Electrochemical stability and neural recording with sputtered ruthenium oxide electrodes subchronically in rat motor cortex.

Abstract

<i>Objective</i>. To investigate sputtered ruthenium oxide (RuO<i>_x</i>) as a low impedance coating for recording neural activity with intracortical microelectrode arrays (MEAs).<i>Approach.</i>RuO<i>_x</i>was sputtered onto active sites of amorphous silicon carbide (a-SiC) MEAs and evaluated electrochemically and as a recording electrode coating over 6 weeks in rat motor cortex. The RuO<i>_x</i>was 250 nm thick with a 200<i>μ</i>m²geometric electrode surface area. We performed weekly electrochemical impedance spectroscopy and cyclic voltammetry (CV) measurements and single-unit action potential recordings.<i>Main results.</i>From 1 to 6 weeks post-implantation, we observed that median 1 kHz impedance decreased from 1.06 MΩ to 0.68 MΩ, median 1 Hz impedance decreased from 24.5 MΩ to 20.7 MΩ, and median 30 kHz impedance remained constant at 0.19 MΩ. Linear regression fits indicated that the 1 kHz and 1 Hz impedance exhibited small decreases whereas the 30 kHz impedance values had no discernible trend. The CV-based charge storage capacity (CSC_c) also exhibited consistency over time. The median CSC_cat 50 mV s^-1was 24.3 mC cm^-2on week 1 and 20.1 mC cm^-2at week 6. At 50 000 mV s^-1sweep rate, the median CSC_cwas 2.4 mC cm^-2at 1 week and 2.8 mC cm^-2at 6 weeks. Regressions for both sweep rates exhibited no significant slope deviations from zero. Neural recordings spanning 6 weeks demonstrated consistent single unit action potential with a 75% single unit active-electrode-yield over 6 weeks. Median<i>V</i>_ppvalues on week 1 were 97.6<i>µ</i>V and 105<i>µ</i>V by week 6. The median and quartile signal-to-noise ratio (SNR) was 12 at week 1 and 12 at week 6. There were no deviations from zero on the regressions for both<i>V</i>_ppand SNR.<i>Significance.</i>The findings suggest that RuO<i>_x</i>is stable over subchronic implantation periods. Future research will explore long-term chronic performance and broader applications of RuO<i>_x</i>in neural interface devices.