Electrochemical Characterization of Microelectrodes for use in Cortical Tissue

Microelectrodes implanted in cortical tissue have been shown to effectively send and record electrical brain signals. The limitation encountered in the past is the robustness of these electrodes. New flexible electrodes are being developed to solve this problem, but electrochemical (EC) measurements are a required step in the characterization of these microelectrodes before they can be implanted in human cortical tissue. Initial cyclic voltammetry (CV) and electronic impedance spectroscopy (EIS) was run in rat tissue in order to understand the charge transfer and impedance characteristics of a model of the flexible microelectrode. Electrodeposited iridium oxide films were deposited on gold electrodes and compared to both gold and polyimide coated stainless steel electrodes. The stainless steel electrodes were found to have a chromium oxide layer that was depleted with each cycle of voltage (-600 mV – 800 mV). This caused a trend towards lower charge transfer abilities, but impedance remained unaffected. Iridium oxide coated gold electrodes resulted in larger charge transfer and lower impedance than pure gold and stainless steel electrodes in saline solution, but loss of the iridium oxide layer made comparison in formalin and rat brain tissue impossible. Both the pure gold and stainless steel electrodes were able to be modeled by an equivalent circuit with mixed kinetic and diffusion controls. The iridium oxide electrode could be modeled by a failed coating model which takes into account the porosity of the surface.