Dielectric elastomer artificial muscle materials advancement and soft robotic applications

Conventional robotic systems are built with rigid materials to deal with large forces and predetermined processes. Soft robotics, however, is an emerging field seeking to develop adaptable robots that can perform tasks in unpredictable environments and biocompatible devices that close the gap between humans and machines. Dielectric elastomers (DEs) have emerged as a soft actuation technology that imitates the properties and performance of natural muscles, making them an attractive material choice for soft robotics. However, conventional DE materials suffer from electromechanical instability (EMI), which reduces their performance and limits their applications in soft robotics. This review discusses key innovations in DE artificial muscles from a material standpoint, followed by a survey on their representative demonstrations of soft robotics. Specifically, we introduce modifications of DE materials that enable large strains, fast responses, and high energy densities by suppressing EMI. Additionally, we examine materials that allow variable stiffness and self‐healing abilities in DE actuators. Finally, we review dielectric elastomer actuator (DEA) applications in soft robotics in four categories, including automation, manipulation, locomotion, and human interaction.


| INTRODUCTION
Since their inception in 1959 by George Devol, robots have brought automation into reality and freed humans from slow, repetitive, and dangerous tasks in fields such as industrial manufacturing, building construction, and medical surgery. [1][2][3] Robots have traditionally been built from stiff materials to deliver large forces and were often designed to perform a single task with adequate efficiency. However, new and emerging technologies, such as autonomous field exploration and wearable health monitoring, 4,5 may render rigid-bodied robots impractical or unsafe as they require adaptable robots that can perform in unpredictable environments and biocompatible mechanisms for direct human-robot interaction. 6 Part of the solution to the above problem is to replace rigid actuators in conventional robots with soft actuation mechanisms. Human muscles with their large strains, high energy density, adaptive stiffness, and self-recovery abilities have inspired scientists and engineers to create similarly soft materials for versatile applications. [7][8][9] These soft materials typically deform when triggered by external stimuli, such as electric field, light, and heat. Among those triggered by electrical stimuli, electroactive polymers (EAPs) have emerged over the past few decades as a lightweight, soft actuation technology that resembles natural muscles in performance, granting them the moniker "artificial muscles." 10 EAPs exhibit significant deformation in response to electrical stimulation and are categorized as either ionic EAPs or electronic EAPs, depending on the nature of their actuation mechanism. 11 Ionic EAPs, including ionic polymer-metal composites, 12 ionic gels, 13 and conductive polymers, 14 are actuated with a biasing voltage and rely on the displacement of mobile ions inside the polymers. 15 Although they can produce large strains at voltages as low as a few volts, they require the presence of electrolytes and a wet operation environment, limiting their practical applications. Moreover, they predominately actuate in a bending mode and suffer from low energy density and actuation speed, further restricting their artificial muscle applications. 16,17 In contrast, the actuation in electronic EAPs is driven by the electrostatic force generated by the applied electric field. Since the activation does not involve the diffusion of charged particles, electronic EAPs possess a much faster response speed. Examples of electronic EAPs include ferroelectric polymers, 18 dielectric elastomers (DEs), 19 and liquid crystal elastomers. 20 Among these, DEs have become an important class of soft actuator materials as they can exhibit actuation strains >1000%, 21 energy densities exceeding 3 J/g, 22 response speeds <1 ms, 23 and a theoretical efficiency up to 90%. 24 These exceptional properties make dielectric elastomer actuators (DEAs) an ideal candidate for artificial muscles and open the door to novel approaches in soft robotics.
This Review discusses core advancements in DEbased artificial muscles for soft robotics and focuses on key innovations from a material perspective as well as representative designs for soft robots. We organize this Review as follows. Section 2 first introduces the working mechanism of DEAs, followed by a discussion on DE materials and how we modify them for improved actuator performance. Next, Section 3 discusses common compliant electrode materials with an emphasis on our use of carbon nanotubes due to their operational stability and self-healing characteristics. Finally, in Section 4, we survey multiple categories of DEA-based soft robots, including automation, manipulation, locomotion, and human interaction.

