 5 ] As of 2022, the cause of the fire is a mystery, including whether the fire was an accident or caused intentionally. [ 4 ] [ 7 ] SCL's revenues in 1999-2000 were about $14 million, with a profit of roughly $400,000. [ 8 ]
SCL came under the administrative control of the Department of Space (DoS) in March 2005, and has since undergone organisational restructuring to become focused on research and development. The society was registered in November 2005. In 2006, Semiconductor Complex Limited was renamed to the Semi-Conductor Laboratory. [ 6 ] As of 2023, SCL is under the administrative control of the Ministry of Electronics and Information Technology (MeitY). [ 9 ]
SCL is India's only integrated device manufacturing facility. [ 9 ] Over 200 of types of CMOS devices can be made by SCL's wafer fabrication facilities. [ 9 ]
SCL's chips have been used in Mangalyaan, India's Mars Orbiter Mission . [ 2 ] Indian Institute of Technology Madras uses SCL as a primary foundry to tapeout SHAKTI processors using its 180 nm technology process. [ 10 ] In 2021, SCL and the Indian Institute of Technology Bombay (IIT Bombay) invented India's first indigenous semiconductor memory technology. [ 11 ] SCL and IIT Bombay demonstrating a CMOS- 180nm -based 8-bit memory technology, that can also be adopted for production. [ 11 ]
On October 20, 2023, the Ministry of Electronics and Information Technology stated that the Bharat Semiconductor Research Centre (BSRC) would be established starting in 2024, working with professionals from the industry and academic institutions. Rajeev Chandrashekhar states that BSRC will function as the Indian counterpart of the MIT Microelectronics Laboratory, Alan G. MacDiarmid NanoTech Institute, Industrial Technology Research Institute , and IMEC . [ 12 ] The Bharat Semiconductor Research Centre will be located at IIT Madras . The center will be established initially as an institution co-located with Semi-Conductor Laboratory, with the potential to split off into a stand-alone semiconductor research organization in the future. [ 13 ]
The Government of India is attempting to modernise SCL. In February 2023, the Indian government informed a Parliamentary Standing Committee that a joint venture with a commercial partner is being sought to modernise SCL's existing facility. [ 14 ] In May 2023, the Indian government announced that it will invest $2 billion in SCL for the purposes of research and prototyping . [ 15 ] The initial amount that Indian government was going to invest was approximately around $1.3 billion, however, the investment amount was later increased to $2 billion. [ 15 ] MeitY also engaged Boston Consulting Group , a management consultancy company, to develop a long-term strategy for SCL. [ 9 ]
In July 2023, Rajeev Chandrasekhar , the Minister of State For Electronics and Information Technology, stated that the Indian Government has approved the modernisation of SCL. [ 3 ] SCL's modernisation will be a brownfield modernisation as a chip manufacturing unit. [ 3 ] | https://en.wikipedia.org/wiki/Semi-Conductor_Laboratory |
Semi-automation is a process or procedure that is performed by the combined activities of man and machine with both human and machine steps typically orchestrated by a centralized computer controller.
Within manufacturing, production processes may be fully manual , semi-automated, or fully automated . In this case, semi-automation may vary in its degree of manual and automated steps.
Semi-automated manufacturing processes are typically orchestrated by a computer controller which sends messages to the worker at the time in which he/she should perform a step. The controller typically waits for feedback that the human performed step has been completed via either a human-machine interface or via electronic sensors distributed within the process. Controllers within semi-automated processes may either directly control machinery or send signals to machinery distributed within the process. Centralized computer controllers within semi-automated processes orchestrate processes by instructing the worker, providing electronic communication and control to process equipment, tools, or machines, as well as perform data management to record and ensure that the process meets established process criteria.
Many manufacturers choose not to fully automate a process, and instead implement semi-automation due to the complexity of the task, or the number of products produced is too low to justify the investment in full automation. Other processes may not be fully automated because it may reduce the flexibility to easily adapt the processes to reflect production needs. | https://en.wikipedia.org/wiki/Semi-automation |
Semi-biotic systems are systems that incorporate biologically derived components/modules – which could range from multi- protein complexes through DNA constructs to multi-cellular assemblies – and integrate them with synthetic components (e.g. microfabricated systems) to produce hybrid devices. One of the potential attractions of these hybrid devices is the possibility that they can be designed to exhibit higher degrees of adaptability and autonomy than is possible with solid-state devices. Examples include: artificial organelle -like systems that could accomplish the synthesis of complex bio macromolecules , or synthetic multi-cellular structures that incorporate specific sensing and reporting functionalities, such that they could be used in hybrid devices for chemical or biological agent sensing.
