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Spider silk

Spider silk

Overview
Spider silk is a protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

 fiber
Fiber
Fiber is a class of materials that are continuous filaments or are in discrete elongated pieces, similar to lengths of thread.They are very important in the biology of both plants and animals, for holding tissues together....

 spun by spider
Spider
Spiders are air-breathing arthropods that have eight legs, and chelicerae with fangs that inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other groups of organisms...

s. Spiders use their silk
Silk
Silk is a natural protein fiber, some forms of which can be woven into textiles. The best-known type of silk is obtained from the cocoons of the larvae of the mulberry silkworm Bombyx mori reared in captivity...

 to make web
Spider web
A spider web, spiderweb, spider's web or cobweb is a device built by a spider out of proteinaceous spider silk extruded from its spinnerets....

s or other structures, which function as nets to catch other animals, or as nests or cocoons for protection for their offspring. They can also suspend themselves using their silk.

Many small spiders use silk threads for ballooning
Ballooning (spider)
Ballooning is a term used for the mechanical kiting that many spiders, especially small species, as well as certain mites and some caterpillars use to disperse through the air. Many small spiders use gossamer or especially fine silk to lift themselves off a surface or use the silk as an anchor in...

, the popular, though technically inaccurate, scientific term for the dynamic kiting spiderlings (mostly) use for dispersal.
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Encyclopedia
Spider silk is a protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

 fiber
Fiber
Fiber is a class of materials that are continuous filaments or are in discrete elongated pieces, similar to lengths of thread.They are very important in the biology of both plants and animals, for holding tissues together....

 spun by spider
Spider
Spiders are air-breathing arthropods that have eight legs, and chelicerae with fangs that inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other groups of organisms...

s. Spiders use their silk
Silk
Silk is a natural protein fiber, some forms of which can be woven into textiles. The best-known type of silk is obtained from the cocoons of the larvae of the mulberry silkworm Bombyx mori reared in captivity...

 to make web
Spider web
A spider web, spiderweb, spider's web or cobweb is a device built by a spider out of proteinaceous spider silk extruded from its spinnerets....

s or other structures, which function as nets to catch other animals, or as nests or cocoons for protection for their offspring. They can also suspend themselves using their silk.

Many small spiders use silk threads for ballooning
Ballooning (spider)
Ballooning is a term used for the mechanical kiting that many spiders, especially small species, as well as certain mites and some caterpillars use to disperse through the air. Many small spiders use gossamer or especially fine silk to lift themselves off a surface or use the silk as an anchor in...

, the popular, though technically inaccurate, scientific term for the dynamic kiting spiderlings (mostly) use for dispersal. They extrude several threads into the air and let themselves be carried away by upward winds. Although most rides will end a few yards later, it seems to be a common way for spiders to invade islands. Many sailors have reported that spiders have been caught in their ship's sails, even when far from land. The extremely fine silk used by spiders for ballooning is known as gossamer.

In some cases, spiders may even use silk as a source of food.

Methods have been developed to silk a spider forcibly.

Uses


All spiders produce silks, and a single spider can produce up to seven different types of silk for different uses. This is in contrast to insect silks, where most often only one type of silk is produced by an individual. Over the 400 million years of evolution, spider silks may be used for a number of different ecological uses, each with properties to match the function of the silk (see Properties section). The evolution of spiders has led to more complex and diverse uses of silk throughout its evolution, for example from primitive tube webs 300-400mya to complex orb webs 110mya.
Ecological use Example Reference
Prey capture The orb webs produced by the Araneidae (typical orb-weavers); tube webs; tangle webs; sheet webs; lace webs, dome webs; single thread used by the Bolas spiders for ‘fishing’
Prey immobilization Silk used as ‘swathing bands’ to wrap up prey. Often combined with immobilising prey using a venom. In species of Scytodes
Scytodes thoracica
Scytodes thoracica is a spitting spider because it spits a poisonous sticky silken substance over its prey. Its size ranges between . The carapace is unusual in sloping upwards towards its rear end, whereas the abdomen slopes downwards....

 the silk is combined with venom and squirted from the chelicerae
Chelicerae
The chelicerae are mouthparts of the Chelicerata, an arthropod subphylum that includes arachnids, Merostomata , and Pycnogonida . Chelicerae are pointed appendages which are used to grasp food, and are found in place of the chewing mandibles most other arthropods have...

