Large Voltage-induced Deformation Research Articles

An equivalent spring-based model to couple the motion of visco-hyperelastic dielectric elastomer with the confined compressible fluid/air mass

Soft materials exhibiting large voltage-induced deformation under external stimuli have gained increasing attention in the recent past owing to their potential applications in soft transducers. Dielectric elastomer actuators (DEAs) coupled with a fluid/air mass have proved to undergo considerable voltage-induced deformation. This article presents a framework to couple the motion of an inflated Dielectric elastomer (DE) membrane with a confined compressible fluid/air mass and develops a nonlinear dynamic analytical model to predict the polar region deflection of an inflated DE membrane attached to an airtight chamber (namely bulging actuators) stimulated by an electric field. In order to derive the differential equations which determine the dynamic behavior of the membrane coupled to an airtight chamber, the Euler–Lagrange equation is employed, which takes into account the effects of membrane pre-stretch, initial inflation pressure, membrane viscoelastic behavior, and transient electric loading. A hypothesis of pseudo-air-spring is incorporated to consider the effect of the volume of the confined fluid/air mass. The experimental investigation conducted for various geometrical and loading conditions in both depressurized and pressurized states of actuation shows that the developed nonlinear model satisfactorily predicts the influence of fluid/air mass on the dynamic response of the considered DE membrane undergoing small voltage-induced deformation. Finally, the proposed framework is incorporated for building insights on several effecting parameters such as inflation pressure, quality, and quantity of the confined fluid mass, etc., on the electrodynamic behavior of the actuator. The results reveal that the volume of the air column appreciably alters the nonlinear dynamic behavior of the bulging actuator. Poincaré plots along with phase diagrams are presented for analyzing the periodicity of the nonlinear oscillations of the actuator. The underlying theoretical framework and the obtained inferences of the present article can be implemented effectively in the design of a futuristic class of structures whose motion is coupled with compressible fluid/ air mass subjected to time-dependent actuation.

Designing Soft Mobile Machines Enabled by Dielectric Elastomer Minimum Energy Structures.

Dielectric elastomers (DE) are ideal electro-active polymers with large voltage-induced deformation for the design and realization of soft machines. Among the diversity of configurations of DE-based soft machines, dielectric elastomer minimum energy structures (DEMES) are unique due to their ease of fabrication, readiness to extend into multiple segments, and versatility of design configurations. Despite many successful demonstrations of DEMES actuators, these DEMES devices are limited to immobile use. We report several possible implementations of soft mobile machines through the combination of DEMES design, finite element simulation, and experiment. Our designs mimic the biomimetic locomotion of inchworms and marry complex components such as ratchet wheels with soft DEMES actuators. We even elucidate that buckling of DE can be harnessed to achieve asymmetric feet, which is otherwise realized via more complicated means. The examples presented here enrich DE devices’ design and provide valuable insights into the design and fabrication of soft machines that other soft-active materials enable. All the codes and files used in this paper can be downloaded from GitHub.

Electrically Adaptive and Shape-Changeable Invertible Microlens.

Existing soft actuators for adaptive microlenses suffer from high required input voltage, optical loss, liquid loss, and the need for assistant systems. In this study, we fabricate a polyvinyl chloride-based gel using a new synergistic plasticization method to achieve simultaneously a high optical transparency and an ultrasoft rubber-like elastic behavior with a large voltage-induced deformation under a weak electric field. By compressing the smooth gel between two sets of annular electrodes, a self-contained biconvex microlens is realized that is capable of considerable shape changes in the optical path. Each surface of the dual-curvature microlens can be independently adjusted to focus or scatter light to capture real or virtual images, yield variable focal lengths (+31.8 to -11.3 mm), and deform to various shapes to improve aberrations. In addition to simple fabrication, our microlens operates silently and consumes low power (0.52 mW), making it superior to existing microlenses.

Electromechanical analysis and simplified modeling of dielectric elastomer multilayer bending actuator

Dielectric elastomers can generate large voltage-induced deformation and have been widely used as soft actuators. Dielectric elastomer multilayer structures function as bending-type actuators, enhancing their stiffness and ability to resist external loads. Modeling a dielectric elastomer multilayer bending actuator allows analyzing its bending performance, and obtaining the stress and strain distributions in each layer of the actuator. It is of practical importance to develop a simple, yet accurate model. This work develops a model to analyze the electromechanical behavior of the dielectric elastomer multilayer bending actuator with various pre-stretch, material, and structural parameters. We also propose a simplified model for a bilayer slender beam, which is a typical multilayer structure. The simplified model describes the relation between the beam bending curvature and the applied voltage, achieving fast calculation with little accuracy sacrificed. Both the model for the dielectric elastomer multilayer and the simplified model for the bilayer beam agree well with the experiment results, validating the models for practical applications in soft robots, flexible sensors, and smart structures.

