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Electroactive polymer transducers have many features that are desirable for MEMS devices. An especially attractive type of electroactive polymer is dielectric elastomer. Dielectric elastomers, transducers that couple the deformation of a rubbery polymer film to an applied electric field, show particular promise with features such as simple fabrication in a variety of size scales, high strain and energy density, high efficiency and fast speed of response, and inherent flexibility, environmental tolerance, and ruggedness. A variety of actuator configurations has been demonstrated at the small size scales needed for MEMS devices, including rolled “artificial muscle" actuators, framed and bending beam actuators for efficient opto-mechanical switches, and diaphragm and thickness mode actuators for pumps and valves. The performance benefits of electroactive polymers can allow for new generations of devices in microrobotics, communications, and biotechnology. Several challenges remain for electroactive polymers, including microfabrication, integration with driving electronics, and operational lifetime.

Dielectric elastomer actuators are “stretchable capacitors” that can offer muscle-like strain and force response to an applied voltage. As generators, dielectric elastomers offer the promise of energy harvesting with few moving parts. Power can be produced simply by stretching and contracting a relatively low-cost rubbery material. This simplicity, combined with demonstrated high energy density and high efficiency, suggests that dielectric elastomers are promising for a wide range of energy-harvesting applications. Indeed, dielectric elastomers have been demonstrated to harvest energy from human walking, ocean waves, flowing water, blowing wind, pushing buttons, and heat engines. While the technology is promising and advances are being made, there are challenges that must be addressed if dielectric elastomers are to be a successful and economically viable energy-harvesting technology. These challenges include developing materials and packaging that sustain a long lifetime over a range of environmental conditions, designing the devices that stretch the elastomer material uniformly, and system issues such as practical and efficient energy-harvesting circuits.

We have developed an flexible electro-active polymer (EAP) actuator and a thin flexible visual display with 3×3 array configuration via polymer technology. The flexible actuator consists of nine EAP cells vertically moving in response to change in their thickness. The flexible display uses polymer based optical waveguide allowing light to scatter only at specific area. A pressure sensor is installed under the integrated module. The performance of the actuator is proved to be sufficient for satisfying perceivable range of human touch sense.

Electroactive polymers (EAPs) that are suitable for actuators undergo changes in size, shape, or stress state upon the application of an electrical stimulus. Much research in the field of EAPs tends to focus on the development and understanding of the polymer materials themselves. However, practical devices require that changes in dimension and stress state be effectively exploited to produce the desired functionalities (e.g., driving the motion of a robot limb or simply changing appearance or surface texture). This chapter focuses on those issues that must be considered in implementing EAP materials in practical devices. For purposes of discussion we will focus on one particular type of electroactive polymer: dielectric elastomers. In the literature [e.g., Liu, Bar-Cohen, and Leary, 1999] and elsewhere in this book, dielectric elastomers are also known as electrostatically stricted polymers. Dielectric elastomers are a type of electronic EAPs as defined in Chapter 1 of this book - €”in that their operation is based on the electromechanical response of polymer materials to the application of an electric field. They have demonstrated good performance over a range of performance parameters and thus show potential for a wide range of applications. Dielectric elastomers were pioneered by SRI International, but several research groups around the world are actively investigating applications of this technology [e.g., Wingert et al., 2002; Sommer-Larsen et al., 2001; Jeon et al., 2001]. While the principle of operation of dielectric elastomer EAPs is not used with all EAPs, many of the issues we will discuss are common to all. These issues include the high compliance and large strains that EAPs can produce, as well as the necessity of simultaneous consideration of both the electrical properties and mechanical properties of materials. This chapter is organized as follows. First, we consider the specifications used to match actuation technologies with applications, and when it makes sense to consider EAPs. Next, we discuss the basic principles of dielectric elastomer technology. We then consider design issues that may affect the actuation performance of dielectric elastomer EAPs, as well as the operational characteristics of EAPs and how they may affect an application. We present several examples of dielectric elastomer actuators for a wide range of applications, highlighting both the potential advantages of EAPs and the challenges associated with their use. Finally, we conclude with a brief summary of the subchapter and a discussion of the future of EAP application.

