High-speed electrically actuated elastomers with strain greater than 100%

Regulating hydrogel mechanical properties with an electric field

Stimuli-responsive polymeric materials have attracted significant attention due to their ability to change properties in response to various external stimuli. Using an electric field as the stimulus is of particular interest as it possesses the potential for seamless integration of materials with electronic systems. While many materials with electric field responsive actuation have an associated mechanical property change, it is beneficial to develop materials that exhibit mechanical property changes without accompanying significant shape deformation. To address this challenge, here we designed a semi-interpenetrating polymer network (semi-IPN) hydrogel system containing both polyelectrolytes and salt ions, which enables electric field induced changes in mechanical properties while minimizing actuation. We first successfully verified the viability of our design by removing salt ions through a diffusion-only method where we witnessed the stiffness increased to 4.5 times the initial value while still being highly deformable. After this, we applied an electric field to transport the salt ions out of the hydrogel, as shown by both Raman spectroscopy and scanning electron microscopy. We were able to show a time-dependent stiffness increase, the maximum of which was 5 times the original stiffness. We quantified ion transport and water-splitting in the hydrogel by both experiments and simulations. Following this, we showed functional system reversibility by reversing the direction of the current to reinject salt ions into the semi-IPN hydrogel and reducing its stiffness to below the initial value. It's worth noting that our simulations enable us to understand the governing mechanisms behind ion generation and salt transport that leads to mechanical property changes. Finally, we were able to fabricate a spatially variable stiffness haptic interface with our hydrogel, with demonstrated reversibility and cyclability. This research can possibly find applications in soft robotics and haptics and also inspire the development of bio-compatible electronics related devices.

Optimization of Core-Shell Ternary Electrodes for High-Performance Ionic Actuator in Soft Gripper

Ionic actuators based on composite electrodes consisting of nanomaterials and conducting polymer typically offer the advantages of low-voltage operation and high stability, however, electrode preparation using conventional mixing suffers from issues of ineffective dispersion of nanomaterials, greatly diminishing their synergistic effects. Here, the ternary electrode system based on SWCNTs/PEDOT: PSS/ionic liquid using the two-step dispersion process is optimized, achieving a uniformly coated core-shell structure with high conductivity (≈392.4 S cm-1). The ions migration process is analyzed according to the core-shell model, further optimization of the ternary electrode and device structure enables the actuator to realize the peak-to-peak strain per volt reaching 1.3% V-1 and normalized blocking force of 0.15 MPa V-1 (≈89.2 times its own weight), with stable performance maintained over 1 million cycles. Therefore, the actuator can be utilized for the assembly of multi-clawed grippers to grasp precision components or larger objects. Multiple connected actuators fulfill a complex deformation, indicating promising applications in smart grippers, bioinspired robotics, and human-machine interaction.

Bioinspired Soft Machines: Engineering Nature's Grace into Future Innovations

This article explores the transformative advances in soft machines, where biology, materials science, and engineering have converged. We discuss the remarkable adaptability and versatility of soft machines, whose designs draw inspiration from nature's elegant solutions. From the intricate movements of octopus tentacles to the resilience of an elephant's trunk, nature provides a wealth of inspiration for designing robots capable of navigating complex environments with grace and efficiency. Central to this advancement is the ongoing research into bioinspired materials, which serve as the building blocks for creating soft machines with lifelike behaviors and adaptive capabilities. By fostering collaboration and innovation, we can unlock new possibilities in soft machines, shaping a future where robots seamlessly integrate into and interact with the natural world, offering solutions to humanity's most pressing challenges.

Design and Characterization of an Equibiaxial Multi-Electrode Dielectric Elastomer Actuator

With the ongoing journey of automation advancements and a trend towards miniaturization, the choice of actuator plays a crucial role. Over recent years, soft actuators have demonstrated their usefulness in various applications, especially where light weight and high strain are required. Dielectric elastomer actuators (DEAs) are a class of soft actuators that provide high-strain actuation possibilities in applications like biomedicine, logistics, or consumer electronics. A variety of work featuring DEAs for actuation has been carried out in recent years, but a single work detailing the design conception, fabrication, modeling and experimental validation is lacking, especially in the context of achieving high strains with the integration of multiple electrodes and their interaction. This work discusses these issues with an equibiaxial DEA, enabling optimized equibiaxial strain patterns due to full use of the available actuation area. The developed DEA can achieve an equibiaxial strain of 12.75% for actuation at 60 V μm-1 over an active area of 7 cm2 which is an improvement of 1.3 times compared to traditional dot actuators. These properties position the device as a promising alternative for various applications like cell cultures or microassembly and provide an advantage of optimized use of passive regions within the actuator.

Vibration study of dielectric elastomer conical structure based on fractional viscoelasticity

Dielectric elastomer (DE) is intelligent soft material which is widely used in actuator and energy harvesting fields such as softs robots and wave energy harvesting. Viscoelasticity of DE is seldom investigated by the fractional derivative modeling which is more suited for describing the non-Newton fluid of viscosity. The fractional dynamical governing equation of a conical structure for energy harvesting is established, the dynamic behaviors such as vibration displacement and velocity, amplitude-frequency character, output voltage are all studied by theory and experiments. The fractional viscoelasticity modeling without electricity fits well with the experiments. Viscoelasticity decreases both the displacement and the velocity of the transit vibration in overall frequency region. However, for the stable vibration, the viscoelasticity reduces the amplitude in low and resonance regions, and it enhances the amplitude in high frequency region. The input voltage effects lightly both the static and dynamic amplitudes. The output voltage predicted by theory is agree with counterparts of the experiment to some extent. This research can give some information for design of the wave energy harvesting.

Bioinspired Mechanisms and Actuation of Soft Robotic Crawlers

Inspired by soft-bodied animals, scientists and roboticists have explored and created soft materials and robots in this last decade to mimic the functionality and behavior of soft-crawling animals. Such effort has originated from the unique ability of soft structures to crawl effectively in complex natural environments. As a result, there is growing interest in the design of highly functional soft robotic crawlers through the creation of new flexible functional materials, understanding the underlying science of crawling mechanisms, and employing advanced actuation strategies. This review investigates the current state-of-the-art in this fascinating area to demonstrate the nexus between materials, mechanisms, actuation, and applications. Bioinspired crawling mechanisms of soft crawlers are initially outlined, which include two-anchor peristaltic and serpentine crawling and undulatory motion as analogs to caterpillars, worms, and snakes, respectively. The fabrication and use of new materials in the design of soft crawlers are also discussed, along with the exploitation of actuation mechanisms to achieve specific crawling locomotion. Finally, insights into future research directions are outlined.

Skin-attached haptic patch for versatile and augmented tactile interaction

Wearable tactile interfaces can enhance immersive experiences in virtual/augmented reality systems by adding tactile stimulation to the skin along with the visual and auditory information delivered to the user. We introduce a flat cone dielectric elastomer actuator (FCDEA) array that is thin, soft, and capable of producing spatiotemporally adjustable and large static-to-dynamic force in response to electric voltage signals on large areas of the skin. Integration of the FCDEA array into a photomicrosensor array enables the implementation of a wearable wireless communication haptic patch. We demonstrate that the developed haptic patch allows users to communicate tactile information in real time while maintaining conformal contact with the skin. The haptic patch can also express the topology of 3D structures and render textures of virtual objects in response to localized vibration of the FCDEA array. We expect that the developed haptic patch will provide an immersive touching experience in virtual reality and facilitate tactile communication between users in various applications.