Semi-biotic systems is an emerging area of research within the broader area of Synthetic Biology . In the European community a programme entitled NEONUCLEI was funded under FP6 whose aim is to generate synthetic analogues of cell nuclei capable of sustaining transcription , in self-assembled systems comprising DNA, macromolecules (or nanoparticles ), and lipids . [ 1 ] | https://en.wikipedia.org/wiki/Semi-biotic_systems |
In mathematical analysis , semicontinuity (or semi-continuity ) is a property of extended real -valued functions that is weaker than continuity . An extended real-valued function f {\displaystyle f} is upper (respectively, lower ) semicontinuous at a point x 0 {\displaystyle x_{0}} if, roughly speaking, the function values for arguments near x 0 {\displaystyle x_{0}} are not much higher (respectively, lower) than f ( x 0 ) . {\displaystyle f\left(x_{0}\right).} Briefly, a function on a domain X {\displaystyle X} is lower semi-continuous if its epigraph { ( x , t ) ∈ X × R : t ≥ f ( x ) } {\displaystyle \{(x,t)\in X\times \mathbb {R} :t\geq f(x)\}} is closed in X × R {\displaystyle X\times \mathbb {R} } , and upper semi-continuous if − f {\displaystyle -f} is lower semi-continuous.
A function is continuous if and only if it is both upper and lower semicontinuous. If we take a continuous function and increase its value at a certain point x 0 {\displaystyle x_{0}} to f ( x 0 ) + c {\displaystyle f\left(x_{0}\right)+c} for some c > 0 {\displaystyle c>0} , then the result is upper semicontinuous; if we decrease its value to f ( x 0 ) − c {\displaystyle f\left(x_{0}\right)-c} then the result is lower semicontinuous.
The notion of upper and lower semicontinuous function was first introduced and studied by René Baire in his thesis in 1899. [ 1 ]
Assume throughout that X {\displaystyle X} is a topological space and f : X → R ¯ {\displaystyle f:X\to {\overline {\mathbb {R} }}} is a function with values in the extended real numbers R ¯ = R ∪ { − ∞ , ∞ } = [ − ∞ , ∞ ] {\displaystyle {\overline {\mathbb {R} }}=\mathbb {R} \cup \{-\infty ,\infty \}=[-\infty ,\infty ]} .
A function f : X → R ¯ {\displaystyle f:X\to {\overline {\mathbb {R} }}} is called upper semicontinuous at a point x 0 ∈ X {\displaystyle x_{0}\in X} if for every real y > f ( x 0 ) {\displaystyle y>f\left(x_{0}\right)} there exists a neighborhood U {\displaystyle U} of x 0 {\displaystyle x_{0}} such that f ( x ) < y {\displaystyle f(x)<y} for all x ∈ U {\displaystyle x\in U} . [ 2 ] Equivalently, f {\displaystyle f} is upper semicontinuous at x 0 {\displaystyle x_{0}} if and only if lim sup x → x 0 f ( x ) ≤ f ( x 0 ) {\displaystyle \limsup _{x\to x_{0}}f(x)\leq f(x_{0})} where lim sup is the limit superior of the function f {\displaystyle f} at the point x 0 . {\displaystyle x_{0}.}
If X {\displaystyle X} is a metric space with distance function d {\displaystyle d} and f ( x 0 ) ∈ R , {\displaystyle f(x_{0})\in \mathbb {R} ,} this can also be restated using an ε {\displaystyle \varepsilon } - δ {\displaystyle \delta } formulation, similar to the definition of continuous function . Namely, for each ε > 0 {\displaystyle \varepsilon >0} there is a δ > 0 {\displaystyle \delta >0} such that f ( x ) < f ( x 0 ) + ε {\displaystyle f(x)<f(x_{0})+\varepsilon } whenever d ( x , x 0 ) < δ . {\displaystyle d(x,x_{0})<\delta .}
A function f : X → R ¯ {\displaystyle f:X\to {\overline {\mathbb {R} }}} is called upper semicontinuous if it satisfies any of the following equivalent conditions: [ 2 ]
A function f : X → R ¯ {\displaystyle f:X\to {\overline {\mathbb {R} }}} is called lower semicontinuous at a point x 0 ∈ X {\displaystyle x_{0}\in X} if for every real y < f ( x 0 ) {\displaystyle y<f\left(x_{0}\right)} there exists a neighborhood U {\displaystyle U} of x 0 {\displaystyle x_{0}} such that f ( x ) > y {\displaystyle f(x)>y} for all x ∈ U {\displaystyle x\in U} .