.
Reproduction Male spiders may produce sperm webs; spider eggs are covered in silk cocoons
Dispersal "Ballooning" or "kiting"
Ballooning (spider)
Ballooning is a term used for the mechanical kiting that many spiders, especially small species, as well as certain mites and some caterpillars use to disperse through the air. Many small spiders use gossamer or especially fine silk to lift themselves off a surface or use the silk as an anchor in...

 used by many small spiders for dispersal
Source of food The kleptoparasitic Argyrodes eating the silk of host spider webs. Some daily weavers of temporary webs also eat their own unused silk daily, thus mitigating a heavy metabolic expense.
Nest lining and nest construction Tube webs used by ‘primitive’ spider such as the European Tube Web Spider (Segestria florentina). Threads radiate out of nest to provide a sensory link to the outside. Silk is a component of the lids of Trapdoor spiders, and the "Water" or "Diving bell" spider Argyroneta aquatica builds its diving bell of silk. It is in fact difficult to think of any spider that does not use silk in constructing its abode.
Guide lines Some spiders that venture from shelter will leave a trail of silk by which to find their way home again.
Drop lines and anchor lines Many spiders, such as the Salticidae, that venture from shelter and leave a trail of silk, use that as an emergency line in case of falling from inverted or vertical surfaces. Many others, even web dwellers, will deliberately drop from a web when alarmed, using a silken thread as a drop line by which they can return in due course. Some, such as species of Paramystaria, also will hang from a drop line when feeding.
Alarm lines Some spiders that do not spin actual trap webs do lay out alarm webs that the feet of their prey (such as ants) can disturb, cuing the spider to rush out and secure the meal if it is small enough, or to avoid contact if the intruder seems too formidable.
Pheromonal trails Some wandering spiders will leave a largely continuous trail of silk impregnated with pheromones that the opposite gender can follow to find a mate.

Types


Meeting the specification for all these ecological uses requires different types of silk suited to different broad properties, as either a fiber, a structure of fibers, or a silk-globule. These types include glues and fibers. Some types of fibers are used for structural support, others for constructing protective structures. Some can absorb energy effectively, whereas others transmit vibration efficiently. In a spider, these silk types are produced in different glands; so the silk from a particular gland can be linked to its use by the spider. See the later section for details on the mechanical properties of silk and how the structure of silk can achieve these different properties.
Gland Silk Use
Ampullate (Major) Dragline silk - used for the web’s outer rim and spokes and the lifeline.
Ampullate (Minor) Used for temporary scaffolding during web construction.
Flagelliform Capture-spiral silk - used for the capturing lines of the web.
Tubuliform Egg cocoon silk - used for protective egg sacs.
Aciniform Used to wrap and secure freshly captured prey; used in the male sperm webs; used in stabilimenta
Aggregate A silk glue of sticky globules
Piriform Used to form bonds between separate threads for attachment points

Mechanical Properties


Each spider and each type of silk has a set of mechanical properties optimised for their biological function.

Most silks, in particular dragline silk, have exceptional mechanical properties. They exhibit a unique combination of high tensile strength
Tensile strength
Ultimate tensile strength , often shortened to tensile strength or ultimate strength, is the maximum stress that a material can withstand while being stretched or pulled before necking, which is when the specimen's cross-section starts to significantly contract...

 and extensibility (ductility). This enables a silk fiber to absorb a lot of energy before breaking (toughness, the area under a stress-strain curve).

A frequent mistake made in the mainstream media is to confuse strength and toughness when comparing silk to other materials. As shown below in detail, weight for weight, silk is stronger than steel, but not as strong as Kevlar. Silk is, however, tougher than both.

Strength


In detail a dragline silks’ tensile strength
Tensile strength
Ultimate tensile strength , often shortened to tensile strength or ultimate strength, is the maximum stress that a material can withstand while being stretched or pulled before necking, which is when the specimen's cross-section starts to significantly contract...

 is comparable to that of high-grade steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...

 (1500 MPa), and about half as strong as aramid
Aramid
Aramid fibers are a class of heat-resistant and strong synthetic fibers. They are used in aerospace and military applications, for ballistic rated body armor fabric and ballistic composites, in bicycle tires, and as an asbestos substitute. The name is a portmanteau of "aromatic polyamide"...

 filaments, such as Twaron
Twaron
Twaron is the brandname of Teijin Aramid for a para-aramid. It is a heat-resistant and strong synthetic fibre developed in the early 1970s by the Dutch company AKZO, division Enka, later Akzo Industrial Fibers. The research name of the para-aramid fibre was originally Fiber X, but it was soon...

 or Kevlar
Kevlar
Kevlar is the registered trademark for a para-aramid synthetic fiber, related to other aramids such as Nomex and Technora. Developed at DuPont in 1965, this high strength material was first commercially used in the early 1970s as a replacement for steel in racing tires...

