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P. Huie, J. Loudin, A. Butterwick, G. Y. McLean, D. Palanker; Integration of the Outer Retina With Subretinal Prosthetic Implants. Invest. Ophthalmol. Vis. Sci. 2007;48(13):2550.
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Retinal stimulation with high spatial resolution requires close proximity of electrodes to the target cells. Implantation of flat prosthetic implants, epiretinally or subretinally, typically results in a separation from target cells by several tens of micrometers. Cellular scale proximity can be achieved through atraumatic penetration of pillar electrodes, and their integration into the target layer of cells in the retinal tissue.
Flat and 3-dimensional arrays of pillars were fabricated lithographically using SU8 UV-sensitive polymer. The pillars were fabricated with 10 µm diameters, heights in the 40 to 70 µm range, and lateral spacing varied from 20 to 60 µm. Special surgical tools have been developed for safe insertion of tethered and non-tethered implants into the subretinal space through a scleral incision. Implantation into RCS rats was performed after complete degeneration of the photoreceptors (45-65 days of age). Histological analysis has been performed in 18 rats 2- and 6- weeks post-operatively.
Six weeks after the implantation, the flat subretinal implants with no coatings were found separated from the inner nuclear layer (INL) by a 5-10 µm fibrotic pre-retinal membrane and a 30-40 µm layer of hypertrophied glia cell processes. In contrast, pillars penetrated completely and non-traumatically into the inner nuclear layer, and no fibrotic encapsulation was observed. We developed an electrically active tethered implant and a corresponding surgical technique for rat implantation. The subretinal portion of the implant is connected to a microcontroller and power source via a subcutaneous cable from the eye to a backpack. The programmable microcontroller periodically sends electrical pulses that stimulate the retina, records the impedance and stores the data without human intervention and anesthesia.
Three-dimensional subretinal implants with pillar electrodes take advantage of the intrinsic plasticity of the retina since migration brings retinal cellular components into intimate proximity of electrodes. Atraumatic subretinal insertion of the implant is very important to assure integration of the device with the retina while avoiding a fibrotic seal. Powered implants allow assessment of the effects of chronic retinal stimulation and implantation in rats.
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