3-D Finite Element Analyses Of The Epi-retinal Electrical Stimulation

Epi-retinal prostheses have received great attention for helping the RP(Retinitis Pigmentosa) or AMD (Age-related Macular Degeneration)patients to restore their vision around the world and gotten the Europe CEMark permission. The prosthesis electrically stimulates the retina tissuethrough implanted microelectrode array to generate perception orphosphene. The objective of this thesis was to systematically investigatethe impacts of the electrode size, electrode-retina distance and electrodegeometry on the stimulation via a novel three-dimensional finite elementmodeling, in order to improve the spatial resolution of artificial vision byoptimizing the electrode design.To better understanding the impacts of electrical stimulation onganglion cell layer (GCL), a five-layered retina model was establishedaccording to the electrophysiological experiment data. A3-D finite elementsimulation model was set up by Pt electrode and retina layers in COMSOLMultiphysics3.5a. Electric field strength was adopted to represent theexcitation levels of ganglion cells, and1000V/m was considered as thethreshold. The input current when evoked electric field at GCL middlepoint reached1000V/m was estimated as the threshold.The research results are as follows:(1)The thresholds increased dramatically with increasing electrodesize when electrode was close to retina surface. The larger electrode wasmore tolerated with the distance changes than smaller one did, which wereconsistent with the animal electrophysiological experiments and clinicaltrials.(2) The spatial resolution was related with evoked electric field on GCL. A new concept “effective evoked responded area” was defined to theganglion cell cluster where the elicited electric field was no less thanthreshold value1000V/m. As a result, a proper-sized electrode (~75μm)with a certain distance (<200μm) and threshold input stimulus would beselected for high resolution.(3) Electrode geometry affects the distribution of charge on electrodesurface and keeping the injected charge density in safety threshold was akey issue for chronic implant. Four different shapes of electrode wereutilized in this study including concave, flat, convex and coated electrode.It was obviously observed that the concave electrode used in ourframework obtained stronger and more focused electric field on targetganglion cell layer with low charge density on electrode surface.