Nacre-inspired composite film with mechanical robustness for highly efficient actuator powered by humidity gradients

Exploring and fabricating smart actuating materials that strike the perfect balance between humidity response and mechanical integrity (especially wet tensile strength) via reasonable structural design and simple yet low-cost preparation is critical for biomimetic devices, soft robotics, artificial muscles and generators, but it remains challenging. Herein, inspired by the structure of natural nacre, we demonstrated a robust yet highly sensitive composite film-based humidity actuator composed of carboxymethyl cellulose (CMC), MXene nanosheets, and multivalent aluminum ions (Al3+) via a facile evaporation-induced self-assembly method. The synergistic reinforcing effects of MXene nanosheets and Al3+ through hydrogen and ionic bonding as well as the densely hierarchical microstructure endow the composite film with both an ultrahigh mechanical strength (273.6 MPa), a desirable toughness (7.95 MJ/m3) and even an impressive wet tensile strength (154.2 MPa) at 97 % humidity. Interestingly, the unique laminated structure and water-induced swelling effect of CMC and MXene synergistically enable the composite film with large shape deformation, sensitive actuation (less than 2.3 s) and exceptional cycling stability (over 1500 cycles) upon exposure to humidity gradients. Based on the above merits, the composite film actuator can be well constructed to simulate a flying dragonfly, human finger, artificial muscle, and has also been preliminarily employed as a moist-electric generator, which provides new insight for designing comprehensive composite film-based actuators and reveals their extensive applications.