ELECTROACTIVE POLYMERS
George Jeronimidis
Centre for Biomimetics, The University of Reading, UK

Polymers which can change shape in response to electrical stimuli have been known for over a hundred years. In the last decade there have been significant developments in electroactive polymers (EAPs) to produce substantial change in size or shape and force generation for actuation mechanisms in a wide range of applications, robotics and smart textiles in particular. In contrast to many conventional actuation systems, many types of EAPs are also capable of providing sensing functions.

The advantages of EAP-based actuation or sensing are several:

EAPs can provide a range of basic actuator mechanisms, force and displacement levels. There are three basic groups involving electronic interactions, ionic interactions and phase transitions with associated conformational changes.

Active polymer gels fall typically in the low stress (low force)-high strain group, together with muscle. Their elastic modulus in the swollen state is low, typically of the order of 1000 Pa and, consequently, the forces that they can generate in unconstrained conditions are low. Measured values of force generation are about 1N/g of swollen gel. Isotropic volumetric free swelling can be very large indeed, with swelling ratios but is omni-directional. Differential swelling, and hence bending, of beam or plate-like shapes can be induced by charge separation techniques involving static or alternating external electrical field. They are essentially soft elastomeric materials. In order to generate higher forces, at the expenses of reduced deformability, and take advantage of their swelling potential and virtual incompressibility, their expansion must be confined. This is analogous to the free expansion of a gas that cannot produce useful work.

Dielectric elastomers actuators exploit the electrostatic Maxwell stress experienced by all dielectrics. These are dry materials based on relatively soft elastomeric films. Essentially the device is a capacitor in which the electrodes are attached to the polymer film. Upon application of a voltage the charges on the opposing electrodes attract each other, reducing film thickness. Since such rubbers deform at almost constant volume this leads to an expansion of the area of the polymer film. Furthermore the like charges on each electrode will repel each other tending to lead to an expansion of the electrode. There is a built in amplification process since as the film thickness decreases the electric field strength increases. As a consequence the actuation is non-linear with a strain approximately proportional to the square of the applied voltage. Strains of up to 400% have been observed in acrylic elastomers exerting a pressure of ~ 7 MPa. Such systems have the highest energy densities observed for any EAP but the voltages required may be as high as 5kV.

EAPs based on conducting polymers utilise mass transport of ions into and out of the polymer. Two key requirements are an electric-field driven diffusion mechanism to transport metal ions into the polymer and polymer conduction to get electrons into the polymer to generate this field. In both cases the polymer change can be used to generate mechanical work. Such materials have been widely fabricated as bending actuators. Polypyrrole and derivatives and polyaniline based systems have been extensively studied. These activator types exhibit modest strains of ~ 10% but can develop high pressures, for example, 450M Pa. However, the overall response times are relatively slow.

Considerable progress still needs to be made with EAP technologies before commercially viable applications are made other than in the area of piezoelectric polymers. A multidisciplinary approach is essential for future developments. Applications such as fabrics and textile structures will require fibre-like EAP actuators and sensors in order to achieve effective integration. The large stimulated displacements that have been observed have encouraged new thinking in terms of both applications and designs. The natural ease of preparing and shaping such materials, coupled with their low mass and large displacements, opens up new approaches in many traditional areas as well as the potential to enable new technologies.

Centre for Biometrics, University of Reading