In Vitro Determination of Stimulus-Induced pH Changes in Visual Prostheses - Artificial Sight: Basic Research, Biomedical Engineering, and Clinical Advances - page 240

Conclusions
Experiments in unbufferd saline solution showed pH shifts significantly when
electrode is pulsed monophasically even at very low charge density 30C/cm 2 .
An increase in pH is detected in surrounding solution as an electrode is pulsed
cathodically, and conversely, a decrease in pH is detected when an anodic
pulse is used. The results support what is previously known about electro-
chemical reactions occurring at the electrode-electrolyte interface. Stimulation
using biphasic charge-balanced pulses did not introduce a notable pH change,
regardless of which leading pulse phase for the charge density up to 035mC/cm 2
for smooth solid Pt electrodes. To minimize such possible pH effects, a biphasic
charge-balanced pulse should be used for neural stimulation. Body fluid has
high pH buffering capacity and it will neutralize pH changes during in vivo
stimulation.
The in vitro pH changes due to electric stimulation were recorded successfully
by the planar micro-pH electrode array in this experiment. A 2D distribution of
pH change was established by using such combined microelectrode arrays. This
in vitro experiment was conducted in a solution when all electrode sites faced up
without any blockage and physical constrain. It is possible that the local pulse-
induced pH change near the electrode site might not completely agree with what
is found under open-space condition, especially when the electrode array is not
followed by the curvature of the retina surface and the distance of electrode to
tissue is different from that of electrode to electrode in the array. Continued study
in vivo may help in better understanding the differences between open space
in vitro testing and actual implanted conditions. The planar thin-film micro-pH
electrode arrays reported in this work hold promise for the detection of possible
stimulus induced pH changes at the electrode-tissue interface.
Acknowledgments. This work was supported by the National Institute of
Health/National Eye Institute , under grant #1R24EY12893-01. The authors wish
to thank Chase Byers, Teresa Swan, and Kjersti Morris for their contributions to
this work.
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