Soft Electroactive Materials Research Articles

Axisymmetric vibration of multilayered electroactive circular plates in contact with fluid

Soft electroactive materials (SEAMs), which possess significant advantages over conventional hard materials, have attracted great attention in recent years. They have been widely used in automotive, aerospace, and biomedical industries, where they frequently come into contact with fluid. However, few analytical studies on the SEAM structures interacting with fluid have been reported. Based on the nonlinear electroelasticity and its relevant incremental theory, this paper investigates the linearized axisymmetric vibration of SEAM plates in contact with a fluid medium and subjected to electromechanical biasing fields. The state-space method and Hankel transform technique are adopted to solve the fully coupled partial differential equations. The numerical examples of a homogeneous plate, a two-layer plate, and a functionally graded plate are considered. We find various interesting phenomena and results, such as the complex competition mechanism, the difference between dry and wet high-order modes, and the influence of electromechanical biasing fields, fluid, and material heterogeneity on the natural frequency and buckling threshold. Our numerical findings provide helpful guidance for the design of dynamic devices made of SEAMs working in a fluid environment.

Al@SiO2 Core-Shell Fillers Enhance Dielectric Properties of Silicone Composites.

Over the past decade, there has been significant interest in polysiloxane-based dielectric elastomers as promising soft electroactive materials. Nevertheless, the natural low permittivity of polydimethylsiloxane has limited its practical applications. In this study, we have developed silicone rubber/Al@SiO2 composites with a high dielectric constant, low dielectric loss, and high electrical breakdown strength by controlling the shell layer thickness and the content of the core-shell filler. We also investigated the dielectric behavior of the composites. The use of core-shell fillers has increased the Maxwell-Wagner-Sillars (MWS) relaxation process while reducing the dielectric loss of direct current conductance in silicone rubber composites. Moreover, the temperature dependence of the MWS relaxation time in the composites follows the Arrhenius equation. This strategy of increasing the permittivity of silicone composites through core-shell structural fillers can inspire the preparation of other high dielectric constant composites.

Spatially Modulus-Patterned dielectric elastomer actuators with oriented electroactuation

The oriented actuation of muscles is an important motion form in the locomotion of living beings. Dielectric elastomers (DE) as soft electroactive materials are considered as one of the leading candidates for artificial muscles. However, the electro-actuated deformations of most DE films are uniformly expanded, since synthetic elastomer films are generally mechanically isotropic. Here, we design an elastomer network with reserved double bonds, which allows post-crosslinking in as-prepared isotropic elastomer films with designed crosslinked patterns to spatially modulate the stiffness, and the elastic modulus in the post-crosslinked areas can be continuously increased by an order of magnitude. We reveal that introducing proper numbers of periodic parallel post-crosslinked strips enables the elastomer film to exhibit strong anisotropic mechanical behavior. Furthermore, we simulate and demonstrate the advantage of oriented electroactuation in DE actuators by using DE films with different numbers of parallel strips. Like the dexterous and elegant muscle-driven motions of creatures, our DE films capable of post-regulating mechanical properties can be potentially programmed for achieving complicated electroactuation via designing local crosslinked density and patterns.

Miniature and tunable high voltage-driven soft electroactive biconvex lenses for optical visual identification

Soft electroactive materials including dielectric elastomer (DE) and polyacrylamide (PAM) hydrogel have recently been investigated, which can provide exciting opportunities for optical imaging and biomedical engineering. We propose a tunable liquid lens based on PAM hydrogels, and the miniature lens is also composed of a dielectric elastomer actuator (DEA) and an ionic liquid enclosed. When a biconvex lens is fabricated, a bubble needs to be voided by controlling the pressure. The lens DEA based on PAM electrodes has various resistances that decrease with the stretch. However, it is a constant of 0.49 Ω for the DEA coupling carbon grease electrodes. In a high voltage-driven state, the curvature radius of the lens increased. As a result, the focal length was tuned and enlarged. Computational models are derived for the soft-actuated liquid lens, which improves the existing related theory by detail. Especially, the relationship between voltage and focal length is deduced and verified by experiments. The computational models and experimental phenomena are consistent. Moreover, an increase in pre-stretch and voltage produces a wider tenability range. This study opens the soft electroactive biconvex lenses in potential optical healthcare rehabilitation and optical visual identification applications.

