High-performance electrically responsive artificial muscle materials for soft robot actuation

Traditional robotic devices are often bulky and rigid, making it difficult for them to adapt to the soft and complex shapes of the human body. In stark contrast, soft robots, as a burgeoning class of robotic technology, showcase exceptional flexibility and adaptability, positioning them as compelling contenders for a diverse array of applications. High-performance electrically responsive artificial muscle materials (ERAMMs), as key driving components of soft robots, can achieve efficient motion and deformation, as well as more flexible and precise robot control, attracting widespread attention. This paper reviews the latest advancements in high-performance ERAMMs and their applications in the field of soft robot actuation, using ionic polymer-metal composites and dielectric elastomers as typical cases. Firstly, the definition, characteristics, and electro-driven working principles of high-performance ERAMMs are introduced. Then, the material design and synthesis, fabrication processes and optimization, as well as characterization and testing methods of the ERAMMs are summarized. Furthermore, various applications of two typical ERAMMs in the field of soft robot actuation are discussed in detail. Finally, the challenges and future directions in current research are analyzed and anticipated. This review paper aims to provide researchers with a reference for understanding the latest research progress in high-performance ERAMMs and to guide the development and application of soft robots. STATEMENT OF SIGNIFICANCE.

Carbon Nanotube-Doped 3D-Printed Silicone Electrode for Manufacturing Multilayer Porous Plasticized Polyvinyl Chloride Gel Artificial Muscles

Plasticized polyvinyl chloride (PVC) gel has large deformation under an applied external electrical field and high driving stability in air and is a candidate artificial muscle material for manufacturing a flexible actuator. A porous PVC gel actuator consists of a mesh positive pole, a planar negative pole, and a PVC gel core layer. The current casting method is only suitable for manufacturing simple 2D structures, and it is difficult to produce multilayer porous structures. This study investigated the feasibility of a 3D-printed carbon nanotube-doped silicone electrode for manufacturing multilayer porous PVC gel artificial muscle. Carbon nanotube-doped silicone (CNT-PDMS) composite inks were developed for printing electrode layers of PVC gel artificial muscles. The parameters for the printing plane and mesh electrodes were explored theoretically and experimentally. We produced a CNT-PDMS electrode and PVC gel via integrated printing to manufacture multilayer porous PVC artificial muscle and verified its good performance.

Cobalt MOF-Based Porous Carbonaceous Spheres for Multimodal Soft Actuator Exhibiting Intricate Biomimetic Motions

The advancement of active electrode materials is essential to meet the demand for multifaceted soft robotic interactions. In this study, a new type of porous carbonaceous sphere (PCS) for a multimodal soft actuator capable of both magnetoactive and electro-ionic responses is reported. The PCS, derived from the simultaneous oxidative and reductive breakdown of specially designed cobalt-based metal-organic frameworks (Co-MOFs) with varying metal-to-ligand ratios, exhibits a high specific surface area of 529 m2 g-1 and a saturated magnetization of 142.7 Am2 kg-1. The size of the PCS can be controlled through the Ostwald ripening mechanism, while the porous structure can be regulated by adjusting the metal-to-ligand mol ratio. Its exceptional compatibility with poly(3,4-ethylene-dioxythiophene)-poly(styrenesulfonate) enables the creation of uniform electrode, crucial for producing soft actuators that work in both magnetic and electrical fields. Operated at an ultralow voltage of 1 V, the PCS-based actuator generates a blocking force of 47.5 mN and exhibits significant bending deflection even at an oscillation frequency of 10 Hz. Employing this simultaneous multimodal actuation ensures the dynamic and complex motions of a balancing bird robot and a dynamic eagle robot. This advancement marks a significant step toward the realization of more dynamic and versatile soft robotic systems.

A Microactuator Array Based on Ionic Electroactive Artificial Muscles for Cell Mechanical Stimulation

Mechanical stimulation is prevalent within organisms, and appropriate regulation of such stimulation can significantly enhance cellular functions. Consequently, the in vitro construction and simulation of mechanical stimulation have emerged as a research hotspot in biomechanics. In recent years, a class of artificial muscles named electroactive polymers (EAPs), especially ionic EAPs, have shown promising applications in biomechanics. While several techniques utilizing ionic EAPs for cell mechanical stimulation have been reported, further research is needed to advance and enhance their practical applications. Here, we prepared a microactuator array based on ionic EAP artificial muscles for cell mechanical stimulation. As a preliminary effort, we created a 5 × 5 microactuator array on a supporting membrane by employing laser cutting. We evaluated the electro-actuation performance of the microactuators through experimental testing and numerical simulations, affirming the potential use of the microactuator array for cell mechanical stimulation. The devised approach could inspire innovative design concepts in the development of miniaturized intelligent electronic devices, not only in biomechanics and biomimetics but also in other related fields.

Self-standing bacterial cellulose-reinforced poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) doped with graphene oxide composite electrodes for high-performance ionic electroactive soft actuators

Flexible electrode films with good film-forming properties, large deformation ability, high conductivity, and strong charge and discharge capabilities are crucial for ionic electroactive polymer soft actuators. However, there are still challenges in preparing high-quality electrode films that can combine well with the intermediate polyelectrolyte to form high-performance soft actuators. Herein, we propose an advanced sandwich ionic electroactive actuator utilizing self-standing bacterial cellulose (BC) reinforced poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PP) doped with graphene oxide (GO) conductive composite electrodes and a Nafion ion-exchange membrane via a hot-pressing method. The prepared BC-PP-GO electrodes have good film-forming properties with a Young's modulus of 1360 MPa and a high conductivity of 150 S cm-1. The hot-pressed BC-PP-GO/Nafion ionic actuator exhibited a large bending displacement of 6.2 mm (1 V, 0.1 Hz) with a long-term actuation stability up to 95% over 360 cycles without degradation. Furthermore, we introduced the actuator's potential applications including bionic grippers, flies, and fish, providing more opportunities for the development of next-generation micromanipulators and biomimetic microrobots in cm-scale space.

Functionalized MXene Films with Substantially Improved Low-Voltage Actuation

Ti3 C2 Tx MXene film is promising for low-voltage electrochemical actuators (ECAs) due to its excellent electrical conductivity, volumetric capacitance, and mechanical properties. However, its in-plane actuation is limited to little intralayer strain of MXene sheets under polarization. Here it is demonstrated that a simple tetrabutylammonium (TBA) functionalization of MXene improves the in-plane actuation strain by 337% and also enhances the mechanical property and stability in air and the electrolyte. Various in situ characterizations reveal that the improved actuation is ascribed to the co-insertion/desertion of TBA and Li ions into/from MXene interlayer galleries and inter-edge gaps that causes a large in-plane sliding of MXene sheets under negative/positive polarizations. The assembled bending actuator has a high strength and modulus and generates a peak-to-peak strain difference of 0.771% and a blocking force up to 51.5 times its own weight under 1 V. The designed soft robotic tweezer can grasp an object under 1 V and hold it firmly under 0 V. The novel sheet sliding mechanism resembling the filament sliding theory in skeletal muscles may inspire the design of high-performance actuators with other nanomaterials.

