The impedance, cyclic voltammetry, and charge-injection properties of rectangular, sputtered iridium oxide (SIROF) electrodes have been measured in buffered physiological saline over a range of geometric surface areas (GSA) and perimeter-to-area ratios (P/A). Electrodes with a higher P/A are expected to have a lower impedance and higher charge injection capacity (Q inj ), and both these effects were evident for SIROF electrodes with a GSA in the range 0.0023-0.0031 mm 2 . However, the magnitude of the effect was modest. The increase in Q inj for rectangular electrodes with a P/A ranging from 94 to 255 mm -1 was 21-26% depending on pulse width. There was a corresponding decrease in impedance (0.1 to 10 5 Hz) with increasing P/A and an increase in the SIROF charge storage capacity calculated from cyclic voltammetry. To assess the full usefulness of high P/A electrodes for increasing the reversible Q inj of an electrode, measurements should now be extended to chronic in vivo preparations.

Penetrating multielectrode arrays with electrode coatings of sputtered iridium oxide (SIROF) have been implanted chronically in cat cortex for periods over 300 days. The ability of these electrodes to inject charge at levels above expected thresholds for neural excitation has been examined in vivo by measurements of voltage transients in response to current-controlled, cathodal stimulation pulsing. The effect of current pulse width from 150 (is to 500 (is and voltage biasing of the electrodes in the interpulse period at two levels, 0.0 V and 0.6 V vs. Ag|AgCl, were also investigated. The results of in vivo characterization of the electrodes by open-circuit potential measurements, cyclic voltammetry and impedance spectroscopy are also reported.

The extent to which oxygen reduction occurs on sputtered iridium oxide (SIROF) and platinum neural stimulation electrodes was quantified by cyclic voltammetry and voltage-transient measurements in oxygen-saturated physiological saline. Oxygen reduction was the dominant charge-admittance reaction on platinum electrodes during slow-sweep-rate cyclic voltammetry, contributing ~12 mC/cm 2 (88% of total charge) to overall cathodal charge capacity. For a 300-nm-thick SIROF electrode, oxygen reduction was a minor reaction contributing 1.3 mC/cm 2 , ~3% of total charge. During current pulsing with platinum electrodes, oxygen reduction was observed at a level of 7% of the total injected charge. There was no indication of oxygen reduction on pulsed SIROF electrodes. A sweep-rate-dependent contribution of oxygen reduction was observed on penetrating SIROF microelectrodes (nominal surface area 2000 μm 2 ) and is interpreted in terms of rate-limited diffusion of oxygen in electrolyte that penetrates the junction between the insulation and electrode shaft. For typical neural stimulation pulses, no oxygen reduction could be observed on penetrating SIROF microelectrodes. Based on the in vivo concentration of dissolved oxygen, it is estimated that oxygen reduction on platinum microelectrodes will contribute less than 0.5% of the total injected charge and considerably less on SIROF electrodes.

Sputtered iridium oxide films (SIROFs) deposited by DC reactive sputtering from an iridium metal target have been characterized in vitro for their potential as neural recording and stimulation electrodes. SIROFs were deposited over gold metallization on flexible multielectrode arrays fabricated on thin (15 microm) polyimide substrates. SIROF thickness and electrode areas of 200-1300 nm and 1960-125,600 microm(2), respectively, were investigated. The charge-injection capacities of the SIROFs were evaluated in an inorganic interstitial fluid model in response to charge-balanced, cathodal-first current pulses. Charge injection capacities were measured as a function of cathodal pulse width (0.2-1 ms) and potential bias in the interpulse period (0.0 to 0.7 V vs. Ag|AgCl). Depending on the pulse parameters and electrode area, charge-injection capacities ranged from 1-9 mC/cm(2), comparable with activated iridium oxide films (AIROFs) pulsed under similar conditions. Other parameters relevant to the use of SIROF on nerve electrodes, including the thickness dependence of impedance (0.05-10(5) Hz) and the current necessary to maintain a bias in the interpulse region were also determined.

Sputtered iridium oxide (SIROF) is a candidate low-impedance coating for neural stimulation and recording electrodes. SIROF on planar substrates has exhibited a high charge-injection capacity and impedance suitable for indwelling cortical microelectrode applications. In the present work, the properties of SIROF electrode coatings deposited onto multi-shank penetrating arrays intended for intracortical and intraneural applications were examined. The charge-injection properties under constant current pulsing were evaluated for a range of pulsewidths and current densities using voltage transients to determine maximum potential excursions in an inorganic model of interstitial fluid at 37 degrees C. The charge-injection capacity of the SIROFs was significantly improved by the use of positive potential biasing in the interpulse period, but even without bias, the SIROFs reversibly inject higher charge than other iridium oxides or platinum. Typical deliverable charge levels of 25 to 160 nC/phase were obtained with 2000 mu m(2) electrodes depending on pulsewidth and interpulse bias. Similar sized platinum electrodes could inject 3 to 8 nC/phase.

