Utah Electrode Array Research Articles

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 7 h (>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.

Influence of System Integration and Packaging on Its Inductive Power Link for an Integrated Wireless Neural Interface

In an integrated wireless neural interface based on the Utah electrode array, the implanted electronics are supplied with power through inductive coupling between two coils. This inductive link is affected by conductive and dielectric materials and media surrounding the implant coil. In this study, the influences of the integration of an implant coil on a silicon-based IC and electrode array, thin-film Parylene-C encapsulation, and physiological medium surrounding the coil were investigated systematically and quantitatively by empirical measurements. A few embodiments of implant coils with different geometrical parameters were made with a diameter of approximately 5.5 mm by winding fine wire with a diameter of approximately 50 mum. The parasitic influences affecting the inductive link were empirically investigated by measuring the electrical properties of coils in different configurations and in different media. The distance of power transmission between the transmit and receive coils was measured when the receive coil was in air and immersed in phosphate buffered saline solution to simulate an implanted physiological environment. The results from this study quantitatively address the influences of factors such as device integration, encapsulation, and implantation on its inductive power link, and suggest how to maximize the efficiency in power transmission for such neural interface devices powered inductively.

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.

Integrated wireless neural interface based on the Utah electrode array

This report presents results from research towards a fully integrated, wireless neural interface consisting of a 100-channel microelectrode array, a custom-designed signal processing and telemetry IC, an inductive power receiving coil, and SMD capacitors. An integration concept for such a device was developed, and the materials and methods used to implement this concept were investigated. We developed a multi-level hybrid assembly process that used the Utah Electrode Array (UEA) as a circuit board. The signal processing IC was flip-chip bonded to the UEA using Au/Sn reflow soldering, and included amplifiers for up to 100 channels, signal processing units, an RF transmitter, and a power receiving and clock recovery module. An under bump metallization (UBM) using potentially biocompatible materials was developed and optimized, which consisted of a sputter deposited Ti/Pt/Au thin film stack with layer thicknesses of 50/150/150 nm, respectively. After flip-chip bonding, an underfiller was applied between the IC and the UEA to improve mechanical stability and prevent fluid ingress in in vivo conditions. A planar power receiving coil fabricated by patterning electroplated gold films on polyimide substrates was connected to the IC by using a custom metallized ceramic spacer and SnCu reflow soldering. The SnCu soldering was also used to assemble SMD capacitors on the UEA. The mechanical properties and stability of the optimized interconnections between the UEA and the IC and SMD components were measured. Measurements included the tape tests to evaluate UBM adhesion, shear testing between the Au/Sn solder bumps and the substrate, and accelerated lifetime testing of the long-term stability for the underfiller material coated with a a-SiC(x):H by PECVD, which was intended as a device encapsulation layer. The materials and processes used to generate the integrated neural interface device were found to yield a robust and reliable integrated package.

Thermal Impact of an Active 3-D Microelectrode Array Implanted in the Brain

A chronically implantable, wireless neural interface device will require integrating electronic circuitry with the interfacing microelectrodes in order to eliminate wired connections. Since the integrated circuit (IC) dissipates a certain amount of power, it will raise the temperature in surrounding tissues where it is implanted. In this paper, the thermal influence of the integrated 3-D Utah electrode array (UEA) device implanted in the brain was investigated by numerical simulation using finite element analysis (FEA) and by experimental measurement in vitro as well as in vivo. The numerically calculated and experimentally measured temperature increases due to the UEA implantation were in good agreement. The experimentally validated numerical model predicted that the temperature increases linearly with power dissipation through the UEA, with a slope of 0.029 degree C/mW over the power dissipation levels expected to be used. The influences of blood perfusion, brain metabolism, and UEA geometry on tissue heating were also investigated using the numerical model.

Characterization of a-SiC x:H thin films as an encapsulation material for integrated silicon based neural interface devices

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.

