Ionic Electroactive Polymer Actuators Research Articles

An Ionic Polymer Actuator Capable of Simultaneously Improving Bandwidth and Blocking Force for Efficient Haptic Feedback

Recently, an ionic electroactive polymer (i-EAP) actuators are spotlighted in various fields because they provide a lot of advantages such as lightweight, flexibility, simple process, and low driving power. However, they suffer from low blocking force, displacement, and wide bandwidth that can be implemented under electric field for apply to various applications. In order to apply these i-EAP actuators to various applications, it is important to provide the optimum characteristics of the i-EAP actuator adaptive to the target application. Particularly, it is required to achieve high blocking force and displacement as well as wide actuation bandwidth for haptic feedback among the various applications. In this talk, we propose an ionic polymer actuator (IPA) with wide bandwidth of over 100Hz and high blocking force based on PEDOT:PSS electrode with additives and ionic polymer with fibrillary nanostructures for effective haptic feedback. In particular, we developed an actuator that improves the interfacial properties between electrodes and an ionic polymer as well as electrical conductivity of electrodes by simple and effective way through spray-coating of PEDOT: PSS solution with additives. As a result, the IPA was successfully operated with a large displacement up to 2.5mm at an operating frequency of 20mHz under an applied voltage of 2V. Further, the IPA shows large blocking force of 0.4mN at operating frequency of 1Hz. These our actuator is capable of driving up to a high frequency of 200Hz and shows an improved blocking force up to 0.4mN. We potentially believe our IPA will provide a rational guide to future haptic feedback.

Fast and Stable Ionic Electroactive Polymer Actuators with PEDOT:PSS/(Graphene⁻Ag-Nanowires) Nanocomposite Electrodes.

Ionic electroactive polymer (IEAP) actuators that are driven by electrical stimuli have been widely investigated for use in practical applications. However, conventional electrodes in IEAP actuators have a serious drawback of poor durability under long-term actuation in open air, mainly because of leakage of the inner electrolyte and hydrated cations through surface cracks on the metallic electrodes. To overcome this problem, a top priority is developing new high-performance ionic polymer actuators with graphene electrodes that have superior mechanical, electrical conductivity, and electromechanical properties. However, the task is made difficultby issues such as the low electrical conductivity of graphene (G). The percolation network of silver nanowires (Ag-NWs) is believed to enhance the conductivity of graphene, while poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), which exhibits excellent stability under ambient conditions, is expected to improve the actuation performance of IEAP actuators. In this study, we developed a very fast, stable, and durable IEAP actuator by employing electrodes made of a nanocomposite comprising PEDOT:PSS and graphene–Ag-NWs (P/(G–Ag)). The cost-effective P/(G–Ag) electrodes with high electrical conductivity displayed a smooth surface resulting from the PEDOT:PSS coating, which prevented oxidation of the surface upon exposure to air, and showedstrong bonding between the ionic polymer and the electrode surface. More interestingly, the proposed IEAP actuator based on the P/G–Ag electrode can be used in active biomedical devices, biomimetic robots, wearable electronics, and flexible soft electronics.

A hot-pressing pretreatment of Cu substrate and dry transfer method of multilayer-stacked graphene for ionic electroactive polymers

We herein report the development of an ionic electroactive polymer (IEAP) actuator based on a high quality graphene electrode with minimized surface damage through the use of a hot-pressing (HP) pretreatment process for copper substrate in addition to the dry transfer method for a multilayer-stacked graphene electrode. This HP process for pretreatment of the Cu foil was developed to ensure high-quality graphene growth, and the production of larger grain sizes than those normally obtained following conventional pretreatment methods. We also demonstrate that compared to the conventional wet transfer process based on poly(methyl methacrylate) and the roll-to-roll direct dry transfer method minimizes surface damage of graphene. A 5-layer stacked graphene electrode was prepared and it exhibited a low sheet resistance and flexibility, in addition to the ability to prevent water evaporation during electrical stimulation, thereby improving both the electrode performance and the durability. This 5-layer stacked graphene-based IEAP actuator prepared using HP pretreatment and a dry transfer method exhibited enhanced actuation performances, including a larger bending curvature, a larger maximum bending deformation, and an improved durability.

