- Spider silk
Spider silk, also known as
gossamer, is a protein fiberspun by spiders. Spiders use their silk to make webs or other structures, which function as nets to catch other creatures, or as nests or cocoons for protection for their offspring. They can also suspend themselves using their silk, normally for the same reasons.
Many small spiders use silk threads for ballooning, the scientific term for the dynamic kiting [ [http://books.google.com/books?id=Gvrt2eIVG7YC&pg=PT24&lpg=PT24&dq=spider+ballooning+is+really+kiting&source=web&ots=wHpCpxCy3r&sig=srQgV7Hf_SzOfhOSjrKvI9YKdwg&hl=en Spiders By Ann R. Heinrichs. Google Books. She observes that the so called ballooning is like a kite or balloon; she is mechanically correct about the kite part, as no true balloon is ever formed by the spider as told in the other references.] ] [ [http://www.snerdey.com/sky/index.html Flying Spiders over Texas! Coast to Coast. Chad B., Texas State University Undergrad: He correctly describes the mechanical kiting of spider "ballooning".] ] spiderlings (mostly) use for dispersal. 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.
In some cases, spiders may even use silk as a source of
food. [cite web |url=http://www.chm.bris.ac.uk/motm/spider/page2h.htm |title=Spider Silk |accessdate=2007-05-22 |format= |work= |publisher= School of Chemistry - Bristol University - UK] [cite journal |last=Miyashita |first=Tadashi |authorlink= |coauthors=Yasunori Maezono, Aya Shimazaki |year=2004 |month=March |title= Silk feeding as an alternative foraging tactic in a kleptoparasitic spider under seasonally changing environments |journal=Journal of Zoology |volume=262 |issue=03 |pages=225–229 |doi =10.1017/S0952836903004540 |id= |url=http://journals.cambridge.org/action/displayAbstract;jsessionid=E5F9929D3300E3C4E54DF68FB169AD62.tomcat1?fromPage=online&aid=204739 |accessdate=2007-05-22 |quote= ]
Types of silk
speciesof spiderhave different glands for different jobs, such as housing and web construction, defense, capturing and detaining prey, or mobility. Thus, different specialized silks have evolved with material properties optimized for their intended use. For example, " Argiope argentata" has five different types of silk, each for a different purpose: [Cunningham, A. (2007), Taken for a Spin. "Science News" vol. 171, pp. 231-233] [Blackledge, T.A., and Hayashi, C.Y. (2006). Silken toolkits: Biomechanics of silk fibers spun by the orb web spider Argiope argentata. "Journal of Experimental Biology" 209(July 1), pp. 2452-2461 ( [http://www.sciencenews.org/articles/20070414/bob8.asp references] )]
* dragline silk: Used for the web's outer rim and spokes, as well as for the
lifeline. As strong as steel, but much tougher.
* capture-spiral silk: Used for the capturing lines of the web. Sticky, extremely stretchy and tough.
* tubiliform 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
silkis a remarkably strong material. Its tensile strengthis superior to that of high-grade steel, and as strong as Aramidfilaments, such as Twaronor Kevlar. Most importantly, spider silk is extremely lightweight: a strand of spider silk long enough to circle the earth would weigh less than 16 ounces (450 g). [Spider dragline silk has a tensile strength of roughly 1.3 GPa. The tensile strength listed for steel might be slightly higher - e.g. 1.65 GPa. [http://www.geocities.com/pganio/materials.html] cite journal
author= Shao, Z. Vollrath, F.
August 15 2002
title=Materials: Surprising strength of silkworm silk
doi=10.1038/418741a , 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.]
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 "equals that of commercial polyaramid (aromatic nylon) filaments, which themselves are benchmarks of modern polymer fiber technology." [cite journal
author= Vollrath, F. Knight, D.P.
title=Liquid crystalline spinning of spider silk
Spider silk is composed of complex
protein molecules. This, coupled with the isolation stemming from the spider's predatory nature, has made the study and replication of the substance quite challenging. Because of the repetitive nature of the DNAencoding the silk protein, it is difficult to determine its sequence and to date, silk-producing sequences have only been decoded for fourteen speciesof spider. In 2005, independent researchers in the University of Wyoming(Tian and Lewis), University of the Pacific (Hu and Vierra), the University of California at Riverside (Garb and Hayashi) and Shinshu University(Zhao and Nakagaki) have uncovered the molecular structure of the gene for the protein that various female spider species 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
glycineand alanine, or alanine alone) that self-assemble into a beta sheetconformation. 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 amorphousdomains. It is the interplay between the hard crystalline segments, and the elastic semi amorphous regions, that gives spider silk its extraordinary properties.
Various compounds other than protein are used to enhance the fiber's properties.
Pyrrolidinehas hygroscopicproperties and helps to keep the thread moist. It occurs in especially high concentration in glue threads. Potassium hydrogen phosphatereleases protons in aqueous solution, resulting in a pHof about 4, making the silk acidic and thus protecting it from fungusand bacteriathat would otherwise digest the protein. Potassium nitrateis believed to prevent the protein from denaturating in the acidic milieu.Heimer, S. (1988). Wunderbare Welt der Spinnen. "Urania". p.12]
The unspun silk dope is pulled through silk
glands, resulting in a transition from stored gel to final solid fiber.
