Sputtered Iridium Oxide Films as Charge Injection Material for Functional Electrostimulation

The paper reports on the deposition of iridium oxide thin films by dc reactive magnetron sputtering and their characterization by surface analysis methods (X-ray diffraction, scanning electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy) and electrochemical techniques (cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic square waves). The sputtering process was investigated by applying the method of generic curves. An optimal combination of sputtering parameters has been established [40 standard cubic centimeters per minute (sccm) argon/8-12 sccm oxygen/40% effective pump power/100 W dc power/"cold" sputtering] that yields stable microporous amorphous films with a highly extended surface. These films have shown an excellent electrochemical reversibility in physiological saline solution with a "safe" potential range of −1.2 to 1.3 V vs. Ag/AgCl reference electrode, a charge delivery capacity of 90-95 mC/cm 2 , a constant impedance of 70-80 Ω in the frequency range of 100 kHz down to 0.5-1.0 Hz, and a quick response to bipolar current pulses with a safe charge per phase of 2 mC/cm 2 /ph. The demonstrated remarkable electrochemical characteristics combined with their established long-term mechanical stability and corrosion resistance allow one to recommend the use of sputtered iridium oxide films as an ideal electrode material for neural stimulation. © 2004 The Electrochemical Society. All rights reserved.