High-charge-capacity sputtered iridium oxide neural stimulation electrodes deposited using water vapor as a reactive plasma constituent.

Abstract

The deposition and properties of sputtered iridium oxide films (SIROFs) using water vapor as a reactive gas constituent are investigated for their potential as high-charge-capacity neural stimulation electrodes. Systematic investigation through a series of optical and electrochemical measurements reveals that the incorporation of water vapor as a reactive gas constituent, along with oxygen, alters the reduction-oxidation (redox) state of the plasma as well as its morphology and the electrochemical characteristics, including the cathodal charge-storage capacity (CSC_c ) and charge-injection capacity (CIC), of the SIROF. An apparent optimal O₂ :H₂ O gas ratio of 1:3 produced SIROF with a CSC_c of 182.0 mC cm^-2 μm^-1 (median, Q1 = 172.5, Q3 = 193.4, n = 15) and a CIC of 3.57 mC cm^-2 (median, Q1 = 2.97, Q3 = 4.58, n = 12) for 300-nm-thick films. These values are higher than those obtained with SIROFs deposited using no water vapor by a factor of 2.3 and 1.7 for the CSC_c and CIC, respectively. Additionally, the SIROF showed minimal changes in electrochemical characteristics over 10⁹ pulses of constant current stimulation and showed no indication of cytotoxicity toward primary cortical neurons in a cell viability assay. These results warrant investigation of the chronic recording and stimulation capabilities of the SIROF for implantable microelectrode arrays.