Ionic Actuators Research Articles

Improved response of ionic liquid-based bending actuators by tailored interaction with the polar fluorinated polymer matrix

Poly(vinylidene fluoride) (PVDF), poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE), poly(vinylidene fluoride-co-hexafluropropylene) (PVDF-HFP) and poly(vinylidene fluoride-co-chlorotrifluoroethylene) (PVDF-CTFE) were evaluated for the development of ionic liquid (IL) - polymer composite bending actuators. The selected guest IL, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, [Emim][TFSI], was incorporated into the different host polymer matrixes in concentrations ranging from 10 to 40 wt%. The IL/polymer composites electrical properties strongly depend on the IL content. The largest bending response was found for the IL/PVDF-TrFE composites with a maximum bending value of ∼3.5 mm using a frequency of 100 mHz at an applied potential difference of 5.0 Vpp. Thus, PVDF-TrFE are attractive candidates for the development of high performance bending actuators.

Modeling, fabrication, and characterization of motion platform actuated by carbon polymer soft actuator

Carbon polymer composites (CPC) are kind of soft actuators belonging to the category of ionic electroactive polymer transducers. In this work, we present design, modeling, fabrication, and characterization of a novel parallel manipulation system actuated by the CPC actuators. In this system, four actuators are operated in parallel to manipulate an end effector or a platform to generate motion along the different axis. The major advantage of the current system is that the platform and the actuators are fabricated as a single structure using the simple layer-by-layer spray-deposition method with selective masking. The proposed system is highly dexterous and is capable of generating three degrees of motions, namely, roll, pitch, and piston. The initial structural design and its deformation are optimized by modeling and simulating using finite element method and followed by fabrication and characterization to examine its workspace. The experimental results have demonstrated high levels of manipulability from the CPC actuators that are outstanding in the class of soft ionic actuators while keeping the fabrication method simple, scalable and cost-effective. The proposed configuration has potential application in micromanipulation systems that require large deformation in a confined space.

Ionic polymer with single-layered electrodes: a novel strategy for ionic actuator design

The current ionic polymer actuators demand symmetric assembly of two electrode layers, which brings much difficulty to the patterned electroding process and thus significantly limits their practical application. We proposed a novel actuating structure of ionic polymer with single-layered electrodes, in which the electrodes are fabricated on one side of the substrate, leading to distinct S-shaped deformation with large displacement and high load capacity. This structure tolerates complex electrode patterning process and can be extended to most ionic polymer actuators. Thereby it provides a brand new strategy for the design of so-based soft electromechanical appliance with versatile deformation behavior.

Supersonic cluster beam fabrication of metal–ionogel nanocomposites for soft robotics

Soft robotics is an emerging field targeting at the development of robotic bodies and architectures characterized by flexibility, adaptability, and motility typical of that of biological systems. The use of electroactive ionic polymer–metal nanocomposites able to reversibly deform in response to low-intensity electric fields constitutes a promising solution for the implementation of actuators into soft robots. Currently, the use of this class of nanocomposites is hampered by several drawbacks, mainly related to the mismatch between the mechanical properties of the polymer and the metallic electrodes compromising their stability and resilience upon cyclic deformation. Here, we report and discuss on the use of supersonic cluster beam implantation (SCBI) as an effective strategy for the fabrication of soft electroactive ionic polymeric nanocomposite actuators. SCBI relies on the use of supersonically accelerated beams of neutral metal nanoparticles that can be aerodynamically collimated and directed onto a polymeric target to generate thin nanostructured metal layers physically interpenetrating with the polymer. Soft electroactive actuators based on engineered ionogel and ionogel-based hybrid nanocomposites provided with monolithically integrated cluster-assembled gold electrodes will be discussed. These systems can undergo long-term bending deformation in a low-voltage regime, due to the nanostructured electrode resilience. The use of cluster-assembled nanostructured electrodes opens new opportunities for the high-throughput manufacturing of soft ionic actuators with excellent mechanical resiliency, high-performance actuation, and high durability.

