Spider silk as rubber

The silks produced by spiders are exceptional structural materials. Although their tensile strengths are similar to those of cellulose, collagen and chitin, their extensibilities are considerably greater 1,2 . Thus, the energy required to break spider silk (that is, its toughness) can be 10 times greater than for these other biological materials. The silks are crystalline proteins, with substantial portions of their polypeptide chains forming crystallites of stacked β -pleated sheets 3–6 . However, the extensibility of silk is attributed to noncrystalline regions, which appear amorphous on X-ray diffraction 7,8 . The mechanical properties and macromolecular conformation of these amorphous regions have been the subject of much speculation 1,2,7–10 , but little direct experimentation. Here we show that these regions can behave as rubber networks and exhibit rubber-like or entropy elasticity.