Bio-Inspired High Sensitivity of Moisture-Mechanical GO Films with Period-Gradient Structures

Untethered Soft Robots Based on 1D and 2D Nanomaterials

Biological structures exhibit autonomous and intelligent behaviors, such as movement, perception, and responses to environmental changes, through dynamic interactions with their surroundings. Inspired by natural organisms, future soft robots are also advancing toward autonomy, sustainability, and interactivity. This review summarizes the latest achievements in untethered soft robots based on 1D and 2D nanomaterials. First, the performance of soft actuators designed with different structures is compared. Then, the development of basic locomotion forms, including crawling, jumping, swimming, rolling, gripping, and multimodal, mimicking biological motion mechanisms under dynamic stimuli, is discussed. Subsequently, various self-sustained movements based on imbalance mechanisms under static stimuli are introduced, including light tracking, self-oscillating, self-crawling, self-rolling, and flying. Following that, the progress in soft actuators integrated with additional functionalities such as sensing, energy harvesting, and storage is summarized. Finally, the challenges faced in this field and the prospects for future development are discussed.

MXene-based solvent-responsive actuators with a polymer-intercalated gradient structure

Actuators based on electrically conductive and hydrophilic two-dimensional (2D) Ti3C2T X MXene are of interest for fast and specific responses in demanding environments, such as chemical production. Herein, Ti3C2T X -based solvent-responsive bilayer actuators were developed, featuring a gradient polymer-intercalation structure in the active layer. These actuators were assembled using negatively charged pristine Ti3C2T X nanosheets as the passive layer and positively charged polymer-tethered Ti3C2T X as the active layer. 2D wide-angle X-ray scattering and simulations related the gradient polymer intercalated microstructure in the polymer/MXene composite active layer to the counterintuitive actuation behavior. The bending of the bilayer films in solvent vapor is triggered by the gradient polymer-intercalation and the differing diffusion rate of solvent molecules through the MX and MX-polymer layers of the bilayer actuator. With their ease of fabrication, remote light-control capabilities, and excellent actuation performance, the Ti3C2T X -based bilayer actuators reported here may find applications in areas such as sensors for monitoring chemical production, infrared camouflage, smart switches, and excavators in toxic solvent environments.

Research Progress on Moisture-Sorption Actuators Materials

Actuators based on moisture-sorption-responsive materials can convert moisture energy into mechanical/electrical energy, making the development of moisture-sorption materials a promising pathway for harnessing green energy to address the ongoing global energy crisis. The deformability of these materials plays a crucial role in the overall energy conversion performance, where moisture sorption capacity determines the energy density. Efforts to boost the moisture absorption capacity and rate have led to the development of a variety of moisture-responsive materials in recent years. These materials interact with water molecules in different manners and have shown diverse application scenarios. Here, in this review, we summarize the recent progress on moisture-sorption-responsive materials and their applications. We begin by categorizing moisture-sorption materials-biomaterials, polymers, nanomaterials, and crystalline materials-according to their interaction modes with water. We then review the correlation between moisture-sorption and energy harvesting performance. Afterwards, we provide examples of the typical applications using these moisture-sorption materials. Finally, we explore future research directions aimed at developing next-generation high-performance moisture-sorption materials with higher water uptake, tunable water affinity, and faster water absorption.

Ultrafast Bi-Directional Bending Moisture-Responsive Soft Actuators through Superfine Silk Rod Modified Bio-Mimicking Hierarchical Layered Structure

Development of stimulus-responsive materials is crucial for novel soft actuators. Among these actuators, the moisture-responsive actuators are known for their accessibility, eco-friendliness, and robust regenerative attributes. A major challenge of moisture-responsive soft actuators (MRSAs) is achieving significant bending curvature within short response times. Many plants naturally perform large deformation through a layered hierarchical structure in response to moisture stimuli. Drawing inspiration from the bionic structure of Delosperma nakurense (D. nakurense) seed capsule, here the fabrication of an ultrafast bi-directional bending MRSAs is reported. Combining a superfine silk fibroin rod (SFR) modified graphene oxide (GO) moisture-responsive layer with a moisture-inert layer of reduced graphene oxide (RGO), this actuator demonstrated large bi-directional bending deformation (-4.06 ± 0.09 to 10.44 ± 0.00 cm-1) and ultrafast bending rates (7.06 cm-1 s-1). The high deformation rate is achieved by incorporating the SFR into the moisture-responsive layers, facilitating rapid water transmission within the interlayer structure. The complex yet predictable deformations of this actuator are demonstrated that can be utilized in smart switch, robotic arms, and walking device. The proposed SFR modification method is simple and versatile, enhancing the functionality of hierarchical layered actuators. It holds the potential to advance intelligent soft robots for application in confined environments.

