Quantum-Confined-Superfluidics-Enabled Multiresponsive MXene-Based Actuators.

MXene, renowned for its natural "quantum-confined-superfluidic" (QSF) channels, demonstrates superior electrical/thermal conductivity, favorable hydrophilicity, and remarkable mechanical strength, rendering it an ideal candidate for multiresponsive actuators, which are promising for soft electronics and robots. Currently, most MXene-based actuators are mainly prepared by combining an active layer and an inner layer, with only a few utilizing regulated QSF channels. However, tailoring QSF channels for multiresponsive actuators is extremely challenging. Herein, we introduce a multiresponsive graphene oxide (GO)&Fe3O4/MXene actuator that can respond to humidity, light, heat, electricity, and magnetic fields by constructing asymmetric QSF channels. The asymmetric water adsorption, transportation, and desorption behaviors, controlled by the different QSF channels between the GO&Fe3O4 layer and the MXene layer, enable the multiresponsiveness of the actuator. As proof-of-concept demonstrations, several smart devices, such as a bionic crab-like crawler, a transporting flower robot, and a smart gripper, are prepared, holding great potential for advancing future soft robotics.

Heterogeneous self-healing assembly of MXene and graphene oxide enables producing free-standing and self-reparable soft electronics and robots.

Self-healing materials (SHMs) with unique mechanical and electronic properties are promising for self-reparable electronics and robots. However, the self-healing ability of emerging two-dimensional (2D) materials, for instance, MXenes, has not been systematically investigated, which limits their applications in self-healing electronics. Herein, we report the homogeneous self-healing assembly (homo-SHA) of MXene and the heterogeneous self-healing assembly (hetero-SHA) of MXene and graphene oxide (GO) under moisture treatments. The self-healing mechanism has been attributed to the hydration induced interlayer swelling of MXene and GO and the recombination of hydrogen bond networks after water desorption. The multiform hetero-SHA of MXene and GO not only enables facile fabrication of free-standing soft electronics and robots, but also endows the resultant devices with damage-healing properties. As proof-of-concept demonstrations, free-standing soft electronic devices including a generator, a humidity sensor, a pressure sensor, and several robotic devices have been fabricated. The hetero-SHA of MXene and GO is simple yet effective, and it may pioneer a new avenue to develop miniature soft electronics and robots based on 2D materials.

Reconfigurable, Reversible, and Redefinable Deformation of GO Based on Quantum-Confined-Superfluidics Effect.

Graphene oxide (GO) films with natural "quantum-confined-superfluidics" (QSF) channels for moisture actuation have emerged as a smart material for actuators and soft robots. However, programming the deformation of GO by engineering QSF nanochannels around 1 nm is extremely challenging. Herein, we report the reconfigurable, reversible, and redefinable deformation of GO under moisture actuation by tailoring QSF channels via moisture-assisted strain-induced wrinkling (MSW). The shape fixity ratio of a general GO film can reach ∼84% after the MSW process, and the shape recovery ratio is ∼83% at room temperature under moisture actuation. The flexible shaping and deformation abilites, as well as the self-healing property of GO make it possible to fabricate soft robots using GO. Besides, as a proof-of-concept, passive electronics and soft robots capable of crawling, turning, switching circuit, and automatic somersault are demonstrated. With unique shaping and deformation abilities, GO may bring great implications for future soft robotics.

Bioinspired Soft Robots Based on the Moisture-Responsive Graphene Oxide.

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.

Programmable deformation of patterned bimorph actuator swarm.

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.

Nacre-inspired moisture-responsive graphene actuators with robustness and self-healing properties.

Moisture-responsive actuators based on graphene oxide (GO) have attracted intensive research interest in recent years. However, current GO actuators suffer from low mechanical strength. Inspired by the robustness of nacre's structure, moisture-responsive actuators with high mechanical strength and self-healing properties were successfully developed based on GO and cellulose fiber (CF) hybrids. The hybrid paper demonstrated significantly improved tensile strength, ∼20 times higher than that of pure GO paper, and self-healing properties. A broken paper can be well cured under moist conditions, and the mechanical properties of the self-healed hybrid paper can still maintain similar tensile strength to the pristine one. After controllable ultraviolet light photoreduction treatment, a hybrid paper with a photoreduction gradient along the normal direction was prepared, which can act as a moisture-responsive actuator. A maximum bending curvature of ∼1.48 cm-1 can be achieved under high relative humidity (RH = 97%). As a proof-of-concept, a butterfly-like actuator that can deform itself with moisture actuation was demonstrated. Our approach may pave a new way for designing robust and self-healable graphene actuators.