 (3000 MPa).

Density


Consisting of mainly protein, silks are about a fifth of the density of steel (1.31 g/cm^3). As a result, a strand long enough to circle the Earth would weigh less than 500 grams (17.6 oz).

Extensibility


Silks are also especially ductile, with some able to stretch up to four times their relaxed length without breaking.

Toughness


The combination of strength and ductility gives dragline silks a very high toughness
Toughness
In materials science and metallurgy, toughness is the ability of a material to absorb energy and plastically deform without fracturing; Material toughness is defined as the amount of energy per volume that a material can absorb before rupturing...

 (or work to fracture), which "equals that of commercial polyaramid (aromatic nylon) filaments, which themselves are benchmarks of modern polymer fiber technology".

Temperature


Whilst unlikely to be relevant in nature, dragline silks can hold their strength below −40 °C and up to 220 °C.

Supercontraction


When exposed to water, dragline silks undergo supercontraction, shrinking up to 50% in length and behaving like a weak rubber under tension. Many hypothesis have been suggested as to its use in nature, with the most popular being to automatically tension webs built in the night using the morning dew.

Highest-Performance


The strongest known spider silk is produced by the species Darwin's bark spider
Darwin's bark spider
Darwin's bark spider is an orb-weaver spider that produces one of the largest known orb webs, web size ranged from 900–28,000 cm2 ,...

 (Caerostris darwini): "The toughness of forcibly silked fibers averages 350 MJ/m3, with some samples reaching 520 MJ/m3. Thus, C. darwini silk is more than twice as tough as any previously described silk, and over 10 times tougher than Kevlar".

Types of silk


Many species of spider have different glands to produce silk with different properties for different purposes, including housing, web
Spider web
A spider web, spiderweb, spider's web or cobweb is a device built by a spider out of proteinaceous spider silk extruded from its spinnerets....

 construction, defense, capturing and detaining prey, egg protection, and mobility (gossamer for ballooning, strands to let the spider drop down on as they are extruded). Different specialized silks have evolved with properties suitable for different uses. For example, Argiope argentata
Argiope argentata
Argiope argentata is a member of the Argiope genus of spiders and is also known as the Silver Argiope.-Description:As with most members of the Argiope genus the female of the species tends to be much larger than the male...

 has five different types of silk, each used for a different purpose:
Silk Use
major-ampullate (dragline) silk Used for the web's outer rim and spokes and the lifeline. Can be as strong per unit weight as steel, but much tougher.
capture-spiral silk Used for the capturing lines of the web. Sticky, extremely stretchy and tough.
tubiliform (aka cylindriform) silk Used for protective egg sacs. Stiffest silk.
aciniform silk Used to wrap and secure freshly captured prey. Two to three times as tough as the other silks, including dragline.
minor-ampullate silk Used for temporary scaffolding during web construction

Macroscopic structure down to protein hierarchy



Silks, as well as many other biomaterials, have a hierarchical structure [e.g., cellulose
Cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....

, hair
Hair
Hair is a filamentous biomaterial, that grows from follicles found in the dermis. Found exclusively in mammals, hair is one of the defining characteristics of the mammalian class....

]. The primary structure
Primary structure
The primary structure of peptides and proteins refers to the linear sequence of its amino acid structural units. The term "primary structure" was first coined by Linderstrøm-Lang in 1951...

 is its amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

 sequence, mainly consisting of highly repetitive glycine and alanine blocks, why silks are often referred to as a block co-polymer. On a secondary structure level, the short side chained alanine is mainly found in the crystalline domains (beta sheets) of the nano fibril, glycine is mostly found in the so called amorphous matrix consisting of helical and beta turn structures. It is the interplay between the hard crystalline segments, and the strained elastic semi amorphous regions, that gives spider silk its extraordinary properties.
Various compounds other than protein are used to enhance the fiber's properties. Pyrrolidine
Pyrrolidine
Pyrrolidine, also known as tetrahydropyrrole, is an organic compound with the molecular formula C4H9N. It is a cyclic secondary amine with a five-membered heterocycle containing four carbon atoms and one nitrogen atom...