The effects of ionic conductivity and buffer concentration of electrolytes used for in vitro measurement of the charge-injection limits of activated iridium oxide (AIROF) neural stimulation electrodes have been investigated. Charge-injection limits of AIROF microelectrodes were measured in saline with a range of phosphate buffer concentrations from [PO43−] = 0 to [PO43−] = 103 mM and ionic conductivities from 2–28 mS cm−1. The charge-injection limits were insensitive to the buffer concentration, but varied significantly with ionic conductivity. Using 0.4 ms cathodal current pulses at 50 Hz, the charge-injection limit increased from 0.5 mC cm−2 to 2.1 mC cm−2 as the conductivity was increased from 2 mS cm−1 to 28 mS cm−1. An explanation is proposed in which the observed dependence on ionic conductivity arises from non-uniform reduction and oxidation within the porous AIROF and from uncorrected iR-drops that result in an overestimation of the redox potential during pulsing. Conversely, slow-sweep-rate cyclic voltammograms (CVs) were sensitive to buffer concentration with the potentials of the primary Ir3+/Ir4+ reduction and oxidation reactions shifting ∼300 mV as the buffer concentration decreased from [PO43−] = 103 mM to [PO43−] = 0 mM. The CV response was insensitive to ionic conductivity. A comparison of in vitro AIROF charge-injection limits in commonly employed electrolyte models of extracellular fluid revealed a significant dependence on the electrolyte, with more than a factor of 4 difference under some pulsing conditions, emphasizing the need to select an electrolyte model that closely matches the conductivity and ionic composition of the in vivo environment.

The use of potential biasing and biphasic, asymmetric current pulse waveforms to maximize the charge-injection capacity of activated iridium oxide (AIROF) microelectrodes used for neural stimulation is described. The waveforms retain overall zero net charge for the biphasic pulse, but employ an asymmetry in the current and pulse widths of each phase, with the second phase delivered at a lower current density for a longer period of time than the leading phase. This strategy minimizes polarization of the AIROF by the charge-balancing second phase and permits the use of a more positive anodic bias for cathodal-first pulsing or a more negative cathodic bias for anodal-first pulsing to maximize charge injection. Using 0.4-ms cathodal-first pulses, a maximum charge-injection capacity of 3.3 mC/cm2 was obtained with an 0.6-V bias (versus Ag/AgCl) and a pulse asymmetry of 1:8 in the cathodal and anodal pulse widths. For anodal-first pulsing, a maximum charge capacity of 9.6 mC/cm2 was obtained with an asymmetry of 1:3 at an 0.1-V bias. These measurements were made in vitro in carbonate-buffered saline using microelectrodes with a 2000 microm2 surface area.

The charge-injection limits of activated iridium oxide electrodes (AIROF) and PtIr microelectrodes with similar geometric area and shape have been compared in vitro using a stimulation waveform that delivers cathodal current pulses with current-limited control of the electrode bias potential in the interpulse period. Charge-injection limits were compared over a bias range of 0.1-0.7 V (versus Ag/AgCl) and pulse frequencies of 20, 50, and 100 Hz. The AIROF was capable of injecting between 4 and 10 times the charge of the PtIr electrode, with a maximum value of 3.9 mC/cm2 obtained at a 0.7 V bias and 20 Hz frequency.

Iridium oxide films formed by electrochemical activation of iridium metal (AIROF) or by electrochemical deposition (EIROF) are being evaluated as low-impedance charge-injection coatings for neural stimulation and recording. Iridium oxide may also be deposited by reactive sputtering from iridium metal in an oxidizing plasma. The characterization of sputtered iridium oxide films (SIROFs) as coatings for nerve electrodes is reported. SIROFs were characterized by cyclic voltammetry, electrochemical impedance spectroscopy, and potential transient measurements during charge-injection. The surface morphology of the SIROF transitions from smooth to highly nodular with increasing film thickness from 80 nm to 4600 nm. Charge-injection -capacities exceed 0.75 mC/cm(2) With 0.75 ms current pulses in thicker films. The SIROF was deposited on both planar and non-planar substrates and photolithographically patterned by lift-off.

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.

The galvanic interaction between electronically conductive polyaniline films doped with camphor sulfonic acid (PAni-CSA), and aluminum alloy (AA) 2024-T3 has been investigated for the purpose of assessing PAni-CSA as a corrosion protection coating on AAs. Potentiodynamic polarization and zero resistance ammeter measurements of galvanic couples between PAni-CSA and AA 2024-T3 in 3.5% NaCl show that the AA dominates the mixed potential response and polarizes the PAni-CSA to approximately the potential of the freely corroding alloy (-0.54 V vs. Ag\AgCl). This observation indicates that conductive PAni coatings on XI are rapidly reduced to the nonconductive form on immersion in an aqueous electrolyte. The results also suggest that any corrosion protection of Al by PAni coatings is due to the barrier properties of the reduced polymer rather than anodic stabilization of passivity as observed with ferrous alloys. Mechanisms by which a PAni-CSA coating might provide corrosion protection of exposed 2024-T3 at defects in the coating are discussed. (C) 2000 The Electrochemical Society. S0013-4651(99)11-002-4. All rights reserved.