Biomechanics of silk - 2452 The Journal of Experimental...

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2452 Introduction Spider silk is an inspiration for biomimetic super fibers because of the high tensile strength and unmatched toughness of major ampullate draglines (Kaplan et al., 1994; Vollrath, 1999). Silk also provides ecologists with links between the behaviors of spiders as they spin webs and selective factors in the environment (Blackledge et al., 2003; Blackledge and Gillespie, 2004; Craig, 2003). Molecular biologists are increasing the known diversity of genes that code for a variety of spider silk proteins, whereas biochemists and structural biologists develop hypotheses for how amino acid sequence affects the mechanical function of silk fibers (e.g. Gatesy et al., 2001; Guerette et al., 1996; Hayashi and Lewis, 1998; Simmons et al., 1996; Tian and Lewis, 2005). Finally, the mechanical process of spinning fibers from liquid dope has been studied from both morphological and functional perspectives (Garrido et al., 2002; Vollrath and Knight, 1999; Vollrath et al., 1998; Vollrath et al., 2001) and genetic engineers are synthesizing recombinant silk proteins (Arcidiacono et al., 1998; Foo and Kaplan, 2002; Huemmerich et al., 2004; Lazaris et al., 2002; Lewis et al., 1996; Winkler et al., 1999). Together, these disparate fields of research indicate the potential of the spider silk system to integrate aspects of organismal biology, molecular evolution and biomechanics (e.g. Craig, 2003; Gosline et al., 1999; Hayashi et al., 1999). Characterization of the mechanical performance of different types of spider silk fibers is a crucial first step for developing hypotheses that relate protein sequence and structure to the ecological function of spider silk. For instance, several studies have demonstrated that orb webs contain two types of silk fibers with radically different mechanical properties. The framework of webs is composed of stiff and strong major ampullate silk. By contrast, the sticky capture spiral is an order of magnitude stretchier and 1000 times more compliant than major ampullate silk (Denny, 1976; Gosline et al., 1999; Kitagawa and Kitayama, 1997; Köhler and Vollrath, 1995). These silks function together to arrest and absorb the kinetic energy of flying insects. However, most spiders spin more than one or two types of silk. Orb-weaving spiders spin a total of seven different kinds of silk, five of which form long fibers (Foelix, 1996). Whereas capture, spiral and major ampullate silks are moderately well characterized, comparative data on the mechanical performance of other silks are limited (Denny, Orb-weaving spiders spin five fibrous silks from differentiated glands that contain unique sets of proteins. Despite diverse ecological functions, the mechanical properties of most of these silks are not well characterized. Here, we quantify the mechanical performance of this toolkit of silks for the silver garden spider Argiope argentata . Four silks exhibit viscoelastic behaviour typical of polymers, but differ statistically from each other by up to 250% in performance, giving each silk a distinctive suite of material properties. Major ampullate silk is 50% stronger than other fibers, but also less extensible.
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