 has hygroscopic properties and helps to keep the thread moist. It occurs in especially high concentration in glue threads. Potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...

 hydrogen phosphate releases proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....

s in aqueous solution, resulting in a pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

 of about 4, making the silk acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...

ic and thus protecting it from fungi and bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 that would otherwise digest the protein. Potassium nitrate
Potassium nitrate
Potassium nitrate is a chemical compound with the formula KNO3. It is an ionic salt of potassium ions K+ and nitrate ions NO3−.It occurs as a mineral niter and is a natural solid source of nitrogen. Its common names include saltpetre , from medieval Latin sal petræ: "stone salt" or possibly "Salt...

 is believed to prevent the protein from denaturing in the acidic milieu.

This first very basic model of silk was introduced by Termonia in 1994 suggested crystallites embedded in an amorphous matrix interlinked with hydrogen bonds. This model has refined over the years: Semi crystalline regions were found as well as a fibrillar skin core model suggested for spider silk, later visualized by AFM and TEM
Transmission electron microscopy
Transmission electron microscopy is a microscopy technique whereby a beam of electrons is transmitted through an ultra thin specimen, interacting with the specimen as it passes through...

. Sizes of the nano fibrillar structure and the crystalline and semi-crystalline regions were revealed by Neutron Scattering
Neutron scattering
Neutron scattering,the scattering of free neutrons by matter,is a physical processand an experimental technique using this processfor the investigation of materials.Neutron scattering as a physical process is of primordial importance...

.

Non-protein composition


Various compounds other than protein are found in spider silks, such as sugars, lipids, ions, and pigments that might affect the aggregation behaviour and act as a protection layer in the final fiber.

Biosynthesis


The production of silks, including spider silk, differs in an important respect from the production of most other fibrous biological materials: rather than being continuously grown as keratin in hair, cellulose in the cell walls of plants, or even the fibers formed from the compacted faecal matter of beetles, it is ‘spun’ on demand from liquid silk precursor sometimes referred to as unspun silk dope, out of specialised glands.

The spinning process occurs when a fiber is pulled away from the body of a spider, be that by the spider’s legs, by the spider's falling and using its own weight, or by any other method including being pulled by humans. The name ‘spinning’ is misleading as no rotation of any component occurs, but the name comes from when it was thought that spiders produced their thread in a similar manner to the spinning wheels of old. In fact the process is a pulltrusion - similar to extrusion, with the subtlety that the force is induced by pulling at the finished fiber rather than being squeezed out of a reservoir of some kind.

The unspun silk dope is pulled through silk gland
Gland
A gland is an organ in an animal's body that synthesizes a substance for release of substances such as hormones or breast milk, often into the bloodstream or into cavities inside the body or its outer surface .- Types :...

s, of which there may be both numerous duplicates and also different types on any one spider species.

Silk gland


The gland's visible, or external, part is termed the spinneret. Depending on the complexity of the species, spiders will have two to eight sets of spinnerets, usually in pairs. There exist highly different specialised glands in different spiders, ranging from simply a sac with an opening at one end, to the complex, multiple-section Major Ampullate glands of the Nephila golden orb weaving spiders.

Behind each spinneret visible on the surface of the spider lies a gland, a generalised form of which is shown in the figure to the right, "Schematic of a generalised gland".
The gland described here will be based upon the major apullate gland from a golden orb weaving spiders as they are the most-studied and presumed to be the most complex.

1. The first section of the gland labelled 1 on Figure 1 is the secretory or tail section of the gland. The walls of this section are lined with cells that secrete proteins Spidroin I and Spidroin II, the main components of this spider’s dragline. These proteins are found in the form of droplets that gradually elongate to form long channels along the length of the final fiber, hypothesized to assist in preventing crack formation or even self-healing of the fiber.

2. The second section is the storage sac. This stores and maintains the gel-like unspun silk dope until it is required by the spider. In addition to storing the unspun silk gel, it secretes proteins that coat the surface of the final fiber.

3. The funnel rapidly reduces the large diameter of the storage sac to the small diameter of the tapering duct.

4. The final length is the tapering duct, the site of most of the fiber formation. This consists of a tapering tube with several tight about turns, a valve almost at the end (mentioned in detail at point No. 5 below) ending in a spigot from which the silk fiber emerges. The tube here tapers hyperbolically, therefore the unspun silk is under constant shear stress, which is an important factor in fiber formation. This section of the duct is lined with cells that exchange ions and remove water from the fiber. The spigot at the end has lips that clamp around the fiber, controlling fiber diameter and further retaining water.

