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From 2013.igem.org

NOTE: This part should contain an introduction to how cool silk is an why it is so good as an coating product (Silk vs collagen)

Silk

Silk is a natural protein fibre that is known for its use in textiles. The best known silk comes from the silk moth pupa but arthropods are also capable of producing silk. One of the arthropods well known for its silk is the spider.

Spider Silk is amazing but currently large scale production is impossible. Various techniques to produce spider silk are being considered and one of them is letting bacteria produce the silk. This idea is promising although one of the main challenges is the low production rate of bacteria.

Silk has some really nice attributes and some of these attributes are feasible for medical devices. Silk is an inalergic biomaterial and it is proven to enhance the healing process when it is used as a cover of implants, allowing for better acceptance by the human body (Vepari & Kaplan 2007, Mandal et al 2012).

The industry from which silk is obtained, however, is less than ideal. Scientists have therefore begun to design silk-producing micro-organisms. The 2012 iGEM team from Utah have indeed successfully designed BioBricks for this very purpose.

Though there has been some success in producing silk in bacteria (Xia et al 2010), currently the bacteria needs to be killed in order to extract the silk. In this project the plan is to have the bacteria secrete the silk so that it can live and continue to produce more silk. Also the the design needs to be made such that most of the silk production happens at the required location to compensate for the low production volume.

The properties of silk

The unique properties of silk are a result of its highly constant and repetitive amino-acid structure. The sequence of amino-acids determines what secondary structures will arise, and thus the final preferred protein conformation. The secondary structures may be beta sheets, beta-spirals, and beta-helices, of which the sheets realize the silk's amazing tensile strength, and the spirals and helices its elongation.

In the figure below a stress-strain diagram can be found (Frank K. Ko, et at. 2001) where Clavipus spider silk is compared to, Kevlar 29, normal silkworm silk, PET (polyethylene terephthalate), Nylon 6, and Merino wool. The stress-strain diagram relates the degree of deformation to the amount of energy absorbed.

When used as clothing, silk has many beneficial properties. Its smooth, compact surface feels and looks nice, and it enables easy removal of dirt. It is a bad conductor of heat, making it cool in the summer and warm in the winter. Furthermore, it has a water absorption efficiency similar to that of wool, and is resistant to insects and mildew.

A final general property of silk it that it can be integrated with the human body - it will not induce an immune response - potentially making it an ideal choice for many biomedical applications. Its compatibility extends to the gastrointestinal tract, that is, it is even safe to eat!

The production of silk

The farming of silk is an arduous, time consuming, and costly process. Although a single cocoon may produce up to one mile of filament, 4 to 8 filaments are needed to produce a single thread, and approximately 5500 cocoons are needed for one kilogram of silk. Eight fully grown mulberry trees would have been needed for this single kilogram, and 48 hours of man-labor required to hand-reel it. Finally, the caterpillars required a full month to mature and three to five days to spin their cocoons, after which they were brutally boiled alive. Harvesting the more desired and rare silk from spiders requires an even more labor-extensive process. Each thread actually has to be pulled individually by hand from the spiders gland - needless to say, not a viable business plan!

The silk industry itself has undergone very little development over the past few millennia. Indeed, the manner in which it is obtained follows the very same process as that since it's initial discovery, albeit at a much grander scale with more specialized equipment. Scientists have therefore begun to design their own silk producing organisms [2]. Moreover, the 2012 iGEM team from Utah successfully designed the first spider-silk producing Biobricks for Escherichia coli (for more information, please visit their [https://2012.igem.org/Team:Utah_State wiki]). Such advancements are needed to provide the industries and manufacturers with sufficient silk proteins for their applications.

Applications for silk

Silk's journey as a product began as luxurious clothing reserved exclusively for the emperor subsequent to its initial discovery. As the sericulture developed, however, it was soon adopted by all classes of society. New applications were discovered, and it was spun into many different products; fishing lines, musical instruments, and bowstrings to name a few. It's utility and value were also recognized by other kingdoms, and a world-wide, ever increasing demand for the material began. Indeed, the western demand for silk was so great, that the main set of trade routes between Europe and Asia became known as the Silk Road.

.... Nowadays, the variety of silk applications is even more extensive; bullet-proof clothing, all sorts of ropes and cables, artificial tendons and ligaments, bandages, sewing thread, seat belts, parachutes, biodegradable bottles, and much more.....

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