Optimization of Sputtered Iridium Oxide Microelectrodes for Long-term Intracortical Stimulation

The combination of excellent stimulation properties and long-term stability of electrodes is key in the development of a clinical neural prosthesis for intracortical stimulation. Iridium oxide combines both characteristics, but its quality is highly dependent on the exact fabrication parameters. In this study, the fabrication of sputtered iridium oxide films (SIROFs) was investigated in terms of oxygen flow, sputtering power and sputtering time. By using electrochemical characterization techniques such as electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV), and pulsing experiments, we show that, between the investigated parameters, the best surface structure and electrochemical properties were obtained for a 5% oxygen flow, at 80 W and for 7 minutes, resulting in a film thickness of 660 nm. The average impedance at 1 kHz of these electrodes was 4.8 kΩ and their maximal charge injection capacity (CIC) was 2.3 mC/cm², measured in vitro for a geometric surface area of 3200 µm². The electrode impedance and CIC also remained stable during long-term (276 million pulses) and high-amplitude (135 thousand pulses) stimulation. Lastly, it was shown that the voltage transients remained stable over three months in vivo in a non-human primate with sufficiently low voltage levels required for clinically relevant stimulation currents. Together, the demonstrated electrochemical characteristics of the sputtered iridium oxide film combined with its robustness and long-term stability indicate that it is an excellent material for efficient and safe intracortical stimulation in chronic clinical applications.