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 thioland tyrosinegroups. After this beginning process, the ampullaacts as a storage sac for the newly created fibers. From there, the spinning ducteffectively removes waterfrom the fiber and through fine channels also assists in its formation. Lipidsecretions 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.
The spinneret apparatus of a "
Araneus diadematus" consists of the following glands:
Glandulae piriformesfor attachment points
Glandulae ampullaceaefor the web frame
* about 300
Glandulae aciniformesfor the outer lining of egg sacs, and for ensnaring prey
Glandulae tubuliformesfor egg sac silk
Glandulae aggregataefor glue
Glandulae coronataefor the thread of glue lines
Peasants in the southern
Carpathian Mountainsused 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 silkHeimer, S. (1988). Wunderbare Welt der Spinnen. "Urania". p.14]
The silk of "
Nephila clavipes" has recently been used to help in mammalian neuronal regeneration. [Allmeling, C., Jokuszies, A., Reimers, K., Kall, S., Vogt, P.M. (2006): Use of spider silk fibres as an innovative material in a biocompatible artificial nerve conduit. "J. Cell. Mol. Med." 10(3):770-777 [http://www.jcmm.ro/download/jcmm010.003.18.pdf PDF] - DOI|10.2755/jcmm010.003.18]
At one time, it was common to use spider silk as a thread for
crosshairs in telescopes, microscopes and similar optical instruments. [Berenbaum, May R., "Field Notes - Spin Control", The Sciences, The New York Academy Of Sciences, September/October 1995]
Artificial spider silk
Spider silk is as strong as many industrial fibers (see
tensile strengthfor common comparisons). There is commercial interest in duplicating spider silk artificially, since spiders use renewable materials as input and operate at room temperature, low pressures and using water as a solvent. However, it has been difficult to find a commercially viable process to mass-produce spider silk.
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 this was tried with "
Nephila" silk). Compared with silkworms, spiders are aggressive and will eat one another, making it inadvisable to keep many spiders together in the same space. Other efforts have involved extracting the spider silk geneand using other organisms to produce the required amount of spider silk. In 2000, Nexia, a Canadian biotechnologycompany, was successful in producing spider silk proteinin transgenic goats. These goats carried the gene for spider silk protein, and the milk produced by the goats contained significant quantities of the protein (1-2 grams of silk proteins / 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/denier, termed "biosteel". [http://www.nexiabiotech.com/en/00_home/index.php] [http://www.sciencedirect.com.libproxy.lib.unc.edu/science?_ob=ArticleURL&_udi=B6TCW-4S4J6G0-3&_user=130907&_coverDate=05%2F31%2F2008&_rdoc=5&_fmt=high&_orig=browse&_srch=doc-info(%23toc%235181%232008%23999739994%23686677%23FLA%23display%23Volume)&_cdi=5181&_sort=d&_docanchor=&_ct=9&_acct=C000004198&_version=1&_urlVersion=0&_userid=130907&md5=4e943bf9f8b13ee1aca065ebce361f56]
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 10-60 μm, compared to diameters of 2.5-4 μm seen in natural spider silk.
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 biopolymerregions). Replicating these complex conditions in lab environment has proved difficult. 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 exact properties of the native spider silk. [Scheibel, T. (2004): Spider silks: recombinant synthesis, assembly, spinning, and engineering of synthetic proteins. "Microb Cell Fact" 3:14 [http://www.microbialcellfactories.com/content/3/1/14] ]
Hagfish- produces similar fiber.
Silk- natural fibre produced by silkworms, the larvae of the moth " Bombyx mori".
*Forbes, Peter (4th Estate, London 2005). "The Gecko's Foot - Bio Inspiration: Engineered from Nature", ISBN 0-00-717990-1 in H/B
*Graciela C. Candelas, José Cintron. "A spider fibroin and its synthesis", Journal of Experimental Zoology (1981), Department of Biology, University of Puerto Rico, Río Piedras, Puerto Rico 00931
* [http://hubcap.clemson.edu/~ellisom/biomimeticmaterials/files/spiderbiology.htm The Silk Gland] - A very nice breakdown of the silk gland, its parts and uses with images and drawings.
* [http://lsda.jsc.nasa.gov/scripts/experiment/exper.cfm?exp_index=428 Spiders in Space] -
NASAarticle and databaseinformation on the research of spiders in space.
* [http://www.isracast.com/tech_news/271204_tech.htm Israeli and German scientists created artificial silk using genetically engineered spider proteins] - Article on IsraCast
* [http://jeb.biologists.org/cgi/reprint/202/23/3295.pdf The mechanical design of spider silks: from fibroin sequence to mechanical function] - Article on The Journal of Experimental Biology
* [http://www.livescience.com/technology/041129_spider_man.html The Real Spider-Man] - Article on forming Spider Silk Fibers from Caterpillars
* [http://www.newscientisttech.com/article.ns?id=dn9316&feedId=online-news_rss20 Genetic tweak boosts stiffness of spider silk]
* [http://www.arachnology.be/pages/Silk.html Silk & Webs - The Arachnology Home Page]
* [http://www.oxfordsilkgroup.com Silk Research Group at Oxford University]
* [http://www.tms.org/pubs/journals/JOM/0502/Elices-0502.html Finding Inspiration in Spider Silk Fibers]
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