Graphene Oxide-Silver Nanowires Paper Electrodes with Poly(3,4-ethylenedioxythiophene)-Poly(styrenesulfonate) to Enhance the Driving Properties of Ionic Electroactive Polymer Actuators

Graphene Oxide-Silver Nanowires Paper Electrodes with Poly(3,4-ethylenedioxythiophene)-Poly(styrenesulfonate) to Enhance the Driving Properties of Ionic Electroactive Polymer Actuators

Modelling and Control of Ionic Electroactive Polymer Actuators under Varying Humidity Conditions

In this work, we address the problem of position control of ionic electroactive polymer soft actuators under varying relative humidity conditions. The impact of humidity on the actuation performance of ionic actuators is studied through frequency response and impedance spectroscopy analysis. Considering the uncertain performance of the actuator under varying humidity conditions, an adaptable model using the neural network method is developed. The model uses relative humidity magnitude as one of the model parameters, making it robust to different environmental conditions. Utilizing the model, a closed-loop controller based on the model predictive controller is developed for position control of the actuator. The developed model and controller are experimentally verified and found to be capable of predicting and controlling the actuators with excellent tracking accuracy under relative humidity conditions varying in the range of 10–90%.

Soft Ionic Electroactive Polymer Actuators with Tunable Non-Linear Angular Deformation.

The most rational approach to fabricate soft robotics is the implementation of soft actuators. Conventional soft electromechanical actuators exhibit linear or circular deformation, based on their design. This study presents the use of conjugated polymers, Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) to locally vary ion permeability of the ionic electroactive polymer actuators and manipulate ion motion through means of structural design to realize intrinsic angular deformation. Such angular deformations are closer to biomimetic systems and have potential applications in bio-robotics. Electrochemical studies reveal that the mechanism of actuation is mainly associated with the charging of electric double layer (EDL) capacitors by ion accumulation and the PEDOT:PSS layer’s expansion by ion interchange and penetration. Dependence of actuator deformation on structural design is studied experimentally and conclusions are verified by analytical and finite element method modeling. The results suggest that the ion-material interactions are considerably dominated by the design of the drop-cast PEDOT:PSS on Nafion.

Development and modeling of a new ionogel based actuator

Ionic electroactive polymer actuators are expected to be one of the most promising driving mechanisms in the future due to their extraordinary features such as their lightweight, flexibility, and low-energy consumption. Traditional ionic electroactive polymer actuators for example, ionic-polymer metal composites have a problem with durability in open air due to the evaporation of water contained in the polymer electrolytes, resulting in a corresponding loss of performance. Electrolysis of the water at relatively low operating voltages may cause deterioration of these materials. Ionic liquids are more thermally and electrochemically stable than water, with unique advantages including negligible volatility, low melting point and high ionic conductivity, therefore they can be used in the application of ionic electroactive polymer actuators. In this work, a new ionic electroactive polymer actuator based on ionogel is developed, which can be operated at low driving voltage with high electrochemical stability. In order to investigate the actuation mechanism of the actuator, a general model consisting of an equivalent electrical circuit, an electromechanical coupling term and a mechanical beam model is built up to characterize its interrelated electrical, mechanical, and chemical properties. This model explains the relationship between input voltage and bending displacement of the actuator. Theoretical and experimental results are demonstrated and documented to validate the conclusion that the model can effectively predict the actuation response of the material. The geometric scalability of the model is also investigated, giving support to the design of the soft mechanism based on ionogel.

Effect of ambient humidity on ionic electroactive polymer actuators

Comparable electromechanical measurements were carried out with carbon-based ionic electroactive polymer actuators in vacuum, dry inert, and in ambient air environment. The results bring forward the effect of ambient humidity on the electrical and mechanical parameters of the laminates of this type. Presence of water decreases the Young’s modulus of the polymer and lowers the viscosity of the ionic liquid, which, in turn, is accompanied by the increase of ionic conductivity of the electrolyte. The factual bending behavior of the actuator is a result of the combined effect of these factors. A four-parameter model was developed for the quantitative estimation of the rates of forward-actuation and back-relaxation as well as the electrical parameters. An important outcome of the experiments is the observation that there is nearly no back-relaxation in vacuum and in dry inert environment.

Parylene-coated ionic liquid-carbon nanotube actuators for user-safe haptic devices.

Simple fabrication, high power-to-weight and power-to-volume ratios, and the ability to operate in open air at low voltage make the ionic electroactive polymer actuators highly attractive for haptic applications. Whenever a direct tactile stimulation of the skin is involved, electrical and chemical insulation as well as a long-term stability of the actuator are required. Because of its inherent physicochemical properties such as high dielectric strength, resistance to solvents, and biological inactivity, Parylene C meets the requirements for making biocompatible actuators. We have studied the displacement and the generated force of Parylene-coated carbon nanotube actuators as well as the encapsulation quality. A 2 μm coating creates an effective electrical insulation of the actuators without altering the blocking force at frequencies from 50 mHz to 1 Hz. Moreover, the generated strain is preserved at higher frequencies (from 0.5 to 5 Hz). We employed a simple mechanical model to explain the relation between the key parameters-flexural stiffness, displacement, and force-for uncoated and coated actuators. In addition, we demonstrated that our Parylene-coated actuators are not damaged by rinsing in liquid media such as 2-propanol or water. In conclusion, our results indicate that Parylene C encapsulated actuators are safe to touch and can be used in contact with human skin and in biomedical applications in direct contact with tissues and physiological fluids.