5. Almost at the end of the tapering duct is a valve, approximate position marked ‘5’ on figure 1. Though discovered some time ago, the precise purpose of this valve is still under discussion. It is believed to assist in restarting and rejoining broken fibers acting much in the way of a helical pump, regulating the thickness of the fiber, and/ or clamping the fiber as a spider falls upon it. There is some discussion on the similarity of the silk worm’s silk press and the roles each of these valves play in the production of silk in these two organisms.

Throughout the process the unspun silk appears to have a nematic texture, in a similar manner to a liquid crystal
Liquid crystal
Liquid crystals are a state of matter that have properties between those of a conventional liquid and those of a solid crystal. For instance, an LC may flow like a liquid, but its molecules may be oriented in a crystal-like way. There are many different types of LC phases, which can be...

. This allows the unspun silk to flow through the duct as a liquid but maintain a molecular order.

As an example of a complex spinning field, the spinneret apparatus of an adult Araneus diadematus (garden cross spider) consists of the following glands:
  • 500 Glandulae piriformes for attachment points

  • 4 Glandulae ampullaceae for the web frame

  • about 300 Glandulae aciniformes for the outer lining of egg sacs, and for ensnaring prey

  • 4 Glandulae tubuliformes for egg sac silk

  • 4 Glandulae aggregatae for glue

  • 2 Glandulae coronatae for the thread of glue lines.

Artificial synthesis


In order to artificially synthesize spider silk into fibers, there are two broad areas that must be covered. These are synthesis of the feedstock (the unspun silk dope in spiders), and synthesis of the spinning conditions (the funnel, valve, tapering duct, and spigot). There have been a number of different approaches discussed below.

Feedstock


As discussed in the Structural section of the article, the molecular structure of unspun silk is both complex and extremely long. Though this endows the silk fibers with their desirable properties, it also makes replication of the fiber somewhat of a challenge. Various organisms have been used as a basis for attempts to replicate some components or all of some or all of the proteins involved. These proteins must then be extracted, purified and then spun before their properties can be tested. The table below shows the results including the true gold standard- actual stress and strain of the fibers as compared to the best spider dragline.
Organism Details Average Maximum breaking stress (MPa) Average Strain
Deformation (mechanics)
Deformation in continuum mechanics is the transformation of a body from a reference configuration to a current configuration. A configuration is a set containing the positions of all particles of the body...

 (%)
Reference
Gold Standard- Darwin’s bark spider (Caerostris darwini) Malagasy spider famed for making webs with strands up to 25m long across rivers. “...C. darwini silk is more than twice as tough as any previously described silk” 1850 +-350 33 +-0.08
Gold Standard- Nephila clavipes Typical golden orb weaving spider 710 -1200 18-27
Bombyx mori Silkworms Silkworms were genetically altered to express spider proteins and fibers measured. A little confusing, as it has been shown that when reeled silkworm fibers have comparable properties to spiders. 660 18.5
E. coli Synthesizing such a large and repetitive molecule (250-320 kDa) is complex. Yet, if this is not achieved, the properties will not match those of actual spiders. Here a 285 kDa protein was produced and spun 508 +-108 15 +-5
Goats Goats were genetically modified to secrete silk proteins in their milk, which could then be purified. Widely reported in the media but strangely no actual scientific paper of the process found. 285-250 30-40
Tobacco & potato plants Spider proteins were inserted into tobacco and potato plants, the rationale being that should this be successful, scaled-up harvesting would be much facilitated. Patents have been granted in this area, but no fibers have yet been described in the literature. n/a n/a

Geometry


As was shown in the biosynthesis section, spider silks with comparatively simple molecular structure need complex ducts to be able to spin an effective fiber. There have been a number of methods used to produce fibers, of which the main types are briefly discussed below.

Syringe & needle


Feedstock is simply forced through a hollow needle using a syringe . This method has been shown to make fibers successfully on multiple occasions.

