Паучий шёлк: структура, свойства и функции

Protein fiber made by spiders

Spider silk is a protein fibre or silk spun by spiders. Spiders use silk to make webs or other structures that function as adhesive traps to catch prey, to entangle and restrain prey before biting, to transmit tactile information, or as nests or cocoons to protect their offspring. They can use the silk to suspend themselves from height, to float through the air, or to glide away from predators. Most spiders vary the thickness and adhesiveness of their silk according to its use. In some cases, spiders may use silk as a food source. While methods have been developed to collect silk from a spider by force, gathering silk from many spiders is more difficult than from silk spinning organisms such as silkworms. All spiders produce silk, although some spiders do not make webs. Silk is 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 common across widespread taxa. The function of male produced silk in mating has received little study.

Silks have a hierarchical structure. The primary structure is the amino acid sequence of its proteins (spidroin), mainly consisting of highly repetitive glycine and alanine blocks, which is why silks are often referred to as a block co polymer. On a secondary level, the short side chained alanine is mainly found in the crystalline domains (beta sheets) of the nanofibril. Glycine is mostly found in the so called amorphous matrix consisting of helical and beta turn structures. The interplay between the hard crystalline segments and the strained elastic semi amorphous regions gives spider silk its extraordinary properties. Various compounds other than protein are used to enhance the fibre's properties. Pyrrolidine has hygroscopic properties that keep the silk moist while warding off ant invasion. It occurs in high concentration in glue threads. Potassium hydrogen phosphate releases hydrogen ions in aqueous solution, resulting in a pH of about 4, making the silk acidic and thus protecting it from fungi and bacteria that would otherwise digest the protein. Potassium nitrate is believed to prevent the protein from denaturing in the acidic milieu. Termonia introduced this first basic model of silk in 1994. He suggested crystallites embedded in an amorphous matrix interlinked with hydrogen bonds. Refinements to this model include: semi crystalline regions were found later visualised by AFM and TEM. Sizes of the nanofibrillar structure and the crystalline and semi crystalline regions were revealed by neutron scattering. The fibres' microstructural information and macroscopic mechanical properties are related. Ordered regions (i) mainly reorient by deformation for low stretched fibres and (ii) the fraction of ordered regions increases progressively for higher fibre stretching.

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 and extensibility (ductility). This enables a silk fibre to absorb a large amount of energy before breaking (toughness, the area under a stress strain curve). Strength and toughness are distinct quantities. Weight for weight, silk is stronger than steel, but not as strong as Kevlar. Spider silk is, however, tougher than both. The variability of spider silk fibre mechanical properties is related to their degree of molecular alignment. Mechanical properties also depend on ambient conditions, i. e. humidity and temperature.

A dragline silk's tensile strength is comparable to that of high grade alloy steel (450−2000 MPa), and about half as strong as aramid filaments, such as Twaron or Kevlar (3000 MPa).

Consisting of mainly protein, silks are about a sixth of the density of steel (1.3 g/cm3). As a result, a strand long enough to circle the Earth would weigh about (Spider dragline silk has a tensile strength of roughly 1.3 GPa. The tensile strength listed for steel might be slightly highere. g. 1.65 GPa, but spider silk is a much less dense material, so that a given weight of spider silk is five times as strong as the same weight of steel.)

The energy density of dragline spider silk is roughly 1.2.

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

The combination of strength and ductility gives dragline silks a high toughness (or work to fracture), which "equals that of commercial polyaramid (aromatic nylon) filaments, which themselves are benchmarks of modern polymer fibre technology".

While unlikely to be relevant in nature, dragline silks can hold their strength below 40 °C ( 40 °F) and up to 220 °C (428 °F). As occurs in many materials, spider silk fibres undergo a glass transition. The glass transition temperature depends on humidity, as water is a plasticiser for spider silk.

To artificially synthesise spider silk into fibres, two broad tasks are required. These are synthesis of the feedstock (the unspun silk dope in spiders), and synthesis of the production conditions (the funnel, valve, tapering duct, and spigot). Few strategies have produced silk that can efficiently be synthesised into fibres.

