Dielectric-elastomer actuators deliver clean energy

Abstract

Last year marked the 10th anniversary of the Stanford Research Institute’s landmark Science paper on deformable capacitors based on elastomeric insulators with highly compliant electrodes.1 It showed that attractive electrostatic forces between positive and negative charges on compliant electrodes lead to high-speed, giant-strain, electrically controlled actuators, which have now exceeded the performance of natural muscles in terms of strain, stress, and elastic-energy density. The perspectives of the field are discussed in a recent Science article,2 showing that such dielectric-elastomer actuators (or electroactive-polymer artificial muscle) have become areas of both intense academic research and industrial development. Electrically controlled optical lenses and phase gratings are already on the market,3 while a first mass-market application has been announced for mobile phones, providing users with vibrotactile feedback.4 Key factors promoting the success of the technology are the simple physics behind the actuation (attractive electrostatic forces acting on soft, deformable dielectrics) and its cost-effective implementation with inexpensive materials. The idea that a solid material deforms when stimulated by electricity originated in the late 18th Century, as documented in a letter by Alessandro Volta discussing observations of Felice Fontana on volume changes in electrified Leyden jars, the first electrical capacitors.2 In the late 19th Century, Wilhelm Conrad Rontgen designed an experiment, which is seen today as a beautiful description of the dielectric-elastomer-actuation principle.2 A rubber strip, when electrified by a corona discharge (which sprays opposite electrical charges on either side of the strip), shrinks in thickness and increases in length. Figure 1 shows a sketch of the experiment and a demonstration using simple household materials. We replicated Rontgen’s experiment with modern materials,5 showing that such charge-controlled actuation overcomes the electromechanical pull-in effect, i.e., the mechanical collapse of Figure 1. (top) Dielectric-elastomer-actuation principle anticipated by Rontgen in 1880 and (bottom) demonstration of the elongation of a stretched rubber strip after corona charging. HV: High voltage.