Nonlinear statistical mechanics drives intrinsic electrostriction and volumetric torque in polymer networks.

Statistical mechanics is an important tool for understanding polymer electroelasticity because the elasticity of polymers is primarily due to entropy. However, a common approach for the statistical mechanics of polymer chains, the Gaussian chain approximation, misses key physics. By considering the nonlinearities of the problem, we show a strong coupling between the deformation of a polymer chain and its dielectric response, that is, its net dipole. When chains with this coupling are cross linked in an elastomer network and an electric field is applied, the field breaks the symmetry of the elastomer's elastic properties and, combined with electrostatic torque and incompressibility, leads to intrinsic electrostriction. Conversely, deformation can break the symmetry of the dielectric response, leading to volumetric torque and asymmetric actuation. Both phenomena have important implications for designing high-efficiency soft actuators and soft electroactive materials, and the presence of mechanisms for volumetric torque, in particular, can be used to develop higher degree of freedom actuators and to achieve bioinspired locomotion.

An Inconspicuous, Integrated Electronic Travel Aid for Visual Impairment

Abstract With a globally aging population, visual impairment is an increasingly pressing problem for our society. Visual disability drastically reduces quality of life and constitutes a large cost to the health care system. Mobility of the visually impaired is one of the most critical aspects affected by this disability, and yet, it relies on low-tech solutions, such as the white cane. Many avoid solutions entirely. In part, reluctance to use these solutions may be explained by their obtrusiveness, a strong deterrent for the adoption of many new devices. Here, we leverage new advancements in artificial intelligence, sensor systems, and soft electroactive materials toward an electronic travel aid with an obstacle detection and avoidance system for the visually impaired. The travel aid incorporates a stereoscopic camera platform, enabling computer vision, and a wearable haptic device that can stimulate discrete locations on the user’s abdomen to signal the presence of surrounding obstacles. The proposed technology could be integrated into commercial backpacks and support belts, thereby guaranteeing a discreet and unobtrusive solution.

Exact axisymmetric adhesive contact analysis for a pre-deformed soft electroactive half-space

In this paper, three-dimensional exact solutions of adhesive contact between a pre-deformed compressible soft electroactive half-space and an axisymmetric rigid indenter are presented. The change of surface adhesion energy during the contact is examined by using the modified JKR model, which accounts for the real contact area instead of the projected area. With the help of new results in the potential theory method, all physical (field) variables are derived in terms of elementary functions for three common types of axisymmetric indenters (flat-ended, conical, and spherical). The analytical contact relations for different indenter geometries and material properties are provided and summarized in Tables 2 and 3 to serve a solid base for revealing the underlying electromechanical mechanism of soft electroactive materials. For numerical illustration, neo-Hookean isotropic electroactive material is considered. The simulation results clearly demonstrate that both the mechanical and electric biasing fields significantly affect the indentation measurement of the electroactive material. Moreover, at either micro- or nano-scale, adhesion plays a prominent role in the indentation responses. It is of interest that, even without adhesion, the normal stress somewhere in the contact region may become tensile under a prescribed pre-stretch when the biasing electric displacement exceeds a certain value. This abnormal phenomenon actually corresponds to surface instability of the half-space under the biasing field. In the case of adhesive contact, other than the surface instability, the value of surface adhesion energy between the probe and the sample will impose a constraint on the validity of indentation analysis within the linear elastic regime.

On Structural Theories for Ionic Polymer Metal Composites: Balancing Between Accuracy and Simplicity