Selaginella lepidophylla-Inspired Multi-Stimulus Cooperative Control MXene-Based Flexible Actuator

Predictable bending deformation, high cycle stability, and multimode complex motion have always been the goals pursued in the field of flexible robots. In this study, inspired by the delicate structure and humidity response characteristics of Selaginella lepidophylla, a new multilevel assisted assembly strategy was developed to construct MXene-CoFe2O4 (MXCFO) flexible actuators with different concentration gradients, to achieve predictable bending deformation and multi-stimulus cooperative control of the actuators, revealing the intrinsic link between the gradient change and the bending deformation ability of the actuator. The thickness of the actuator shows uniformity compared with the common layer-by-layer assembly strategy. And, the bionic gradient structured actuator shows high cycle stability, and it maintains excellent interlayer bonding after bending 100 times. The flexible robots designed based on the predictable bending deformation and the multi-stimulus cooperative response characteristics of the actuator initially realize conceptual models of humidity monitoring, climbing, grasping, cargo transportation, and drug delivery. The designed bionic gradient structure and unbound multi-stimulus cooperative control strategy may show great potential in the design and development of robots in the future.

Silicon Distribution-Induced Actuation Film with Bidirectional Bending Deformation and Versatile Bionic Applications

As an important branch of intelligent materials, the research and development of stimulus-responsive flexible intelligent actuation materials is of great significance to promote the industrialization of intelligent materials. In this study, the asymmetric PVA-co-PE/silicon nanoparticle (PPSN) composite films and PVA-co-PE/silicon sol (PPSS) composite film with different silicon distributions were prepared by a simple spraying method. The silicon nanoparticle layer in the PPSN composite film was similar to the sand-like water-absorbing layer, which can quickly absorb water and permeate it into the interior region, leading to the hygroscopic expansion behavior on one side of the nanofiber film. Then, the PPSN composite film would quickly bend and deform to the silicon nanoparticle side. However, in the PPSS composite film, due to the excellent hygroscopicity and swelling characteristics of the silica sol layer, the composite film can be rapidly deformed to the PVA-co-PE nanofiber film side under moisture stimulation. The above results subvert the traditional asymmetric actuation film, which mainly depends on the hydrophilicity difference to determine the hygroscopic responsiveness and deformation direction, and ignore that the swelling degree is the main factor determining the bending direction of actuator. In addition, both the composite films can quickly respond to moisture stimulation (<1 s) and produce large-scale bending deformation (180°). Furthermore, due to the excellent interface adhesion formed by the continuity structure in the PPSS composite film, it has better actuation stability than the PPSN composite film. The excellent actuation characteristics and different bending directions of the PPSN and PPSS composite films make it a great application prospect in the field of bionics in the future.

Development and challenges of smart actuators based on water-responsive materials

Water-responsive (WR) materials, due to their controllable mechanical response to humidity without energy actuation, have attracted lots of attention to the development of smart actuators. WR material-based smart actuators can transform natural humidity to a required mechanical motion and have been widely used in various fields, such as soft robots, micro-generators, smart building materials, and textiles. In this paper, the development of smart actuators based on different WR materials has been reviewed systematically. First, the properties of different biological WR materials and the corresponding actuators are summarized, including plant materials, animal materials, and microorganism materials. Additionally, various synthetic WR materials and their related applications in smart actuators have also been introduced in detail, including hydrophilic polymers, graphene oxide, carbon nanotubes, and other synthetic materials. Finally, the challenges of the WR actuator are analyzed from the three perspectives of actuator design, control methods, and compatibility, and the potential solutions are also discussed. This paper may be useful for the development of not only soft actuators that are based on WR materials, but also smart materials applied to renewable energy.

