ABSTRACT
Acknowledgments .............................................................................................................. 400
References .......................................................................................................................... 401
Silk is an externally spun fibrous protein secretion formed into fibers, usually resulting in
material structures such as cocoons or webs [1-4]. Silks are essentially pure proteins, with only
in some Of all the natural silks represent the
only ones that are spun. Silk fibers from silkworms have been used in textiles for nearly
5000 years. The primary reasons for this longtime use have been the unique luster, tactile
properties, durability, and dyability of silks. Silk fibers are remarkable materials displaying
unusual mechanical properties: strong, extensible, and mechanically compressible. Silks also
display interesting thermal and electromagnetic responses, particularly in the UV range for
insect entrapment and form crystalline phases related to processing. Silk fibers were used in
optical instruments as late as the mid-1900s because of their fine and uniform diameter and
high strength and stability over a range of temperatures and humidity. Naturalist reports
suggest that some spider silks were used in the South Pacific for gill nets, dip nets, and
fishing-a testimony to the remarkable mechanical properties and durability of this family of
protein polymers. Silks have historically been used in medicine as sutures over the past 100
years and are currently used today in this mode along with a variety of consumer product
applications. Commercially, silkworm cocoons are mass produced in a process termed
‘‘sericulture’’. The cocoons are extracted in hot soapy water to remove the sericin glue-
like protein. The remaining fibroin or structural silk is reeled onto spools, yielding appro-
ximately 300-1200 m of usable thread per cocoon. These threads can be dyed or modified
for textile applications. The annual world production of raw silk is about 60,000 tons,
with China producing half of the world supply followed by India, Korea, and Japan
(http:==www.dawn.com=2005=06=27=ebr6.htm). Silks represent one member of a larger class of fibrous proteins in nature, which include
keratins, collagens, elastins, and others [5]. These types of proteins can be considered nature’s
equivalent of synthetic block copolymers. Aside from their direct use in materials applica-
tions, fibrous proteins provide experimentally accessible model systems with simpler and well-
controlled genetic template-based protein synthesis. The highly repetitive structure allows key
features of the primary sequences of these proteins to be captured in shorter consensus
sequences at the corresponding genetic level. Short synthetic genetic variants can then be
combined to generate larger genes and thus proteins that represent mimics of the native
protein. This technique is useful in simplifying the complex behavior of these proteins to an
intelligible level, while retaining their biological relevance and materials function. These
shorter genetic variants, when polymerized (multimerized) into longer genes, can be used to
explore protein sequence and size relationships.
