Ionic Polymer Metal Composite Actuators Research Articles

Manufacturing a soft actuator/sensor integrated structure via multi-material direct writing processes technology

This paper describes a new actuator/sensor integrated structure fabricated via direct writing technique in which ionic polymer–metal composites (IPMCs) are used for actuators, and carbon nanotubes (CNTs) doped in polydimethylsiloxane (PDMS) were used for the sensors. First, we developed Nafion inks and CNT-PDMS composite inks with rheological properties tailored for direct writing. CNT-PDMS composite inks with a 4 wt% CNT concentration were three-dimensionally-printed in a Nafion structure to create an IPMC actuator structure; the CNT-PDMS composite was fabricated into a large array of strain sensors that were able to measure the bending deformation of the IPMC. A CNT-PDMS scaffold structure was semi-embedded in the Nafion structure via direct writing using CNT-PDMS composite inks with a 7 wt% CNT concentration to sense the external pressure and temperature of the IPMC actuator. Palladium and gold electrodes were applied on the surfaces of the Nafion/CNT-PDMS structure through an electroless plating and electroplating process, which enabled the three-dimensionally-printed Nafion/CNT-PDMS integrated structure to form an IPMC/CNT-PDMS integrated structure that could be actuated by voltage signals. The actuating/sensing integrated performances were tested to determine their feasibility and for using direct writing to create a smart structure that had both actuator and sensor functions and that could be used in a wide range of fields, such as micro-robotics, biomedicine, and medical science.

Axial Motion Characterization of a Helical Ionic Polymer Metal Composite Actuator and Its Application in 3-DOF Micro-Parallel Platforms

In this work, a helical ionic polymer metal composite (IPMC) was fabricated by thermal treatment in a mold with helix grooves. The axial actuation behaviors of the helical IPMC actuator were observed, and the electromechanical and electrochemical characteristics were evaluated. The experimental results showed that as the voltage increased and the frequency decreased, the axial displacement, axial force, and electric current of the actuator all increased. Compared with square wave and sinusoidal signals, the actuator exhibited the most satisfactory motion under the direct current (DC) signal. For the electrochemical test, as the scanning rate decreased, the gravimetric specific capacitance increased. Within a suitable voltage range, the actuator was chemically stable. In addition, we coupled the Electrostatics module, Transport of Diluted Species module, and Solid Mechanics module in COMSOL Multiphysics software to model and analyze the helical IPMC actuator. The simulation data obtained were in good agreement with the experimental data. Finally, by using three helical IPMC actuators as driving components, an innovative three-degree-of-freedom (3-DOF) micro-parallel platform was designed, and it could realize a complex coupling movement of pitch, roll, and yaw under the action of an electric field. This platform is expected to be used in micro-assembly, flexible robots, and other fields.

Port-Hamiltonian Modeling and IDA-PBC Control of an IPMC-Actuated Flexible Beam

In this paper, the infinite-dimensional port-Hamiltonian modelling and control problem of a flexible beam actuated using ionic polymer metal composite (IPMC) actuators is investigated. The port-Hamiltonian framework is used to propose an interconnected control model of the mechanical flexible beam and the IPMC actuator. The mechanical flexible dynamic is modelled as a Timoshenko beam, and the electric dynamics of the IPMCs are considered in the model. Furthermore, a passivity-based control-strategy is used to obtain the desired configuration of the proposed interconnected system, and the closed-loop stability is analyzed using the early lumped approach. Lastly, numerical simulations and experimental results are presented to validate the proposed model and the effectiveness of the proposed control law.

A hyperelastic porous media framework for ionic polymer-metal composite actuators and sensors: thermodynamically consistent formulation and nondimensionalization of the field equations

Ionic polymer-metal composites (IPMCs) are smart materials that exhibit large deformation in response to small applied voltages, and conversely generate detectable electrical signals in response to mechanical deformations. The study of IPMC materials is a rich field of research, and an interesting intersection of material science, electrochemistry, continuum mechanics, and thermodynamics. Due to their electromechanical and mechanoelectrical transduction capabilities, IPMCs find many applications in robotics, soft robotics, artificial muscles, and biomimetics. The current literature is sparce in multiphysics models that account for large deformations, coupled ion-solvent transport, the porous structure of the membrane, and lossy electrodes under finite-strains. This study addresses this by developing a new, hyperelastic porous media modeling framework for IPMCs using the principles of continuum thermodynamics and multiphasic materials. This framework compactly captures the broadest strokes of IPMC modeling methodologies, summarizing them in the form of general constitutive requirements placed on thermodynamic potential describing the polymer skeleton. A simple IPMC model is derived under this framework which captures the finite-strain deformation of a hyperelastic material, accounting for the coupled ion-solvent transport through the porous polymer, and resistive electrodes deforming with the skeleton. The resulting governing equations are further cast into a generalized nondimensional formulation, and an expansive set of unique dimensionless -groups are derived for IPMC actuator and sensor devices. This new framework and the nondimensional formulation lay the groundwork for future research and an expansive characterization of IPMC transduction phenomena via dimensional analysis.

