(J-372) Ruthenium Oxide Neural Recording and Stimulation Electrode Coatings for Implantable Medical Devices

Introduction:: The use of chronically implanted multi-electrode arrays and devices for electrical stimulation and recording of neural activity is increasing with anticipated applications in sensory restoration, treatment of paralysis, epilepsy, and depression, among others. The introduction of microfabrication techniques has led to the miniaturization of these implants, reducing surgical insertion trauma and increasing the spatial selectivity of neural stimulation and recording. In addition, miniaturized devices with ultramicroelectrode dimensions have also been associated with reduced chronic foreign body response (FBR), further improving the selectivity and longevity of the electrical neural interface. However, one of the biggest technical challenges for these microscale implants is the long-term stability of the electrode coatings for neural recording and stimulation. As the physical size of the implants gets smaller, the size of the electrode will decrease. Thus, it is necessary to provide electrode coatings with low impedance and high charge injection capacity while maintaining a relatively small size. Transition metal oxides, such as sputtered iridium oxide (SIROF) and sputtered ruthenium oxide RuO_x, have been used previously as electrode coatings due to their stability, high charge storage capacity, and high charge injection capacity. This study further investigates the relationship between the structure of SIROF and RuO_x electrodes and their charge storage (CSC_c) and stimulation charge injection capacity (Q_inj) using electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and stimulation current pulsing. The objective is to establish structure-property relations that allow the selection and optimization of electrode coatings for electrical microstimulation.


Materials and Methods:: SIROF and RuO_x coatings were deposited and tested on wafer-based test structures made via microfabrication. The size of the electrodes ranged from 25 um² to 4000 um². Ti-Au-Ti metal traces were sputtered on top of a 2-mm amorphous silicon carbide (a-SiC) base layer and then encapsulated by a second a-SiC overlayer. An AJA ATC 2200 sputter deposition system was used to sputter deposit the SIROF and RuO_x coatings. To study the effect of the thickness, SIROF coatings were deposited at 30 mTorr with various deposition time. To study the effect of the film density, RuO_x coatings were deposited at pressures ranging from 5 mTorr to 60 mTorr. All coatings were electrochemically tested in Ar-sparged phosphate buffer saline solution (PBS) at 37°C. EIS and CVs were measured with a Gamry Reference 600+ potentiostat. EIS was measured from 10⁵ Hz to 0.05 Hz at 10 point/decade in the potentiostatic mode. CVs were measured at 50 mV/s and 50000 mV/s sweep rate. All potentials were reported versus Ag|AgCl reference electrodes. Symmetrical biphasic voltage transients of SIROF and RuO_x electrodes were obtained using a Plexon Plexstim stimulator with 200 ms pulse width and 100 ms interphase delay. Cross-sectional and surface images of SIROF and RuO_x coatings were obtained using a Zeiss SIGMA 500 VP Scanning Electron Microscope. Additional optical images of all coatings were obtained using a ZEISS Axio Imager 2 optical microscope.


Results, Conclusions, and Discussions:: We examined the thickness dependence of SIROF charge-injection properties. Fracture cross-section SEM images show that SIROF has a columnar growth morphology that matches that of Zone 1 described in the structure zone models (SZMs). The CSC_c of thicker SIROF electrodes was proportionally higher than those of thinner coatings, suggesting that the iridium redox centers within the SIROF coating remain accessible at thickness up to at least 650 nm. Although Q_inj of thicker coatings was generally higher, the percentage utilization (Q_inj/CSC_c) decreased compared to thinner films, suggesting that access to the redox capacity of the SIROF becomes increasingly limited at the high current densities associated with stimulation current pulsing. We also examined the pressure dependence of RuO_x electrochemical properties. The deposition rate of RuO_x was inversely proportional to pressure. SEM images show RuO_x has a columnar growth morphology that is more defined in coatings deposited at higher pressures, suggesting that lower pressures result in more compact coatings. The columnar structure of the RuO_x has a similar Zone 1 columnar growth morphology. EIS and CV behavior varied with deposition pressure, which we ascribe to pressure-dependent differences in film density. Based on mixed-conductor transport in RuO_x coatings, the results suggest that the microstructure of the RuO_x affects the balance between ionic and electronic conduction within the coatings. The high CSC_c observed in lower-density films is likely due to electrolyte access within the 3D structure of the coatings. Additional potential dependence of EIS was investigated over 0.7 V to -0.7 V range for RuO_x deposited at 30 mTorr. The EIS and post-EIS CVs indicate that RuO_x electrodes may be damaged if biased more negatively than -0.5 V or more positively than 0.5 V. These putative damage thresholds should inform further studies of RuO_x electrodes under neural stimulation conditions.

These observations should inform further study of optimizing coatings with certain structure properties that are physically robust and have good electrochemical properties for neural recording and stimulation.




Acknowledgements (Optional): : Research reported in this publication was supported by the National Institute on Deafness and Other Communication Disorders of the National Institutes of Health under Award Number 1U01NS126052-01. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health. The authors would also like to thank Dr. Kevin Otto and Dr. Mark Orazem from University of Florida for their help.

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