Nanocellulose‐based hydrogel membranes for underwater osmotic actuators

Nanocelluloses, as one of the most abundant natural biomass materials on the planet, are conventionally extracted from natural lignocellulosic fibers and present superior combined properties such as high hydrophilicity, low density, high mechanical strength, etc., which show attractive potential as the building blocks to construct different nanocellulose‐based hydrogel membranes for different and even for underwater actuator applications. Notably, the reported nanocellulose‐based hydrogel membranes for underwater actuators driven by osmotic pressure to date exhibit superior uniaxial actuation behaviors. We herein comprehensively summarize the present progress of nanocellulose‐based hydrogel membranes for underwater osmotic actuators, from the building blocks of nanocelluloses to the underwater osmotic actuation (OA) mechanism, including OA, electrochemical OA (ECOA), and corresponding underwater OA applications. Finally, we outline current challenges faced by nanocellulose‐based hydrogel membranes for advanced underwater osmotic actuators, such as ion/water transport kinetics in hydrogel membranes, controllability, and output power density. We also propose corresponding promising strategies for addressing these challenges, including the engineering of the nano‐building blocks, assembly techniques for nanocellulose‐based hydrogel membrane microstructures, programmable actuator configurations, and the integration into remote intelligent actuation systems, aiming to facilitate the high‐quality development of the next generation nanocellulose‐based underwater osmotic actuators.