| Activated iridium oxide film (AIROF) microelectrodes have been widely used for neural stimulation applications. They are capable, relative to bare metal electrodes, of higher charge capacity due to their reversible Ir(+III)/Ir(+IV) redox reaction mechanism. Traditionally, the limits of safe use for all stimulating metal microelectrodes have been characterized by defining the maximum charge density (mC/cm2) limit, for each metal type, for which irreversible electrochemical reaction, such as water electrolysis, can be avoided during the neural stimulation current delivery.;Such safe charge density limit (also called charge capacity) is often set by measuring the performance of a candidate electrode while placed within a relatively high concentration/buffered electrolyte such as phosphate buffered saline (PBS). In PBS the charge capacity of AIROF is typically about 10 times higher than that of a similarly-sized Platinum metal electrode. However, when testing our AIROF stimulating microelectrodes in vivo, and using the same limiting criteria as was defined in vitro, the electrodes had only about 1/10 of their in vitro charge capacity, thus raising the question: how can we characterize the electrode/neural tissue interface and ensure the safe use of AIROF for neural stimulation in vivo?;To address this question, we performed both acute and chronic AIROF microelectrode experiments in the bird cortex. Analysis of the experimental data resulted in the identification of a more sophisticated approach to assessing the safe charge delivery protocol of AIROF microelectrodes. The inter-phase electrode voltage, which is a direct measure of the polarization at the electrode/electrolyte interface, in combination with the transition time are used to distinguish between safe and unsafe charge delivery mechanisms. These serve as a better safety measure for AIROF charge delivery than the traditional mC/cm 2 value.;The general principles that define this new safety paradigm apply not only to AIROF microelectrodes, but also to other metal-based neural stimulation microelectrodes as well. Importantly, defining methods for electrochemically safe charge delivery during neural stimulation, in vivo, is a significant step towards understanding how to maintain a stable and reliable artificial neural interface for chronic neural prosthesis applications, as well as for long-term neuroscience experiments. |