A Low-Power Integrated Circuit for a Wireless 100-Electrode Neural Recording System

Recent work in field of neuroprosthetics has demonstrated that by observing the simultaneous activity of many neurons in specific regions of the brain, it is possible to produce control signals that allow animals or humans to drive cursors or prosthetic limbs directly through thoughts. As neuroprosthetic devices transition from experimental to clinical use, there is a need for fully-implantable amplification and telemetry electronics in close proximity to the recording sites. To address these needs, we developed a prototype integrated circuit for wireless neural recording from a 100-channel microelectrode array. The design of both the system-level architecture and the individual circuits were driven by severe power constraints for small implantable devices; chronically heating tissue by only a few degrees Celsius leads to cell death. Due to the high data rate produced by 100 neural signals, the system must perform data reduction as well. We use a combination of a low-power ADC and an array of "spike detectors" to reduce the transmitted data rate while preserving critical information. The complete system receives power and commands (at 6.5 kb/s) wirelessly over a 2.64-MHz inductive link and transmits neural data back at a data rate of 330 kb/s using a fully-integrated 433-MHz FSK transmitter. The 4.7times5.9 mm2 chip was fabricated in a 0.5-mum 3M2P CMOS process and consumes 13.5 mW of power. While cross-chip interference limits performance in single-chip operation, a two-chip system was used to record neural signals from a Utah Electrode Array in cat cortex and transmit the digitized signals wirelessly to a receiver

Switchable polymer-based thin film coils as a power module for wireless neural interfaces

Reliable chronic operation of implantable medical devices such as the Utah Electrode Array (UEA) for neural interface requires elimination of transcutaneous wire connections for signal processing, powering and communication of the device. A wireless power source that allows integration with the UEA is therefore necessary. While (rechargeable) micro-batteries as well as biological micro-fuel cells are yet far from meeting the power density and lifetime requirements of an implantable neural interface device, inductive coupling between two coils is a promising approach to power such a device with highly restricted dimensions. The power receiving coils presented in this paper were designed to maximize the inductance and quality factor of the coils and microfabricated using polymer-based thin film technologies. A flexible configuration of stacked thin film coils allows parallel and serial switching, thereby allowing to tune the coil's resonance frequency. The electrical properties of the fabricated coils were characterized and their power transmission performance was investigated in laboratory condition.

Cochlear nerve stimulation with a 3-dimensional penetrating electrode array.

An array of penetrating microelectrodes can be implanted into the cochlear nerve to produce stable evoked responses with important electrophysiologic advantages over conventional electrode technology. A totally implantable cochlear implant system would benefit from new electrode technology that lowers the current required for stimulation. Modern cochlear implant arrays placed in the scala tympani have an appreciable distance between the electrodes and the cochlear nerve, the site of intended stimulation. This distance can create the problem of cross-talk, limiting the number of electrodes that can represent discrete frequencies over a given length as well as significantly increasing stimulation thresholds and producing nonfocal stimulation of the fibers in the nerve. An electrode in direct contact with neurons in the cochlear nerve could reduce these problems. The Utah Electrode Array is a novel, three-dimensional, penetrating electrode array intended for direct neural stimulation with the potential ability to implant up to 200 electrodes directly into the cochlear nerve. Arrays containing 6 to 19 electrodes were implanted acutely into six separate cat cochlear nerves for analysis. Thresholds and input/output functions were measured with electrically induced auditory brainstem responses. Current injections in 38 of 70 implanted electrodes produced stable brainstem responses after implantation. The median threshold was 15 microA. An array of penetrating electrodes can be implanted into the cochlear nerve and used to evoke brainstem responses. The responses are of low threshold and are stable. Arrays of electrodes, inserted into auditory nerve, could form the neural interfaces for the next generation of auditory prostheses.

Development of a novel eighth-nerve intraneural auditory neuroprosthesis.

