Combining Living Microorganisms with Regenerated Silk Provides Nanofibril-Based Thin Films with Heat-Responsive Wrinkled States for Smart Food Packaging.

Regenerated silk (RS) is a protein-based "biopolymer" that enables the design of new materials; here, we called "bionic" the process of regenerated silk production by a fermentation-assisted method. Based on yeast's fermentation, here we produced a living hybrid composite made of regenerated silk nanofibrils and a single-cell fungi, the yeast extract, by fermentation of such microorganisms at room temperature in a dissolution bath of silkworm silk fibers. The fermentation-based processing enhances the beta-sheet content of the RS, corresponding to a reduction in water permeability and CO₂ diffusion through RS/yeast thin films enabling the fabrication of a mechanically robust film that enhances food storage durability. Finally, a transfer print method, which consists of transferring RS and RS/yeast film layers onto a self-adherent paraffin substrate, was used for the realization of heat-responsive wrinkles by exploiting the high thermal expansion of the paraffin substrate that regulates the applied strain, resulting in a switchable coating morphology from the wrinkle-free state to a wrinkled state if the food temperature overcomes a designed threshold. We envision that such efficient and smart coatings can be applied for the realization of smart packaging that, through such a temperature-sensing mechanism, can be used to control food storage conditions.