Bending response optimization of an ionic polymer-metal composite actuator using orthogonal array method

In the recent past decades, Ionic polymer-metal composites (IPMCs) have been researched in detail due to their flexible characteristics and light in weight. IPMC is most well-known electro-active polymers (EAPs), which can be used in the medical applications, biomimetic, micro robotic actuators, artificial muscles, dynamic sensors and underwater propulsions. Due to their large bending actuation with low voltage sources and biocompatibility, IPMCs are receiving more and more attention to many researchers in recent days. To reduce the experimental costs, the principal factors which are responsible for more bending response are designed in this work using Taguchi orthogonal array method. Multiple process parameters include input voltage and thickness of the fabricated Ag-IPMC actuator was optimized to produce a maximum bending response. Experiments were carried out using L9 orthogonal array to recognize the optimal process parameters. In the optimized conditions, the IPMC actuator exhibited a noteworthy improvement in bending deflection compared to the normal conditions. Under the application of 1.0 V, the bending response increases around 30% in the optimised condition compared to normal IPMC actuator. This study lays a solid foundation for developing high performance IPMC actuator where higher bending displacement is required i.e., in the robotic and biomimetic application with a very small input voltage.

Facile and effective repair of Pt/Nafion IPMC actuator by dip-coating of PVP@AgNPs

Ionic polymer metal composite (IPMC) always takes big risks of electrode cracking and peeling, which lead to energy wasting, waterloss, and uneven electric field distribution, thus hamper its commercial applications. To address this issue, we propose a facile and effective technique to repair the electrode fatigue by coating polyvinylpyrrolidone (PVP) encapsulated Ag nanoparticles (PVP@AgNPs) on the long-term used IPMC surface. To improve the electrochemical stability, the silver nanoparticles (Ag NPs) with a diameter of ∼34 nm are encapsulated by a 1.3 nm thick PVP film, thus forming a shell–core structure to resist corrosion from the electrolyte solution. Physiochemical investigations reveal that, PVP@AgNPs closely attach to the interior and exterior surfaces of the original Pt nanograin electrode, thus refreshing its electronic conductivity; the repaired IPMC actuator exhibits better electromechanical properties compared to its precursor actuator: 7.62 folds in displacement output, 9.38 folds in force output, and 9.73 folds in stable working time.

Artificial accommodating intraocular lens powered by an ion polymer-metal composite actuator.

The current method of controlling the focus of an accommodating intraocular lens is based on ciliary muscle contraction and cannot be used in older patients with presbyopia. We aimed to develop a dynamically accommodating intraocular lens powered by a membrane-shaped ion polymer metal composite actuator that is thin enough to be inserted in the eye. This study addresses two key problems identified in our previous accommodating intraocular lens prototype: the lack of repeatability due to the use of swine lenses instead of artificial lenses and the occurrence of a sixth order aberration. Thus, we present a new accommodating intraocular lens design and a method to transfer energy to actuators. To accommodate lens deformation and depth of focus, we used a membrane-shaped ion polymer metal composite actuator, thin enough to be inserted in the eye, and used an artificial silicone lens. To prevent the sixth order aberration, we included a ring between the ion polymer metal composite actuator and the lens. Different voltage patterns were applied to the IPMC actuator and changes in focus were observed. We were able to obtain repeatability and prevent the sixth order aberration. The dioptric power changed to ±0.23 D when ±1.5 V was used; however, at >1.5 V, a large accommodating range occurred, in addition to astigmatic vision. Thus, we have developed a novel prototype that is completely artificial, allowing reproducible and repeatable results. Visual accommodative demands were successfully met; however, although astigmatic vision was lessened, it was not completely eradicated.