Ionic polymer metal composites (IPMCs) are soft electroactive materials that are finding increasing use as actuators in several engineering domains, where there is a need of large compliance and low activation voltage. Similar to traditional sandwich structures, an IPMC comprises a hydrated ionomer core that is sandwiched by two stiffer electrodes. The application of a voltage across the electrodes drives charge migration within the ionomer, which, in turn, contributes to the development of an eigenstress, associated with osmotic pressure and Maxwell stress. Critical to IPMC actuation is the variation of the eigenstress through the thickness of the ionomer, which is responsible for strain localization at the ionomer-electrode interfaces. Despite considerable progress in the development of reliable continuum theories and finite element tools, accurate structural theories that could beget physical insight into the inner workings of IPMC actuation are lacking. Here, we seek to bridge this gap by contributing a principled methodology to structural modeling of IPMC actuation. Our approach begins with the study of the IPMC electrochemistry through the method of matched asymptotic expansions, which yields a semi-analytical expression for the eigenstress as a function of the applied voltage. Hence, we establish a total potential energy that accounts for the strain energy of the ionomer, the strain energy of the electrodes, and the work performed by the eigenstress. By projecting the IPMC kinematics on select beam-like representations and imposing the stationarity of the total potential energy, we formulate rigorous structural theories for IPMC actuation. Not only do we examine classical low-order and higher-order beam theories, but we also propose enriched theories that account for strain localization near the electrodes. The accuracy of these theories is assessed through comparison with finite element simulations on a plane-strain problem of non-uniform bending. Our results indicate that an enriched Euler-Bernoulli beam theory, with three independent field variables, is successful in capturing the main features of IPMC actuation at a limited computational cost.

A dielectric elastomer membrane integrated with protective passive layers under explicit and implicit prestretch

Dielectric elastomers (DEs) are a category of soft electro-active materials that can be used as sensors, actuators and generators. In order to provide protection, insulation layers are essential and thicker layers can lead to stronger protecion. However, the presence of the additional layers can certainly constrain the deformation of DEs. Thus a balance between protection and performance is needed. Prestretch is widely used for the enhancement of actuation performance. Thus the DE membrane integrated with protective passive layers is studied here, focusing on the effect of the coexistence of the prestretch and passive layers. We identify two approaches of prestretches, where the DE membrane and protective passive layers can be explicitly prestretched by external loads, or the DE membrane can be implicitly prestretched by solely protective passive layers. For the latter approach, it does not require additional components such as rigid clamps or dead weights. We then establish a coupled model for the DE membrane and passive layers. The effects of the prestretches and passive layers on the electromechanical behavior and voltage-induced deformation of the DE membrane are then revealed numerically. Furthermore, we theoretically study the detailed effects of specific mechanical properties of passive layers such as the thickness and shear modulus, which are useful for the design of the system. This theoretical research thus sheds insights for the design of dielectric elastomer applications where both safety and performance are required.

Coupled vibro-acoustic modeling of a dielectric elastomer loudspeaker.

Dielectric elastomer membranes are soft electro-active materials capable of large deformations. When inflated over a cavity, the membrane radiates sound and can therefore be used as a loudspeaker. This type of device has been studied both experimentally and numerically. However, most studies on the dynamics of dielectric elastomer membranes either focus on the very low frequency behavior to analyse viscosity effects for example, or try to maximise the overall radiated sound pressure level. Here the mid-frequency range is analysed in detail, by setting up a fully coupled finite element model of an inflated dielectric elastomer membrane. Electrostatics, vibro-acoustics, free-field radiation, and pre-stressed linear dynamics are solved together, to find the fluid loaded resonance modes. The dynamics of the membrane and the sound radiation are then computed using this resonance mode basis. Perfectly matched layers are used to implement the Sommerfeld radiation boundary condition. The model is validated by a comparison with measurements of the pressure radiated by a prototype, and predicts accurately the radiated pressure and the directivity. This model should therefore help the development of optimized dielectric elastomer loudspeakers, with improved frequency responses and directivity.

Printed PEDOT:PSS Trilayer: Mechanism Evaluation and Application in Energy Storage.

Combining ink-jet printing and one of the most stable electroactive materials, PEDOT:PSS (poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)), is envisaged to pave the way for the mass production of soft electroactive materials. Despite its being a well-known electroactive material, widespread application of PEDOT:PSS also requires good understanding of its response. However, agreement on the interpretation of the material’s activities, notably regarding actuation, is not unanimous. Our goal in this work is to study the behavior of trilayers with PEDOT:PSS electrodes printed on either side of a semi-interpenetrated polymer network membrane in propylene carbonate solutions of three different electrolytes, and to compare their electroactive, actuation, and energy storage behavior. The balance of apparent faradaic and non-faradaic processes in each case is discussed. The results show that the primarily cation-dominated response of the trilayers in the three electrolytes is actually remarkably different, with some rather uncommon outcomes. The different balance of the apparent charging mechanisms makes it possible to clearly select one electrolyte for potential actuation and another for energy storage application scenarios.

