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E. Basham, M. Sivaprakasam, W. Liu; Functional Magnetic Stimulation for Implantable Epiretinal Prosthesis . Invest. Ophthalmol. Vis. Sci. 2005;46(13):1485.
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Purpose:Prosthetic devices which employ electrical stimulation are in development to restore vision in patients with Retinitis Pigmentosa and Age related Macular Degeneration. Functional magnetic stimulation offers advantages in biocompatibility, bioresistance and operational biotoxicity effects compared to functional electrical stimulation. The goal of this work is to demonstrate functional magnetic stimulation on the scale required for implantable epiretinal prosthesis. Methods: Stimulator arrays employed for epiretinal prosthetic devices commonly employ microelectrodes which stimulate via a biphasic voltage pulse leading to a current exchange. Several issues exist with this approach including the long term biofouling of the electrode, bioresistance of the implant and the possibility of biotoxic byproducts of the quasi–irreversible reactions caused by the stimulation waveforms at the electrode interface. In addition, scaling limits make the development of a 1000 electrode interfaces challenging, while still maintaining the resolution of the stimulation field. A coil is used to create the magnetic field for stimulation. Microelectronic manufacturing processes for microelectromechanical devices have matured to the point where components difficult to integrate previously have become more common place. This allows the development of an integrated coil possible. Since the coil surface may be hermetically sealed biocompatibility and bioresistance issues are improved. Because the magnetic permeability of tissue is near that of free space, protein deposition on the surface of the device is not an issue. Biotoxic electrolysis products are reduced because there is no direct electrical contact with the tissue. Because the structure of an individual cell and the retina is complex and not easily modeled using electromagnetic theory, the first appropriate step is demonstration. Results:A prototype inductive stimulator has been designed. To reduce power consumption, an asymmetric waveform resulting in asymmetric current flow was used. The devices are currently being tested in vitro. Conclusions: Magnetic stimulation represents a significant departure from conventional retinal stimulation strategies. While it offers several advantages, many challenges need to be overcome.
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