What makes spider silk fibers so strong? From molecular-crystallite network to hierarchical network structures.

A Hierarchical Network (HN) model of soft matter was put forward to explain the mechanical properties of animal silk fibers. At the nano-micro level, the silk fibers consist of a bundle of twisted nano-fibrils with strong friction among them. At the nano-fibril level, β-crystallites together with silk molecular chains constitute the molecular networks. According to the model, the influences of different structural parameters, i.e. the ordering, and the density of β-nanocrystallites, on the breaking stress of silk fibers were analyzed quantitatively. It turns out that a better alignment of β-crystallites, a larger number of β-crystallites within the cross-section of a nano-fibril and a smaller effective loading area of a peptide chain will correlatively lead to stronger silk fibers. This is in excellent agreement with our observations for both spider dragline and silkworm silk fibers, and explains the fact that the spider dragline silk fibers having a lower crystallinity are much stronger than silkworm silk fibers. Furthermore, it was found that at the nanofibril scale, the interlock among the adjacent nanofibrils in the nanofibril bundle serves as a crack-stopper, which restricts the propagation of cracks. Such a structure reinforces the silk fibers significantly. The knowledge obtained will shed light on how to obtain ultra-strong fibrous materials from the structural point of view.