High-stress dielectric elastomer actuators with oil encapsulation

Dielectric elastomer actuators (DEAs) are usually operated at high voltage to induce sufficient electric pressure between two compliant electrodes sandwiching the dielectric elastomer. However, a harsh environment (e.g. humid environment combined with high voltage) often induces electrical breakdown (EB) of the DEAs, which results in pinhole formation or even tearing of the device, followed by macroscopic failure. Therefore, it is ideal for DEAs to be self-healing to extend robustness and lifetime, such as observed for biological muscles, which can be healed from injuries by inherent biological processes. Herein, we prepared a soft (Young’s modulus: 187 kPa) polydimethylsiloxane (PDMS) thermoplastic elastomer (TPE) to demonstrate an autonomous self-healing ability. The system exploits hydrogen bonding (H-bonding) in two types of transient cross-linkers: urea group serves as a sacrificial bond under loading and ureidopyrimidone (UPy) serves as a strong, load-carrying crosslinker. The PDMS TPE shows a crossover of the elastic moduli at 119 °C and highly frequency dependent elastic modulus. The DEA is prepared with compliant, corrugated silver electrodes on both sides of a corrugated PDMS TPE film. A maximum actuation strain (6.8 % in longitudinal direction) is achieved at 18.8 V μm-1. A further increase in potential does not increase the actuation strain, but EBs are observed on the electrodes and the actuation is maintained without any detrimental effects to the film despite the previous breakdown. The EBs do not form permanent pinholes, nor do they cause tearing of films. Instead, only the removal of the silver electrode is observed, which is the so-called self-clearing effect commonly observed for metallic electrodes. In combination with the self-clearing effect, the heat generated by the EBs allows the PDMS TPE to soften. The polymer molecules are then capable of flowing into the voids that were created at the initiation of the EBs. As a result, further propagation of the EB is hindered, and instead, an instantaneous and autonomous self-healing of the DEA is observed.

As a widely used configuration for dielectric elastomer (DE) actuators, DE tube actuators (or cylindrical actuators) are also found to be susceptible to electromechanical instability (EMI), which may lead to a premature electrical breakdown (EB), and inhibit the potential actuation of DE actuators. This work investigates the electromechanical response of a DE tube actuator with and without boundary constraints to demonstrate an alternative to avoid EMI while achieving large actuation. Our simulation results based on the Gent strain energy model show that the EMI of a DE tube actuator can be eliminated, and larger actuation deformation can be achieved by applying boundary constraints. As a result of these constraints, consideration is also given to the possible mechanical buckling failure that may occur. Mechanisms of possible failure modes of constrained and unconstrained DE tube actuators, such as electromechanical instability, electrical breakdown and mechanical buckling, are elucidated. This paper should provide better theoretical guidance on how to improve the actuation performance of DE actuators, thus leading to the optimal design of DE-based devices.

Dielectric elastomer actuators (DEA) have gained increased attention in the design of electrically driven soft and lightweight robotic devices. They rely on Coulomb forces for their actuation and are able to undergo large displacements as a function of the applied electrical field. Fabricating DEAs with additive manufacturing opens the way to a personalized and cost‐efficient fabrication of soft active devices. In particular, extrusion‐based techniques provide a versatile strategy to multi‐material 3D print DEAs by using electrically conductive and soft filaments with high elongation. One key aspect of DEAs is the electrode's performance, which has to provide a high electrical conductivity and a good adhesion to the sandwiched membrane. Herein, selected 3D printing parameters were varied to optimize the electrode infill density and electrode infill direction. Mechanical and electrical properties, as well as actuation performance, of fully 3D‐printed DEA demonstrators were studied comprehensively. With the optimized parameter set, a maximum DEA displacement of 91% was achieved with respect to the free length of the actuator. This is comparable to the performance of non‐3D‐printed DEAs and demonstrates the potential of 3D printing for the production of DEAs undergoing large deformations while leveraging the design and manufacturing freedom of multi‐material 3D printing.

