Electric field concentration in hydrogel–elastomer devices

Hydrogels and elastomers are being integrated to make stretchable, transparent, electromechanical devices. In such a device, a dielectric elastomer functions as an electric insulator, and a salt-containing hydrogel functions as an electric conductor. Here we report several experimental observations associated with electric field concentration along the edges of the hydrogels. We apply cyclic voltage to a large number of samples, and record the numbers of cycles when the elastomer suffers electric breakdown. In most samples, the elastomer breaks down at the edges of the hydrogels. Before the elastomer breaks down, we observe salting out, localized heating, and plasma along the edges of the hydrogels. These observations are consistent with the hypothesis that the concentrated electric field at the edges of the hydrogels breaks down the air when the elastomer is intact. Remarkably, the breakdown of air makes the electric field less concentrated, and protects the elastomer. When the sample is coated with an elastomer, air no longer breaks down, but the elastomer breaks down at a reduced applied voltage. We discuss the significance of these observations in applications.