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From 2013.igem.org

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 a lot of animals such as arthropods are also capable of producing silk. One of the arthropods well known for its silk is the spider and spider silk is our material of choice for our implant coating.

The spider silk we used is the so called dragline silk from the Argiope aurantia (MaSp2) from Brooks et al (2008). It is the strongest silk produced inside the spider. The spider silk consists out of a big repetitive domain (around 2500 base pairs) with an N and a C terminus (figure 1 b).

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 2 below a stress-strain diagram can be found. where Clavipus

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

Figure 2: Properties of spidersilk versus other materials (Frank K. Ko, et at. 2001). Spider silk is compared to, Kevlar 29, normal silkworm silk, polyethylene terephthalate (PET), Nylon 6, and Merino wool. The stress-strain diagram relates the degree of deformation to the amount of energy absorbed.

Silk as a coating material

Silk can be regarded as a superb version of collagen. Collagen is a material used a lot for the coating of implants. Although collagen is a naturally occurring protein found inside the human body, the implants are coated with proteins harvested from cows, which can give an immune response. On the other hand spider silk is an in allergic material and will never give an immune response. Spider Silk is biodegradable, has a low bacterial adherence and can rival any artificial material with the respect of cell growth (1-3). Therefore spider silk is a good candidate for the coating of medical implants.

For example one can think of a cartilage implant. The implant consists of a biodegradable polymer coated with spider silk. The human cells will grow into the implant, slowly degrading the implant. After a certain amount of time the implant is fully degraded, leaving only a new piece of cartilage in for example a knee.

References

(1) Silk-based materials for biomedical applications, A.Leal-Egana (2010)
(2) Silk as biomaterial, C. Vepari (2007)
(3) Interactions between spider silk and cells, J.W. Kuhlier (2010)