Spider silk is a fibre secreted by spiders. Spider silk is a remarkably strong material. 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.
Properties of spider silk
Spider silk is also especially elastic, 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 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 Riverside, Jessica Garb and Cheryl Hiyashi, 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.
How does the spider make the silk?
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. 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 any number of spinnerets, usually in pairs. The beginning of the gland is rich in sulfhydryl 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.
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 spiders. Unlike silkworms, spiders are aggressive and will eat one another, making it impossible to keep many 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 protein 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.
References
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. Note: Use of frames - adjust browser settings if appropriate.
- Israeli and German scientists created artificial silk using genetically engineered spider proteins - Article on IsraCast