In implantable devices, a major problem for functional electrical stimulation (FES) is the charge imbalance due to imperfections of the stimulator electronics. Blocking capacitors have been used to achieve charge balanced stimulation for a long time, because of capacitor's characteristic - allow AC and block DC. In multi-channel stimulator, however, there's no room for blocking capacitors, and active charge balancing has to be taken into consideration in order to avoid electrolysis.

Recently, we developed two active charge balancing circuits. Firstly, a drain of short current pulses is generated after the mismatched biphasic stimulation cycle. Therewith, the charge imbalance will be compensated after each stimulation cycle. This technique was successfully implemented in a retinal implant chip [M. Ortmanns, et al., IEEE J. Solid-State Circuits, Dec 2007]. Second technique is based on an offset regulation [K. Sooksood, T. Stieglitz and M. Ortmanns, IEEE ISCAS 2009], which shows a great long-term stability. The offset regulation active charge balancing was implemented on a PCB board with discrete components and tested in vitro with a platinum black electrode in saline solution.

One easiest way to check the stability of FES is to compare the electrode-electrolyte interface impedances before and after stimulation. Here, the electrode impedances are measured by two-electrodes method (working and counter electrodes) using a VersaSTAT 4 potentiostat with VersaStudio program from Princeton Applied Research.