A Tripolar Stimulator with Return-Electrode-Based Charge-Pack Injection Technique for Charge Imbalance Correction in Spatiotemporal Stimulation

Spatiotemporal stimulation (STS) has been proved to be an effective method for neuromodulation therapies, e.g., producing coherent percepts of visual forms [1] and restoring walking after spinal cord injury [2], [3] in humans. However, such multichannel simultaneous stimulation with different current amplitudes on each channel causes severe crosstalk, affecting electric field distribution across the tissue [4] and current distribution among stimulation channels [5]. Fig. 1 (right) shows three conventional stimulation configurations. Compared with monopolar stimulation, bipolar stimulation exhibits higher spatial resolution (i.e., less electric field distribution distorts) especially when utilizing a hexagonal array [4], [6], making it more suitable for STS. However, current distribution cannot be avoided, and it will affect the residual voltage <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\Delta V)$</tex> on the electrodes, leading to an offset voltage (Vos) on charge imbalance. This problem is more serious in single-current-source (SCS) bipolar stimulation than in double-current-source (DCS) bipolar stimulation. Active charge balancing (ACB) techniques are promising ways to restore charge balance. Conventional ACB techniques in bipolar stimulation conduct compensation according to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\Delta\vee$</tex> between the working electrode (WE) and the counter electrode (CE) (i.e., <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\Delta V_{\text{WE}-}$</tex> CE) [7], [8], which causes error detection and compensation when applied in STS. To address these shortcomings, this paper demonstrates a DCS tripolar stimulator with the return-electrode-based charge-pack injection (RECPI) technique.