Dynamic Interactions of Retinal Prosthesis Electrodes with Neural Tissue and Materials Science in Electrode Design - Artificial Sight: Basic Research, Biomedical Engineering, and Clinical Advances - page 217

principle and corroborate observed results in in vivo neural stimulation experi-
ments that current reversal and charge balance are the determinants of safe stimu-
lation of neural tissue. The charge density at which breakthrough electrolysis
occurs on a 1.05mm diameter Pt sphere electrode with 1msec biphasic pulses
and 1msec intraphase delay is just below 1mC/cm 2 (Figure 11.5).
The assumption that a charge density of 1mC/cm 2 will initiate interfacial
electrolysis is, however, not based on data derived from tests with small diameter
electrodes like those proposed for retinal implants. Benchmark reference points
derived from tests with microdiameter Pt electrodes are essential for optimizing
electrode design and function. To this point, there are no well-established
standard protocols for predicting the onset of unwanted faradaic transformations
in the vitreous. The biomimetic eye model and the isolated gas flow apparatus
were used to test the limits of stimulation of 150m Pt electrodes. The electrode
disks were electropolished until shiny and smooth to define the surface geometry
for calculations of charge density. The electrodes were driven with balanced
biphasic pulses with an intraphase delay for 20min. Pulse amplitude and dwell
time were variable and charge densities remained within the prescribed range
of 01-10mC/cm 2 . Table 11.1 shows a compilation of experimental pulse
parameters and steady-state hydrogen production. First appearance of H 2 was at
1mC/cm 2 , consistent with earlier results from larger electrodes. The onset of
electrolysis was reproducible and specific for retinal implant models.
It is conjectured that, if electron transfer initiates electrolysis at 1mC/cm 2 ,it
is likely that other vitreous solutes are also involved in redox reactions at the
interface. Table 11.2 shows hydrogen gas/production when a spherical Pt electrode
is pulsed with a biphasic 500A current at variable dwell times (3-500msec)

in saline. In these experiments, total chlorine from charge-induced oxidation of
saline was measured using an Orion ion-selective electrode. Saline samples were
drawn from the reaction cell before and after 1-hour pulsing. In general, when

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Figure 11.5. Threshold electrolysis with biphasic pulsing of a 1mm Pt electrode.