| Working principle and material background
DEs are highly deformable and incompressible elastomeric dielectric material. When coated with compliant electrodes on their opposite sides, a DE forms a variable capacitor that can convert electrostatic energy into mechanical work. Specifically, under an external electric potential, the electrostatic attraction between opposite charges on the opposing electrodes induces a stress, known as the Maxwell stress, causing the film to contract in thickness and expand in area ( Figure 1A).
In a DEA, the Maxwell stress, p, is given by where ε is the relative permittivity of the DE, ε 0 is the vacuum permittivity, and E is the applied electric field. 22,25 The DE's thickness strain s z can be approximated by where Y is its modulus of elasticity. 11,22,26 Therefore, the elastic strain energy density in a DEA, u elastic , is written as Although Equations (2) and (3) are only valid for linear DEs under small strains, they provide useful predictions about the behavior of DEs based on their dielectric and mechanical properties. For example, since the electrostatic energy density of the DE can be expressed as the ratio of the elastic strain energy density to the electrostatic energy density is εε 0 E 2 /Y, which indicates that an elastomer with a low elastic modulus can convert more input electrical energy into mechanical energy and generate a higher strain. 27,28 Therefore, DEA performance relies heavily on the selection of DE materials. In addition to a low elastic modulus, a high-performance DE requires a large dielectric constant, low viscoelasticity, and high dielectric breakdown strength. 26,28 Typical DE materials used in research are acrylic and silicone elastomers. Commercially available very high bond (VHB) adhesive tapes from 3M are the most common acrylic elastomers, thanks to their large strains and high energy densities. 11,22,26,29 However, the high viscoelastic loss of the VHB films can severely delay their response time. 30,31 On the other hand, silicone elastomers, such as Dow Corning HS3 and NuSil CF19-2186, have lower viscoelasticity and show faster responses to rapid voltage change. 32 However, they tend to have lower maximum strains compared to VHB tapes. 22,29 These conventional elastomers additionally exhibit long plateaus in their stress-strain curves and experience electromechanical instability (EMI), which reduces their performance. [33][34][35] When a DE reduces in thickness under an applied voltage, the same voltage induces a larger electric field that further thins down the elastomers, creating a positive feedback loop that persists until failure. Hence, it is critical to resolve EMI in DEs to obtain stable actuation performance.
It has been reported that prestretching DEs before activation can help suppress or remove EMI. 22,34,36,37 Pelrine et al. 22 reported significantly improved actuation responses in DEs that are highly prestretched ( Figure 1B). Actuators based on VHB 4910 with 300% biaxial prestrain showed area strains of 158%, while actuators with 15% biaxial prestrain expanded only 40% in area. Also, prestretching drastically increases the breakdown field of VHB films, from 25 to 412 V/µm. 38 However, bulky and rigid frames are required to maintain the tension on prestretched films, limiting DEAs' application in soft robotics. This is especially true when multiple prestretched films are stacked to scale up their output: the stack contains even higher tension and is more likely to experience loss of tension, fatigue, and stress concentration along the rigid frame, leading to reduced lifetime. 39 Attempts to stack unprestretched silicone films were made to enhance processability, 40,41 but they exhibit lower breakdown strength and lower maximum strains. 42 Consequently, a fabrication process to produce freestanding DEs while keeping the benefit of prestretch is highly desirable.

| Interpenetrating polymer networks (IPNs)
One method to make frame-free DE films with suppressed EMI is by synthesizing an IPN. By diffusing polymerizable and cross-linkable monomers into a prestretched DE, one can form a support structure inside the elastomer network upon curing (Figure 2A). 44 When relaxed, the highly prestretched elastomer network compresses the stiff additive network until both networks are in balance. The final free-standing film preserves a portion of the initial prestrain and exhibits EMI suppression. For example, Ha et al. 43 incorporated a poly(trimethylolpropane trimethacrylate) (TMPTMA) network into a 400% biaxially prestretched VHB film and preserved a permanent prestrain of 245%. The prestrain-locked IPN film generated a 300% maximum area strain and achieved a dielectric breakdown strength of 420 V/μm. Nevertheless, acrylic IPN films suffer from low actuation speeds due to the viscoelastic nature of acrylic elastomers. During a deformation process, polymer chains in the elastomer network slide and entangle with each other, resulting in nonnegligible viscoelastic relaxation times and slow response speeds. 45 Zhang et al. 46 proposed that adding plasticizers, such as dibutoxyethoxyethyl formal, facilitates the sliding of polymer chains by breaking intermolecular interactions, thus reducing the viscoelastic loss of the IPN film. In addition, a high plasticizer concentration lowers the elastic modulus of the elastomer, which improves the actuation strain at a given electric field and broadens the operating temperature range. The plasticized IPN film showed improved response speed at high frequencies and produced 215% area strain at −40°C.
In another effort to obtain IPN films with faster responses, Brochu et al. 47 developed all-silicone IPNs to leverage the significantly lower viscoelastic losses in silicone elastomers. 32 In the IPN, a soft room temperature vulcanizing (RTV) silicone serves as the host matrix, while a rigid high-temperature vulcanizing (HTV) silicone acts as the supporting network. Because of the difference in the curing temperature, the RTV silicone is allowed to be cured and prestretched before the HTV silicone network is formed to lock the prestrain ( Figure 3A). Compared to a pristine RTV silicone film with no prestrain, the resulting IPN film demonstrates higher maximum actuation strains and breakdown strength ( Figure 3B,C).

