Electromechanically Active Polymers

Smart hydrogels are soft polymer particles that swell and deswell by taking up water according to external stimuli. After a general introduction, we extensively discuss the thermodynamics that governs the swelling equilibrium of neutral and polyelectrolyte gels. The kinetics of gel swelling is then presented in two models: The Tanaka-Fillmore model that is based on pure mechanics and the more advanced model by Doi which includes thermodynamic processes as the reason for swelling and deswelling. In the following section, the possible sensitivities with which smart hydrogels have been equipped are discussed. Finally we outline the current challenges of fabricating hydrogels with improved mechanical properties.

Smart hydrogels, exhibiting response to various stimuli such as temperature, pH, light, electric field, etc., have been extensively explored due to their high potential in different areas ranging from actuators to biomedical applications. A number of synthetic pathways have been developed to synthesize hydrogels with desired chemical structure and to tune the mechanical properties and the swelling degree of these switchable materials. These synthetic approaches also provide the possibility of incorporating various functionalities inside hydrogel network and thus in turn controlling their response. The available methods for the fabrication of various types of functional hydrogels and their characteristic properties will be reviewed in this chapter.

Polyelectrolyte gels, often referred as ionic polymer gels are quite attractive intelligent materials. They consist of a solid phase, i.e., a polymer network with fixed charges, and a liquid phase with mobile ions. Typically these gels are immersed in a solution bath. An application of different kinds of stimuli – e.g., chemical (change of salt concentration or pH), thermal, magnetical, or electrical – leads to a new equilibrium between the different forces, such as osmotic pressure forces, electrostatic forces, and (visco-)elastic forces. This occurs in cooperation with absorption or delivery of the solvent resulting in a (local) change of volume.In the present chapter, an overview over different modeling alternatives for chemically and electrically stimulated polyelectrolyte gels, placed in a solution bath, are given.First, the statistical theory – a theory in which only the global swelling is of interest – is reviewed. By refining the scale, two different mesoscopic models are presented: first, the chemo-electro-mechanical transport model and second, a continuum model based on porous media. These models are capable of describing the changes of the local variables: concentrations, electric field, and displacement. So, e.g., by the application of an electric field, a bending movement of the polymer gel can be realized which is in excellent correlation with experimental investigations.Concluding, the statistical theory is an efficient method to easily model the chemical stimulation of polyelectrolyte gels and the two continuum-based formulations are capable of simulating both chemically and electrically induced swelling or bending. So, they are an excellent technique to model hydrogel bending actuators or grippers.

Stimuli-responsive hydrogels display a variety of interesting features that make them ideal candidates for technological applications. The applicable stimuli range from temperature, pH, and (bio)chemical species to electric fields and light; some materials can even be controlled by multiple stimuli. Hydrogel materials can be synthesized by a single-step free-radical polymerization, and various methods to introduce them into a final system are discussed. This chapter covers applications of smart hydrogels in various (micro-)systems starting from transparent conductors over stimuli-sensitive optical components and drug delivery devices for medical applications. Intensively discussed are microfluidic applications starting from single components as thermostats, chemostats, and valves toward complex integrated systems. Finally, we outline the implications of autonomous microfluidic devices to the field of chemical information processing.

Electromechanically active polymers (EAP) show great potential for many actuator applications. In this context, hydrogels which are also considered as active polymers have shown also actuator and sensor applications due to their volume phase transition. Nevertheless, in the general term of electromechanically active polymers there is not an exact definition about what is an active polymer. Hydrogels can be considered as active polymer materials not only because of their volume phase transition but also due to their electrical and dielectric properties depending on their internal or chemical modification. The most spread definition of hydrogels is that they are soft and wet materials which show very intriguing properties regarding their volume phase transition. Applications of hydrogels are tightly restricted due to their relative mechanical weakness. In the past 10 years a lot of research has been done in the field of modifying the mechanical properties of hydrogels in order to adapt these materials to daily life requirements. They have been used as sensors and actuators in many fields of science and engineering including microfluidics and biomedicine. In this chapter, we briefly present the main properties of hydrogels, some of the methods used to characterize them as well as the principal applications from an engineering and general point of view. This chapter could be used as a general introduction to the topic of hydrogels, more specifically thermal responsive ones, and also represents an opportunity for all those who want to enter to the field of hydrogels.

This chapter reviews the fundamentals of ionic polymer–metal composites (IPMCs), which are used for sensors and actuators. First, the basic structure of IPMCs is described, and a brief review of their development is provided. Then, the configurations of various devices based on them, including those of the electrode materials and ionic polymers, are described. Then, the basic techniques used to characterize IPMCs are described. In the next section, electromechanical models and, in particular, a physics-based model of an IPMC actuator are discussed. Finally, electrochemical models, including an Alternating Current impedance equivalent circuit model and an electrode reaction model, and a mechanical model are discussed.

