Recreating Silk's Fibrillar Nanostructure by Spinning Solubilized, Undegummed Silk

The remarkable toughness (>70 MJ m^-3) of silkworm silk is largely attributed to its hierarchically arranged nanofibrillar nanostructure. Recreating such tough fibers through artificial spinning is often challenging, in part because degummed, dissolved silk is drastically different to the unspun native feedstock found in the spinning gland. The present work demonstrates a method to dissolve silk without degumming to produce a solution containing undegraded fibroin and sericin. This solution exhibits liquid-liquid phase separation above 10% (wt/wt), a behavior observed in the silk gland but not in degummed silk solutions to date. This partitioning enhances the stability of the undegummed solution, delaying gelation two-fold compared with degummed silk at the same concentration. When spun under identical conditions, undegummed solutions produces fibers 8× stronger and 218× tougher than degummed silk feedstocks. Through ultrasonication, undegummed wet spun fibers are seen to possess hierarchical structure of densely packed ≈20 nm nanofibrils, similar to native silks, although completely absent from fibers wet-spun from degummed silk solutions. This work demonstrates that the preservation of molecular weight, presence of sericin and stimulation of liquid-liquid phase separation underpin a new pathway to recreate a hierarchical fiber with structures akin to native silk.