Investigation on the dynamic performance of viscoelastic dielectric elastomer oscillators considering nonlinear material viscosity

As a typical kind of soft electroactive materials, dielectric elastomers are capable of producing large deformation under external stimuli, which makes them desirable materials for many practical applications in transduction technology, including tunable oscillators and resonators. The dynamic performance of such dielectric elastomer–based vibrational devices is strongly affected by material viscosity as well as electromechanical coupling. Moreover, as suggested by experiments and theoretical studies, dielectric elastomers exhibit deformation-dependent relaxation process, which makes the modeling of the dynamic performance of dielectric elastomer–based devices more challenging. In this work, by adopting the state-of-art modeling framework of finite-deformation viscoelasticity, the effect of the nonlinear material viscosity on the in-plane oscillation and the frequency tuning of dielectric elastomer membrane oscillators is investigated. From the simulation results, it is found that the nonlinear viscosity only affects the transient state of the frequency tuning process. The modeling framework developed in this work is expected to provide useful guidelines for predicting the dynamic performance of dielectric elastomer–based vibrational devices as well as their optimal design.

Free vibration and active control of pre-stretched multilayered electroactive plates

Investigations on soft electro-active materials (SEAMs) have attracted increasing attention in the fields of mechanics and related engineering disciplines. Multi-field coupling and finite deformation are both regarded as effective means to actively control the material properties and geometry of SEAM structures. In this paper, the free vibration analysis of a pre-stretched SEAM plate is investigated based on the linearized biasing theory of electro-elasticity. A three-dimensional solution is obtained by using the new-type state-space formulations. Two decoupled state equations are derived, which correspond to the in-plane vibration and the flexural vibration, respectively. In the numerical part, the single-layered plate is assumed to be loaded in two ways, i.e. by the fixed in-plane stretches or the fixed in-plane forces. As an attempt on structural optimization, a few pre-stretched layers are bonded together to get a layered configuration of SEAM plate. The influences of pre-voltage and pre-stretch on the natural frequencies are studied for single-layered, two-layered and graded plates. The active control of SEAM plates is demonstrated and some interesting phenomena, such as frequency jumping, pre-stretch stiffening, and pre-voltage softening effects, are observed. Optimal frequency is also obtained by the layered configuration of SEAM plate. Two different kinds of layered plates with laminations achieved through mechanical and electrical means, respectively, are introduced. The effective material properties in both plates can vary along the thickness direction in a controllable way. The present paper reports more possibilities to control the free vibration characteristics of SEAM plates.

Adaptive Lenses Based on Soft Electroactive Materials

Soft electroactive materials including dielectric elastomer actuators (DEAs) and polyvinyl chloride (PVC) gels have recently been extensively investigated. These smart materials can effectively respond to an electric field, resulting in shape deformation. In addition to artificial muscles, actuators, sensors, and micro-electromechanical systems, they can be used to prepare various adaptive lenses with unique features such as a simple fabrication, compact structure, good flexibility, and light weight. In contrast to DEAs, PVC gels can provide exciting opportunities for emerging applications in imaging, sensing, optical communication, biomedical engineering, and displays. In this review paper, the underlying physical mechanisms of these two electroactive materials are explained first, and then some recent progress in their application in macro-sized lenses and microlens arrays is presented. Finally, future perspectives of the PVC gels are discussed.