Automatic Design of Soft Dielectric Elastomer Actuators With Optimal Spatial Electric Fields

Dielectric elastomer actuators (DEAs) are a promising actuation technology in soft robotics owing to their large voltage-induced deformation and rapid response. However, most existing DEA design paradigms are empirical or intuitive, lacking the mathematical modeling and optimization methodology to exploit their actuation capabilities for prescribed motion tasks. In this paper, we present an automatic design methodology to maximize the concerned displacement(s) of DEAs by topology optimization of the applied spatial electric fields (SEFs). Our method is enabled by integrating the freeform SEF profile captured by implicit level sets, and the constitutive model of DEAs incorporating geometric and material nonlinearities and the electromechanical coupling effect, into a gradient-based optimizer. We implement our method for motions of single and multiple degrees of freedom (DOFs) of planar DEAs, and the optimized SEFs have been found to improve the output displacements by more than 75% compared with their intuitive counterparts. We further demonstrate a proof-of-concept application in which our designed two-DOF DEAs can actively drive various host structures to shape-morph from flat sheets to desired three dimensional configurations. Overall, our paper represents the first step toward automatic design of soft DEAs for diverse potential applications in soft machines and robots.

Bioinspired Soft Actuators for Eyeball Motions in Humanoid Robots

Technological progress in advanced materials and artificial intelligence has given an impetus to the development of humanoid robots that can mimic human appearance and interact with people effectively. Eyeball motions play a key role in facial expression of humanoid robots, and can greatly improve the relationship between the robots and the users. In this paper, we develop soft actuators which can achieve horizontal, vertical, and circular motions, for the application of robotic eyeballs. Inspired by nature, this soft actuator combines three linear dielectric elastomer actuators (DEAs). By controlling voltages applied to the three DEAs, this soft actuator is capable of realizing the horizontal, vertical, and circular motions. Each DEA is developed with optimal designs to achieve large voltage-induced deformation. The calculations on the optimal prestretches of the DEA are qualitatively consistent with the experiments. By combining two DEAs together, the soft actuator can mimic the horizontal motion of human eyeballs. By combing three DEAs together, the soft actuator can track desired motion trajectories effectively. This project can contribute to the development of humanoid robots with bioinspired design, lightweight, and low cost. The humanoid robot based on soft actuators can also improve the interaction with the users, due to its natural appearance, excellent safety and adaptability, and quiet operation.

Fabrication of Dielectric Elastomer Composites by Locking a Pre-Stretched Fibrous TPU Network in EVA.

Dielectric elastomer (DE) composites with high electrical breakdown strength and large voltage-induced deformation were developed by retaining pre-stretched thermoplastic polyurethane (TPU) fibers in ethylene vinyl acetate copolymer (EVA). The microstructure of the candidate E-TPU fiber membrane and EVA coated E-TPU (E-TPU/EVA) film were characterized by scanning electron microscopy (SEM). The quasi-static and dynamic mechanical property, and the electromechanical properties, including the dielectric constant, dielectric loss tangent, and electromechanical sensitivity, of the DE composites were evaluated. Initially, tensile tests demonstrated that the DE composites based on E-TPU/EVAs had a higher elongation at break of above 1000% but a low elastic modulus of approximately 1.7 MPa. Furthermore, dielectric spectroscopy showed that the E-TPU/EVA had a dielectric constant of 4.5 at the frequency of 1000 Hz, which was 1.2 times higher than that of pure EVA film. Finally, it was found from electromechanical test that the voltage induced strain of E-TPU/EVA rose to 6%, nearly 3 times higher than that of pure TPU film, indicating an excellent electromechanical property. The DE composites developed have demonstrated the potential to be good candidate materials in the fields of artificial intelligence, biomimicry and renewable energy.

Untethered soft robot capable of stable locomotion using soft electrostatic actuators

This paper reports an untethered soft robot using soft electrostatic actuators. The robot consists of a deformable body driven by dielectric elastomer actuators, and two paper-based feet driven by electroadhesion actuators. The use of light-weight batteries and small-volume amplifiers contributes to the development of this untethered soft robot. Inspired by inchworms, this untethered soft robot can achieve locomotion through alternate expansion/contraction of its deformable body and adhesion/detachment of its two paper-based feet. The strong electroadhesion ensures a stable locomotion, and the large voltage-induced deformation and fast response of the robotic body leads to a velocity of 0.02 body length/s. This velocity is higher than that of untethered soft crawling robots based on pneumatic actuators or ionic polymer metal composites. The deformation of the robotic body is studied through finite element analysis. Compared to traditional hard robots, soft robots are found to be more susceptible to dissipative processes, including charging/discharging the actuators, dielectric relaxation and viscoelastic effects. These dissipative effects on this untethered soft robot are also investigated in this paper.