Controllable Multimodal Actuation in Fully Printed Ultrathin Micro-Patterned Electrochemical Actuators

Submillimeter or micrometer scale electrically controlled soft actuators have immense potential in microrobotics, haptics, and biomedical applications. However, the fabrication of miniaturized and micropatterned open-air soft actuators has remained challenging. In this study, we demonstrate the microfabrication of trilayer electrochemical actuators (ECAs) through aerosol jet printing (AJP), a rapid prototyping method with a 10 μm lateral resolution. We make fully printed 1000 × 5000 × 12 μm3 ultrathin ECAs, each of which comprises a Nafion electrolyte layer sandwiched between two poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) electrode layers. The ECAs actuate due to the electric-field-driven migration of hydrated protons. Due to the thinness that gives rise to a low proton transport length and a low flexural rigidity, the printed ECAs can operate under low voltages (∼0.5 V) and have a relatively fast response (∼seconds). We print all the components of an actuator that consists of two individually controlled submillimeter segments and demonstrate its multimodal actuation. The convenience, versatility, rapidity, and low cost of our microfabrication strategy promise future developments in integrating arrays of intricately patterned individually controlled soft microactuators on compact stretchable electronic circuits.

Air-Working Electrochromic Artificial Muscles

Artificial muscles are indispensable components for next-generation robotics to mimic the sophisticated movements of living systems and provide higher output energies when compared with real muscles. However, artificial muscles actuated by electrochemical ion injection have problems with single actuation properties and difficulties in stable operation in air. Here, air-working electrochromic artificial muscles (EAMs) with both color-changing and actuation functions are reported, which are constructed based on vanadium pentoxide nanowires and carbon tube yarn. Each EAM can generate a contractile stroke of ≈12% during stable operation in the air with multiple color changes (yellow-green-gray) under ±4 V actuation voltages. The reflectance contrast is as high as 51%, demonstrating the excellent versatility of the EAMs. In addition, a torroidal EAM arrangement with fast response and high resilience is constructed. The EAM's contractile stroke can be displayed through visual color changes, which provides new ideas for future artificial muscle applications in soft robots and artificial limbs.

Multivapor-Responsive Controlled Actuation of Starch-Based Soft Actuators

Multivapor-responsive biocompatible soft actuators have immense potential for applications in soft robotics and medical technology. We report fast, fully reversible, and multivapor-responsive controlled actuation of a pure cassava-starch-based film. Notably, this starch-based actuator sustains its actuated state for over 60 min with a continuous supply of water vapor. The durability of the film and repeatability of the actuation performance have been established upon subjecting the film to more than 1400 actuation cycles in the presence of water vapor. The starch-based actuators exhibit intriguing antagonistic actuation characteristics when exposed to different solvent vapors. In particular, they bend upward in response to water vapor and downward when exposed to ethanol vapor. This fascinating behavior opens up new possibilities for controlling the magnitude and direction of actuation by manipulating the ratio of water to ethanol in the binary solution. Additionally, the control of the bending axis of the starch-based actuator, when exposed to water vapor, is achieved by imprinting-orientated patterns on the surface of the starch film. The effect of microstructure, postsynthesis annealing, and pH of the starch solution on the actuation performance of the starch film is studied in detail. Our starch-based actuator can lift 10 times its own weight upon exposure to ethanol vapor. It can generate force ∼4.2 mN upon exposure to water vapor. To illustrate the vast potential of our cassava-starch-based actuators, we have showcased various proof-of-concept applications, ranging from biomimicry to crawling robots, locomotion near perspiring human skin, bidirectional electric switches, ventilation in the presence of toxic vapors, and smart lifting systems. These applications significantly broaden the practical uses of these starch-based actuators in the field of soft robotics.

Functionally antagonistic polyelectrolyte for electro-ionic soft actuator

Electro-active ionic soft actuators have been intensively investigated as an artificial muscle for soft robotics due to their large bending deformations at low voltages, small electric power consumption, superior energy density, high safety and biomimetic self-sensing actuation. However, their slow responses, poor durability and low bandwidth, mainly resulting from improper distribution of ionic conducting phase in polyelectrolyte membranes, hinder practical applications to real fields. We report a procedure to synthesize efficient polyelectrolyte membranes that have continuous conducting network suitable for electro-ionic artificial muscles. This functionally antagonistic solvent procedure makes amphiphilic Nafion molecules to assemble into micelles with ionic surfaces enclosing non-conducting cores. Especially, the ionic surfaces of these micelles combine together during casting process and form a continuous ionic conducting phase needed for high ionic conductivity, which boosts the performance of electro-ionic soft actuators by 10-time faster response and 36-time higher bending displacement. Furthermore, the developed muscle shows exceptional durability over 40 days under continuous actuation and broad bandwidth below 10 Hz, and is successfully applied to demonstrate an inchworm-mimetic soft robot and a kinetic tensegrity system.

Highly Aligned Ternary Nanofiber Matrices Loaded with MXene Expedite Regeneration of Volumetric Muscle Loss

Current therapeutic approaches for volumetric muscle loss (VML) face challenges due to limited graft availability and insufficient bioactivities. To overcome these limitations, tissue-engineered scaffolds have emerged as a promising alternative. In this study, we developed aligned ternary nanofibrous matrices comprised of poly(lactide-co-ε-caprolactone) integrated with collagen and Ti3C2Tx MXene nanoparticles (NPs) (PCM matrices), and explored their myogenic potential for skeletal muscle tissue regeneration. The PCM matrices demonstrated favorable physicochemical properties, including structural uniformity, alignment, microporosity, and hydrophilicity. In vitro assays revealed that the PCM matrices promoted cellular behaviors and myogenic differentiation of C2C12 myoblasts. Moreover, in vivo experiments demonstrated enhanced muscle remodeling and recovery in mice treated with PCM matrices following VML injury. Mechanistic insights from next-generation sequencing revealed that MXene NPs facilitated protein and ion availability within PCM matrices, leading to elevated intracellular Ca2+ levels in myoblasts through the activation of inducible nitric oxide synthase (iNOS) and serum/glucocorticoid regulated kinase 1 (SGK1), ultimately promoting myogenic differentiation via the mTOR-AKT pathway. Additionally, upregulated iNOS and increased NO- contributed to myoblast proliferation and fiber fusion, thereby facilitating overall myoblast maturation. These findings underscore the potential of MXene NPs loaded within highly aligned matrices as therapeutic agents to promote skeletal muscle tissue recovery.

Ionic Polymer-Metal Composites: From Material Engineering to Flexible Applications

ConspectusIonic polymer-metal composites (IPMCs) are one kind of artificial muscles that can realize energy conversions in response to external stimulus with merits of lightweight, scalability, quick response, and flexibility and have been treated as an important platform in artificial intelligence, such as bionic robotics, smart sensors, and micro-electromechanical systems. It is well-known that IPMC devices are mainly composed of one electrolyte layer laminated with symmetric electrode layers and realize energy conversion based on ion migration and redistribution inside the devices. However, several critical issues have greatly impeded the practical applications of IPMC devices, including metal electrode cracks, metal-polymer interface detachment, and water loss in the electrolyte. In the past decade, our group and collaborators have made attempts to address the mentioned critical issues with the purpose of accelerating practical applications of IPMC devices. First, in order to address the metal electrode cracks, we have developed various electrode materials to replace the metal electrode material, such as black phosphorus and graphdiyne. These materials display superior electrical and mechanical properties with enhanced material stability without cracking. Second, to address metal-polymer interface detachment, we have designed robust interfaces for IMPC devices with vertical array structures. The as-prepared interfaces present high ionic conductivity with excellent mechanical stability under bending states. As a result, the IPMC devices deliver high working stability exceeding a million cycles under air conditions. Third, in order to avoid water loss in the electrolyte, especially at ambient conditions, we have developed ionogel electrolytes containing highly stable ionic liquids as active ion sources. The ionogel electrolytes effectively prevent water loss in conventional water-containing electrolytes, just like Nafion electrolytes and greatly improve the working stability of IPMC devices. After addressing the key issues of IPMC devices, we finally obtained many high-performing IPMC devices and explored various intelligent applications of them. For instance, we have demonstrated the smart functions of IPMC devices as sensitive strain sensors, such as sign language recognition, handwriting detection, and human muscle monitoring. In addition, we have developed bionic flying robots with a vibration frequency as high as 30 Hz with the aid of high-performance IPMC actuators. A medical catheter based on IPMC actuators has been put forward by our group and can realize multiple degrees of freedom deformation under a low driving voltage of 2.5 V, which presents great potential for application in medical instruments. Lastly, a perspective on critical challenges and future research directions on IPMC materials and devices is highlighted for accelerating practical application.