Equivalently, f {\displaystyle f} is lower semicontinuous at x 0 {\displaystyle x_{0}} if and only if lim inf x → x 0 f ( x ) ≥ f ( x 0 ) {\displaystyle \liminf _{x\to x_{0}}f(x)\geq f(x_{0})} where lim inf {\displaystyle \liminf } is the limit inferior of the function f {\displaystyle f} at point x 0 . {\displaystyle x_{0}.}
If X {\displaystyle X} is a metric space with distance function d {\displaystyle d} and f ( x 0 ) ∈ R , {\displaystyle f(x_{0})\in \mathbb {R} ,} this can also be restated as follows: For each ε > 0 {\displaystyle \varepsilon >0} there is a δ > 0 {\displaystyle \delta >0} such that f ( x ) > f ( x 0 ) − ε {\displaystyle f(x)>f(x_{0})-\varepsilon } whenever d ( x , x 0 ) < δ . {\displaystyle d(x,x_{0})<\delta .}
A function f : X → R ¯ {\displaystyle f:X\to {\overline {\mathbb {R} }}} is called lower semicontinuous if it satisfies any of the following equivalent conditions:
Consider the function f , {\displaystyle f,} piecewise defined by: f ( x ) = { − 1 if x < 0 , 1 if x ≥ 0 {\displaystyle f(x)={\begin{cases}-1&{\mbox{if }}x<0,\\1&{\mbox{if }}x\geq 0\end{cases}}} This function is upper semicontinuous at x 0 = 0 , {\displaystyle x_{0}=0,} but not lower semicontinuous.
The floor function f ( x ) = ⌊ x ⌋ , {\displaystyle f(x)=\lfloor x\rfloor ,} which returns the greatest integer less than or equal to a given real number x , {\displaystyle x,} is everywhere upper semicontinuous. Similarly, the ceiling function f ( x ) = ⌈ x ⌉ {\displaystyle f(x)=\lceil x\rceil } is lower semicontinuous.
Upper and lower semicontinuity bear no relation to continuity from the left or from the right for functions of a real variable. Semicontinuity is defined in terms of an ordering in the range of the functions, not in the domain. [ 4 ] For example the function f ( x ) = { sin ( 1 / x ) if x ≠ 0 , 1 if x = 0 , {\displaystyle f(x)={\begin{cases}\sin(1/x)&{\mbox{if }}x\neq 0,\\1&{\mbox{if }}x=0,\end{cases}}} is upper semicontinuous at x = 0 {\displaystyle x=0} while the function limits from the left or right at zero do not even exist.
If X = R n {\displaystyle X=\mathbb {R} ^{n}} is a Euclidean space (or more generally, a metric space) and Γ = C ( [ 0 , 1 ] , X ) {\displaystyle \Gamma =C([0,1],X)} is the space of curves in X {\displaystyle X} (with the supremum distance d Γ ( α , β ) = sup { d X ( α ( t ) , β ( t ) ) : t ∈ [ 0 , 1 ] } {\displaystyle d_{\Gamma }(\alpha ,\beta )=\sup\{d_{X}(\alpha (t),\beta (t)):t\in [0,1]\}} ), then the length functional L : Γ → [ 0 , + ∞ ] , {\displaystyle L:\Gamma \to [0,+\infty ],} which assigns to each curve α {\displaystyle \alpha } its length L ( α ) , {\displaystyle L(\alpha ),} is lower semicontinuous. [ 5 ] As an example, consider approximating the unit square diagonal by a staircase from below. The staircase always has length 2, while the diagonal line has only length 2 {\displaystyle {\sqrt {2}}} .
Unless specified otherwise, all functions below are from a topological space X {\displaystyle X} to the extended real numbers R ¯ = [ − ∞ , ∞ ] . {\displaystyle {\overline {\mathbb {R} }}=[-\infty ,\infty ].} Several of the results hold for semicontinuity at a specific point, but for brevity they are only stated for semicontinuity over the whole domain.
Let f , g : X → R ¯ {\displaystyle f,g:X\to {\overline {\mathbb {R} }}} .
For set-valued functions , several concepts of semicontinuity have been defined, namely upper , lower , outer , and inner semicontinuity, as well as upper and lower hemicontinuity .
