the temptation to apply results from studies of one electrode design to other electrode­ designs that use the same metal or coating often results in the discovery of electrode failure, or damage to the surrounding neurons, only after the elect­ rode is implanted and chronically stimulated. Using the stability of the functional response of the stimulated neurons as a measure of safe charge injection is risky, since shifts in neuronal thresholds may occur only after damage to either the electrode or the tissue has already occurred.

11.4  Intracortical Electrode Coatings

Recognizing the charge injection limitations of intracortical metal electrodes, the National Institutes of Health, under the administration of the Neural Prosthesis Program, funded research to identify coatings that could be placed over metal electrodes towards the goal of limiting the charge injection reactions to within the water window [30]. This resulted in the identification and development of Activated IRidium Oxide Film (AIROF) at EIC Laboratories (Norwood, MA). AIROF is a faradaic coating that is based upon the electrochemical growth of a three-dimensional film of hydrated iridium oxide [1, 30]. Presently, AIROF, and Sputtered IRidium Oxide Film (SIROF) [15, 27, 41] have emerged as the preferred coating materials for many neural stimulating electrodes.

AIROF is formed upon pure iridium metal electrodes using an electrochemical activation process. The attractiveness of AIROF as a stimulation electrode coating was recognized by Brummer and first reported in 1983 [8, 35]. Charge injection limits for AIROF electrodes in physiological buffer were reported in 1988 by Beebe and Rose [6].

The electrochemistry of the activation process has been studied extensively and models to explain the observed accumulation of oxide and the charge propagation mechanism have been suggested [10, 11, 30, 32]. It is known that thick anodic oxide films can be formed on the surface of an iridium electrode by continuously cycling the electrode potential with a triangular or square waveform in an aqueous electrolyte. The potential limits are typically between values slightly positive of hydrogen evolution and just negative of the onset of oxygen evolution. It has been shown that AIROF formation is influenced by the chemical composition of the electrolyte; the geometry and morphology of the iridium metal substrate; and, the duration and form of the voltage/current activation waveform [23, 45]. The electrolyte composition influences the rate of formation as well as oxide morphology through the pH, the ionic strength, the conductivity and structure of the double layer at the Ir/electrolyte interface [17].

The benefit of using AIROF on intracortical electrodes stems from the premise that the redox reactions needed for charge transfer from the electrode to the tissue can take place exclusively within the film. Thus the film serves as a buffer zone for charge transfer between the metal surface and the biological electrolyte. AIROF is known for demonstrating significantly higher maximum charge injection limits,