Herbert Waite
Marine Science Institute & MCDB Department, University of California, Santa Barbara, CA 93106 USA

In the manufacture of multi-component products, the proper way to join parts with mismatched critical mechanical properties remains a great technical challenge. Insufficient attention to mismatch can and does lead to product failure due to the build-up of residual stresses at joint interfaces. Biological organisms often indulge in joining ostensibly mismatched materials such as tendon and bone, dentin and enamel, nail and skin. Yet surprising little is known about how these materials are connected. We have investigated adaptations for coping with modulus mismatch in a simple biological material - mussel byssus. Byssal threads consist of two joined parts: a stiff distal and extensible proximal portion. The Young's moduli of these portions differ by a factor of ten. The molecular transition from one portion to another is mediated by block copolymers that are essentially collagens with flanking silk- or elastin-like domains. While maintaining the collagen domain throughout, these exhibit a graded substitution of stiff silk-like blocks in the distal portion for softer rubbery ones in the proximal portion. The transition occurs over the course of 5 to 10 mm in the middle of a fiber that averages 40 mm in total length. The modulus mismatch would thus appear to be reduced or eliminated by the abolition of a sharp interface.