This chapter provides an overview of the materials used for manufacturing IPMC actuators and sensors. Recently, considerable effort has been put into investigating various electrode materials and ionic polymer membranes to increase the actuation performance of IPMC and overcome some of the shortcomings to improve their reliability and stability. Various metallic and nonmetallic electrode materials with notable electrochemical and electromechanical properties have been considered for IPMC electrodes. Herein, some of the more commonly used noble metal-based electrodes and recently introduced nonmetallic  conductive material (such as transition metal oxide and various carbon derivatives)-based electrode designs along with specific manufacturing approaches are introduced, highlighting their key aspects and design challenges. Also, several representative ionic polymer membranes used for IPMC fabrication such as sulfonated aromatic hydrocarbon, block copolymers, biopolymers, and nanocomposites capable of providing higher electro-chemo-mechanical properties have been investigated. Herein, more recently developed membrane materials including self-assembled sulfonated polyimide block copolymers, functional cellulose-based biopolymers, and graphene-reinforced nanocomposites are introduced, considering their main advantages, facile synthesis process (such as freeze drying method, all-solution process, and electrospinning technique) and actuation performance.

The primary objectives of modeling IPMCs are to facilitate researchers with IPMC fabrication and performance prediction, to better understand the underlying principles of IPMC actuation and sensing, and to develop functional real-time control systems for IPMC devices. This chapter provides an overview of IPMC modeling with specific emphasis on physics-based models and control models. The underlying governing equations are presented and methods of solving are discussed.

IPMC actuators have number of advantages for the applications such as low drive voltage (less than 3 V), relatively high response (100 Hz), large displacement, soft material, capability of activation in water or in wet condition, possibility to work in dry condition, durability, and easy to miniaturize. In recent years, a great number of applications based on the IPMCs have been carried out by many workers. In this chapter, IPMC research on biomedical applications, biomimetic robotics, sensor/actuator integration, and energy harvesting is reviewed.

This chapter describes how to start experiments with ionic polymer–metal composite (IPMC) actuators. In the first part, a fabrication of IPMC actuator element is summarized. In the next part, how to setup a measurement system of IPMC actuator and test the actuator performance is described. In the last part, a control method of IPMC actuator is discussed. From the information in this chapter, experiments with IPMC actuators can be started.Ionic polymer-metal composites (IPMCs)fabrication of

Films of conducting polymers (CPs) follow reversible volume variations by electrochemical oxidation/reduction in liquid electrolytes: the actuation principles are presented. Actuators, or artificial muscles, transducing those volume variations into large macroscopic movements are presented here. The same reaction gives exchange of anions, or cations, and opposed film volume variations (here described) from different families of CPs. Transduction from the small local volume changes to macroscopic movements has required different designs and structures. A good control of the movement requires a good theoretical description. Two different approaches, as mechanical-based devices or as electrochemical devices, are presented. Moreover the electrochemical reaction driving the muscles movement also senses any physical or chemical variable acting on the reaction energy. The sensing principle is presented giving dual sensing-actuators: an actuator, a mechanical sensor, a chemical sensor, a thermal sensor, and an electrical sensor work simultaneously, driven by the same reaction, in a physically uniform device. Only haptic muscles from mammals are dual actuating-sensor originating proprioception: the mammal brain is aware of position, movement rate and direction, trailed weight, muscle fatigue state, or working temperature during movements. The artificial proprioceptive equations, attained from electrochemical, mechanical, and polymeric principles, allow an easy description and control of the multi-tool device.

This chapter focuses on device configurations based on conjugated polymer transducers. After the actuation and sensing configurations in the literature are presented, some successful device configurations are reviewed, and a detailed account of their operation principles is described. The chapter is concluded with critical research issues. With reference to the significant progress made in the field of EAP transducers in the last two decades, there is an increasing need to change our approach to the establishment of new device configurations, novel device concepts, and cutting-edge applications. To this aim, we should start from the performance specifications and end up with the material synthesis conditions and properties which will meet the performance specifications (top-to-down approach). The question should be “what electroactive material or materials can be used for a specific purpose or application,” rather than looking for an application or a device concept suitable to the unique properties of the EAPs and transducers already made of these materials. The field is mature enough to undertake this paradigm change.