Although very cheap and easy to assemble, the shape and conditions of the gland are very loosely approximated. Fibers created using this method may need encouragement to change from liquid to solid by removing the water from the fiber with such chemicals as the environmentally undesirable methanol
Methanol
Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...

 or acetone
Acetone
Acetone is the organic compound with the formula 2CO, a colorless, mobile, flammable liquid, the simplest example of the ketones.Acetone is miscible with water and serves as an important solvent in its own right, typically as the solvent of choice for cleaning purposes in the laboratory...

, and also may require post-stretching of the fibe to attain fibers with desirable properties

Microfluidics


As the field of microfluidics
Microfluidics
Microfluidics deals with the behavior, precise control and manipulation of fluids that are geometrically constrained to a small, typically sub-millimeter, scale.Typically, micro means one of the following features:* small volumes...

 matures, it is likely that more attempts to spin fibers will be made using microfluidics. These have the advantage of being very controllable and able to test spin very small volumes of unspun fiber but setup and development costs are likely to be high. A patent has been granted in this area for spinning fibers in a method mimicking the process found in nature, and fibers are successfully being continuously spun by a commercial company http://www.spintec-engineering.de/spintec-engineering.de/Home.html

Electrospinning


Electrospinning
Electrospinning
Electrospinning uses an electrical charge to draw very fine fibres from a liquid. Electrospinning shares characteristics of both electrospraying and conventional solution dry spinning of fibers. The process does not require the use of coagulation chemistry or high temperatures to produce solid...

 is a very old technique whereby a fluid is held in a container in a manner such that it is able to flow out through capillary action. A conducting substrate is positioned below, and a large difference in electrical potential is applied between the fluid and the substrate. The fluid is attracted to the substrate, and tiny fibers jump almost instantly from their point of emission, the Taylor cone
Taylor cone
A Taylor cone refers to the cone observed in electrospinning, electrospraying and hydrodynamic spray processes from which a jet of charged particles emanates above a threshold voltage...

, to the substrate, drying as they travel. This method has been shown to create nano-scale fibers from both silk dissected from organisms and regenerated silk fibroin.

Other artificial shapes formed from silk


Silk can be formed into other shapes and sizes such as spherical capsules for drug delivery, cell scaffolds and wound healing, textiles, cosmetics, coatings, and many others.

Research milestones


Due to spider silk being a scientific research field with a long and rich history, there can be unfortunate occurrences of researchers independently rediscovering previously published findings. What follows is a table of the discoveries made in each of the constituent areas, acknowledged by the scientific community as being relevant and significant by using the metric of scientific acceptance, citations. Thus, only papers with 50 or more citations are included.
Table of significant papers (50 or more citations)
Area of contribution Year Main researchers [Ref] Title of paper Contribution to the field
Chemical Basis 1960 Fischer, F. & Brander, J. “Eine Analyse der Gespinste der Kreuzspinne” (Amino acid composition analysis of spider silk)
1960 Lucas, F. & et al. “The Composition of Arthropod Silk Fibrons; Comparative studies of fibroins”
Gene Sequence 1990 Xu, M. & Lewis, R. V. “Structure of a Protein Superfiber - Spider Dragline Silk”
Mechanical Properties 1964 Lucas, F. “Spiders and their silks” First time compared mechanical properties of spider silk with other materials in a scientific paper.
1989 Vollrath, F. & Edmonds, D. T. “Modulation of the Mechanical Properties of Spider Silk by Coating with Water” First important paper suggesting the water interplay with spider silk fibroin modulating the properties of silk.
2001 Vollrath, F. & Shao, Z.Z. “The effect of spinning conditions on the mechanics of a spider's dragline silk”
Structural Characterization 1992 Hinman, M.B. & Lewis, R. V “Isolation of a clone encoding a second dragline silk fibroin. Nephila clavipes dragline silk is a two-protein fiber”
1994 Simmons, A. & et al. “Solid-State C-13 Nmr of Nephila-Clavipes Dragline Silk Establishes Structure and Identity of Crystalline Regions” First NMR study of spider silk.
1999 Shao, Z., Vollrath, F. & et al. “Analysis of spider silk in native and supercontracted states using Raman spectroscopy” First Raman study of spider silk.
1999 Riekel, C., Muller, M.& et al. “Aspects of X-ray diffraction on single spider fibers” First X-ray on single spider silk fibers.
2000 Knight, D.P., Vollrath, F. & et al. “Beta transition and stress-induced phase separation in the spinning of spider dragline silk” Secondary structural transition confirmation during spinning.
2001 Riekel, C. & Vollrath, F. “Spider silk fibre extrusion: combined wide- and small-angle X- ray microdiffraction experiments” First X-ray on spider silk dope.
2002 Van Beek, J. D. & et al. “The molecular structure of spider dragline silk: Folding and orientation of the protein backbone”
Structure-Property Relationship 1986 Gosline, G.M. & et al. “The structure and properties of spider silk” First attempt to link structure with properties of spider silk
1994 Termonia, Y “Molecular Modeling of Spider Silk Elasticity” X-ray evidence presented in this paper; simple model of crystallites embedded in amorphous regions.
1996 Simmons, A. & et al. “Molecular orientation and two-component nature of the crystalline fraction of spider dragline silk” Two types of alanine-rich crystalline regions were defined.
2006 Vollrath, F. & Porter, D. “Spider silk as an archetypal protein elastomer” New insight and model to spider silk based on Group Interaction Modelling.
Native Spinning 1991 Kerkam, K., Kaplan, D. & et al. “Liquid Crystallinity of Natural Silk Secretions”
1999 Knight, D.P. & Vollrath, F. “Liquid crystals and flow elongation in a spider's silk production line”
2001 Vollrath, F. & Knight, D.P. “Liquid crystalline spinning of spider silk” The most cited paper on spider silk
Reconstituted /Synthetic Spider Silk and Artificial Spinning 1995 Prince, J. T., Kaplan, D. L. & et al. “Construction, Cloning, and Expression of Synthetic Genes Encoding Spider Dragline Silk” First successful synthesis of Spider silk by E. coli.
1998 Arcidiacono, S., Kaplan, D.L. & et al. “Purification and characterization of recombinant spider silk expressed in Escherichia coli”
1998 Seidel, A., Jelinski, L.W. & et al. “Artificial Spinning of Spider Silk” First controlled wet-spinning of reconstituted spider silk.

