Contents
1 Sources and classification of silk 1
1.1 Introduction 1
1.2 Mulberry and non-mulberry silks 1
1.3 Spider silks 2
1.4 Marine silks 3
1.5 Ant and honey bee silks 7
References 12
2 Structure and properties of silk fibers 13
2.1 Structure of silk fibers produced by Bombyx mori 13
2.2 Spider silk 34
References 44
Further reading 47
3 New developments in degumming silk 49
3.1 Conventional degumming using alkali 49
3.2 Degumming with the aid of surfactants/detergents 53
3.3 Infrared assisted degumming 57
3.4 Degumming using enzymes 61
3.5 Removal of sericin using ionic liquids 65
3.6 Degumming using steam 66
3.7 Comparison of various degumming approaches 66
3.8 Demineralization of wild silks before degumming 68
References 70
Further reading 71
4 Regenerated silk fibers 73
4.1 Regenerated silk using ionic liquids as solvents 73
4.2 Formic acid as a solvent for silk fibroin 81
4.3 N-Methylmorpholine N-oxide (NMMO) based solvent system
for producing regenerated silk fibers 85
4.4 Regenerated fibers produced from spider silk 88
4.5 Novel approaches for producing regenerated silk fibers 93
References 106
5 Electrospun silk fibers 109
5.1 Electrospun fibers from B. mori silk fibroin 109
5.2 Silk fiber blends 116
5.3 New systems of electrospinning 118
5.4 Electrospun fibers from wild silks 121
5.5 Electrospun fibers from spider silk proteins 122
References 129
Further reading 131
6 Applications of silk 133
6.1 Medical applications of silk fibroin 133
6.2 Medical applications of spider silk proteins 155
6.3 Clinical uses of silk 157
6.4 Biotechnological applications of silk 161
6.5 Cosmetic applications 162
6.6 Miscellaneous applications 166
References 172
Further reading 177
7 3D printing silk 179
7.1 Scaffolds developed from silk fibroin and blends with other
biopolymers 179
7.2 3D printing silk with bacterial cellulose and microalgae 186
7.3 Scaffolds developed from silk fibroin and blends with
synthetic polymers 190
7.4 3D printed silk fibroin and bioactive glass 193
7.5 Sericin 3D scaffolds 194
7.6 3D printed scaffolds from spider silks 197
References 198
8 Future trends in the sources, processing and applications of silk 201
8.1 New sources and structure of silk 201
8.2 New approaches in processing silk into biomaterials 201
8.3 Emerging and futuristic applications of silk 202
References 212
Index 215
Sources and classification of silk
1.1 Introduction
Unlike any other fiber, silk is produced on land, water and air. More than 23 different silk lineages in 17 insect orders have been recorded and classified (Table 1.1) (Sutherland et al., 2010). Most of the silk is produced by the Lepidoptera order of insects and specifically from the Bombycidae and Saturniidae species. Extensive studies have been done to further classify the silks produced by the different species. One example of classifying silk, based on the sequence of amino acids, is given in Fig. 1.1.
In addition, silk can also be classified based on the gland in which it is produced. Similarly, silk species have been classified based on the differences in the FTIR spectra (Boulet-Audet et al., 2015) which also was able to distinguish silk based on their composition as seen from Fig. 1.2.
1.2 Mulberry and non-mulberry silks
The primary source of silk is from the cocoons of the insect Bombyx mori which has been domesticated for over 5000 years. Before B. mori was domesticated and used for silk production, it has been reported that silk was generated from Bombyx mandarina considered to be the wild ancestor of the B. mori silkworm. The two insects differ by one chromosome number with B. mori having 28 and B. mandarina having 27. However, these two species are considered to be infertile and hence produce distinct cocoons and resulting silk fibers. During the process of domestication, B. mori has evolved as the more suitable option to obtain silk fibers and although B. mandarina is prevalent, it is now considered as wild silk. More than 400 phenotypes and 4310 silkworm germplasm strains have been recorded world wide (Zanatta et al., 2009). The common mulberry silk worm belongs to the Bombycidae family with B. mori being the most common strain. B. mori silk worms feed on mulberry leaves and hence silk produced by B. mori is also called as mulberry silk. In addition to classification based on feed, B. mori silkworms have also been distinguished depending on the number of cocoon producing cycles. For example, univoltine silkworms have only one cocoon producing cycle compared to two cycles for bivoltine and multiple cycles for multivoltine silk (Table 1.2).
Silkworms which feed on non-mulberry leaves are called wild silks and mostly belong to the saturniidae family and further classified into the Attacini sub-group (Fig. 1.3). Some of the common saturniidae insects (Fig. 1.4) also produce cocoons and silk as shown in figure (Chen et al., 2014). Tasar, muga and eri are the silks can also be further classified based on their cocoon characteristics or habitat (Padaki et al., 2015). Wild silks are generally categorized as those that feed on non-mulberry plants. Wild silks can be classified broadly as temperate and tropical. Antheaea pernyi found in China, A. yamamai found in Japan and A. roylei, A. frithi and A. pernyi are prevalent in temperate conditions whereas A. mylitta is found in tropical conditions and mostly in India. Images of some of the cocoons produced by different wild silk worms are given in Fig. 1.4. The wild silks not only differ in terms of their composition and structure but also have to be processed using harsher conditions than mulberry silks. wild silks reared and commercially sold in relatively large quantities.
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