The Cribellate Nanofibrils of the Southern House Spider—Extremely Thin Natural Silks with Outstanding Extensibility

Fibrils are fundamental building blocks of the most prevalent structural materials in the living world. This includes spider silks, featuring a strength akin to steel and extensibility reaching that of rubber. The most basic building blocks of these silks are their nanofibrils, formed from proteins in a variety of secondary structures. Despite their importance, the mechanical properties of individual spider silk nanofibrils have not been tested before. The cribellate silk of the <i>Kukulcania hibernalis</i> spider features a very sophisticated structure, including the thinnest known individualized natural silk fibrils, only 5 nm thick. Here we test the nanomechanics of these thin individualized silks through three-point bending using force spectroscopy. They showed an outstanding extensibility of ~1,100%, more than double the highest extensibility, ~475%, currently achieved by any spider silk. Nanofibrils are not much larger than the protein secondary structures found in silk, yet they represent the outstanding mechanical properties of spider silk. Therefore, this research provides a unique opportunity to relate the macroscopic properties of spider silk to its molecular structure. For instance, the large extensibility may be directly related to unfolding of the protein structure.