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  • Writer's pictureArcadian Jones

Spider silk


Spider silk is a protein fibre spun by spiders. Spiders use their silk to make webs or other structures, which function as sticky nets to catch other animals, or as nests or cocoons to protect their offspring, or to wrap up prey. They can also use their silk to suspend themselves, to float through the air, or to glide away from predators. Most spiders vary the thickness and stickiness of their silk for different uses. In some cases, spiders may even use silk as a source of food.[1] While methods have been developed to collect silk from a spider by force, it is difficult to gather silk from many spiders compared to silk-spinning organisms such as silkworms. All spiders produce silk, and even in non-web building spiders, silk is intimately tied to courtship and mating. Silk produced by females provides a transmission channel for male vibratory courtship signals, while webs and draglines provide a substrate for female sex pheromones. Observations of male spiders producing silk during sexual interactions are also common across phylogenetically widespread taxa. However, the function of male-produced silk in mating has received very little study.

  • Schematic of the spider's orb web, structural modules, and spider silk structure. On the left is shown a schematic drawing of an orb web. The red lines represent the dragline, radial line, and frame lines, the blue lines represent the spiral line, and the centre of the orb web is called the “hub”. Sticky balls drawn in blue are made at equal intervals on the spiral line with viscous material secreted from the aggregate gland. Attachment cement secreted from the piriform gland is used to connect and fix different lines. Microscopically, the spider silk secondary structure is formed of spidroin and is said to have the structure shown on the right side. In the dragline and radial line, a crystalline β-sheet and an amorphous helical structure are interwoven. The large amount of β-spiral structure gives elastic properties to the capture part of the orb web. In the structural modules diagram, a microscopic structure of dragline and radial lines is shown, composed mainly of two proteins of MaSp1 and MaSp2, as shown in the upper central part. In the spiral line, there is no crystalline β-sheet region.

Non-protein composition[edit] 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 fibre. Biosynthesis and fibre spinning[edit] The production of silks, including spider silk, differs in an important aspect 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 fibres formed from the compacted faecal matter of beetles; it is "spun" on demand from liquid silk precursor out of specialised glands. The spinning process occurs when a fibre is pulled away from the body of a spider, whether by the spider's legs, by the spider's falling under its own weight, or by any other method including being pulled by humans. The term "spinning" is misleading because no rotation of any component occurs, but rather comes from analogy to the textile spinning wheels. Silk production is a pultrusion,[39] similar to extrusion, with the subtlety that the force is induced by pulling at the finished fibre rather than being squeezed out of a reservoir. The unspun silk fibre is pulled through silk glands of which there may be both numerous duplicates and different types of gland on any one spider species. Silk gland[edit] Schematic of the spiders spinning apparatus and structural hierarchy in silk assembling related to assembly into fibers. In the process of dragline production, the primary structure protein is secreted first from secretory granules in the tail. In the ampullate (neutral environment, pH = 7), the proteins form a soft micelle of several tens of nanometers by self-organization because the hydrophilic terminals are excluded. In ampullate, the concentration of the protein is very high. Then, the micelles are squeezed into the duct. The long axis direction of the molecules is aligned parallel to the duct by a mechanical frictional force and partially oriented. The continuous lowering of pH from 7.5 to 8.0 in the tail to presumably close to 5.0 occurs at the end of the duct. Ion exchange, acidification, and water removal all happen in the duct. The shear and elongational forces lead to phase separation. In the acidic bath of the duct, the molecules attain a high concentration liquid crystal state. Finally, the silk is spun from the taper exterior. The molecules become more stable helixes and β-sheets from the liquid crystal. The gland's visible, or external, part is termed the spinneret. Depending on the complexity of the species, spiders will have two to eight 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 golden silk orb-weavers. 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". Schematic of a generalised gland of a Golden silk orb-weaver. Each differently coloured section highlights a discrete section of the gland. Gland characteristics

  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 fibre, hypothesised to assist in preventing crack formation or even self-healing of the fibre.

  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 fibre.

