Charge injection characteristics of sputtered ruthenium oxide electrodes for neural stimulation and recording
B. Chakraborty, Alexandra Joshi‐Imre, Stuart F. Cogan
Abstract
Abstract 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/cm 2 (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/cm 2 (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/cm 2 (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/cm 2 (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.