Page 177 - Handbook of Properties of Textile and Technical Fibres

154 Handbook of Properties of Textile and Technical Fibres

for spidroin 2 (Kaplan et al., 1994) with 38 and also with 72 amino acids (labels are
given in Table 5.2):
GPGGYGPGQQGPGGYGPGQQGPSGGPGSAAAAAAAAAAGPGGYGPGQQ
GPGGYGPGQQGPSGGPGSAAAAAAAAAAGPGGYGPGQQGPGGYGPGQQG
PSGGPGSAAAAAAAAAA
The amino acid composition of this sequence is 36.8% of glycine, 26.3% of alanine,
15.8% of proline, 10.5% of tyrosine, 5.3% of serine, and 5.3% of tyrosine with 10
hydrophobic and 6 hydrophilic residues for the 38 amino acid. A helical conformation
has been obtained from 27 to 36 positions (26%), whereas the 1 to 26 (74%) is a
random coil part in the involved chain (Fig. 5.4(a)). The same result is obtained for
the double sequence, an alpha helix from 27 to 36 positions and also from 65 to 74
0
corresponding to the GSAAAAAAAAAA part (Fig. 5.4(a )). The addition of NH 2
to close the chain decreases or even suppresses the flat ribbon part. It is reasonable
to think that chemical treatments such as degumming can modify or interact with
some grafts that will modify the structure and associated phase transition. Neverthe-
less, the model goes some way to explain the low crystallinity of silk fibers. Rigid
(composed of aromatic rings) or long amino acid side grafts can impose specific
bond angles in the polyamide chain and hence may destroy the regular structure.
Untwisting of the helix (Fig. 5.4(b)) leads to an important lengthening of the chain
(Paquin and Colomban, 2007) and the availability of untwisted ribbons. These ribbons
need to be associated to form a b-sheet.

5.2.2.2 Technology and silk production
Spiders are not amenable to being cultivated on an industrial scale as they are quite
aggressive to one another and cannibalistic. Hence the use of spider silk is still mainly
limited to laboratory studies. At the laboratory, special devices allow the extraction of
long spider fibers with small diameters. A day’s rest is required to permit the spider to
be ready again for the forced spinning. An experimental farm was managed from the
1970s in Madagascar and this N. madagascariensis silk production has been studied
by Colomban et al. (2008a, 2012b), Wojciesjak (2014), and Wojcieszak et al.
(2014). However, Nephila fiber has been woven locally for centuries by inhabitants
and even exported to France (Bon de Saint-Hilaire, 1710). The most extensively stud-
ied spider silk is that produced by Nephila clavipes.

5.2.3 Regenerated and synthetic silk

It has been possible to manufacture rayon fibers from solubilized cellulose for over a
century (Michel, 2009), and this has inspired present attempts to produce regenerated
silk. For the manufacture of regenerated fibroin, the (degummed) silkworm (or
as-received spider) silk fibers must be dissolved (Fig. 5.7). The first step is the elimi-
nation of the sericin sheath, called degumming, in a soap solution brought to boiling
point. Fibroin is then dissolved in a strong concentrated ionic solution, typically by
using LiBr, KCl, CaNO 3 , LiSCN (Ha et al., 2003; Ki et al., 2007a; Lu et al., 2010;
Percot et al., 2014), or more complex mixtures such as CaCl 2 /H 2 O/CH 3 CH 2 OH