The fractal self-assembly of the silk protein sericin

Silk consists primarily of two proteins—a fibrous core protein, fibroin, and a glue protein, sericin, which envelops the fibroin fibers with sticky layers thus helping in the formation of cocoon, achieved by cementing the silk fibers together. Sericin, a water soluble protein, has traditionally been discarded in silk processing, despite great potential for use as a biomaterial. Here we show that this glue protein, sericin, has the ability to form self-assembled nano and microstructures with hierarchical self-similarity across length scales in the form of a diffusion-limited, fractal assembly. Sericin obtained from two silkworms, a domesticated mulberry Bombyx mori and a wild non-mulberry Antheraea mylitta, were studied, with particular insight into its structure and morphology as it dried on a surface. High-resolution atomic force microscopy was used to image the self-assembled protein, with investigations on the various factors that influenced this process. We describe the self-assembly patterns formed by the sericin protein and analyze the images based on the theory of diffusion limited aggregation to explain the fractal nature of these architectures observed. The unique physical behavior and fractal nature of this glue protein may represent a key step in understanding its biological relevance as well as the role it plays in the assembly and formation of silks.