Polymorphic Phase-Dependent Vapor-Responsive Actuation in Silk Fibroin Films: Control of Bending Angle and Axis

The composition of polymorphic phases, such as amorphous α-helices, crystalline β-sheets, and other secondary structures, in protein films plays a pivotal role in defining their physical, chemical, and mechanical properties. <i>Bombyx mori</i> silk, which inherently possesses tunable secondary structures, can be easily tailored by processes such as mechanical stretching, water annealing, or exposure to specific solvents. In this work, we investigate the vapor-responsive actuation behavior of silk fibroin films fabricated via a nontoxic, aqueous-based processing route. In particular, we elucidate the influence of the polymorphic phase composition on the water vapor actuation characteristics of the films. The different fractions of secondary structures are achieved by exposing the silk films to various alcohols. We also introduce a novel approach to modulate crystallinity by controlling the ambient humidity during the drying process of the silk aqueous solutions. Our experiments clearly reveal a nonmonotonic dependence of actuation performance on the content of crystalline β-sheets. The actuation characteristics initially improve with increasing β-sheet crystallinity but deteriorate once the fraction exceeds approximately 35%. Furthermore, we demonstrate that the directionality of actuation (bending axis) can be precisely controlled by imprinting periodic microgrooves onto the film surface. As a proof of concept, we present an autonomously actuated "autobreathe" window designed to dynamically regulate humidity levels within a sealed chamber.