Quasi-static Analysis of Electric Field Distributions by Disc Electrodes in a Rabbit Eye Model

Abstract

We developed a compartmentalized finite element model (FEM) of the electric fields generated in the rabbit retina due to a biphasic stimulus pulse. The model accounts for the different resistivities and capacitances of the retina, pigment epithelium (PE), and sclera. Axiosymmetric 2-D FEMs were created for monopolar stimulation electrodes using COMSOL. 250 μm diameter electrodes with 10 μm thick insulation were placed at three different locations near the retina: the inner limiting membrane (epiretinal), the subretinal space (PE/retina) (subretinal), and the choroid layer behind the PE/retina (suprachoroidal). A broad return electrode was located at the back of the eye (sclera). The relative dielectric constants of each eyewall layer with linearly varying resistivity for the retina layers were incorporated into the model. Biphasic 1 mA/cm² current pulses with pulse widths of either 0.5 ms (0.5 μC/cm²), 1ms (1 μC/cm²), and 5 ms (5 μC/cm²) were passed through the tip of the electrode for stimulation. We found that these waveforms, which match waveforms commonly used to activate the retina in retinal implants, show a transient-sustained electric field profile due to charging of the high capacitance and resistivity of the PE. The PE develops high electric fields in all three electrode models. Wider pulses induce greater electric fields in the PE than shorter pulses. This needs to be accounted for when determining safe levels of stimulation. Simulation models that assume constant resistivity (4k Ω-cm) for the retina calculate larger electric fields across the retina than Gaussian resistivity models (3k-7k Ω-cm). Electric field strength is known to be greatly enhanced at the electrode edges. We found that the electric fields at the electrode edge can cause significant damage to the retina even when the nominal current density is below the damage threshold.