Highly stretchable and transparent ionogels as nonvolatile conductors for dielectric elastomer transducers

Large deformation of soft materials is harnessed to provide functions in the nascent field of soft machines. This paper describes a new class of systems enabled by highly stretchable, transparent, stable ionogels. We synthesize an ionogel by polymerizing acrylic acid in ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate ([C2mim][EtSO4]). The ionogel exhibits desired attributes of adequate conductivity (0.22 S m(-1)), low elastic modulus (∼3 kPa), large rupturing stretch (∼4.6), and negligible hysteresis and degradation after cyclic stretches of large amplitude. Using the ionogel and a dielectric elastomer, we fabricate electromechanical transducers that achieve a voltage-induced areal strain of 140%. The ionogel is somewhat hygroscopic, but the transducers remain stable after a million cycles of excitation in a dry oven and in air. The transparency of the ionogels enable the transducers with conductors placed in the path of light, and the nonvolatility of the ionogels enable the transducers to be used in open air.