Loren Rieth

Encapsulation of an Integrated Neural Interface Device With Parylene C

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 37degC 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.

In vitro comparison of sputtered iridium oxide and platinum-coated neural implantable microelectrode arrays.

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.

Neural Electrode Degradation from Continuous Electrical Stimulation: Comparison of Sputtered and Activated Iridium Oxide

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 (UEAs) 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 7h (>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/cm(2) per phase and 80 nC with a charge density of 1.0 mC/cm(2) per phase. While for AIROF coated electrode tips, the threshold was between 40 nC with a charge density of 0.9 mC/cm(2) per phase and 50 nC with a charge density of 0.5 mC/cm(2) per phase. Compared to AIROF, SIROF showed higher damage threshold and therefore is highly recommended to be used as a stimulation material.

A new high-density (25 electrodes/mm2) penetrating microelectrode array for recording and stimulating sub-millimeter neuroanatomical structures

OBJECTIVE Among the currently available neural interface devices, there has been a need for a penetrating electrode array with a high electrode-count and high electrode-density (the number of electrodes/mm(2)) that can be used for electrophysiological studies of sub-millimeter neuroanatomical structures. We have developed such a penetrating microelectrode array with both a high electrode-density (25 electrodes/mm(2)) and high electrode-count (up to 96 electrodes) for small nervous system structures, based on the existing Utah Slanted Electrode Array (USEA). Such high electrode-density arrays are expected to provide greater access to nerve fibers than the conventionally spaced USEA especially in small diameter nerves. APPROACH One concern for such high density microelectrode arrays is that they may cause a nerve crush-type injury upon implantation. We evaluated this possibility during acute (<10 h) in vivo experiments with electrode arrays implanted into small diameter peripheral nerves of anesthetized rats (sciatic nerve) and cats (pudendal nerve). MAIN RESULTS Successful intrafascicular implantation and viable nerve function was demonstrated via microstimulation, single-unit recordings and histological analysis. Measurements of the electrode impedances and quantified electrode dimensions demonstrated fabrication quality. The results of these experiments show that such high density neural interfaces can be implanted acutely into neural tissue without causing a complete nerve crush injury, while mediating intrafascicular access to fibers in small diameter peripheral nerves. SIGNIFICANCE This new penetrating microelectrode array has characteristics un-matched by other neural interface devices currently available for peripheral nervous system neurophysiological research.

Long-term reliability of Al2O3?and Parylene C bilayer encapsulated Utah electrode array based neural interfaces for chronic implantation

OBJECTIVE We focus on improving the long-term stability and functionality of neural interfaces for chronic implantation by using bilayer encapsulation. APPROACH We evaluated the long-term reliability of Utah electrode array (UEA) based neural interfaces encapsulated by 52 nm of atomic layer deposited Al2O3 and 6 µm of Parylene C bilayer, and compared these to devices with the baseline Parylene-only encapsulation. Three variants of arrays including wired, wireless, and active UEAs were used to evaluate this bilayer encapsulation scheme, and were immersed in phosphate buffered saline (PBS) at 57 °C for accelerated lifetime testing. MAIN RESULTS The median tip impedance of the bilayer encapsulated wired UEAs increased from 60 to 160 kΩ during the 960 days of equivalent soak testing at 37 °C, the opposite trend to that typically observed for Parylene encapsulated devices. The loss of the iridium oxide tip metallization and etching of the silicon tip in PBS solution contributed to the increase of impedance. The lifetime of fully integrated wireless UEAs was also tested using accelerated lifetime measurement techniques. The bilayer coated devices had stable power-up frequencies at ∼910 MHz and constant radio-frequency signal strength of -50 dBm during up to 1044 days (still under testing) of equivalent soaking time at 37 °C. This is a significant improvement over the lifetime of ∼100 days achieved with Parylene-only encapsulation at 37 °C. The preliminary samples of bilayer coated active UEAs with a flip-chip bonded ASIC chip had a steady current draw of ∼3 mA during 228 days of soak testing at 37 °C. An increase in the current draw has been consistently correlated to device failures, so is a sensitive metric for their lifetime. SIGNIFICANCE The trends of increasing electrode impedance of wired devices and performance stability of wireless and active devices support the significantly greater encapsulation performance of this bilayer encapsulation compared with Parylene-only encapsulation. The bilayer encapsulation should significantly improve the in vivo lifetime of neural interfaces for chronic implantation.

A 3D glass optrode array for optical neural stimulation

This paper presents optical characterization of a first-generation SiO2 optrode array as a set of penetrating waveguides for both optogenetic and infrared (IR) neural stimulation. Fused silica and quartz discs of 3-mm thickness and 50-mm diameter were micromachined to yield 10 × 10 arrays of up to 2-mm long optrodes at a 400-μm pitch; array size, length and spacing may be varied along with the width and tip angle. Light delivery and loss mechanisms through these glass optrodes were characterized. Light in-coupling techniques include using optical fibers and collimated beams. Losses involve Fresnel reflection, coupling, scattering and total internal reflection in the tips. Transmission efficiency was constant in the visible and near-IR range, with the highest value measured as 71% using a 50-μm multi-mode in-coupling fiber butt-coupled to the backplane of the device. Transmittance and output beam profiles of optrodes with different geometries was investigated. Length and tip angle do not affect the amount of output power, but optrode width and tip angle influence the beam size and divergence independently. Finally, array insertion in tissue was performed to demonstrate its robustness for optical access in deep tissue.

