Page 164 - Handbook of Properties of Textile and Technical Fibres

Silk: fibers, films, and compositesdtypes, processing, structure, and mechanics 141

(a) (b)

(c) (d)

Figure 5.2 Optical (a and d) and scanning electron (b and c) microphotographs of silk fibers:
(a) section of a Bombyx mori silk yarn made of 8 2 fibers (note the two fibers from the same
bave e.g., on the top left and bottom right); (b) as-spun bave consisting of two fibers in the
same sericin envelope; (c) two Nephila madagascariensis fibers, free of any sericin coating;
(d) dragline of an araneid consisting of a straight fiber with a second one wrapped around it.

(Vollrath and Knight, 2001). In particular, the dragline fiber has a nice cylinder shape
and a sericin-free smooth surface. In contrast the shape of the silkworm fiber is very
irregular and the worm produces a bifiber coated with sericin (Fig. 5.2). However
farming Arachnids is very difficult (cannibalism), and alternative production routes
are being investigated so as to benefit from this type of silk and to overcome the
problems involved.
Production routes such as regenerated silk spinning and gene modifications of the
B. mori (Grenier et al., 2004; Royer et al., 2005) is being actively pursued. Attempts to
product a silk precursor from the milk of genetically modified goats has even been
studied but failed to produce fibers exhibiting significant mechanical properties.
Finally, recombinant silk filaments (diameter w80 mm, length, a few mm) are also
produced from bacteria such as Escherichia coli in which silk sequence have been
inserted (Prince et al., 1995; Lewis et al., 1996; Scheller et al., 2001; Lazaris et al.,
2002; Stark et al., 2007; Grip et al., 2009).