Hybrid tail excitation for robotic fish: Modeling and performance analysis

We investigate the potential of a novel hybrid excitation of the tail for a robotic fish. The tail consists of an ionic polymer-metal composite (IPMC) beam excited at its base. A model of the beam dynamics under base excitation, IPMC actuation mechanism, and hydrodynamic forces is derived. Lighthill's theory of elongated bodies is used to estimate the forward speed from the tail's oscillations and associated power for different tail dimensions and actuation configurations. The model of the robotic fish propelled by only an IPMC actuator is validated using previously reported experimental frequency responses. Our results show that incorporating a small-amplitude base excitation has the potential to significantly improve the forward speed of the robotic fish over a broader range of frequency excitations while consuming minimal additional power. The results also show that this improvement is very significant at specific aspect ratios.

Highly efficient structure design of bending stacking actuators based on IPMC with large output force

Ionic electroactive polymers (iEAPs) are electroactive polymers that have been used as artificial muscles and have broad application potential. Although iEAPs show large bending deformation under low electric stimulus, the low output force significantly restricts their application. Based on an ionic polymer–metal composite (IPMC) actuator, we proposed a bending stacking structure principle which adapts the bending deformation cooperation conditions. According to the principle, the width stacking structure and thickness stacking structure were designed that can improve the output force, maintain a high stacking efficiency, and eliminate degradation to the deformation ability. The advantages of the proposed stacking structures were identified using theoretical analysis and experimental verification based on IPMC actuators. Compared with an ultra-wide actuator, the wide stacking structure can resist undesirable width deformation. Compared with a traditional stacking structure, the proposed thickness stacking structures solved the bending deformation coordination problem to get a high stacking efficiency and significantly enhance the output force.

Printing ionic polymer metal composite actuators by fused deposition modeling technology

ABSTRACT In this work, we printed a Nafion precursor membrane by fused deposition modeling (FDM) rapid prototyping technology and further fabricated IPMCs by electroless plating. The ion-exchange capacity of the Nafion membrane was tested, and the morphology of IPMCs was observed. The electro-mechanical properties of IPMCs under AC voltage inputs were studied, and grasping experiments were performed. The results show that the Nafion membrane after hydrolysis has a good ion-exchange ability and water-holding capacity. SEM observed that the thickness of the IPMC’s electrode layer was about 400 nm, and the platinum layer was tightly combined with the substrate membrane. When using a square wave input of 3.5 V and 0.1 Hz, the maximum current of IPMCs reached 0.30 A, and the displacement and blocking force were 7.57 mm and 10.5 mN, respectively. The new fabrication process ensures the good driving performance of the printed IPMC. And two pieces of IPMCs can capture the irregular objects successfully, indicating the feasibility of printing IPMCs by FDM technology. This paper provides a new and simple method for the fabrication of three-dimensional IPMCs, which can be further applied in flexible grippers and soft robotics.

A 3D analytical ion transport model for ionic polymer metal composite actuators in large bending deformations

Ionic polymer metal composites (IPMCs) are a kind of soft electroactive polymer composites. An IPMC strip commonly has a thin polymer membrane coated with a noble metal as electrodes on both sides. Whenever an electric voltage is applied to the IPMC, it bends and whenever it is deformed, a low voltage is measurable between its electrodes, hence IPMC is an actuator as well as a sensor. They are well known for their promising features like low density, lightness, high toughness and remarkable stimulus strain, also, they have the potential for low-voltage operation while exhibiting acceptable large bending deformation. In this paper, a three-dimensional (3D), dynamic and physics-based model is presented analytically and experimentally for IPMC actuators. The model combines the ion transport dynamics within the IPMC and the bending dynamics of it as a beam under an electrical stimulation. In particular, we present an analytical model to create a relation between the input voltage and the output tip displacement of an IPMC actuator for large bending deformations. Experimental results show that the proposed model captures well the tip displacement.

Liquid Metal Logic for Soft Robotics

While there are many soft matter sensing and actuation technologies, there is far less choice when it comes to soft material devices for control and computation. One solution is the Soft Matter Computer (SMC) which can perform both analogue and digital computations in soft materials. This computer processes a fluidic input pattern consisting of alternating regions of conducting and insulating fluids into an electronic output signal. However, the use of salt water as the conductive fluid means that the Soft Matter Computer has high electrical resistance and requires an AC voltage, making untethered operation impractical. In this letter, we introduce the liquid metal Soft Matter Computer (LM-SMC), which uses galinstan as an alternative conductive fluid. We show that by switching to a liquid metal-sodium hydroxide fluidic input, we reduce the electrical resistance of the SMC by three orders of magnitude, allowing operation at DC voltages of 2 Volts and under. We characterise the stability of the liquid metal input patterns and demonstrate the potential of the LM-SMC by using it to control bipolar ionic polymer metal composite and shape memory alloy actuators. By enabling fully soft computation and control of multiple actuators from a single low voltage DC source, the LM-SMC enables a new class of intelligent and untethered soft machines.

