Stability of Ruthenium Oxide Coated Microelectrode Arrays for Chronic Stimulation of Rat Somatosensory Cortex

Intracortical microstimulation (ICMS) of the primary somatosensory cortex offers the potential for restoring sensory function in neuroprosthetic applications; however, achieving stable long-term performance from implanted electrode materials remains a challenge. Recently, ruthenium oxide (RuO_x) has emerged as a viable faradaic coating material suitable for ICMS application due to its high charge injection capacity, low impedance, and stable electrochemical properties. However, its long-term in vivo performance remains an area of active investigation. In this study, we evaluated the viability of RuO_x-coated amorphous silicon carbide microelectrode arrays (MEAs) for producing ICMS-evoked percepts over a 25-week chronic implantation period with an established rodent-based go/no-go behavioral paradigm. Weekly behavioral, electrochemical, and local electrophysiological measurements were analyzed to assess the stability of RuO_x for long-term stimulation and recording studies. Results showed an exponential-like reduction in perception thresholds, stabilizing around the ninth post-implantation week at ~0.4 nC/ph per electrode ×16 electrodes (simultaneous multi-channel stimulation). Throughout this period, behavioral performance remained consistently high (~91%), with stable reaction times, indicating reliable sensory perception up to 25 weeks. Electrochemical measurements, including 1 kHz impedance and driving voltage, alongside neural recording metrics like spike rate and noise floor, remained relatively stable throughout the study. Although minor degradations in the number of detected units per electrode and the peak-to-peak voltages of those units were observed (consistent with other silicon-based MEA studies), RuO_x-coated electrodes demonstrated the ability to reliably evoke sensory percepts over the duration of the experiment. Overall, these findings highlight RuO_x as a durable MEA-coating material for future ICMS applications.