Polysulfonated covalent organic framework as active electrode host for mobile cation guests in electrochemical soft actuator

Tailoring transfer dynamics of mobile cations across solid-state electrolyte-electrode interfaces is crucial for high-performance electrochemical soft actuators. In general, actuation performance is directly proportional to the affinity of cations and anions in the electrolyte for the opposite electrode surfaces under an applied field. Herein, to maximize electrochemical actuation, we report an electronically conjugated polysulfonated covalent organic framework (pS-COF) used as a common electrolyte-electrode host for 1-ethyl-3-methylimidazolium cation embedded into a Nafion membrane. The pS-COF-based electrochemical actuator exhibits remarkable bending deflection at near-zero voltage (~0.01 V) and previously unattainable blocking force, which is 34 times higher than its own weight. The ultrafast step response shows a very short rising time of 1.59 seconds without back-relaxation, and substantial ultralow-voltage actuation at higher frequencies up to 5.0 hertz demonstrates good application prospects of common electrolyte-electrode hosts. A soft fluidic switch is constructed using the proposed soft actuator as a potential engineering application.

Flexible Actuators Based on Conductive Polymer Ionogels and Their Electromechanical Modeling

High-performance flexible actuators, integral components of soft robotics, hold promise for advancing applications in safe human-robot interactions, healthcare, and various other fields. Notable among these actuators are flexible electrochemical systems, recognized for their merits in low-voltage manipulation, rapid response speed, and cost-effectiveness. However, the optimization of output strain, response speed, and stability presents a significant challenge in this domain. Despite the application of diverse electrochemically active materials to enhance actuation performance, a critical need persists for corresponding electrical-mechanical models to comprehensively grasp actuation mechanisms. In this study, we introduce a novel electrochemical actuator that utilizes conductive polymer ionogel as active electrodes. This ionogel exhibits exceptional properties, including high conductivity, flexibility, and electrochemical activity. Our electrochemical actuators exhibit noteworthy bending strain capabilities and rapid response rates, achieving frequencies up to 10 Hz at a modest voltage of 1 V. An analytical model integrating ion migration and dynamic processes has been established to elucidate actuator behavior. Simulation results highlight that electrodes characterized by low resistance and high capacitance are optimal for simultaneous enhancement of bending strain and blocking force. However, the augmentation of Young's modulus, while increasing blocking force, compromises bending strain. Furthermore, a larger aspect ratio proves beneficial for unidirectional stress output, leading to increased bending strain, while actuator blocking force diminishes with greater length. These findings underscore the intricate interplay between material properties and dimensions in optimizing the performance of flexible electrochemical actuators. This work provides important practical and theoretical guidance for the manufacture of high-performance flexible actuators and the search for new smart materials.

An Ultra-Thin MXene Film for Multimodal Sensing of Neuroelectrical Signals with Artifacts Removal

Neuroelectrical signals transmitted onto the skin tend to decay to an extremely weak level, making them highly susceptible to interference from the environment and body movement. Meanwhile, for comprehensively understanding cognitive nerve conduction, multimodal sensing of neural signals, such as magnetic resonance imaging (MRI) and functional near-infrared spectroscopy (fNIRS), is highly required. Previous metal or polymer conductors cannot either provide a seamless on-skin feature for accurate sensing of neuroelectrical signals or be compatible with multimodal imaging techniques without opto- and magnet- artifacts. Herein, a ≈20 nm thick MXene film that is able to simultaneously detect electrophysiological signals and perform imaging by MRI and fNIRS with high fidelity is reported. The ultrathin film is made of crosslinked Ti3 C2 Tx film via poly (3,4-ethylene dioxythiophene): polystyrene sulfonate (PEDOT: PSS), showing a record high electroconductivity and transparency combination (11 000 S cm-1 @89%). Among them, PEDOT: PSS not only plays a cross-linking role to stabilize MXene film but also shortens the interlayer distance for effective charge transfer and high transparency. Thus, it can achieve a low interfacial impedance with skin or neural surfaces for accurate recording of electrophysiological signals with low motion artifacts. Besides, the high transparency originating from the ultrathin feature leads to good compatibility with fNIRS and MRI without optical and magnetic artifacts, enabling multimodal cognitive neural monitoring during prolonged use.

Impermeable Graphene Skin Increases the Heating Efficiency and Stability of an MXene Heating Element

A Joule heater made of emerging 2D nanosheets, i.e., MXene, has the advantage of low-voltage operation with stable heat generation owing to its highly conductive and uniformly layered structure. However, the self-heated MXene sheets easily get oxidized in warm and moist environments, which limits their intrinsic heating efficiencies. Herein, an ultrathin graphene skin is introduced as a surface-regulative coating on MXene to enhance its oxidative stability and Joule heating efficiency. The skin layer is deposited on MXene using a scalable solution-phased layer-by-layer assembly process without deteriorating the excellent electrical conductivity of the MXene. The graphene skin comprises narrow and hydrophobic channels, which results in ≈70 times higher water impermeability of the hybrid film of graphene and MXene (GMX) than that of the pristine MXene. A complementary electrochemical analysis confirms that the graphene skin facilitates longer-lasting protection than conventional polymer coatings owing to its tortuous pathways. In addition, the sp2 planar carbon surface with a low heat loss coefficient improves the heating efficiency of the GMX, indicating that this strategy is promising for developing adaptive heating materials with a tractable voltage range and high Joule heating efficiency.

2D Materials Beyond Post-AI Era: Smart Fibers, Soft Robotics, and Single Atom Catalysts

Recent consecutive discoveries of various 2D materials have triggered significant scientific and technological interests owing to their exceptional material properties, originally stemming from 2D confined geometry. Ever-expanding library of 2D materials can provide ideal solutions to critical challenges facing in current technological trend of the fourth industrial revolution. Moreover, chemical modification of 2D materials to customize their physical/chemical properties can satisfy the broad spectrum of different specific requirements across diverse application areas. This review focuses on three particular emerging application areas of 2D materials: smart fibers, soft robotics, and single atom catalysts (SACs), which hold immense potentials for academic and technological advancements in the post-artificial intelligence (AI) era. Smart fibers showcase unconventional functionalities including healthcare/environmental monitoring, energy storage/harvesting, and antipathogenic protection in the forms of wearable fibers and textiles. Soft robotics aligns with future trend to overcome longstanding limitations of hard-material based mechanics by introducing soft actuators and sensors. SACs are widely useful in energy storage/conversion and environmental management, principally contributing to low carbon footprint for sustainable post-AI era. Significance and unique values of 2D materials in these emerging applications are highlighted, where the research group has devoted research efforts for more than a decade.

