(35) Influence of Microstructure on the Charge-injection Properties of SIROF Neural Stimulation Electrode Coatings

Introduction: Multielectrode arrays (MEAs) with ultrasmall electrode sizes are of interest for neural stimulation in both behavioral studies and future clinical applications.[1-3] Here we report on the effect of microstructure and thickness of sputtered iridium oxide (SIROF) coatings.[4-6] As MEAs evolve towards smaller insertion footprints and higher spatial selectivity, electrode performance faces the challenge of a reduced electrode geometric surface area while maintaining sufficient charge-injection capability to elicit a physiological functional response.[6-9] In this study, we investigate the relationship between impedance, cathodal charge-storage capacity (CSCc), and charge-injection capacity (Qinj) as a function of deposition thickness for SIROF coatings having microelectrode (GSA = 2000 µm2) and ultrasmall microelectrode (GSA = 80 µm2) sizes.

Methods: We employed electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and voltage transients (VT) in phosphate buffered saline (PBS) at 37oC. Electrode potential was recorded versus an Ag|AgCl reference. SIROF was sputtered via reactive sputtering using a customized 2200 magnetron sputtering system.[5] Five thicknesses of SIROF were studied: 320 nm, 650 nm, 850 nm, 1030 nm, and 1320 nm.

Results: The SIROF growth was found to be linear with respect to deposition time. Fracture-cross-sectional SEM images revealed a columnar growth morphology with inter-column voids having a hierarchical surface morphology. Overall, the microstructure can be described as Zone 1 in the structure zone model (SZMs) of vacuum-deposited thin-films.[10] The focused-ion-beam (FIB) cross-sectional SEM images further showed a highly porous, arborized structure that was not apparent in SEMs of fracture cross-sections. These observations indicate that SIROF is a highly porous film with multiple levels of porosity: inter-column voids, micromorphology on columnar surfaces, and columnar arborization. This microstructure, observed at all thickness levels, promotes a high ESA to GSA ratio. We report impedance, CSCc, and Qinj for all five thicknesses. Impedance (1 kHz) decreased and Qinj increased monotonically with thickness. When pulsing the electrodes to the maximum cathodic potentials (-0.6V versus Ag|AgCl), Qinj for SIROF also increased with SIROF thickness, although the rate of increase was less than that of CSCc.

Conclusion: These results suggest that SIROF porosity remains electrochemically accessible from 320 nm to 1320 nm, significantly affecting its electrochemical performance and potential applications in ultrasmall electrodes.