A three-electrode structured ionic polymer carbon-composite actuator with improved electromechanical performance

Ionic polymer carbon-composite (IPCC) is a kind of ionic electroactive polymer with a Nafion/ionic liquid electrolyte layer between two multi-walled carbon nanotube (MWCNT) film electrodes. We have fabricated a three-electrode structured IPCC (3E-IPCC) that exhibited a higher safe working voltage and improved electromechanical performance when compared with a traditional two-electrode laminar actuator. An intermediate electrode that was fabricated using MWCNTs was introduced into the matrix membrane, which was then subjected to hot-pressing to form the 3E-IPCC actuator. The effects of the intermediate electrode on the performance of the 3E-IPCC were analyzed. The experiment results indicated that the intermediate electrode connected the two electrolyte layers in series and successfully broadened the window of working voltage by distributing the voltage over the two electrolyte layers. The broadened window of working voltage endowed a higher safe working voltage and then led to enhanced electromechanical performance when compared with the traditional IPCC actuator. The 3E-IPCC exhibited desirable properties, including a large deformation, high output force and excellent stability under high working voltages and so these actuators show great potential for use in a wide range of fields.

Preparation and electromechanical properties of the chitosan gel polymer actuator based on heat treating

As a new kind of smart materials for fabricating ionic electric actuator, Chitosan Gel Polymer (CGP) known as bionic artificial muscle, has broad application prospects and high academic value with advantages of simple structure, good biocompatibility and large deflection under low driving voltage (≤5 V). In this paper, effects and enhancement mechanism of the heat treating optimization technology on response speed performance of the CGP actuator were mainly investigated. Furthermore, its preparation process including material selection, membrane fabrication and the assembly along with fabrication principle were deeply researched. The CGP actuator consisted of two parts; electric actuating membrane which is sandwiched between non-metallic electrode membranes. On one hand, high viscosity chitosan inside the actuating membrane served as framework which gives access to anions and cations within the electrode membranes. On the other hand, Multi-walled Carbon Nanotube (MCNT) was adopted to modify chitosan in the electrode membrane for obtaining good conductivity and stable chemical character. Moreover, response speed of the CGP actuator that underwent repeated heat treating for a short time, would be continuously improved with the increase of heat treating times. But as the heat treating time prolonged, electromechanical properties of the CGP actuator began to degrade. In addition, since conduction velocity of microscopic ions inside the CGP actuator was presented as macroscopic current magnitude, the experimental results revealed that the changing trend of response speed was almost in accord with the current trend of the CGP actuator. Both of these are fitted in a quadratic function relationship.

Effect of CaCl2 and glycerol on response performance of biological gel electric actuator

Currently problems of large toxicity, poor biocompatibility and high cost between polymer and crosslinking agent are found in traditional ionic gel electric actuator. To conquer such challenges, in this work, sodium alginate(SA) and calcium chloride(CaCl2) were used to produce the biological gel electric actuator (BGEA). Furthermore, to achieve better response performance of the BGEA, two kinds of BGEAs with different concentrations of CaCl2 and different concentrations of glycerol were prepared experimentally, and then the response performance of the BGEA was studied. The effect of calcium chloride on the BGEA showed that the concentration of calcium chloride had a great influence on the response performance of the BGEA, and the best concentration of calcium chloride was 0.95 g L−1. Compared with the BGEA without crosslinking, the response speed and deflection displacement were increased by 1.64 times and 1.52 times respectively. The effect of glycerol on BGEA showed that although the response speed of the BGEA without glycerol was faster, the BGEA itself was dry, easy to break and weak to the external shock. Therefore, adding 1 ml of glycerol into the electrolyte membrane of the BGEA, not only increases the deflection displacement of the BGEA, but also avoids large reduction in its response speed.

Highly Conductive, Photolithographically Patternable Ionogels for Flexible and Stretchable Electrochemical Devices.

An ionic conducting membrane is an essential part in various electrochemical devices including ionic actuators. To miniaturize these devices, micropatterns of ionic conducting membrane are desired. Here, we present a novel type of ionogel that can be patterned using standard photolithography and soft imprinting lithography. The ionogel is prepared in situ by UV-initiated free-radical polymerization of thiol acrylate precursors in the presence of ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. The resultant ionogel is very flexible with a low Young's modulus (as low as 0.23 MPa) and shows a very high ionic conductivity (up to 2.4 × 10-3 S/cm with 75 wt % ionic liquid incorporated) and has a reactive surface due to the excess thiol groups. Micropatterns of ionogel are obtained by using the thiol acrylate ionogel solution as an ionic conducting photoresist with standard photolithography. Water, a solvent immiscible with ionic liquid, is used as the photoresist developer to avoid complete removal of ionic liquid from thin micropatterns of the ionogel. By taking advantage of the reactive surface of ionogels and the photopatternability, ionogels with complex three-dimensional microstructure are developed. The surface of the ionogels can also be easily patterned using UV-assisted soft imprinting lithography. This new type of ionogels may open up for building high-performance flexible electrochemical microdevices.

