Stimulus induced pH changes in retinal implants

Electrical stimulation of neural tissue requires charge injection into the biological environment. This is achieved through both Faradaic and non-Faradaic reactions at the electrode/tissue interface. Some Faradaic reactions have the potential to dramatically alter pH levels, leading to tissue damage. The present study looked to investigate the effects of stimulus induced pH changes for a variety of stimulation parameters in a retinal implant. Electrodes were stimulated using monophasic and biphasic pulses at different intensities and in different mediums. Stimulus frequency and pulse width were maintained consistent for all tests. pH levels were recorded using a pH microelectrode and verified using a pH color indicator (phenol red). As expected, no significant pH change could be detected in buffered saline or balanced salt solution. However, stimulation parameters causing pH changes could be detected in unbuffered saline solution. While electrode stimulation using biphasic charge-balanced current pulses showed minimal pH change, stimulation using monophasic pulses showed significant pH shifts. The extent of pH change was related to duration of stimulation. The results from this study provide an insight to the electrochemical mechanisms at the interface of the electrolyte medium and retinal stimulation electrodes.