Autonomously self-healing dielectric elastomer actuators from thermoplastic polydimethylsiloxane elastomer

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.

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.

Dielectric elastomer actuators (DEAs) can generate a large strain, but they are often short of strength and stress change for robotic applications. Their maximum actuation is limited by pre-mature electrical breakdown. This paper showed that dielectric oil encapsulation helps stop pre-mature breakdowns. Consequently, the oil encapsulated DEAs can sustain an ultra-high electrical breakdown field up to 835MV/m, which is 46% higher than 570MV/m in air without oil encapsulation. These oil-encapsulated DEA can produce muscle-like high stress change up to 1.05MPa, which is 75% higher than that produced by the DEA in air. Furthermore, oil encapsulation is also found effective to suppress electrical breakdown of tubular DEAs. These high-stress dielectric elastomer actuators show good potential to drive human-inspired robots.

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 elastomers are novel soft smart material that could deform sustainably when subjected to external electric field. That makes dielectric elastomers promising materials for actuators. In this paper, a spring-roll actuator that would bend when a high voltage is applied was fabricated based on dielectric elastomer. Using such actuators as active parts, the flexible grippers and inchworm-inspired crawling robots were manufactured, which demonstrated some examples of applications in soft robotics. To guide the parameters design of dielectric elastomer based spring-roll bending actuators, the theoretical model of such actuators was established based on thermodynamic theories. The initial deformation and electrical induced bending angle of actuators were formulated. The failure of actuators was also analyzed considering some typical failure modes like electromechanical instability, electrical breakdown, loss of tension and maximum tolerant stretch. Thus the allowable region of actuators was determined. Then the bending angle-voltage relations and failure voltages of actuators with different parameters, including stretches of the dielectric elastomer film, number of active layers, and dimensions of spring, were investigated. The influences of each parameter on the actuator performances were discussed, providing meaningful guidance to the optical design of the spring-roll bending actuators.

Dielectric elastomers (DEs) are the polymers capable of inducing deformation under electrical stimulation. When subject to a voltage across its thickness, the material reduces in thickness and expands in area. This paper presents a new method to analyze deformation and stress distribution response of the dielectric elastomer actuator (DEA) model under different applied voltage. An equal-biaxial pre-strained circular actuator model was built. The Yeoh strain energy potential and the collocation method are used for describing the large strain actuation response and stress distribution. The study in this paper has shown that: the stress and the stretch distributions in the passive region of the DE actuator depend on the radial distance from the center at the calculation point of the passive region and the magnitude of the applied voltage; with the same excitation applied voltage, we can get a larger deformation actuation by choosing an appropriate pre-stretch ratio; the influence of the non-ideal material has seriously affected the actual deformation of the DE actuators. This analytical model has a reference potential for the design optimization of high performance DEA systems and the model-based control of the DEA robot.

Currently used dielectric elastomers do not have the ability to self-heal after detrimental events such as tearing or electrical breakdown, which are critical issues in relation to product reliability and lifetime. In this paper, we present a self-healing dielectric elastomer that additionally possesses high dielectric permittivity and consists of an interpenetrating polymer network of silicone elastomer and ionic silicone species that are cross-linked through proton exchange between amines and acids. The ionically cross-linked silicone provides self-healing properties after electrical breakdown or cuts made directly to the material due to the reassembly of the ionic bonds that are broken during damage. The dielectric elastomers presented in this paper pave the way to increased lifetimes and the ability of dielectric elastomers to survive millions of cycles in high-voltage conditions.

Dielectric elastomer (DE) is widely used in various fields because of its advantages of large deformation, lightweight, and good flexibility. In this article, based on our previous research work, the actuation performance of the cone dielectric elastomer actuator (DEA) is studied first, and the influence of loading voltage, domain ratio, and additional load on the displacement and output force of DEA is analyzed and discussed. Then, a three-dimensional model of displacement and force of DEA is established. At last, the relationship between the structure and the performance of DE is discussed from the microscopic point of view. The results show that the output displacement and force of the cone DEA increase with the increase of loading voltage and additional load. The three-dimensional model graph of cone DEA can reflect the performance change of DEA well. The design with different domain ratios is the key factor that influences the final DEA actuator, which has a great influence on the cross-link density and chain length in the DE structure. By making clear the regulating function of external environment factors, we can design DEA with different configurations, which lays a good foundation for the further development of DEA and enlarges the potential application scope of DEA.

Soft actuation technology has attracted considerable interest in recent decades, with the light weight, high response speed, and large deformation of dielectric elastomer actuators (DEAs) showing particular promise. DEAs composed of compliant frames and prestrained dielectric elastomers (DEs) can be considered as a soft actuation system, which is termed minimum-energy structure. Most existing DEAs come from a well-known design with a simple configuration but complex design process. In this article, we propose an integrated design method for the actuation and mechanism of DEs based on topology optimization using fat Bezier curves. In this method, the compliant frame is represented by fat Bezier curves, and the prestrained DE is distributed in the region bounded by the curves. This article first describes the theoretical modeling process, and then presents three design examples that validate the optimization algorithm. Finite element analysis (FEA) and experiments are conducted to assess the performance of the optimized configurations. The FEA simulations and experimental results show that the topology optimization algorithm results in DEAs that exhibit the desired motion.

