Stuart F. Cogan, University of Texas at Dallas, Richardson, TX
There is growing interest in multielectrode arrays that electrically record and stimulate neural activity with a spatial selectivity that is on the order of a single cell or nerve fiber. Emerging applications, particularly those employing neural recording for volitional control or neural stimulation for somatosensory feedback, also contemplate multielectrode arrays with hundreds and likely thousands of individually addressable electrodes. Because these electrodes have a small surface area (<2000 μm²), they are typically coated with an electrode material that is capable of delivering charge to neural tissue via reversible reduction-oxidation reactions that are confined within the three-dimensional structure of the electrode coating. These electrode coatings, typified by sputtered iridium oxide (SIROF), are on the order of 100-1000 nm thick. To achieve a low impedance for recording and high levels of charge-injection at high current densities for stimulation, the coatings are hydrated, mixed electron-ion conductors with a density that is usually <70% of bulk crystalline values. Sputter deposition methods for achieving the desired electrochemical properties of these neural electrode coatings through control of film morphology, density and, when appropriate, hydration are described. Emphasis is placed on SIROF and a comparatively new electrode coating based on ruthenium oxide. The selection of reactive plasma constituents based on hydrogen/oxygen or water/oxygen gas mixtures during DC magnetron sputtering is discussed. The manner in which the plasma chemistry might control the properties of the SIROF and ruthenium oxide films is investigated through optical emission spectroscopy and mass spectroscopy (RGA), using x-ray diffraction, XPS, and Raman spectroscopy, as well as electrochemical measurements, to characterize the electrode coatings. In addition, the neural stimulation properties of the electro-active coatings are compared with those of high-surface-area sputtered titanium nitride (TiN) that is commonly used in cardiac pacing and operates through charging and discharging of the electrochemical double-layer to provide charge delivery for neural stimulation.