A novel masking method for high aspect ratio penetrating microelectrode arrays

Neural interfaces connect signal processing electronics to the nervous system via implanted microelectrode arrays such as the Utah electrode array (UEA). The penetrating electrodes of the UEA are encapsulated with parylene-C. In order to form active electrode sites, parylene-C must be removed from the electrode tips. Masking the electrodes to selectively de-insulate the tips has been accomplished by poking the electrode through aluminum foil. This mechanical poking process lacks precise control over the length of exposed tips. The non-uniformity in tip exposure can change the electrode impedance across the array, which causes variability in recording and stimulating characteristics. This paper focuses on the development of a fabrication technology to produce uniformly exposed tip lengths, over a range of 30–350 µm. A novel batch-oriented and wafer-scale method to de-insulate the electrode tips of the UEA is presented which uses a photoresist to mask the samples and an oxygen plasma to etch parylene-C. The tip exposure is controlled by varying the spin speed during photoresist coating of the electrode array. At 100 rpm, the non-uniformity of the batch-processed arrays was 14.74 ± 7.49% while in the case of a wafer-scale process it was 3.0 ± 0.2% for 10 × 10 electrodes; this indicates that the wafer-scale process leads to better uniformity in tip exposure. A set of photoresist-based process parameters were also tested to ensure that the electrical and mechanical properties of parylene-C were not compromised by the process.

Plasma-assisted atomic layer deposition of Al2O3 and parylene C bi-layer encapsulation for chronic implantable electronics

Encapsulation of biomedical implants with complex three dimensional geometries is one of the greatest challenges achieving long-term functionality and stability. This report presents an encapsulation scheme that combines Al(2)O(3) by atomic layer deposition with parylene C for implantable electronic systems. The Al(2)O(3)-parylene C bi-layer was used to encapsulate interdigitated electrodes, which were tested invitro by soak testing in phosphate buffered saline solution at body temperature (37 °C) and elevated temperatures (57 °C and 67 °C) for accelerated lifetime testing up to 5 months. Leakage current and electrochemical impedance spectroscopy were measured for evaluating the integrity and insulation performance of the coating. Leakage current was stably about 15 pA at 5 V dc, and impedance was constantly about 3.5 MΩ at 1 kHz by using electrochemical impedance spectroscopy for samples under 67 °C about 5 months (approximately equivalent to 40 months at 37 °C). Alumina and parylene coating lasted at least 3 times longer than parylene coated samples tested at 80 °C. The excellent insulation performance of the encapsulation shows its potential usefulness for chronic implants.

Long-Term Bilayer Encapsulation Performance of Atomic Layer Deposited Al

We present an encapsulation scheme that combines atomic layer deposited (ALD) Al2O3 and Parylene C for the encapsulation of implantable devices. The encapsulation performances of combining alumina and Parylene C was compared to individual layers of Parylene C or alumina and the bilayer coating had superior encapsulation properties. The alumina-Parylene coated interdigitated electrodes (IDEs) soaked in PBS for up to nine months at temperatures from 37 to 80 °C for accelerated lifetime testing. For 52-nm alumina and 6-μm Parylene C, leakage current was ~20 pA at 5 VDC, and the impedance was about 3.5 MΩ at 1 kHz with a phase near -87° from electrochemical impedance spectroscopy for samples soaked at 67 °C for equivalent lifetime of 72 months at 37 °C. The change of impedance during the whole soaking period (up to 70 months of equivalent soaking time at 37 °C) over 1 to 10 6 Hz was within 5%. The stability of impedance indicated almost no degradation of the encapsulation. Bias voltage effect was studied by continuously applying 5 VDC, and it reduced the lifetime of Parylene coating by ~75% while it showed no measurable effect on the bilayer coating. Lifetime of encapsulation of IDEs with topography generated by attaching a coil and surface mount device (SMD) capacitor was about half of that of planer IDEs. The stable long-term insulation impedance, low leakage current, and better lifetime under bias voltage and topography made this double-layer encapsulation very promising for chronic implantable devices.

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This paper characterizes the Utah Slant Optrode Array (USOA) as a means to deliver infrared light deep into tissue. An undoped crystalline silicon (100) substrate was used to fabricate 10 × 10 arrays of optrodes with rows of varying lengths from 0.5 mm to 1.5 mm on a 400-μm pitch. Light delivery from optical fibers and loss mechanisms through these Si optrodes were characterized, with the primary loss mechanisms being Fresnel reflection, coupling, radiation losses from the tapered shank and total internal reflection in the tips. Transmission at the optrode tips with different optical fiber core diameters and light in-coupling interfaces was investigated. At λ = 1.55μm, the highest optrode transmittance of 34.7%, relative to the optical fiber output power, was obtained with a 50-μm multi-mode fiber butt-coupled to the optrode through an intervening medium of index n = 1.66. Maximum power is directed into the optrodes when using fibers with core diameters of 200 μm or less. In addition, the output power varied with the optrode length/taper such that longer and less tapered optrodes exhibited higher light transmission efficiency. Output beam profiles and potential impacts on physiological tests were also examined. Future work is expected to improve USOA efficiency to greater than 64%.