Since ancient times spider silk has fascinated man because of the elegant way it combines strength and elasticity. It can even enable the regeneration of peripheral nerves in rats and sheep when used to replace nerve deficits. As such spider silk presents a very attractive material for use in medical applications – Here Anna Rising tells us about her love affair with silk
All spiders produce at least one type of silk and some, like orb weaving spiders, can have up to seven different types of silk glands each producing a silk with a specific purpose and unique mechanical properties. The dragline silk, a strong and extendible fibre used for construction of the framework of the webs and as a lifeline, displays a toughness never attained in synthetic or other natural fibres. For example, spider silk shows the same tensile strength as Kevlar, seven times the extensibility and takes three times the energy to break. It also shows a low density and is biodegradable.
While the outstanding mechanical properties of spider silk are well characterised, the production remains problematic. Since spiders are territorial and produce low amounts of silk they cannot be employed as such for large scale silk production. It would need 1 million spiders to produce enough silk for a 1 x 3m2 piece of silk textile, and take 70 people four years to weave! Instead, bacteria (or other organisms) that have received a piece of DNA encoding the spider silk proteins may be used for industrial production of synthetic spider silk. However, spider silk proteins are extremely prone to aggregate and therefore they are problematic to produce in bacteria and difficult to keep soluble during purification, often requiring harsh solvents for solubilisation.
In 2003 I started working at the Swedish University of Agricultural Sciences as a PhD student in a project aiming to find a way to produce synthetic spider silk. To do so we wanted to use a spider that was relatively large and produced a strong dragline. Based on data in the literature we decided on Euprosthenops australis. To get hold of the spider I went to South Africa and managed to catch 100 spiders in the wilderness. The spiders are about 10 cm in diameter (including legs) but are still difficult to spot in the bush! Luckily, I received excellent help and field work assistance from The Spider Club of South Africa. The spiders we caught were used for extracting DNA encoding the silk genes.
When I got back to Sweden an arduous time followed for my colleagues and me. The DNA was mapped and analysed and different ways of producing the proteins were tested and evaluated. Finally we could determine the smallest entity of the spider silk protein that could be efficiently produced and still retains most of its native properties. This part of the protein corresponded to only 10% of the whole spider silk protein, but still somehow “knew” that it should assemble into a fibre. Making a smaller protein is advantageous since it is easier for the bacteria to produce and the protein is also easier to handle during purification compared to longer variants. In 2007 we published the first report on how to produce synthetic spider silk in E.coli that spontaneously assembles into metre long fibres. The fibres are equally well tolerated as clinically used suture materials when implanted subcutaneously in rats. Apart from fibres we are also able to produce transparent films and foams from the silk protein solution. Furthermore, human cells can be cultured on the material which enables applications in the fields of tissue engineering and regenerative medicine.
After receiving my PhD in 2007, my colleagues and I decided to start a company based on our research findings. Spiber Technologies was founded in 2008 and today it has seven employees. The company focuses on scaling up the production of the material, IPR protecting the technology, as well as the development of scaffolds and implants for tissue engineering and regenerative medicine. In particular, the company is working on human stem cells. In 2012 Spiber Technologies was elected to the “33-list” of Sweden’s hottest young technological companies and was represented at the World Expo in Shanghai 2010.
Synthetic spider silk is superior to available materials today since it is defined with no animal or human derived components and so shows biocompatibility, there is no risk of spreading disease and better batch-to-batch variations. It is mechanically robust, with a tensile strength greater than mammalian tendons, is stable up to 260?C and so can be sterilised, is stable in most solvents, e.g. urea, degrades when implanted and is well tolerated by living tissue. In addition, it can be decorated with specific signals or protein fragments in order to produce an “intelligent” biomaterial. Its suitability as a matrix in cell culture has been shown with human primary fibroblasts attaching and growing well on all matrix types7, even in the absence of serum proteins or animal-derived additives, and with the highest cell counts obtained on the matrices combining film and fibre mesh. The cells showed an elongated shape that followed the structure of the matrices and exhibited prominent actin filaments. Moreover, the fibroblasts produced, secreted and deposited collagen type 1 onto the matrices.
Spider silk is an attractive material for medical applications, and recently we have learnt how to produce it in a scalable system. The synthetic silk can be used for cell culture applications and has proven to be biocompatible in animal trials, offering the potential to be used in implants and tissue engineering.
I would like to thank Professor Wilhelm Engström for introducing me to spider silk research, Professor Jan Johansson for valuable discussions and guidance, and Sara Amandusson for photographing the fibres (Figure 2).
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5. Stark, M., et al., Macroscopic fibers self-assembled from recombinant miniature spider silk proteins. Biomacromolecules, 2007. 8(5): p. 1695-701.
6. Fredriksson, C., et al., Tissue Response to Subcutaneously Implanted Recombinant Spider Silk: An in Vivo Study. Materials, 2009. 2(4): p. 1908-1922.
7. Widhe, M., et al., Recombinant spider silk as matrices for cell culture. Biomaterials, 2010. 31(36): p. 9575-85.
Anna Rising, DVM PhD, Assistant Professor at Karolinska Institutet and Lecturer in Translational Veterinary Medicine at SLU.