Dependence of mechanical properties of lacewing egg stalks on relative humidity.

Silk fibers are well known for their mechanical properties such as strength and toughness and are lightweight, making them an interesting material for a variety of applications. Silk mechanics mainly rely on the secondary structure of the underlying proteins. Lacewing egg stalk silk proteins obtain a cross-β structure with individual β strands aligned perpendicular to the fiber axis. This structure is in contrast with that of silks of spiders or silkworms with β strands parallel to the fiber axis and to that of silks of honeybees with α helices arranged in coiled coils. On the basis of the cross-β structure the mechanical properties of egg stalks are different from those of other silks concerning extensibility, toughness, and bending stiffness. Here we show the influence of relative humidity on the mechanical behavior of lacewing egg stalks and propose a model based on secondary structure changes to explain the differences on a molecular level. At low relative humidity, the stalks rupture at an extension of 3%, whereas at high relative humidity the stalks rupture at 434%. This dramatic increase corresponds to breakage of hydrogen bonds between the β strands and a rearrangement thereof in a parallel-β structure.