Electrical performance of penetrating microelectrodes chronically implanted in cat cortex

Abstract

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 μs to 500 μs 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.

Fig. 1.

Scanning electron micrograph of a 16-shaft penetrating microelectrode array with a 400 μm spacing between electrode tips

Fig. 2.

Representative cyclic voltammograms from a SIROF-coated microelectrode in vitro and after 32, 117, and 168 days in vivo.

Fig. 3.

Averaged CV response for five electrodes on one array in model-ISF prior to implantation and after 194 days in vivo. Sweep rate 50 mV/s.

Fig. 4

Cathodal charge storage capacity measured at 50 mV/s averaged over the total number of implanted electrodes studies as a function of time. Error bars are standard deviations.

Fig. 5.

Comparison of the average CV response of six electrodes pre-implantation in model-ISF and after 194 days in vivo. Sweep rate 50,000 mV/s.

Fig. 6.

A representative example of the change in electrode impedance over the course of 168 days implantation. The pre-implantation, in vitro impedance is shown for comparison. The arrow indicates the direction of impedance change with implantation time.

Fig. 7.

Mean impedance magnitude at 1 kHz averaged over the total number of implanted electrodes studies as a function of time. Error bars are standard deviations.

Fig. 8.

Voltage transients for 12 electrodes on one array pulsed cathodally at 8 nC/ph (400 μs pulse width, 20 μA current, 50 Hz) corresponding to a nominal charge-injection density of 400 μC/cm². The measurements were made 194 days post-implantation.