Role of pH gradients in the actuation of electro-responsive polyelectrolyte gels

Polyelectrolyte gels are able to mimic artificial muscles, swelling, shrinking or bending in response to environmental stimuli. Mechanical response is also observed in the presence of an electric field, in which case electrical energy is directly converted into mechanical energy. Although several mechanisms have been proposed to describe electro-actuating in polyelectrolyte gels, no consensus yet exists on which mechanisms are responsible for this phenomenon. Here we use a universal pH indicator to investigate the role of localized pH changes in the gel during bending electro-actuation. We show that, when the gel is not in contact with the electrodes, a pH wave propagating from the electrodes is not the factor that triggers or determines the amplitude of electro-actuation. We also show that, surprisingly, the direction of actuation depends on the salt concentration in the surrounding electrolyte. The polarity of actuation is consistent with models based on dynamic enrichment and depletion of electrolyte for low salt conditions, but reverses at physiological salt concentrations. This suggests that not all experimental observations can be described in terms of a single simple model, and that further theoretical work is needed in the case of physiological salt conditions.