| Bimodal-networked DEs
Although IPNs eliminate the need for rigid supports in a DEA, they still require prestretching and relaxing of the elastomer layer during fabrication, which adds complexity and limits the manufacturability of a F I G U R E 2 (A) Synthesis of an acrylic IPN film. First, the acrylic elastomer network (black) is highly prestretched to an external support frame. Next, a multifunctional monomer additive is added to the elastomer and subsequently polymerized to form the supporting network (red). Finally, the film is allowed to relax with zero external stress, and the resultant free-standing film preserves a portion of the initial prestrain. 43 (B) VHB-poly(TMPTMA) IPN film at zero biasing voltage, and (C) at 420 V/µm with a 300% area strain. 43 Reproduced with permission: Copyright 2007, IOP Publishing. IPN, interpenetrating polymer network; TMPTMA, trimethylolpropane trimethacrylate. multilayer device. Therefore, it is preferable to have a DE material that requires no prestretch at all. However, creating a prestrain-free DE material demands a deep understanding of the role of prestrain in suppressing EMI. By quantitatively modeling the actuation mechanism of a DE, Suo 33 revealed that the strain stiffening of a prestretched elastomer is the essence of eliminating the instabilities. Accordingly, Niu et al. 48 synthesized a UV-curable DE (UV-DE) with a tuned stress-strain relationship. By varying the crosslink density, the electromechanical properties of UV-DE can be tuned to exhibit a strain-stiffening effect. UV-DE showed complete removal of EMI, producing high actuation strains >100%.
Recently, Shi et al. 49 reported a processable, highperformance dielectric elastomer (PHDE) with a tailored stress-strain behavior. PHDE has a bimodal network structure consisting of two crosslinkers of different chain lengths. A carefully adjusted amount of crosslinkers give rise to PHDE's rapid stiffening above a critical stretch ratio ( Figure 4B). Without prestretch, PHDE showed total EMI suppression and achieved a maximum strain of 189%. Additionally, acrylic acid was added to the network to form weak physical crosslinks that can dissociate during deformation, leading to higher chain mobility and lower viscoelasticity. Under a 2-Hz squarewave voltage, PHDE maintained a stable strain at 110%, significantly higher than most existing DE materials. 49 The successful synthesis of PHDE also enables the scalable fabrication of multilayer DEAs. Since the force and energy output of a single-layer DE film is too small to be useful in most real-world devices, a large number of DE layers needs to be stacked to scale up the output. Currently, most multilayer DEAs use a wet stacking technique, in which an uncured layer is formed on a cured film. [50][51][52] This is inefficient as individual steps cannot be conducted in parallel. In addition, a wet stacking process tends to produce layers with nonuniform thickness that leads to inferior actuation performance. Furthermore, wet stacking is not applicable to preformed films. Table 1 summarizes and compares multiple works on stacked DEAs. In the PHDE study, the authors developed an efficient dry stacking method applicable to all DEs regardless of the material form (resin or precured films). 49 Briefly, the dry stacking process involves the formation of individual DE layers, spray deposition of electrodes, and lamination. All steps are compatible with existing large-scale, automatic manufacturing techniques, and can be carried out in parallel to improve efficiency. More importantly, individual layers can be inspected and screened before stacking, allowing a higher actuator yield during fabrication.
It is worthwhile to note that the interlayer adhesion of dry-stacked multilayer DEs relies solely on the Van der Waals force between individual layers. Therefore, without special interlayer treatment, the adhesion may be insufficient and can cause layer delamination during cyclic actuation, reducing the actuator's performance. One solution to this problem is to deposit a thin adhesive layer on each inner DE layer before lamination. In the PHDE case, for example, the authors sprayed a thin, uncured acrylic polymer precursor on the DE before each lamination. After lamination and UV curing, these thin polymer layers act as adhesive layers, or binding layers, to improve interlayer adhesion of the stack. An SEM image of a drystacked 10-layer PHDE showed uniform layer thickness and solid lamination ( Figure 4D), indicating the effectiveness of the stacking method. Under the same driving voltage, the 10-layer PHDE achieved comparable actuation strains and frequency responses to a single-layer PHDE ( Figure 4E,F).