In situ scanning electron microscopy study of strains of ionic electroactive polymer actuators

We have developed a technique to determine the bending strain of ionic electroactive polymer actuators without the use of the macroscopic bending geometry. In situ comparisons of the scanning electron microscope micrographs from a bending ionic electroactive polymer actuator, using a digital image correction methodology, identify its bi-directional deformation field. The developed technique allows verification of the factual axial and thickness strains of any notional layer of the actuator, including the outer surfaces of the electrodes. Thus, calculation of the bending and thickness strains of the ionic electroactive polymer laminate becomes possible. Moreover, the technique allows the determination of the position of the neutral layer of bending that is an important requirement for the calculation of the second area and bending moments of the beam. The four examples presented demonstrate the potential variations of the bending schemes, in cases where the neutral layer is at the centroid and shifted away from the centroid.

Ionic electroactive polymer actuators as active microfluidic mixers

On-chip sample processing is integral to the continued development of lab-on-a-chip devices for various applications. We have demonstrated an active microfluidic mixer using ionic electroactive polymer actuators as artificial cilia.

Ionic electroactive polymer artificial muscles in space applications

A large-scale effort was carried out to test the performance of seven types of ionic electroactive polymer (IEAP) actuators in space-hazardous environmental factors in laboratory conditions. The results substantiate that the IEAP materials are tolerant to long-term freezing and vacuum environments as well as ionizing Gamma-, X-ray, and UV radiation at the levels corresponding to low Earth orbit (LEO) conditions. The main aim of this material behaviour investigation is to understand and predict device service time for prolonged exposure to space environment.

Evidence of counterion migration in ionic polymer actuators via investigation of electromechanical performance

Functional ionomeric polymer membranes are the backbone of a wide range of ionic devices; the mobility of ions through the ionomeric membrane is the principle of operation of these devices. Drift and diffusion of ions through ionomeric membranes strongly depend on the ionic properties of host membrane, as well as the physical and chemical properties of the ions. It is well-established that cations and anions provided via a dopant (e.g. electrolyte or ionic liquid) are mobilized under stimulation. However, in this study, we report that in addition to ions sourced by the dopant, counterions of the ionomeric membrane are also mobilized when stimulated. In particular, we have investigated the electromechanical response of ionic electroactive polymer actuators consisting of Nafion ionomeric membranes with different counterions and have demonstrated that those with cation counterions of larger Van der Waals volume exhibit stronger actuation due to motion of the larger cation counterions compared to actuators consisting of Nafion with counterion of smaller Van der Waals volumes.

Influence of ionic liquid concentration on the electromechanical performance of ionic electroactive polymer actuators

We have investigated influence of ionic liquid concentration on the electromechanical response of ionic electroactive polymer actuators. Actuators were fabricated from ionomeric membrane and doped with different concentrations of 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ionic liquid. Samples were investigated for their electromechanical and electrochemical characteristics; and it was observed that the maximum electromechanical strain of approximately 1.4% is achieved at 22wt% ionic liquid content. Increasing ionic liquid concentration results in saturation of the electrode–ionomer interface and formation of ionic double/multi layers, which in turn result an inward accumulation of ions; hence, generate strain in an undesired direction that deteriorates the electromechanical response of the actuator.

Lifetime measurements of ionic electroactive polymer actuators

This article is focused on proposing a unified methodology for automating the measurement procedures of ionic electroactive polymer actuators. The proposed methodology and large-scale automation would make testing ionic electroactive polymer actuators less labor-intensive and allow analyzing many ionic electroactive polymer actuators simultaneously. Defining a clear framework for testing ionic electroactive polymer actuators performance and reliability would make the testing process reproducible and provide better comparison between ionic electroactive polymer actuators of either different or similar classes. Our methodology separates two types of degradation: degradation during operation and spontaneous self-degradation.

