Electronic neural interfaces have been developed to restore function to the nervous system for patients with neural disorders. A conformal and chronically stable dielectric encapsulation is required to protect the neural interface device from the harsh physiological environment and localize the active electrode tips. Chemical vapor deposited Parylene-C films were studied as a potential implantable dielectric encapsulation material using impedance spectroscopy and leakage current measurements. Both tests were performed in 37 degrees C saline solution, and showed that the films provided an electrically insulating encapsulation for more than one year. Isotropic and anisotropic oxygen plasma etching processes were compared for removing the Parylene-C insulation to expose the active electrode tips. Also, the relationship between tip exposure and electrode impedance was determined. The conformity and the uniformity of the Parylene-C coating were assessed using optical microscopy, and small thickness variations on the complex 3-D electrode arrays were observed. Parylene C was found to provide encapsulation and electrical insulation required for such neural interface devices for more than one year. Also, oxygen plasma etching was found to be an effective method to etch and pattern Parylene-C films.

Neural interfaces connect signal processing electronics to the nervous system via implanted microelectrode arrays such as the Utah electrode array (UEA). The active sites of the UEA are coated with thin films of either platinum (Pt) or iridium oxide (IrOx). Pt and IrOx have attracted attention as a stimulating or recording material due to their ability to transfer between ionic and electronic current and to resist corrosion. The physical, mechanical, chemical, electrical and optical properties of thin films depend on the method and deposition parameters used to deposit the films. In this work, surface morphology, impedance and charge capacity of Pt and sputtered iridium oxide film (SIROF) were investigated and compared with each other. UEAs with similar electrode area and shape were employed in this study. DC sputtering was used to deposit Pt films and pulsed-dc reactive sputtering was used to deposit SIROF. The electrodes coated with SIROF and Pt were characterized by scanning electron microscopy, cyclic voltammetry, electrochemical impedance spectroscopy and potential transient measurements to measure charge injection capacity (CIC). SIROF and Pt selectively deposited on the electrode tip had dendrite and granular microstructure, respectively. The CIC of unbiased SIROF and Pt was 2 and 0.3 mC cm(-2), respectively. The average impedance at 1 kHz, of SIROF and Pt electrodes, was 6 kOmega and 125 kOmega, respectively. Low impedance and high CIC make SIROF promising stimulation/recording material for neural prosthetic applications.

The performance of neural electrodes in physiological fluid, especially in chronic use, is critical for the success of functional electrical stimulation devices. Tips of the Utah Electrode Arrays (UEA) were coated with sputtered iridium oxide film (SIROF) and activated iridium oxide film (AIROF) to study the degradation during charge injection consistent with functional electrical stimulation (FES). The arrays were subjected to continuous biphasic, cathodal first, charge balanced (with equal cathodal and anodal pulse widths) current pulses for 7 hours (> 1 million pulses) at a frequency of 50 Hz. The amplitude and width of the current pulses were varied to determine the damage threshold of the coatings. Degradation was characterized by scanning electron microscopy, inductively coupled plasma mass spectrometry, electrochemical impedance spectroscopy and cyclic voltammetry. The injected charge and charge density per phase were found to play synergistic role in damaging the electrodes. The damage threshold for SIROF coated electrode tips of the UEA was between 60 nC with a charge density of 1.9 mC/cm2 per phase and 80 nC with a charge density of 1.0 mC/cm2 per phase. While for AIROF coated electrode tips, the threshold was between 40 nC with a charge density of 0.9 mC/cm2 per phase, and 50 nC with a charge density of 0.5 mC/cm2 per phase. Compared to AIROF, SIROF showed higher damage threshold and therefore is highly recommended to be used as a stimulation material.

Background: Cervical vagus nerve stimulation (VNS) is an emerging bioelectronic treatment for brain, metabolic, cardiovascular and immune disorders. Its desired and off-target effects are mediated by different nerve fiber populations and knowledge of their engagement could guide calibration and monitoring of VNS therapies. Objective: Stimulus-evoked compound action potentials (eCAPs) directly provide fiber engagement information but are currently not feasible in humans. A method to estimate fiber engagement through common, noninvasive physiological readouts could be used in place of eCAP measurements. Methods: In anesthetized rats, we recorded eCAPs while registering acute physiological response markers to VNS: cervical electromyography (EMG), changes in heart rate (DHR) and breathing interval (DBI). Quantitative models were established to capture the relationship between A-, Band C-fiber type activation and those markers, and to quantitatively estimate fiber activation from physiological markers and stimulation parameters. Results: In bivariate analyses, we found that EMG correlates with A-fiber, DHR with B-fiber and DBI with C-fiber activation, in agreement with known physiological functions of the vagus. We compiled multivariate models for quantitative estimation of fiber engagement from these markers and stimulation parameters. Finally, we compiled frequency gain models that allow estimation of fiber engagement at a wide range of VNS frequencies. Our models, after calibration in humans, could provide noninvasive estimation of fiber engagement in current and future therapeutic applications of VNS.

A fully integrated, wireless neural interface device is being developed to free patients from the restriction and risk of infection associated with a transcutaneous wired connection. This device requires a hermetic, biocompatible encapsulation layer at the interface between the device and the neural tissue to maintain long-term recording/stimulating performance of the device. Hydrogenated amorphous silicon carbide (a-SiC x :H) films deposited by a plasma enhanced chemical vapor deposition using SiH 4 , CH 4 , and H 2 precursors were investigated as the encapsulation layer for such device. Si-C bond density, measured by Fourier transform infrared absorption spectrometer, suggests that deposition conditions with increased hydrogen dilution, increased temperature, and low silane flow typically result in increase of Si-C bond density. From the variable angle spectroscopic ellipsometry measurement, no dissolution of a-SiC x :H was observed during soaking tests in 90°C phosphate buffered saline. Conformal coating of the a-SiC x :H in Utah electrode array was observed by scanning electron microscope. Electrical properties were studied by impedance spectroscopy to investigate the performance of a-SiC x :H as an encapsulation layer, and the results showed long term stability of the material.