Smart devices with abilities of perceiving, processing, and responding are attracting more and more attentions due to the emerging development of artificial intelligent systems, especially in biomimetic and intelligent robotics fields. Designing a smart actuator with high flexibility and multistimulation responsive behaviors to simulate the movement of creatures, such as weight lifting, heavy objects carrying via simple materials, and structural design is highly demanded for the development of intelligent systems. Herein, a soft actuator that can produce reversible deformations under the control of light, thermal, and humidity is fabricated by combining high photothermal properties of CNT/PDMS layer with the natural hydrophilic GO layer. Due to the asymmetric double-layer structure, the novel bilayer membrane-based actuator showed different bending directions under photothermal and humidity stimulations, resulting in bidirectional controllable bending behaviors. In addition, the actuation behaviors can be well controlled by directionally aligning the graphene oxide onto carbon nanotube/PDMS layer. The actuator can be fabricated into a series of complex biomimetic devices, such as, simulated biomimetic fingers, smart "tweezers", humidity control switches, which has great potential applications in flexible robots, artificial muscles, and optical control medical devices.

Biomimetic actuators with rapid response speed, high sensitivity, and selectivity to external stimulus have found potential applications in smart switches, artificial muscles, and soft robots. The nanoscale structures of actuators enhance the exposed area to stimulus as well as enable versatile control of the actuation behaviors. Freestanding, flexible, and porous water-driven actuators with poly­(vinyl alcohol-co-ethylene) (EVOH) nanofibers as the substrate and super hydrophilic nanoscale cellulose materials (cellulose nanofibers, cellulose nanocrystals, bacterial cellulose) as the active substance via uniform mixing or surface depositing were fabricated. The effects of the EVOH nanofiber substrate, the structures and concentrations of nanoscale cellulose materials, as well as the different environmental stimuli like humidity and temperature on the performance of actuators were studied. The water-driven actuation mechanism was proposed from the macroscopic and molecular aspects and the analysis of Gibbs free energy and mechanical energy. The actuator could bend to an angle of 180° and recovered less than 1 s for more than 100 circles without compromising properties when the environmental moisture changed. Furthermore, the multidimensional deformation behaviors of the water-stimulated actuators could also be well tuned by varying the orientations of the nanoscale materials. Additionally, the applications of the prepared actuator were demonstrated.

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Smart actuators with excellent flexibility, sensitive responsiveness, large-scale bending-deformation, and rapid deformation-recovery performance have been sought after by researchers. Two-dimensional graphene oxide (GO) is considered as an ideal candidate for humidity-responsive actuators because of its excellent moisture sensitivity. Herein, a flexible membrane-based actuator was prepared by evenly dispersing GO sheets into a three-dimensional network formed by onedimensional PVA-co-PE nanofibers (NFs) and silver nanowires (AgNWs). The three-dimensional interlaced pore structure of the AgNWs/NFs/GO composite membrane ensured its larger contact area (19.33 m 2 /g), faster moisture exchange rate, and large bending deformation under moisture stimulation. In addition, a new explanation for the spatial distribution of adsorbed water molecules and their actuating effect on the bending behaviors of composite membranes is proposed. The adsorbed water lies between the interlayer and surface layer of the composite membrane. The interlayer water molecules make the film volume expand, resulting in a large bending angle of the membrane. On the other hand, the water on the surface layers of the membrane only leads to the change in film weight, having little effect on the bending behavior. Moreover, to make the soft actuator more practical and multifunctional, a conductive AgNWs-NFs/GO bilayer membrane-based actuator was prepared by layered spraying of a AgNW on the NFs/GO membrane, which can be directly used in switching control circuits. The novel flexible membrane-based actuators are of great significance for the soft robot and intelligent control systems in the future.

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PVA-co-PE nanofiber-based membrane facilely decorated using chitosan/graphene-oxide nanosheets for the dual-purpose of filtration and inactivation of bacteria.