Hollow fiber-based strain sensors with desirable modulus and sensitivity at effective deformation for dexterous electroelastomer cylindrical actuator

The electroelastomer cylindrical actuators, a typical representation of soft actuators, have recently aroused increasing interest owing to their advantages in flexibility, deformability, and spatial utilization rate. Proprioception is crucial for controlling and monitoring the shape and position of these actuators. However, most existing flexible sensors have a modulus mismatch with the actuation unit, hindering the free movement of these actuators. Herein, a low-modulus strain sensor based on laser-induced cellular graphitic flakes (CGF) onto the surface of hollow TPU fibers (HTF) is present. Through the electrostatic self-assembly technology, the flexible sensor features a unique hybrid sensing unit including soft HTF as substrate and rigid CGF as conductive path. As a result, the sensor simultaneously possesses desirable modulus (~0.155 MPa), a gauge factor of 220.3 (25% < ε < 50%), fast response/recovery behaviors (31/62 ms), and a low detection limit (0.1% strain). Integrating the sensor onto the electroelastomer cylindrical actuators enables precise measurement of deformation modes, directions, and quantity. As proof-of-concept demonstrations, a prototype soft robot with high-precision perception is successfully designed, achieving real-time detection of its deformations during the crawling process. Thus, the proposed scheme sheds new light on the development of intelligent soft robots.

Effects of Polymeric Crosslinker on Network Structure, Morphology, and Properties of Liquid Isoprene Rubber

In this study, we investigated the influence of an epoxy end-capped polypropylene oxide crosslinker (epoxy-PPO) on the formation of the crosslinked network structure, the stress-strain response, and the electro-mechanical actuation performance of a maleic anhydride functionalized liquid isoprene rubber (LIR). The crosslinker amount varied from 10 (C-LIR-10) to 50 (C-LIR-50) weight parts per hundred parts (phr) of LIR. The swelling test of the cured rubbers revealed that C-LIR-20 formed the densest crosslinked network with the lowest chloroform uptake value within this series. The crosslinked rubber became stiffer in tensile response upon increasing the epoxy-PPO amount from C-LIR-10 to C-LIR-20 and then softened at higher amounts. The SEM measurements were used to relate this composition-induced softening of the rubbers to the phase morphology evolution from nanoscale homogeneity in C-LIR-10 to microscale segregations of excess crosslinkers in C-LIR-50. The use of epoxy-PPO improved the dielectric constant value of LIR; however, the leakage current through the films also increased from 25 µA DC to 320 µA DC for LIR-30 and LIR-50, respectively, during DEA operation. The electro-mechanical actuation tests with circular actuators showed that the C-LIR-10 elastomer film demonstrated a radial strain of 1.7% on activation at an electric field strength of 17.5 V/µm. At higher crosslinker amounts, the close proximity of excess epoxy-PPO molecules caused leakage current across elastomer films thus diminishing the actuation strain of otherwise relatively softer elastomers with higher dielectric constant values.

Electrically Driven, Bioluminescent Compliant Devices for Soft Robotics

Soft robotics, a research field wherein robots are fabricated from compliant materials, has sparked widespread research interest because of its potential applications in a variety of scenarios. In soft robots, luminescence is an important functionality for communication and information transmission, and it is typically achieved through electroluminescence, which relies on synthetic substances activated by external electric sources, such as batteries. This paper focuses on the use of luciferase, a biologically derived luminescent enzyme, as a luminescent material. Bioluminescence, which is triggered by the luciferin-luciferase reaction, is highly energy-efficient, nontoxic, and eco-friendly. In this regard, a mammalian cell-derived secreted luciferase bioluminescent liquid was developed. This bioluminescent liquid is strongly bright, stable, freezable, and scalable for use as a soft robotic material. To investigate the applicability of this bioluminescent liquid to soft robotics, it was incorporated as an electrode in electrically driven soft actuators, sensors, and robots. Specifically, dielectric elastomer sensors (DESs) and dielectric elastomer actuators (DEAs) were fabricated and characterized using established fabrication processes. The resistivity of the bioluminescent liquid was found to be 448.1 Ω·cm. When the DES was subjected to uniaxial strain, it exhibited a linear response and large deformation of up to 200% strain, with a simultaneous luminance change of 27%. The DEA displayed an areal strain of 46.0% and a luminance change of 31% at an applied voltage of 3.4 kV. The waterproof bending DEA generated a tip angle of 21.8° at 10 kV and was applied to a jellyfish robot that could swim in water at a speed of 2.1 mm/s. The experimental results demonstrated the successful operation of these devices, validating the concept of energy-efficient, safe, and environmentally friendly bioluminescent soft robots.

Integrated thermal management-sensing-actuation functional artificial muscles

Electrothermal-driven polymer fiber-based artificial muscles with helical or twisted structures are promising due to their low cost and high energy density output. However, the current cooling methods for these muscles, such as natural cooling or cold-liquid baths, limit their actuation frequency, especially for large-diameter artificial muscles, posing a technical barrier to their broader application. In this study, we developed an advanced tubular fluidic pump by introducing carbon nanotube electrodes, achieving pumping capabilities over 2 times that of conventional electrodes. We integrated this pump with tubular fiber artificial muscles, creating fluid pump-cooled electrothermal artificial muscle systems with parallel and series configurations. This integration reduced cooling time to about one-ninth of the original and increased mechanical energy output power density by 3 times, expanding the effective actuation frequency range by 3.5 times. Additionally, to effective control artificial muscle actuation, we incorporated a resistive sensing layer directly onto the surface of the artificial muscles, enabling position monitoring. On the application front, we demonstrated the potential of these artificial muscles in thermally responsive functional composite materials, deformable mechanical components, and bionic origami wrist joints.

Dual-Modal Dielectric Elastomer System for Simultaneous Energy Harvesting and Actuation

Dielectric elastomers (DEs) have promising capabilities for soft electromechanical systems, including those for actuation and energy generation. However, their widespread application is restricted by electromechanical instability (EMI) and the requirement for high-voltage operation. This study presents a dual-modal DE system that effectively overcomes these limitations by leveraging a dual-membrane structure. The proposed structure not only suppresses EMI through charge sharing but also enables simultaneous energy harvesting and actuation, enhancing the overall electrical performance of the system. The system demonstrated a remarkable improvement in output performance, exceeding that of traditional single-modal DE generators by up to 30%. The practicality of the system is developed by integrating it into a mechanically powered soft robot capable of locomotion and environmental monitoring using a wireless temperature sensor. This study paves the way for the development of advanced DE-based systems with enhanced stability, functionality, and potential for diverse applications in soft robotics, energy harvesting, and other areas that require coupled electromechanical capabilities.