| Bistable electroactive polymers (BSEPs)
Although their low elastic moduli grant them high actuation strain, soft DEs cannot withstand high F I G U R E 4 (A) Photo of a 10-layer PHDE stack with 20 actuators. 49 (B) Stress-strain behavior of PHDE, unprestretched VHB 4905, and bimodal-networked acrylic elastomers with different concentrations of crosslinkers (propoxylated neopentyl glycol diacrylate (PNPDA) or 1,6-hexanediol diacrylate (HDDA)). Acrylic elastomers with no crosslinkers (PNPDA 0 and VHB 4905) exhibit long plateaus and are prone to EMI. 49 (C) Electrically induced actuation of diaphragm actuators made with PHDE, 300%-biaxially prestretched VHB 4905, and bimodal-networked acrylic elastomers with different concentration of PNPDA. 49 (D) Scanning electron microscopy (SEM) image of the cross section of a dry-stacked PHDE multilayer showing seamless lamination. 49 (E) Electrically induced actuation of diaphragm actuators made with single-layer and 10-layer PHDE. 49 (F) Frequency responses of single-layer and 10-layer PHDE actuators at 3.5 kV. 49 Reproduced with permission: Copyright 2022, The American Association for the Advancement of Science. EMI, electromechanical instability; PHDE, high-performance dielectric elastomer. mechanical loads, and their tactile applications are consequently limited. Moreover, DEAs require a continuous supply of electric fields to remain in their deformed states, leading to energy waste and reduced device lifetime. To this end, BSEPs are synthesized to incorporate shape memory properties into DEs, allowing rigidto-rigid bistable actuation. 54 An overview of the working mechanism of BSEPs is shown in Figure 5. Under room temperature, BSEPs behave like rigid plastics. However, when heated above their transition temperature (T t ), BSEPs become soft and can be actuated similarly to regular DEs. If a BSEP is cooled below T t while being deformed, it can retain its actuated shape after removing the voltage bias. Finally, the actuation can be reversed by simply reheating the BSEP above T t .
Bistable actuation was first demonstrated with poly (tert-butyl acrylate) (PTBA), a semicrystalline thermoplastic. 55 PTBA is classified as a glass-transition BSEP because its rigid-to-rubbery transition stems from the glass transition of the amorphous domains ( Figure 6A). In addition, nanocrystalline domains in PTBA act as physical crosslinkers and contribute to their rubbery elasticity. 58 When heated from room temperature to 70°C, the storage modulus of PTBA reduces from 1.5 GPa to 0.42 MPa, allowing a maximum area strain of 335% at 260 V/µm. 55 However, since the nanocrystalline domains have various sizes and melting points, the modulus of PTBA drops as temperature further increases above T t ( Figure 6C, open symbols), resulting in nonuniform and highly unstable actuation. Niu et al. 56 proposed to solve the instability by introducing chemical crosslinkers in PTBA to form IPNs. The modified PTBA was able to maintain a stable modulus above its T t , though the rigid-to-rubbery transition still spans a rather broad temperature range of around 40°C ( Figure 6C, solid symbols).
Later, Ren et al. 57 synthesized a phase-changing BSEP that exhibits a much narrower transition temperature range. The polymer consists of stearyl acrylate (SA), a long-chain urethane diacrylate (UDA), and trimethylolpropane triacrylate (TMPTA). The authors selected SA as the phase-changing moiety due to its sharp transition between its molten and crystalline states. 59 The longchain UDA forms the polymer matrix with a high tensile strength and improves the toughness of the BSEP in the rubbery state. By varying the ratio between SA and UDA, the transition temperature can be tuned between 34°C and 46°C, making it more suitable for human interaction applications. Additionally, since BSEPs at their soft states behave like conventional DEs and are prone to EMI, 56,57 TMPTA was introduced as the small-molecule chemical crosslinks to enable strain stiffening in the BSEP. With an optimal SA and UDA ratio, the phase-changing BSEP produced 70% maximum area strain and achieved a maximum energy density of 0.14 J/g.