Dielectric elastomer actuators (DEA) are a class of eletro-active polymers with promising properties for a number of applications, however, such actuators are prone to failure. One of the leading failure mechanisms is the electrical breakdown. It is already well-known that the electro-mechanical actuation properties of DEA are strongly influenced by the mechanical properties of the elastomer and compliant electrodes. It was recently suggested that also the electrical breakdown in such soft materials is influenced by the mechanical properties of the elastomer. Here, we present stress-strain measurements obtained on two tri-block thermoplastic elastomers (SEBS 500040 and SEBS 500120, poly-styrene-ethylene-butadiene-styrene), with resulting large differences in mechanical properties, and compare them to measurements on the commonly used VHB 4910. Materials were prepared by either direct heat-pressing of the raw material, or by dissolving in toluene, centrifuging and drop-casting. Experiments showed that materials prepared with identical processing steps showed a difference in stiffness of about 20%, where centrifuged and drop-casted films were seen to be softer than heat-pressed films. Electric breakdown measurements showed that for identically processed materials, the stiffness seemed to be a strong indicator of the electrical breakdown strength. It was therefore found that processing leads to differences in both stiffness and electrical breakdown strength. However, unexpectedly, the softer drop-cast films had a much higher breakdown strength than the heatpressed films. We attribute this effect to impurities still present in the heat-pressed films, since these were not purified by centrifuging.

Dielectric elastomer actuators have been widely studied theoretically and experimentally thanks to its superior attributes such as large deformation. The actuator failure has long been an issue placing barriers to large deformation in practical applications. In this paper, we investigate the various failure modes and the maximum stretch of the circular dielectric elastomer actuator. An analytical model depicting the actuation mechanism of the dielectric elastomer actuator is proposed to help analyze the failure onset. Several types of normally observed failure modes, i.e. electromechanical instability, loss of tension, and electrical breakdown, are analyzed theoretically based on the proposed model. A 3D figure that interprets the relationship of the corresponding factors and the possible failure modes are proposed as well, by which we could know the possible failure onset, as well as the associated maximum stretch under specific conditions. Experiments are carried out and the results are in accordance with the prediction by the proposed 3D figure.

Azobenzene‐functionalized liquid crystalline polymer networks (azo‐LCNs) are promising candidates for light‐fueled contactless manipulation of miniaturized soft robots through embedding photoactive molecular switches into alignment‐programmable LCNs. In particular, the 3D helical geometry of azo‐LCNs is reported to achieve rapid photomotility by introducing rolling resistance. However, the maximum height of the obstacle that soft robot can overcome is limited by the helix diameter and the stress–strain responsivity. Herein, the helical diameter per unit length and photogenerated stress through molecular engineering of photoactive molecular switches are maximized. The carbon number of aliphatic spacers in the photoactive molecular switches is varied from two to eight to systematically investigate the structure–property–performance relations by studying the molecular geometry, physical properties of polymers, and photomotility of polymers. Furthermore, a finite‐element analysis simulation is presented to understand the rolling locomotion of helical torsional soft robots. Through molecular engineering, the helix diameter per unit length of 0.2 mg soft robots is maximized, demonstrating high Young's modulus (≈2 GPa) and photogenerated stress (>1 MPa), as well as large velocity per body length, compared with the previously reported soft robots. Finally, the molecularly engineered soft robots successfully climb stairs, which is a key task in robotic systems.

Optical imaging systems using varifocal lenses have been widely used in many applications over the past several decades, such as machine vision devices, consumer electronic products, and medical instruments. Traditional varifocal lenses often consist of multiple solid focal length refractive optical elements. The varifocal ability is obtained by dislocating these optical elements along the optical axis over specific distances using mechanical driving mechanisms. It makes the traditional optical varifocal imaging systems suffer from bulky dimensions, slow response speed, complicated configuration, and discrete magnifications. Adaptive varifocal lenses have been a better choice to address the aforementioned limitations of traditional varifocal lenses. Dielectric elastomer actuators (DEA), which can effectively respond to an electric field and result in shape deformation, have been used to develop various adaptive lenses. This paper aims to give a brief review of adaptive varifocal lenses based on DEA. First, this paper describes the basic physical mechanism of DEA. Second, this paper reviews adaptive varifocal liquid lenses based on DEA and introduces their material, structure, and fabrication process, focusing on their unique advantages, such as fast response speed and compactness. However, despite these merits, the adaptive varifocal liquid lens still has challenges in environment stability and liquid leakage. To address these challenges, adaptive varifocal soft solid lenses based on DEA have been proposed, which are also reviewed. In addition, other adaptive varifocal lenses, including metalens, Fresnel lens, microlens array, and Alvarez lens, are also presented. Finally, the prospects and challenges for the development of adaptive varifocal lenses based on DEA are discussed.

