Introduction to Natural Protein Fibers

Abstract

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 (25–30 % 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.