Hydrogen-Bonding-Dependent Water-Responsive Actuation of Bacillus subtilis Cell Walls

Water-responsive (WR) materials can exert significant forces when they deform in response to changes in the relative humidity. Recent studies on biological WR materials have brought attention to the potential influence of nanoconfined water on high-energy WR actuation. Here, we investigated the effects of nanoconfined liquids on the WR actuation of Bacillus subtilis cell walls by introducing chaotropic or kosmotropic solutes, known for their impact on the H-bonding network and biomolecule stabilities in aqueous solutions. We discovered that cell walls treated with low-concentration kosmotropic solutes exhibited a significant increase in the WR actuation energy density, reaching 103.3 MJ m^-3, surpassing that of existing actuators. However, higher concentrations of kosmotropic or chaotropic solutes led to decreased WR performance. These findings could be explained by the impact of the solutes on hydration forces and intermolecular interactions, which affect the ultimate WR pressure. This, in turn, provides a pathway toward achieving superior WR actuation performance and advancing the development of high-work-density actuator materials.