Relationships between supercontraction and mechanical properties of spider silk

Typical spider dragline silk tends to outperform other natural fibres and most man-made filaments(1). However, even small changes in spinning conditions can have large effects on the mechanical properties of a silk. bre(2-6) as well as on its water uptake. Absorbed water leads to significant shrinkage in an unrestrained dragline. bre(7,8) and reversibly converts the material into a rubber(9). This process is known as supercontraction(10) and may be a functional adaptation for the silk's role in the spider's web(11). Supercontraction is thought to be controlled by specific motifs in the silk proteins(12,13) and to be induced by the entropy-driven recoiling of molecular chains(9,14). In analogy, in man-made fibres thermal shrinkage induces changes in mechanical properties(15-17) attributable to the entropy-driven disorientation of 'unfrozen' molecular chains (as in polyethylene terephthalate) 15,18 or the `broken' intermolecular hydrogen bonds (as in nylons)(17). Here we show for Nephila major-ampullate silk how in a biological fibre the spinning conditions affect the interplay between shrinkage and mechanical characteristics. This interaction reveals design principles linking the exceptional properties of silk to its molecular orientation.