Charge injection characteristics of sputtered ruthenium oxide electrodes for neural stimulation and recording

We have studied the charge-injection characteristics and electrochemical impedance of sputtered ruthenium oxide (RuO_x ) films as electrode coatings for neural stimulation and recording electrodes. RuO_x films were deposited by reactive DC magnetron sputtering, using a combination of water vapor and oxygen gas as reactive plasma constituents. The cathodal charge storage capacity of planar RuO_x electrodes was found to be 54.6 ± 9.5 mC/cm² (mean ± SD, n = 12), and the charge-injection capacity in a 0.2-ms cathodal current pulse was found to be 7.1 ± 0.3 mC/cm² (mean ± SD, n = 15) at 0.6 V positive bias versus Ag|AgCl, in phosphate buffer saline at room temperature for ~250 nm thick films. In general, the RuO_x films exhibited high charge-injection capacities, with or without a positive interpulse bias, comparable to sputtered iridium oxide (SIROF) coatings. The charge-injection capacity increased monotonically with film thickness from 120 to 630 nm, and reached 11.30 ± 0.34 mC/cm² (mean ± SD, n = 5) at 0.6 V bias versus Ag|AgCl at 630 nm film thickness. In addition, RuO_x films showed minimal changes in electrochemical characteristics over 1.5 billion cycles of constant current pulsing at a charge density of 408 μC/cm² (8 nC/phase, 200 μs pulse width). The findings of low-impedance, high charge-injection capacity, and long-term pulsing stability suggest the suitability of RuO_x as a comparatively inexpensive and favorable choice of electrode material for neural stimulation and recording.