Molecular mechanistic origin of the toughness of natural adhesives, fibresand composites

Natural materials are renowned for their strength and toughness(1-5). Spide r dragline silk has a breakage energy per unit weight two orders of magnitu de greater than high tensile steel(1,6), and is representative of many othe r strong natural fibres(3,7,8). The abalone shell, a composite of calcium c arbonate plates sandwiched between organic material, is 3,000 times more fr acture resistant than a single crystal of the pure mineral(4,5). The organi c component, comprising just a few per cent of the composite by weight(9), is thought to hold the key to nacre's fracture toughness(10,11). Ceramics l aminated with organic material are more fracture resistant than non-laminat ed ceramics(11,12), but synthetic materials made of interlocking ceramic ta blets bound by a few weight per cent of ordinary adhesives do not have a to ughness comparable to nacre(13). We believe that the key to nacre's fractur e resistance resides in the polymer adhesive, and here we reveal the proper ties of this adhesive by using the atomic force microscope(14) to stretch t he organic molecules exposed on the surface of freshly cleaved nacre. The a dhesive fibres elongate in a stepwise manner as folded domains or loops are pulled open. The elongation events occur for forces of a few hundred picon ewtons, which are smaller than the forces of over a nanonewton required to break the polymer backbone in the threads. We suggest that this 'modular' e longation mechanism might prove to be quite general for conveying toughness to natural fibres and adhesives, and we predict that it might be found als o in dragline silk.