Conductive grease and powder are commonly applied as compliant electrodes for dielectric elastomer actuators (DEAs). Unfortunately, they can be rubbed off easily and DEAs based on them cannot self-heal from localised electrical breakdowns. Metallic thin films are cleaner and more resilient alternatives for electrodes. They are currently widely used in metalized plastic capacitors, which are known for their self-healing capability. However, they are not widely used in DEAs due to limitations in strain. In this paper, we demonstrate that a metalized DEA is capable of areal strains of up to 21%. The inexpensive and simple method of electroless silver deposition had been used to create the electrodes for the single-layer DEA. The lightly pre-stretched 80μm thick dielectric film demonstrated a 21% areal strain, which is a 17% reduction in thickness, with an applied voltage of 2.5kV. Self-healing properties of the silver electrodes have also been observed. Localised breakdowns of the dielectric film self-healed, thereby averting electrical breakdown and allowing actuation to continue, even at higher applied voltages. With higher breakdown voltages, larger breakdown fields were obtained, which would in turn lead to greater electrostatic forces. Relatively high breakdown fields of up to 75 MV/m were obtained. This is in contrast to the 35 MV/m obtained by silver grease under the same conditions. In mechanical strain tests, the silver films remained conductive while subjected to a uni-axial mechanical strain of up to 50%, which ascertains the ability of such electrodes to sustain high strains.

Robots have been around us for quite some time now. Robots are machines capable of performing tasks on their own with high accuracy and precision and minimal supervision to reduce human effort. Our daily lives are touched by different forms of robots around us, be it a direct interaction in form of robotic helpers at the airport or indirectly in the case of industrial arm robots assembling the electronics and machines. Traditionally, there are several component systems to robotics and one of the prominent one is actuation, which is made from hard and rigid components in these conventional robots. However, this is responsible for their intermittent motion. A new field of research, soft robotics, makes of soft materials for their fabrication and draws inspiration from natural organisms and how they produce fluidic and complex motions. There are different ways by which these soft materials can be actuated and include techniques like applying pneumatic and hydraulic pressure, applying heat, making use of a chemical process like combustion, applying varying magnetic fields and applying electric fields. Among these, the class of actuators employing soft materials deformed by applying the electric field, known as Dielectric elastomer actuators, are preferred owing to their advantages of simple fabrication, large actuation strain and control by electrical stimulus. The architecture of these actuators resembles a variable capacitor with electrodes on either sides of the elastomer, and their performance depends on material properties like dielectric constant and mechanical stiffness. Generally, researchers have tried to modify either of these properties with addition of solid conductive or ceramic fillers, or chemical modification of the elastomer, and this has led to undesirable changes on the other material properties. Drawing inspiration from biological materials like skin, which is a soft material system filled with fluids, a novel composite is synthesized utilizing a solid polymer matrix and a high dielectric constant liquid filler, an ionic liquid. An increase in dielectric constant is observed accompanied with significant reduction in mechanical stiffness. The novel composites show significant improvement upon the actuation performance of the commonly used elastomeric system at considerably low electric fields. Owing to the judicious choice of material systems, the composites also show high stability and transparency. With emergence of new generation technologies such as display touchscreens, touchpads, virtual reality and augmented reality, human machine interaction and haptics has become ever more important. Soft actuators and dielectric elastomer actuators (in particular), owing to their inherent softness, have convincing advantage over the conventional rigid actuators. Additionally, state-of-the-art technologies for haptics primarily seek to simulate the perception of texture change with sensory manipulation. Furthermore, dielectric elastomer actuators in their current form suffer from visible obstructiveness and non-integrable architecture. A new device architecture for dielectric elastomer actuators, capable of producing on-demand surface texture change and local topographic features, is demonstrated. Owing to its unique architecture, a highly transparent, flexible and integrable configuration of the device is showed. Out-of-plane deformations and force feedback from the devices are shown to be well above the perceptual threshold of sensors, known as mechanoreceptors, present in our fingertips. Another attribute of the biological appendages in natural organisms is the ability to reversibly modulate its mechanical properties upon demand, which still eludes the field of soft robotics. One of the best examples of this is found in human body, where muscles modulate their stiffness in accordance to the load distribution and movement. Most of the current approaches mange to only demonstrate reversible stiffness and involves combination of a passive reversible stiffness component with the existing actuator. Hence, a novel low-temperature phase change material is identified for enabling reversible rigidity in the soft actuators owing to its low melting point, significant gap between melting and solidification, superior thermal stability, high mechanical strength and high dielectric constant. Thermal control is chosen the modulation of mechanical properties, owing to its lightweight design and scalability. The fabricated reversible rigidity composites show impressive variations in stretchability, bendability and hardness. Owing to its ability to melt and re-solidify, the composites also demonstrate a healing behaviour with the ability to regain complete structural integrity and mechanical strength. These novel ultra-soft, high-k liquid filler-based polymer composites, reversible rigidity composites and transparent and integrable device architectures for field-driven soft actuators could pave the way for next generation applications such as shape morphing, adaptive surfaces and transparent haptic devices.

