Breakthrough Assembly of a Silk Fibroin Composite for Application in Resistive Pressure Sensing

Driven by the dictates of sustainability, we have designed, realized, and optimized a method for easy development of biocompatible, highly porous, and electrically conductive 3D structures from the combination of natural and synthetic polymers for pressure sensing applications. In particular, a foaming method followed by a fast freezing step, both performed on blends made of silk fibroin (SF) aqueous solution, PEDOT:PSS electrically conductive polymer, and water-soluble PVA, has allowed the fabrication of conductive electrosponges, intrinsically integrating the structural and electrical counterparts of a resistive pressure sensor in a single "green" material. An exhaustive analysis of their structural (with FTIR), morphological (with μ-CT), and mechanical (by means of stress-strain measurements) properties has been performed, of which the latter was coupled with the electrical characterization of the electrosponges while undergoing compression-decompression cycles. PVA addition has been recognized as crucial for conferring to the material the right compromise among elasticity, recovery attitude, and resilience/durability to the proposed constructs. The fabricated electrosponges show a promising combination of mechanical and electrical properties, with the former induced by both the highly porous structure of the foamed/frozen compound and the elasticity enhancement induced by PVA, whose concentration influences the electrosponge resilience and recovery attitude. Based on the results from the material characterization, the composite with 1% v/v PVA content has shown the best compromise among elasticity, resilience, and shape recovery. The related sensor shows a sensitivity comparable to other hybrid SF composites (10^-3 kPa/mA vs 10^-3-10^-2 kPa/decade), an applied stress magnitude-dependent swiftness (from hundreds of milliseconds to few seconds), and an exhaustive current recovery on numerous repeated compression-decompression cycles in wet conditions.