A set-valued function F {\displaystyle F} from a set A {\displaystyle A} to a set B {\displaystyle B} is written F : A ⇉ B . {\displaystyle F:A\rightrightarrows B.} For each x ∈ A , {\displaystyle x\in A,} the function F {\displaystyle F} defines a set F ( x ) ⊂ B . {\displaystyle F(x)\subset B.} The preimage of a set S ⊂ B {\displaystyle S\subset B} under F {\displaystyle F} is defined as F − 1 ( S ) := { x ∈ A : F ( x ) ∩ S ≠ ∅ } . {\displaystyle F^{-1}(S):=\{x\in A:F(x)\cap S\neq \varnothing \}.} That is, F − 1 ( S ) {\displaystyle F^{-1}(S)} is the set that contains every point x {\displaystyle x} in A {\displaystyle A} such that F ( x ) {\displaystyle F(x)} is not disjoint from S {\displaystyle S} . [ 10 ]
A set-valued map F : R m ⇉ R n {\displaystyle F:\mathbb {R} ^{m}\rightrightarrows \mathbb {R} ^{n}} is upper semicontinuous at x ∈ R m {\displaystyle x\in \mathbb {R} ^{m}} if for every open set U ⊂ R n {\displaystyle U\subset \mathbb {R} ^{n}} such that F ( x ) ⊂ U {\displaystyle F(x)\subset U} , there exists a neighborhood V {\displaystyle V} of x {\displaystyle x} such that F ( V ) ⊂ U . {\displaystyle F(V)\subset U.} [ 10 ] : Def. 2.1
A set-valued map F : R m ⇉ R n {\displaystyle F:\mathbb {R} ^{m}\rightrightarrows \mathbb {R} ^{n}} is lower semicontinuous at x ∈ R m {\displaystyle x\in \mathbb {R} ^{m}} if for every open set U ⊂ R n {\displaystyle U\subset \mathbb {R} ^{n}} such that x ∈ F − 1 ( U ) , {\displaystyle x\in F^{-1}(U),} there exists a neighborhood V {\displaystyle V} of x {\displaystyle x} such that V ⊂ F − 1 ( U ) . {\displaystyle V\subset F^{-1}(U).} [ 10 ] : Def. 2.2
Upper and lower set-valued semicontinuity are also defined more generally for a set-valued maps between topological spaces by replacing R m {\displaystyle \mathbb {R} ^{m}} and R n {\displaystyle \mathbb {R} ^{n}} in the above definitions with arbitrary topological spaces. [ 10 ]
Note, that there is not a direct correspondence between single-valued lower and upper semicontinuity and set-valued lower and upper semicontinuouty.
An upper semicontinuous single-valued function is not necessarily upper semicontinuous when considered as a set-valued map. [ 10 ] : 18 For example, the function f : R → R {\displaystyle f:\mathbb {R} \to \mathbb {R} } defined by f ( x ) = { − 1 if x < 0 , 1 if x ≥ 0 {\displaystyle f(x)={\begin{cases}-1&{\mbox{if }}x<0,\\1&{\mbox{if }}x\geq 0\end{cases}}} is upper semicontinuous in the single-valued sense but the set-valued map x ↦ F ( x ) := { f ( x ) } {\displaystyle x\mapsto F(x):=\{f(x)\}} is not upper semicontinuous in the set-valued sense.
A set-valued function F : R m ⇉ R n {\displaystyle F:\mathbb {R} ^{m}\rightrightarrows \mathbb {R} ^{n}} is called inner semicontinuous at x {\displaystyle x} if for every y ∈ F ( x ) {\displaystyle y\in F(x)} and every convergent sequence ( x i ) {\displaystyle (x_{i})} in R m {\displaystyle \mathbb {R} ^{m}} such that x i → x {\displaystyle x_{i}\to x} , there exists
a sequence ( y i ) {\displaystyle (y_{i})} in R n {\displaystyle \mathbb {R} ^{n}} such that y i → y {\displaystyle y_{i}\to y} and y i ∈ F ( x i ) {\displaystyle y_{i}\in F\left(x_{i}\right)} for all sufficiently large i ∈ N . {\displaystyle i\in \mathbb {N} .} [ 11 ] [ note 2 ]
A set-valued function F : R m ⇉ R n {\displaystyle F:\mathbb {R} ^{m}\rightrightarrows \mathbb {R} ^{n}} is called outer semicontinuous at x {\displaystyle x} if for every convergence sequence ( x i ) {\displaystyle (x_{i})} in R m {\displaystyle \mathbb {R} ^{m}} such that x i → x {\displaystyle x_{i}\to x} and every convergent sequence ( y i ) {\displaystyle (y_{i})} in R n {\displaystyle \mathbb {R} ^{n}} such that y i ∈ F ( x i ) {\displaystyle y_{i}\in F(x_{i})} for each i ∈ N , {\displaystyle i\in \mathbb {N} ,} the sequence ( y i ) {\displaystyle (y_{i})} converges to a point in F ( x ) {\displaystyle F(x)} (that is, lim i → ∞ y i ∈ F ( x ) {\displaystyle \lim _{i\to \infty }y_{i}\in F(x)} ). [ 11 ] | https://en.wikipedia.org/wiki/Semi-continuity |
Semi-deciduous or semi-evergreen is a botanical term which refers to plants that lose their foliage for a very short period, when old leaves fall off and new foliage growth is starting. This phenomenon occurs in tropical and sub-tropical woody species, for example in Dipteryx odorata . [ 1 ] Semi-deciduous or semi-evergreen may also describe some trees, bushes or plants that normally only lose part of their foliage in autumn/winter or during the dry season , but might lose all their leaves in a manner similar to deciduous trees in an especially cold autumn/winter or severe dry season (drought). [ 2 ]
This botany article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Semi-deciduous |
In calculus , the notions of one-sided differentiability and semi-differentiability of a real -valued function f of a real variable are weaker than differentiability . Specifically, the function f is said to be right differentiable at a point a if, roughly speaking, a derivative can be defined as the function's argument x moves to a from the right, and left differentiable at a if the derivative can be defined as x moves to a from the left.
