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J. S. George, S. Srinivas, G. Lazzi; Micromagnetic Stimulation of Retina: Numerical Studies and Preliminary Results. Invest. Ophthalmol. Vis. Sci. 2008;49(13):1784.
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Stimulation of the neural retina by current injection from an electrode array can partially restore vision to people affected by degenerative retinal diseases, however there are significant technical obstacles. We explored the possibility of exciting neurons in the retina using currents induced by time varying magnetic fields generated by an array of microcoils. This approach has several potential advantages: imperfect contact between the retina and the microcoil array should not severely compromise system performance, and an array of microcoils (unlike an array of microelectrodes) can be fully insulated, avoiding the major source of damage to the device.
We formulate and employ a novel finite difference scheme to analyze the performance of microcoils for magnetic stimulation of retina, solving the diffusion equation for magnetic vector potential in cylindrically symmetric geometries used to model retinal neurons. After obtaining the rate of change of current inside the inductor coil, we estimate eddy currents induced in tissue. Spatially resolved transmembrane potential was compared to thresholds for neural stimulation.
Timecourses of current in the inductor coil and the predicted eddy currents were calculated. We describe numerical results for simple coils carrying time-varying currents as well as coils with embedded ferrite cores or sandwiched between ferrite materials. We describe assembly and testing of a basic magnetic stimulator circuit, where a capacitor charged with a DC power supply is discharged through an inductor coil. Studies of the feasibility of stimulation of isolated retina with this system are beginning.
Simulations suggest that magnetic stimulation of retinal neurons with a miniature (mm scale) is possible with sub-millisecond pulses of 100-250 mA. Smaller coils and drive capacitors might allow stimulation with mA currents or less, providing a feasible alternative to electrodes for a retinal prosthetic device. Our studies also suggest that induced eddy currents might be focused or steered by an array of stimulating microcoils, potentially allowing such devices to achieve effective stimulation resolution greater than the geometrical resolution of the coil array.
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