mechanical design, spidersilks

mechanical design, spidersilks - The Journal of...

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Orb-web-spinning spiders produce a variety of high- performance structural fibres with mechanical properties unmatched in the natural world and comparable with the very best synthetic fibres produced by modern technology. As a result, there is considerable interest in the design of these materials as a guide to the commercial production of protein- based structural polymers through genetic engineering. To understand the design of silks, we must understand the relationships between structure and function, and this will require information from molecular biology, polymer physics, materials science and spider biology. We now have good information on the amino acid sequence motifs present in spider fibroins, and our understanding of the molecular architecture of spider silks is growing daily. Unfortunately, in many cases, our understanding of silk design is flawed because we do not know which mechanical properties are crucial to its function. To quote Wainwright (1988), ‘Identification of the properties that are important to a particular function requires all the intuitive insights and creative abilities of objective scientists.’ Only when we understand the true function of spider silks will be able determine whether a spider’s dragline silk offers an appropriate model for man-made materials produced through genetic engineering. The function of spider silks The orb-web-weaving araneid spiders provide ideal model organisms for studying the functional design of protein-based structural materials. These spiders have seven different gland–spinneret complexes, each of which synthesizes a unique blend of structural polymers and produces a fibre with a unique set of functional properties (Gosline et al., 1986; Vollrath, 1992). Unfortunately, our knowledge of functional relationships for most of these materials is very limited, with the exception of the major ampullate (MA) gland fibres and to some extent the viscid silk fibres produced by the flagelliform (FL) gland. The organization of these fibres in the spider’s orb- web is illustrated in Fig. 1, where it can be seen that MA silk fibres form the web frame and the spider’s dragline. The viscid silk forms the glue-covered catching spiral. Mechanical properties of MA and viscid silks Fig. 1 shows typical tensile test data for MA and viscid silk from the spider Araneus diadematus plotted as stress–strain curves. The stress ( σ ) is the normalized force ( F ), defined as σ = F / A , where A is the initial cross-sectional area of the silk fibre. The strain ( ε ) is the normalized deformation, defined as, ε = L / L 0 , where L 0 is the fibre’s initial length, and L is the change in fibre length. The slope of the stress–strain curve gives the stiffness of the material, and the maximum values of stress and strain at the point where the material fails give the strength ( σ max ) and extensibility ( ε max ), respectively. The area under the stress–strain curve gives the energy required to break the material, and this variable can be used to quantify toughness. Values for the stiffness, strength, extensibility and
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mechanical design, spidersilks - The Journal of...

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