Design of Dielectric Elastomer Actuator and Its Application in Flexible Gripper

Dielectric elastomer actuators (DEAs) are difficult to apply to flexible grippers due to their small deformation range and low output force. Hence, a DEA with a large bending deformation range and output force was designed, and a corresponding flexible gripper was developed to realize the function of grasping objects of different shapes. The relationship between the pre-stretch ratio and DEA deformation degree was tested by experiments. Based on the performance test results of the dielectric elastomer (DE), the bending deformation process of DEAs with different shapes was simulated by Finite Element Method (FEM) simulation. DEAs with different shapes were prepared through laser cutting and the relationship between the voltage and the bending angle, and the output force of the DEAs was measured. The result shows that under uniaxial stretching, the deformation of the DEA in the stretching direction gradually increases and decreases in the unstretched direction with the increase in the pre-stretch ratio. Under biaxial stretching, DEA deformation increases with the increase in the pre-stretch ratio. The shape of the DEA has a certain influence on the bending deformation range under the same conditions, and the elliptical DEA has a larger bending deformation range and higher output force compared with the rectangular DEA and the trapezium DEA. The elliptical DEA can produce a bending deformation of 40° and an output force of 37.2 mN at a voltage of 24 kV. The three-finger flexible gripper composed of an elliptical DEA can realize the grasping of a paper cup.

Biomimetic, Interface-Free Stiffness-Gradient PDMS-Co-Polyimide-Based Soft Materials for Stretchable Electronics and Soft Robotics

Soft materials play a pivotal role in the efficacy of stretchable electronics and soft robotics, and the interface between the soft devices and rigid counterparts is especially crucial to the overall performance. Herein, we develop polyimide-polydimethylsiloxane (PI-PDMS) copolymers that, in various ratios, combine on a molecular level to give a series of chemically similar materials with an extremely wide Young's modulus range starting from soft 2 MPa and transitioning to rigid polymers with up to 1500 MPa. Of particular significance is the copolymers' capacity to prepare seamless stiffness gradients, as evidenced by strain distribution analyses of gradient materials, due to them being unified on a molecular level. The copolymers and gradient materials were successfully used as substrates for stretchable thin-film conductors and tested as dielectric elastomer actuators, demonstrating their potential application as enabling components in stretchable electronics and soft robots.

The Effects of Chain Conformation and Nanostructure on the Dielectric Properties of Polymers

The dielectric properties of polymers play a pivotal role in the development of advanced materials for energy storage, electronics, and insulation. This review comprehensively explores the critical relationship between polymer chain conformation, nanostructure, and dielectric properties, focusing on parameters such as dielectric constant, dielectric loss, and dielectric breakdown strength. It highlights how factors like chain rigidity, free volume, molecular alignment, and interfacial effects significantly influence dielectric performance. Special emphasis is placed on the impact of nanofillers, molecular weight, crystallinity, and multilayer structures in optimizing these properties. By synthesizing findings from recent experimental and theoretical studies, this review identifies strategies to enhance energy efficiency, reliability, and mechanical stability of polymer-based dielectrics. We also delve into techniques such as electrostatic force microscopy (EFM) and focused ion beam (FIB) milling for characterizing breakdown mechanisms, offering insights into molecular design for next-generation high-performance polymers. Despite considerable progress, critical challenges such as achieving an optimal balance between dielectric permittivity and breakdown strength, understanding nanoscale interfacial phenomena, and scaling these materials for industrial applications persist. These gaps can be addressed by systematic structure-property relations, advanced processing techniques, and environmental studies.

Liquid-Gas Phase Transition Actuator: Rejuvenation Procedure Extended and Open-Air Performance

To achieve the actuation of silicone-based foamed composites, a liquid-gas phase transition of the liquid captured in its pores is employed. The uncertainty of key parameters for a single or sequential open-air performance of such soft actuators limits their application. To define the main characteristics of the composites, in this work, two functions of the liquid there were separated: the pore-forming agent (FPA) and working liquid (WL). It was demonstrated that the composites can be fabricated using either ethanol or methanol as the PFA, while any of the C1-C4 alcohols can be used as the WL. The results of the sequential actuation tests of the composites revealed that pore formation depends on the composite viscosity during curation, while their expansion in single heat experiments can be approximated by a unified linear relation. Based on a Mendeleev-Clapeyron equation, the qualitative model for predicting the actuator strain is proposed. It was found that the composites with C3-C4 alcohols as the WL outperform ethanol-containing composites on the number of cycles survived under open-air conditions. These findings pave the way to control the operation of soft actuators by manipulating WL variation and PFA content during the composite cure to set the operation temperature and degree of expansion of pre-formed actuators.

Mechanics of Surface Instabilities in Soft Dielectrics Subject to Electromechanical Loading

As a category of polymeric materials, soft dielectrics, such as most elastomers and rubber-like materials, have shown great potential for extensive applications in various fields. Owing to their intriguing electromechanical coupling behaviors, the morphological instabilities in soft dielectrics have been an active research field in recent years. In this work, the recent progress in experimental and theoretical research on their electromechanical morphological instabilities is reviewed, especially regarding the theoretical aspect. First, we revisit the theoretical framework for the electroelasticity of soft dielectrics. Then, the typical configurations of soft dielectric membranes used to generate two typical types of surface instabilities, namely wrinkles and creases, are introduced. Three commonly used modeling approaches (i.e., the stress balance method, the incremental method, and the energy method) for surface instabilities are reviewed with specific examples. Moreover, discussions on the difference between these methods and the corresponding critical loading conditions are presented. Furthermore, this review also covers the relation and transition between wrinkling and creasing phenomena.

Insights into multi-scale structural evolution and dielectric response of poly(methyl acrylate) under pre-strain: A simulation study

The structural evolution of dielectric elastomer induced by pre-strain is a complex, multi-scale process that poses a significant challenge to a deep understanding of the effect of pre-strain. Through simulation results, we identify the variation in the dielectric constant and multi-scale (electronic structure, molecular chain conformation, and aggregation structure) response of poly(methyl acrylate). As the pre-strain increases, the dielectric constant initially rises (below 200% pre-strain) and then declines (above 200% pre-strain). Analysis of the charge distribution, surface electrostatic potential, HOMO-LUMO bandgap, and electron density differences reveal that adjusting chain conformation appropriately could enhance polarity domain and electron polarization. The correlation between permittivity and segment dynamics of deformed molecules is explored, encompassing segment orientation, mean shift displacement, and diffusion coefficient. Following molecular chain orientation, the kinematic capability of the chain segment improves, which leads to an increase in the number and activity of effective dipoles and the enhancement of orientation polarization. Excessive stretching restricts the polymer molecular chain mechanically, reducing the number and activity of effective dipoles and negatively impacting electron polarization. The permittivity transitions from isotropic to anisotropic behavior when the system is subjected to strain. This study provides an interesting solution for research on multiscale responses and intrinsic mechanisms of pre-strain.

A high-response-frequency bimodal network polyacrylate elastomer with ultrahigh power density under low electric field

Dielectric elastomers, used as driver modules, require high power density to enable fast movement and efficient work of soft robots. Polyacrylate elastomers usually suffer from low power density under low electric fields due to limited response frequency. Here, we propose a bimodal network polyacrylate dielectric elastomer which breaks the intrinsic coupling relationship between dielectric and mechanical properties, featuring relatively high dielectric constant, low Young's modulus, and wide driving frequency bandwidth (~200 Hz) like silicones. Therefore, an ultrahigh power density (154 W kg-1@20 MV m-1, 200 Hz) is realized at low electric field and high resonance frequency, 75 times greater than at 10 Hz. Further, a rotary motor is developed, reaching an impressive speed of 1245 rpm at 19.6 MV m-1 and 125 Hz, surpassing previous acrylate-based motors and entering the high-speed domain of silicone-based motors. These findings offer a versatile strategy to fabricate high-power-density dielectric elastomers for low-electric-field soft actuators.

