Fragile-Tough Mechanical Reversion of Silk Materials via Tuning Supramolecular Assembly.

Regenerated silk nanofibers are interesting as protein-based material building blocks due to their unique structure and biological origin. Here, a new strategy based on control of supramolecular assembly was developed to regulate interactions among silk nanofibers by changing the solvent, achieving tough mechanical features for silk films. Formic acid was used to replace water related to charge repulsion of silk nanofibers in solution, inducing interactions among the nanofibers. The films formed under these conditions had an elastic modulus of 3.4 ± 0.3 GPa, an ultimate tensile strength of 76.9 ± 1.6 MPa, and an elongation at break of 3.5 ± 0.1%, while the materials formed from aqueous solutions remained fragile. The mechanical performance of the formic acid-derived nanofiber films was further improved through post-stretching or via the addition of graphene. In addition, the silk nanofiber films could be functionalized with various bioactive ingredients such as curcumin. These new silk nanofiber films with a unique combination of mechanical properties and functions provide new biomaterials achieved using traditional solvents and processes through insight and control of their assembly mechanisms in solution.