The influence of electrodeposited conducting polymer electrode structure on the actuation performance of muscle-like ionic actuators

This study demonstrates the influences of physical and electrochemical characteristics of conducting polymer (CP) film as electrode material on the actuation performances of ionic actuators which is of considerable interest for increasing the application of metal-free ionic actuators in different fields. Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and polypyrrole (PPy) were applied as electrode materials in fabrication of metal-free Nafion-based electromechanical actuators. These polymers were electrochemically deposited on both surfaces of polymeric membrane and variations in physical, electrochemical and electromechanical characteristics of actuators were followed as a function of the applied polymer type. Pt-electrode Nafion based actuators were prepared as standard. Electrochemical studies revealed that the mechanism of actuation is mainly related to double-layer capacitance (Cdl) and that faradic capacitance (CF) plays less significant role. PPy-electrode actuators showed higher ion and water permeability due to formation of thinner electrode layers with finer morphology. The largest tip displacement (25 mm) was obtained in PPy-electrode actuators, in response to 6 V dc potential. These actuators exhibited the highest specific capacitance of 146.26 m F.cm−2 and capacitance retention of 87.41% after 100 cycles. The electro-mechanical energy efficiency and the maximum tip displacement of PPy-electrode actuators were 46.42% and 31.57% higher than that considered for PEDOT:PSS-electrode actuators, respectively.

Study on actuation enhancement for ionic-induced IL-cellulose based biocompatible composite actuators by glycerol plasticization treatment method

For the growing demand of actuation performance, a highly powerful IL-cellulose based biocompatible ionic actuator was developed by the plasticization treatment method. In view of the effects of plasticizing treatment, the electromechanical properties and electrochemical properties of biocompatible ionic actuator were mainly studied in this paper. The characteristic peaks scanned by FT-IR and the phase analysis tested by XRD showed that no regenerated substances in the IL-cellulosed membrane were generated under different plasticizing parameters. With the increase of the plasticizing bath concentration and plasticizing time, the porosity of the IL-cellulose layer obviously decreased with a good flexibility, which improved the inner ions movement channels. CV results revealed that the specific capacitance under plasticization parameter of 30%—120 min was a significantly higher than that other IL-cellulose membranes. The results of EIS and GCD demonstrated that the ions diffusion rate increased under different plasticizer concentrations, and the charge and discharge speed was faster than the initial state. However, the ion diffusion rate remained unchanged under different plasticizing time, and the power density was declining along with a deteriorated permeability of IL-cellulose membrane. Additionally, through plasticization treatment, actuators under plasticization parameter of 30%—120 min exhibited largest single cycle peak displacement and peak to peak displacement, which were 219 and 393% over un-plasticized actuators.

Programmable and functional electrothermal bimorph actuators based on large-area anisotropic carbon nanotube paper

Electro-active polymer (EAP) actuators, such as electronic, ionic and electrothermal (ET) actuators, have become an important branch of next-generation soft actuators in bionic robotics. However, most reported EAP actuators could realize only simple movements, being restricted by the small area of flexible electrodes and simple designs. We prepared large-area flexible electrodes of high anisotropy, made of oriented carbon nanotube (CNT) paper, and carried out artful graphic designs and processing on the electrodes to make functional ET bimorph actuators which can realize large bending deformations (over 220°, curvature > 1.5 cm−1) and bionic movements driven by electricity. The anisotropy of CNT paper benefits electrode designs and multiform actuations for complex actuators. Based on the large-area CNT paper, more interesting and functional actuators can be designed and prepared which will have practical applications in the fields of artificial muscles, complicated actuations, and soft and bionic robotics.

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%.

Ultrathin electrochemically driven conducting polymer actuators: fabrication and electrochemomechanical characterization