Flexible Piezoelectric Nanocolumnar Composite Films as Flat-Panel Loudspeakers: Application and Modeling

Highly anisotropic piezoelectric composites promise to progress electroacoustic devices as a class by combining the advantages of both piezoceramics and polymers. Fundamentally, piezoelectric loudspeakers employ the converse piezoelectric effect to convert electrical to mechanical energy. Quasi-1-3 piezoceramic/polymer composites enable flat-panel loudspeakers that are tunable in elastic modulus, with opportunities for mechanical flexibility, optical transparency, and large-area coverage. Their processing route enables relatively flexible design parameters, such as the particle loading, polymer-matrix modulus, film thickness, film size, and electrode-material stiffness. Alternative processing routes of electric field (E-field) aligned-piezoelectric composites are demonstrated, including using the relaxor ferroelectric lead magnesium niobate-lead titanate (PMN-PT) to enhance the acoustic performance and photocurable resins to accelerate the materials processing. Material properties critical for dielectrophoresis are characterized, and loudspeakers were fabricated based on the optimal processing conditions. Subsequently, electroacoustic characterization explores the effect of loudspeaker size, substrate stiffness, the microphone distance, the piezoceramic material, and the matrix modulus. Finally, finite-element (FE) modeling of the electromechanical behavior validates the natural frequencies and modes shapes of the loudspeakers via the analytical solution and frequency response to electrical and mechanical excitation. Good correspondence between the predicted electroacoustic performance and experimentally validated model is observed.

Compressible dielectric elastomer actuators in high hydrostatic pressures: Models and experiments

Dielectric elastomer actuators (DEAs) are an emerging type of soft actuators based on intelligent electroactive polymers. Compared with conventional rigid actuators, DEAs can adapt to extreme hydrostatic pressures without any bulky protective vessels and, therefore, have demonstrated great promises in high-hydrostatic pressure applications such as deep-sea explorations. However, the effects of the enormous hydrostatic compressions on the mechanical and electromechanical coupling properties and electrical breakdown strengths of DEAs remain unclear due to the restrictions in the existing theoretical models and limitations in the experimental techniques developed for DEAs. To bridge these gaps, this paper develops a hydrostatic pressure-coupled DEA model and introduces a series of experimental characterization techniques for accurately quantifying such effects introduced by the hydrostatic pressures. A wide range of hydrostatic pressures up to 105 MPa (close to the hydrostatic pressure in Mariana Trench) are investigated and its effects on the stiffening of the elastomer, reduction of the actuation strain, and increase of the breakdown electric field of the DEAs are reported in both experiments and models. The contributions of this paper can offer guidelines for characterizing the performances of DEAs under high-hydrostatic compressions and for designing the next-generation soft robotic systems for deep-sea applications.

Haptic artificial muscle skin for extended reality

Existing haptic actuators are often rigid and limited in their ability to replicate real-world tactile sensations. We present a wearable haptic artificial muscle skin (HAMS) based on fully soft, millimeter-scale, multilayer dielectric elastomer actuators (DEAs) capable of significant out-of-plane deformation, a capability that typically requires rigid or liquid biasing. The DEAs use a thickness-varying multilayer structure to achieve large out-of-plane displacement and force, maintaining comfort and wearability. Experimental results demonstrate that HAMS can produce complex tactile feedback with high perception accuracy. Moreover, we show that HAMS can be integrated into extended reality (XR) systems, enhancing immersion and offering potential applications in entertainment, education, and assistive technologies.

Wearable and Implantable Soft Robots

Soft robotics presents innovative solutions across different scales. The flexibility and mechanical characteristics of soft robots make them particularly appealing for wearable and implantable applications. The scale and level of invasiveness required for soft robots depend on the extent of human interaction. This review provides a comprehensive overview of wearable and implantable soft robots, including applications in rehabilitation, assistance, organ simulation, surgical tools, and therapy. We discuss challenges such as the complexity of fabrication processes, the integration of responsive materials, and the need for robust control strategies, while focusing on advances in materials, actuation and sensing mechanisms, and fabrication techniques. Finally, we discuss the future outlook, highlighting key challenges and proposing potential solutions.

Review of Flexible Robotic Grippers, with a Focus on Grippers Based on Magnetorheological Materials

Flexible grippers are a promising and pivotal technology for robotic grasping and manipulation tasks. Remarkably, magnetorheological (MR) materials, recognized as intelligent materials with exceptional performance, are extensively employed in flexible grippers. This review aims to provide an overview of flexible robotic grippers and highlight the application of MR materials within them, thereby fostering research and development in this field. This work begins by introducing various common types of flexible grippers, including shape memory alloys (SMAs), pneumatic flexible grippers, and dielectric elastomers, illustrating their distinctive characteristics and application domains. Additionally, it explores the development and prospects of magnetorheological materials, recognizing their significant contributions to the field. Subsequently, MR flexible grippers are categorized into three types: those with viscosity/stiffness variation capabilities, magnetic actuation systems, and adhesion mechanisms. Each category is comprehensively analyzed, specifying its unique features, advantages, and current cutting-edge applications. By undertaking an in-depth examination of diverse flexible robotic gripper types and the characteristics and application scenarios of MR materials, this paper offers a valuable reference for fellow researchers. As a result, it facilitates further advancements in this field and contributes to the provision of efficient gripping solutions for industrial automation.

Emerging innovations in electrically powered artificial muscle fibers

This review systematically explores the inherent structural advantages of fiber over conventional film or bulk forms for artificial muscles, emphasizing their enhanced mechanical properties and actuation, scalability, and design flexibility. Distinctive merits of electrically powered artificial muscle fiber actuation mechanisms, including electrothermal, electrochemical and dielectric actuation, are highlighted, particularly for their operational efficiency, precise control capabilities, miniaturizability and seamless integration with electronic components. A comprehensive overview of significant research driving performance enhancements in artificial muscle fibers through materials and structural innovations is provided, alongside a discussion of the diverse design methodologies that have emerged in this field. A detailed comparative assessment evaluates the performance metrics, advantages and manufacturing complexities of each actuation mechanism, underscoring their suitability for various applications. Concluding with a strategic outlook, the review identifies key challenges and proposes targeted research directions to advance and refine artificial muscle fiber technologies.

