Neural interface dynamics following insertion of hydrous iridium oxide microelectrode arrays

Studies examining traumatic brain injury have suggested a 'window of opportunity' exists for therapeutic agents to mitigate edema and cellular toxicity effectively. However, successful therapy also relies on identifying the extent of blood-brain barrier disruption, which is associated with excessive extra-cellular concentrations of ions, excitatory amino acids, and serum proteins. The following study investigates the use of pH-selective hydrous iridium oxide microelectrodes to assess trauma following insertion of a neural probe. Electrochemical activation of iridium microelectrode arrays was performed in either acidic (0.5 M H2SO4) or weak basic (0.3 M Na2HPO4, pH=8.56) solutions. Both oxides demonstrated super-Nernstian pH sensitivity (-88.5 mV/pH and -77.1 mV/pH, respectively) with little interference by other cations. Data suggest that acid-grown oxide provides better potential stability than base-grown oxide (sigma=2.8 versus 4.9 mV over 5 hours). Implantation of these electrodes into motor cortex and dorsal striatum revealed significant acidosis during and following insertion. Variability in the spatiotemporal pH profile included micro-scale inhomogeneities along the probe shank and significant differences in the averaged pH response between successive insertions using the same depth and speed. This diagnostic technology has important implications for intervention therapies in order to more effectively treat acute surgical brain trauma.