Designing Actuation in pH-Responsive Hydrogels

Polyelectrolyte hydrogels can reversibly swell and shrink in response to environmental stimuli such as pH and temperature, making them useful as smart materials with tunable properties. However, their application has been limited by slow response time and low power density. Here, we employ hybrid molecular dynamics and Monte Carlo simulations to investigate the actuation behavior of pH-responsive nanogels, explicitly accounting for dynamic protonation and hydrodynamic interactions. We find that polyelectrolyte hydrogels exhibit a closed-loop phase behavior bounded by two critical points. Near a critical point, nanogels undergo conformational switching on a microsecond time scale and generate work densities of approximately 10 5 J/m 3, exceeding those of skeletal muscles. Importantly, the power density scales as L –2, where L is the linear size of the gel. This suggests that high-performance actuators can be realized by keeping L small, for example, by stacking nanoscale gel components. These results establish a fundamental connection between microscopic structure and dynamic response, and offer quantitative guidelines for designing responsive hydrogels.