The dielectric elastomer, a particularly attractive type of electroactive polymer, uses commercial polymers such as acrylic and silicone elastomers. The technology has been limited in application by perceived lifetime issues. By addressing several lifetime issues, lifetimes of more than one million cycles, and in some cases beyond ten million cycles, were achieved with a variety of transducer configurations (including operation in generator mode) under a variety of operating conditions (including high humidity). Dielectric elastomers can produce maximum actuation strains of more than 100% and specific energy density exceeding that of known electric-field induced technology. Performance testing for dielectric elastomer actuators has typically been for peak-performance or “over-driven” conditions with short operational lifetimes (typically 100s or 1000s of cycles), particularly under conditions such as high humidity. By minimizing electric field and mechanical strain concentration factors, long lifetimes (>1 million cycles) with acrylic transducers were achieved with actuation strains as great as 40% areal strain (and up to 100% areal strain in generator mode). Actuators in a dry environment had an almost 20x increase in lifetime over actuators at ambient humidity (about 50% RH) at the same driving field conditions. Long actuation lifetimes were also achieved in a 100% RH environment and when fully submerged in salt water at reduced operating strain and field. In 100% RH, lifetimes of several million cycles were achieved at 4% strain. In underwater operation, 6 out of 11 actuators survived for >10 million cycles with an electric field limited to 32 MV/m and approximately 2% strain. The demonstrated lifecycle improvements are applicable to a variety of uses of dielectric elastomers, including haptic interface devices, pumps (implantable and external), optical positioners, and “artificial muscles” to replace small damaged muscles. Continued improvements in materials, actuator design, and packaging, combined with management of operational conditions as described here, should support new practical application of this promising technology.

Many different actuator configurations based on SRI International’s dielectric elastomer (DE) type of electroactive polymer (EAP) have been developed for a variety of applications. These actuators have shown excellent actuation properties including maximum actuation strains of up to 380% and energy densities of up to 3.4 J/g, using the planar mode of actuation. Recently, SRI has investigated different configurations of DE actuators that allow complex changes in surface shape and thus the creation of active surface texture. In this configuration, the “active” polymer film is bonded or coated with a thicker passive layer, such that changes in the polymer thickness during actuation of the DE device are at least partially transferred to (and often amplified by) the passive layer. Although the device gives out-of-plane motion, it can nonetheless be fabricated using two-dimensional patterning. The result is a rugged, flexible, and conformal skin that can be spatially actuated by subjecting patterned electrodes on a polymer substrate to an electric field. Using thickness-mode DE, we have demonstrated thickness changes of the order of 0.5 - 2 mm by laminating a passive elastomeric layer to a DE polymer that is only 60 μm in thickness. Such thickness changes would otherwise require a very large number of stacked layers of the DE film to produce comparable surface deformations. Preliminary pressures of 4.2 kPa (0.6 psi) in a direction normal to the plane of the DE film have been measured. However, theoretical calculations indicate that pressures of the order of 100 kPa are feasible using a single layer of DE film. Stacking multiple layers of DE film can lead to a further increase in achievable actuation pressures. Even with current levels of thickness change and actuation pressures, potential applications of such surface texture change are numerous. A thin, compliant pad made from these actuators can have a massaging or sensory augmentation function, and can be incorporated into garments if desired. The bumps and troughs could act as valves or pumping elements in a fluidic or microfluidic system. Such a device could also be the basis of a smart skin that controls boundary-layer flow properties in a boat or airplane so as to reduce overall drag. The DE elements of the pad can also be used as sensors to make a touch-sensitive skin for recording human interaction with the environment. By driving a thin, compliant vibrating layer at resonant frequencies, one can also configure these devices as solid or fluidic conveyors that transport material on a macroscopic or microscopic scale.

Composite materials have increased the range of mechanical properties available to the design engineer compared with the range afforded by single component materials, leading to a revolution in capabilities. Nearly all commonly used engineering materials, including these composite materials, however, have a great limitation; that is, once their mechanical properties are set they cannot be changed. Imagine a material that could, under electric control, change from rubbery to rigid. Such composite "meta-materials" with stiffness and damping properties that can be electrically controlled over a wide range would find widespread application in areas such as morphing structures, tunable and conformable devices for human interaction, and greatly improved vibration control. Such a technology is a breakthrough capability because it fundamentally changes the paradigm of composite materials having a fixed set of mechanical properties. These electronically controllable composites may be the basis of discrete devices with tunable impedance. The composites can also be multifunctional materials: They can minimize size and mass by acting not only as a tunable impedance device, but also as a supporting structure or protective skin. Current approaches to controllable mechanical properties include composites with materials that have intrinsically variable properties such as shape memory alloys or polymers, or magnetorheological fluids, or composites that have active materials such as piezoelectrics, magnetostrictives, and newly emerging electroactive polymers. Each of these materials is suitable for some applications, but no single technology is capable of fast and efficient response that can produce a very wide range of stiffness and damping with a high elongation capability, that is, go from rubber to rigid. Such a material would be capable of a change in its maximum elastic energy of deformation of 50,000 J/cm3. No existing material is within three orders of magnitude of this value. Similarly, no material appears capable of going from a very lightly damped to a very heavily damped condition over a wide range of motion. We suggest an approach based on composites whose meso-scale structure can be changed with actuation or change in intrinsic properties. Passive composite meta-materials have been demonstrated, however, such active composite meta-materials have not yet been demonstrated.

