The Chemical Composition and Microstructure of Silk

Silks are produced by a number of insect and spider species, but the major source of commercial textile silks are the silk moths of the family Bombyci-dae, of which the most important is the domesticated silkworm, Bombyx mori. The first known references to the culture of silkworms (sericulture) come from China in the second or third millennium BC, from where it spread through Asia, then on to the Middle East and eventually Europe. 

The highly crystalline nature of fibroin leads to the characteristic properties of silk fibres a good mechanical strength and limited extensibility, arising from the extensively inter-bonded, fully extended nature of the protein chains, and 

As with other natural fibres, silk has a hierarchical microstructure - about five anti-parallel (f-sheets, each with around 12 chains, aggregate to form parallel, crystalline microfibrils (approximately 10 nm in diameter), bundles of which make up fibrillar elements (roughly 1 p,m across), which in turn associate to comprise the individual fibroin filaments (7-12 xm) at each level of organisation, the ordered elements are embedded within amorphous matrices derived from the non-crystalline components. Once again, then, the behaviour of the structural composite can be understood in terms of the semi-crystalline array of its component parts. 

Sericin, the protein that binds the pairs of fibroin filaments as they emerge from the silkworm, and which may have a role in dehydrating the fibroin and encouraging its crystallisation, has a markedly different composition and structure to that of fibroin. It is largely amorphous and is rich in serine (—32%), aspartic acid (—14%) and glycine (—13%) there is a much greater proportion of residues with polar and/or bulky side-chains. The predominance of these polar, hydrophilic groups means that sericin is readily soluble in hot water. 

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