In this chapter, first some basic principles of photolithography and general microfabrication are introduced. These methods have been adapted to fit the microfabrication of conducting polymer actuators, resulting in a toolbox of techniques to engineer microsystems comprising CP microactuators, which will be explained in more detail. CP layers can be patterned using both subtractive and additive techniques to form CP microactuators in a variety of configurations including bulk expansion, bilayer, and trilayer actuators. Methods to integrate CP microactuators into complex microsystems and interfaces to connect them to the outside world are also described. Finally, some specifications, performance, and a short introduction to various applications are presented.

This chapter outlines the various methods that have been developed in the past three decades to characterize the electroactive performance of conducting polymers (CP) to provide fundamental metrics such as strain, strain rate, stress, force, modulus of elasticity, and work capacity. In addition to providing metrics, these characterization techniques have served as valuable tools for studying CPs, providing a greater understanding of the actuation process, optimizing synthesis conditions, and geometric parameters for optimal device performance. The issues associated with the determination of metrics and the need for standardization are discussed.

Conducting polymer actuators and sensors employ the coupling between electrochemistry and mechanics. The aim of this chapter is to equip the reader with the basic models needed in order to assess the feasibility of using conducting polymers, design devices, describe the device response, and predict behavior. The chapter begins with an overview of the basic observations and phenomena on which physical models are based, briefly describes models employed, and gives some references to literature presenting the models. The use of system identification techniques is then presented, and it is shown that these can very effectively be employed to create and validate models, as well as extract physical phenomena and enable predictions.

Artificial muscles are the longtime dream of human being to replace the existing engines, motors, and piezoelectric actuators because of the low-noise, environment-friendly, and energy-saving actuators (or power force generators). This chapter describes applications of conducting polymers (CPs) to EAPs such as bending actuators, microactuators, and linear actuators. The bending actuators were applied to diaphragm pumps, swimming devices, and flexural-jointed grippers with the trilayer configurations. On the other hand, the microactuators have the advantage of short diffusion times and thus fast actuation. Since the CP actuators operate in any salt solutions, such as a saline solution, cell culture media, and biological liquid, the PPy microactuators have potential applications in microfluidics and drug delivery, cell biology, and medical devices. Furthermore, the linear actuators were developed for the applications to the Braille cells, artificial muscles for soft robots.

The goal of this chapter is to give initial practical information on experimentation with conducting polymer based actuators. The chapter is dedicated to all kind of audience, from confirmed researchers starting in the field of EAPs to teachers wanting to propose practical works on such materials. It will focus on bi(tri)layer bending actuators operating in liquid electrolyte and on two kinds of trilayer bending devices able to operate in open-air. Experimental details will be provided on how to fabricate each of them step by step giving examples. Electrical and electromechanical characterizations will be also described from basic measurements to some more complicated concepts.

The varieties of different carbon structures offer a great basis for EAPs. They are most widely used electrode materials in low-voltage actuation generation. So far, carbon nanotubes (CNTs) have gained unrivaled attention. Their popularity is reflected in a production capacity that presently exceeds several thousand tons per year. In addition to carbon nanotubes, other electrically conductive carbon allotropes that contain electron-rich conjugated double bonds can be also successfully used as actuator electrodes. The following chapter focuses on most common carbon material-based EAPs. More specifically, on graphite which consists of multiple stacked conducting layers of graphene, on porous amorphous carbons, on fullerenes (C60, C70) and on carbon nanotubes (CNT). The porous amorphous carbons will include activated carbons, carbon nanofibers and filaments, carbide-derived conductive carbons synthesized from different precursors, and carbon aerogels. The chapter introduces these carbons as independently standing actuators or as materials for actuator electrodes and presents different approaches toward actuator fabrication. The ability to manipulate the morphology of these carbons, thus tuning the mechanical performance of the actuators, is also discussed. Essential differences in electrochemical, electro(chemo)mechanical properties and overall device configuration of carbon material-based actuators compared to other EAPs will be revealed.Electrochemically-driven carbon based materialsresearch motivation and application

Inserting twist into carbon nanotube fibers has enabled novel actuation mechanisms that result in both rotating and linear translational movement. Heating the carbon nanotube fibers generates a volume increase that drives a partial untwist of the fiber. The process is reversible upon cooling with the aid of a return spring mechanism. The actuation can be magnified by incorporation of a guest material, such as paraffin wax. The torsional stroke and/or torque can be used to perform useful work, such as the rotation of an attached paddle for fluid mixing. Various device configurations are possible and can be modeled by torsion mechanics. Tensile contraction also occurs during fiber untwist and can be greatly magnified by overtwisting the yarns to form spring-like coils. The high conductivity of the carbon nanotube yarns facilitates the convenient electrical heating and control giving high stroke, long-life, and rapid tensile and torsional actuation. This chapter summarizes the methods to produce guest-filled carbon nanotube yarns and the configurations that can be employed to generate either torsional or tensile actuation.

