Page 209 - Handbook of Properties of Textile and Technical Fibres

186 Handbook of Properties of Textile and Technical Fibres

(Slotta et al., 2007), and fourier-transform infrared spectroscopy (FTIR) (Slotta
et al., 2007). The protein structure and genetics of spider silk, especially the silks
from N. clavipes, have been intensively studied as well (Xu and Lewis, 1990;
Hinman et al., 1992; Beckwitt and Arcidiacono, 1994; Guerette et al., 1996;
Beckwitt et al., 1998; Colgin and Lewis, 1998; Hayashi and Lewis, 1998; Craig
and Riekel, 2002; Kenney et al., 2002). However, because of the fineness of spider
silk, less than 4 mm in diameter, the characterization of the mechanical properties of
spider silks has been limited to the tensile mode (Vollrath, 2000; Liu et al., 2005a;
Gosline et al., 1986, 1999; Blackledge et al., 2005; Guinea et al., 2005; Van
Nimmen et al., 2006; Vehoff et al., 2007; Hayashi et al., 2004; Ko et al., 1999).
Little is known about the response of spider silks to other modes of deformation in
the transverse direction and in torsion. This article offers a summary of the structure
of spider silks and introduces an initial attempt to characterize the stress-strain behavior
of silk from N. clavipes spiders under simple tension, transverse compression, and
torsional deformation. These engineering properties of spider silk will provide a basis
for the structural analysis of structures made of spider silk. The unique toughness or
capability of spider silk to absorb energy owes it origin to its fundamental nonlinear
viscoelastic behavior. The engineering properties of the spider silk under various
modes of deformation depend on strain rate or the time domain over which the defor-
mation is applied. The time-dependent mechanical properties of A. aurantia spider
dragline silk under tension is also presented in this paper, forming the basis for an
attempt to establish a constitutive relation for spider dragline silk.

6.2 Structure

Of the more than 1,200,000 species of spiders on earth, about 2500 species are silk
makers (orb weavers). Properties of spider silks vary by species and by functions.
These spiders produce at least seven different silks, each synthesized and spun by
different silk glands and spinnerets located on the posterior end of the spider’s
abdomen (Fig. 6.1; Vollrath and Knight, 2001; Gosline et al., 1986). The most
extensively characterized spider silk is the major ampullate dragline silk (MA silk).
The dragline originates in the ampullate gland and is extruded through the anterior
spinnerets. As the structural framework of a spider web and the lifeline of a spider,
spider dragline is the strongest and toughest of the silks a spider makes.
One of the outstanding characteristics of spider silk is its fineness. For example, the
dragline is between 3 and 4 microns in diameter. The cribellate silk was found to be as
fine as 0.03 mm in diameter. The diameter of the A. aurantia spider dragline measured
by scanning electron microscopy was 3.1 microns, which corresponds to 0.085 denier,
assuming a fiber density of 1.25 gm/cc. Scanning electron microscope pictures indi-
cated that the dragline silks have a circular fiber cross-section. Table 6.1 presents
the diameter of spider dragline silk in comparision to other textile fibers.
Spider silks are made up of proteins, termed spidroins. Usually, silk protein is
regarded as a biological representative of the nylon family, Nylon 2 (O’Brien et al.,
1998; Yang et al., 2005). The MA silk of the most-studied spider genera such as