Cochlear nerve stimulation using a linear array of electrodes, the cochlear implant, has become an accepted treatment for profound deafness. Major limitations of this technology are high threshold of stimulation, poor performance in a noisy background, cross-talk between electrodes, unsatisfactory channel selectivity, and variable reconstruction of frequency space. A novel auditory neuroprosthesis is proposed that is expected to overcome these problems by implanting an array of three-dimensional microelectrodes, the Utah Electrode Array, directly into the cochlear nerve. We have conducted acute, extending for up to 12 hours and semichronic, extending for up to 52 hours, electrophysiological experiments, radiologic and histologic studies in 12 cats. The electrically evoked auditory brainstem response was used as a means to characterize the threshold, dynamic range, and stability of cochlear nerve stimulation through the implanted Utah Electrode Array neuroprosthesis. Plain film, computed tomographic, and histological studies were conducted to determine the result of the implant. The electrically evoked auditory brainstem response thresholds were approximately one to two orders of magnitude lower than those evoked with conventional cochlear implants. We were able to close the cochleostomy, bring the cat into normal anatomical position, and obtain stable electrically evoked auditory brainstem responses for up to 52 hours. Plain film and computed tomographic studies indicated that the Utah Electrode Array neuroprosthesis was in the intended position in the nerve. Histological studies did not reveal hemorrhage or significant damage to the nerve. Because the presented stimulation paradigm appears to significantly mitigate some of the problems of conventional cochlear implants, it may offer a new therapeutic approach to profound deafness.

A multielectrode array for intrafascicular recording and stimulation in sciatic nerve of cats

The feasibility of implanting an array of penetrating electrodes into peripheral nerves is studied in acute experiments in the cat sciatic nerve. A novel, silicon-based array of microelectrodes, the Utah Electrode Array, was used, which contains 25 or 100 1-mm long electrodes that project out from a silicon substrate. Electrode arrays of this complexity, when inserted in the peripheral nerve, could cause significant compression of the nerve and block the conduction of action potentials. Using a high velocity insertion technique, the electrode array was implanted into the sciatic nerve. Compound action potentials were evoked by and recorded with cuff electrodes. Compound action potentials recorded 1 h after insertion were only slightly altered from those recorded before insertion. Single units were readily extracted from evoked multiunit neural recordings in response to cutaneous stimulation and limb rotation around joints. Current injections into the nerve through the electrodes evoked muscle twitches with currents in the 10 μA range. Recording and stimulation stability were demonstrated for periods of up to 60 h. We have shown that high density arrays of electrodes can be inserted into the peripheral nerve and can provide a stable recording and stimulating interface to individual peripheral nerve axons. Such an array may be useful in future neuroscience research and potential neuroprosthetic applications.

A 100-channel system for real time detection and storage of extracellular spike waveforms

As extracellular electrode arrays with 100 or more active recording sites become more widely used for simultaneous recording of neural ensembles, practical data acquisition systems that can efficiently accommodate high electrode counts are needed. To reduce the high data rates associated with extracellular recordings from these arrays, various algorithms and systems have been designed to provide complete online detection and classification of extracellular spike waveforms. However, many of these algorithms require significant user supervision to ensure accurate performance. In this paper, we discuss the design and validation of a 100-channel PC-based system that can be used with arrays of extracellular electrodes such as the Utah Electrode Array. Instead of comprehensive online spike analysis, the system performs online detection and storage of the spike waveforms for offline classification. This strategy preserves the data of interest, reduces system complexity, and requires less user supervision during experiments.

A neural interface for a cortical vision prosthesis

The development of a cortically based vision prosthesis has been hampered by a lack of basic experiments on phosphene psychophysics. This basic research has been hampered by the lack of a means to safely stimulate large numbers of cortical neurons. Recently, a number of laboratories have developed arrays of silicon microelectrodes that could enable such basic studies on phosphene psychophysics. This paper describes one such array, the Utah electrode array, and summarizes neurosurgical, physiological and histological experiments that suggest that such an array could be implanted safely in visual cortex. We also summarize a series of chronic behavioral experiments that show that modest levels of electrical currents passed into cortex via this array can evoke sensory percepts. Pending the successful outcome of biocompatibility studies using such arrays, high count arrays of penetrating microelectrodes similar to this design could provide a useful tool for studies of the psychophysics of phosphene perception in human volunteers. Such studies could provide a proof-of-concept for cortically based artificial vision.