Spider silk

Spider silk, also known as gossamer, is a fiber spun by spiders. Spider silk is a remarkably strong material. Its tensile strength is comparable to that of high-grade steel — according to Nature^[1], spider dragline silk has a tensile strength of roughly 1.3 GPa, while one source [2] lists a tensile strength for one form of steel at 1.65 GPa. However, spider silk is much less dense than steel; its tensile strength to density ratio is roughly five times higher than that of steel (i.e. it is five times as strong as steel of the same density — as strong as Aramid filaments, such as Twaron or Kevlar.) In fact, a strand of spider silk long enough to circle the earth would weigh less than 16 ounces (450 g).

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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.

Argiope argentata has five different types of silk, each for a different purpose:^[2][3]

• 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

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 "equals that of commercial polyaramid (aromatic nylon) filaments, which themselves are benchmarks of modern polymer fiber technology."^[4]

The notion that spider silk is stronger than any industrial fiber is a common misconception as whilst some may be stronger, none are tougher (total energy to break). Numerous artificial fibers are similar or stronger, notably aramids like Kevlar and carbon fibre materials (see tensile strength for common comparisons). Nonetheless, there is much interest in duplicating the silk process artificially, since spiders use renewable materials as input and operate at room temperature, low pressures and using water as a solvent. Spider silk can be harvested in large scale quantities if one has proper harvesting equipment. One can also make near-indestructible spider silk threads by weaving the fine threads into thicker and more durable weaves in the same fashion as other industrial threads.

Spider silk is composed of complex protein molecules. This, coupled with the isolation relating from the spider's predatory nature, has made the study and replication of the substance quite challenging. Because of the repetitive nature of the DNA encoding the silk protein, it is difficult to determine its sequence and to date, silk-producing sequences have only been decoded for fourteen species of 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 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 semi amorphous regions, that gives spider silk its extraordinary properties.

Synthesis

The unspun silk dope is pulled through silk glands, resulting in a transition from stored gel to final solid fiber. 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.^{[5] [6]} Thus, different specialized silks have evolved with material properties optimized for their intended use.

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. 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.^[7]

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
• about 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^[8]

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)^[7] Some fishermen in the indo-pacific ocean use the web of Nephila to catch small fish.^[7]

The silk of Nephila clavipes has recently been used to help in mammalian neuronal regeneration. ^[9]

At one time, it was common to use spider silk as a thread for crosshairs in telescopes, microscopes and similar optical instruments.^[10]

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^[7]). 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 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. 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.^[11]

See also

• Hagfish - produces similar fiber.

References