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Robert G. Wilke, Gita Khalili Moghaddam, Nigel Lovell, Socrates Dokos, Gregg Suaning; Multipolar Return Configurations In Microelectrode Arrays Designed For Retinal Implants: Modeling Effects On Threshold Levels And Dynamic Range. Invest. Ophthalmol. Vis. Sci. 2012;53(14):295.
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© ARVO (1962-2015); The Authors (2016-present)
Estimating the effect of current focusing in the retina using local guard electrodes in conjunction with a distant return electrode. Current thresholds to elicit ganglion cell activation and respective dynamic range are estimated using a finite-element modeling approach.
a microelectrode array in a hexagonal configuration was modeled solving for required current at the stimulating electrode yielding an electric field magnitude at the ganglion cell layer previously reported to activate ganglion cells. The following parameters has been investigated:- Ratio of h/D (distance from electrode to ganglion cells / electrode diameter): 0.5, 1, and 2- Return electrode configuration: monopolar (distant active return electrode), hexagonal (6 local active return electrodes), and quasi-monopolar (QMP, a mix of hexagonal and distant return configuration)All return electrodes are designed to actively pull current. In the QMP configuration 50% of injected current is recovered through distant and local return electrodes, respectively. Electrodes were chosen to be of 100µm diameter and 110µm pitch.Dynamic range was defined as the difference between threshold current and the safe current injection limit for SIROF electrodes.
The dynamic range systematically decreased as the electrode configuration varied from MP to QMP to hexagonal. This effect is more significant for larger electrode/target cell distances (h/D of 2), and hardly noticeable for small distances. This is due to elevated threshold levels for QMP and hexagonal configuration due to partial shunting of stimulating current directly to return electrodes.
Current focusing using multipolar configuration like the hex-return has the benefit of more localized stimulation that is less prone to electric crosstalk. However, depending on electrode geometry and distance from target cells it also has higher threshold levels. We investigated if a mix of return configuration using distant return and local hex return can compensate for this limitation. QMP configuration with 50% distant return was found to lower thresholds and yield higher dynamic ranges as compared to pure hex-configuration. If target cells are very close to stimulating electrodes as could be achieved with penetrating electrodes, return configuration has no influence on threshold levels.
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