The development of accurate sensing strategies for soft robotic systems represents an ongoing challenge. In general, it is difficult to integrate a sensory system that preserves the robot's compliance while simultaneously providing accurate measurements. Dielectric elastomer (DE) transducers offer a potential solution to this problem owing to their ability to function as soft actuators and sensors at the same time, thus enabling the realization of self-sensing soft robotic systems. In this work, we propose a bi-modal sensory scheme for a DE-based soft robot, which allows simultaneous estimation of the robot's own configuration and detection of external contacts without relying on any external sensors. Self-sensing proprioception and contact detection are performed at the same time while actuating the robot, based on online processing of electrical signals measured directly at the actuators' level, i.e., DEs' voltage and current. Both estimation tasks are achieved in real time by means of artificial neural networks. After describing the system and the bi-modal self-sensing architecture, the approach is validated experimentally on a DE soft robot.

The availability of compliant actuators is essential for the development of soft robotic systems. Dielectric elastomers (DEs) represent a class of smart actuators which has gained a significant popularity in soft robotics, due to their unique mix of large deformation (>100%), lightweight, fast response, and low cost. A DE consists of a thin elastomer membrane coated with flexible electrodes on both sides. When a high voltage is applied to the electrodes, the membrane undergoes a controllable mechanical deformation. In order to produce a significant actuation stroke, a DE membrane must be coupled with a mechanical biasing system. Commonly used spring-like bias elements, however, are generally made of rigid materials such as steel, and thus they do not meet the compliance requirements of soft robotic applications. To overcome this issue, in this paper we propose a novel type of compliant mechanism as biasing elements for DE actuators, namely a three-dimensional polymeric dome. When properly designed, such types of mechanisms exhibit a region of negative stiffness in their force-displacement behavior. This feature, in combination with the intrinsic softness of the polymeric material, ensures large actuation strokes as well as compliance compatibility with soft robots. After presenting the novel biasing concept, the overall soft actuator design, manufacturing, and assembly are discussed. Finally, experimental characterization is conducted, and the suitability for soft robotic applications is assessed.

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.

Biomimetic, entirely soft robots with animal-like behavior and integrated artificial nervous systems will open up totally new perspectives and applications. However, until now, most presented studies on soft robots were limited to only partly soft designs, since all solutions at least needed conventional, stiff electronics to sense, process signals and activate actuators. We present a novel approach for a set up and the experimental validation of an artificial pace maker that is able to drive basic robotic structures and act as artificial central pattern generator. The structure is based on multi-functional dielectric elastomers (DEs). DE actuators, DE switches and DE resistors are combined to create complex DE oscillators (DEOs). Supplied with only one external DC voltage, the DEO autonomously generates oscillating signals that can be used to clock a robotic structure, control the cyclic motion of artificial muscles in bionic robots or make a whole robotic structure move. We present the basic functionality, derive a mathematical model for predicting the generated signal waveform and verify the model experimentally.

Soft robotics, a research field wherein robots are fabricatedfromcompliant materials, has sparked widespread research interest becauseof its potential applications in a variety of scenarios. In soft robots,luminescence is an important functionality for communication and informationtransmission, and it is typically achieved through electroluminescence,which relies on synthetic substances activated by external electricsources, 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–luciferasereaction, is highly energy-efficient, nontoxic, and eco-friendly.In this regard, a mammalian cell-derived secreted luciferase bioluminescentliquid 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 tosoft robotics, it was incorporated as an electrode in electricallydriven soft actuators, sensors, and robots. Specifically, dielectricelastomer 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 448.1Ω·cm. When the DES was subjected to uniaxial strain, itexhibited a linear response and large deformation of up to 200% strain,with a simultaneous luminance change of 27%. The DEA displayed anareal strain of 46.0% and a luminance change of 31% at an appliedvoltage of 3.4 kV. The waterproof bending DEA generated a tip angleof 21.8° at 10 kV and was applied to a jellyfish robot that couldswim in water at a speed of 2.1 mm/s. The experimental results demonstratedthe successful operation of these devices, validating the conceptof energy-efficient, safe, and environmentally friendly bioluminescentsoft robots.

Metallic thin films have not often been used as electrodes in dielectric elastomer actuators (DEAs) as the reported actuated strains have been small. This is especially so when compared to commonly used conductive greases and powders. Here, the use of thin silver films formed by electroless deposition (ELD silver) as electrodes in DEAs is studied. As electroless deposition involves only the use of chemicals, expensive equipment is not needed. That, coupled with the fact that the thin silver electrodes require only a small amount of silver per unit area, means that such electrodes are simple and inexpensive to fabricate. In addition, unlike conductive powders and greases, these silver films adhere well to most substrates that are or have been made hydrophilic. This is especially useful in maintaining structural integrity of the actuator, such as when DEA units need to be stacked up one on top of each other. Most importantly, thin silver film electrodes have the ability to self heal. Self-healing not only averts actuator failure brought about by localised breakdowns, it also enables actuation to resume, even allowing higher driving voltages to be reached. In this paper, we demonstrate that DEAs with corrugated ELD silver electrodes can allow actuated area strains of up to 125% at a relatively low driving voltage of 1.9 kV. This is due to the low stiffening effect that the corrugated ELD silver electrodes have on the dielectric layer, which was found to be close to that of graphite.