Human uses


Peasants in the southern Carpathian Mountains
Carpathian Mountains
The Carpathian Mountains or Carpathians are a range of mountains forming an arc roughly long across Central and Eastern Europe, making them the second-longest mountain range in Europe...

 used to cut up tubes built by Atypus
Atypus
Atypus or Purse web spiders is a genus of mygalomorph spiders. It occurs in Eurasia, with one species reaching into North Africa, and one species in the USA. Only three of the described species occur in Europe: A. piceus, A. affinis and A...

 and cover wounds with the inner lining. It reportedly facilitated healing, and even connected with the skin. This is believed to be due to antiseptic properties of spider silk and because the silk is rich in vitamin K
Vitamin K
Vitamin K is a group of structurally similar, fat soluble vitamins that are needed for the posttranslational modification of certain proteins required for blood coagulation and in metabolic pathways in bone and other tissue. They are 2-methyl-1,4-naphthoquinone derivatives...

, which can be effective in clotting blood.

Some fishermen in the Indo-Pacific ocean use the web of Nephila to catch small fish.

The silk of Nephila clavipes
Nephila clavipes
Nephila clavipes is a species of golden orb-web spider. It lives in the warmer regions of the Americas. The large size and bright colours of the species make it distinctive...

 has recently been used to help in mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

ian neuron
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...

al regeneration.

At one time, it was common to use spider silk as a thread for crosshair
Crosshair
A reticle is a net of fine lines or fibers in the eyepiece of a sighting device, such as a telescope, a telescopic sight, a microscope, or the screen of an oscilloscope. The word reticle comes from the Latin "reticulum," meaning "net." Today, engraved lines or embedded fibers may be replaced by a...

s in optical instruments such as telescopes, microscopes, and telescopic rifle sights
Telescopic sight
A telescopic sight, commonly called a scope, is a sighting device that is based on an optical refracting telescope. They are equipped with some form of graphic image pattern mounted in an optically appropriate position in their optical system to give an accurate aiming point...

.

Due to the difficulties in extracting and processing substantial amounts of spider silk, there is currently only one known piece of cloth made of spider silk, an 11 by textile with a golden tint made in Madagascar
Madagascar
The Republic of Madagascar is an island country located in the Indian Ocean off the southeastern coast of Africa...

 in 2009. 82 people worked for four years to collect over one million golden orb spiders and extract silk from them.

In 2011, spider silk fibers were used in the field of optics to generate very fine diffraction patterns over N-slit interferometric signals
N-Slit interferometer
The N-slit interferometer is an extension of the double-slit interferometer also known as Young's double-slit interferometer. One of the first known uses of N-slit arrays in optics was illustrated by Newton...

 utilized in optical communications.