Role of MXenes in advancing soft robotics

MXenes with their unique electronic, optical, chemical, and mechanical properties have shown great promise in soft robotics. MXene-based soft actuators have been designed to display ultrafast actuations and recovery speeds as well as angle-independent structural colors in response to vapor. Several studies have developed soft actuators by combining MXenes with other materials to mimic the movement of natural organisms. Thus, MXene-based soft actuators have the potential to revolutionize the field of soft robotics and flexible electronics (e.g., wearable devices and artificial muscles). MXene-based artificial muscles have been explored for use in kinetic soft robotics as actuators in microsystems requiring exceptional compliance. MXene-based sensors and actuators have already been developed for human-like sensors and photodetection. However, there are still challenges that need to be addressed in such applications, such as the design of stretchable and compliant robotic skins with a high-level functional integration for soft robotics. The integration of various devices, such as power sources, sensors, and actuators, into soft robotics is another crucial challenge. Despite the excellent stretchability and tensile strength of MXene-based composites, there is a vital need to develop their mechanical and electrochemical features and grant them multi-functionalities. Herein, recent developments pertaining to the applications of MXenes and their composites in soft robotics are discussed with a focus on the important challenges and future perspectives.

Boosting Charge Utilization in Self-Powered Photodetector for Real-Time High-Throughput Ultraviolet Communication

Ultraviolet (UV) communication is a cutting-edge technology in communication battlefields, and self-powered photodetectors as their optical receivers hold great potential. However, suboptimal charge utilization has largely limited the further performance enhancement of self-powered photodetectors for high-throughput communication application. Herein, a self-powered Ti3 C2 Tx -hybrid poly(3,4 ethylenedioxythiophene):poly-styrene sulfonate (PEDOT:PSS)/ZnO (TPZ) photodetector is designed, which aims to boost charge utilization for desirable applications. The device takes advantage of photothermal effect to intensify pyro-photoelectric effect as well as the increased conductivity of the PEDOT:PSS, which significantly facilitated charge separation, accelerated charge transport, and suppressed interface charge recombination. Consequently, the self-powered TPZ photodetector exhibits superior comprehensive performance with high responsivity of 12.3 mA W-1 and fast response time of 62.2 µs, together with outstanding reversible and stable cyclic operation. Furthermore, the TPZ photodetector has been successfully applied in an integrated UV communication system as the self-powered optical receiver capable of real-time high-throughput information transmission with ASCII code under 9600 baud rate. This work provides the design insight of highly performing self-powered photodetectors to achieve high-efficiency optical communication in the future.

Magnetically and Electrically Responsive Soft Actuator Derived from Ferromagnetic Bimetallic Organic Framework

The advancement in smart devices and soft robotics necessitates the use of multiresponsive soft actuators with high actuation stroke and stable reversibility for their use in real-world applications. Here, this work reports a magnetically and electrically dual responsive soft actuator based on neodymium and iron bimetallic organic frameworks (NdFeMOFs@700). The ferromagnetic NdFeMOFs@700 exhibits a porous carbon structure with excellent magnetization saturation (166.96 emu g^-1 ) which allows its application to a dual functional material in both magnetoactive and electro-ionic actuations. The electro-ionic soft actuator, which is fabricated using NdFeMOFs@700 and PEDOT-PSS, demonstrates 4.5 times higher ionic charge storage capacity (68.21 mF cm^-2 ) and has excellent cycle stability compared with the PEDOT-PSS based actuator. Under a low sinusoidal input voltage of 1 V, the dual-responsive actuator displays bending displacement of 15.46 mm and also generates deflection of 10 mm at 50 mT. Present results show that the ferromagnetic bimetallic organic frameworks can open a new way to make dual responsive soft actuators due to the hierarchically porous structures with its high redox activity, superior magnetic properties, and larger electrochemical capacitance. With the NdFeMOFs@700 based soft actuators, walking movement of a starfish robot is demonstrated by applying both the magnetic and electric fields.

Scalable multi-dimensional topological deformation actuators for active object identification

Rarely are bionic robots capable of rapid multi-dimensional deformation and object identification in the same way as animals and plants. This study proposes a topological deformation actuator for bionic robots based on pre-expanded polyethylene and large flake MXene, inspired by the octopus predation behavior. This unusual, large-area topological deformation actuator (easily reaching 800 cm² but is not constrained to this size) prepared by large-scale blow molding and continuous scrape coating exhibits different distribution states of molecular chains at low and high temperatures, causing the actuator's deformation direction to change axially. With its multi-dimensional topological deformation and self-powered active object identification capabilities, the actuator can capture objects like an octopus. The contact electrification effect assists the actuator to identify the type and size of the target object during this multi-dimensional topological deformation that is controllable and designable. This work demonstrates the direct conversion of light energy into contact electrical signals, introducing a new route for the practicality and scaling of bionic robots.

Low-Voltage Driven Ionic Polymer-Metal Composite Actuators: Structures, Materials, and Applications

With the characteristics of low driving voltage, light weight, and flexibility, ionic polymer-metal composites (IPMCs) have attracted much attention as excellent candidates for artificial muscle materials in the fields of biomedical devices, flexible robots, and microelectromechanical systems. Under small voltage excitation, ions inside the IPMC proton exchange membrane migrate directionally, leading to differences in the expansion rate of the cathode and the anode, which in turn deform. This behavior is caused by the synergistic action of a three-layer structure consisting of an external electrode layer and an internal proton exchange membrane, but the electrode layer is more dominant in this process due to the migration and storage of ions. The exploration of modifications and alternatives for proton exchange membranes and recent advances in the fabrication and characterization of conductive materials, especially carbon-based materials and conductive polymers, have contributed significantly to the development of IPMCs. This paper reviews the progress in the application of proton exchange membranes and electrode materials for IPMCs, discusses various processes currently applied to IPMCs preparation, and introduces various promising applications of cutting-edge IPMCs with high performance to provide new ideas and approaches for the research of  new generation of low-voltage ionic soft actuators.

Toward Smart Sensing by MXene

The Internet of Things era has promoted enormous research on sensors, communications, data fusion, and actuators. Among them, sensors are a prerequisite for acquiring the environmental information for delivering to an artificial data center to make decisions. The MXene-based sensors have aroused tremendous interest because of their extraordinary performances. In this review, the electrical, electronic, and optical properties of MXenes are first introduced. Next, the MXene-based sensors are discussed according to the sensing mechanisms such as electronic, electrochemical, and optical methods. Initially, biosensors are introduced based on chemiresistors and field-effect transistors. Besides, the wearable pressure sensor is demonstrated with piezoresistive devices. Third, the electrochemical methods include amperometry and electrochemiluminescence as examples. In addition, the optical approaches refer to surface plasmonic resonance and fluorescence resonance energy transfer. Moreover, the prospects are delivered of multimodal data fusion toward complicated human-like senses. Eventually, future opportunities for MXene research are conveyed in the new material discovery, structure design, and proof-of-concept devices.

