Spider silk
From Wikipedia, the free encyclopedia
Spider silk is a fibre secreted by spiders. Spider silk is a remarkably strong material; the strongest naturally-occurring fiber known[citation needed]. Its tensile strength is comparable to that of high-grade steel — according to Nature (see reference below), spider silk has a tensile strength of roughly 1.3 GPa, while one source [1] lists a tensile strength for one form of steel at 1.65 GPa. However, spider silk is much less dense than steel; its ratio of tensile strength to density is perhaps 5 times better than steel — as strong as aromatic nylon filaments, such as DuPont's Kevlar. In fact, a strand of spider silk long enough to circle the earth would weigh less than 16 ounces[citation needed].
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[edit] Usage
Spiders normally use their silk to make structures, either for protection for their offspring, or for predation on other creatures. They can also suspend themselves using their silk, normally for the same reasons.
The Trapdoor spider will burrow into the ground and weave a trapdoor-like structure with spindles around so it can tell when prey arrives and take it by surprise.
Many small spiders use silk threads for ballooning. They extrude several threads into the air and let themselves become carried away with upward winds. Although most rides will end a few meters 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.
[edit] Properties
Spider silk is also especially ductile, able to stretch up to 40% of its length without breaking. This gives it a very high toughness (or work to fracture), which according to "Liquid crystalline spinning of spider silk" (Nature, vol 410, p. 541), "equals that of commercial polyaramid (aromatic nylon) filaments, which themselves are benchmarks of modern polymer fiber technology." The notion that spider silk is stronger than any other fiber now known is thus erroneous, especially considering current research with carbon nanotubes that have yielded stronger fibers. Nonetheless, there is much interest in duplicating the silk process artificially, since spiders use renewable materials as input and operate at room temperature and low pressure. Spider silk can be harvested in large scale quantities if one has proper harvesting equipment. One can also make near indestructible spidersilk threads by weaving the fine threads into thicker and more durable ones in the same fashion as industrial threads are made today.
Spider silk is made of complex protein molecules. This, coupled with the spider's preference—as a predatory animal—for isolation from other species, has made the study and replication of this substance quite challenging. Because of the repetitive nature of the DNA encoding the silk protein, it is difficult to determine its sequence, and the silk from only 14 species has been decoded. As of 2001 ten such sequences have been completed through a collaboration between the University of California at Riverside and the University of Wyoming. In 2005 two biology researchers from the University of California at Riverside, Jessica Garb and Cheryl Hayashi, uncovered the molecular structure of the gene for the protein that female spiders use to make their silken egg cases.
Although different species of spider, and different types of silk, have different protein sequences, a general trend in spider silk structure is a sequence of amino acids (usually alternating glycine and alanine, or alanine alone) that self-assemble into a beta sheet conformation. These "Ala rich" blocks are separated by segments of amino acids with bulky side-groups. The beta sheets stack to form crystals, whereas the other segments form amorphous domains. It is the interplay between the hard crystalline segments, and the elastic amorphous regions, that gives spider silk its extraordinary properties.
[edit] Synthesis
The thread is released through silk glands. Many species of spider have different glands for different jobs, such as housing and web construction, defense, capturing and detaining prey, mobility and in extreme cases even as food.[citation needed] Thus, the silk needs to be specialized for the task at hand so success is guaranteed.
The gland's visible, or external, part is termed the spinneret. Depending on the species, spiders will have anything from two to eight spinnerets, usually in pairs. The beginning of the gland is rich in thiol and tyrosine groups, the main ingredient to silk fiber. After this beginning process, the ampulla acts as a storage sac for the newly created fibers. From there, the spinning duct effectively removes water from the fiber and through fine channels also assists in its formation. Lipid secretions take place just at the end of the distal limb of the duct, and proceeds to the valve. The valve is believed to assist in rejoining broken fibers, acting much in the way of a helical pump.
Various compounds other than protein are used to enhance the fiber's properties. Pyrrolidine has hygroscopic properties and helps to keep the thread moist. It occurs in especially high concentration in glue threads. Potassium hydrogen phosphate releases protons in aqueous solution, resulting in a pH of about 4, making the silk acidic and thus protecting it from fungus and bacteria that would otherwise digest the protein. Potassium nitrate is believed to prevent the protein from denaturating in the acidic milieu.[1]
The spinneret apparatus of a Araneus diadematus consists of the following glands:
- 500 Glandulae piriformes for attachment points
- 4 Glandulae ampullaceae for the web frame
- ca. 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[2]
[edit] Human use
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 (which is made of protein)[1] Some fishermen in the indopacific ocean use the web of Nephila to catch small fish.[1]
[edit] Artificial spider silk
Spider silk's properties have made it the target of industrial research efforts. It is not generally considered possible to use spiders themselves to produce industrially useful quantities of spider silk, due to the difficulties of managing large quantities of small spiders (although it was tried with Nephila silk[1]). It is thought that using one large genetically engineered "super-spider" would be more practical. Compared to silkworms, spiders are aggressive and will eat one another, making it impossible to keep many small spiders together. Other efforts have involved extracting the spider silk gene and using other organisms to produce the required amount of spider silk. In 2000, Nexia, a Canadian biotechnology company, was successful in producing spider silk protein in transgenic goats. These goats carried the gene for spider silk protein, and the milk produced by the goats contained significant quantities of the protein. Attempts to spin the protein into a fiber similar to natural spider silk failed, however. The spider's highly sophisticated spinneret is instrumental in organizing the silk proteins into strong domains. Specifically, the spinneret creates a gradient of protein concentration, pH, and pressure, which drive the protein solution through liquid crystalline phase transitions, ultimately generating the required silk structure (which is a mixture of crystalline and amorphous biopolymer regions). Replicating these complex conditions in lab environment has proved difficult. Nexia attempted to press the pherotein solution through small extrusion holes in order to simulate the behavior of the spinneret, but this was insufficient to properly organize the fibers. Ultimately, Nexia was forced to abandon research on artificial spider silk, despite having successfully created the silk protein in genetically modified organisms.
[edit] See also
- Hagfish - produces similar fiber.
[edit] References
- ^ a b c d Heimer, S. (1988). Wunderbare Welt der Spinnen. Urania. p.12
- ^ Heimer, S. (1988). Wunderbare Welt der Spinnen. Urania p.12
- Forbes, Peter (4th Estate, London 2005) 'The Gecko's Foot - Bio Inspiration: Engineered from Nature' ISBN 0007179901 in H/B
[edit] External links
- The Silk Gland - A very nice breakdown of the silk gland, its parts and uses with images and drawings.
- Spiders in Space - NASA article and database information on the research of spiders in space.
- Spider Silk Analysis - Detailed research based investigation into spider silk, considering properties, structure and uses. Diagrams and images included.
- Production of spider silk proteins in tobacco and potato - Article on nature.com
- Israeli and German scientists created artificial silk using genetically engineered spider proteins - Article on IsraCast
- The mechanical design of spider silks: from fibroin sequence to mechanical function - Article on The Journal of Experimental Biology
- The Real Spider-Man - Article on forming Spider Silk Fibers from Caterpillars
- Genetic tweak boosts stiffness of spider silk
- Silk & Webs - The Arachnology Home Page
