Water-Responsive Actuation of Bombyx Mori silk/Silica Nanocomposites

Water-responsive (WR) materials can convert chemical potential of water into mechanical energy when they swell and shrink in response to humidity changes. Nature has shown notable WR materials such that spider silk can forcefully and reversibly deforms in response to changes in relative humidity, and its WR actuation energy density reaches ~500 kJ m^-3, higher than those of conventional actuators and artificial muscles. However, the fundamental mechanisms leading to spider silk’s outstanding water-responsiveness remain unclear. Notably, spider silk contains a large volume ratio stiff β-sheet crystal domains (36-37%) that could play a crucial role in spider silk’s WR actuation. Here, we showed that, by simply adding stiff silica nanoparticles that mimic β-sheet crystals, Bombyx (B.) mori silk’s WR actuation can be dramatically increased from ~200 kJ m^-3 to ~700 kJ m^-3, surpassing that of spider silk. When the volume concentration of silica nanoparticles is increase from 0% to 50%, silk/silica nanocomposites show slightly increased Young’s moduli from 3.2 GPa to 4.1 GPa, and silk’s β-sheet crystallinity increased from 21% to 32%. We found that, when the silica nanoparticle’s concentration reaches 17%, silk/silica nanocomposites show a maximum WR energy density of 698 kJ m^-3. However, when the silica nanoparticle’s concentration keeps increasing, the energy density decreases, which could attribute to a higher degree aggregation of silica nanoparticles. Our findings suggest that stiff silica nanoparticles can dramatically reduce energy dissipation during hydration and dehydration processes and enhance amorphous silk’s water-responsiveness, and the simple and scalable silk/silica nanocomposites could be used as powerful WR actuators for broad applications.