A Single Electronic Tattoo for Multisensory Integration

Wearable electronics are garnering growing interest in various emerging fields including intelligent sensors, artificial limbs, and human-machine interfaces. A remaining challenge is to develop multisensory devices that can conformally adhere to the skin even during dynamic-moving environments. Here, a single electronic tattoo (E-tattoo) based on a mixed-dimensional matrix network, which integrates two-dimensional  MXene nanosheets and one-dimensional cellulose nanofibers/Ag nanowires, is presented for multisensory integration. The multidimensional configurations endow the E-tattoo with excellent multifunctional sensing capabilities including temperature, humidity, in-plane strain, proximity, and material identification. In addition, benefiting from the satisfactory rheology of hybrid inks, the E-tattoos are able to be fabricated through multiple facile strategies including direct writing, stamping, screen printing, and three-dimensional printing on various hard/soft substrates. Especially, the E-tattoo with excellent triboelectric properties also can serve as a power source for activating small electronic devices. It is believed that these skin-conformal E-tattoo systems can provide a promising platform for next-generation wearable and epidermal electronics.

Photocured Liquid-Crystalline Polymer Electrolytes with 3D Ion Transport Pathways for Electromechanical Actuators

Self-assembly of ionic molecules into hierarchical ordered structures is a promising route to new types of solid electrolytes with enhanced ion transport. Herein, we report a liquid-crystalline polymer electrolyte membrane that contains three-dimensionally (3D) interconnected ionic pathways. To build this membrane, we used wedge-shaped amphiphilic molecules that have two ionic heads and a lipophilic tail. These molecules were combined with a low content of ionic liquid (5.6 wt %) to form a hexagonal columnar phase, where the self-assembled lipophilic cylinders were surrounded by the ionic shell. Photopolymerization of this phase produced flexible nanostructured films with 3D ionic pathways, which can serve as an electrolyte layer in soft robotic actuators. Ionic transport in the 3D pathways leads to shape memory capability as well as durable bending actuation with a voltage-controllable blocking force. Furthermore, we find a significant enhancement of actuation for the nanostructured electrolyte compared with the corresponding amorphous electrolyte.

Advances in artificial muscles: A brief literature and patent review

Background: Artificial muscles are an active research area now. Methods: A bibliometric analysis was performed to evaluate the development of artificial muscles based on research papers and patents. A detailed overview of artificial muscles' scientific and technological innovation was presented from aspects of productive countries/regions, institutions, journals, researchers, highly cited papers, and emerging topics. Results: 1,743 papers and 1,925 patents were identified after retrieval in Science Citation Index-Expanded (SCI-E) and Derwent Innovations Index (DII). The results show that China, the United States, and Japan are leading in the scientific and technological innovation of artificial muscles. The University of Wollongong has the most publications and Spinks is the most productive author in artificial muscle research. Smart Materials and Structures is the journal most productive in this field. Materials science, mechanical and automation, and robotics are the three fields related to artificial muscles most. Types of artificial muscles like pneumatic artificial muscles (PAMs) and dielectric elastomer actuator (DEA) are maturing. Shape memory alloy (SMA), carbon nanotubes (CNTs), graphene, and other novel materials have shown promising applications in this field. Conclusion: Along with the development of new materials and processes, researchers are paying more attention to the performance improvement and cost reduction of artificial muscles.

Bioinspired MXene-Based Soft Actuators Exhibiting Angle-Independent Structural Color

In nature, many living organisms exhibiting unique structural coloration and soft-bodied actuation have inspired scientists to develop advanced structural colored soft actuators toward biomimetic soft robots. However, it is challenging to simultaneously biomimic the angle-independent structural color and shape-morphing capabilities found in the plum-throated cotinga flying bird. Herein, we report biomimetic MXene-based soft actuators with angle-independent structural color that are fabricated through controlled self-assembly of colloidal SiO2 nanoparticles onto highly aligned MXene films followed by vacuum-assisted infiltration of polyvinylidene fluoride into the interstices. The resulting soft actuators are found to exhibit brilliant, angle-independent structural color, as well as ultrafast actuation and recovery speeds (a maximum curvature of 0.52 mm-1 can be achieved within 1.16 s, and a recovery time of ~ 0.24 s) in response to acetone vapor. As proof-of-concept illustrations, structural colored soft actuators are applied to demonstrate a blue gripper-like bird's claw that can capture the target, artificial green tendrils that can twine around tree branches, and an artificial multicolored butterfly that can flutter its wings upon cyclic exposure to acetone vapor. The strategy is expected to offer new insights into the development of biomimetic multifunctional soft actuators for somatosensory soft robotics and next-generation intelligent machines.

Artificial neuromuscular fibers by multilayered coaxial integration with dynamic adaption

Integrating sense in a thin artificial muscle fiber for environmental adaption and actuation path tracing, as a snail tentacle does, is highly needed but still challenging because of the interfacing mismatch between the fiber's actuation and sensing components. Here, we report an artificial neuromuscular fiber by wrapping a carbon nanotube (CNT) fiber core in sequence with an elastomer layer, a nanofiber network, and an MXene/CNT thin sheath, achieving the ingenious sense-judge-act intelligent system in an elastic fiber. The CNT/elastomer components provide actuation, and the sheath enables touch/stretch perception and hysteresis-free cyclic actuation tracing due to its strain-dependent resistance. As a whole, the coaxial structure builds a dielectric capacitor that enables sensitive touchless perception. The key to seamless integration is to use a nanofiber interface that allows the sensing layer to adaptively trace but not restrict actuation. This work provides promising solutions for closed-loop control for future intelligent soft robots.

High-performance electrified hydrogel actuators based on wrinkled nanomembrane electrodes for untethered insect-scale soft aquabots

Hydrogels have diverse chemical properties and can exhibit reversibly large mechanical deformations in response to external stimuli; these characteristics suggest that hydrogels are promising materials for soft robots. However, reported actuators based on hydrogels generally suffer from slow response speed and/or poor controllability due to intrinsic material limitations and electrode fabrication technologies. Here, we report a hydrogel actuator that operates at low voltages (<3 volts) with high performance (strain > 50%, energy density > 7 × 10 5 joules per cubic meter, and power density > 3 × 10 4 watts per cubic meter), surpassing existing hydrogel actuators and other types of electroactive soft actuators. The enhanced performance of our actuator is due to the formation of wrinkled nanomembrane electrodes that exhibit high conductivity and excellent mechanical deformation through capillary-assisted assembly of metal nanoparticles and deswelling-induced wrinkled structures. By applying an electric potential through the wrinkled nanomembrane electrodes that sandwich the hydrogel, we were able to trigger a reversible and substantial electroosmotic water flow inside a hydrogel film, which drove the controlled swelling of the hydrogel. The high energy efficiency and power density of our wrinkled nanomembrane electrode–induced actuator enabled the fabrication of an untethered insect-scale aquabot integrated with an on-board control unit demonstrating maneuverability with fast locomotion speed (1.02 body length per second), which occupies only 2% of the total mass of the robot.

