(79c) The Influence of Water Structure on Water-Responsive Actuation of Bombyx Mori Silk

Water-responsive materials have recently shown record-high energy densities as they swell and shrink in response to changes in environmental humidity. These materials hold great potential as high-performance actuators for a broad range of engineering applications, including smart textiles, robotics and new types of energy-harvesting devices. However, the physical mechanisms governing stress generation are still not clear, which limits our ability to design or engineer these materials. In our study, we used three samples of differently post-treated regenerated Bombyx mori silk fibroin, which have various secondary structures. Consequently, each sample presented humidity-dependent water structures with distinct populations of bulk-like mobile water and structured bound water. These samples also showed different water-responsive properties, as well as the inception of dehydration-induced stress as the samples started actuating at different relative humidity levels. However, despite their different secondary structures, all three samples share a similar bound-to-mobile water ratio, above which the materials start to induce force. This transition was found to correspond to a viscous to rubbery phase transition. Our findings suggest that the water structure plays an important role in silk’s water-responsiveness, and they could serve as a predictive guideline for the water-responsive behavior of regenerated silk fibroin, and perhaps more generally, for other water-responsive materials.