We describe a potentiostat designed for in situ electrochemical control of MEMS actuators. This module is tailored for integration into a hybrid CMOS-MEMS system-on-a-chip to confine cells and measure signals from them. The design has been fabricated in a commercially available 0.5 mum CMOS process. The fabricated chip has been employed for the control of off-chip electroactive polymer films and micro-actuators