Electrical-Modulated Flexible Acoustic Metamaterial: Enhancing Low-Frequency Absorption via an Ionic Electroactive Polymer

The growing concern over low-frequency noise pollution resulting from global industrialization has posed substantial challenges in noise attenuation. However, conventional acoustic metamaterials, with fixed geometries, offer limited flexibility in the frequency range adjustment once constructed. This research unveiled the promising potential of ionic electroactive polymers, particularly ionic polymer-metal composites (IPMCs), as a superior candidate to design tunable acoustic metamaterial due to its bidirectional energy conversion capabilities. The previously perceived limitations of the IPMC, including slow reaction and high energy expenditure, owning to its inherent sluggish intermediary ionic mass transport process, were astutely leveraged to expedite the attenuation of low-frequency sound energy. Both our experimental and simulation results elucidated that the IPMC can generate voltage potentials in response to acoustic pressure at frequencies significantly higher than those previously established. In addition, the peak absorption frequency can be effectively shifted by up to 45.7% with the application of a 4 V voltage. By further integration with a microperforated panel (MPP) structure, the developed metamaterial absorbers can achieve complete sound absorption, which was continuously tunable under minimal voltage stimulation across a wide frequency spectrum. In addition, a microslit structure IPMC metamaterial absorber was designed to realize modulation of the perforation rate, and the absorption peak can be shifted by up to 79.2%. These findings signify a pioneering application of ionic intelligent materials and may pave the way for further innovations of tunable low-frequency acoustic structures, ultimately advancing the pragmatic deployment of both soft intelligent materials and acoustic metamaterials.

Microscopic in situ observation of electromechanical instability in a dielectric elastomer actuator utilizing transparent carbon nanotube electrodes

Electromechanical instability (EMI) restricts the performance of dielectric elastomer actuators (DEAs), leading to premature electrical breakdown at a certain voltage. However, macro-level observations using traditional carbon grease electrodes have failed to capture the detailed features of EMI. In this study, we investigated EMI at the microscopic scale by fabricating transparent and conductive single-walled carbon nanotube (SWCNT) electrodes. Our findings reveal that EMI predominantly occurs in highly localized regions with dimensions on the order of tens of micrometers. This snap-through instability is likely induced by pre-existing defects within the elastomer, such as air voids or conductive particles, which reduce the critical voltage required for EMI in the flawed areas. From the perspective of phase transition principles, these defects act as heterogeneous nucleation sites for new phase embryos, thereby lowering the energy barrier for the electromechanical phase transition (i.e., EMI) compared to homogeneous nucleation in an ideally impurity-free elastomer. This study clarifies the longstanding discrepancy between theoretically predicted deformation bursts and the experimentally observed macroscopic continuous expansion of DEAs under low pre-stretch conditions. Additionally, it underscores the critical importance of material purity in mitigating electromechanical instability.

Soft Gripping Fingers Made of Multi-Stacked Dielectric Elastomer Actuators with Backbone Strategy

Soft grippers, a rapidly growing subfield of soft robotics, utilize compliant and flexible materials capable of conforming to various shapes. This feature enables them to exert gentle yet, if required, strong gripping forces. In this study, we elaborate on the material selection and fabrication process of gripping fingers based on the dielectric elastomer actuation technique. We study the effects of mixing the silicone elastomer with a silicone thinner on the performance of the actuators. Inspired by nature, where the motion of end-effectors such as soft limbs or fingers is, in many cases, directed by a stiff skeleton, we utilize backbones for translating the planar actuation into a bending motion. Thus, the finger does not need any rigid frame or pre-stretch, as in many other DEA approaches. The idea and function of the backbone strategy are demonstrated by finite element method simulations with COMSOL Multiphysics® 6.5. The paper describes the full methodology from material choice and characterization, design, and simulation to characterization to enable future developments based on our approach. Finally, we present the performance of these actuators in a gripper demonstrator setup. The developed actuators bend up to 68.3° against gravity, and the gripper fingers hold up to 10.3 g against gravity under an actuation voltage of 8 kV.

Confining Nano-ZrO2 in Nanodomains Leads to Electroactive Artificial Muscle with Large Deformation

Dielectric elastomers generate muscle-like electroactive actuation, which is applicable in soft machines, medical devices, etc. However, the actuation strain and energy density of most dielectric elastomers, in the absence of prestretch, have long been limited to ∼20% and ∼10 kJ m-3, respectively. Here, we report a dielectric elastomer with ZrO2 nanoparticles confined in nanodomains, which achieves an actuation strain >100% and an energy density of ∼150 kJ m-3 without prestretch. We decorate the surface of each nanoparticle with a layer of a diblock oligomer, poly(acrylic acid-b-styrene). The surface-decorated nanoparticles coassemble with a triblock copolymer elastomer, poly(styrene-b-(2-ethylhexyl acrylate)-b-styrene) during cosolvent casting. Consequently, the nanoparticles are confined in the polystyrene nanodomains, which results in a dielectric elastomer nanocomposite with a low modulus, high breakdown strength, and intense strain-hardening behavior. During the actuation, the nanocomposite avoids the snap-through instability that most elastomers would suffer and achieves a superior actuation performance.

Robust control of electrohydraulic soft robots

This article introduces a model-based robust control framework for electrohydraulic soft robots. The methods presented herein exploit linear system control theory as it applies to a nonlinear soft robotic system. We employ dynamic mode decomposition with control (DMDc) to create appropriate linear models from real-world measurements. We build on the theory by developing linear models in various operational regions of the system to result in a collection of linear plants used in uncertainty analysis. To complement the uncertainty analyses, we utilizeH ∞ ("H Infinity") synthesis techniques to determine an optimal controller to meet performance requirements for the nominal plant. Following this methodology, we demonstrate robust control over a multi-input multi-output (MIMO) hydraulically amplified self-healing electrostatic (HASEL)-actuated system. The simplifications in the proposed framework help address the inherent uncertainties and complexities of compliant robots, providing a flexible approach for real-time control of soft robotic systems in real-world applications.

Soft Polymer-Actuated Compliant Microgripper with Adaptive Vibration-Controlled Grasp and Release

Microgrippers that incorporate soft actuators are appropriate for micromanipulation or microsurgery owing to their ability to grasp objects without causing damage. However, developing a microgripper with a large gripping range that can produce a large force with high speed remains challenging in soft actuation mechanisms. Herein, we introduce a compliant microgripper driven by a soft dielectric elastomer actuator (DEA) called a spiral flexure cone DEA (SFCDEA). The submillimeter-scale SFCDEA exhibited a controllable linear displacement over a high bandwidth and the capability of lifting 100.9 g, which was 670 times higher than its mass. Subsequently, we developed a compliant microgripper based on the SFCDEA using smart composite microstructure technology to fabricate three-dimensional gripper linkages. We demonstrated that the microgripper was able to grasp various millimeter-scale objects with different shapes, sizes, and weights without a complex feedback control owing to its compliance. We proved the versatility of our gripper in robotic manipulation by demonstrating adaptive grasping and releasing of small objects using vibrations owing to its high bandwidth.

Fiber-Reinforced Equibiaxial Dielectric Elastomer Actuator for Out-of-Plane Displacement

Dielectric elastomer actuators (DEAs) have gained significant attention due to their potential in soft robotics and adaptive structures. However, their performance is often limited by their in-plane strain distribution and limited mechanical stability. We introduce a novel design utilizing fiber reinforcement to address these challenges. The fiber reinforcement provides enhanced mechanical integrity and improved strain distribution, enabling efficient energy conversion and out-of-plane displacement. We discuss an analytical model and the fabrication process, including material selection, to realize fiber-reinforced DEAs. Numerical simulations and experimental results demonstrate the performance of the fiber-reinforced equibiaxial DEAs and characterize their displacement and force capabilities. Actuators with four and eight fibers are fabricated with 100 μm and 200 μm dielectric thicknesses. A maximal out-of-plane displacement of 500 μm is reached, with a force of 0.18 N, showing promise for the development of haptic devices.

