Finite Element Analysis Of Electric And Temperature Field During Electrical Stimulation Of Retina With Microelectrode Array

Objectives:Epi-retinal prosthesis is used to help the blind to restore part of vision by stimulating the retinal ganglion cells with the microelectrode array. During the stimulation, different selections of stimulation configurations (stimulating and return electrodes configuration) can affect the spatial resolution. On the other hand. Joule heat will be produced during long-term electrical stimulating the retina, and the temperature of retina will increase, then the retina may be damaged by the heat. In order to improve the stimulation configuration and improve the spatial resolution, using the finite element method, we analysed the effect of the geometry factors of the return electrode contact on the activated area of retinal ganglion cells. At the same time, in order to prevent the thermal damage of retina and keep safety of electrical stimulation, we investigated the steady-state temperature field of electrical stimulation of retina with microelectrode array, and considered how the different parameters of the microelectrode array affect the temperature field in the retina during electrical stimulation.Methods:1. A3D model of electrical stimulation of retina with microelectrode array was established. A3D model of electrical stimulation of retina with microelectrode array had been established with the finite element software (Comsol Multiphysics). The model included the vitreous, retina, choroid, sclera and the microelectrode array (stimulating electrode and the return electrode contact).2. The effect of geometry factors of the return electrode contact on the active area. The tripolar stimulation configuration was composed of one stimulating electrode contact and two return electrode contact. In the tripolar stimulation configuration, the retina was composed of different layers. According to the different position of the return and stimulating electrode contacts, equilateral triangle and parallel tripolar stimulation configuration were chosen. The distribution of electric field of the different center-to-center distance between the stimulating electrode contact and the return electrode contact, the center-to-center distance between two return electrode contacts, and the area of the return electrode contact were calculated. The cross-sectional active area of the activated retinal ganglion cells were used to reflect the focalizing of the electric field.3. The temperature field of electrical stimulation of retina with microelectrode array. Using monopolar stimulation configuration, the steady-state temperature field of electrical stimulation of the retina with4×4microelectrode array was solved by the modified bio-heat transfer equation. The effect of the electrode parameters such as the center-to-center distance between the stimulating electrode contacts, the area of the electrode contact, different stimulating electrode contacts and the electrode materials on temperature field were considered, with the help of parametric sweep.Results:1. The result of tripolar stimulation configuration. During tripolar stimulation configuration, cells activated in Ganglion cell layer are in the vicinity of the stimulating microelectrode, and the closer to electrode contacts the easier to be activated. Keep the same area of the stimulating electrode contact and the return electrode contact, at the same time keep the same center-to-center distance between the stimulating electrode contact and the return electrode contact, equilateral triangle stimulation configuration had smaller active area. When the center-to-center distance between the stimulating electrode contact and the return electrode contact increased from520μm to1430μm, the cross-sectional active area increased8.4%-14.4%, the electric field was more focalized, the area of active ganglion cells increased. Keep the center-to-center distance between the stimulating electrode contact and the return electrode contact, the center-to-center distance between the return electrode contacts increased from2×520sin(15°)μm to2x520sin(45°)μm, the cross-sectional active area decreased17.2%-38.7%, the electric field was more focalized, the area of active ganglion cells decreased. When the area (the radius) of the return electrode contact increased, the electric field was less focalized, the number of cells activated in the vicinity of the stimulating microelectrode decreased.2. The temperature field distribution of monopolar stimulation configuration. The maximum increase in the retina steady temperature was about0.0039℃with electrical stimulating by16electrode contacts. With electrical stimulating of different4electrode contacts, the scattered4electrode contacts had the minimum temperature increase, and the focused4electrode contacts had the maximum temperature increase, and their difference in temperature increase was about0.0005℃.When the center-to-center distance between the stimulating electrode contacts was changed from390μm to780μm or the contact radius was changed from130μm to260μm, the temperature decrease was over0.005℃. There was no significant difference between the effects of electrode materials on the temperature increase.Conclusion:1. Keeping the same area of the stimulating electrode contact and the return electrode contact, at the same time keeping the same center-to-center distance between the stimulating electrode contact and the return electrode contact, compared with parallel stimulation configuration, equilateral triangle stimulation configuration has a focused electric field, so it has a better spatial resolution.2. Tripolar stimulation configuration has a simple structure and is easier to design. A more focal stimulation can be gotten by decreasing the center-to-center distance between the stimulating electrode contact and the return electrode contact, or increasing the center-to-center distance between the return electrode contacts and the area of the return electrode contact, so the spatial resolution is improved.3. There is small temperature increase in the retina with a4×4epiretinal microelectrode array based on monopolar stimulation configuration; the temperature increase is in the vicinity of the stimulating microelectrode array. The maximum temperature increase in the retina decreases with increasing the center-to-center distance between the stimulating electrode contacts or the electrode contact area, however the maximum temperature increase decreases significantly when the distance became more than two times the diameter of electrode contact. Rational selection of the center-to-center distance between the stimulating electrode contacts and their area in electrode design can reduce the temperature rise induced by electrical stimulation. When the number of stimulating electrode contacts becomes four times, the temperature increases0.0017℃, not increased exponentially, so we can use hundreds of electrode contacts to stimulate the retina with monopolar stimulation configuration.