Activation of Iridium Electrodes for Neural Stimulation

Abstract

Introduction: Iridium oxide films have been investigated for multiple uses including electrochromic materials, pH sensors, and neural stimulation electrodes. In neural interfaces, activated iridium oxide film (AIROF) electrodes with planar or 3-dimensional geometries have been used to increase charge storage capacity and charge injection capacity for electrical stimulation of neural tissue. General properties of iridium oxide films have been investigated by several publications, leading to proposed film structures, observations on differences between anhydrous and hydrous iridium oxide films, and reported super-Nernstian changes in potential with pH. [1,2,3] In the present work, we investigated the activation of sharpened iridium microwire electrodes to AIROF using the same approach discussed in prior work. [4,5] We first sought to establish the charge-injection properties of AIROF electrodes in phosphate buffered saline and in an inorganic model of interstitial fluid as a function of the number of activation pulses. As the electrochemical properties of AIROF are known to be highly dependent on the prior oxidation state of the film, [1] we also sought to understand the charge injection Qinj capacity of AIROF when held at 0V compared to +0.6V vs. Ag|AgCl reference. Film cracking and delamination was observed in some electrodes during characterization, and so, we sought to understand the origin of the observed film instability and the role of limiting the maximum current allowed during activation. Methods: Iridium wire probes with 2000µm2 surface area were electrochemically cleaned in phosphate buffered saline (PBS). Cyclic voltammetry (CV) was recorded at 0.05V/s and 50V/s scan rates before and after cleaning. Each probe was activated at 25°C in air equilibrated PBS with 0.05Hz square wave potential pulsing between +0.8V and -0.6V vs. Ag|AgCl. Electrochemical impedance spectroscopy, CV, and voltage transient measurements at 0V and +0.6V bias vs. Ag|AgCl were recorded in both PBS and model interstitial fluid (m-ISF) at 37°C. Changes to the electrode surface were evaluated through cyclic voltammetry at multiple scan rates followed by scanning electron microscope (SEM) images of the film after soaking in DI water and drying. Results: Charge storage capacity (CSC) and maximum charge injection (Qinj) capacity increase significantly within the first 20 activation pulses. While CSC values in m-ISF are similar to values in PBS, Qinj capacity measured in m-ISF is significantly lower. Growth of the iridium oxide film was influenced by limiting the maximum current allowed during potential pulsing for activation. CSC and Qinj capacity of the 2000µm2 AIROF electrodes increased with increasing activation up to 1300 pulses. However, SEM images and CV profiles of probes activated without current limiting showed evidence of cracking in the microstructure of the film, and delamination of the film after 1300 pulses. Conclusions: Kinetics that are more informative for evaluating potential performance of neural stimulation devices in vivo can be observed when AIROF electrodes are measured in m-ISF in addition to PBS. Despite the long history of iridium oxide film characterization, parameters influencing activation are not yet fully understood as the dependence of film growth on current limiting found in this study was not expected. The degree to which activation parameters influence film growth currently appear to be specific to the electrode geometry and the electrochemical cell used and must be further understood to accomplish reliable transition to a stable oxide film covering the metal surface. Further study of the iridium oxide film structure and activation process is needed in order to standardize the activation process for the use of AIROF in neural devices.