  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 fibre 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 solid silk fibre emerges. The tube here tapers hyperbolically, therefore the unspun silk is under constant elongational shear stress, which is an important factor in fibre formation. This section of the duct is lined with cells that exchange ions, reduce the dope pH from neutral to acidic, and remove water from the fibre. Collectively, the shear stress and the ion and pH changes induce the liquid silk dope to undergo a phase transition and condense into a solid protein fibre with high molecular organisation. The spigot at the end has lips that clamp around the fibre, controlling fibre 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 fibres, acting much in the way of a helical pump, regulating the thickness of the fibre, and/or clamping the fibre as a spider falls upon it. There is some discussion of 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, arising in part due to the extremely high protein concentration of silk dope (around 30% in terms of weight per volume). 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 glands shown below. Similar multiple gland architecture exists in the black widow spider.

  • 500 pyriform glands for attachment points

  • 4 ampullate glands for the web frame

  • about 300 aciniform glands for the outer lining of egg sacs, and for ensnaring prey

  • 4 tubuliform glands for egg sac silk

  • 4 aggregate glands for adhesive functions

  • 2 coronate glands for the thread of adhesion lines

Artificial synthesis[edit] Single strand of artificial spider silk produced under laboratory conditions To artificially synthesise spider silk into fibres, 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 but few of these methods have produced silk that can efficiently be synthesised into fibres. Feedstock[edit] The molecular structure of unspun silk is both complex and extremely long. Though this endows the silk fibres with their desirable properties, it also makes replication of the fibre 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. OrganismDetailsAverage Maximum breaking stress (MPa)Average Strain (%)ReferenceDarwin's bark spider (Caerostris darwini)Malagasy spider famed for making webs with strands up to 25 m long, across rivers. "C. darwini silk is more than twice as tough as any previously described silk"1850 ±35033 ±0.08[21]Nephila clavipesTypical golden orb weaving spider710–120018–27 Bombyx mori SilkwormsSilkworms were genetically altered to express spider proteins and fibres measured. E. coliSynthesising a large and repetitive molecule (~300 kDa) is complex, but required for the strongest silk. Here E. coli was engineered to produce a 556 kDa protein. Fibers spun from these synthetic spidroins are the first to fully replicate the mechanical performance of natural spider silk by all common metrics.1030 ±11018 ±6 Goats.Goats were genetically modified to secrete silk proteins in their milk, which could then be purified.285–25030–40 Tobacco & potato plantsTobacco and potato plants were genetically modified to produce silk proteins. Patents were granted,[69] but no fibres have yet been described in the literature.n/an/a Geometry Spider silks with comparatively simple molecular structure need complex ducts to be able to spin an effective fibre. There have been a number of methods used to produce fibres, of which the main types are briefly discussed below. Syringe and needle Feedstock is simply forced through a hollow needle using a syringe. This method has been shown to make fibres successfully on multiple occasions. Although very cheap and easy to produce, the shape and conditions of the gland are very loosely approximated. Fibres created using this method may need encouragement to change from liquid to solid by removing the water from the fibre with such chemicals as the environmentally undesirable methanol or acetone, and also may require post-stretching of the fibre to attain fibres with desirable properties. Microfluidics[edit] As the field of microfluidics matures, it is likely that more attempts to spin fibres will be made using microfluidics. These have the advantage of being very controllable and able to test spin very small volumes of unspun fibre but setup and development costs are likely to be high. A patent has been granted in this area for spinning fibres in a method mimicking the process found in nature, and fibres are successfully being continuously spun by a commercial company. Electrospinning Electrospinning 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 fibres jump almost instantly from their point of emission, the Taylor cone, to the substrate, drying as they travel. This method has been shown to create nano-scale fibres from both silk dissected from organisms and regenerated silk fibroin. Other artificial shapes formed from silk[edit] 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.[78][79] Spider silk proteins can also self-assemble on superhydrophobic surfaces to generate nanowires, as well as micron-sized circular sheets. It has recently been shown that recombinant spider silk proteins can self-assemble at the liquid air interface of a standing solution to form protein permeable, super strong, and super flexible membranes that support cell proliferation. Suggested applications include skin transplants, and supportive membranes in organ-on-a-chip. 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. Peasants in the southern Carpathian Mountains used to cut up tubes built by Atypus 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, which can be effective in clotting blood. Due to the difficulties in extracting and processing substantial amounts of spider silk, the largest known piece of cloth made of spider silk is an 11-by-4-foot (3.4 by 1.2 m) textile with a golden tint made in Madagascar in 2009. Eighty-two people worked for four years to collect over one million golden orb spiders and extract silk from them. The silk of Nephila clavipes was used in research concerning mammalian neuronal regeneration. Spider silk has been used as a thread for crosshairs in optical instruments such as telescopes, microscopes, and telescopic rifle sights. In 2011, spider silk fibres were used in the field of optics to generate very fine diffraction patterns over N-slit interferometric signals used in optical communications. In 2012, spider silk fibres were used to create a set of violin strings. Development of methods to mass-produce spider silk has led to manufacturing of military, medical and consumer goods, such as ballistics armour, athletic footwear, personal care products, breast implant and catheter coatings, mechanical insulin pumps, fashion clothing, and outerwear. Spider silk is used to suspend inertial confinement fusion targets during laser ignition, as it remains considerably elastic and has a high energy to break at temperatures as low as 10–20 K. In addition, it is made from "light" atomic number elements that won't emit x-rays during irradiation that could preheat the target so that the pressure differential required for fusion is not achieved. Spider silk has been used to create biolenses that could be used in conjunction with lasers to create high-resolution images of the inside of the human body. Attempts at producing synthetic spider silk. Proposed framework for producing artificial skin from spider silk to help patients with burns. Replicating the complex conditions required to produce fibres that are comparable to spider silk has proven difficult in research and early-stage manufacturing. Through genetic engineering, Escherichia coli bacteria, yeasts, plants, silkworms, and animals other than silkworms have been used to produce spider silk proteins, which have different, simpler characteristics than those from a spider. Extrusion of protein fibres in an aqueous environment is known as "wet-spinning". This process has so far produced silk fibres of diameters ranging from 10 to 60 μm, compared to diameters of 2.5–4 μm for natural spider silk. Artificial spider silks have fewer and simpler proteins than natural dragline silk, and are consequently half the diameter, strength, and flexibility of natural dragline silk.

