Page 268 - Advances in Textile Biotechnology

Developments in recombinant silk and other elastic protein fi bers 249

technology (Gosline et al., 1999). Spider silks are excellent biomaterials,
being lightweight and exceptionally strong, and having elastic properties,
including high impact resistance. The molecular structure of spider silks
consists of regions of protein crystals separated by less organized protein
chains. The primary structural modules give rise to diverse secondary struc-
tures that, in their turn, direct functions of different silks (Kluge et al., 2008)
(Table 10.1).
Biomaterials based on silks have applications ranging from medical

(micro-sutures, artificial ligaments, coatings), military (body armor, light-
weight gear) to textiles fi elds. Although variable in their primary structure,
silk proteins in general reveal a high amphiphilicity and a highly repetitive
nature. The key properties of web silks involve a balance between strength
and extensibility, giving a high degree of toughness and internal molecular
friction. Spider silks are able to absorb a large quantity of kinetic energy
with a minimal volume of silk. The viscoelastic nature of the spider silks
transforms this energy into heat instead of being available through elastic
recoil (Gosline et al., 1999). The combination of strength and elasticity is
judged to be similar or even superior to that of synthetic high-tech fi bers
made of polyamide or polyester. Indeed, compared with Kevlar, which must
be spun from almost boiling sulfuric acid, the spider silks are produced in
aqueous solutions at ambient temperature. Spider silks exhibit mechanical

properties similar to the best synthetic fibers produced by modern technol-
ogy, commonly employed to transmit and support tensile forces (Gührs
et al., 2000) (Table 10.2).
All silks contain protein crystals, and the majority of these silks contain
β-pleated sheet crystals that form from tandem repeated amino acid
sequences rich in small amino acid residues. The β-sheets contribute to the
high tensile strength of silk fi bers and form through natural physical cross-
linking of amino-acid sequences, which on spider and silkworm silk consist
of multiple repeats of mainly alanine: poly(A) n , glycine–alanine (GA) n or
glycine–alanine–serine (GAGAGS). The non-crystalline regions of silk are
commonly made up of: β-spirals similar to a β-turn composed of GPGXX
repeats (where X is mostly glutamine) and helical structures composed of
GGX. These semi-amorphous regions provide silk with elasticity. In addi-
tion to elastic fiber and semi-amorphous regions, non-repetitive regions are

present at the amino- and carboxyl termini of the proteins and they prob-
ably play a role in the controlled assembly of silk proteins (Gosline et al.,
1999, Kluge et al., 2008).
Although spiders produce seven different types of silks, only the

major ampullate gland silk and flagelliform silk have been produced
as heterologous proteins using genetic engineering techniques. Heterolo-
gous expression of spider-silk proteins has been achieved, as well as the
formation of new materials from spider-silk proteins derived recombinant
DNA.

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