'High Performance Fibers from Nature: Silk Processing and Assembly by Insects and Spiders'
Silks are externally spun protein fibers generated by spiders and insects (Kaplan et aI., 1994). Reeled silkworm silk (Bombyx mori) has been used in the textile industry for over 5,000 years. Unlike silkworm silks, spider silk production has not been domesticated because spiders are more difficult to raise in large numbers due to their solitary and predatory nature. In addition, orb webs are not reelable as a single fiber and they generate only small quantities of silk. Silkworms can be raised in large numbers and generate one type of silk at one stage in their lifecycle, forming the basis for the sericulture industry. Many spiders have evolved families of silk proteins (different polymer chain chemistries- primary amino acid sequences) with different functions. For example, the spider, Nephila clavipes, generates at least six different silks from sets of different glands, each silk specifically matched to function- such as for environmental glues, strong or flexible web components, prey capture, and encapsulation (cocoons) for offspring development.
Silks are of interest for their remarkable mechanical properties as well as their durability, luster, and "feel." Silk fibers generated by spiders and silkworms represent the strongest natural fibers known, even rivaling synthetic high performance fibers in terms of mechanical properties (Gosline et al., 1986) The best properties of N. clavipes native dragline fibers collected and tested at quasi static rates were 60 and 2.9 GPa for initial modulus and ultimate tensile strength, respectively. In addition, these fibers display resistance to mechanical compression that distinguishes them from other high performance fibers (Cunniff et aI., 1994). Based on microscopic evaluations of knotted single fibers, no evidence of kink-band failure on the compressive side of a knot curve was observed. Synthetic high performance fibers fail by this mode even at relatively low stress levels. Silks are mechanically stable up to almost 200C (Cunniff et aI., 1994).
Spider dragline and silkworm cocoon silks are considered semi crystalline materials with the crystalline components termed b-sheets (Gosline et aI., 1986). Most silks assume a range of different secondary structures during processing from water-soluble protein in the glands to water-insoluble spun fibers. Marsh et al. (1955) first described the crystalline structure of silk as an anti-parallel hydrogen bonded b-sheet. The unit cell parameters in the silk II structure (the spun form of silk that is insoluble in water) are: 0.94 nm (inter chain), 0.697 nm (fiber axis), 0.92 nm (inter sheet). These unit cell dimensions are consistent with a crystalline structure in which the protein chains run anti parallel with inter chain hydrogen bonds perpendicular to the chain axis between carbonyl and amine groups, and van der Waal forces stabilizing the inter sheet interactions (based on the predominance of short side chain amino acids such as glycine, alanine, and serine in b-sheet regions). Generally, silkworm fibroin in cocoons contains a higher content of crystallinity (b-sheet content) than spider dragline silks such as from N. clavipes.
As more protein sequence data from various spiders and silkworms has been elucidated, it is clear that these families of silk proteins are similar but also encompass a range of sequence variations that reflect their functional properties. Silkworm fibroin is the protein that forms the structural aspects of the fibers. These fibers are encased in a family of glue-like sericin proteins. The primary sequence of amino acids of these proteins is found to be highly repeated, thus, small regions of sequence chemistry in the protein chain are found elsewhere in other regions of the chains. This design feature is critical to the function of this group of proteins as structural materials. This design feature also allows the sequences of these large proteins to be represented in relatively short sequences in terms of polymer design. This has led to the option of forming synthetic genetic variants to represent the larger proteins, a useful laboratory technique to enhance the ability to understand these proteins in a simplified form. These shorter genetic pieces can be polymerized (multimerized) into longer genes to explore sequence and size relationships.
These powerful tools in molecular biology facilitate direct insight into the role of sequence chemistry, protein block sizes and distributions, and protein polymer chain length on materials structure and function. Native and synthetic silk clones have been generated in a variety of heterologous expression systems, including bacteria, yeast, insect cells, plants, and mammalian cells (Wong and Kaplan, 2002).