Silk: Materials, Processes, and Applications

By Narendra Reddy

Silk: Materials, Processes, and Applications addresses the latest research on the structure and properties of silk fibers, properties of silk-based materials, and cutting edge-related industrial practices. It pays particular attention to mulberry silk, but unconventional silks such as spider silk and marine silk fibers are also covered.

Although silk is one of the oldest known fibers, new research continues to shed light on its properties, leading it to be applied in new contexts particularly in the medical field, and new non-textile areas. In addition to structural and mechanical qualities, this book also includes a great deal of new research on the chemical modifications of silk fibers, and other processing methods.

With a focus on practical methodologies, this is the most readable and readily applicable book on silk so far, making it a perfect guide for readers with a range of backgrounds.

• Addresses the fundamental differences between mulberry, spider, and wild silks
• Describes silk fiber and non-fiber forms, including hydrogels and films
• In-depth coverage of silk-processing methods provides the perfect starting point for biotechnologists interested in the use of silk for non-textile applications

• Language: English
• Publisher: Elsevier Science
• Release date: Nov 16, 2019
• ISBN: 9780128196878

Narendra Reddy

Dr. Narendra Reddy is a professor and Ramalingaswami fellow at the Centre for Incubation, Innovation, Research and Consultancy, at Jyothy Institute of Technology, Bengaluru, India. His work has been reported by CNN, Discovery, Nature, American Chemical Society and other major news agencies. He has received funds from his research from United States Department of Agriculture, DST, DBT and CEFIPRA (India-France joint funding). His recent interest is in plant and animal proteins for non-food applications particularly for biotechnology. His group has demonstrated that proteins can be useful for energy generation and storage, and for biosensing and electronic applications

Book preview

Silk: Materials, Processes, and Applications

First Edition

Narendra Reddy

Center for incubation innovation research and consultancy, Jyothy institute of technology, Bengaluru, Karnataka, India

Table of Contents

Cover image

Title page

Copyright

1: Sources and classification of silk

Abstract

1.1 Introduction

1.2 Mulberry and non-mulberry silks

1.3 Spider silks

1.4 Marine silks

1.5 Ant and honey bee silks

2: Structure and properties of silk fibers

Abstract

2.1 Structure of silk fibers produced by Bombyx mori

2.2 Spider silk

3: New developments in degumming silk

Abstract

3.1 Conventional degumming using alkali

3.2 Degumming with the aid of surfactants/detergents

3.3 Infrared assisted degumming

3.4 Degumming using enzymes

3.5 Removal of sericin using ionic liquids

3.6 Degumming using steam

3.7 Comparison of various degumming approaches

3.8 Demineralization of wild silks before degumming

4: Regenerated silk fibers

Abstract

4.1 Regenerated silk using ionic liquids as solvents

4.2 Formic acid as a solvent for silk fibroin

4.3 N-Methylmorpholine N-oxide (NMMO) based solvent system for producing regenerated silk fibers

4.4 Regenerated fibers produced from spider silk

4.5 Novel approaches for producing regenerated silk fibers

5: Electrospun silk fibers

Abstract

5.1 Electrospun fibers from B. mori silk fibroin

5.2 Silk fiber blends

5.3 New systems of electrospinning

5.4 Electrospun fibers from wild silks

5.5 Electrospun fibers from spider silk proteins

6: Applications of silk

Abstract

6.1 Medical applications of silk fibroin

6.2 Medical applications of spider silk proteins

6.3 Clinical uses of silk

6.4 Biotechnological applications of silk

6.5 Cosmetic applications

6.6 Miscellaneous applications

7: 3D printing silk

Abstract

7.1 Scaffolds developed from silk fibroin and blends with other biopolymers

7.2 3D printing silk with bacterial cellulose and microalgae

7.3 Scaffolds developed from silk fibroin and blends with synthetic polymers

7.4 3D printed silk fibroin and bioactive glass

7.5 Sericin 3D scaffolds

7.6 3D printed scaffolds from spider silks

8: Future trends in the sources, processing and applications of silk

Abstract

8.1 New sources and structure of silk

8.2 New approaches in processing silk into biomaterials

8.3 Emerging and futuristic applications of silk

Index

Copyright

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Notices

Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary.

Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility.

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1

Sources and classification of silk

Abstract

Silk is an eternal fiber and unparalled in terms of luster, handle, comfort and many other unique properties. From being a primary source for apparel and other textiles, applications of silk has diversified into medical, cosmetic, biotechnology and other uses. Bombyx mori (B. mori) has been the predominant source of silk for centuries and continues to be the most common source for silk fibers. However, many other insects, plants and even marine organisms produce various forms of silk with distinct properties. It is not an overstatement to mention that silk is probably the only fiber that is produced on land, air and in water. In this chapter, an overview of the different sources for silk and their classification is provided. The rest of the chapters in the book mostly focus on B. mori silk and common wild silks but other forms of silk have also been discussed as and when appropriate.

Keywords

B. mori silk; Wild silks; Marine silks; Ant silks; Spider silks

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.

Table 1.1

Fig. 1.1 Phylogeny of lepidopteran insects based on the amino acid sequence of the gene 13 PCGs ( Liu et al., 2013). Reproduced with permission through Elsevier Open Access Publication.

Fig. 1.2 Classification of silk producing Lepidopteran insects based on the differences in FTIR spectra ( Boulet-Audet et al., 2015). Published through open access publication under the terms of the Creative Commons Attribution License.

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).

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 wild silks reared and commercially sold in relatively large quantities. The wild 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.

Fig. 1.3 Classification of silk producing insects based on phylogeny of Bombycidae species ( Chen et al., 2014). Reproduced with permission from Elsevier.

Fig. 1.4 Images of some of the cocoons produced by different wild silkworms ( Kundu et al., 2012). Reproduced with permission from John Wiley and Sons.

1.3 Spider silks

Archanids to which spiders belong consists of about 37,000 species and are known to produce silk with extraordinary properties. Different species of spiders produce silk from different glands such as ampullate (dragline), flagelliform etc. (Table 1.3, Fig. 1.5). The extraordinary properties of spider silks depend heavily on the species and glands and shows substantial variations.

Table 1.3

Reproduced with permission from Elsevier.

Fig. 1.5 Picture shows the various glands that extrude silk in spiders ( N. clavipes ), classification of silk and its uses ( Tokareva et al., 2014). Reproduced with permission from Elsevier.

1.4 Marine silks

Unique and distinct silks have been discovered in several marine animals but are probably the least studied among all the sources of silk. For instance, the amphipod (Crassicorophium bonelli) produces fine silk from its legs as an adhesive underwater (Kronenberger et al., 2012). A classification of the possible amphipods that produce marine silks are given in Fig. 1.6. Similar to C. bonelli, caddisflies belonging to the Tricoptera family produces silk based adhesives that are used to prepare structures for storing food.

Fig. 1.6 Classification of anthropods that produce silk underwater ( Kronenberger et al., 2012). Reproduced with permission from Springer Nature.

About 12,000 species of Trichoptera have been discovered and classified into sub orders of Annulipalpia, Spicipalpia and Integripalpia. Each species forms distinct cocoons usually for storage of food using stones and debris found underwater. These silks also differ in composition and properties compared to regular silks. Lack of alanine, higher amounts of arginine are distinct features. A mean hydrated net strength of 221 mN/m² for Hydropsyche siltalia was reported. Ability of these insects to produce underwater silk using various substrates was demonstrated in an aquarium and also in a flow chamber (Ashton et al., 2012) (Fig. 1.7). Ability of the insects to form insoluble silk under water is quiet intriguing and is being studied further.

Fig. 1.7 Different approaches of forming silk under water in aquariums using PTFE case (A) and between two glass slides (B) ( Ashton et al., 2012). Reproduced with permission from John Wiley and Sons.

Mussels which belong to the Bivalvia class are another species of animals that produce silk under water. Commonly referred to as byssal threads, they help in anchoring of the mussels to various substrates. The byssal threads are quiet unique since one end is stiff and strong whereas the other end is soft and flexible. However, the structure and properties of the mussels from different species (Fig. 1.8) vary considerably. For instance, Pinna noblis generates thousands of fine fibers known as sea silk (Fig. 1.9). Comparatively, Mytilus species produces silk made up to globular proteins organized into nanofibrils (Pasche et al., 2018).

