Discharge Correction Under Asymmetric Electrode-Electrolyte Interfaces in Bipolar Stimulation

Neural electrical stimulators have been widely used in biomedical applications, especially those with multiple channels. However, the electrode-electrolyte interfaces are usually not identical, which is rarely discussed in stimulator design. This paper first analyzes the stimulation and discharge phases in bipolar stimulation when the interfaces are not identical, proving that the asymmetry will lead to incomplete discharge. To address this issue, we propose a tripolar stimulator by introducing a return electrode to discharge the working and counter electrodes separately. The prototype of a 4-channel tripolar stimulator is designed and fabricated in a 180-nm CMOS technology, with each stimulation channel occupying 0.176 mm2. The maximum stimulation current is 2.8 mA. After discharge, residual voltages are near-zero in both electrical and in vitro tests.