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Measured |
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Polarization |
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Electrode Polarization |
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2nd Phase |
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Electrode Current |
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Interphase - zero current |
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Fig. 11.4 Depiction of an alternate current waveform for delivering stimulation pulses to microelectrodes. During the zero-current interphase portion of the waveform the effect of access resistance upon the voltage waveform is eliminated, and the residual voltage, during the interphase, is a reasonable measure of the electrode polarization. To remain within the water window the measured polarization should be more positive than −0.6 V with respect to Ag|AgCl
Too often, the injectable charge capacity of a particular electrode is estimated from the use of a priori published material type-based charge densities, and this approach does take into account the actual dynamic behavior of the electrode since the wrong parameter, i.e. charge density rather than electrode polarization, is being considered. An alternate method of estimating the electrode polarization is depicted in Fig. 11.4 and involves adding a third interphase region to the stimulation waveform. If a short period of zero-current is imposed between the first and second phases, then a measurement of the electrode voltage during this time of zero-current should be free from true IR drops, and should be a better estimate of the polarization caused by the delivery of charge during the first phase of the biphasic waveform [13]. The disadvantage to this approach is that the measurement is made after the polarization has occurred. There exists some debate about whether a typical AIROF intracortical electrode can tolerate single-stimulus conditions that transgress the water window without damage, and use of the interphase voltage measurement as a continuous measure of electrode safety may be inadequate to protect an AIROF electrode. However, there presently exists no implantable stimulator that uses leading-edge voltage measurements in a predictive manner to protect either AIROF, or bare metal, intracortical electrodes from damage.
11.6 Contrasts of In Vitro and In Vivo Behavior
Most available data for intracortical electrodes come from in vitro studies that were carried out in model physiological fluid. Based upon those studies, the maximum injectable charge capacity for AIROF intracortical electrodes whose tip areas are under 2,000 mm2 is well within the anticipated stimulation charge thresholds for visual
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cortex neurons. For example, Schmidt et al. [40] observed that stable phosphenes could be obtained in a human volunteer when using 0.4–4.6 nC/phase of stimulation, whereas in vitro measurements of 2,000 mm2 AIROF electrodes in phosphatebuffered saline typically show up to ten times this required charge capacity, while maintaining operation within the water window. This led to the historical conclusion that AIROF intracortical electrodes were more than adequate for long-term stimulation of the visual cortex in visual protheses.
More recently, this view has been challenged, as in vivo studies of AIROF and SIROF electrodes have been performed. Cogan et al., compared the in vitro and in vivo charge injection behavior of large area (~125,000 mm2) AIROF electrodes intended for a retinal visual prosthesis [12], and found that the charge capacity of the electrodes, once implanted subretinally in rabbits, required three times the total electrode voltage excursion as had been observed in vitro, for delivery of the same charge. Hu et al. [26] compared the performance of 2,000 mm2 intracortical electrodes implanted within the cortex of a zebra finch with their performance in dilute phosphate buffered saline, and found a factor of four decrease in their charge injection capacity in vivo for equal in vitro and in vivo voltage excursions. Even more disturbing, is the observation that electrode polarization, in vivo, appears to increase by a factor of two, over that seen in vitro, for equal charge injection [12].
Figure 11.5 shows a dramatic demonstration of the loss of charge capacity, relative to in vitro behavior, for intracortical electrodes placed within the in vivo cortical environment. Two electrodes were measured in vitro, immediately placed in vivo, then immediately replaced into the in vitro environment. The stimulator circuitry was specially designed to limit the total electrode voltage to less than ±0.6 V (water window) in order to prevent electrode damage. On the left of Fig. 11.5 are shown the pulse voltage excursions for the two AIROF intracortical
Fig. 11.5 AIROF intracortical electrodes tested in vitro and in vivo. Electrodes were transferred between a beaker of PBS and the cortex of a Zebra Finch during the same experiment. In vitro current and voltage excursions for two electrodes are shown on the right and left set of plots. In vivo waveforms are shown in the center plots. Note the dramatic decrease in the in vivo injectable charge capacity, relative to the in vitro behavior, as seen in the center plots by the larger voltage excursions for the smaller stimulation currents