(2) Return Electrode Considerations for Potential Transients in Neural Stimulation

Introduction: Neural stimulation devices are widely used in clinical applications such as cochlear implants, deep brain stimulation, and spinal cord stimulation to treat conditions including hearing loss, movement disorders, and chronic pain. These devices rely on electrodes to deliver electrical pulses, where electrochemical properties influence charge injection and electrode stability. Return electrodes play a key role in shaping voltage transients (VTs) and potential transients (PTs), which determine stimulation efficacy, patient safety, and device longevity. Optimizing return electrode material and stimulation configuration is crucial for improving therapeutic outcomes and ensuring reliable long-term performance of neural interfaces.

Methods: VTs and PTs were recorded from 18-pin floating microelectrode arrays (FMAs) with 2,000-μm² activated iridium oxide film (AIROF) electrodes. Tested return electrodes consisted of platinum-iridium (PtIr), platinum (Pt), stainless steel (SS), titanium (Ti), and tungsten (W). Stimulation configurations comprised monopolar (MP), bipolar (BP), partial bipolar (PBP), and partial tripolar (PTP). The effect of pulse polarity (cathodic- and anodic-first) was also tested. Measurements were conducted in phosphate-buffered saline (PBS) at 37°C and automated in MATLAB using the Plexon PlexStim Electrical Stimulator system and Tektronix TBS1104 oscilloscope. Potential was obtained versus a Ag|AgCl reference electrode using a Plexon Differential Amplifier.

Results: Return electrode material notably impacted polarization and charge-injection capacity. The return electrode directly influences the interpulse potential during pulsing by establishing a potential bias. AIROF electrodes paired with return electrodes of lower open-circuit potentials (OCPs) exhibited improved charge injection, particularly with anodic-first pulsing. However, excessive negative bias from W return electrodes decreased charge injection due to polarization of the AIROF electrode to a high impedance state. Partial multipolar configurations showed greater energy efficiency for stimulation by providing higher charge-injection capacity with smaller driving voltage due to the implementation of a remote return electrode. These results emphasize the importance of optimizing return electrode selection for improved stimulation efficiency and power consumption.

Conclusion: This study underscores the importance of the return electrode material and stimulation configuration in neural stimulation. The PTs exhibited different electrode behaviors under different configurations. The OCP of the return electrode plays a role in the amount of deliverable charge for stimulation as potential bias. Furthermore, the lower power consumption of partial multipolar configurations is further demonstrated by the smaller driving voltages required for current pulsing. These insights aid in designing more effective stimulation protocols for clinical devices such as cochlear implants, deep brain stimulators, and spinal cord stimulators.