The molecular structure of unspun silk is both complex and long. Though this endows the fibres with desirable properties, it also complicates replication. Various organisms have been used as a basis for attempts to replicate necessary protein components. These proteins must then be extracted, purified, and then spun before their properties can be tested. + Organism Details Average Maximum breaking stress (MPa) Average Strain (%) Reference Darwin's bark spider (Caerostris darwini) Malagasy spider famed for making 25 m long strands. "C. darwini silk is more than twice as tough as any previously described silk" 1850 ±350 33 ±0.08 Bombyx mori Silkworms Silkworms genetically altered to express spider proteins and fibres. 660 18.5 Escherichia coli Synthesising 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 ±110 18 ±6 Goats Genetically modified to secrete silk proteins in their milk. 285–250 30–40 Tobacco & potato plants Genetically modified to produce silk proteins. Patents were granted in 2010, but no fibres have yet been described. n/a n/a

Spider silks with comparatively simple molecular structure need complex ducts to be able to form an effective fibre. Approaches:

Feedstock is forced through a hollow needle using a syringe. Although cheap and easy to produce, gland shape and conditions are loosely approximated. Fibres created using this method may need encouragement to solidify by removing water from the fibre with chemicals such as (environmentally undesirable) methanol or acetone,

Self assembly of silk at standing liquid gas interphases of a solution tough and strong sheets. These sheets are now explored for mimicking the basal membrane in tissue modeling.

Microfluidics have the advantage of being controllable and able to test spin small volumes of unspun fibre, but setup and development costs are high. A patent has been granted and continuously spun fibres have achieved commercial use.

Electrospinning is an old technique whereby a fluid is held in a container such that it flows out through capillary action. A conducting substrate is positioned below, and a difference in electrical potential is applied between the fluid and the substrate. The fluid is attracted to the substrate, and tiny fibres jump from their point of emission, the Taylor cone, to the substrate, drying as they travel. This method creates nano scale fibres from silk dissected from organisms and regenerated silk fibroin.

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. Spider silk proteins can self assemble on superhydrophobic surfaces into nanowires, as well as micron sized circular sheets. These nanomembranes have been used to create a static in vitro model of a blood vessel.

Replicating the complex conditions required to produce comparable fibres has challenged research and early stage manufacturing. Through genetic engineering, E. coli bacteria, yeasts, plants, silkworms, and animals other than silkworms have been used to produce spider silk like proteins, which have different characteristics than those from a spider. A 556 kDa spider silk protein was manufactured from 192 repeat motifs of the N. 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). Bolt Threads produced 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. Kraig Biocraft Laboratories used research from the Universities of Wyoming and Notre Dame to create silkworms genetically altered to produce spider silk. Defunct Canadian biotechnology company Nexia produced spider silk protein in transgenic goats; 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. Nexia used wet spinning and squeezed the silk protein solution through small extrusion holes to simulate the spinneret, but this was not sufficient to replicate native spider silk properties. Spiber produced a synthetic spider silk (Q/QMONOS). In partnership with Goldwin, a ski parka made from this was in testing in 2016. Researchers from Japan's RIKEN Center constructed an artificial gland that reproduced spider silk's molecular structure. Precise microfluidic mechanisms directed proteins to self assemble into functional fibers. The process used negative pressure to pull (rather than push) a spidroin solution through the device. The resulting fibers matched the hierarchical structure of natural fiber.

+ Significant papers (50 or more citations) Area of contribution Year Main researchers Title 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 Fibroins; 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 mechanical properties of spider silk compared 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 silk properties. 2001 Vollrath, F. & Shao, Z. Z. "The effect of spinning conditions on the mechanics of a spider's dragline silk" 2006 Plaza, G. R., Guinea, G. V., Pérez Rigueiro, J. & Elices, M. "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 fibres. 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 structure and properties of spider silk" First attempt to link structure with properties of spider silk 1994 Termonia, Y "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. "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.

The earliest recorded attempt to weave fabric from spider silk was in 1709 by François Xavier Bon who, using a process similar to creating silkworm silk, wove silk derived spider's egg cocoons into stockings and gloves. Fifty years later Jesuit missionary Ramón María Termeyer, invented a reeling device for harvesting spider silk directly from spiders, allowing it to be spun into threads. Neither Bon nor Termeyer were successful in producing commercially viable quantities. The development of methods to mass produce spider silk led to the manufacturing of military, medical, and consumer goods, such as ballistic armour, athletic footwear, personal care products, breast implant and catheter coatings, mechanical insulin pumps, fashion clothing, and outerwear. However, due to the difficulties in extracting and processing, the largest known piece of cloth made of spider silk is an 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. In 2012, spider silk fibres were used to create a set of violin strings.

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 connected with the skin. This is believed to be due to the silk's antiseptic properties, and because silk is rich in vitamin K, which can aid in clotting blood. N. clavipes silk 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, silk fibres were used to generate fine diffraction patterns over N slit interferometric signals used in optical communications. 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. Silk has been 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 emit no x rays during irradiation that could preheat the target, limiting the pressure differential required for fusion.