supercontraction - 1981 The Journal of Experimental Biology...

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1981 INTRODUCTION Most spiders spin a variety of different silks that are utilized for diverse ecological functions (Blackledge and Hayashi, 2006). Dragline silk, produced from major ampullate silk glands, evolved early in the diversification of spiders, perhaps in the Jurassic (Vollrath and Selden, 2007), and it was the first silk to be spun as discrete structural threads, rather than sheets of fibers. The origin of dragline silk is a hypothesized key innovation in spiders’ evolutionary success (Bond and Opell, 1998). More than 40,000 species of spiders now use dragline silk for a variety of functions such as lifelines and the frames of webs. In addition to its clear evolutionary importance, dragline silk exhibits many desirable qualities that make it a focus of biomimetic research (Hakimi et al., 2007; Vollrath and Porter, 2006b). Dragline silk is mechanically impressive. It combines high tensile strength and elasticity in a low density fiber, achieving a strength to weight ratio ~5 ± greater than steel and a toughness ~3 ± greater than Kevlar (Booth and Price, 1989; Gosline et al., 1986; Guan, 2007; Vollrath, 2000; Vollrath and Knight, 2001; Vollrath and Porter, 2006b). Dragline silk is also spun under environmentally benign conditions and is immunologically compatible with living tissue (Hakimi et al., 2007; Vadlamudi, 1995; Vollrath and Porter, 2006a). Finally, spider dragline silk ‘supercontracts’ (Work, 1981). The silk absorbs water at high humidity, altering its material properties and shrinking up to 50% of its original length, if unrestrained, while increasing in overall volume. This process generates substantial stress in silk when it is restrained and the potential to perform work. Supercontraction may provide a mechanism that tensions webs as they become loaded with dew or rain (Elices et al., 2004; Guinea et al., 2003). Potentially more important, supercontraction probably plays an essential role in determining the molecular orientation of silk during the spinning process as the still wet fiber is extruded through the spinning duct, thereby increasing the alignment of silk molecules along the fiber axis (Pérez-Rigueiro et al., 2003). But, supercontraction may also be exploited by materials scientists to tailor silk’s already impressive properties (Guinea et al., 2005). Our understanding of the mechanics of supercontraction is growing (Guinea et al., 2003; Guinea et al., 2005; Pérez-Rigueiro et al., 2003; Pérez-Rigueiro et al., 2005; Savage et al., 2004; Work, 1981; Yang et al., 2000). Spider dragline silk is a hierarchically structured material composed of a blend of multiple types of proteins (Hinman and Lewis, 1992; Xu and Lewis, 1990). Among species spanning millions of years of evolutionary history, the amino acid sequences of dragline silk proteins, called major ampullate spidroins (MaSp), are highly conserved, so that they can be classified into two groups often termed MaSp1 and MaSp2 (Gatesy et al., 2001).
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supercontraction - 1981 The Journal of Experimental Biology...

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