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M. Djilas, H. Lorach, G. Lissorgues-Bazin, L. Rousseau, L. Cadetti, P. Bergonzo, R. Benosman, S.-H. Ieng, S. Picaud; Three-Dimensional Electrode Arrays for Retinal Prostheses: From Modeling to in vivo Testing. Invest. Ophthalmol. Vis. Sci. 2010;51(13):3036.
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Retinal diseases can cause photoreceptor degeneration following which other layers of the retina, including bipolar and ganglion cells, partially remain. The concept of retinal prostheses was developed to restore useful vision in blind patients by activating this remaining inner retinal network. Planar electrode arrays show limited activation selectivity of cells underlying the electrode array. We here present a modeling study showing that original 3D electrodes configurations may provide more focal stimulation, and an in vivo examination on P23H rat retina.
Finite-element models of 3D electrode geometries were created using the Comsol Multiphysics Modeling and Simulation software package (Comsol AB). Model parameter optimization was done using MATLAB (The Mathworks) where the model parameters were the cathode and anode sizes, geometries and distance. Optimization constraints were defined in relation with microfabrication constraints. Electrode geometry was considered optimal if a set of model parameters defining it produced the most selective stimulation in the pixel area. Stimulation selectivity was quantified by dividing the integral of the electrical potential in this pixel area by the integral of the potential outside this area. Polyimide implant prototypes were then prepared for in vivo evaluation of the tissue reaction to 3D designs.
A 3D electrode geometry was found to yield a selectivity 12 times better compared to an optimal planar geometry of similar dimensions. 3D electrodes limit the activation of ganglion cells to a smaller cell number allowing more focalized images. When 3D electrode designs were introduced in the subretinal space of blind P23H rats, they appeared well integrated into the tissue. Immunostaining for GFAP and retinal neurons enabled us to localize neurons with respect to electrodes confirming the advantage of 3D electrodes. No further increase in GFAP expression was noted in the vicinity of the 3D electrodes.
3D electrode structures could provide more focal stimulation in retinal prostheses, resulting in restoration of vision with greater detail. Further work will concentrate on experimental validation of 3D electrode designs with implant fabrication, histological studies and functional analyses.
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