Carbon is a distinctive electrode material for actuators, as it is available in a wide variety of forms, ranging from monoliths to powders, fibers, and yarns. The diversity in the properties of different carbonaceous materials is also expressed in a variety of actuation mechanisms. This chapter considers two classes of actuators – electrochemically and electrothermally driven actuators – which both make use of carbonaceous materials as active elements. In both of the listed types of actuators, carbon is especially advantageous because of its chemical and thermal inertness and also because of its high intrinsic electrical conductivity. The working principles of different actuators, having carbonaceous electrodes, are drastically different and so are the optimization criteria for selecting a particular type of carbon for a particular type of actuator. This chapter is to explain some important practical considerations for successful experimentation with the carbon-based actuators. Special attention is bestowed on the choice of materials and the choice of appropriate electrical driving signal. The effects caused by the ambient environment are discussed. Finally, a selection of commonly used characterization methods is suggested.

In this chapter, fundamental aspects of piezoelectricity and electrostriction in dielectric materials and especially in polymers will be outlined. In order to make the introduction into the subject easier to access, basic and schematic ways of describing the complex matter have been chosen instead of an elaborate or comprehensive theoretical approach. For more detailed and more precise information, the interested reader is referred to the large volume of available original and review literature and to other relevant chapters of this book.

In this chapter a brief introduction on piezoelectric polymers and electrostrictive polymers is presented, and some representative polymers are given with their essential properties. The information should provide knowledge for readers to know the origin of modern research on piezoelectric and electrostrictive polymers and recent advances in this area.

This chapter presents a brief overview on sensor and transducer applications of piezoelectric and electrostrictive polymers. Piezoelectric and electrostrictive polymers are smart electromechanical materials which have already found commercial applications in various transducer configurations. Novel applications may arise in the emerging fields of autonomous robots, electronic skin, and flexible energy generators. This chapter focuses on recent device demonstrations of piezoelectric and electrostrictive polymers in these novel fields of research to stimulate and to facilitate the exchange of ideas between disciplines. The applications considered include piezoelectric sensors for electronic skin, piezoelectric loudspeakers and transducers for mechanically flexible energy harvesters, as well as electrostrictive transducers for haptic feedback in displays.

Electrets are dielectric materials capable of quasi-permanently storing electric charges at their surface or in their bulk. This chapter presents a brief history of electret research, followed by a classification and introduction of the most important electret materials. The chapter also discusses ferroelectrets and recent developments in charge stability.

Recent progress relating to polymer electret and ferroelectret materials is reviewed. As for polymer electret materials, the development is described in two aspects: (i) Modified conventional polymer electret materials with improved electret properties. The improvement of the electret properties is achieved by incorporating suitable additives, by blending different polymer compounds, or by modifying with certain chemicals. Sometimes, the properties can be further enhanced by physical aging. (ii) Newly introduced high-performance polymer electrets. Parylene HT® and CYTOP are two examples. They can not only retain high surface charge densities but also show exceptional high temperature stability. Moreover, they are compatible with MEMS technology and therefore are particularly attractive for applications in micro power electret generators.The research of ferroelectret, as a relatively new branch in the field of electret, has been advanced significantly in recent years. A considerable number of cellular polymer foams and polymer film systems containing internal cavities have been developed and identified as ferroelectrets. Following the early example of cellular polypropylene (PP) ferroelectret, cellular foam ferroelectrets have been developed from polyesters (polyethylene terephthalate PETP and poly(ethylene naphthalate) PENP), cyclo-olefin copolymer (COC), and fluoroethylenepropylene (FEP). The cellular structures, formed by techniques such as stretching filler-loaded polymer melt and foaming with supercritical CO₂, can be adjusted and optimized with gas-diffusion expansion process. Besides, the number of ferroelectrets of polymer film systems with internal cavities is rapidly increasing. These are layer structures, composed of hard (solid) and soft (highly porous) polymer layers, and polymer film systems containing regular cavities. Polytetrafluoroehylene (PTFE) (solid or porous) and polycarbonate (PC) are also added to the list of candidate materials for making this type of ferroelectrets. These exciting developments significantly enlarge the range of functional space-charge polymer electrets and bring forth numerous novel applications.

The successful development and optimization of EAPs requires quick, reliable, and precise characterization techniques for all relevant materials and device properties. This chapter gives a concise overview about specific techniques for the characterization of mechanical, electrical, and electromechanical properties of polymeric electrets with some emphasis on ferroelectrets.