Bio-Inspired Bianisotropic Magneto-Sensitive Elastomers with Excellent Multimodal Transformation

Magneto-sensitive soft materials that can accomplish fast, remote, and reversible shape morphing are highly desirable for practical applications including biomedical devices, soft robotics, and flexible electronics. In conventional magneto-sensitive elastomers (MSEs), there is a tradeoff between employing hard magnetic particles with costly magnetic programming and utilizing soft magnetic particle chains causing tedious and small deformation. Here, inspired by the shape and movement of mimosa, a novel soft magnetic particle doped shape material bianisotropic magneto-sensitive elastomer (SM bianisotropic MSE) with multimodal transformation and superior deformability is developed. The high-aspect-ratio shape anisotropy and the material anisotropy in which the magnetic particles are arranged in a chainlike structure together impart magnetic anisotropy to the SM bianisotropic MSE. A magneto-elastic analysis model is proposed, and it is elucidated that magnetic anisotropy leads to peculiar field-direction-dependent multimodal transformation. More importantly, a quadrilateral assembly and a regular hexagon assembly based on this SM bianisotropic MSE are designed, and they exhibit 2.4 and 1.7 times the deformation capacity of shape anisotropic samples, respectively. By exploiting the multidegree of freedom and excellent deformability of the SM bianisotropic MSE, flexible logic switches and ultrasoft magnetic manipulators are further demonstrated, which prove its potential applications in future intelligent flexible electronics and autonomous soft robotics.

Localizable, Identifiable, and Perceptive Untethered Light-Driven Soft Crawling Robot

Soft robots based on bionics have attracted extensive attention in recent years. However, most of previous works focused on the motion of robots that were incapable of communication and perception. In this work, an untethered crawling robot is proposed with integration of motion, communication, and location based entirely on a flexible material, which is capable of being utilized as a sensing platform. The hydrophilic graphene oxide film, capable of photothermal conversion, allows the robot to undergo a large deformation stimulated by near-infrared light. Conductive fabric with low resistivity and high mechanical strength, replacing the traditional rigid circuit, is utilized to complete the communication of the robot. The designed communication module allows an electrical signal to be inductively coupled to the soft robot instead of being generated by batteries or through transmission lines. The perception of the robot is demonstrated by covering sensitive materials. Furthermore, the positioning and identification of the robot are verified by an external coil array. The proposed soft crawling robot provides an innovative strategy for the integration of multifunctional robots and shows great potential in bionic devices, intelligent robots, and advanced sensors.

Graphene Oxide/Nanofiber-Based Actuation Films with Moisture and Photothermal Stimulation Response for Remote Intelligent Control Applications

The rapid development of intelligent technology and industry has induced higher requirements for multifunctional materials, especially intelligent materials with stimulus-responsive self-actuation behavior. In this study, a Cu@PVA-co-PE/GO composite actuation film, with an asymmetric sandwich structure, was prepared by attaching graphene oxide (GO) to the surface of a polyvinyl alcohol ethylene copolymer (PVA-co-PE) nanofiber composite film containing copper nanoparticles (Cu) through layer-on-layer adsorption. This unique structural design endowed the composite film with not only excellent structural stability but also different bending directions (in response to moisture and infrared light). The actuation performance shows that when the adsorption time was 4 h, the maximum bending angle of the Cu@PVA-co-PE/GO composite film was up to 90° within 5.99 s. Furthermore, the actuation behavior was stable after 100 cycles of reversible moisture stimulation. Additionally, the maximum actuation strain of the composite film was up to 1.35 MPa during the illumination time of 6.8 s and maintained an excellent stability for 400 s under continuous infrared stimulation of 0.53 W/cm². The rapid and sensitive stimulus response of the Cu@PVA-co-PE/GO composite film exhibited self-actuation behavior under the remote control of moisture and infrared light. This, in turn, suggests prospects for wide applications in emerging technologies, such as intelligent switches, artificial muscles, intelligent medical treatment, and flexible robots.

Multifunctional Ti3C2Tx MXene/Low-Density Polyethylene Soft Robots with Programmable Configuration for Amphibious Motions

To diversify the motion modes of multifunctional soft robots capable of shape programming, we fabricate a biomimetic and programmable Ti₃C₂T_x MXene/low-density polyethylene (LDPE) bilayer actuator by spraying an aqueous dispersion of MXenes onto a plasma-activated LDPE film, followed by optimal thermal regulations. Because of the eminent light absorption and photothermal/electrothermal features of MXenes and the extremely mismatched thermal expansion coefficients between the two layers, the MXene/LDPE actuator can be sensitively driven by many stimuli of near-infrared light, electricity, and heat. The initial configuration of the bilayer actuator can be programmed by tuning the thermal regulation temperature, thereby assembling multiple actuation units to achieve biomimetic functions, such as artificial iris, mechanical arms, and flying birds. More importantly, in virtue of free shape cutting and programmable configuration, the MXene/LDPE bilayer actuator can perform untethered locomotion including crawling, rolling, and sailing. The soft robots can not only move on the ground in different forms but also sail on water along any designated routes and complete the surface cargo transportation driven by a near-infrared laser via the photothermal Marangoni effect. The shape-programmable methodology for the three amphibious motion modes lays foundations for wide applications of the MXene-based soft robots.