Roles of Low-Dimensional Nanomaterials in Pursuing Human-Machine-Thing Natural Interaction

A wide variety of low-dimensional nanomaterials with excellent properties can meet almost all the requirements of functional materials for information sensing, processing, and feedback devices. Low-dimensional nanomaterials are becoming the star of hope on the road to pursuing human-machine-thing natural interactions, benefiting from the breakthroughs in precise preparation, performance regulation, structural design, and device construction in recent years. This review summarizes several types of low-dimensional nanomaterials commonly used in human-machine-thing natural interactions and outlines the differences in properties and application areas of different materials. According to the sequence of information flow in the human-machine-thing interaction process, the representative research progress of low-dimensional nanomaterials-based information sensing, processing, and feedback devices is reviewed and the key roles played by low-dimensional nanomaterials are discussed. Finally, the development trends and existing challenges of low-dimensional nanomaterials in the field of human-machine-thing natural interaction technology are discussed.

High-frequency and rapid response tungsten sulfide nano onion-based electrochemical actuators

Poor rate capability, the biggest barrier to potential applications of electrochemical actuators (ECAs), is primarily resulted from symmetric electrochemical reactions. This makes it extremely difficult for ECAs to actuate above 1 Hz while maintaining sufficient displacement retainability compared with their actuations at relatively low frequencies, particularly when working in liquids. Here, tungsten trisulfide (WS₃) assisted tungsten disulfide nano onions are synthesized through a one-step laser-assisted strategy. Using the irreversibility of WS₃ in adsorbing hydrogen in an acidic solution, the electrochemical reaction of tungsten sulfide nano onions is tailored to realize an asymmetric redox reaction for breaking the symmetry of the electrical double layer and battery-like process. Experiments demonstrate that the ECA's response rate (0.24 mm^-1 s^-1) is at least 10 times faster than that of the previously reported ECAs. Moreover, this ECA can actuate at 30 Hz and reaches top performance in liquids at 4 Hz with long-term durability (>90% after 23 000 cycles), which is comparable to that of electromagnetic and electrothermal actuators. To understand the electrochemical actuation of tungsten sulfide from the atomic scale to the macroscopic scale, density functional theory calculations are conducted and an electrochemomechanical coupling model is proposed. A new generation of subvolt electric-driven actuators used in underwater robotics can be developed by modulating the electrochemical response and chemomechanical coupling effect.

A Dual-Responsive Magnetoactive and Electro-Ionic Soft Actuator Derived from a Nickel-Based Metal-Organic Framework

There is growing demand for multiresponsive soft actuators for the realization of natural, safe, and complex motions in robotic interactions. In particular, soft actuators simultaneously stimulated by electrical and magnetic fields are always under development owing to their simple controllability and reliability during operation. Herein, magnetically and electrically driven dual-responsive soft actuators (MESAs) derived from novel nickel-based metal-organic frameworks (Ni-MOFs-700C), are reported. Nanoscale Ni-MOFs-700C has excellent electrochemical and magnetic properties that allow it to be used as a multifunctional material under both magnetoactive and electro-ionic actuations. The dual-responsive MESA exhibits a bending displacement of 30 mm and an ultrafast rising time of 1.5 s under a very low input voltage of 1 V and also exerts a bending deflection of 12.5 mm at 50 mT under a high excitation frequency of 5 Hz. By utilizing a dual-responsive MESA, the hovering motion of a hummingbird robot is demonstrated under magnetic and electrical stimuli.

Molecular-Level Methylcellulose/MXene Hybrids with Greatly Enhanced Electrochemical Actuation

Ti₃ C₂ T_x MXene film is promising for electrochemical actuators due to its high electrical conductivity and volumetric capacitance. However, its actuation performance is limited by the slow ion diffusion through the film and poor mechanical property in aqueous electrolytes. Here, molecular-level methylcellulose (MC)/MXene hybrid films are assembled with obviously enlarged layer distance, improved wet strength, and ambient stability. The hybrid films show significantly higher in-plane actuation strain in a liquid electrolyte. Based on direct strain measurements, in situ X-ray diffraction (XRD) and ex situ X-ray photoelectron spectroscopy (XPS) analyses, the actuation enhancement can be ascribed to the enlarged layer distance allowing more water and ions to be intercalated/de-intercalated and MC-induced sliding of MXene sheets. The assembled soft actuator has a high Young's modulus of 1.93 GPa and can be operated in air, generating a peak-to-peak strain difference up to 0.541% under a triangular wave voltage of ±1 V and a blocking force of 4.7 times its own weight.

Strain-Driven Auto-Detachable Patterning of Flexible Electrodes

Flexible electrodes that are multilayer, multimaterial, and conformal are pivotal for multifunctional wearable electronics. Traditional electronic circuits manufacturing requires substrate-supported transfer printing, which limits their multilayer integrity and device conformability on arbitrary surfaces. Herein, a "shrinkage-assisted patterning by evaporation" (SHAPE) method is reported, by employing evaporation-induced interfacial strain mismatch, to fabricate auto-detachable, freestanding, and patternable electrodes. The SHAPE method utilizes vacuum-filtration of polyaniline/bacterial cellulose (PANI/BC) ink through a masked filtration membrane to print high-resolution, patterned, and multilayer electrodes. The strong interlayer hydrogen bonding ensures robust multilayer integrity, while the controllable evaporative shrinking property of PANI/BC induces mismatch between the strains of the electrode and filtration membrane at the interface and thus autodetachment of electrodes. Notably, a 500-layer substrateless micro-supercapacitor fabricated using the SHAPE method exhibits an energy density of 350 mWh cm^-2 at a power density of 40 mW cm^-2 , 100 times higher than reported substrate-confined counterparts. Moreover, a digital circuit fabricated using the SHAPE method functions stably on a deformed glove, highlighting the broad wearable applications of the SHAPE method.

Inhibition Effect of Ti3C2Tx MXene on Ice Crystals Combined with Laser-Mediated Heating Facilitates High-Performance Cryopreservation

The phenomena of ice formation and growth are of great importance for climate science, regenerative medicine, cryobiology, and food science. Hence, how to control ice formation and growth remains a challenge in these fields and attracts great interest from widespread researchers. Herein, the ice regulation ability of the two-dimensional MXene Ti₃C₂T_x in both the cooling and thawing processes is explored. Molecularly speaking, the ice growth inhibition mechanism of Ti₃C₂T_x MXene is ascribed to the formation of hydrogen bonds between functional groups of -O-, -OH, and -F distributed on the surface of Ti₃C₂T_x and ice/water molecules, which was elucidated by the molecular dynamics simulation method. In the cooling process, Ti₃C₂T_x can decrease the supercooling degree and inhibit the sharp edge morphology of ice crystals. Moreover, taking advantage of the outstanding photothermal conversion property of Ti₃C₂T_x, rapid ice melting can be achieved, thus reducing the phenomena of devitrification and ice recrystallization. Based on the ice restriction performance of Ti₃C₂T_x mentioned above, Ti₃C₂T_x is applied for cryopreservation of stem-cell-laden hydrogel constructs. The results show that Ti₃C₂T_x can reduce cryodamage to stem cells induced by ice injury in both the cooling and thawing processes and finally increase the cell viability from 38.4% to 80.9%. In addition, Ti₃C₂T_x also shows synergetic antibacterial activity under laser irradiation, thus realizing sterile cryopreservation of stem cells. Overall, this work explores the ice inhibition performance of Ti₃C₂T_x, elucidates the physical mechanism, and further achieves application of Ti₃C₂T_x in the field of cell cryopreservation.

