Page 324 - Fiber Fracture

306 C. Viney

Right handed
a- el ix
h

Microfibril
double helices)
Fig. I. Hierarchical structure of a keratin microfibril, showing the molecular (A), double-helical (B) and
supercoil (C) twists in the constituent protofibrils. The representation of a molecular a-helix shows only the
[-N-C-C-I,, backbone for clarity. Note that the twists A occur in the opposite sense to twists B and C. If
an attempt is made to stretch the microfibrils, unwinding of twists A is resisted by tightening of twists B and
C, and unwinding of twists B and C is resisted by tightening of twists A. The hierarchy of structural order
therefore confers stability on the structure. In topological respects, we can regard this hierarchical structure
as a well-engineered small-scale version of a rope or yarn. Nature got there first.

supramolecular twists further decouple the net macroscopic mechanical properties of the
material from the intrinsic properties of the constituent polymer. Macroscopic properties
can therefore be modzjied by subjecting the native fibre to microstructurally invasive
processes such as weighting (silk: Chittick, 191 3), mercerising (cotton: Nishimura and
Sarko, 1987) or ‘mothproofing’ (wool: Billmeyer, 1984), but the degree of reproducible
property control in each case is limited.

Nature Revisited

Over the past two decades, we have substantially increased our understanding of
how nature produces organic fibres by polymerising available monomers into controlled
sequences and then self-assembling the product macromolecules into hierarchical mi-
crostructures. Progress has been catalysed by a renaissance in interdisciplinary science,
drawing on knowledge from the traditionally distinct fields of physiology, engineering,
materials characterisation and textile science, and incorporating convergent develop-
ments from the emerging disciplines of biotechnology and nanotechnology. Lessons
derived from observing nature, along with discoveries about how to manipulate nature
at the molecular level, have significantly expanded our expectations for fibrous proteins,
polysaccharides and other natural polymers.
(1) The primary structure (amino acid sequence) and the molecular weight of fibre-
forming proteins can be controlled exactly by genetic engineering. The amino acids
need not necessarily be those that are found in nature (Tirrell et al., 1997). In the case
of polysaccharides (Linton et al., 1991) and polyesters (Steinbuchel, 1991), the yield
and/or composition of the polymer can be controlled.