  • In March 2010, researchers from the Korea Advanced Institute of Science & Technology succeeded in making spider silk directly using the bacteria E. coli, modified with certain genes of the spider Nephila clavipes. This approach eliminates the need to milk spiders and allows the manufacture of the spider silk in a more cost-effective manner.

  • A 556 kDa spider silk protein was manufactured from 192 repeat motifs of the Nephila clavipes dragline spidroin, having similar mechanical characteristics as their natural counterparts, i.e., tensile strength (1.03 ± 0.11 GPa), modulus (13.7 ± 3.0 GPa), extensibility (18 ± 6%), and toughness (114 ± 51 MJ/m3).[67]

  • The company AMSilk developed spidroin using bacteria, making it into an artificial spider silk.

  • The company Bolt Threads produces a recombinant spidroin using yeast, for use in apparel fibers and personal care. They produced the first commercial apparel products made of recombinant spider silk, trademarked Microsilk™, demonstrated in ties and beanies. They have also partnered with vegan activist and luxury designer Stella McCartney as well as Adidas to produce Microsilk™ garments.[114][115]

  • The company Kraig Biocraft Laboratories used research from the Universities of Wyoming and Notre Dame to create silkworms that were genetically altered to produce spider silk.

  • The now defunct Canadian biotechnology company Nexia successfully produced spider silk protein in transgenic goats 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 litre of milk. Attempts to spin the protein into a fibre similar to natural spider silk resulted in fibres with tenacities of 2–3 grams per denier.[118] 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 procedure was not sufficient to replicate the stronger properties of native spider silk.

  • The company Spiber has produced a synthetic spider silk that they are calling Q/QMONOS. In partnership with Goldwin, a ski parka made from this synthetic spider silk is currently in testing and is to be in mass production soon for less than $120,000 YEN.

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