Spider fiber

The structures of some natural protein-based materials, such as silk and wool, result in strong, tough fibers. Spiders and silkworms use proteins as a structural material of remarkable strength (Fig. 19.22). Chemists are duplicating nature by making artificial spider silk (Fig. 19.23), which is one of the strongest fibers known. 

Fiber, spider silk These DuPont fibers, on an equal weight basis, are stronger than steel. They are also very elastic and tough. Their combination of strength and stretch makes the energy-to-break very... 

Dunaway, D.L., Thiel, B.L., Srinivasan, S.G. and Viney, C. (1995a) (Characterizing the cross-sectional geometry of thin, non-cylindrical, twisted fibers (spider silk). J. Mater. Sci., 30 4161-4170. 

Micro-diffraction techniques have been developed mainly at the ID 13 beamline of the European Synchrotron Radiation Facility (ERSF) with a beam size of 3-10 pm for viscose rayon fibers, spider silk, spherulites of P(3HB), and a poly(lactic acid)/(atactic-P(3HB)) blend. Recently, we developed the micro-diffraction techniques with 0.5 pm beam size for analysis ultra-high-molecular-weight-P(3HB) mono-filament ° and P(3HB) copolymer spherulites. To reveal the detail fiber structure and the distribution of two types of molecular conformations in drawn P(3HB-co-8%-3HV) mono-filament, a micro-beam X-ray diffraction experiment was performed with synchrotron radiation at SPring-8, Japan. The beam size was focused to 0.5 pm with the Fresnel Zone Plate technique and the P(3HB-co-8%-3HV) mono-filament was scanned linearly perpendicular to the fiber axis with a step of 4 pm. 

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. 

Spider silk is nature s high-performance fiber... 

An elegant NMR experiment by the group of Lynn Jelinski at Cornell University has established that at least part of the microcrystals is built up from the polyalanine repeats in the protein chains. These experiments, which were made on C-enriched proteins produced by feeding the spiders C-labeled alanine, showed that there were two populations of alanine side chains, one ordered and oriented perpendicular to the fiber axis and a second less ordered. Jelinski s interpretation is that parts of the polyalanine sequences are incorporated as p strands in the microcrystals with an orientation parallel to the fiber axis. Whether or not the Gly-Gly-X repeats also form P strands in the microcrystals remains an open question. 

Bram, A., et al. X-ray diffraction from single spider silk fibers. J. Appl. Cryst. 30 390-394, 1997. 

FIGURE 19.10 Synthetic fibers are made by extruding liquid polymer from small holes in an industrial version of a spider s spinneret. 

Lazaris, A., Arcidiacono, S., Huang, Y., Zhou, J.F., Duguay, F., Chretien, N., Welsh, E.A., Soares, J.W., and Karatzas, C.N., Spider silk fibers spun from soluble recombinant silk produced in mammalian cells. Science, 295(5554), 472- 76, 2002. 

Parkhe AD, Seeley SK, Gardner K (1997) Structural studies of spider silk proteins in the fiber. J Mol Recognit 10 1-6... 

The feeling of a spider web may be unsettling, but a similar natural material has been used for centuries to make silk fabric that is prized for its smooth texture. Silkworms produce the silk fibers used to make clothing. They feast on mulberry leaves and convert the molecules from these leaves into silk, from which they spin cocoons. 

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