In mathematics , a left derivative and a right derivative are derivatives (rates of change of a function) defined for movement in one direction only (left or right; that is, to lower or higher values) by the argument of a function.
Let f denote a real-valued function defined on a subset I of the real numbers.
If a ∈ I is a limit point of I ∩ [ a ,∞) and the one-sided limit
exists as a real number, then f is called right differentiable at a and the limit ∂ + f ( a ) is called the right derivative of f at a .
If a ∈ I is a limit point of I ∩ (–∞, a ] and the one-sided limit
exists as a real number, then f is called left differentiable at a and the limit ∂ – f ( a ) is called the left derivative of f at a .
If a ∈ I is a limit point of I ∩ [ a ,∞) and I ∩ (–∞, a ] and if f is left and right differentiable at a , then f is called semi-differentiable at a .
If the left and right derivatives are equal, then they have the same value as the usual ("bidirectional") derivative. One can also define a symmetric derivative , which equals the arithmetic mean of the left and right derivatives (when they both exist), so the symmetric derivative may exist when the usual derivative does not. [ 1 ]
If a real-valued, differentiable function f , defined on an interval I of the real line, has zero derivative everywhere, then it is constant, as an application of the mean value theorem shows. The assumption of differentiability can be weakened to continuity and one-sided differentiability of f . The version for right differentiable functions is given below, the version for left differentiable functions is analogous.
Theorem — Let f be a real-valued, continuous function , defined on an arbitrary interval I of the real line. If f is right differentiable at every point a ∈ I , which is not the supremum of the interval, and if this right derivative is always zero, then f is constant .
For a proof by contradiction , assume there exist a < b in I such that f ( a ) ≠ f ( b ) . Then
Define c as the infimum of all those x in the interval ( a , b ] for which the difference quotient of f exceeds ε in absolute value, i.e.
Due to the continuity of f , it follows that c < b and | f ( c ) – f ( a ) | = ε ( c – a ) . At c the right derivative of f is zero by assumption, hence there exists d in the interval ( c , b ] with | f ( x ) – f ( c ) | ≤ ε ( x – c ) for all x in ( c , d ] . Hence, by the triangle inequality ,
for all x in [ c , d ) , which contradicts the definition of c .
Another common use is to describe derivatives treated as binary operators in infix notation , in which the derivatives is to be applied either to the left or right operands . This is useful, for example, when defining generalizations of the Poisson bracket . For a pair of functions f and g, the left and right derivatives are respectively defined as
In bra–ket notation , the derivative operator can act on the right operand as the regular derivative or on the left as the negative derivative. [ 2 ]
This above definition can be generalized to real-valued functions f defined on subsets of R n using a weaker version of the directional derivative . Let a be an interior point of the domain of f . Then f is called semi-differentiable at the point a if for every direction u ∈ R n the limit
with h ∈ {\displaystyle h\in } R exists as a real number.
Semi-differentiability is thus weaker than Gateaux differentiability , for which one takes in the limit above h → 0 without restricting h to only positive values.
For example, the function f ( x , y ) = x 2 + y 2 {\displaystyle f(x,y)={\sqrt {x^{2}+y^{2}}}} is semi-differentiable at ( 0 , 0 ) {\displaystyle (0,0)} , but not Gateaux differentiable there. Indeed, f ( h x , h y ) = | h | f ( x , y ) and for h ≥ 0 , f ( h x , h y ) = h f ( x , y ) , f ( h x , h y ) / h = f ( x , y ) , {\displaystyle f(hx,hy)=|h|f(x,y){\text{ and for }}h\geq 0,f(hx,hy)=hf(x,y),f(hx,hy)/h=f(x,y),} with a = 0 , u = ( x , y ) , ∂ u f ( 0 ) = f ( x , y ) {\displaystyle a=0,u=(x,y),\partial _{u}f(0)=f(x,y)}
(Note that this generalization is not equivalent to the original definition for n = 1 since the concept of one-sided limit points is replaced with the stronger concept of interior points.)
Instead of real-valued functions, one can consider functions taking values in R n or in a Banach space . | https://en.wikipedia.org/wiki/Semi-differentiability |
A semi-drying oil is an oil which partially hardens when it is exposed to air. This is as opposed to a drying oil , [ 1 ] which hardens completely, or a non-drying oil , which does not harden at all. Oils with an iodine number of 115–130 are considered semi-drying. | https://en.wikipedia.org/wiki/Semi-drying_oil |
In nuclear physics , the semi-empirical mass formula ( SEMF ; sometimes also called the Weizsäcker formula , Bethe–Weizsäcker formula , or Bethe–Weizsäcker mass formula to distinguish it from the Bethe–Weizsäcker process ) is used to approximate the mass of an atomic nucleus from its number of protons and neutrons . As the name suggests, it is based partly on theory and partly on empirical measurements . The formula represents the liquid-drop model proposed by George Gamow , [ 1 ] which can account for most of the terms in the formula and gives rough estimates for the values of the coefficients. It was first formulated in 1935 by German physicist Carl Friedrich von Weizsäcker , [ 2 ] and although refinements have been made to the coefficients over the years, the structure of the formula remains the same today.
