Application of MEMS in Optobionics: Retinal Implant By Alessandro Beghini PhD Student Northwestern University

Outline Eye physiology and retinal diseases Approaches to the problem: epiretinal and subretinal microimplant Characteristic of the approaches (descriptions, microfabrication,..) Biocompatibility Comparison of the two approaches Applications Conclusion (feasibility)

Human Eye

Retina Physiology Eye Retina neural layer Photoreceptors

Retinal Diseases Principal diseases: Retinitis Pigmentosa (RP) and Age related Macular Degeneration (AMD); Symptoms: night blindness, lost peripheral vision (tunnel vision), loss of the ability to discriminate color; Possible cure: use of vitamin A; Current research on the genes which causes RP.

Approaches to Retinal Diseases The epiretinal approach stimulates the ganglion cells. The subretinal approach replaces photoreceptors and photodiodes.

Epiretinal Microimplant (I)

Epiretinal Microimplant: Components (II) Main components: Retina encoder Telemetry link Stimulator device

Characteristics (III) Photodiode with light sensitivity higher than 140 dB Spatial filtering Convolution of the of pixel parameters Generation of spike trains Receiver units: rectification, demodulation, decoding

Microfabrication (IV) The most important point in epiretinal implant is the microfabrication of polymide film:

Subretinal Microimplant (I) The device resembles the degenerated photoreceptors, therefore the retina must be only partially damaged to apply this approach Final device

Microfabrication (II) Oxidation (TEOS) Photoresist layer Etching of contact hole Titanium nitride deposited and micropatterned by lift off Grooves for chip separation

Characteristics (III) 2000-5000 photodiode cells on a single device Cell size: 20x20 µm2 up to 200x200 µm2 Improved coupling between photoreceptors and bipolar cell Contact layer: p-doped SI:H, monocrystalline SI, metal induced crystallization (high perpendicular conductivity and low lateral parasitic loss)

Biocompatibility Main concern: chronic inflammation and cellular reaction Muller cell could scar the retinal surface and generate traction forces which could detach the retina Stabilization of the electrode matrix By electrodes By adhesives

Epiretinal and Subretinal Device: Pros and Cons Epiretinal Approach: No need for intact neurons In-vivo experiment must be conducted Low number of electrode sites Subretinal Approach: Simpler structure No need for an external camera Not influenced from outside

Applications and Experiments Implantation in pigs and rabbits revealed the decay of the passivation layer for a subretinal device: Titanium nitride electrodes are biostable for a period of 18 month Application in human of the subretinal implant is an important on going research

Conclusion This research has shown the possible applications of MEMS technology in curing important retinal diseases. Both epiretinal and subretinal approaches has been analyzed and microfabrication processes has been described. However, the implemented systems are still far from nature’s sophistication.

Future Research Extend the number of active microchips to three and glue them to a PI foil. Improve biostability. Increase the number of electrode. Perform more experiment. Study in genetics and tissue engineering.

Thank you!