Electronic conducting polymer based-actuators have attracted lots of interest as alternative materials to traditional piezoelectric and electrostatic actuators. Their specific characteristics such as their low operating voltages and large strains should allow them to adapt better to soft microstructures. Recently, poly (3,4-ethylenedioxythiophene) (PEDOT) – based ionic actuators have overcome some initial stumbling blocks to widespread applications in the microfabricated devices field. These trilayer bending microactuators were fabricated (i) by sequential stacking, using a layer by layer polymerization (LbL) of conducting polymer electrodes and a solid polymer electrolyte and (ii) by micro-patterning, using standard microsystems processes. While microfabrication processing of a trilayer actuator, involving no manual handling has been demonstrated, their bending performances remain limited for practical applications. Moreover, the complete characterization of their electrical, electrochemical, and mechanical properties has never been investigated. This paper describes the optimization of PEDOT electroactive electrodes synthesized with a vapor phase polymerization process. Influence of synthesis parameters on thickness, electronic conductivity and volumetric charge density were studied to determine the guidelines for synthesizing highly efficient electrodes. Afterwards, these parameters are used to guide the LbL synthesis process of ultrathin trilayer actuators. Electrochemical and mechanical properties of the resulting microactuators have been thoroughly characterized. Bending deformation and output force generation have been measured and reached 0.5% and 11 μN respectively. This constitutes the first characterization of ionic PEDOT-based microactuators operating in air of such a thin thickness (11 μm dry and 18.3 μm swelled in 1-Ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMImTFSI)). These actuators and their actuation properties are promising for future soft microsystem devices where the use of polymer actuators should be essential.

Effect of porosity and tortuosity of electrodes on carbon polymer soft actuators

This work presents an electro-mechanical model and simulation of ionic electroactive polymer soft actuators with a porous carbon electrode, polymer membrane, and ionic liquid electrolyte. An attempt is made to understand the effects of specific properties of the porous electrodes such as porosity and tortuosity on the charge dynamics and mechanical performance of the actuator. The model uses porous electrode theory to study the electrochemical response of the system. The mechanical response of the whole laminate is attributed to the evolution of local stresses caused by diffusion of ions (diffusion-induced stresses or chemical stresses). The model indicates that in actuators with porous electrode, the diffusion coefficient of ions, conductivity of the electrodes, and ionic conductivity in both electrodes and separator are altered significantly. In addition, the model leads to an obvious deduction that the ions that are highly active in terms of mobility will dominate the whole system in terms of resulting mechanical deformation direction and rate of deformation. Finally, to validate the model, simulations are conducted using the finite element method, and the outcomes are compared with the experimental data. Significant effort has been put forward to experimentally measure the key parameters essential for the validation of the model. The results show that the model developed is able to well predict the behavior of the actuator, providing a comprehensive understanding of charge dynamics in ionic polymer actuator with porous electrodes.

Actuators: Functionally Antagonistic Hybrid Electrode with Hollow Tubular Graphene Mesh and Nitrogen‐Doped Crumpled Graphene for High‐Performance Ionic Soft Actuators (Adv. Funct. Mater. 5/2018)

In article number 1705714, Il-Kwon Oh and co-workers design a functionally antagonistic graphene-mesh-nitrogen doped crumpled graphene hybrid electrode for a high-performance ionic soft actuator. The ionic soft actuator exhibits large bending actuation, fast rise time and low phase delay in a broad range of input frequencies due to the outstanding electro-chemo-mechanical properties of the hybrid electrode.

3D structured polypyrrole/reduced graphene oxide (PPy/rGO)-based electrode ionic soft actuators with improved actuation performance

We report a facile approach to fabricate 3D polypyrrole/reduced graphene oxide (PPy/rGO)-based electrodes for Nafion-based ionic soft actuators.

Self-standing cellulose nanofiber/poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate)/ionic liquid actuators with superior performance.

This paper describes new actuators with cellulose nanofiber/poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate)/ionic liquid (CNF/PEDOT:PSS/IL) structures. Devices containing these structures exhibit higher strain and maximum generated stress than those based on only PEDOT:PSS/IL. The new actuator system contains an electrode, which is an electrochemical capacitor, and which consists of both a faradaic capacitor (FC) and a small electric double-layer capacitor (EDLC), i.e., PEDOT:PSS. This combined capacitor plays the role of an FC and a base polymer, and the CNF skeleton is used in the place of carbon nanotubes (CNTs). This device therefore functions differently from traditional CNT/PVdF–HFP/IL actuators, which are only used as EDLC units and from PEDOT:PSS/vapor-grown carbon nanofibers (VGCF)/IL actuators, which are used as hybrid (FC and EDLC) units. The developed films are novel, robust, and flexible, and demonstrate potential as actuator materials for wearable energy-conversion devices. A double-layer charging kinetic model, which is similar to that previously proposed for PEDOT:PSS/CNT/IL actuators, is developed to explain the oxidation and reduction of PEDOT:PSS. This model successfully simulates the frequency-dependent displacement response of actuators.