| COMPLIANT ELECTRODE MATERIALS
Compliant electrodes are another crucial element in a high-performance DEA. Electrodes must be stretchable to avoid constraining the elastomer film and maintain sufficient conductivity under millions of deformation cycles ranging from 10% to more than 200%. 26,60 They must also be patternable and relatively thin to provide processability in practical DEA devices with multilayer configurations. In addition, the adhesion between the electrodes and the DEs needs to be strong to prevent interlayer delamination in multilayer DEAs. Figure 7 shows four commonly used compliant electrode materials which are further described below.

| Carbon grease and carbon powders
Although thin metallic electrodes are very conductive, they are not compliant enough and tend to lose conductivity under low strains. 64 Carbon-based electrode materials such as carbon grease and carbon powders are used in most DEA studies. 22

| Carbon nanotubes
On the other hand, percolation networks of single-walled carbon nanotubes (SWNTs) offer superior mechanical compliance, thermal stability, electrical conductivity, and optical transparency. [69][70][71] Typical diameters of SWNTs are on the order of 1 nm, whereas AgNWs have diameters in the range of 40-100 nm. 72,73 Therefore, with the same optical transmittance, SWNT networks have more coverage over the DE surface than AgNWs, resulting in higher actuation strains. 74 In a study by Shian et al., 74 SWNT electrodes achieve 224% maximum area strain with 72% optical transmittance, while optimized AgNWs produce a lower area strain of 156% with a transmittance of 38%. Additionally, SWNT electrodes exhibit remarkable "self-clearing" characteristics that are absent in other compliant electrode materials. The term "self-clearing," also called "selfhealing," was originally described in film capacitors with metal electrodes thinner than 100 nm. 75 Due to defects in the polymer dielectric, such as foreign particles, nonuniform thickness, and microcracks, localized breakdowns can occur when a high voltage is applied, leading to a sharp increase in temperature near the defect sites. Consequently, the dielectric is burnt until punctured around the breakdown sites, and the surrounding electrodes are rapidly vaporized, preventing catastrophic failure of the dielectric film ( Figure 8A). Such a selfclearing process is critical for the long-term operational stability of DEAs that are subject to constant supplies of high voltage. By depositing 60-nm-thick SWNT electrodes on a prestretched VHB film, Yuan et al. 62 obtained a DEA with excellent fault-tolerant properties. In their experiment, a small hole was created on the SWNTcoated acrylic film with a sharp pin ( Figure 8D). When a voltage of 3 kV was applied, the SWNT electrodes started to self-clear around the hole, characterized by sparking events and numerous current spikes in the actuator during the first actuation cycle ( Figure 8E). After the self-clearing process, the actuation stabilized at 80% area strain without further sparking events. Similar phenomena were also observed in silicone DEAs with SWNT electrodes. 40,76 There still remain challenges with SWNT electrodes. In their highly porous structure, SWNTs contain sharp tips that can locally amplify the electric field during activation, resulting in corona discharging of the surrounding air and localized breakdowns near the tips. 62 Even though such breakdowns can be selfcleared, the electrodes will gradually lose conductivity after multiple self-clearing events, leading to a reduced lifetime. To address this issue, Yuan et al. 77 quenched the corona discharge by coating dielectric oil over the SWNT surface. With a dielectric oil coating on each electrode layer, a five-layer DEA achieved a stable actuation strain >100% and a lifetime of over 1000 min. Nonetheless, the oil coatings limit the processability of the overall device and may permeate into the elastomer network to cause undesirable effects. Alternatively, Peng et al. 78 proposed to shield the sharp tips in SWNTs with water-based polyurethane (WPU). After a thin WPU layer is deposited over the SWNT electrode, the WPU matrix penetrates through the porous SWNT network, forming a WPU/ SWNT interpenetrating bilayer structure ( Figure 9B). The WPU matrix covers the SWNT surface and fills the pores in the SWNT network, effectively isolating the sharp tips from the air. As a result, DEAs with the bilayer electrodes showed minimum localized breakdown events and survived more than 1000 actuation cycles.

| SOFT ROBOTICS ENABLED BY DEAS
It has been a great challenge to unleash the full potential of soft robotics and design soft actuators with diverse functions. Owing to their large strains, high energy densities, variable stiffness, and self-healing abilities, DEAs show promise as the muscle-mimicking actuation technology for multifunctional soft robots. Additionally, the soft nature of DEAs allows them to be built into various configurations to maximize their performance in diverse applications. As shown in Figure 10, typical DEA configurations include stacked actuators, 52,53 springrolled actuators, 79,80 rolled actuators, 40,81,82 and balloon actuators. 83,84 As trade-offs between displacement, force, lifetime, and device compatibility exist and vary among different configurations, deliberate designs are required to construct soft robotic systems with optimal efficiency and output. In this section, we survey representative research on DEA-based soft robots that fulfill different objectives. They include automating weight-lifting processes through safe yet powerful output, manipulating objects with subtle control, executing rapid maneuvers in dynamic environments, and providing direct interactions between humans and robots.