Recent attempts to develop materials with comparable properties to spider silk


Replicating the complex conditions in a laboratory environment required to produce fibers that are comparable to spider silk has proved difficult. What follows is a miscellaneous list of attempts on this problem, though, without providing hard data accepted by the relevant scientific community, it is difficult to judge whether these attempts have been successful or constructive.
  • One approach that does not involve farming spiders is to extract the spider silk gene
    Gene
    A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

     and use other organisms to produce the spider silk. In 2000, Canadian biotechnology
    Biotechnology
    Biotechnology is a field of applied biology that involves the use of living organisms and bioprocesses in engineering, technology, medicine and other fields requiring bioproducts. Biotechnology also utilizes these products for manufacturing purpose...

     company Nexia successfully produced spider silk protein in transgenic goat
    Goat
    The domestic goat is a subspecies of goat domesticated from the wild goat of southwest Asia and Eastern Europe. The goat is a member of the Bovidae family and is closely related to the sheep as both are in the goat-antelope subfamily Caprinae. There are over three hundred distinct breeds of...

    s that carried the gene for it; the milk produced by the goats contained significant quantities of the protein, 1–2 grams of silk proteins per liter of milk. Attempts to spin the protein into a fiber similar to natural spider silk resulted in fibers with tenacities of 2–3 grams per denier (see BioSteel
    BioSteel
    BioSteel is a trademark name for a high-strength based fiber material made of the recombinant spider silk-like protein extracted from the milk of transgenic goats, made by Nexia Biotechnologies....

    ).

Nexia used wet spinning and squeezed the silk protein solution through small extrusion holes in order to simulate the behavior of the spinneret, but this has so far not been sufficient to replicate the properties of native spider silk.
  • Extrusion of protein fibers in an aqueous environment is known as "wet-spinning". This process has so far produced silk fibers of diameters ranging from 10 to 60 μm, compared to diameters of 2.5–4 μm for natural spider silk.

  • In March 2010, researchers from the Korea Advanced Institute of Science & Technology (KAIST) have succeeded in making spider silk directly with the bacteria E.coli, modified with certain genes of the spider Nephila clavipes
    Nephila clavipes
    Nephila clavipes is a species of golden orb-web spider. It lives in the warmer regions of the Americas. The large size and bright colours of the species make it distinctive...

    . Thus, this eliminates dependency on the spider for milking and allows to manufacture the spider silk at a more cost-effective manner.

  • The company Kraig Biocraft Laboratories
    Kraig Biocraft Laboratories
    Kraig Biocraft Laboratories, Inc. is a biotechnology company focused on the commercialization of new textiles and high performance fibers including spider silks. Kraig Biocraft Laboratories was founded in April 2006 to develop and commercialize spider silks and other high performance polymers using...

     has used research from the Universities of Wyoming
    University of Wyoming
    The University of Wyoming is a land-grant university located in Laramie, Wyoming, situated on Wyoming's high Laramie Plains, at an elevation of 7,200 feet , between the Laramie and Snowy Range mountains. It is known as UW to people close to the university...

     and Notre Dame
    University of Notre Dame
    The University of Notre Dame du Lac is a Catholic research university located in Notre Dame, an unincorporated community north of the city of South Bend, in St. Joseph County, Indiana, United States...

     in a collaborative effort to create a silkworm that is genetically altered to produce spider silk. In September 2010 it was announced at a press conference at the University of Notre Dame that the effort had been successful.

See also

  • Hagfish
    Hagfish
    Hagfish, the clade Myxini , are eel-shaped slime-producing marine animals . They are the only living animals that have a skull but not a vertebral column. Along with lampreys, hagfish are jawless and are living fossils whose next nearest relatives include all vertebrates...

     – produces similar fiber.
  • Silk
    Silk
    Silk is a natural protein fiber, some forms of which can be woven into textiles. The best-known type of silk is obtained from the cocoons of the larvae of the mulberry silkworm Bombyx mori reared in captivity...

     – natural fiber produced by silkworms, the larvae of the moth Bombyx mori
    Bombyx mori
    The silkworm is the larva or caterpillar of the domesticated silkmoth, Bombyx mori . It is an economically important insect, being a primary producer of silk...

    .
  • "The Silk Spinners", a BBC program about silk-producing animals.

External links