Spider Silk: Structure, Engineering, and Applications - Soft Fibrillar Materials: Fabrication and Applications - page 189

(a) (b) (c)

Figure 6.6 The mechanism of fiber
supercontraction. (a) Static linkers (green
extended chains) in as-drawn dragline silk
connect crystalline regions (gold pleated
blocks) and are stabilized by interchain hy-
drogen bonds (red dotted lines). (b) As the
hydrated fiber is allowed to contract, an
abrupt transition to a highly mobile phase

(blue blurred chains) occurs in some re-
gions, increasing local strain and enforcing
quiescence in neighboring regions. (c) For
a fiber allowed to fully supercontract, more
regions become mobile. (Reprinted with
permission from Ref. [56]. Copyright (2004)
American Chemical Society.)

another remarkable property of this accomplished material and demonstrates the
potential usefulness of such fibers in harsh environments.

6.3
Mimicking Spider Silk

To mimic spider dragline silk successfully one must copy the crucial design features
of both the feedstock dope proteins and the spinning process. Molecular biology
provides the potential to extract, synthesize, and assemble artificial genes to supply
feedstocks for the production of silks [57-60], whereas classical structural studies
provide construction details of the spider's extrusion system [61-64]. Together,
these two biological disciplines can lead to the design of first prototypes for a
highly advanced and benign fiber extrusion technology, and close collaboration
with process engineering should eventually allow commercialization.

6.3.1
Genetic Engineering

The potential use of spider silk as a new biomaterial has led to the evaluation of
various heterologous expression systems for the production of recombinant spider
silk-like proteins [65]. Partial cDNA constructs of dragline silk protein were cloned
and expressed in Escherichia coli [66], mammalian cell lines (MAC-T/bovine and
BHK (baby hamster kidney)/hamster) [67], insect-cell lines [68, 69], and transgenic
silkworm larvae [69]. Designer synthetic genes based on Nephila clavipes spider
dragline and flagelliform protein sequences have also been expressed in E. coli