| Automation
Until now, most industries still employ large and rigid robots to automate the lifting and displacement of objects. Although rigid-bodied robots provide reliable and consistent automation, they pose injury risks to anyone within their range of motion. While being soft to absorb certain collision forces, human muscles can actuate to allow the lifting of relatively heavy items. The resemblance between DEAs and human muscles has inspired researchers to build soft DEA arms that can produce high mechanical power output for automation.
In the early stage of DEA development, Kovacs et al. 85 demonstrated an arm wrestling robot capable of large force output ( Figure 11A). Inspired by human arms, the authors constructed the robot with two groups of spring-rolled DEAs that form an agonist-antagonist pair, allowing reversible rotational motion ( Figure 11B). Each spring-rolled DEA was made by rolling a piece of prestretched VHB 4910 film around a coil spring. Interestingly, the authors selected the dimension of the rolls not only to produce sufficient wrestling force but to mimic the size of an average human arm. The final wrestling robot includes a total of 256 spring rolls and can produce a maximum of 200 N of unidirectional wrestling force. Later, Lu et al. 86 developed an artificial arm based on a fiber-constraint linear dielectric elastomer actuator (FCDEA) using an antagonistic arrangement. With an optimized prestretch, FCDEA can produce up to 142% unidirectional strain. By integrating the DEA into the artificial arm and incorporating an antagonistic spring, the forearm achieved a maximum rotation of 70°under an activation voltage of 6.3 kV (Figure 11C,D).
However, the wrestling robot and the artificial arm still contain stiff springs that contribute to the rigidity of the overall devices. To make softer artificial  arms, Duduta et al. 52 demonstrated fully soft artificial muscles that can raise a polymer replica of a human arm bone ( Figure 11E,F). The authors realized entirely soft artificial muscles by fabricating stacked DEAs with a strain-stiffening DE, which removes the need for prestrain and rigid frames. A stack with 1170 layers was able to lift a 1-kg mass by 8 mm, around 13% of the actuator length. Notably, the DEA artificial muscle showed a maximum energy density of 19.8 J/kg, approaching the upper limit of a natural muscle (40 J/kg). Although enhancing the load capacity of DEAs remains challenging, the above demonstrations of DEA artificial arms show the potential of realizing soft and safe yet powerful automation.

| Manipulation
Dexterous manipulation is one of the cornerstones of soft robotics. With appropriate compliant materials, soft grippers can readily conform to the uneven surface of an object, allowing efficient yet delicate grasping of multiple items with unknown geometry, moduli, and weights. Researchers have exploited the soft nature of DEAs and designed DEA-based grippers capable of dexterous manipulation.
Kofod et al. 87 were the first to demonstrate grasping with DEAs using a configuration called self-organized dielectric elastomer minimum energy structure (DEMES). A DEMES consists of a prestretched DEA, which is fixed to a flexible frame. When relaxed, the structure bends until the tension on the prestretched film and the bending moment of the frame reach equilibrium, forcing a gripper-like shape. As shown in Figure 12A, applying a biasing voltage releases some tension of the elastomer and thus opens the gripper, while removing the voltage allows the film to contract and achieve grasping. To realize more dexterous manipulation, Lau et al. incorporated a long, segmentized flexible frame to make a DEMES gripper with extended fingers. 88 When actuated in the back-to-back configuration shown in Figure 12B, two bent DEMES fingers unfold and meet each other, performing a subtle pinching action. The DEMES fingers successfully picked up an egg yolk, indicating the potential of DEAs in the dexterous handling of delicate materials. In another exploration of a DEA gripper with a different mechanism, Shian et al. 89 introduced stiff fibers to constrain the in-plane deformation of the DEA, leading to out-of-plane movement of the gripper ( Figure 12C). During actuation, the gripper performs a wrapping motion and conforms to objects with different shapes. Meanwhile, the soft gripper generates sufficient force to support the weight, allowing subsequent displacement of the items.