Electroactive polymer (EAP) transducers are an emerging technology with many features that are desirable for MEMS devices. These advantages include simple fabrication in a variety of size scales, and ruggedness due to their inherent flexibility. Dielectric elastomer, a type of EAP transducer that couples the deformation of a rubbery polymer film to an applied electric field, shows particular promise because it can produce high strain and energy density, high efficiency and fast speed of response, and inherent environmental tolerance. A variety of proof-of-principle dielectric elastomer actuator configurations have been demonstrated at the small size scales needed for MEMS devices, including rolled "artificial muscle" actuators for insect-inspired microrobots, framed and bending beam actuators for efficient opto-mechanical switches, diaphragm and enhanced-thickness-mode actuators for microfluidic pumps, and valves and arrays of diaphragms for haptic displays. Several challenges remain for EAPs, including integration with driving electronics, and operational lifetime.

Dielectric elastomer actuators offer unprecedented opportunities for actuation in a wide range of applications. To make appealing large scale and efficient systems, new electronic devices combining high voltage and flexibility need to be designed. In this paper we report the first mechatronic system, made of an array of electro-active polymer based actuators integrated with thin film photoconductive high voltage switches fabricated on a plastic film substrate. The actuator is an acrylic elastomer diaphragm that expands under electrical stimulation. Each actuator is connected to the high voltage power supply through a photoconductive switch, which is addressed and closed by illumination. The amorphous silicon switches are made on flexible and transparent polyimide (Kapton E®) substrates. Individual switches were tested up to 8 kV and a nine-element array was successfully working at 5 kV.

Space-based astronomy and remote sensing systems would benefit from extremely large aperture mirrors that can permit greater-resolution images. To be cost effective and practical, such optical systems must be lightweight and capable of deployment from highly compacted stowed configurations. Such gossamer mirror structures are likely to be very flexible and therefore present challenges in achieving and maintaining the required optically precise shape. Active control based on dielectric elastomers was evaluated in order to address these challenges. Dielectric elastomers offer potential advantages over other candidate actuation technologies including high elastic strain, low power dissipation, tolerance of the space environment, and ease of commercial fabrication into large sheets. The basic functional element of dielectric elastomer actuation is a thin polymer film coated on both sides by a compliant electrode material. When voltage is applied between electrodes, a compressive force squeezes the film, causing it to expand in area. We have explored both material survivability issues and candidate designs of adaptive structures that incorporate dielectric elastomer actuation. Experimental testing has shown the operation of silicone-based actuator layers over a temperature range of -100 °C to 260 °C, suitable for most earth orbits. Analytical (finite element) and experimental methods suggested that dielectric elastomers can produce the necessary shape change when laminated to the back of a flexible mirror or incorporated into an inflatable mirror. Interferometric measurements verified the ability to effect controllable shape changes less than the wavelength of light. In an alternative design, discrete polymer actuators were shown to be able to control the position of a rigid mirror segment with a sensitivity of 1800 nm/V, suggesting that sub-wavelength position control is feasible. While initial results are promising, numerous technical challenges remain to be addressed, including the development of shape control algorithms, the fabrication of optically smooth reflective coatings, consideration of dynamic effects such as vibration, methods of addressing large-numbers of active areas, and stowability and deployment schemes.

Dielectric elastomers exhibit remarkable elastic and electroactive properties. Upon kilovolt range electrical stimulation, the polymeric actuator deforms with very large mechanical strain. We made thin film amorphous silicon (a-Si:H) photoconductive switches on 50-μm thick flexible polyimide films to control the polymer actuation. 155–310-nm thick a-Si:H films were grown by plasma enhanced chemical vapor deposition at 150 °C substrate temperature. The photoconductive switches were made with 250-, 500- or 1000-μm gaps formed by electrodes of evaporated aluminum. When properly passivated, the switches have a dielectric breakdown strength of ∼10 kV/mm across the gap. We present optical and electrical characteristics of these a-Si:H switches in the 1–10 kV range. The integration of a nine-element a-Si:H high voltage photoconductor switch array with a nine-element electroactive polymer actuator array is demonstrated. The time constants measured on this array agree well with the values modeled using the a-Si:H photoconductor characteristics.