Spider dragline fibers

Figure 6.5 A schematic model demonstrating how the silkworm and spider dragline fibers respond when they are subjected to stretching. There are two components in the...

Secondary Structure. The silkworm cocoon and spider dragline silks are characterized as an antiparaHel P-pleated sheet wherein the polymer chain axis is parallel to the fiber axis. Other silks are known to form a-hehcal (bees, wasps, ants) or cross- P-sheet (many insects) stmctures. The cross-P-sheets are characterized by a polymer chain axis perpendicular to the fiber axis and a higher serine content. Most silks assume a range of different secondary stmctures during processing from soluble protein in the glands to insoluble spun fibers. 

Crystallinity. Generally, spider dragline and silkworm cocoon silks are considered semicrystalline materials having amorphous flexible chains reinforced by strong stiff crystals (3). The orb web fibers are composite materials (qv) in the sense that they are composed of crystalline regions immersed in less crystalline regions, which have estimates of 30—50% crystallinity (3,16). Eadier studies by x-ray diffraction analysis indicated 62—65% crystallinity in cocoon silk fibroin from the silkworm, 50—63% in wild-type silkworm cocoons, and lesser amounts in spider silk (17). 

Thermal Properties. Spider dragline silk was thermally stable to about 230°C based on thermal gravimetric analysis (tga) (33). Two thermal transitions were observed by dynamic mechanical analysis (dma), one at —75° C, presumed to represent localized mobiUty in the noncrystalline regions of the silk fiber, and the other at 210°C, indicative of a partial melt or a glass transition. Data from thermal studies on B. mori silkworm cocoon silk indicate a glass-transition temperature, T, of 175°C and stability to around 250°C (37). The T for wild silkworm cocoon silks were slightly higher, from 160 to 210°C. 

If smart fibers began to be widely used, perhaps clothes and other fabrics would become more adaptable. Nature is one place to look for inspiration. Olivier Emile and Albert Le Floch of the Universite de Rennes in France and Fritz Vollrath at Oxford University in England studied why spiders rarely spin around when hanging from their silk threads. Most fibers turn and twist, as a climber dangling from a rope knows all too well, yet a suspended spider is stable. The researchers discovered that spider silk has a kind of shape memory in which it rapidly recovers its shape, resisting any twisting motion. This research, Shape Memory in Spider Draglines, was published in Nature in 2006. 

As to fibers, it was reported that the inferior mechanical properties of silk from cocoons compared to spider silk result from the silkworm spinning process. If silkworm silk is processed at a constant pulling speed rather than constant force pulling, it possesses excellent properties, approaching the spider dragline silk (Shao and Vollrath, 2002). This suggests that the silkworm silk has the potential to produce better fibers, and the regenerated fibroin, which is easy to harvest, has the possibility to be fabricated into a reconstituted super-fiber. 

The Tyr residue in the B. inori silk fibroin present in the repetitive region is about 80% of the total Tyr content. Besides its presumed role in hydrogen bonding, the Tyr residue also plays an important role as an active site for covalently immobili/ing enzymes to silk fibers." S.c. ricini consists of a polyalanine ((Ala)n n = 10-14) which is followed by a Gly-rich sequence containing the bulky residues, as reminiscent of spider dragline silk. The Tyr residue exists mainly in the Gly-rich environment and approximately 60% of all Tyr in the sequence is present as -Tyr-Gly-Gly-Gly- or -Gly-Gly-Gly-Tyr-. ... 

Most silkworm cocoon and spider dragline silk fibers contain assembled antiparallel p-pleated-sheet crystalline structures [33-35]. Silks are considered semicrystalline materials with 30-50% crystallinity in spider silks, 62-65% in cocoon silk fibroin from the silkworm B. mori, and 50-63% in wild-type silkworm cocoons. In the -sheet erystals the polymer chain axis is parallel to the fiber axis. The extent to which these struetures form, as well as their orientation and size, directly impact the mechanical features of silk fibers. Furthermore, the polyalanine repeats or... 

Comparison of Mechanical Properties of Common Silks (Silkworm and Spider Dragline) to Several Types of Biomaterial Fibers and Tissues Commonly Used Today... 

T. Izdebski, P. Akhenblit, J.E. Jenkins, J.L. Yarger, G.P. Holland, Structure and dynamics of aromatic residues in spider sHk 2D carbon correlation NMR of dragline fibers. Biomacromolecules 11 (2010) 168-174. 

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