Tuning Elastic Waves in Soft Phononic Crystal Cylinders Via Large Deformation and Electromechanical Coupling

Soft electroactive materials can undergo large deformation subjected to either mechanical or electrical stimulus, and hence, they can be excellent candidates for designing extremely flexible and adaptive structures and devices. This paper proposes a simple one-dimensional soft phononic crystal (PC) cylinder made of dielectric elastomer (DE) to show how large deformation and electric field can be used jointly to tune the longitudinal waves propagating in the PC. A series of soft electrodes, which are mechanically negligible, are placed periodically along the DE cylinder, and hence, the material can be regarded as uniform in the undeformed state. This is also the case for the uniformly prestretched state induced by a static axial force only. The effective periodicity of the structure is then achieved through two loading paths, i.e., by maintaining the longitudinal stretch and applying an electric voltage over any two neighboring electrodes or by holding the axial force and applying the voltage. All physical field variables for both configurations can be determined exactly based on the nonlinear theory of electroelasticity. An infinitesimal wave motion is further superimposed on the predeformed configurations, and the corresponding dispersion equations are derived analytically by invoking the linearized theory for incremental motions. Numerical examples are finally considered to show the tunability of wave propagation behavior in the soft PC cylinder. The outstanding performance regarding the band gap (BG) property of the proposed soft dielectric PC is clearly demonstrated by comparing with the conventional design adopting the hard piezoelectric material. One particular point that should be emphasized is that soft dielectric PCs are susceptible to various kinds of failure (buckling, electromechanical instability (EMI), electric breakdown (EB), etc.), imposing corresponding limits on the external stimuli. This has been carefully examined for the present soft PC cylinder such that the applied electric voltage is always assumed to be less than the critical voltage except for one case, in which we illustrate that the snap-through instability of the axially free PC cylinder made of a generalized Gent material may be used to efficiently trigger a sharp transition in the BGs.

On buckling of a soft incompressible electroactive hollow cylinder

Soft electroactive materials show great potential for device and robot applications. However, these materials are apt to experience buckling and pull-in instability under critical pressure or voltage, and, therefore, their practical applications are more or less prevented. In this paper, buckling behavior of incompressible soft electroactive hollow cylinders is investigated based on the nonlinear theory of electroelasticity and the associated linear incremental field theory. Hollow cylinders including or excluding the effects of exterior electric field are studied in a comparison manner. The equations governing the linearized incremental motion upon a finitely deformed configuration in the presence of an electric field are derived and exactly solved by introducing three displacement functions. As an illustrative example, the generic isotropic electroactive materials are considered and results are presented for a simple model of ideal electroelastic material. Numerical calculations show that the buckling of electroactive hollow cylinders is significantly influenced by the biasing fields, the electromechanical coupling parameters, the geometrical parameters of the cylinder, and the electric field outside the cylinder. In particular, a phase diagram is constructed based on the numerical results to clearly identify the dominant buckling modes and the transition between them in the κ−−ν (axial wave number versus radius ratio) plane.

Dynamic analysis of a tunable viscoelastic dielectric elastomer oscillator under external excitation

As a category of soft electroactive materials, dielectric elastomers (DEs) show great potential for the development of tunable oscillators and resonators for actuating and sensing purposes. However, the dynamic performance of these DE-based vibration devices could be very susceptible to external environment (external loads and excitations) and material viscoelasticity of the DEs. Based on the finite-deformation viscoelasticity theory, this work first investigates the frequency tuning process of a viscoelastic DE membrane oscillator. A comparison of the frequency tuning process and the tunable frequency range between a viscoelastic and a purely elastic DE oscillator is presented. Moreover, particular considerations have been given to the nonlinear response of the oscillator to external harmonic excitation. It is found that the displacement transmissibility of the oscillator can also be actively tuned by changing the static voltage applied to the DE membrane. Under harmonic excitation, various vibration patterns of the oscillator could be actively achieved with the application of both static and alternating electric voltage. Simulation results in this work demonstrate that the material viscoelasticity has a significant effect on the electromechanical coupling and the dynamic performance of the DE-based vibration devices.