The formula gives a good approximation for atomic masses and thereby other effects. However, it fails to explain the existence of lines of greater binding energy at certain numbers of protons and neutrons. These numbers, known as magic numbers , are the foundation of the nuclear shell model .
The liquid-drop model was first proposed by George Gamow and further developed by Niels Bohr , John Archibald Wheeler and Lise Meitner . [ 3 ] It treats the nucleus as a drop of incompressible fluid of very high density, held together by the nuclear force (a residual effect of the strong force ), there is a similarity to the structure of a spherical liquid drop. While a crude model, the liquid-drop model accounts for the spherical shape of most nuclei and makes a rough prediction of binding energy.
The corresponding mass formula is defined purely in terms of the numbers of protons and neutrons it contains. The original Weizsäcker formula defines five terms:
The mass of an atomic nucleus, for N {\displaystyle N} neutrons , Z {\displaystyle Z} protons , and therefore A = N + Z {\displaystyle A=N+Z} nucleons , is given by
where m n {\displaystyle m_{\text{n}}} and m p {\displaystyle m_{\text{p}}} are the rest mass of a neutron and a proton respectively, and E B {\displaystyle E_{\text{B}}} is the binding energy of the nucleus. The semi-empirical mass formula states the binding energy is [ 4 ]
The δ ( N , Z ) {\displaystyle \delta (N,Z)} term is either zero or ± δ 0 {\displaystyle \pm \delta _{0}} , depending on the parity of N {\displaystyle N} and Z {\displaystyle Z} , where δ 0 = a P A k P {\displaystyle \delta _{0}={a_{\text{P}}}{A^{k_{\text{P}}}}} for some exponent k P {\displaystyle k_{\text{P}}} . Note that as A = N + Z {\displaystyle A=N+Z} , the numerator of the a A {\displaystyle a_{\text{A}}} term can be rewritten as ( A − 2 Z ) 2 {\displaystyle (A-2Z)^{2}} .
Each of the terms in this formula has a theoretical basis. The coefficients a V {\displaystyle a_{\text{V}}} , a S {\displaystyle a_{\text{S}}} , a C {\displaystyle a_{\text{C}}} , a A {\displaystyle a_{\text{A}}} , and a P {\displaystyle a_{\text{P}}} are determined empirically; while they may be derived from experiment, they are typically derived from least-squares fit to contemporary data. While typically expressed by its basic five terms, further terms exist to explain additional phenomena. Akin to how changing a polynomial fit will change its coefficients, the interplay between these coefficients as new phenomena are introduced is complex; some terms influence each other, whereas the a P {\displaystyle a_{\text{P}}} term is largely independent. [ 5 ]
The term a V A {\displaystyle a_{\text{V}}A} is known as the volume term . The volume of the nucleus is proportional to A , so this term is proportional to the volume, hence the name.
The basis for this term is the strong nuclear force . The strong force affects both protons and neutrons, and as expected, this term is independent of Z . Because the number of pairs that can be taken from A particles is A ( A − 1 ) / 2 {\displaystyle A(A-1)/2} , one might expect a term proportional to A 2 {\displaystyle A^{2}} . However, the strong force has a very limited range, and a given nucleon may only interact strongly with its nearest neighbors and next nearest neighbors. Therefore, the number of pairs of particles that actually interact is roughly proportional to A , giving the volume term its form.
The coefficient a V {\displaystyle a_{\text{V}}} is smaller than the binding energy possessed by the nucleons with respect to their neighbors ( E b {\displaystyle E_{\text{b}}} ), which is of order of 40 MeV . This is because the larger the number of nucleons in the nucleus, the larger their kinetic energy is, due to the Pauli exclusion principle . If one treats the nucleus as a Fermi ball of A {\displaystyle A} nucleons , with equal numbers of protons and neutrons, then the total kinetic energy is 3 5 A ε F {\displaystyle {\tfrac {3}{5}}A\varepsilon _{\text{F}}} , with ε F {\displaystyle \varepsilon _{\text{F}}} the Fermi energy , which is estimated as 38 MeV . Thus the expected value of a V {\displaystyle a_{\text{V}}} in this model is E b − 3 5 ε F ∼ 17 M e V , {\displaystyle E_{\text{b}}-{\tfrac {3}{5}}\varepsilon _{\text{F}}\sim 17~\mathrm {MeV} ,} not far from the measured value.