| Locomotion
Soft robots offer a natural advantage of compliant locomotion in confined spaces and unconstructed environments. In addition, the lightweight of the soft actuators enables a variety of efficient movements and agile maneuvers. DEAs have shown their promise to drive soft robots with a variety of movements, including walking, flying, and swimming.
In the early 2000s, Pei et al. 80 designed a bioinspired walking robot called MERbot ( Figure 13A). MERbot consists of six DEA spring rolls as its legs, each with 2-degree of freedom (2-DOF). In a regular 1-DOF springrolled DEA, a prestretched acrylic film is coated with continuous electrodes and rolled around a compression spring, resulting in a linear actuation. On the other hand, the acrylic film in a 2-DOF spring roll is patterned with electrodes that align radially on two circumferential spans upon rolling. The pattern allows additional bending actuation of the roll while retaining the linear actuation of a 1-DOF spring roll. Under a biasing voltage of 5.5 kV at 7 Hz, MERbot achieved a maximum walking speed of 13.6 cm/s, equivalent to two-thirds of its body length per second. Using a DEMES as an inchworm-like locomotive body, Cao et al. 93 reported a soft robot with a battery and control circuit packaged on board. The untethered robot has two paper-based feet operating under an electrostatically driven adhesion/detachment mechanism. Later, Ji et al. 90 demonstrated another untethered soft robotic insect ( Figure 13B). Driven by a multilayer of ultrathin DE films, the robotic insect can achieve speeds up to 30 mm/s at 450 V, and can carry a weight of up to five times its body weight. Furthermore, the robotic insect also demonstrates decent impact resistance. After being completely flattened by a fly swatter, the robot can be peeled from the surface and continue to perform its moving action.
The lightweight of DEAs presents potential advantages in powering flying robots. In an effort to pursue aerial robots, Zhao et al. 91 fabricated a flapping wing system by joining three DEMES, a configuration introduced in the previous subsection. As shown in Figure 13B,C, the rotary joint can perform a wing-flapping motion upon activation, achieving a joint rotation angle over 180°at its resonant Intriguingly, DEs have mass densities close to the water and exhibit little to no water absorption when immersed. 94 This makes DEA-powered biomimetic swimming robots very attractive. Inspired by snailfish, Li et al. 92 designed a self-powered DEA robotic fish for deep sea exploration ( Figure 13F). Due to the inherent resilience of silicone actuators and the careful design of the robotic body, the robot can withstand hydrostatic pressure of 110 MPa and swim freely in the Mariana Trench, the deepest point on earth.

| Human-robot interaction
Providing safe interactions with human bodies is the most important benefit that soft robotics can bring. Breakthroughs in tactile interactive soft robots begin to bridge the gap between humans and robots, augmenting the information people can receive from machines. The following subsections discuss representative work in DEA-based Braille displays and haptic feedback devices.

| Refreshable Braille displays (RBD)
Tactile communication is vital for people with vision impairment. The development of RBD provides blind people with an opportunity for active learning and interaction with the written world. Light-weighted BSEPs, with their rigid-to-rigid transition, show promise to replace the bulky piezoelectric actuators in current Braille devices, allowing multiple lines of Braille cells to be packaged into a compact, portable, and affordable device. By forming a layer of PTBA-IPN as the Braille surface, Niu et al. 56 demonstrated an RBD with an array of 4 × 10 Braille cells ( Figure 14A). The refreshable BSEP surface meets the Braille standard with sufficient stability and provides enough force to support the interaction with human fingers. However, the bistable actuation of this RBD requires a high-voltage input and an external heating source, adding safety risks and bulkiness to the overall device. To this end, Qiu et al. 95 developed a BSEPbased RBD using a pneumatic actuation mechanism. As shown in Figure 14B, the BSEP layer is sealed onto a pneumatic chamber and is coated with serpentine SWNT electrodes to allow phase transition of the polymer through Joule heating. Subsequently, the authors can pump the chamber with air and actuate the softened BSEP, lifting the Braille pins above the BSEP. This RBD reduces the driving voltage to 30 V and removes the need for an external heating source, yet still achieves a high actuation and sufficient force support to meet the Braille standard.