Elastomers with high dielectric permittivity that self‐heal after electric breakdown and mechanical damage are important in the emerging field of artificial muscles. Here, a one‐step process toward self‐healable, silicone‐based elastomers with large and tunable permittivity is reported. Anionic ring‐opening polymerization of cyanopropyl‐substituted cyclic siloxanes yields elastomers with polar side chains. The equilibrated product is composed of networks, linear chains, and cyclic compounds. The ratio between the components varies with temperature and allows realizing materials with largely different properties. The silanolate end groups remain active, which is the key to self‐healing. Elastomeric behavior is observed at room temperature, while viscous flow dominates at higher temperatures (typically 80 °C). The elasticity is essential for reversible actuation and the thermoreversible softening allows for self‐healing and recycling. The dielectric permittivity can be increased to a maximum value of 18.1 by varying the polar group content. Single‐layer actuators show 3.8% lateral actuation at 5.2 V µm–1 and self‐repair after a breakdown, while damaged ones can be recycled integrally. Stack actuators reach an actuation strain of 5.4 ± 0.2% at electric fields as low as 3.2 V µm–1 and are therefore promising for applications as artificial muscles in soft robotics.

This work investigates the reliability of dielectric elastomer actuators (DEAs) based on silicone elastomers and carbon nanotube (CNT) electrodes from two key perspectives: intrinsic dielectric breakdown and long-term reliability associated with CNT electrode stability, particularly under high-frequency actuation. Experimental results reveal that the breakdown behavior is strongly influenced by voltage polarity, waveform, frequency, and material thickness, while high-frequency bipolar voltages result in significant insulation degradation. To facilitate systematic testing, a multi-channel reliability testing platform was developed, enabling continuous monitoring of voltage, current, and breakdown events for multiple DEAs under various conditions. It is found that high-frequency mechanical vibrations can cause CNT detachment from the silicone surfaces, reducing the overlapping area of the top and bottom electrodes to nearly zero. Furthermore, it is demonstrated that actuation current plays a critical role in accelerating DEA performance decline through CNT degradation. Coating encapsulation layers and using relatively higher-areal-density CNT electrodes can substantially enhance long-term reliability. In addition, self-healing current waveforms in the milliamperes range were observed, regardless of whether the electrodes have coated layers or CNT areal densities. These insights offer fundamental guidance for the design and control of DEA systems under sustained high-frequency electrical stress.

Study on the bio-inspired electrochromic device enabled via dielectric elastomer actuator

Distinguished from traditional robots made of rigid materials, soft robots have great potential in fields such as human–robot interaction and biomedical applications due to their mechanical pliability. Soft actuators are one of the most important components in soft robotics systems and can be activated by external stimuli such as heat, light, humidity, electric fields, or magnetic fields. Among them, magnetically responsive soft actuators composed of strong magnetic composite can achieve multi‐degree‐of‐freedom wireless remote control and reversible driving action, which have considerable application potential. In this perspective, we first discuss the properties and actuation mechanisms of the strong‐magnetic flexible composites. Then, the preparation method of magnetic actuators by strong‐magnetic flexible composites are briefly introduced. Next, various applications of magnetically responsive soft actuators, such as grasping, walking, swimming, and sensing control, are highlighted. Finally, current opportunities and challenges for stimulus‐responsive soft robots are discussed. This review provides scientists with the latest technological advances and information needed to design and develop the next generation of soft bionic robotic systems, and further demonstrates the future applications of these robots are vast and promising.

Dielectric elastomer actuators have attracted a great deal of attention thanks to their remarkably large actuation strain and energy density. Several different design concepts and configurations have been fabricated for many proposed applications. However, high rates of failure and short lifetime caused by dielectric breakdown have prevented their use in commercial applications. Employing single-walled carbon nanotube electrodes of tens of nanometer thickness and a coating of dielectric oil on the electrode surface, the actuators can be operated continuously at larger than 150% area strain for longer than 1500 minutes without terminal failure. As a comparison, under the same test conditions, actuators with carbon grease electrodes can be driven for less than 60 minutes before terminal failure. It has been demonstrated that the carbon nanotube electrodes endow the actuators with the ability to self-heal following localized dielectric failure, which should make them more amenable to practical applications.

Dielectric elastomer (DE) is a soft material that can deform to a large degree under the action of an electric field. In this paper, multilayer DE films were stacked in parallel to prepare a 20-layer dielectric elastomer actuator (DEA). This DEA could provide a peak output force of 30 N, which significantly improves the driving performance of the DEA and provides conditions for large load driving of the DEA. As a new driving method, the DEA was applied to a jumping robot, and the heavy-weight robot accomplished jumping motion after several cycles of energy storage.