The growing interest of engineers in gels has arisen from the promising perspectives and possibilities of provoking drastic phase transitions by inducing small changes in their external conditions [Hamlen, 1965; Tanaka, 1982; Yannas, 1973]. For technical applications gels, which respond with considerable swelling or shrinkage, are predestinate. These changes in volume can be obtained by external stimulation, e.g., by applying an external electric field, through temperature changes, light, or a change of the chemical milieu of the solvent in which the gels are immersed. By combining sensoric and actuatoric properties, gels can even be classified as typical adaptive materials. They can fill the gap between the conventional mechanical and solid-state actuators, although at the cost of precision.
Electroactive polymers (EAP) are a particularly attractive class of actuation materials with great similarity to biological contractile tissues [De Rossi et al., 1992]. The backbone of these gels consists of polymers or other long-chain molecules, chemically or physically cross-linked to create a tangled network (Fig. 1).
The shape stability of the gel strongly depends on the interaction between its elastic network and the liquid medium in which it is immersed. The liquid prevents the network from collapsing. On the other hand, the network retains the liquid and imparts solidity to the gel.