Investigation of Ionic EAP Actuators with Metal and Polymer Electrodes in Aqueous Medium

Electroactive polymers (EAP) are promising materials for creating electromechanical transducers. Among ionic EAP, ionic polymer-metal composites (IPMC), which are an ion-exchange membrane with metal electrodes on both sides, have been widely spread and well studied. The evolutionary development of IPMC results in ionic polymer-polymer composites (IP2C), in which polymer electrodes are used. To obtain IPMC actuators with platinum electrodes, the method of chemical reduction from the salt solution was chosen, and to obtain IP2C actuators with PEDOT electrodes, the method of in situ polymerization of the monomer on the membrane surface was chosen. Samples of 2x0.5 cm in size based on the MF-4SK membrane with a thickness of 290 μm were preliminarily kept in deionized water (H+ form) and in 0.1 M CuSO4 aqueous solution (Cu2+ form), after which their performance was studied in air, in deionized water, as well as in aqueous solutions of CuSO4 and NaCl. When applying a DC voltage and a sine wave AC voltage, a decrease in the maximum displacement and peak-to-peak displacement of the IPMC actuators and IP2C actuators with an increase in the ionic strength of the liquid was observed, except for the case of the IPMC actuator operation in CuSO4 aqueous solutions. In all considered media, the IPMC actuators and IP2C actuators in Cu2+ form displaced more strongly than the corresponding samples in H+ form, except for the IP2C actuators in deionized water. The largest peak-to-peak displacement was demonstrated by the IPMC actuators in Cu2+form when operating in air (5 mm) and the IP2С actuators in H+ form when operating in deionized water (8.4 mm).

Control of an IPMC Soft Actuator Using Adaptive Full-Order Recursive Terminal Sliding Mode

The ionic polymer metal composite (IPMC) actuator is a kind of soft actuator that can work for underwater applications. However, IPMC actuator control suffers from high nonlinearity due to the existence of inherent creep and hysteresis phenomena. Furthermore, for underwater applications, they are highly exposed to parametric uncertainties and external disturbances due to the inherent characteristics and working environment. Those factors significantly affect the positioning accuracy and reliability of IPMC actuators. Hence, feedback control techniques are vital in the control of IPMC actuators for suppressing the system uncertainty and external disturbance. In this paper, for the first time an adaptive full-order recursive terminal sliding-mode (AFORTSM) controller is proposed for the IPMC actuator to enhance the positioning accuracy and robustness against parametric uncertainties and external disturbances. The proposed controller incorporates an adaptive algorithm with terminal sliding mode method to release the need for any prerequisite bound of the disturbance. In addition, stability analysis proves that it can guarantee the tracking error to converge to zero in finite time in the presence of uncertainty and disturbance. Experiments are carried out on the IPMC actuator to verify the practical effectiveness of the AFORTSM controller in comparison with a conventional nonsingular terminal sliding mode (NTSM) controller in terms of smaller tracking error and faster disturbance rejection.

Ionic polymer–metal composite actuators driven by methylammonium formate for high-voltage and long-term operation

Ionic liquids (ILs) have been exploited as the internal solvent in the development of ionic polymer–metal composite (IPMC) actuators capable of long-term operation. However, a major issue is that most IPMCs impregnated with conventional ILs are yet to be practically applied due to insufficient actuation performance. Herein, methylammonium formate (MAF), an IL with low viscosity, small ionic size, high ionic conductivity, and excellent electrochemical stability, is introduced as a new internal solvent for IPMCs to achieve both excellent operation characteristics and long-term stability. The actuation characteristics of a series of IPMCs impregnated with four types of ILs, including MAF, were analyzed. The MAF-based IPMCs had more than twice the displacement and three-fold higher blocking forces compared with other IL-based IPMCs. Moreover, unlike conventional water-driven IPMCs, the MAF-driven IPMCs operated even at high voltages (4V) without solvent loss and electrolysis. Their initial displacement remained above 81% even after operating for 2160 cycles (6h). This simple change of media in IPMC actuators is expected to advance the design of IPMCs with improved displacement, force generation, and stability.

