Actuator materials that can reproduce the multifunctionality of natural muscles have long been desired for the development of biologically inspired robots. Electroactive polymers (EAPs) have attracted increasing attention as potential candidates for artificial muscles. While several types of EAPs have been investigated, [1][2][3][4] electroelastomers, also known as dielectric elastomers, have been particularly attractive for largestrain and high-power applications. [5][6][7] Electroelastomers based on acrylic copolymer elastomers (e.g., 3M VHB 4910) and compliant electrodes have been shown to exhibit electromechanical strains of up to 380 % in terms of area expansion. Furthermore, the specific elastic energy density (3.4 J g -1), stress (up to 8 MPa), and electromechanical conversion efficiency (60-90 %) are all extraordinarily high. However, this outstanding performance is only observed when the acrylic films are highly prestrained. The reported specific elastic-energy density and stress are calculated from the weight or volume of the active acrylic elastomers. The performance of the packaged actuators is substantially lower.A number of actuator configurations, such as bow, bowtie, rigid-frame, diaphragm, and spring-roll actuators, have been designed to support the required high prestrain. [8,9] Each of these designs has its own unique advantages for certain applications, but without exception, the prestrain-supporting structures occupy significantly more space and weigh significantly more than the films themselves. The consequence of this is that the supporting structures cause a large performance gap between the active material and the packaged actuators. In addition, the lifetimes of the actuators are limited by the concentration of stress at the interfaces between the soft polymer film and the rigid supporting structure. The shock tolerance of the actuators is also reduced because of the introduction of rigid structural components. The prestrained films exhibit stress relaxation that affects the subsequent actuation.[10]Therefore, it would be highly desirable to eliminate mechanical prestraining while still retaining its performance benefits. We report here the development of new electroelastomers that exhibit high strain without requiring high prestrain. Electrically induced strain is proportional to the square of the applied electric field. High strain necessitates high breakdown strength. Prestrain enhances the dielectric-breakdown field of the elastomer films.[11] Mechanistic reasons for the enhancement of dielectric strength via mechanical prestraining are not well understood; we attribute it to the increased probability of hot electrons colliding with polymer chains realigned in parallel to the film surfaces. The prestrain also realigns defects, such as fibrous impurities, non-spherical voids, and gel particles, which may be responsible for premature dielectric breakdown. Here, we describe new interpenetrating elastomeric networks in which the benefits resulting from prestrain are obtained without exte...