| Haptic feedback devices
Haptic feedback devices can simulate or enhance the feeling of touch in environments that a person's senses cannot reach, such as in prosthetics and virtual environments. DEAs' large strain and fast responses, as well as their mechanical compliance with human skin, make them an appealing choice for these devices. In 2008, Koo et al. 51 designed a millimeter-scale wearable DEA tactile display array ( Figure 15A). This haptic device exploits the fast responses of silicone elastomers, achieving addressable vibrotactile feedback at a maximum frequency of 100 Hz in a 4 × 5 array. However, as DEAs require a high-voltage input to produce sensible displacement and force, the control circuit tends to be bulky, and it is challenging to package it with the haptic array. To solve this issue, Marette et al. 96 integrated flexible thin-film transistors with a 4 × 4 DEA matrix to enable individual high-voltage switching at only 30 V of gate voltage ( Figure 15B). Alternatively, Ji et al. 97 fabricated a wearable haptic device using an 18-µmthick multilayer DEA, allowing actuation at a voltage lower than 450 V ( Figure 15C). The haptic device generates sufficient force and displacement to exceed the tactile perception of the index finger, providing rich tactile feedback with a broad bandwidth from 1 to 500 Hz.
Although the above fingertip haptic devices seem promising, they can be bothersome when users need to perform important tasks with their hands, such as manipulating objects in the real environment. Zhao et al. 98 developed an arm-based DEA haptic communicator that can free the hands. As human arms are less sensitive to touch, 99 arm-based haptic devices require higher force and displacement than fingertip or handbased haptics. Considering the high force output of rolled DEAs, the authors incorporated four DEA rolls as the actuators in their haptic device, which produced a maximum block force of 0.6 N and displacement of 300 µm. 98 In addition, they conducted psychophysical testing and verified human perception of the generated force with a moderate frequency range from 10 to 200 Hz.

| CHALLENGES AND PROSPECTS
DEA artificial muscles exhibit a unique combination of lightweight, large strain, high energy density, fast response, and exceptional mechanical compliance, making them an extremely favorable material for soft robotics. Carefully engineered DEAs can also manifest adaptable stiffness, enabling more efficient robotic designs and biocompatible human-machine interaction. In addition, the successful synthesis of new DE materials with suppressed EMI provides both high performance and processability, paving the way for the commercialization of multilayer DEA devices. Furthermore, with SWNTs as compliant electrodes, DEAs can exhibit self-healing capabilities, allowing stable and long-lasting device operations.
Nonetheless, many challenges still exist in DEAs and hinder their further applications in soft robotics. In current DEA studies, acrylic DEs are implemented in applications that require large displacement, 85 whereas silicone-based DEs are adopted in devices that demand broader operation bandwidth. 82 However, it remains challenging to design a DE material that can maintain large and stable strains (>100%) at high operation frequencies (on the order of 100 Hz). Therefore, the successful synthesis of DE materials with both high strains and broad bandwidth can significantly expand the scope of DEA applications.
The commercialization of DEAs requires not only high actuation performance but a stable operation for a long period of time. While DEAs have shown maximum actuation performance comparable to or even exceeding that of natural muscles, 11,26 such results are typically obtained at electric fields close to their dielectric breakdown strength, and continuous operation tends to cause catastrophic failure of the DEA. Although researchers have conducted lifetime studies on DEAs and identified multiple external factors and intrinsic material flaws as the cause of premature failure, there are always trade-offs between the actuation performance, the processability of the device, and the durability of the actuators. 39,[100][101][102][103] Another limitation of DEAs is the safety issue resulting from the high-voltage input. Although DEAs require a rather low operation current far below the human safety threshold, the use of high voltage can pose the risk of electrical discharge around the actuators, limiting their wearable applications. 104 Proper encapsulations of the DEA device can alleviate this problem, but it risks diminishing the performance and lifetime of the actuators. 100 It is more promising to mitigate the high-voltage issue by fabricating ultrathin DE films <10 µm so that a high electric field can be obtained at a lower biasing voltage. 105 Nevertheless, manufacturing very thin DEs remains remarkably challenging, and potential defects in the thin elastomer films become more likely to cause failure compared to their thick counterparts. 106 An alternative solution is to increase the dielectric constant of the DE material by incorporating organic dipole molecules, adding high permittivity ceramic fillers, or introducing conductive nanofillers. [107][108][109][110] Since the Maxwell stress is proportional to the dielectric constant but quadratic to the electric field, reducing the field by a factor of 3 requires the dielectric constant to be increased by ninefold, which is challenging without incurring high leakage current.
Despite the drawbacks and challenges of DEAs, researchers have continuously improved the properties and performance of DEAs ever since their debut in the late 1990s. The resemblance between DEAs and natural muscles has thus far shown notable advantages of DEAs over traditional rigid actuators in modern robotics. To truly revolutionize the field of soft robotics and bring DEAs into our daily life, further improvement in the reliability and application designs of DEAs remains critical.

CONFLICTS OF INTEREST STATEMENT
The authors declare no conflicts of interest.