Sputtered Electrode Coatings for Neural Stimulation and Recording




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Electrode coatings form an integral part of implantable microelectrode arrays (MEAs) which are the basis for chronic neural interfaces capable of providing feedback though neural recording and eliciting functional response through electrical stimulation in patients with lost or impaired neurological functionality. The aim of this dissertation was to develop and assess chronically stable electrode materials with low-impedance and high charge-injection capacity for neural recording and stimulation respectively. To this end, we have investigated three types of transition metal oxide thin-films, iridium oxide, ruthenium oxide and a mixed ruthenium/titanium oxide formed by a DC magnetron reactive sputtering process. We have characterized these films in terms of their microstructure, composition and electrochemistry in order to establish the relationship between film properties and electrochemical charge-injection capacities. Sputtered iridium oxide films (SIROF) and ruthenium oxide films (RuOx) were deposited by reactive sputtering in a DC magnetron sputtering system using water-vapor as a reactive plasma constituent. The films deposited using a combination of oxygen and water-vapor showed lower impedance and higher charge-injection capacity than the films deposited using oxygen alone. The films deposited using only water-vapor as the reactive gas showed the presence of nano-sized metallic iridium and ruthenium on the SIROF and RuOx films respectively. Systematic investigation by plasma optical spectroscopy, surface characterization and electrochemical measurements revealed that the incorporation of water-vapor as a reactive gas constituent, along with oxygen, alters the reduction-oxidation (redox) state of the plasma as well as the microstructure and the electrochemical characteristics for the SIROF and RuOx films. A 3:1 water-vapor to oxygen plasma condition formed a hydrous, low-crystallinity and nodular film microstructure that enabled both electronic and ionic conduction producing an apparent optimal electrode coating with low impedance for recording and high charge-injection capacity for stimulation. Electrochemical measurements on SIROF and RuOx electrodes, deposited using a ratio of watervapor to oxygen 3:1, demonstrated that these have minimal contribution towards oxygen reduction, an unwanted side reaction and also showed rapid reversibility of their respective Faradaic processes during stimulation current pulsing. These films were found to be electrochemically stable for ~1 billion pulses at biphasic 8 nC/phase (0.4 mC/cm2 ) constant current stimulation in an inorganic model of interstitial fluid at 37o C and also showed no indication of cytotoxicity towards primary cortical neurons in a cell viability assay. Additionally, we investigated the deposition and characterization of sputtered mixed oxide films consisting of ruthenium/titanium oxide (RuTiOx) as a neural stimulation and recording electrode. This study was motivated by the observation that RuOx films deposited at optimal watervapor:oxygen gas flow ratios were more friable and thus less physically stable than SIROF deposited under the same conditions. Incorporation of titanium resulted in an increased the hardness of the film, by a factor of two with respect to RuOx films, while still maintaining an adequately low enough impedance for recording and a charge-injection capacity suitable for clinical neural stimulation.



Engineering, Materials Science, Engineering, Biomedical