Energy-Based Modeling of Ionic Polymer–Metal Composite Actuators Dedicated to the Control of Flexible Structures

This article deals with the control-oriented energy-based modeling of ionic polymer–metal composite (IPMC) patches using multiscale infinite dimensional port-Hamiltonian formulations and Lagrange multipliers. Inspired by the work of Nishida <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">et al.</i> in 2012, but considering different assumptions, this article focuses on the constraints arising from the coupling between the polymer gel and the compliant mechanical structure of the actuator, under the quasi-static mechanical assumption for the gel, leading to a constrained port-Hamiltonian system. The geometric structure of the overall system and the associated energy balance are derived. The proposed energy-based model of the IPMC actuator allows deriving controllers via energy-based control design methods with a clear physical interpretation. The proposed actuator model is further discretized in space using a structure preserving finite difference method. The Lagrange multipliers are eliminated using coordinate projections. Simulations are compared with experimental results. With proper discretization numbers, our model is consistent with the physical system. Finally, Lagrange multipliers are exploited to connect the actuator to a 2-D flexible structure stemming from the modeling of a flexible endoscope.

Modeling actuation and sensing in ionic polymer metal composites by electrochemo-poromechanics

Ionic polymer metal composites (IPMCs) consist of an electroactive polymeric membrane plated with metal electrodes. They hold promise as actuators and sensors for soft robotics and biomedical applications. Their capabilities ensue from the motion, within the membrane, of a fluid phase consisting of ions dispersed in a solvent. Toward a thorough understanding of IPMC multiphysics, we propose a large deformation theory combining electrochemistry and poromechanics. Namely, we modify the theory recently developed by Porfiri’s group by introducing the transport of the solvent, whose redistribution determines the volumetric deformation of the fluid-saturated membrane, and we further account for the cross-diffusion of solvent and ions. In actuation, the imposed voltage drop across the electrodes triggers ion migration, such that the solvent is transported toward the cathode by electro-osmosis. This determines the initial bending toward the anode; then, back-relaxation occurs because of both the solvent counter-diffusion and the asymmetric redistribution of ions near the electrodes. In short-circuit sensing, the applied load triggers solvent motion, such that ions are mainly transported toward the cathode by convection with the solvent. This determines charge accumulation; then, ion counter-diffusion leads to a decrease of the charge stored at the electrodes. We demonstrate that these behaviors can be predicted by the proposed theory on the basis of relevant finite element benchmarks. Additionally, our analysis encompasses the assessment of the role of the membrane elastic moduli in the counter-diffusion of solvent and ions in IPMC actuation and sensing.

A fractional order model for electrochemical impedance of IPMC actuators based on constant phase element

The accurate modeling of electrical impedance over a wide range of frequency is essential for precise dynamic modeling and control problems of Electroactive Polymer (EAP) actuators. Recently, fractional order modeling has attracted more attention due to the high accuracy. This paper deals with a fractional order electrical impedance model and its identification procedure for a class of EAP actuator named Ionic Polymer Metal Composite (IPMC). To take IPMC’s fractional characteristic into account, constant phase element (CPE) is used to construct a model structure according to Electrochemical Impedance Spectroscopy (EIS). By employing the Levy’s method in combination with genetic optimization algorithm, the unknown parameters of the resulting fractional transfer function are identified. Finally the proposed model is verified, by comparing with experimental EIS data. The results show that the fractional order model has high accuracy for representing the electrical impedance of IPMC actuator. The proposed modeling procedure is general and can also be used for any type of EAPs.

Fabrication of Macroporous Nafion Membrane from Silica Crystal for Ionic Polymer-Metal Composite Actuator

Nafion membrane with macropores is synthesized from silica crystal and composited with Pt nanoparticles to fabricate macroporous ionic polymer-metal composite (M-IPMC) actuator. M-IPMC shows highly dispersed small Pt nanoparticles on the porous walls of Nafion membrane. After the electromechanical performance test, M-IPMC actuator demonstrates a maximum displacement output of 19.8 mm and a maximum blocking force of 8.1 mN, far better than that of IPMC actuator without macroporous structure (9.6 mm and 2.8 mN) at low voltages (5.8–7.0 V). The good electromechanical performance can be attributed to interconnected macropores that can improve the charge transport during the actuation process and can allow the Pt nanoparticles to firmly adsorb, leading to a good electromechanical property.

Developing next generation ionic polymer–metal composite materials: perspectives for enabling robotics and biomimetics

AbstractIn this article, we present recent advancements in Ionic Polymer‐Metal Composite (IPMC) technology. The current trajectory of the research shows immense promise for a future with highly capable IPMC actuators and sensors being used in a wide range of robotic, soft‐robotic, and biomimetic technologies.. © 2020 Society of Industrial Chemistry