The behaviour of Dielectric Elastomer Actuators (DEA) can be predicted using hyperelastic models that are based on strain energy density functions. The parameters used in the hyperelastic models are generally obtained via a uni-axial pull test. However, DEAs are most commonly used in an biaxially stretched configuration. This is an appropriate assumption if the modelled parameters translate accurately to different stretch configurations. We have conducted stress-stretch experiments on silicone membranes in two different configurations; uni-axial and pure shear stretch. Fitting common hyperelastic models, such as Gent, to the experimental data shows that the modelling parameters depend on the stretch configuration. In addition, we show that the Mullins effect, where the stress-stretch response is dependent on the maximum stretch previously experienced by the sample, is predominant in the silicone membranes. This means that the model parameters depend on the loading configuration and the stretch history of the sample making it difficult to predict the behaviour of highlyprestretched DEAs. One way to tackle this issue is to carry out testing as close to the original configuration as possible which is difficult in the case of highly prestretched DEAs. We have created a model that takes into account both the loading configuration and the Mullins effect and used this to optimize the prestretch and stretch of the cell stretching device.
We present a method for the patterning of compliant electrodes for dielectric elastomer actuators (DEA) using drop-on-demand (DoD) printing and a lift-off process. DoD is a very appealing method for the patterning of electrodes, due to its high resolution, and the design versatility brought by printing from computer files. However, it has very narrow requirements regarding the viscosity, surface tension, and agglomeration size of the solution to be printed, and a new jetting waveform must be developed for each ink. This makes experimenting with new compliant electrode formulations difficult and time-consuming. Our approach consists in printing a watersoluble sacrificial layer on the elastomer, which serves as a mask selectively protecting portions of the membrane. Compliant electrodes can then be applied on the mask by different means (brush, spray coating, stamping etc.), and the mask can subsequently be dissolved to wash away the excess of ink and reveal the pattern, similar to a lift-off process. The inkjet printing process must only be developed and optimized for a single solution (the sacrificial layer), whereas many different electrodes formulations can then rapidly be patterned and tested, without having to meet the requirements of the printer regarding viscosity, surface tension or agglomeration size. We demonstrate the method by patterning an Polyvinylpyrrolidone (PVP) mask. We then use an airbrush to apply a carbon black/silicone mixture over the whole membrane. Finally, we wash away the mask to reveal the compliant electrodes.