Unlocking giant negative thermal expansion through reversible conformational changes in regenerated silk fibroin film

Developing thermally contractive polymeric materials with controlled conformational changes that enable more efficient actuation mechanisms has long been an ambitious goal. However, the utilization of naturally derived assemblies with biodegradable and reversible capabilities has, thus far, been hindered by the challenges associated with organizing the hierarchical nanostructures. Herein, hierarchically structured building blocks in regenerated fibroin were utilized to construct a thermal-responsive protein film through a friction-induced assemble strategy. By preserving the protofibrils within the regenerated silk film, we enable the efficient storage and transfer of elastic energy to external loads. The freestanding fibroin film, with pre-extended molecular chains, thus produced exhibited an exceedingly high negative thermal expansion (−1220 ppm K−1) and work capacity (∼203 J kg−1). The comprehensive analysis involving synchrotron infrared spectroscopy, in-situ Raman spectroscopy, and molecular dynamics simulations has confirmed that the actuating mechanism entails a reversible conformational change initiated by H-bonds, which is then amplified into macroscopic deformations by the hierarchical structure. This newly discovered, low-energy-driven mechanism paves the way for the creation of flexible protein assemblies with high-performance actuation capabilities.