Effect of temperature and pre-stretch on the dynamic performance of dielectric elastomer minimum energy structure

Dielectric Elastomer Minimum Energy Structures (DEMES) have the ability of actively adjusting their shape to accommodate complex scenarios, understanding the actuation mechanism of DEMES is essential for their effective design and control, which has rendered them a focus of research in the field of soft robotics. The actuation ability of DEMES is usually influenced by external conditions, among which the electromechanical properties of DE materials are highly sensitive to temperature changes, and the pre-stretch ratio of DE materials has a significant impact on the dynamic performance of DEMES. Therefore, it is necessary to study the effects of temperature and pre-stretch ratio on the nonlinear dynamic behavior of DEMES. In this paper, in response to the lack of research on the influence of DE pre-stretch ratio on the actuation characteristics of DEMES, this paper proposes a systematic modeling and analysis framework that comprehensively considers pre-stretch factors, temperature factors, and viscoelastic factors, and establishes the motion control equation of DEMES affected by the coupling effect of DE pre-stretch ratio and temperature. The proposed analytical framework is used to analyze the evolution of the electromechanical response of DEMES under voltage excitation under the coupling of DE pre-stretch ratio and temperature. The results indicate that the bending angle, inelastic deformation, resonant frequency, and dynamic stability of DEMES can be jointly adjusted by the DE pre-stretch ratio and ambient temperature. A low pre-stretch ratio of DE can lead to dynamic instability of DEMES, while appropriate temperature conditions and higher pre-stretch ratios can significantly improve the actuation ability of DEMES. This can provide theoretical guidance for the design and deformation control of DEMES.

Electromechanical Performances of Polyvinyl Chloride Gels Using (Polyvinyl Chloride-Co-Vinyl Acetate) (P(VC-VA)) Synergistic Plasticization

The current polyvinyl chloride (PVC) gel flexible actuators are facing challenges of high input voltage and an insufficient elastic modulus. In this study, we conducted a detailed study on the properties of PVC gel prepared by introducing the modifier polyvinyl chloride-vinyl acetate (P(VC-VA)). We compared a modified PVC gel with the traditional one in terms of the relative dielectric constant, mechanical modulus, and electromechanical actuation performance. Experimental results demonstrated that the introduction of P(VC-VA) enhanced the dielectric constant and reduced the driving electric field strength of PVC gels. The dielectric constant increased from 4.77 to 7.3. The electromechanical actuation performance increased by 150%. We employed the Gent model to fit the experimental results, and the actual experimental data aligned well with the expectations of the Gent model. The research results show that this type of plasticizing method effectively balanced the mechanical and electrical performance of PVC gels. This study summarizes the experimental results and performance analysis of PVC gels prepared using innovative plasticization methods, revealing the potential engineering applications of polymeric gels.

Modular multi-channel high voltage arbitrary waveform generator and imaging setup for dielectric elastomer actuator characterisation

Various applications require multi-channel high-voltage sources for their control, e.g. electrostatic adhesion, electrophoresis and artificial muscles such as piezoelectric, hydraulically amplified self-healing electrostatic(HASEL) and dielectric elastomer actuators(DEAs). Further, the ability to simultaneously monitor the state of the actuators either with images, or voltage and current sensing is crucial to characterise their behaviour. In this work, we present the design of a versatile characterisation setup, capable of generating eight HV (15 kV) arbitrary waveforms(rise time of 8 ms and fall time of 80 ms for 60 M Ω load), while synchronously monitoring voltage and current, and record high-speed (120 fps) video. The setup ensures modularity and customisability by consisting of three independent modules: (1) The imaging module includes a Raspberry Pi and a Pi Camera; (2) A 3.3 V analogue interface 16-bit resolution data acquisition module on a PCB that accommodates a microcontroller board, two 8-channel analogue-to-digital converters, and an 8-channel digital-to-analogue converter; (3) Up to 8 DC-to-HVDC converter boards powered by 12 V DC, with 3.3 V analogue interface.

A large-strain and ultrahigh energy density dielectric elastomer for fast moving soft robot

Dielectric elastomer actuators (DEAs) with large actuation strain and high energy density are highly desirable for actuating soft robots. However, DEAs usually require high driving electric fields (>100 MV m-1) to achieve high performances due to the low dielectric constant and high stiffness of dielectric elastomers (DEs). Here, we introduce polar fluorinated groups and nanodomains aggregated by long alkyl side chains into DE design, simultaneously endowing DE with a high dielectric constant and desirable modulus. Our DE exhibits a maximum area strain of 253% at a low driving electric field of 46 MV m-1. Notably, it achieves an ultrahigh specific energy of 225 J kg-1 at only‍‌ ‌‌40‍‌ MV m-1, around 6 times higher than natural muscle and twice higher than the state-of-the-art DE. Using our DE, soft robots reach an ultrafast running speed of 20.6 BL s-1, 60 times higher than that of commercial VHB 4910, representing the fastest DEA-driven soft robots ever reported.

Dielectric Elastomer Actuators with Enhanced Durability by Introducing a Reservoir Layer

A Dielectric Elastomer Actuator (DEA) consists of electrodes with a dielectric layer between them. By controlling the design of the electrodes, voltage, and frequency, the operating range and speed of the DEA can be adjusted. These DEAs find applications in biomimetic robots, artificial muscles, and similar fields. When voltage is applied to the DEA, the dielectric layer undergoes compression and expansion due to electrostatic forces, which can lead to electrical breakdown. This phenomenon is closely related to the performance and lifespan of the DEA. To enhance stability and improve dielectric properties, a DEA Reservoir layer is introduced. Here, stability refers to the ability of the DEA to perform its functions even as the applied voltage increases. The Reservoir layer delays electrical breakdown and enhances stability due to its enhanced thickness. The proposed DEA in this paper is composed of a Reservoir layer and electrode layer. The Reservoir layer is placed between the electrode layers and is independently configured, not subjected to applied voltage like the electrode layers. The performance of the DEA was evaluated by varying the number of polymer layers in the Reservoir and electrode designs. Introducing the Reservoir layer improved the dielectric properties of the DEA and delayed electrical breakdown. Increasing the dielectric constant through the DEA Reservoir can enhance output characteristics in response to electrical signals. This approach can be utilized in various applications in wearable devices, artificial muscles, and other fields.

Multimodal Soft Robotic Actuation and Locomotion

Diverse and adaptable modes of complex motion observed at different scales in living creatures are challenging to reproduce in robotic systems. Achieving dexterous movement in conventional robots can be difficult due to the many limitations of applying rigid materials. Robots based on soft materials are inherently deformable, compliant, adaptable, and adjustable, making soft robotics conducive to creating machines with complicated actuation and motion gaits. This review examines the mechanisms and modalities of actuation deformation in materials that respond to various stimuli. Then, strategies based on composite materials are considered to build toward actuators that combine multiple actuation modes for sophisticated movements. Examples across literature illustrate the development of soft actuators as free-moving, entirely soft-bodied robots with multiple locomotion gaits via careful manipulation of external stimuli. The review further highlights how the application of soft functional materials into robots with rigid components further enhances their locomotive abilities. Finally, taking advantage of the shape-morphing properties of soft materials, reconfigurable soft robots have shown the capacity for adaptive gaits that enable transition across environments with different locomotive modes for optimal efficiency. Overall, soft materials enable varied multimodal motion in actuators and robots, positioning soft robotics to make real-world applications for intricate and challenging tasks.

