1. Retinal prostheses aim to restore vision for those with retinal dystrophies like retinitis pigmentosa which destroy photoreceptors. They work by using a camera to capture images and then stimulating remaining inner retinal cells or ganglion cells with electrodes to generate visual perceptions. 2. Two types of implants exist - epiretinal which sit on top of the retina and subretinal underneath. Issues include biocompatibility and stability of electrode materials over long periods. 3. Carbon nanotubes show potential as an electrode material as they are biocompatible, robust, and flexible with superior electrochemical properties for long-term stimulation. However, their toxicity is a concern requiring further

1. ‘ MEMS in visual prosthesis’ Nanotecnologias 2009/2010 David Conceição Nº 64405 MBioNano

5. Retinitis pigmentosa A type of progressive retinal dystrophy Causes the loss of up to 95% of the photoreceptor layer, but spares up to 80% of the inner nuclear layer and ~ 30% of the ganglion celllayer Macular degeneration Patients with age-related macular degeneration (AMD) can lose up to 70% of photoreceptors with no loss of other retinal cell types Medical condition which usually affects older adults that results in a loss of vision in the center of the visual field (the macula) because of damage to the retina

6. Previousstudies ... Epiretinal implants sit on top of the retina, directly stimulating ganglia using signals sent from the external camera and power sent from an external transmitter Subretinalimplantssit under the retina, stimulating bipolar or ganglion cells from underneath It has been demonstrated that electrical stimulation of the retina can produce visual percepts in blind patients suffering from macular degeneration and retinitis pigmentosa.

7. A videocameratransmits640 × 480 pixel images at 25–50 Hz to a pocket PC. The computer processes the data and displays the resulting video on an LCD matrix mounted on goggles worn by the patient The LCD screen is illuminated with pulsed near-infrared (NIR, 800–900 nm) light, projecting each video image through the eye optics onto the retina The NIR light is then received by a photodiode array on a ∼3 mm implanted chip

8. Effect of cellular migration !! Two basic geometries of sub-retinal implants presented: perforated membranes and protruding electrode arrays Photodiodes convert light into pulsed electric current Electrodes must inject enough charge to stimulate nerve cells, within electrochemically safe voltage limits

9. Lithographically fabricated 10 μm wide pillars penetrating into the inner plexiform layer in the retina of RCS rat 15 days after the implantation. Rat retina grown on the three-layered structure for 7 days in vitro. Common problems include biocompatibility and electrochemical stability - Electrodes height: 70 µm;

10. Carbon Nanotubes as na alternative ... Superior electrochemical properties Robust Flexible Biocompatible? CNT protruding electrodes may be able to provide a safer solution for long-term retinal stimulation and implantation They could also act as recording units to sense electrical and chemical activities in neural systems for fundamental neuroscience research

11. - Their usage may be limited by their potential toxicity; - A large number of variables has considerable impact on the reactivity of carbon nanotubes. Carbon nanotubes Insulating layer Silicon wafer

12. Several considerations... - For functional restoration of sight a retinal implant should ideally cover a larger field of view, up to 10◦ (3 mm in diameter), and support a visual acuity of at least 20/80 (corresponding to a pixel size of 20 μm) – 18 000 pixels – Tissue hyperthermia! -Minimal distance between electrodes and target cells (for visual acuity of 20/80): 7 µm; - The optimal size of the electrode designed for selective stimulation of a single cell should be comparable to the cellular size (L ≈ 10 μm)