Soft Freestanding Planar Artificial Muscle Based on Dielectric Elastomer Actuator

Dielectric elastomer actuators (DEAs) exhibit interesting muscle-like attributes including large voltage-induced deformation and high energy density, thus can function as artificial muscles for soft robots/devices. This paper focuses on soft planar DEAs, which have extensive applications such as artificial muscles for jaw movement, stretchers for cell mechanotransduction, and vibration shakers for tactile feedback, etc. Specifically, we develop a soft planar DEA, in which compression springs are employed to make the entire structure freestanding. This soft freestanding actuator can achieve both linear actuation and turning without increasing the size, weight, or structural complexity, which makes the actuator suitable for driving a soft crawling robot. Furthermore, its simple structure and homogeneous deformation allow for analytic modeling, which can be used to interpret the large voltage-induced deformation and interesting mechanics phenomenon (i.e., wrinkling and electromechanical instability). A preliminary demonstration showcases that this soft planar actuator can be employed as an artificial muscle to drive a soft crawling robot.

Interactions Between Dielectric Elastomer Actuators and Soft Bodies

Abstract Soft robots have recently attracted much interest, and dielectric elastomers have shown great promise for soft actuators because of their large voltage-induced deformation. The actuation performance is primarily characterized by the largest deformation, based on the material failure analyses. However, most previous related work usually neglected the fact that, different from their counterpart in the field of traditional rigid robots, soft actuators are deeply coupled with the environment. Interactions between soft actuators and the environment are significant and under-researched, playing an important role in evaluating their actuation capabilities and in matching them with prescribed soft robotic systems. This article investigates the interactions between a dielectric elastomer balloon actuator and an actuated soft body. We present a computational model of the coupled system and look at the effects of mechanical and material designs on the performance of the balloon actuator. Parametric studies ...

Giant voltage-induced deformation in dielectric elastomers near the verge of snap-through instability

Dielectric elastomers are capable of large voltage-induced deformation, but achieving such large deformation in practice has been a major challenge due to electromechanical instability and electric breakdown. The complex nonlinear behavior suggests an important opportunity: electromechanical instability can be harnessed to achieve giant voltage-induced deformation. We introduce the following principle of operation: place a dielectric elastomer near the verge of snap-through instability, trigger the instability with voltage, and bend the snap-through path to avert electric breakdown. We demonstrate this principle of operation with a commonly used experimental setup—a dielectric membrane mounted on a chamber of air. The behavior of the membrane can be changed dramatically by varying parameters such as the initial pressure in the chamber, the volume of the chamber, and the prestretch of the membrane. We use a computational model to analyze inhomogeneous deformation and map out bifurcation diagrams to guide the experiment. With suitable values of the parameters, we obtain giant voltage-induced expansion of area by 1692%, far beyond the largest value reported in the literature.

Dielectric elastomer actuators under equal-biaxial forces, uniaxial forces, and uniaxial constraint of stiff fibers

A membrane of a dielectric elastomer deforms when a voltage is applied through its thickness. The achievable voltage-induced deformation is strongly affected by how mechanical loads are applied. Large voltage-induced deformation has been demonstrated for a membrane under equal-biaxial forces, but only small voltage-induced deformation has been observed for a membrane under a uniaxial force. This difference is interpreted here theoretically. The theory also predicts that, when the deformation of a membrane is constrained in one direction, a voltage applied through the thickness of the membrane can cause it to deform substantially in the other direction. Experiments are performed on membranes under equal-biaxial forces and uniaxial forces, as well as on fiber-constrained membranes of two types: a dielectric elastomer membrane with carbon fibers on both faces, and two dielectric elastomer membranes sandwiching nylon fibers. The experimental observations are compared with the theory.

Extension limit, polarization saturation, and snap-through instability of dielectric elastomers

A dielectric elastomer is capable of large voltage-induced deformation, particularly when the voltage is applied on the verge of snap-through instability. A model is described which shows that the snap-through instability is markedly affected by both the extension limit of polymer chains and the polarization saturation of dipoles. The model may guide the search for high-performance dielectric elastomer transducers.