Tough and Multifunctional Composite Film Actuators Based on Cellulose Nanofibers toward Smart Wearables

Although humidity-responsive actuators serve as a promising candidate in smart wearables, artificial muscles, and biomimetic devices, most of them derived from synthetic polymers could not simultaneously achieve multifunctional properties. In this work, a cellulose nanofiber (CNF)-based film actuator with high mechanical properties, excellent Joule heating, and antibacterial capability is successfully constructed by integrating with Ti₃C₂T_x (MXene) and tannic acid (TA) via a vacuum-assisted filtration approach. Owing to the unique nacrelike structure and strong hydrogen bonds, the tensile strength and toughness of the composite film could reach 275.4 MPa and 10.2 MJ·m^-3, respectively. Importantly, the hydrophilic nature of CNFs and alterable interlayer spacing of MXene nanosheets endow the composite film with sensitive humidity response and extraordinary stability (1000 cycles). With the assistance of MXene nanosheets and TA, the composite film could not only present outstanding Joule heating but also possess remarkable antibacterial properties against both Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus. Benefiting from the above merits, the proof-of-concept smart garment is assembled by the as-prepared film and is capable of regulating humidity and temperature.

Design of MXene Composites with Biomimetic Rapid and Self-Oscillating Actuation under Ambient Circumstances

Although responsive actuators have been intensively investigated, it remains challenging to enable rapid and self-oscillating actuation under ambient circumstances without human intervention analogous to living organisms. By hybridizing a unique type of two-dimensional nanomaterials (i.e., MXene) with a particular hydrophilic polymer, a smart and flexible conductive composite was produced with rapid actuation and spontaneous oscillation near a moist surface. Due to the presence of layered microstructures and the moisture-sensitivity improved by surface roughness and intercalated polymeric layers, the composites could reversibly bend up to 180° in 2 s or 210° in 10 s on demand when the circumstantial humidity was varied, being superior or comparable to many actuators in the literature. More importantly, the composite was capable not only of flipping upside down repeatedly on the moist surface but also of self-oscillating ceaselessly under ambient gradient humidity without human intervention, e.g., an oscillation between 30 and 100° with an oscillation frequency of 0.08 Hz. This self-oscillation resulted from the occurrence of rapid asymmetrical hydration and dehydration of the composite between the regions of high and low humidity, which could further be modulated both by different hydrophilic polymers and by photoradiation owing to the photothermal effect of MXene nanosheets. Because of the ubiquitous presence of humidity gradient near the moist surface, this type of smart composite may not only offer a strategy for designing artificial materials that are capable of spontaneous actuation under ambient circumstance without human intervention but also promise potential applications in artificial muscles, autonomous robotics, and energy harvesting from environments.

Self-Healing and Reconfigurable Actuators Based on Synergistically Cross-Linked Supramolecular Elastomer

Stimulus-responsive soft actuators show great potential in intelligent robot systems for their various virtues, such as arbitrary shape morphing, outstanding adaptability to environment, and multidegrees of freedom. However, it is extremely challenging to achieve a combination of excellent actuating performance and robust mechanical strength as well as self-healing property. Herein we report a near-infrared light-responsive soft actuator based on the synergistic effects of a crystalline physical cross-linked network and a hydrogen bonding supramolecular network. The actuator exhibits outstanding comprehensive performance including fast and reliable light-responsive behavior (bending angle over 90° within 1.6 s), robust mechanical strength (12.52 MPa), superfast self-healing speed (2 s), and satisfactory self-healing efficiency in both mechanical (87.68%) and actuating (99.50%) performance. In addition, it is convenient to fabricate and reconfigure the actuators by a mild-temperature molding strategy to acquire various three-dimensional structures, thus achieving diverse actuating locomotion. This work provides a powerful and facile strategy to prepare soft actuators with intriguing performance, allowing significant progress in broadening their practical application.