Page 338 - Fiber Fracture

320 C. Viney

ECHINODERM COLLAGENS: FIBRE OPTIMISATION IN SMART
COMPOSITES

We have reflected on several issues that are important to fracture characterisation
and control in the context of natural self-assembled fibrous materials. To conclude this
discussion, it is appropriate to consider some of nature’s best examples of optimisation
and versatility in fibre-reinforced composites: sea cucumber body and the ‘catch’
ligament associated with the ball-and-socket joint at the base of the sea urchin spine
(Fig. 7). These materials are among the living world’s oldest fibrous composites, which
is testimony to their successful design. They offer several time-tested and thought-
provoking lessons for the materials engineer.
(1) Their mechanical properties are not permanent; they change on a physiological
timescale typically measured in seconds (Trotter et al., 2000b).
(2) They demonstrate how controlled, reversible ‘melting’ of one phase in a mi-
crostructure need not cause an overall loss of cohesion, and can enable useful mechani-
cal functions.
(3) They demonstrate how the failure of interfibrillar bonds in a composite can be
controlled to tailor the net load-bearing properties of the composite.
(4) They demonstrate how water plays a central role in controlling the load-bearing
ability and failure resistance of natural materials.
(5) They make use of tapered rather than cylindrical fibres, to provide reinforcement
with the minimum amount of ‘expensive’ material.

Tensile Property Control

Sea cucumbers and sea urchins control the tensile properties of some connective
tissues by regulating stress transfer between collagen fibrils (Trotter et al., 2000b).
Strength and stiffness can vary by more than an order of magnitude between the stiff
and pliant states. Interactions between the fibrils are regulated by water-soluble macro-
molecules. Based on these observations, progress is being made towards developing a
synthetic material that exhibits dynamically controlled tensile properties. The artificial
route involves identifying reagents that bind covalently to fibril surfaces and reversibly
form cross-links with other (synthetic) reagents. At present, that work is focussed on a
chemically controlled stress-transfer capacity. However, molecular processes which can
be effected by chemical control can often be achieved by thermodynamically equivalent
triggers (e.g. electric fields, light, pressure, temperature, pH: Urry, 1992). In principle,
the artificial analogue could contain self-assembled fibres made from a genetically engi-
neered polymer. In this way, detailed tailoring of structure and properties across several
length scales could be coupled with dynamic property control of the bulk material.

Tapered Fibres

The collagen fibres in sea cucumber dermis and sea urchin ligament are tapered rather
than cylindrical. The shape is ensured by the nucleation and growth mechanism (Trotter
et al., 1998; Trotter et al., 2000a) by which the fibres are formed (Fig. 8). We will see