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Biological Materials in Engineering Mechanisms 367

Table 14.1 Comparison between Biological and Nonbiological Polymer or Materials
Synthesis and Assembly

Feature Laboratory Nature
Synthesis
Monomer Usually racemates Stereochemically pure
Blocks or domains Usually mono or diblocks Mono to highly diverse blocks
Polymerization Comparatively rapid — mostly Comparatively slow template
polydisperse control — monodisperse (pro-
teins, nucleic acids); others
polydisperse (polysaccharides)
Processing or assembly
Plasticizers Varied, mostly organic Water
Polymer interactions Chain entanglements, fringed Less chain entanglements,
micelle model extensive hydrogen bonding and
other weak interactions
Higher order structures features Varied, rare Common, controlled by chain
interactions
Organic–inorganic composites Usually mixtures, composites Molecular-level interfaces
controlled by weak bonds
Fate
Environmental stability Wide range of temperature Narrower range of temperature
Degradability Varies with polymer, most Universally degradable, rate
nondegradable matches function

14.2.1 Silk Processing and Assembly by Insects and Spiders
— High Performance Fibers from Nature

Background — Silks are externally spun protein fibers generated by spiders and insects (Kaplan
et al., 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 al., 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 2008C (Cunniff et al., 1994).
Spider dragline and silkworm cocoon silks are considered semicrystalline materials with the
crystalline components termed b-sheets (Gosline et al., 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 antiparallel hydrogen bonded b-sheet. The unit cell parameters in the silk II structure (the spun