Covalent incorporation of biomolecules for improving functional properties of freestanding conductive hydrogels

Event Abstract Back to Event Covalent incorporation of biomolecules for improving functional properties of freestanding conductive hydrogels Alexander Patton1, Rylie A. Green1 and Laura A. Poole-Warren1 1 University of New South Wales, Graduate School of Biomedical Engineering, Australia Introduction: Soft conductive materials have great potential for biomedical applications, in particular those dependent on electrical stimulation of neural and cardiac tissues. Previous research has demonstrated the feasibility of growth of the conducting polymer (CP), poly(3,4-ethylenedioxythiophene) (PEDOT) through a hydrogel with chemically synthesised dispersions of coupled CP:dopant for nucleation[1]. Despite proof-of-concept that such soft electroactive materials can be formed without a metal substrate, the electrodeposition is lengthy and the resulting electrical properties of the conducting hydrogels (CH) are significantly lower than CPs alone[1]. The aim of this study was to investigate the impact of biological molecule incorporation on the properties of freestanding conductive hydrogels. Two aspects were investigated: (i) the use of the anionic glycosaminoglycan (GAG), heparin, as a doping molecule and (ii) the addition of the non-doping cell attachment protein, gelatin. Previous studies evidence the incorporation of gelatin into hydrogels as a support for the adherence of neural cells[2]. Methods: Methacrylate modified poly(vinyl alcohol) (PVA-MA) control hydrogels (20 wt%) with 0.5 wt% chemically synthesised PEDOT:poly(styrene sulfonate) (PEDOT:PSS)[3] were formed by photo-initiated crosslinking[1],[4]. As reported previously the PEDOT:PSS is added to enable secondary nucleation sites within the gels. Test hydrogels were formed in the same way but with the addition of 18 wt% PVA-MA and 2 wt% methacrylated heparin[1],[4]. Electrodeposition of CP in control and test hydrogels was conducted at 1mA/cm2 for 0 to 160 minutes in 0.03 M EDOT monomer solution. Electrical properties were analysed using electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). Hydrogel stiffness was measured by compression tests (50 N load cell, 0.5 mm/min, loaded to failure). PC12 cells were used as a neuron-like cell model to assess adhesion and neurite viability at 96 hours. This was examined on 4 different gels: with and without gelatin loading; and with and without 40 minutes of electrodeposited PEDOT. Results and Discussion: As shown in Table 1, no differences to the modulus of elasticity were seen when undertaking electrodeposition of CP through the gels, likely caused by the fact that the mass of the PVA is much greater relative to the PEDOT component. The incorporation of heparin as a bound dopant significantly improved the electroactivity of the standalone hydrogels as shown in Table 2. This is hypothesised to be due to the presence of bound heparin which has been shown to act as an effective dopant in CPs[4]. Gelatin was observed to promote cellular adherence across gels both with and without PEDOT grown through them, with greater viability observed in these cases. Table 1: Characteristics of non-heparin loaded PEDOT:PSS hydrogels Table 2: Electroactivity differences produced with addition of heparin to PEDOT:PSS gels Conclusions: The incorporation of heparin into these gels significantly improves the ability of electrodeposited conducting polymer to form and develop conductive pathways. The incorporation of gelatin significantly promotes cell adhesion and viability. It was also seen here that the formation of PEDOT through the gels does not significantly impact the mechanics, making them a conductive system with soft, tailorable mechanics.

Electrical impedance controls mechanical sensing in ionic polymer metal composites

Ionic polymer metal composites (IPMCs) are a class of soft electroactive materials that are recently finding extensive application as mechanical sensors and energy harvesters in liquids. In their most fundamental form, IPMCs are composed of a hydrated ionomeric membrane that is sandwiched between two electrochemically deposited metal electrodes. Ionomer swelling, counterion diffusion, and the formation of electric double layers are some of the physical phenomena underpinning energy transduction in IPMCs; however, a thorough understanding of the relative influence of such phenomena is yet to be established. Here, we propose a physics-based modeling framework, based on the Poisson-Nernst-Planck system, to describe IPMC chemoelectrical response to an imposed time-varying flexural deformation. We utilize the method of matched asymptotic expansions to compute a closed-form solution for the electric potential and counterion concentration in the IPMC. The model predicts that IPMC sensing is independent of the time rate of deformation and linearly correlated to the mechanical curvature, with a coefficient of proportionality that is a function of the ionomer thickness and the temperature. Thus, our results demonstrate that the characterization of IPMC electrical impedance suffices to identify all the parameters that are relevant to sensing, in contrast with the current state of knowledge. Theoretical results are validated through experiments on patterned in-house fabricated IPMC samples that are subject to time-varying flexural deformations.