Application of MEMS in Optobionics: Retinal

ImplantBy Alessandro Beghini

PhD StudentNorthwestern 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

Photoreceptors

Retina neural layer

Eye

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 approachstimulates the ganglion cells.

• The subretinal approachreplaces 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

• Receiver units: rectification, demodulation, decoding

• Generation of spike trains

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 monthApplication in human of the subretinal implant is an important on going research

Conclusion

This research has shown the possible applicationsof 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!