(Invited) Long-Term Stimulation Stability of IrOx Tip Metallization for the Utah Electrode Array
Authors: Baker Brian , Caldwell Ryan , Sharma Rohit , Crosland Davis , Rieth Loren 
Published: February 2020

Long-term neural stimulation and recording efficacy is critical for clinical translation of neural interface technologies for use in therapeutic and diagnostic applications. As part of the DARPA HAPTIX program, we will be performing neural stimulation and recording in peripheral nerves of human subjects for more than 1 year, driving the need to improve the electrode metallization. This motivated the development of aggressive in vitro test to evaluate and improve the electrode material and its electrochemical stability. Utah Electrode Arrays (UEAs) with thin film IrOx electrode sites were pulsed at 1,667 Hz for 109 pulses using cathodal-first square biphasic current with 200 µs cathodal and anodal phases, a 100 µs interphase period, and amplitudes ranging from 100 µA to 2,100 µA. The pulse frequency and current werewell outside the typical ranges of 50 to 200 Hz and 1 to 100 µA used for physiological stimulation in order to accelerate failure. The voltage waveforms were sampled, and used in combination with electrochemical impedance spectroscopy and cyclic voltammetry to monitor the electrochemical properties of the electrodes. In addition, physical characterization of the electrodes was performed using back-scattered scanning electron microscopy and dual-beam focused ion beam analysis to investigate changes in surface morphology and the layer structure of the electrodes. Electrodes were stimulated at amplitudes up to 1,600 µA without observable degradation of the electrochemical properties or changes morphology. This stimulation had cathodic excursions (Emc) up to -2 V, which is well outside the water window. Some electrodes stimulated at amplitudes of 2,100 µA experienced modest degradation, and rarely an electrode was damaged catastrophically, with the majority of the tip metal delaminated. The damaged electrodes were investigated with electron microscopy. The dendritic morphology of the IrOx films was fully preserved except during catastrophic failure where no metal remained to observe. Delamination of the films, ranging from either small regions for the majority of samples to nearly complete metal delamination in rare cases, was observed. This strongly suggests that the IrOx material is highly robust and electrochemically stable during aggressive stimulation testing in-vitro. In addition, with cross-sectional analysis of the tip metal using db-FIB, we observed delamination between the tip metal and silicon electrode. We are investigating if this delamination is occurring through a physical process, or is the result of electrochemical etching of the silicon substrate resulting in undercutting of the electrode. In addition, we have developed a highly sensitive method for electrochemically testing neural electrode dielectric encapsulation. By fully encapsulating devices such that the active electrode region is completely insulated by the dielectric film under investigation, we have been able to resolve the time course and evolution of shunt pathways through dielectric defects with much great sensitivity. The fully encapsulated electrodes are characterized using EIS and current leakage measurements as a function of time to monitor changes in the encapsulation. In addition, electrodes that experience failure as, indicated by leakage currents > 1 nA, were decorated by electrochemical Cu plating to localize failures. Device lifetimes and failure modes characterized by these techniques will be reported.