The term a S A 2 / 3 {\displaystyle a_{\text{S}}A^{2/3}} is known as the surface term . This term, also based on the strong force, is a correction to the volume term.
The volume term suggests that each nucleon interacts with a constant number of nucleons, independent of A . While this is very nearly true for nucleons deep within the nucleus, those nucleons on the surface of the nucleus have fewer nearest neighbors, justifying this correction. This can also be thought of as a surface-tension term, and indeed a similar mechanism creates surface tension in liquids.
If the volume of the nucleus is proportional to A , then the radius should be proportional to A 1 / 3 {\displaystyle A^{1/3}} and the surface area to A 2 / 3 {\displaystyle A^{2/3}} . This explains why the surface term is proportional to A 2 / 3 {\displaystyle A^{2/3}} . It can also be deduced that a S {\displaystyle a_{\text{S}}} should have a similar order of magnitude to a V {\displaystyle a_{\text{V}}} .
The term a C Z ( Z − 1 ) A 1 / 3 {\displaystyle a_{\text{C}}{\frac {Z(Z-1)}{A^{1/3}}}} or a C Z 2 A 1 / 3 {\displaystyle a_{\text{C}}{\frac {Z^{2}}{A^{1/3}}}} is known as the Coulomb or electrostatic term .
The basis for this term is the electrostatic repulsion between protons. To a very rough approximation, the nucleus can be considered a sphere of uniform charge density. The potential energy of such a charge distribution can be shown to be
where Q is the total charge, and R is the radius of the sphere. The value of a C {\displaystyle a_{\text{C}}} can be approximately calculated by using this equation to calculate the potential energy, using an empirical nuclear radius of R ≈ r 0 A 1 3 {\displaystyle R\approx r_{0}A^{\frac {1}{3}}} and Q = Ze . However, because electrostatic repulsion will only exist for more than one proton, Z 2 {\displaystyle Z^{2}} becomes Z ( Z − 1 ) {\displaystyle Z(Z-1)} :
where now the electrostatic Coulomb constant a C {\displaystyle a_{\text{C}}} is
Using the fine-structure constant , we can rewrite the value of a C {\displaystyle a_{\text{C}}} as
where α {\displaystyle \alpha } is the fine-structure constant, and r 0 A 1 / 3 {\displaystyle r_{0}A^{1/3}} is the radius of a nucleus , giving r 0 {\displaystyle r_{0}} to be approximately 1.25 femtometers . R P {\displaystyle R_{\text{P}}} is the proton reduced Compton wavelength , and m p {\displaystyle m_{\text{p}}} is the proton mass. This gives a C {\displaystyle a_{\text{C}}} an approximate theoretical value of 0.691 MeV , not far from the measured value.
The term a A ( N − Z ) 2 A {\displaystyle a_{\text{A}}{\frac {(N-Z)^{2}}{A}}} is known as the asymmetry term (or Pauli term ).
The theoretical justification for this term is more complex. The Pauli exclusion principle states that no two identical fermions can occupy exactly the same quantum state in an atom. At a given energy level, there are only finitely many quantum states available for particles. What this means in the nucleus is that as more particles are "added", these particles must occupy higher energy levels, increasing the total energy of the nucleus (and decreasing the binding energy). Note that this effect is not based on any of the fundamental forces ( gravitational , electromagnetic, etc.), only the Pauli exclusion principle.
Protons and neutrons, being distinct types of particles, occupy different quantum states. One can think of two different "pools" of states – one for protons and one for neutrons. Now, for example, if there are significantly more neutrons than protons in a nucleus, some of the neutrons will be higher in energy than the available states in the proton pool. If we could move some particles from the neutron pool to the proton pool, in other words, change some neutrons into protons, we would significantly decrease the energy. The imbalance between the number of protons and neutrons causes the energy to be higher than it needs to be, for a given number of nucleons . This is the basis for the asymmetry term.
The actual form of the asymmetry term can again be derived by modeling the nucleus as a Fermi ball of protons and neutrons. Its total kinetic energy is
where ε F,p {\displaystyle \varepsilon _{\text{F,p}}} and ε F,n {\displaystyle \varepsilon _{\text{F,n}}} are the Fermi energies of the protons and neutrons. Since these are proportional to Z 2 / 3 {\displaystyle Z^{2/3}} and N 2 / 3 {\displaystyle N^{2/3}} respectively, one gets
The leading terms in the expansion in the difference N − Z {\displaystyle N-Z} are then
At the zeroth order in the expansion the kinetic energy is just the overall Fermi energy ε F ≡ ε F,p = ε F,n {\displaystyle \varepsilon _{\text{F}}\equiv \varepsilon _{\text{F,p}}=\varepsilon _{\text{F,n}}} multiplied by 3 5 A {\displaystyle {\tfrac {3}{5}}A} . Thus we get
The first term contributes to the volume term in the semi-empirical mass formula, and the second term is minus the asymmetry term (remember, the kinetic energy contributes to the total binding energy with a negative sign).
