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  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.
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 work to fracture (or toughness), 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." While "five times as strong as steel" sounds more impressive than "just as strong as nylon", both are accurate. 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.
The proteins in the silk are complex molecules of amino acid. 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.
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.
- "Materials: Surprising strength of silkworm silk" in Nature 418:741 (15 August 2002)
Last updated: 10-24-2005 09:23:59