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MXene, which is known for its high electrical/thermal conductivity, surface hydrophilicity, excellent mechanical flexibility, and chemical stability, is a versatile and smart material for soft actuators. However, most MXene actuators are fabricated by combining MXene with other inert materials to form a bilayer or multilayer structure. Considering the strain mismatch at multimaterial interfaces under frequent deformation, MXene‐based actuators are generally associated with poor stability, which limits their practical applications. Herein, inspired by the natural quantum‐confined superfluidic (QSF) effect, a multiresponsive MXene actuator that can be driven by moisture, light, and electricity by engineering an asymmetric QSF structure on both sides of the MXene film is reported. The actuation mechanism of the MXene film can be attributed to nonuniform water adsorption, transport, and desorption within the asymmetric QSF channels under moisture, photothermal, and electrothermal stimuli. Interestingly, MXene actuators can be flexibly formed into various shapes under moisture‐assisted mechanical compression, which not only enhances their multiresponsive actuation, but also permits a more complex deformation. As proof‐of‐concept demonstrations, various intriguing applications including a dual‐role robot, a smart shielding curtain, and a dragonfly robot, are fabricated, revealing the potential of MXene actuators for soft robotics. [ABSTRACT FROM AUTHOR]

Graphene oxide (GO), which has many oxygen functional groups, is a promising candidate for use in moisture‐responsive sensors and actuators due to the strong water–GO interaction and the ultrafast transport of water molecules within the stacked GO sheets. In the last 5 years, moisture‐responsive actuators based on GO have shown distinct advantages over other stimuli‐responsive materials and devices. Particularly, inspired by nature organisms, various moisture‐enabled soft robots have been successfully developed via rational assembly of the GO‐based actuators. Herein, the milestones in the development of moisture‐responsive soft robots based on GO are summarized. In addition, the working mechanisms, design principles, current achievement, and prospects are also comprehensively reviewed. In particular, the GO‐based soft robots are at the forefront of the advancement of automatable smart devices. [ABSTRACT FROM AUTHOR]

Graphene-based actuators featuring fast and reversible deformation under various external stimuli are promising for soft robotics. However, these bimorph actuators are incapable of complex and programmable 3D deformation, which limits their practical application. Here, inspired from the collective coupling and coordination of living cells, we fabricated a moisture-responsive graphene actuator swarm that has programmable shape-changing capability by programming the SU-8 patterns underneath. To get better control over the deformation, we fabricated SU-8 micropattern arrays with specific geometries and orientations on a continuous graphene oxide film, forming a swarm of bimorph actuators. In this way, predictable and complex deformations, including bending, twisting, coiling, asymmetric bending, 3D folding, and combinations of these, have been achieved due to the collective coupling and coordination of the actuator swarm. This work proposes a new way to program the deformation of bilayer actuators, expanding the capabilities of existing bimorph actuators for applications in various smart devices. [ABSTRACT FROM AUTHOR]

• A complementary strategy can work as a universal design principle for dual-responsive actuators. • Combining moisture-active/alkane-inert GO with alkane-active/moisture-inert PDMS enables dual-responsive actuation. • GO and PDMS bimorph features fast and reversible response to both moisture and alkane. Smart actuators that enable deforming in a predictable manner under external stimuli have revealed great potential for both traditional and emerging industries. Generally, an asymmetric bilayer structure with one layer active and the other inert to a certain stimulus is essential to realize bending behavior. However, towards the development of dual- or multi-responsive actuators, it still lacks universal and effective strategies for rational design and fabrication of such devices through the simplest way. In this paper, we report a complementary strategy to produce dual-responsive bilayer actuators by combining the moisture-active/alkane-inert graphene oxide (GO) layer with the alkane-active/moisture-inert polydimethylsiloxane (PDMS) layer. The GO@PDMS bimorph actuator can switch its active and inert layers in response to moisture and alkane, respectively, realizing dual-responsive deformation under different actuations. Typical dual-responsive actuators, including a selective air valve and a grip and hook smart claw, are successfully fabricated, demonstrating the capability of effective gases and objects transmission. The complementary bimorph actuator may hold great promise for developing intelligent devices and portable delivery systems. [ABSTRACT FROM AUTHOR]

Abstract: Actuators that can convert environmental stimuli into mechanical work are widely used in intelligent systems, robots, and micromechanics. To produce robust and sensitive actuators of different scales, efforts are devoted to developing effective actuating schemes and functional materials for actuator design. Carbon‐based nanomaterials have emerged as preferred candidates for different actuating systems because of their low cost, ease of processing, mechanical strength, and excellent physical/chemical properties. Especially, due to their excellent photothermal activity, which includes both optical absorption and thermal conductivities, carbon‐based materials have shown great potential for use in photothermal actuators. Herein, the recent advances in photothermal actuators based on various carbon allotropes, including graphite, carbon nanotubes, amorphous carbon, graphene and its derivatives, are reviewed. Different photothermal actuating schemes, including photothermal effect–induced expansion, desorption, phase change, surface tension gradient creation, and actuation under magnetic levitation, are summarized, and the light‐to‐heat and heat‐to‐work conversion mechanisms are discussed. Carbon‐based photothermal actuators that feature high light‐to‐work conversion efficiency, mechanical robustness, and noncontact manipulation hold great promise for future autonomous systems. [ABSTRACT FROM AUTHOR]

