Dielectric elastomers consist of a flexible elastomer sandwiched between two compliant electrodes. When a voltage is supplied to the dielectric elastomer, the elastomer is squeezed in the electrical field and it subsequently expands perpendicular to the electrical field. This electro-mechanical energy transduction is useful for many applications and it closely resembles the way human muscles work.
This close resemblance also earned the materials the nickname artificial muscles in the 1990s. However, dielectric elastomers, despite ≈30 years of intensive research, still cannot compete with human muscle. Performances exceeding that of human muscles have been shown, however, the dielectric elastomer is not reliable as performance is lost rather rapidly over time and thus can not compete with the lifetime of human muscle.
This shortcoming of artificial muscles is mainly due to the complexity of materials optimization. Optimization of one parameter usually leads to compromising another parameter. Silicone elastomers are in themselves very reliable and robust materials which can easily be cycled more than 10 million times at high strains without losing elasticity. However, when elastomers are functionalized with relatively incompatible high-permittivity moieties, the microscopic structure of the resulting elastomer is changed to a large extent and the material is most likely no longer a simple one-phase elastomer.
In a review Yu and Skov present a detailed analysis of existing optimization methods, and discuss virtues and shortcomings of each method with respect to the various transduction principles such as energy harvesting and actuation.
To meet the growing commercial demand for dielectric elastomers, simple, cost effective and reproducible methods for preparing high-permittivity and very soft materials with mechanical integrity are needed. For energy generation materials softness is not required and thus formulation is simpler, but optimized electrical breakdown strength with a narrow distribution of breakdowns is crucial to achieve large-quantity products with high reliability and life-time. Formulation of both types of dielectric elastomers with high reliability is a major challenge to the field.