A Computing Method to Determine the Performance of an Ionic Liquid Gel Soft Actuator.

A new type of soft actuator material—an ionic liquid gel (ILG) that consists of BMIMBF4, HEMA, DEAP, and ZrO2—is polymerized into a gel state under ultraviolet (UV) light irradiation. In this paper, we first propose that the ILG conforms to the assumptions of hyperelastic theory and that the Mooney-Rivlin model can be used to study the properties of the ILG. Under the five-parameter and nine-parameter Mooney-Rivlin models, the formulas for the calculation of the uniaxial tensile stress, plane uniform tensile stress, and 3D directional stress are deduced. The five-parameter and nine-parameter Mooney-Rivlin models of the ILG with a ZrO2 content of 3 wt% were obtained by uniaxial tensile testing, and the parameters are denoted as c10, c01, c20, c11, and c02 and c10, c01, c20, c11, c02, c30, c21, c12, and c03, respectively. Through the analysis and comparison of the uniaxial tensile stress between the calculated and experimental data, the error between the stress data calculated from the five-parameter Mooney-Rivlin model and the experimental data is less than 0.51%, and the error between the stress data calculated from the nine-parameter Mooney-Rivlin model and the experimental data is no more than 8.87%. Hence, our work presents a feasible and credible formula for the calculation of the stress of the ILG. This work opens a new path to assess the performance of a soft actuator composed of an ILG and will contribute to the optimized design of soft robots.

Functionally Antagonistic Hybrid Electrode with Hollow Tubular Graphene Mesh and Nitrogen‐Doped Crumpled Graphene for High‐Performance Ionic Soft Actuators

AbstractIonic soft actuators, which exhibit large mechanical deformations under low electrical stimuli, are attracting attention in recent years with the advent of soft and wearable electronics. However, a key challenge for making high‐performance ionic soft actuators with large bending deformation and fast actuation speed is to develop a stretchable and flexible electrode having high electrical conductivity and electrochemical capacitance. Here, a functionally antagonistic hybrid electrode with hollow tubular graphene meshes and nitrogen‐doped crumpled graphene is newly reported for superior ionic soft actuators. Three‐dimensional network of hollow tubular graphene mesh provides high electrical conductivity and mechanically resilient functionality on whole electrode domain. On the contrary, nitrogen‐doped wrinkled graphene supplies ultrahigh capacitance and stretchability, which are indispensably required for improving electrochemical activity in ionic soft actuators. Present results show that the functionally antagonistic hybrid electrode greatly enhances the actuation performances of ionic soft actuators, resulting in much larger bending deformation up to 620%, ten times faster rise time and much lower phase delay in a broad range of input frequencies. This outstanding enhancement mostly attributes to exceptional properties and synergistic effects between hollow tubular graphene mesh and nitrogen‐doped crumpled graphene, which have functionally antagonistic roles in charge transfer and charge injection, respectively.

Finite element analysis of ionic liquid gel soft actuator**Project supported by the National Natural Science Foundation of China (Grant Nos. 51538009 and 51605334) and the Natural Science Foundation of Shanghai Municipality, China (Grant No. 08002360285).

A new type of soft actuator material-ionic liquid gel (ILG), which consists of HEMA, BMIMBF4, and TiO2, can be transformed into gel state under the irradiation of ultraviolet (UV) light. In this paper, Mooney–Rivlin hyperelastic model of finite element method is proposed for the first time to study the properties of the ILG. It has been proved that the content of TiO2 has a great influence on the properties of the gel, and Young’s modulus of the gel increases with the increase of its content, despite of reduced tensile deformation. The results in this work show that when the TiO2 content is 1.0 wt%, a large tensile deformation and a strong Young’s modulus can be obtained to be 325% and 7.8 kPa, respectively. The material parameters of ILG with TiO2 content values of 0.2 wt%, 0.5 wt%, 1.0 wt%, and 1.5 wt% are obtained, respectively, through uniaxial tensile tests, including C10, C01, C20, C11, C02, C30, C21, C12, and C03 elements. In this paper, the large-scaled general finite element software ANSYS is used to simulate and analyze the ILG, which is based on SOLID186 element and nonlinear hyperelastic Mooney–Rivlin model. The finite element simulation analysis based stress-strain curves are almost consistent with the experimental stress–strain curves, and hence the finite element analysis of ILG is feasible and credible. This work presents a new direction for studying the performance of soft actuator for the ILG, and also contributes to the design of soft robot actuator.