Dynamic pattern of wrinkles in a dielectric elastomer

Biomimetic human eyes in adaptive lenses with conductive gels

To investigate the imaging effect, adaptive robust lenses are prepared by sealing transparent liquid or gel. Lenses are fabricated using the negative-pressure method, which is a benefit for a stable biconvex shape. Under the action of an electric field, the soft lens deforms following the dielectric elastomer actuator (DEA). DE (dielectric elastomer) membranes expand in the plane perpendicular to the electric field lines. The toroidal driving area leads to a decrease in lens diameter and an increase in convex curvature. Therefore, the focal length of the lens becomes shorter. The experimental measurement utilizes the double focal length method. As a result, the largest focal length change that could be achieved was 44.7% (190 mm→105 mm) of the soft lens using a DEA with carbon grease electrodes. Furthermore, the ECG (electrocardiogram) conductive gel could replace traditional carbon grease for DEA electrodes in optics. This type of transparent electrode is creatively applied to a biomedical lens. Under the same conditions, the electrostriction rate in a DEA with ECG gel was achieved at 33%, which was greater than that of 28% in a DEA coupled with carbon grease electrode. Adaptive lenses have characteristics such as easy fabrication, low cost, and strong operability, and they possess great potential application value in biomedical feild.

Nonlinear electromechanical coupling in graded soft materials: Large deformation, instability, and electroactuation

Subject to an applied electric field, soft dielectrics with intrinsic low moduli can easily achieve a large deformation through the so-called electrostatic Maxwell stress. Meanwhile, the highly nonlinear electromechanical coupling between the mechanical and electric loads in soft dielectrics gives a variety of failure modes, especially pull-in instability. These failure modes make the application of soft dielectrics highly limited. In this paper, we investigate the large deformation, pull-in instability, and electroactuation of a graded circular dielectric plate subject to the in-plane mechanical load and the applied electric load in the thickness direction. The results obtained herein cover, as special cases, the electromechanical behaviors of homogeneous dielectrics. There is a universal physical intuition that stiffer dielectrics can sustain higher electromechanical loads for pull-in instability but achieve less deformation, and vice versa. We show this physical intuition theoretically in different homogeneous dielectrics and graded dielectrics. Interestingly, we find that the ability to sustain a high electric field or a large deformation in a stiff or soft homogeneous circular dielectric plate can be achieved by just using a graded circular dielectric plate. We only have to partly change the modulus of a circular plate, with a stiff or soft outer region. The change makes the same electromechanical behavior as that of a homogeneous dielectric, even increases the maximum electroactuation stretch from 1.26 to 1.5. This sheds light on the effects of the material inhomogeneity on the design of advanced dielectric devices including actuators and energy harvestors.

Insights into the Complex Prebreakdown Actuation of Silicone Elastomers and its Influence on Breakdown Behavior

Dielectric elastomer transducers can be applied in many different applications, but their current use is limited by either their electrical breakdown strength or by electromechanical instabilities in the case of soft elastomers. The breakdown process is never a single, simple process but rather-most likely-an ensemble of thermoelectric processes taking place in both elastomer and electrode materials, coupled with mechanical and potentially also chemical degradation. In this work, by using a high-speed camera, we follow silicone-based dielectric elastomers undergoing a ramp-up in voltage close to electrical breakdown strength, with differently constructed elastomers and electrodes. As such, we present experimental insights into the electromechanical processes immediately before the dielectric breakdown of elastomers and identify three different actuation mechanisms taking place prior to electrical breakdown, denoted prebreakdown actuation in the following. The prebreakdown actuation mechanisms observed herein include film thinning and stretching, as well as the formation of bubble- and ring-shaped structures from the elastomer surface, respectively. We furthermore present a theoretical explanation for the observed actuation mechanisms.

Maximal Performance of an Antagonistically Coupled Dielectric Elastomer Actuator System

Dielectric elastomer actuators (DEAs) have been shown to produce electrically induced strains beyond 500%. The ability to undergo large deformation allows the DEA to store large amounts of elastic energy by electrical actuation; it also allows the DEA to perform flexibly in a diverse range of motions. Existing studies used different methods to maximize actuation strain for soft robotic applications. In this article, we examine the actuation of our antagonistically coupled DEAs, reminiscent to that of human muscles. We perform an analysis to reveal optimal conditions that maximize its actuation stroke, actuation force, and output energy. We quantify actuation stroke by the displacement sweep due to electrical actuation, between two fixed points, expressed as a percentage, and refer to this as "actuation sweep." From the analysis, we predicted an optimal prestretch for the DEA that corresponds to a 59% actuation sweep. In our experiment, we realized a 55% actuation sweep. We further characterized the output force and the mechanical work done for complete performance appraisal of the antagonistic system both theoretically and experimentally. We realized an antagonistic soft actuator system with simple geometry that provides significant electrically induced displacement, force, and work done, similar to that of biological muscle systems, and demonstrated its efficacy.

Instabilities in dielectric elastomers: buckling, wrinkling, and crumpling

Instabilities in a thin sheet are ubiquitous and can be induced by various stimuli, such as a uniaxial force, liquid-vapor surface tension, etc. This paper investigates voltage-induced instabilities in a membrane of a dielectric elastomer. Instabilities including buckling, wrinkling, and crumpling are observed in the experiments. The prestretches of the dielectric elastomer are found to play a significant role in determining its instability mode. When the prestretch is small, intermediate, or large, the membrane may undergo buckling, wrinkling, or crumpling, respectively. Finite element analysis is conducted to study these instability modes, and the simulations are well consistent with the experimental observations. We hope that this investigation of mechanical and physical properties of dielectric elastomers can enhance their extensive and significant applications in soft devices and soft robots.

Electromechanical Instability of Dielectric Elastomer Actuators With Active and Inactive Electric Regions

Electrically driven dielectric elastomers (DEs) suffer from an electromechanical instability (EMI) when the applied potential difference reaches a critical value. A majority of the past investigations address the mechanics of this operational instability by restricting the kinematics to homogeneous deformations. However, a DE membrane comprising both active and inactive electric regions undergoes inhomogeneous deformation, thus necessitating the solution of a complex boundary value problem. This paper reports the numerical and experimental investigation of such DE actuators with a particular emphasis on the EMI in quasistatic mode of actuation. The numerical simulations are performed using an in-house finite element framework developed based on the field theory of deformable dielectrics. Experiments are performed on the commercially available acrylic elastomer (VHB 4910) at varying levels of prestretch and proportions of the active to inactive areas. In particular, two salient features associated with the electromechanical response are addressed: the effect of the flexible boundary constraint and the locus of the dielectric breakdown point. To highlight the influence of the flexible boundary constraint, the estimates of the threshold value of potential difference on the onset of electromechanical instability are compared with the experimental observations and with those obtained using the lumped parameter models reported previously. Additionally, a locus of localized thinning, near the boundary of the active electric region, is identified using the numerical simulations and ascertained through the experimental observations. Finally, an approach based on the Airy stress function is suggested to justify the phenomenon of localized thinning leading to the dielectric breakdown.