Multiresponsive Ti3C2Tx MXene-Based Actuators Enabled by Dual-Mechanism Synergism for Soft Robotics

Multiresponsive and high-performance flexible actuators with a simple configuration, high mechanical strength, and low-power consumption are highly desirable for soft robotics. Here, a novel mechanically robust and multiresponsive Ti₃C₂T_x MXene-based actuator with high actuation performance via dual-mechanism synergistic effect driven by the hygroexpansion of bacterial cellulose (BC) layer and the thermal expansion of biaxially oriented polypropylene (BOPP) layer is developed. The actuator is flexible and shows an ultrahigh tensile strength of 195 MPa. Unlike the conventional bimorph-structured actuators based on a single-mechanism, the actuator developed provides a favorable architecture for dual-mechanism synergism, resulting in exceptionally reversible actuation performance under electricity and near-infrared (NIR) light stimuli. Typically, the developed actuator can produce the largest bending angle (∼400°) at the lowest voltage (≤4 V) compared with that reported previously for single mechanism soft actuators. Furthermore, the actuator also can be driven by a NIR light at a 2 m distance, displaying an excellent long-distance photoresponsive property. Finally, various intriguing applications are demonstrated to show the great potential of the actuator for soft robotics.

Ultralow-Voltage-Drivable Artificial Muscles Based on a 3D Structure MXene-PEDOT:PSS/AgNWs Electrode

The main challenge in manufacturing an ionic actuator of large bending displacement and great response sensitivity is to design a flexible electrode with great electrochemical characteristics and conductivity. This research reports the MXene-PEDOT:PSS/AgNWs (MPA) electrode with a three-dimensional (3D) network structure formed by a hybrid method of the one-dimensional (1D) silver nanowires (AgNWs) and the two-dimensional (2D) Ti₃C₂T_x MXene. Here, a soft actuator based on the ionic cross-linked hybrid electrode was designed. The results show that the MPA electrode-based soft actuator achieves a large bending strain (0.48%, ±0.5 V sine voltage), wide frequency (0.1-10 Hz), 5 h durability (91.9% retention), fast response time (≈5 s), great power density (7.53 kW m^-3), and great energy density (18.83 kJ m^-3). These excellent performances contribute to the 3D structure of electrodes formed by MXene and AgNWs creating an unhindered ion channel, which facilitates short diffusion and rapid injection of ions and provides higher capacitance and mechanical integrity. This 3D network layered structure hybrid electrode provides an opportunity for the development of ultralow-voltage-drivable artificial muscles.

3D MXene/PEDOT:PSS Composite Aerogel with a Controllable Patterning Property for Highly Sensitive Wearable Physical Monitoring and Robotic Tactile Sensing

MXene based composite conductive aerogels have been extensively investigated as sensitive materials for wearable pressure sensors owing to their effective 3D network microstructures and the excellent conductivity of MXene. In this work, we fabricated a 3D porous Ti3C2Tx MXene/poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) composite aerogel (MPCA) with a controllable patterning property utilizing the Cu-assisted electrogelation method. The prepared composite aerogel can be assembled into pressure sensors for wearable physical monitoring and high-resolution sensor microarrays for robotic tactile sensing. The multi-interactions between MXene and PEDOT:PSS enable the MPCA to have a stable 3D conductive network, which consequently enhances both the mechanical flexibility and the piezoresistive property of the MPCA. Thus, the fabricated pressure sensor demonstrating high sensitivity (26.65 kPa-1 within 0-2 kPa), fast response ability (106 ms), and excellent stability can be further applied for wearable physical monitoring. Moreover, due to the controllable patterning property of the electrogelation preparation method, a high-resolution pressure sensor microarray was successfully prepared as an artificial tactile interface, which can be attached to a robotic fingertip to directly recognize the tactile stimuli from human fingers and identify braille letters like human fingers. The proposed MPCA, endowed with a remarkable comprehensive property, particularly the highly sensitive sensing performance and controllable patterning property, demonstrates an enormous advantage and a great potentiality toward wearable electronics.

MXene Enhanced the Electromechanical Performance of a Nafion-Based Actuator

Ionic electroactive polymer-based actuators have attracted much attention due to their low potential stimuli. In this work, MXene-Nafion composite actuators were fabricated, and the actuation performances were tested. The morphology of the as-made MXene-Nafion composite showed that the composite membrane was homogeneous, with an MXene doping level up to 5 wt%. In addition, the results of blocked force, response speed, and durability demonstrated that the actuation behavior of the composite-based actuator was enhanced due to the efficient dispersion of the two-dimensional nanofiller MXene. In addition, the blocking force of the composite actuator with a doping level of 0.5 wt% was about 6 times that of the pure Nafion without back-relaxation and durability degradation during the testing period.

Maximizing Performance of a Hybrid MnO2/Ni Electrochemical Actuator through Tailoring Lattice Tunnels and Cation Vacancies

Electrochemical actuators play a key role in converting electrical energy to mechanical energy. However, a low actuation stress and an unsatisfied strain response rate strongly limit the extensive applications of the actuators. Here, we report hybrid manganese dioxide (MnO₂) fabricated by introducing ramsdellite (R-MnO₂) and Mn vacancies into birnessite (δ-MnO₂) nanosheets, which in situ grew on the surface of a nickel (Ni) film, forming a hybrid MnO₂/Ni actuator. The actuator demonstrated a rapid strain response of 0.88% s^-1 (5.3% intrinsic strain in 6 s) and a large actuation stress of 244 MPa owing to the special R-MnO₂ with a high density of sodium ion (Na⁺)-accessible lattice tunnels, Mn vacancies, and also a high Young's modulus of the hybrid MnO₂/Ni composite. Besides, the cyclic stability of the actuator was realized after 1.2 × 10⁴ cycles of electric stimulation under a frequency of 0.05 Hz. The finding of the novel hybrid MnO₂/Ni actuator may provide a new strategy to maximize the actuating performance evidently through tailoring the lattice tunnel structure and introducing cation vacancies into electrochemical electrode materials.

Additive Manufacturing of Ti3 C2 -MXene-Functionalized Conductive Polymer Hydrogels for Electromagnetic-Interference Shielding

The ongoing miniaturization of devices and development of wireless and implantable technologies demand electromagnetic interference (EMI)-shielding materials with customizability. Additive manufacturing of conductive polymer hydrogels with favorable conductivity and biocompatibility can offer new opportunities for EMI-shielding applications. However, simultaneously achieving high conductivity, design freedom, and shape fidelity in 3D printing of conductive polymer hydrogels is still very challenging. Here, an aqueous Ti₃ C₂ -MXene-functionalized poly(3,4-ethylenedioxythiophene):polystyrene sulfonate ink is developed for extrusion printing to create 3D objects with arbitrary geometries, and a freeze-thawing protocol is proposed to transform the printed objects directly into highly conductive and robust hydrogels with high shape fidelity on both the macro- and microscale. The as-obtained hydrogel exhibits a high conductivity of 1525.8 S m^-1 at water content up to 96.6 wt% and also satisfactory mechanical properties with flexibility, stretchability, and fatigue resistance. Furthermore, the use of the printed hydrogel for customizable EMI-shielding applications is demonstrated. The proposed easy-to-manufacture approach, along with the highlighted superior properties, expands the potential of conductive polymer hydrogels in future customizable applications and represents a real breakthrough from the current state of the art.