Fig. 1.8 Classification of the various Bivalvia species that produce sea silk ( Pasche et al., 2018). Reproduced from Royal Society of Chemistry under Creative Commons Attribution 3.0 Unported Licence.

Fig. 1.9 Morphology of the various types of silk threads produced by Bivalvia class of sea organisms ( Pasche et al., 2018). Reproduced from Royal Society of Chemistry under Creative Commons Attribution 3.0 Unported License.

Novel silk like proteins were discovered from Nematostella vectensis a sea anemone which produces the silk as a means to capture prey. Like mussels, the nematocysts are capable of withstanding high mechanical stresses and the shape and properties of the silk produced are highly dependent on external factors and stimulations (Fig. 1.10). The proteins in the nematocysts could be made into regenerated and electrospun fibers (Yang et al., 2013) (Fig. 1.11).

Fig. 1.10 Digital image of the silk producing N. vectensis (A), before (C) and after stimulation (D) and amino acid sequence (B) of the silk produced ( Yang et al., 2013). Reproduced with permission through Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.

Fig. 1.11 Images of Aneroin fibers produced by wet spinning (A); SEM image of the fibers (B), fibers made into a skein (C) and properties of the fibers drawn to 30 k and 60 k vectors ( Yang et al., 2013). Reproduced with permission through Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.

1.5 Ant and honey bee silks

A unique category of silk is produced by ants, wasps and other insects which belong to the insect order Hymenoptera considered to be derived from spiders and silkworms (Fig. 1.12). This insect order consists of 144,000 species and the silk produced is used for building covers from predatory or parasite hosts. Unlike other forms, the Hymenoptera insects produce coiled-coil silks with a unique molecular structure (Sutherland et al., 2010, 2012). Some of the species that produce coiled-coil silk and their related information are presented in Fig. 1.12 and Table 1.4. Images of the common silks produced by three Hymenoptera are shown in Fig. 1.13 (Kameda et al., 2014). Silks produced by these species have relatively low molecular weights (30–50 kDa) with alanine rich core. Ants are another class of insects that produce silk with highly distinct structure, color and properties (Fig. 1.14) (Campbell et al., 2014). For instance, weaver ants were reported to produce silk in the form of nanofibers that were further made into the form of a non-woven web (Reddy et al., 2011). These nanofiber webs had mechanical properties higher than similar webs made from collagen, regenerated silk and other biopolymers.

Fig. 1.12 Evolutionary relationship between silk producing insects ( Sutherland et al., 2012). Reproduced with permission from John Wiley and Sons.

Table 1.4

Reproduced with permission from Springer Nature.

Fig. 1.13 Images of the coiled coil silks produced by hornets (A); mantis (B) and lacewings (C) ( Kameda et al., 2014). Reproduced with permission from Springer Nature.

Fig. 1.14 Images of silk/cocoons being produced by Oecophylla (top left); Brachymyrmex patagonicus (top right), Leptogenys crustosa (bottom left) and Rhytidoponera victoriae (bottom right) ( Campbell et al., 2014). Reproduced with permission from Elsevier.

References

Ashton N.N., Taggart D.S., Stewart R.J. Silk tape nanostructure and silk gland anatomy of trichoptera. Biopolymers. 2012;97(6):432–445.

Boulet-Audet M., Vollrath F., Holland C. Identification and classification of silks using infrared spectroscopy. J. Exp. Breiol. 2015;218(19):3138–3149.

Campbell P.M., Trueman H.E., Zhang Q., Kojima K., Kameda T., Sutherland T.D. Cross-linking in the silks of bees, ants and hornets. Insect Biochem. Mol. Biol. 2014;48:40–50.

Chen M.-M., Li Y., Chen M., Wang H., Li Q., Xia R.-X., Zeng C.-Y., Li Y.-P., Liu Y.-Q., Qin L. Complete mitochondrial genome of the