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William A. Drohan, Douglas B. Shire, Joseph F. Rizzo, III, John L. Wyatt, Jr., Shawn Kelly; Stimulation Effectiveness Of Various Retinal Implant Designs. Invest. Ophthalmol. Vis. Sci. 2011;52(14):4940.
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This work is related to the efforts of the Boston Retinal Implant Project to develop a sub-retinal prosthesis to restore vision to the blind. The specific purpose of this effort is toCompare several implant designs to assess the relative merits of various design choices, using FEM (Finite Element Modeling) techniques.
The stimulation effectiveness of a retinal implant is related to the strength of the electric field that can be produced at the site of a target neuron. We chose to use the electrical power to a single electrode as a normalization factor so that the comparison is fair; i.e. we compare the electric field (Volts/Meter) produced at the site of a target neuron of various designs with different electrodes and electrode placements with the requirement that in each case exactly 1 uWatt is dissipated by the electrode. We also examine slope of the field, which indicates the focusing effect. The method has two steps: 1) apply one volt to the electrode and obtain the current map of the field. Integrating over the current field we get the electrode current and impedance. 2) Adjust the electrode voltage so that the power used is exactly 1 uWatt. Rerun the field analysis and look at the magnitude of the electric field at the site of the target neuron. This was done for various size electrodes and varying placement . The same three layer anatomical model (choroid, retina, vitreous) was used for each case. All electrodes were modeled as having been built on a polyimide substrate. The penetrating electrode was extended from the substrate by an SU-8 epoxy support pillar.
The line plots of the electric field along the electrode axis from its center were plotted as a function of distance from the electrode, with special focus on the area of the target neuron. A comparison of the effectiveness of all the design options was apparent from these plots.
Of the cases examined, the epi-retinal planar electrode model produced about twice the electric field strength at the target neuron as the sub-retinal planar electrode model except that if there was an interposing 2 uM fluid layer in the epi model the field strength was about the same. The penetrating electrode field strength was about an order of magnitude greater than the other models.
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