'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
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.
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).