Hydrogels such as poly(ethylene glycol) diacrylate are a class of cross-linked polymers that have the ability to absorb water and change volume. Here, a humidity responsiveness of hydrogel film fabricated by the photopolymerization of poly(ethylene glycol) diacrylate (PEG-DA) monomer is reported. This kind of film could be driven by a small humidity gradient, thus spontaneous deformation and motion can be achieved by placing the film onto a moist filter paper. The influence factors of humidity response sensitivity such as the exposure time and number-average molecular weight of PEG-DA monomer used during the fabrication are investigated. Under different relative humidity, the film displays different degrees of pink color and the intensities of fluorescence under 365 nm illumination are also different, which renders the film to be used as a 'humidity test strip.' A humidity-driven walking device is fabricated and the walking velocity is about 3 mm min−1 under the actuating water vapor. Vapor of volatile polar solvents could also drive the actuator to achieve fast deformation and shows the variety of the film responsivity. [ABSTRACT FROM AUTHOR]

Inspired by natural autonomous systems that demonstrate controllable shape, appearance, and actuation under external stimuli, a facile preparation of moisture responsive graphene-based smart actuators by unilateral UV irradiation of graphene oxide (GO) papers is reported. UV irradiation of GO is found to be an effective protocol to trigger the reduction of GO; however, due to the limited light transmittance and thermal relaxation, thick GO paper cannot be fully reduced. Consequently, by tuning the photoreduction gradient, anisotropic GO/reduced GO (RGO) bilayer structure can be easily prepared toward actuation application. To get better control over the responsive properties, GO/RGO bilayer paper with a certain curvature and RGO patterns are successfully prepared for actuator design. As representative examples, smart humidity-driven graphene actuators that mimic the cilia of respiratory tract and tendril climber plant are successfully developed for controllable objects transport. [ABSTRACT FROM AUTHOR]

Soft robots enabling controllable movement in a predicable manner have attracted enormous research interests. However, current researches on soft robots are mainly limited to single actuation mode, and thus complex and multidimensional motility is significantly limited. Here, we propose and demonstrate for the first time a multi-field-coupling energy conversion strategy for flexible manipulation of soft robots, by which 3D-positioning, rotating, levitating, capturing and releasing of a small object by a smart claw are synergistically realized via magnetic, ultrasonic, humidity and light fields. The essence of the strategy is coupling design of the robot compositional materials that can convert energy from different fields into efficient mechanical works. As a typical example, we developed a magnetic-field-assisted gradient assembly method for preparing asymmetric Fe 3 O 4 nanoparticles (NPs) and graphene oxide (GO) composite film. The asymmetric distribution of Fe 3 O 4 NPs in GO leads to an reasonable mechanical stiffness and alters the water adsorption capability of the two sides, which not only imparts multi-responsiveness but also suitably addresses the problem of interlayer detachment in the case of bimorph actuators. The robust soft robots demonstrate good durability, revealing great potential for developing advanced robotic systems that permit direct conversion of multi-field energies to mechanical works. Image 1 • The strategy of multi-field-coupling energy conversion has been adopted for manipulating soft robots. • External magnetic field assists the assembly of Fe 3 O 4 NPs and GO sheets into an asymmetric composite film. • Magnetic, ultrasonic, humidity and light energies can be synergistically coupled with the robot. • A smart claw capable of flexible motility and multi-form deformation is demonstrated. [ABSTRACT FROM AUTHOR]

Graphene oxide (GO) with tunable physical/chemical properties is a versatile material for smart bimorph actuators. Using GO or its derivatives (e.g., reduced GO, RGO) as an active material, actuators can be manipulated under various external stimuli including moisture, light, temperature, and electricity. However, most of these GO‐based actuators respond to a solo stimulus, which limits its cutting‐edge applications in soft robotics. Here, the programmable patterning of RGO/GO Janus paper using a threshold‐controlled direct laser writing (DLW) technology is reported. By combining the RGO/GO Janus paper with a common thermally expansive polymer layer, a moisture, light, and electricity multiresponsive actuator is fabricated, in which the RGO interlayer plays multiroles including moisture‐passive layer, photothermal/electrothermal layer, thermal conductive layer, and inert layer for thermal expansion. As a proof of concept, multiresponsive actuators including a bidirectional cantilever and a smart curtain are fabricated, demonstrating the great potential for developing soft robots. [ABSTRACT FROM AUTHOR]

The strong interaction between water molecules and graphene oxide (GO) enables moisture‐responsive graphene actuators, revealing great potential for soft robots. However, current strategies for developing smart graphene actuators fail to tailor their material property gradient in a controlled manner, and the driving manner is usually limited to single stimulus actuation. Here, a facile preparation of humidity/thermal/light multiresponsive graphene actuators by sequential vacuum filtration of GO and reduced GO (RGO) aqueous solutions is reported. The photoreduction degree of RGO layer is tuned precisely beforehand by changing ultraviolet (UV) light irradiation time, and thus a pretailored reduction gradient along the normal direction of the GO/RGO bilayer paper would form in a highly controlled manner. Taking advantage of the competitive water adsorption between the GO and RGO layers, as well as the thermal‐, light‐promoted desorption, the GO/RGO bilayers deform in response to moisture, light, and temperature changes; and the deformation degree can be modulated by controlling the gradient of oxygen‐containing groups (OCGs). As a proof of principle, a humidity‐responsive graphene mimosa and a humidity/thermal/light multiresponsive graphene actuators are fabricated. The GO/RGO bilayer paper with pretailored reduction gradient holds great promise for easy fabrication of biomimetic actuators that enable performing predictable deformation. Humidity/thermal/light multiresponsive graphene actuators are prepared by sequential vacuum filtration of graphene oxide (GO) and reduced GO (RGO) with a pretailored reduction gradient. GO/RGO bilayer actuators with tunable curving performance are achieved by controlling the photoreduction degree. Humidity responsive mimosa and multiresponsive paper robot are designed based on the competitive water adsorption between GO/RGO layers and the thermal‐, light‐promoted water desorption. [ABSTRACT FROM AUTHOR]