Natural Protein Fibers

Keywords

Silk • Protein fiber • Insect • Secretion • Molecular weight • Bombyx mori

Silk is one of the most ancient fibers known to mankind and has been extensively used for various applications. Silk refers to the proteins secreted by insects in fiber form. Interestingly, silk fibers are made by the insects from proteins in an aqueous solution, but the proteins become semicrystalline and insoluble when formed into fibers [10Sut]. To produce fibers, insects accumulate proteins (25­30 % proteins) in their glands to obtain a viscosity nearly 3.5 million times that of water. Such high viscosity allows the insects to extrude continuous fibers. Fibers are formed by expelling a droplet of the protein onto a substrate and then pulling and drawing the solution away from the substrate. Typically, silk fibers are composed of two filaments containing the main protein fibroin that are glued together by the protein sericin. Fibroin found in fibers is classified as heavy fibroin (200-350 kDa), light fibroin (25-30 kDa), and glycoprotein P25 (25 kDa). The heavy chain fibroin is connected to the light chain fibroin through disulfide bonds and to P25 through hydrophobic interactions in a 6:6:1 ratio [13Lin]. Most silks contain high levels of the nonessential amino acids glycine, alanine, and serine avoiding the use of these proteins as diet by the insects.

Based on the type of silk gland, molecular structure, and phylogenetic relation­ship, silk producing insects have been classified into 23 different groups. Silk produced by the insect Bombyx mori is the most common silk available on the market and is commonly referred to as mulberry silk. B. mori silk is predominantly obtained from univoltine insects that produce only one cocoon during their life cycle in contrast to multivoltine insects that produce multiple cocoons. Currently, about 1.5 million tons of silk are produced across the world every year. Although it is reported that silk is produced in nearly 60 countries, China and India account for more than 90 % of the total world silk production. Table 34.1 lists the annual © Springer-Verlag Berlin Heidelberg 2015

N. Reddy, Y. Yang, Innovative Biofibers from Renewable Resources, DOI 10.1007/978-3-662-45136-6_34

production of silk by various countries. Countries such as Brazil, Thailand, and Uzbekistan produce limited quantities but exotic silks that are used for unique and distinct applications.

References

[10Sut] Sutherland, T. D., Young, J. H., Weisman, S., Hayashi, C. Y., Merritt, D. J.: Annu. Rev. Entomol. 55, 171 (2010)

[13Lin] Lintz, E. S., Scheibel, T. R.: Adv. Funct. Mater. 23, 4467 (2013)