A Reversible Photoresponsive Forward Osmosis-Based Actuator

Fluid-based (hydraulic or pneumatic) actuators are widely used in the field of soft robotics due to their high force output and large strokes, but their reliance on external pumps makes them bulky and limits their use in remotely actuated systems. Turgor-driven deformation and stiffness modulation represent emerging bioinspired actuation strategies that enable untethered soft actuators. These systems show unique promise for photomechanical devices because of the possibility of using optically thick samples without sacrificing photochemical conversion. Here, we demonstrate reversible, photoresponsive modulation of osmotic pressure in an aqueous solution of an azobenzene-containing polymer, arising from the polarity difference between trans- and cis-isomers. Building on this, we developed a forward-osmosis-based bulging-disk actuator that exhibits nonlinear deformation under load and a work capacity of 0.3 kJ/m3 at room temperature. Finally, supported by strong agreement between model predictions and experimental observations, we analyze actuator performance through the lens of thermodynamic principles and material properties.