Spider Silk Fibers Solely Composed out of 20-nm Thin Fibrils*

The origin of spider silk’s outstanding mechanical properties, combining high strength and high extensibility, has been intensively studied for decades. Its hierarchical structure has been recognized to play a significant role. Due to the lack of unambiguous experimental evidence, competing and incompatible structural models of natural silk fibers have been proposed, some of them including various types of fibrillar components. To fully understand the hierarchical structure of silk, we study the silk of the recluse (Loxosceles) spider. While exhibiting the outstanding mechanical properties of a typical spider silk, Loxosceles silk is not cylindrical, but ribbon-like, with a width of 6–8 μm and a thickness of less than 50 nm, only a few protein layers thin. Our work found that the Loxosceles ribbons are entirely composed of 20-nm thin protein fibrils that are several micrometers long and oriented parallel to the fiber axis. Hence, a single nanofibril embodies all key properties of spider silk. The idea of nanofibrils as the major building blocks suggests a paradigm change in both the investigation and the understanding of structure-property relationships of silk, which will further the state of the art in silk research and silk-inspired high-performance materials.