Residual Voltage in Biphasic Electrical Stimulation: Cause, Clues, and Control

2018-12-14T19:06:19Z (GMT) by Ashwati Krishnan
Electrical stimulation of neural tissue has been known to evoke functional responses in ani-<br>mals. Stimulation is primarily performed by passing controlled, symmetric biphasic current<br>pulses to an electrode placed near the neural tissue. The biphasic current pulse consists of a<br>negative pulse, followed by a positive pulse to maintain charge neutrality. A theoretical anal-<br>ysis on a rst order electrode-electrolyte (tissue) interface model has shown that perfectly<br>balanced input current signals do not ensure net neutrality at the interface, due to an unre-<br>coverable loss of charge via the faradaic impedance. In chronically implanted devices, there is<br>currently no practical way to quickly identify changes that occur at the electrode-tissue inter-<br>face, especially in high-density electrode arrays. This work explores the extent to which the<br>residual voltage can act as a preliminary indicator of electrode degradation, because residual<br>voltage is essentially a characteristic of the interface. While residual voltage provides timely<br>feedback when sampled at regular intervals, it can accumulate and a DC voltage on a stimu-<br>lation electrode can be potentially unsafe in a chronically implanted device. The method of<br>delivering current signals has traditionally been implemented as an open loop system. This<br>work also demonstrates that one can control the residual voltage by correcting the positive<br>pulse of the biphasic signal based on the existing state of the electrode-electrolyte/tissue<br>interface. While the resulting biphasic stimulation waveform is imbalanced, the interface<br>is electrochemically neutral. The updated value of the imbalanced anodic pulse width can<br>provide us meta-data about the interface, in case there is any degradation. By controlling<br>the residual voltage at the electrode actively with feedback, the proposed closed loop system<br>will improve the safety of neural stimulator systems.