Functional soft robotic composites based on organic photovoltaic and dielectric elastomer actuator

Improving the energy efficiency of robots remains a crucial challenge in soft robotics, with energy harvesting emerging as a promising approach to address it. This study presents a functional soft robotic composite called OPV-DEA, which integrates flexible organic photovoltaic (OPV) and dielectric elastomer actuator (DEA). The composite can simultaneously generate electrostatic bending actuation and harvest energy from external lights. Owing to its simplicity and inherent flexibility, the OPV-DEA is poised to function as a fundamental building block for soft robots. This study aimed to validate this concept by initially establishing the fabrication process of OPV-DEA. Subsequently, experimental samples are fabricated and characterized. The results show that the samples exhibit a voltage-controllable bending actuation of up to 15.6° and harvested power output of 1.35 mW under an incident power irradiance of 11.7 mW/cm2. These performances remain consistent even after 1000 actuation cycles. Finally, to demonstrate the feasibility of soft robotic applications, an untethered swimming robot equipped with two OPV-DEAs is fabricated and tested. The robot demonstrates swimming at a speed of 21.7 mm/s. The power consumption of the robot is dominated by a high-voltage DC-DC converter, with a value approximately 1.5 W. As a result, the on-board OPVs cannot supply the necessary energy during locomotion simultaneously. Instead, they contribute to the overall system by charging a battery used for the controller on board. Nevertheless, these findings suggest that the OPV-DEA could pave the way for the development of an unprecedented range of functional soft robots.

Actuation for flexible and stretchable microdevices

Flexible and stretchable microdevices incorporate highly deformable structures, facilitating precise functionality at the micro- and millimetre scale. Flexible microdevices have showcased extensive utility in the fields of biomedicine, microfluidics, and soft robotics. Actuation plays a critical role in transforming energy between different forms, ensuring the effective operation of devices. However, when it comes to actuating flexible microdevices at the small millimetre or even microscale, translating actuation mechanisms from conventional rigid large-scale devices is not straightforward. The recent development of actuation mechanisms leverages the benefits of device flexibility, particularly in transforming conventional actuation concepts into more efficient approaches for flexible devices. Despite many reviews on soft robotics, flexible electronics, and flexible microfluidics, a specific and systematic review of the actuation mechanisms for flexible and stretchable microdevices is still lacking. Therefore, the present review aims to address this gap by providing a comprehensive overview of state-of-the-art actuation mechanisms for flexible and stretchable microdevices. We elaborate on the different actuation mechanisms based on fluid pressure, electric, magnetic, mechanical, and chemical sources, thoroughly examining and comparing the structure designs, characteristics, performance, advantages, and drawbacks of these diverse actuation mechanisms. Furthermore, the review explores the pivotal role of materials and fabrication techniques in the development of flexible and stretchable microdevices. Finally, we summarise the applications of these devices in biomedicine and soft robotics and provide perspectives on current and future research.

Soft octopus-inspired suction cups using dielectric elastomer actuators with sensing capabilities

Bioinspired and biomimetic soft grippers are rapidly growing fields. They represent an advancement in soft robotics as they emulate the adaptability and flexibility of biological end effectors. A prominent example of a gripping mechanism found in nature is the octopus tentacle, enabling the animal to attach to rough and irregular surfaces. Inspired by the structure and morphology of the tentacles, this study introduces a novel design, fabrication, and characterization method of dielectric elastomer suction cups. To grasp objects, the developed suction cups perform out-of-plane deflections as the suction mechanism. Their attachment mechanism resembles that of their biological counterparts, as they do not require a pre-stretch over a rigid frame or any external hydraulic or pneumatic support to form and hold the dome structure of the suction cups. The realized artificial suction cups demonstrate the capability of generating a negative pressure up to 1.3 kPa in air and grasping and lifting objects with a maximum 58 g weight under an actuation voltage of 6 kV. They also have sensing capabilities to determine whether the grasping was successful without the need of lifting the objects.

Towards high performance and durable soft tactile actuators

Soft actuators are gaining significant attention due to their ability to provide realistic tactile sensations in various applications. However, their soft nature makes them vulnerable to damage from external factors, limiting actuation stability and device lifespan. The susceptibility to damage becomes higher with these actuators often in direct contact with their surroundings to generate tactile feedback. Upon onset of damage, the stability or repeatability of the device will be undermined. Eventually, when complete failure occurs, these actuators are disposed of, accumulating waste and driving the consumption of natural resources. This emphasizes the need to enhance the durability of soft tactile actuators for continued operation. This review presents the principles of tactile feedback of actuators, followed by a discussion of the mechanisms, advancements, and challenges faced by soft tactile actuators to realize high actuation performance, categorized by their driving stimuli. Diverse approaches to achieve durability are evaluated, including self-healing, damage resistance, self-cleaning, and temperature stability for soft actuators. In these sections, current challenges and potential material designs are identified, paving the way for developing durable soft tactile actuators.

Repeatedly Programmable Liquid Crystal Dielectric Elastomer with Multimodal Actuation

Dielectric elastomers (DEs) are actuatable under an electric field, whose large strain and fast response speed compare favorably with natural muscles. However, the actuation of DE-based devices is generally limited to a single mode and cannot be reconfigured after fabrication, which pales in comparison to biological counterparts given the ability to alter actuation modes according to external conditions. To address this, liquid crystal dielectric elastomers (LC-DEs) that can alter the dielectric actuation modes based on the thermally triggered shape-changing are prepared. Specifically, the two shapes through the LC phase transition possess different bending stiffness, which leads to distinct actuation modes after an electric field is applied. Moreover, the two shapes can be individually programmed/reprogrammed, that is, the one before the transition is regulated through force-directed solvent evaporation and the one after the transition is via bond exchange-enabled stress relaxation. As such, the multimodal dielectric actuation behaviors upon temperature change can be readily diversified. Meanwhile, the space charge mechanism endows LC-DEs with the significantly reduced driving e-field (8 V µm-1) and bidirectional actuation manners. It is believed this unique adaptivity in the actuation modes under a low electric field shall offer versatile designs for practical soft robots.

