Biomedical Engineering Reference
In-Depth Information
of prosthesis electrical components, modeling of neural electrode/vitreous
interactions, and telemetric external electronic imaging and signal-processing
technology. To avoid damage when the prosthesis is driven, there must be good
understanding of the chemical and electrical properties of the microenvironment
where the electrodes contact living ocular tissue. Reaching electrical threshold
potentials for axonal depolarization in retinal tissue requires that electrodes be
close to target neurons and have sufficient charge-carrying capacity to induce
depolarization of targeted neuronal cell membranes. The charge per phase trans-
ferred and the electrode surface area determine the tissue focal area stimulated
and the concentration of the charge. In microelectrode arrays mimicking the
function of myriads of defective retinal photoreceptor cells, charge density at the
surface is an increasing hazard to safe, efficacious retinal stimulation. Charge
injection above thresholds for purely capacitive processes at the interface initiates
irreversible electrochemical reactions in tissue fluids, resulting in localized
electrolysis of water, oxidation of vitreous solutes, pH shifts, and dissolution
of the electrode material. The safe window for stimulation, i.e. the interval
where only reversible reactions occur in the vitreous, can be extended by
choosing waveform stimulation patterns which are reversible. Reversible pulses
are biphasic (bipolar), symmetrical and balanced, passing charge in one direction
and then in the other, with no net charge injection. Charge injection limits and
thresholds for redox reactions in the vitreous can be determined in vitro by
modeling of the eye in a biomimetic eye-cell electrolysis apparatus. From the
results of those studies, standard protocols for safe retinal stimulation can be
established and applied to the next generation of epiretinal visual prostheses.
References
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