Quantum-Confined-Superfluidics-Enabled Moisture Actuation Based on Unilaterally Structured Graphene Oxide Papers.

The strong interaction between graphene oxides (GO) and water molecules has trigged enormous research interest in developing GO-based separation films, sensors, and actuators. However, sophisticated control over the ultrafast water transmission among the GO sheets and the consequent deformation of the entire GO film is still challenging. Inspired from the natural "quantum-tunneling-fluidics-effect," here quantum-confined-superfluidics-enabled moisture actuation of GO paper by introducing periodic gratings unilaterally is reported. The folded GO nanosheets that act as quantum-confined-superfluidics channels can significantly promote water adsorption, enabling controllable and sensitive moisture actuation. Water-adsorption-induced expansion along and against the normal direction of a GO paper is investigated both theoretically and experimentally. Featuring state-of-the-art of ultrafast response (1.24 cm-1 s-1 ), large deformation degree, and complex and predictable deformation, the smart GO papers are used for biomimetic mini-robots including a creeping centipede and a smart leaf that can catch a living ladybug. The reported method is simple and universal for 2D materials, revealing great potential for developing graphene-based smart robots.

Direct laser writing of flexible planar supercapacitors based on GO and black phosphorus quantum dot nanocomposites.

The research interest in wearable electronics has continuously stimulated the development of flexible energy storage systems with high performance and robustness. However, open problems with respect to energy storage efficiency and device integration are still challenging. Here, we demonstrate the laser fabrication of flexible planar supercapacitors based on graphene oxide (GO) and black phosphorus quantum dot (BPQD) nanocomposites. By combining graphene and BPQDs, the resultant supercapacitors feature high conductivity and activity, demonstrating enhanced specific capacity and superior rate performance, compared to those based on reduced GO (RGO) alone. Furthermore, the as-obtained devices present outstanding flexibility. Their performance shows unobvious degradation after repeated cycles of bending and straightening. Additionally, with the help of direct laser writing (DLW) technology, integration of the supercapacitors has been achieved without the need for any metal interconnection. The integrated devices delivered reasonable performance uniformity with a voltage extension of 3 V, which could easily power a LED. The supercapacitor-based RGO and BPQD nanocomposites demonstrate great potential for practical applications in flexible and wearable electronics.

Facile fabrication of flexible graphene FETs by sunlight reduction of graphene oxide.

We reported here a facile fabrication of flexible graphene-based field effect transistors (FETs) by sunlight reduction of graphene oxide (GO) as channel material. As a mask-free and chemical-free method, sunlight photoreduction of GO without the use of any complex equipments is simple and green. The resultant FET demonstrated excellent electrical properties (e.g., an optimized Ion/Ioff ratio of 111, hole mobility of 0.17 cm2 V-1 s-1), revealing great potential for development of flexible microelectrics. Additionally, since the reduced GO channel could be fabricated by sunlight treatment between two pre-patterned electrodes, the process features post-fabrication capability, which makes it possible to integrate graphene-based devices with given device structures.

Light-Mediated Manufacture and Manipulation of Actuators.

Recent years have seen a considerable growth of research interests in developing novel technologies that permit designable manufacture and controllable manipulation of actuators. Among various fabrication and driving strategies, light has emerged as an enabler to reach this end, contributing to the development of actuators. Several accessible light-mediated manufacturing technologies, such as ultraviolet (UV) lithography and direct laser writing (DLW), are summarized. A series of light-driven strategies including optical trapping, photochemical actuation, and photothermal actuation for controllable manipulation of actuators is introduced. Current challenges and future perspectives of this field are discussed. To generalize, light holds great promise for the development of actuators.