Analysis of pH and electrically controlled swelling of hydrogel-based micro-sensors/actuators

In this paper, we carry out the theoretical electro-chemo-mechanical investigation into water swollen ionic polymer gels under the simultaneous influence of electrical and chemical stimuli. In addition to these hydrogels being deployed as active sensing/actuating elements in MEMS/BioMEMS devices, this work can also serve as the basis of illustrating synthetic analogs of physiological muscles with possible applications in orthotics, prosthetics, and as artificial muscles. An electro-chemo-mechanical model or Multi-Effect-Coupling of pH Stimulus (MECpH) model, which was developed earlier by the present authors, is significantly extended to handle nonlinear deformation and implemented numerically to simulate the deformation characteristics of the pH-stimulus responsive hydrogel under the application of an externally applied voltage in different buffered pH solutions. The nonlinear deformation theory provides more accurate results especially when the deformations are large. The hydrogel is observed to experience swelling and bending when pH and external electric field stimuli coexist. The mode and degree of deformation are found to be highly dependent on changes of environmental pH, external electrical potential and bathing ionic strength. As an anionic hydrogel is considered in the simulation, it shows larger changes in deformation characteristic in basic than in acidic solutions. More importantly, the average curvatures of the swollen hydrogel are found to be a linear function with the applied electric potential, making the hydrogel an ideal actuator. However, we also note a significant decrease in the swelling equilibrium degree as the ionic strength becomes concentrated.