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Characterization and variability of intracortical iridium oxide microelectrodes
Abstract
Over 200 iridium microelectrodes with activated iridium oxide (AIROF) charge-injection sites were evaluated in vitro by cyclic voltammetry and voltage transient measurements during current pulsing. Variability in the electrochemical behavior of these nominally identical microelectrodes was correlated with variations in the amount of AIROF present and the physical condition of the charge-injection tip. Electrolyte leakage under polymer insulation and delamination of AIROF both gave rise to characteristic shapes in cyclic voltammograms which could be used to identify their presence.
Objective:
Iridium oxide films are commonly used as a high charge-injection electrode material in neural devices. Yet, few studies have performed in-depth assessments of material performance versus film thickness, especially for films grown on three-dimensional (instead of planar) metal surfaces in neutral pH electrolyte solutions. Further, few studies have investigated the driving voltage requirements for constant-current stimulation using activated iridium oxide (AIROF) electrodes, which will be a key constraint for future use in wirelessly powered neural devices.
Approach:
In this study, iridium microwire probes were activated by repeated potential pulsing in room temperature phosphate buffered saline (pH 7.1-7.3). Electrochemical measurements were recorded in three different electrolyte conditions for probes with different geometric surface areas (GSAs) as the AIROF thickness was increased.
Main results:
Maintaining an anodic potential bias during the inter-pulse interval was required for AIROF electrodes to deliver charge levels considered necessary for neural stimulation. Potential pulsing for 100-200 cycles was sufficient to achieve charge injection levels of 2.5 mC cm-2 (50 nC/phase in a biphasic pulse) in PBS with 2000 µm2 iridium probes. Increasing the electrode surface area to 3000 µm2 and 4000 µm2 significantly increased charge-injection capacity, reduced the driving voltage required to deliver a fixed amount of charge, and reduced polarization of the electrodes during constant-current pulsing.
Significance:
This study establishes methods for choosing an activation protocol and a desired GSA for three-dimensional iridium electrodes suitable for neural tissue insertion and stimulation, and provides guidelines for evaluating electrochemical performance of AIROF using model saline solutions.
A new area saving stimulator cell is described, which is suitable for implantable microstimulators with large numbers of electrodes. The cell generates biphasic charge balanced voltage waveforms by continuous monitoring of the microelectrode voltage during the cathodic and anodic phases. The stimulator provides high voltage compliance and is intended for use with indium oxide micro electrodes with a limited electrochemical potential window. Thus, the new approach can improve electrode count, microelectrode reliability, stimulation efficiency, and power supply longevity. It saves chip area, since large w/l transistors or large capacitors are not required for a CMOS integration of the stimulator cell. The system can be easily extended for action potential recording.
Cyclic voltammetry (CV) at slow scan rate is usually performed to characterize the electrochemical behavior of microelectrodes. Early in vitro study on over 200 activated iridium oxide film (AIROF) microelectrodes showed variability of their electrochemical behavior. In this paper, we reported CV measurements under 3 different scan rates for 96 AIROF microelectrodes. We traced their behavior during 7 stages of fabrication and processing starting from prior to activation, and ending with post-implantation in the experimental animal. The results show that even at the early stage of electrode activation, variations between near identical electrodes existed.
Although implantable medical microsystems like painkillers, deep brain stimulators and the promising cochlear implants, have reached a high degree of reliability and usability for the patient, the implementation of neural stimulators with hundreds or thousands of microelectrodes still suffers by the interconnection problem between the electrode array and the electronic stimulator device. A mechanically flexible interconnection of all electrodes of a large 2D-electrode array to a monolithically integrated stimulator/recorder device is almost impossible, since the integrated circuit can only be attached using either bumps or bonds. In this contribution, the usage of organic semiconductors for the implementation of an addressable active stimulator cell is proposed to overcome these limitations. This stimulator electronic can be fabricated either with printable long chain organic semiconductors or, as described here, with short chain semiconductors (oligomers) like pentacene. The mechanical flexibility of arbitrarily sized and shaped 2D multicontact array is completely preserved, if this flexible semiconductor electronic is interlaced between the electrode rows and columns.
Development of an intracortical visual prosthesis for restoration of vision, has been, and continues to be an elusive goal of neural prosthesis researchers. Our multi-institutional team has tested the feasibility of implanting and evaluating large numbers of stimulation/recording electrodes in an animal model. Using a combination of 8-electrode arrays and individual electrodes, 152 activated iridium microelectrodes were implanted in area V1 of a macaque. Visual stimuli were used to define a retinotopic map. Spatial coordinates for each electrode were used to train the animal to use electrical stimulation in performing a visual psychophysical task.
Localized, long-lasting stimulation-induced depression of neuronal excitability (SIDNE) is a consequence of prolonged, high-frequency microstimulation in the central nervous system (CNS). It represents a persisting refractory state in the neurons and axons near the stimulating microelectrode, that occurs in the absence of histologically detectable tissue injury. It does not involve a change in synaptic efficacy and, in this respect, it differs from the more familiar phenomenon of long-term depression (LTD). Although SIDNE is ultimately reversible (after several days), it must be taken into account in the design of neural prostheses based on microstimulation in the central nervous system and in animal studies that require prolonged microstimulation in the CNS. In this study, we have characterized the phenomenon, using as the paradigm, iridium microelectrodes implanted chronically in the cat's posteroventral cochlear nucleus. Although the SIDNE may persist for several days after the end of the stimulation protocol, it does not become more severe from day to day when the stimulation protocol is repeated on successive days. The severity of the SIDNE is strongly dependent upon both the instantaneous frequency and the duty cycle of the electrical stimulation. The character of the SIDNE, including its localization to the immediate vicinity of the stimulating microelectrodes, suggests that the phenomenon is a direct consequence of the prolonged electrical excitation of the neurons close to the microelectrode. The problem of designing microstimulation systems that allow high-frequency stimulation of a neural substrate, while minimizing SIDNE are discussed.
The maximum charge injection without electrode solution decomposition for activated iridium wire electrodes in bicarbonate buffered saline was 2.1 and 1.0 mC/cm<sup>2</sup> geometric for anodic-first and cathodic-first, respectively, 0.2-ms balanced-charge biphasic current pulses. Electrodes biased at +0.8 V vs. SCE (saturated calomel electrode) accepted charge up to 3.5 mC/cm<sup>2</sup> geometric with monophasic cathodal pulses.