Page 161 - Handbook of Properties of Textile and Technical Fibres

Silk: fibers, films, and compositesdtypes, processing, structure, and mechanics 139

but it is also exported to Europe where companies manufacture high-quality silk yarns
and cloth products. Japanese and Korean companies are also important producers but
silkworm farms are often located in other countries (Brazil, Vietnam, etc.).
There is a renewal in the interest in this type of natural fiber with the attempts to
promote sustainable production, not only for traditional textile products but also for
a variety of technical and biomedical applications as potential support for tissue engi-
neering. Ancient technical uses include fishing line and musical strings for musical
instruments (antiquity), parachute cords and canopies, ropes (19the20th centuries),
dentistry and suture materials, the so-called Crins de Florence (Altman et al., 2003;
Colomban et al., 2012b) were largely used before the development of synthetic
polymer fibers. In the near future, silk is expected to be a major biopolymer in biomed-
ical applications. Regenerated, silk can be used in different shapes: electrospun fibers,
foams, or sheets improved by encapsulation or coating. Furthermore, silk can be modi-
fied by biotechnology (Grenier et al., 2004; Royer et al., 2005). Its potential is large:
intraarticular ligament (Bartow, 1916; Liu et al., 2007), cartilage and bone scaffold
(Meinel et al., 2004; Meinel et al., 2005; Meinel et al., 2006; Luan et al., 2006;
Kirker-Head et al., 2007; Meechaisue et al., 2007), skin (Min et al., 2004), artificial
blood vessel (Lovett et al., 2007; Priestley, 2007; Yang et al., 2007), hepatic tissues
(Hu et al., 2006b), and nerves (Wang et al., 2007) are proposed applications. Sericin,
the coating of B. mori bave, a textile industry by-product, also receives attention for
use in cosmetic products (Padamwar and Pawar, 2004; Kundu et al., 2008; Sehnal,
2011; Aramwit et al., 2012; Kundu et al., 2014; Polaskova et al., 2015).

5.1.3 Creatures producing silk

Technically, silks are fibrous protein polymers containing highly repetitive sequences
of amino acid residues. As sketched in Fig. 5.1(b), the macromolecular polypeptide
chain consists of a polyamide backbone [eR n1 CHe(NeH)e(C]O)eR n2 CHe(Ne
H)e(C]O)eR n3 CHe.], the side chains R ni consist of 20 differing amino acid
residues (Table 5.1). Glycine, alanine, and serine represent more than 75% of these
residues (Asakura et al., 2015a,b).
Silk fiber production is not confined to butterflies, moths, and spiders, but produced
by creatures of a variety of different classes: arachnids, insects, but also mollusks
(mussels). Bees and wasps (Hymenoptera) embed simple silk fibers into wax of their
combs, fleas (Siphonaptera) use silk in their nests and cocoons, lacewings (Neuroptera)
employ it to cement eggs onto stalks, caddis fly larvae (Trichoptera) use it to bind
debris into nests or make underwater webs, etc. Among the arachnids, silk is used
for nest building in the pseudoscorpions and mites and for many uses by true spiders
(Araneids). The precursor of the silk is produced and stored in the creature’s complex
gland as a liquid and converted into fiber(s) through the narrow ducts and then the
spinneret(s). The production of silk fibers, either as regenerated silk or by genetic
modification of silkworms with spider DNA, potentially opens up new engineering
applications for these fibers. In the last decade, spider silk has received considerable
attention because the variety of silk types produced by the specialized glands of
Araneids result in outstanding properties, from being very strong to highly extensible