Investigation of spray-coated silver-microparticle electrodes for ionic electroactive polymer actuators

We have employed the easy-to-scale-up method of spray-coating in combination with layer-by-layer self-assembly technique to fabricate ionic electroactive polymer actuators (IEAPAs). IEAPAs with spray-coated silver microparticle electrodes demonstrate enhanced strain and response time when compared to nearly identical, optimized conventional IEAPA with gold leaf electrodes. The results demonstrate that strain of these IEAPAs increases with the decrease of thickness of the outer silver microparticle electrodes. In addition, the response time of the actuators at frequencies of 1 and 10 Hz improves compared to optimized conventionally fabricated IEAPA. It was found that samples consisting of spray-coated silver electrodes can charge up to ∼3 times faster than conventional actuators at 1 Hz frequency. Faster charging/discharging results in higher mobility of ions within the actuator and thus, faster actuation. Given the relatively large thickness of the silver microparticle electrodes (∼50× gold leaf), similar strain was observed due to the lower Young's modulus of spray-coated layers compared to that of bulk material.

Ionic electroactive polymer actuators based on nano-carbon electrodes

This review paper focuses on the recent studies of electroactive polymer actuators that have a triple-layered configuration composed of an ionic-gel electrolyte layer sandwiched by nano-carbon dispersed ionic-liquid gel electrode layers (bucky-gel actuator) for the purpose of development of practical devices. The review covers recent studies of the developments of the materials of the bucky-gel actuators and their electromechanical modeling. In the final section, the application to an ultra-thin and ultra-light Braille display based on the bucky-gel actuator is described. © 2013 Society of Chemical Industry

Mechanical interpretation of back-relaxation of ionic electroactive polymer actuators

The mechanical model obtained by a non-traditional approach to the basic components of the traditional viscoelastic models—spring and damper—elucidates the back-relaxation of ionic electroactive polymer actuators. The corresponding PDE characterizes the curvature or bending moment of the actuators throughout stimulated bending forward followed by relaxation back towards the initial shape. Combining series of short entities containing the lumped electrical circuit, and the transient bending moment generated according to the PDE, results in a new model of the ionic electroactive polymer actuators. This model takes into account the back-relaxation of the actuators as well as the superposition principle. The experiments carried out with three actuators of different ionic electroactive polymer materials show excellent accordance with the model.

Influence of conductive network composite structure on the electromechanical performance of ionic electroactive polymer actuators

The influence of the nanostructure of the conductive network composite (CNC) on the performance of ionic electroactive polymer (IEAP) actuators has been examined in detail. We have studied IEAP actuators consisting of CNCs with different volume densities of gold nanoparticles (AuNPs) and the polymer network. Varying the concentration of AuNPs in CNC thin films was used as a means to control the CNC–ion interfacial area and the electrical resistance of the CNC, with minimum effect on the mechanical properties of the actuator. Increasing the interfacial area and reducing the resistance, while maintaining porosity of the composite, provide means for generating motion of more ions into the CNC at a significantly shorter time, which results in generation of strain at a faster rate. We have demonstrated that cationic strain in actuators with denser CNCs is improved by more than 460%. Denser CNC structures have larger interfacial areas, which results in attraction/repulsion of more ions in a shorter time, thus generation of a larger mechanical strain at a faster rate. Also, time-dependent response to a square-wave voltage was improved by increasing the AuNP concentration in the CNC. Under 0.1 Hz frequency, the cationic strain was increased by 64% when the AuNP concentration was increased from 4 to 20 ppm.

Enhanced Electromechanical Response of Ionic Polymer Actuators by Improving Mechanical Coupling between Ions and Polymer Matrix

Poly[(vinylidene difluoride)-co-(chlorotrifluoroethylene)] (P(VDF–CTFE)) and P(VDF–CTFE)/poly(methyl methacrylate) (PMMA) cross-linked blends were exploited as ionic electroactive polymer (i-EAP) actuators with electrolyte ionic liquid 1-ethyl-3-methylimidazolium trifluoromethanesulfonate [C2mim][TfO] for the first time. Compared to the traditional i-EAPs that are based on Nafion or Aquivion (Hyflon) ionomers, i-EAP actuators made with P(VDF–CTFE) and P(VDF–CTFE)/PMMA polymers exhibited remarkably enhanced actuation strain and significantly reduced electrical capacitance. The increase in the strain–charge ratio e/C indicates that this improvement is a direct result of improved elastic coupling between the [C2mim][TfO] electrolyte and the polymer matrix. As a result, an order-of-magnitude improvement in electromechanical energy conversion efficiency is achieved for both P(VDF–CTFE) and P(VDF–CTFE)/PMMA polymer-based actuators, which makes them promising polymer matrices for i-EAP actuators.