Fibroin filaments

Figure 1 (a) the sericin (outer layer) and fibroin filaments of Bombyx mori silkworm (b) the... 

The silk fiber protein is synthesized by the silk gland cells and stored in the lumen of silk gland. Subsequently it is converted into silk fibers. Each strand of silk fiber is a double stmeture with 2 fibroin filaments covered by sericin (Figure 18). 

Vigorous movement, contact with rough surfaces, and exposure to mechanical stresses and strains are undesirable at all times during degumming, but especially when sodium carbonate has been added to the liquor. Under such circumstances the fibroin filament becomes ruptured, and bundles of the fibrils of which it is composed project from the surface giving lousy silk. 

Moth larvae produce the polymeric material from a liquid crystalline phase to form a protective cocoon silk is extruded from the spinneret as a bave , consisting of two roughly triangular fibroin filaments ( brins ), about 20 pm across, bound together by a second protein, sericin (Figure 13). In commercial production, the chrysalid is killed before it can hatch (which would damage the thread), and a continuous filament can then be unreeled from each cocoon, giving typically 500-800 m of useable thread. 

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. 

Bombyx mori silkworm silk—determined from single brins (individual fibroin filaments following extraction of sericin). 

Sericin ser-o-son [ISV, fr. L sericum silk] (ca. 1868) n. Silk gum. The gelatinous protein that cements the fibroin filaments in a silk fiber. It is removed in the process called degumming. 

The question whether silk fibroin filaments are resorbable or permanent is open to interpretation. Having a polypeptide chemical structure, silk fibroin, like any other protein, is susceptible to proteolytic degradation, and will become weaker and eventually over a period of 2 years will be totally resorbed in vzvo. However, given the definition for an absorbable suture in the United States Pharmacopeia as a material that loses most of its tensile strength within 60 days post-implantation silk can therefore be classified as a permanent biomaterial. 

An alternative approach is to blend the silk fibroin filaments with more biostable materials. Initial trials in our laboratory have demonstrated that weaving silk fibroin filaments with polyester (PET) yarns to fabricate a 4 mm diameter tubular prosthesis provides both superior mechanical properties and improved cytocompatibility compared to current commercial ePTFE devices. Zhang s group at Soochow University has taken a tissueengineering approach to electrospin a tubular composite scaffold from a... 

Figure 12.2 Structure of a silkworm thread. Two cote fibroin filaments are surrounded by sericin. Each fibroin filament is assembled of numerous fibrils, which are formed by c. 1000 microfibrils. Microfibrils are composed of microcrystalline arrays (giving strength) alternating...

Hirano and Hayashi have introduced a novel fragrant fiber and yam based on chitosan. A bundle of two silk fibroin filaments was coated with a layer of N-modified chitosan using fragrant aldehydes such as cinnamaldehyde. A portion of chitosan fiber was suspended in methanol and a fragrant aldehyde was added. The mixture was kept at room temperature for 18 hours, washed with methanol, filtered, and air-dried. The fragrant derivatives, fibers, and yarns are suggested to be useful as a novel biomaterials in a wide field of applications, including air-filters, cosmetics, and textiles [52]. 

The fibroin filaments, which are attached to each other with sericin, can be separated during removal of sericin (degumming). With this procedure, a weight reduction is unavoidable, which is often compensated for by coating with metallic salts or other synthetic substances (aggravation). 

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