Dielectric elastomers for artificial muscles

Natural muscles have self-repair capability providing billions of work cycles with more than 20% of contractions, contraction speed of 50% per second, stresses of ∼0.35 MPa, and adjustable strength and stiffness [1]. Artificial muscles have been sought for artificial hearts, artificial limbs, humanoid robots, and air vehicles. Various artificial muscles have been investigated for large strain, high response rate, and high output power at low strain using their own material characteristics [2]. Among the candidates for artificial muscles, the dielectric elastomer has typical characteristics of light weight, flexibility, low cost, easy fabrication, etc., which make it attractive in many applications. Applications of dielectric elastomer include artificial muscles and also mobile robots, micro-pumps, micro-valves, disk drives, flat panel speakers, intelligent endoscope, etc. [3-7]. Dielectric elastomer actuators have been known for their unique properties of large elongation strain of 120-380%, large stresses of 3.2 MPa, high specific elastic energy density of 3.4 J/g, high speed of response in 10-3 s, and high peak strain rate of 34,000%/sec [1,2,4,8]. They transform electric energy directly into mechanical work and produce large strains. Their actuators are composed primarily of a thin passive elastomer film with two compliant electrodes on the surfaces, exhibiting a typical capacitor configuration. As with most rubbery materials, the elastomer used in actuator application is incompressible (Poisson's ratio = 0.5) and iscoelastic, which consequently exhibits time- or frequency-dependent characteristics that could be represented by stress relaxation, creep, and dynamicmechanical phenomena under stressed and deformed states [9-10]. When the electrical voltage is applied to the electrodes, an electrostatic force is generated between the electrodes. The force is compressive, and thus the elastomer film expands in the in-plane direction. As an advantage of dielectric elastomer actuators, the performance of elastomer actuators can be tailored by choosing different types of elastomers, changing the cross-linking chemistry of polymer chains, adding functional entities, and improving fabrication techniques with ease and versatility in most cases. The deformation of elastomers complies with the theories of rubber elasticity and nonlinear viscoelasticity. When an electrical field is applied, the elastomer deformaton is influenced primarily by the intrinsic properties of moduli and dielectric constants of elastomers in a coupled manner. In addition, maximum actuation capabilities are often restricted by the dielectric strength (or breakdown voltage) of elastomer films. Although the low stiffness of elastomers may increase strain, maximum actuator stroke, and work per cycle, it should be considered that the maximum stress generation decreases with decreased moduli. Accordingly, the property-processing-structure relationship of elastomers especially under the electrical field and large deformation should be understood on the basis of the fundamental principles of deforming elastomers and practical experience in actuator fabrication.