A bioinspired monolayer gel with efficient omnidirectional moisture-driven actuation for humidity sensing

Given the ubiquitous availability of moisture as an energy source, moisture-responsive actuators have attracted significant attention in recent years. Most reported designs feature bilayer architectures that respond to uniform moisture, or monolayered structures that rely on humidity gradients. However, achieving mechanical motion in monolayers under uniform moisture remains challenging. Here, we present a monolayered moisture-responsive actuator that bends mechanically upon uniform exposure to moisture or in water. The actuator is fabricated from branched polyethylenimine (BPEI) and polyethene glycol diacrylate (PEGDA) via an in situ copolymerization and casting-evaporation process. A coating of octadecyl acrylate (ODA) is applied covalently on one side of the gel to achieve the actuation. The resulting actuator exhibits an unidirectional bending response under uniform moisture conditions. The extent of bending was systematically studied by varying PEGDA content, hydrophobic coating type, and the temperature of the aqueous environment. Importantly, the actuation mechanism differs fundamentally from previously reported moisture-driven systems. Upon contact with water, surface amine groups undergo partial protonation, forming a dense hydrogen-bonded network involving amide groups, ammonium ions, free amines, and water molecules. This network induces surface shrinkage rather than swelling, thereby driving motion. The highest bending angle achieved is 140°. The strategic covalent integration of hydrophilic and hydrophobic components imparts anisotropy to the monolayer, enabling excellent bending even under omnidirectional moisture exposure. Several proof-of-concept demonstrations, including biomimetic and soft-robotic applications, were conducted. Additionally, the actuator's performance as a contactless electrical switch underscores its promise for next-generation smart and adaptive devices.