Dielectric Elastomer Actuator-Based Valveless Impedance-Driven Pumping for Meso- and Macroscale Applications

Impedance pumps are simple designs that allow the generation or amplification of flow. They are fluid-filled systems based on flexible tubing connected to tubing with different impedances. A periodic off-center compression of the flexible tubing causes the fluid to move and generate flow. Wave reflection at the impedance mismatch is the primary driving mechanism of the flow. In addition to their straightforward design, impedance pumps are bladeless, valveless, and pulsatile. These properties are highly sought after by demanding and challenging applications, such as the biomedical field, as they present less risk of damage, disruption, and obstruction when handling viscous and delicate fluids/matter. In this study, we propose a high-performance impedance-driven pumping concept with embedded actuation based on a multilayered tubular dielectric elastomer. This pumping system is made of three parts, a dielectric elastomer actuator tube, a passive tube, and a rigid ring that binds and decouples the two subsystems. The system is able to generate net fluid flow rates up to 1.35 L/min with an internal pressure of 125 mmHg. The soft simplistic design, self-contained concept, and high performances of these pumping systems could make them disruptive in many challenging meso- and macroscale applications in general and in the biomedical field in particular.

Stretchable Electronics with Strain-Resistive Performance

Stretchable electronics have attracted tremendous attention amongst academic and industrial communities due to their prospective applications in personal healthcare, human-activity monitoring, artificial skins, wearable displays, human-machine interfaces, etc. Other than mechanical robustness, stable performances under complex strains in these devices that are not for strain sensing are equally important for practical applications. Here, a comprehensive summarization of recent advances in stretchable electronics with strain-resistive performance is presented. First, detailed overviews of intrinsically strain-resistive stretchable materials, including conductors, semiconductors, and insulators, are given. Then, systematic representations of advanced structures, including helical, serpentine, meshy, wrinkled, and kirigami-based structures, for strain-resistive performance are summarized. Next, stretchable arrays and circuits with strain-resistive performance, that integrate multiple functionalities and enable complex behaviors, are introduced. This review presents a detailed overview of recent progress in stretchable electronics with strain-resistive performances and provides a guideline for the future development of stretchable electronics.

Active Materials for Functional Origami

In recent decades, origami has been explored to aid in the design of engineering structures. These structures span multiple scales and have been demonstrated to be used toward various areas such as aerospace, metamaterial, biomedical, robotics, and architectural applications. Conventionally, origami or deployable structures have been actuated by hands, motors, or pneumatic actuators, which can result in heavy or bulky structures. On the other hand, active materials, which reconfigure in response to external stimulus, eliminate the need for external mechanical loads and bulky actuation systems. Thus, in recent years, active materials incorporated with deployable structures have shown promise for remote actuation of light weight, programmable origami. In this review, active materials such as shape memory polymers (SMPs) and alloys (SMAs), hydrogels, liquid crystal elastomers (LCEs), magnetic soft materials (MSMs), and covalent adaptable network (CAN) polymers, their actuation mechanisms, as well as how they have been utilized for active origami and where these structures are applicable is discussed. Additionally, the state-of-the-art fabrication methods to construct active origami are highlighted. The existing structural modeling strategies for origami, the constitutive models used to describe active materials, and the largest challenges and future directions for active origami research are summarized.

In silico optimization of aligned fiber electrodes for dielectric elastomer actuators

Dielectric elastomer actuators (DEAs) exhibit fast actuation and high efficiencies, enabling applications in optics, wearable haptics, and insect-scale robotics. However, the non-uniformity and high sheet resistance of traditional soft electrodes based on nanomaterials limit the performance and operating frequency of the devices. In this work, we computationally investigate electrodes composed of arrays of stiff fiber electrodes. Aligning the fibers along one direction creates an electrode layer that exhibits zero stiffness in one direction and is predicted to possess high and uniform sheet resistance. A comprehensive parameter study of the fiber density and dielectric thickness reveals that the fiber density primary determines the electric field localization while the dielectric thickness primarily determines the unit cell stiffness. These trends identify an optimal condition for the actuation performance of the aligned electrode DEAs. This work demonstrates that deterministically designed electrodes composed of stiff materials could provide a new paradigm with the potential to surpass the performance of traditional soft planar electrodes.

Bioinspired Stimuli-Responsive Materials for Soft Actuators

Biological species can walk, swim, fly, jump, and climb with fast response speeds and motion complexity. These remarkable functions are accomplished by means of soft actuation organisms, which are commonly composed of muscle tissue systems. To achieve the creation of their biomimetic artificial counterparts, various biomimetic stimuli-responsive materials have been synthesized and developed in recent decades. They can respond to various external stimuli in the form of structural or morphological transformations by actively or passively converting input energy into mechanical energy. They are the core element of soft actuators for typical smart devices like soft robots, artificial muscles, intelligent sensors and nanogenerators. Significant progress has been made in the development of bioinspired stimuli-responsive materials. However, these materials have not been comprehensively summarized with specific actuation mechanisms in the literature. In this review, we will discuss recent advances in biomimetic stimuli-responsive materials that are instrumental for soft actuators. Firstly, different stimuli-responsive principles for soft actuators are discussed, including fluidic, electrical, thermal, magnetic, light, and chemical stimuli. We further summarize the state-of-the-art stimuli-responsive materials for soft actuators and explore the advantages and disadvantages of using electroactive polymers, magnetic soft composites, photo-thermal responsive polymers, shape memory alloys and other responsive soft materials. Finally, we provide a critical outlook on the field of stimuli-responsive soft actuators and emphasize the challenges in the process of their implementation to various industries.

Self-Powered and Self-Healable Extraocular-Muscle-Like Actuator Based on Dielectric Elastomer Actuator and Triboelectric Nanogenerator

Although dielectric elastomer actuators (DEAs) are promising artificial muscles for use as visual prostheses in patients with oculomotor nerve palsy (ONP), high driving voltage coupled with vulnerable compliant electrodes limits their safe long-term service. Herein, a self-healable polydimethylsiloxane compliant electrode based on reversible imine bonds and hydrogen bonds is prepared and coated on an acrylic ester film to develop a self-healable DEA (SDEA), followed by actuation with a high-output triboelectric nanogenerator (TENG) to construct a self-powered DEA (TENG-SDEA). Under 135.9 kV mm-1 , the SDEA exhibits an elevated actuated strain of 50.6%, comparable to the actuation under DC power. Moreover, the mechanically damaged TENG-SDEA displays a self-healing efficiency of over 90% for 10 cycles. The TENG ensures the safe using of TENG-SDEAs and an extraocular-muscle-like actuator with oriented motion ability integrated by several TENG-SDEAs is constructed. Additionally, the SDEA is directly used as a flexible capacitive sensor for real-time monitoring of the patient's muscle movement. Accordingly, a medical aid system based on a conjunction of the extraocular-muscle-like actuator and a flexible capacitive sensor is manufactured to help the patients suffering from ONP with physical rehabilitation and treatment.

Autonomous indication of electrical degradation in polymers

Dielectric polymers are ubiquitous as electrical insulation in electronic devices and electrical systems. Electrical degradation of dielectric polymers tends to initiate catastrophic failure of numerous devices and systems, but its detection and early warning remain challenging. Here we report a general material strategy that signals the electrical degradation of dielectric polymers by autonomously presenting a visually discernible warning in the form of a pronounced colour change. This colour change is induced by the chromogenic response of molecular indicators blended with the polymer, which are chemically activated by the oxygen radicals generated in situ during the electrical degradation of the polymer. We unveil that the structural degradation and electrical properties of the dielectric polymer are quantitatively correlated with the colour difference. Such a chromogenic process is autonomous without the need of human intervention or other external energy, thus offering the convenience to lower or even eliminate the risk of dielectric failure.