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
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).