ε F {\displaystyle \varepsilon _{\text{F}}} is 38 MeV , so calculating a A {\displaystyle a_{\text{A}}} from the equation above, we get only half the measured value. The discrepancy is explained by our model not being accurate: nucleons in fact interact with each other and are not spread evenly across the nucleus. For example, in the shell model , a proton and a neutron with overlapping wavefunctions will have a greater strong interaction between them and stronger binding energy. This makes it energetically favourable (i.e. having lower energy) for protons and neutrons to have the same quantum numbers (other than isospin ), and thus increase the energy cost of asymmetry between them.
One can also understand the asymmetry term intuitively as follows. It should be dependent on the absolute difference | N − Z | {\displaystyle |N-Z|} , and the form ( N − Z ) 2 {\displaystyle (N-Z)^{2}} is simple and differentiable , which is important for certain applications of the formula. In addition, small differences between Z and N do not have a high energy cost. The A in the denominator reflects the fact that a given difference | N − Z | {\displaystyle |N-Z|} is less significant for larger values of A .
The term δ ( A , Z ) {\displaystyle \delta (A,Z)} is known as the pairing term (possibly also known as the pairwise interaction). This term captures the effect of spin coupling. It is given by [ 7 ]
where δ 0 {\displaystyle \delta _{0}} is found empirically to have a value of about 1000 keV, slowly decreasing with mass number A . Odd-odd nuclei tend to undergo beta decay to an adjacent even-even nucleus by changing a neutron to a proton or vice versa. The pairs have overlapping wave functions and sit very close together with a bond stronger than any other configuration. [ 7 ] When the pairing term is substituted into the binding energy equation, for even Z , N , the pairing term adds binding energy, and for odd Z , N the pairing term removes binding energy.
The dependence on mass number is commonly parametrized as
The value of the exponent k P is determined from experimental binding-energy data. In the past its value was often assumed to be −3/4, but modern experimental data indicate that a value of −1/2 is nearer the mark:
Due to the Pauli exclusion principle the nucleus would have a lower energy if the number of protons with spin up were equal to the number of protons with spin down. This is also true for neutrons. Only if both Z and N are even, can both protons and neutrons have equal numbers of spin-up and spin-down particles. This is a similar effect to the asymmetry term.
The factor A k P {\displaystyle A^{k_{\text{P}}}} is not easily explained theoretically. The Fermi-ball calculation we have used above, based on the liquid-drop model but neglecting interactions, will give an A − 1 {\displaystyle A^{-1}} dependence, as in the asymmetry term. This means that the actual effect for large nuclei will be larger than expected by that model. This should be explained by the interactions between nucleons. For example, in the shell model , two protons with the same quantum numbers (other than spin ) will have completely overlapping wavefunctions and will thus have greater strong interaction between them and stronger binding energy. This makes it energetically favourable (i.e. having lower energy) for protons to form pairs of opposite spin. The same is true for neutrons.
The coefficients are calculated by fitting to experimentally measured masses of nuclei. Their values can vary depending on how they are fitted to the data and which unit is used to express the mass. Several examples are as shown below.
The formula does not consider the internal shell structure of the nucleus.
The semi-empirical mass formula therefore provides a good fit to heavier nuclei, and a poor fit to very light nuclei, especially 4 He . For light nuclei, it is usually better to use a model that takes this shell structure into account.
By maximizing E b ( A , Z ) with respect to Z , one would find the best neutron–proton ratio N / Z for a given atomic weight A . [ 10 ] We get
This is roughly 1 for light nuclei, but for heavy nuclei the ratio grows in good agreement with experiment .
By substituting the above value of Z back into E b , one obtains the binding energy as a function of the atomic weight, E b ( A ) .
Maximizing E b ( A )/ A with respect to A gives the nucleus which is most strongly bound, i.e. most stable. The value we get is A = 63 ( copper ), close to the measured values of A = 62 ( nickel ) and A = 58 ( iron ).
The liquid-drop model also allows the computation of fission barriers for nuclei, which determine the stability of a nucleus against spontaneous fission . It was originally speculated that elements beyond atomic number 104 could not exist, as they would undergo fission with very short half-lives, [ 12 ] though this formula did not consider stabilizing effects of closed nuclear shells . A modified formula considering shell effects reproduces known data and the predicted island of stability (in which fission barriers and half-lives are expected to increase, reaching a maximum at the shell closures), though also suggests a possible limit to existence of superheavy nuclei beyond Z = 120 and N = 184. [ 12 ] | https://en.wikipedia.org/wiki/Semi-empirical_mass_formula |
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