Humidity‐Responsive Actuator with Viewable and Programmable Deformation Enabled by Tannic Acid as Physical Cross‐linker

Abstract Programmable humidity‐driven actuators can autonomously transform into predefined forms in reaction to environmental stimuli, which are appealing for the creation of intelligent devices. Herein, this study reports a viewable in situ programmed humidity‐driven monolayer actuator consisting of hydrophilic poly(vinyl alcohol) (PVA), polyacrylamide (PAAm), and tannic acid (TA). Benefiting from the multiple hydrogen bonding of TA with PVA and PAAm, TA acts as a physical cross‐linker to precisely control the modulus distribution of the monolayer film when introducing patterns by TA in the polymer film. Moreover, the physical anchoring effect of TA to PVA and PAAm makes TA molecules highly fluorescent in solid state. The resulting PVA/PAAm/TA film simultaneously exhibits high optical transparency (>80%), good mechanical property (tensile stress at break of 75.0 MPa, Young’s modulus of 7.1 GPa), bright cyan fluorescence, and reversible fast humidity‐response (0.04 s) under the relative humidity difference (ΔRH) of 55%. After in‐plane patterning, the film actuator can bend toward controllable directions in response to humidity gradient, and its programmed mode can be viewed by the fluorescence under UV light. This viewable and programmable approach will substantially aid the design of humidity‐driven actuators for applications in the fields of biomimicry, artificial muscles, and intelligent switches.