Polymer electret and ferroelectrets have unique characteristics such as electrostatic transduction without external voltage, light weight, flexibility, and so on. Their most successful applications are microphones and air filters, but various other types of devices have also been proposed. In this chapter, after giving overview of their applications, developments of acoustic devices and power generators/energy harvesters are discussed.

Ferroelectrets are a new class of electroactive polymers. These materials are highly heterogeneous and represent, in essence, a composite made of nonpolar polymer and air. The surfaces of inner cavities obtained via foaming, inflation, or templating can be charged with dielectric barrier discharges, thus creating large macroscopic dipoles with high piezoelectric response. The stability of the piezoelectricity is determined by how well the polymer material can retain these charges, i.e., by its electret properties. Thus, thermal and temporal stability of the ferroelectret is governed by processes of charge transport and storage in the polymer it is made of. In this chapter we will discuss several alternative models of charge decay in polymers as well as a simple model for piezoelectric coefficient, based on electret microphone principle.

Ferroelectrets are used in capacitive piezoelectric sensors. When a force is applied to such a sensor, an electrical signal is generated. We first describe the equivalent circuit representation of a ferroelectret sensor element and proceed with the coupling of the sensor element to a measurement device. Based on the governing equations for the measurement signal following a given time-dependent force, we show that the sensors measure either force or force rate. We illustrate the use of ferroelectrets by proposing a reaction game using ferroelectret sensor elements. Several sensor elements are coupled to a measurement instrument, such as an oscilloscope, while forces are applied by the gamers as fast as possible with their hands. The sensor element is easily prepared by students within a few minutes, immediately enabling playing with the elements, followed by a fundamental understanding of ferroelectret force sensing.

Dielectric elastomer transducers are a class of electroactive polymers that operate based on the interaction of quasi-static electric charges with deformable dielectric and electrode materials. The basic functional element of a dielectric elastomer transducer is a thin dielectric film sandwiched between electrodes. When a dielectric elastomer transducer is used as an actuator, voltage applied across the film causes unlike charges on opposite electrodes to attract and like charges on each electrode to repel. The net result is compressive stress in the thickness direction and tensile stresses in the planar directions. When a dielectric elastomer transducer is used as a generator or sensor, stretching the film will vary the voltage and energy state of applied charges (i.e., the functional element is a stretchable capacitor). Analysis of a simplified model derives an expression for the effective total strain that is twice that for a rigid plate capacitor. The performance of dielectric elastomer transducers is limited by lifetime issues associated with dielectric breakdown and mechanical failure of the film or electrodes. The basic functional element can be incorporated into practical transducers in a wide range of configurations. The particular configuration chosen depends on many factors, including the magnitude of the desired strain or force, the available space or desired form factor, and the required bandwidth (speed of response).

Dielectric elastomer actuators (DEAs) consist of a thin elastomer with even thinner compliant electrodes, both of equal importance for obtaining actuation. In this chapter, materials for both elastomers and electrodes will be discussed.

The effective use of dielectric elastomers (DE) in actual transducers requires the definition of reliable design tools which correctly predict their electromechanical behavior. In this chapter, we present two different approaches for modeling DE. The first approach is focussed on describing the electroelastic behavior of DEs in the framework of finite-strain electromechanics. The second approach, based on lumped parameters, is motivated by the desire to provide a simple and adequate description of the behavior of DE actuators under the influence of an electrical voltage applied to the electrodes.Dielectric elastomers (DEs)definition

The unique advantages of dielectric elastomers have stimulated a great number of applications which can be categorized into actuators, energy harvesters, and sensors. This chapter presents multiple electromechanical transduction systems including biologically inspired robotics, tactile feedback and displays, tunable optics, fluid control and microfluidics, capacitive sensors, and energy harvesters.

As can be seen from the large number of videos of home-made dielectric elastomer actuators (DEAs) on YouTube, getting started making DEAs is straightforward and can be done at low cost. This chapter provides information on making two types of basic dielectric elastomer actuators, as well as detailed information on using DE for energy harvesting (converting mechanical energy into electrical energy).A word of caution about high voltages: Voltages of several thousand volts are required to operate DEAs. The user must therefore exercise caution to avoid electrocution or electrical fires. In addition to taking steps to limit the current delivered by the power supply in the event of a short circuit or of accidental contact, one must also keep in mind that a DEA is a large capacitor capable of storing very large electrical charge, which means that even a “human-safe” current-limited power supply can expose the user to lethal shocks. Arcing in air can easily occur, and short-circuits through the thin membrane are common, linking the high-voltage side to the low-voltage side. Do not use high-voltage circuits unless you have appropriate safety training and experience, as they can be dangerous.