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

2. 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)

3. Human Eye

4. Retina Physiology
Eye
Retina neural layer
Photoreceptors

5. 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.

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

7. Epiretinal Microimplant (I)

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

9. 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

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

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

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

13. Characteristics (III)
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)

14. 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

15. 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

16. 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

17. 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.

18. 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.

19. Thank you!