Page 178 - Handbook of Properties of Textile and Technical Fibres

Silk: fibers, films, and compositesdtypes, processing, structure, and mechanics 155

(Nam and Park, 2001; Hu et al., 2006a). Dialysis is then performed to remove the
ions. This method results in the total elimination of the sericin protein. Obtaining
fibroin solutions allows for the preparation of materials with different morphologies,
suitable for the intended applications (Yamada et al., 2001; Vepari and Kaplan, 2007;
Colomban et al., 2008a,b; Rockwood et al., 2011). The evaporation of water leads to
the formation of fibroin films (Colomban et al., 2008b). However, the dissolution
process cuts the macromolecules and changes/degrades their organization (Colom-
ban et al., 2008a,b; Wojcieszak et al., 2017). Some authors also create fibers, gels,
sponges, nonwoven mats, tubes, or microspheres by using different solvents and pro-
cesses (Ayutsede et al., 2005; Lawrence et al., 2010; Li et al., 2003; Hardy and Schei-
bel, 2010). Several procedures, which generate more or less organized materials, are
used to influence the secondary structure of fibroin films: (1) by using an organic
solvent with a low dielectric constant such as methanol, ethanol, or dioxane (Law-
rence et al., 2010; Kweon et al., 2000; Nam and Park, 2001; Yin et al., 2010); (2)
by varying the pH (Ayub et al., 1993); (3) by varying the temperature (Freddi
et al., 1995); and (4) the relative humidity (Lawrence et al., 2010). Whatever the
manufactured materials, strengths and ultimate strains are much lower than those
of the natural fibers (only a few % of the original strength) and attempts to form a
chemical cross-linking of fibroin chains (Li et al., 2003)oranenhancement of these
films by different polymeric fibers from natural (proteins, polysaccharides, etc.) or
synthetic (polyacrylonitrile, polyester, etc.) origin have been made (Hardy and Schei-
bel, 2010).
Biotechnology also permits the synthesis of spidroin or fibroin filaments through
the biochemical synthesis of recombinant proteins obtained from microorganisms
genetically programmed to produce particular sequences of amino acids (Lazaris
et al., 2002; Scheller et al., 2001; Stark et al., 2007). Many teams use the vector
E. coli to produce spider silk (Stark et al., 2007; Arcidiacono et al., 1998; Fahnestock
and Irwin, 1997; Lewis et al., 1996; Prince et al., 1995). To date only very few
filaments have been produced.
Incorporation of spider genes in B. mori silkmoth was also tested (Grenier et al.,
2004; Royer et al., 2005), but no significant increase of the tensile ultimate fiber
strength was observed (Dinh, 2010; Jauzein, 2010; Colomban et al., 2012a). As
expected the contribution of side chains to the mechanical properties was weak.

5.3 Mechanical properties and microstructure

5.3.1 Mechanical properties of traditional silk

Silkworm fibers possess useful mechanical properties but the variability is huge and
problematic. Notably, the fiber cross section permitting the stresses supported by the
fiber, which is not circular and variable along the fiber length is difficult to measure.
Nevertheless, silk shows competitive moduli and failure stress compared to synthetic
fibers. However, silk presents interesting deformability with a superior strain at break
(see Table 5.1).