Engineering Bombyx Mori Silk into High-Energy Water Responsive Actuators

Water-responsive (WR) materials that expand and contract in response to changes in relative humidity (RH) have shown a great potential to serve as powerful actuator components for various applications, including soft robotics, sensors, and energy harvesting systems. Spider silk is a natural material that has been demonstrated a high WR actuation energy density, surpassing actuation energy densities of natural muscles and conventional actuators. However, spider silk’s relatively low production efficiency limits their usage for practical applications. Here, we present that Bombyx mori (B. mori) silk with similar protein structures as spider silk was processed to exhibit higher WR energy densities. To mimic spider silk’s secondary structures, we treat regenerated B. mori silk fibroin with water vapor and methanol under various conditions to increase B. mori silk's beta-sheet crystallinity to different degrees. By immersing regenerated B. mori silk fibroin in methanol for 1 min, its beta-sheet volume ratios increase from 19.5% to 57.6%, and its energy density increases by 8-fold from 202 kJ m^-3 to 1626 kJ m^-3 which exceed WR energy density of spider silk (500 kJ m^-3). We speculate that, during the hydration/dehydration processes of silk, a higher crystallinity of silk reduces energy dissipation and translates the chemical potential of water induced pressure to external loads more efficiently. The processability, low-cost, and availability of B. mori silk opens up the possibilities for simple and scalable production of high-energy WR actuators for various applications.