Spider silk
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 , 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, such as DuPont's
Kevlar.
Encyclopedia
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 , 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, such as DuPont's
Kevlar.
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.
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 , which according to "Liquid crystalline spinning of spider silk" , "equals that of commercial polyaramid 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 indestructable 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 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.
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. 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 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.
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, , 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 . 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.
See also
References
External links
- - A very nice breakdown of the silk gland, its parts and uses with images and drawings.
- - NASA article and database information on the research of spiders in space.
- - Detailed research based investigation into spider silk, considering properties, structure and uses. Diagrams and images included.
- - Article on nature.com
- - Article on IsraCast
- - Arcticle on The Journal of Experimental Biology
- - Article on forming Spider Silk Fibers from Caterpillars