Nanostructure Engineering of Graphitic Carbon Nitride for Electrochemical Applications

Graphitic carbon nitride with ordered two-dimensional structure displays multiple properties, including tunable structure, suitable bandgap, high stability, and facile synthesis. Many achievements on this material have been made in photocatalysis, but the advantages have not yet been fully explored in electrochemical fields. The bulk structure with low conductivity impedes charge-transfer kinetics during electrochemical processes. Excessive nitrogen content leads to insufficient charge transfer, while bulk structures produce tortuous channels for mass transport. Some attempts have been made to address these issues by nanostructure engineering, such as ultrathin structure design, heterogeneous composition, defect engineering, and morphology control. These structure-engineered nanomaterials have been successfully applied in electrochemical fields, including ionic actuators, flexible supercapacitors, lithium-ion batteries, and electrochemical sensors. Herein, a timely review on the latest advances in graphitic carbon nitride through various engineering strategies for electrochemical applications has been summarized. A perspective on critical challenges and future research directions is highlighted for graphitic carbon nitride in electrochemistry on the basis of existing research works and our experimental experience.

Electro-Active and Photo-Active Vanadium Oxide Nanowire Thermo-Hygroscopic Actuators for Kirigami Pop-up

Emerging technologies such as soft robotics, active biomedical devices, wearable electronics, haptic feedback systems, and healthcare systems require high-fidelity soft actuators showing reliable responses under multi-stimuli. In this study, the authors report an electro-active and photo-active soft actuator based on a vanadium oxide nanowire (VONW) hybrid film with greatly improved actuation performances. The VONWs directly grown on a cellulose fiber network increase the surface area up to 30-fold and boost the hydrophilicity owing to the presence of oxygen-rich functional groups in the nanowire surfaces. Taking advantage of the high surface area and hydrophilicity of VONWs, a soft thermo-hygroscopic VONW actuator capable of being controlled by both light and electric sources shows greatly enhanced actuation deformation by almost 70% and increased actuation speed over 3 times during natural convection cooling. Most importantly, the proposed VONW actuator exhibits a remarkably improved blocking force of up to 200% compared with a bare paper actuator under light stimulation, allowing them to realize a complex kirigami pop-up and to accomplish repeatable shape transformation from a 2D planar surface to a 3D configuration.

Electro-responsive actuators based on graphene

Electro-responsive actuators (ERAs) hold great promise for cutting-edge applications in e-skins, soft robots, unmanned flight, and in vivo surgery devices due to the advantages of fast response, precise control, programmable deformation, and the ease of integration with control circuits. Recently, considering the excellent physical/chemical/mechanical properties (e.g., high carrier mobility, strong mechanical strength, outstanding thermal conductivity, high specific surface area, flexibility, and transparency), graphene and its derivatives have emerged as an appealing material in developing ERAs. In this review, we have summarized the recent advances in graphene-based ERAs. Typical the working mechanisms of graphene ERAs have been introduced. Design principles and working performance of three typical types of graphene ERAs (e.g., electrostatic actuators, electrothermal actuators, and ionic actuators) have been comprehensively summarized. Besides, emerging applications of graphene ERAs, including artificial muscles, bionic robots, human-soft actuators interaction, and other smart devices, have been reviewed. At last, the current challenges and future perspectives of graphene ERAs are discussed.

A comparative review of artificial muscles for microsystem applications

Artificial muscles are capable of generating actuation in microsystems with outstanding compliance. Recent years have witnessed a growing academic interest in artificial muscles and their application in many areas, such as soft robotics and biomedical devices. This paper aims to provide a comparative review of recent advances in artificial muscle based on various operating mechanisms. The advantages and limitations of each operating mechanism are analyzed and compared. According to the unique application requirements and electrical and mechanical properties of the muscle types, we suggest suitable artificial muscle mechanisms for specific microsystem applications. Finally, we discuss potential strategies for energy delivery, conversion, and storage to promote the energy autonomy of microrobotic systems at a system level.

Host-Guest Intercalation Chemistry in MXenes and Its Implications for Practical Applications

The ever-increasing demand on developing layered materials for practical applications, such as electrochemical energy storage, responsive materials, nanofluidics, and environmental remediation, requires the profound understanding and artful exploitation of interlayer engineering or intercalation chemistry. The past decade has witnessed the massive exploration of a recently discovered 2D material-transition metal carbides, carbonitrides, and nitrides (referred to as MXenes), which began to take hold of a myriad of applications owing to the abundant possibilities on their compositions and intercalation states. However, application-targeted manipulation of the material performance of MXenes is constrained by the dearth of deep comprehension on fundamental intercalation chemistry/physics. To this end, the aim of this review is to provide a holistic discussion on the intercalation chemistry in MXenes and the physical properties of MXene intercalation compounds. On the basis of this, potential solutions for the challenges confronted in the synthesis, tuning of material properties, and practical applications are proposed, which are also expected to reinvigorate the exploration of layered materials that are similar to MXenes.

Axial Motion Characterization of a Helical Ionic Polymer Metal Composite Actuator and Its Application in 3-DOF Micro-Parallel Platforms

In this work, a helical ionic polymer metal composite (IPMC) was fabricated by thermal treatment in a mold with helix grooves. The axial actuation behaviors of the helical IPMC actuator were observed, and the electromechanical and electrochemical characteristics were evaluated. The experimental results showed that as the voltage increased and the frequency decreased, the axial displacement, axial force, and electric current of the actuator all increased. Compared with square wave and sinusoidal signals, the actuator exhibited the most satisfactory motion under the direct current (DC) signal. For the electrochemical test, as the scanning rate decreased, the gravimetric specific capacitance increased. Within a suitable voltage range, the actuator was chemically stable. In addition, we coupled the Electrostatics module, Transport of Diluted Species module, and Solid Mechanics module in COMSOL Multiphysics software to model and analyze the helical IPMC actuator. The simulation data obtained were in good agreement with the experimental data. Finally, by using three helical IPMC actuators as driving components, an innovative three-degree-of-freedom (3-DOF) micro-parallel platform was designed, and it could realize a complex coupling movement of pitch, roll, and yaw under the action of an electric field. This platform is expected to be used in micro-assembly, flexible robots, and other fields.

Flexible Photodriven Actuator Based on Gradient-Paraffin-Wax-Filled Ti3C2Tx MXene Film for Bionic Robots

Due to their high flexibility and adaptability, bionic robots have great potential in applications such as healthcare, rescue, and surveillance. The flexible actuator is an essential component of the bionic robot and determines its performance. Even though much progress has been achieved in bionic robot research, there still exists a great challenge in preparing a flexible actuator with a large stroke, high sensitivity, fast response, low triggering power, and long lifetime. This study presents a flexible actuator based on a paraffin wax and Ti₃C₂T_x MXene (PW-MX) film composite. Such a flexible actuator delivers an excellent actuation performance, including a large curvature change (2.2 × 10² m^-1), high thermal sensitivity (4.6 m^-1/°C), low triggering power of light (76 mW/cm²), wavelength selectivity, fast response (0.38 s), and long lifetime (>20000 cycles). Due to the high thermal sensitivity and the strong infrared absorption of the PW-MX film, crawling motion of an inchworm robot based on PW-MX film can be triggered by infrared irradiation from the human finger. To mimic living organisms with bioluminescence, we prepared a PW-MX actuator with green fluorescence by doping PW-MX film with CdSe/ZnS quantum dots. The integration of luminescent function enables the PW-MX actuator to deliver information under light stimulation and to camouflage under a background of green foliage actively. With its merits of ease of fabrication and high actuation performance, the flexible PW-MX actuator is expected to lend itself to more applications in the future.