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E. D. Cohen, S. Minnikanti, B. Huang, N. Peixoto; Measurement, Impedance Analysis, and Modeling of the Electric Fields Developed in Rabbit Retina by Retinal Prosthesis Electrodes. Invest. Ophthalmol. Vis. Sci. 2010;51(13):4323.
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To measure, model and examine the electric fields developed in the retinal layers by biphasic pulse stimulation from subretinal and epiretinal prosthesis electrodes.
An in-vitro eyecup preparation of the rabbit retina (1.13cm2) was used for recording the impedance and permittivity of the eyewall layers. Using a stimulus isolator and Pt-Ir electrodes, 30msec steps and sine waves 3Hz-10KHz of electrical current (10-20µA) were passed across the eyecup. The resistance and the permittivity across the eyewall layers were calculated from voltage measurements using a saline-filled recording electrode at different depths in the retina, pigment epithelium (PE), choroid, and the sclera.
The charging of the eyewall layers was dominated by the high capacitance and resistivity of the pigment epithelium (see also Faber, 1969 SUNY Buffalo thesis), which had an average time constant and resistance of 3.3msec, 224ohms respectively (n=10). Based upon the step and sine wave measurements, we performed finite element analysis to determine the electric fields developed across the eyewall layers during typical biphasic stimulation pulses (0.5-5msec, 1mC/cm2/ph) applied through insulated epiretinal and subretinal electrodes (250µm diam). During biphasic current pulse stimulation modeling, large electric field transients were developed across the local PE during the cathodic/anodic transition phase by epiretinal stimulus electrodes. Smaller fields were also observed in the model across the PE during stimulation by subretinal electrodes. In the model, the vitreous humor formed a prominent current shunt for epiretinal prosthesis electrodes not closely apposed to the retina. When a stimulating electrode was as close as 100µm from the retina surface, a cross-sectional profile of the electric field at the inner retinal surface was dome-like at the center, while electrodes directly on the retinal surface had profiles with centers increased ~10X in magnitude and strong ring-shaped maxima at the electrode edges.
AC measurement and modeling of activation of the retinal layers and PE during biphasic current pulse stimulation by epiretinal and subretinal stimulation electrodes revealed large electric field transients. These transients reflect capacitative charging of the PE layer. The peak electric fields developed across the retina during pulses may be significantly underestimated by static electric field analysis.
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