Application of MEMS in Optobionics: Retinal Implant

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Application of MEMS in Optobionics Retinal
Implant

• By Alessandro Beghini
• PhD Student
• Northwestern University

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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)

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Human Eye
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Retina Physiology
Eye
Retina neural layer
Photoreceptors
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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.

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Approaches to Retinal Diseases

• The epiretinal approach
• stimulates the ganglion cells.

• The subretinal approach
• replaces photoreceptors
• and photodiodes.

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Epiretinal Microimplant (I)
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Epiretinal Microimplant Components (II)

• Main components
• Retina encoder
• Telemetry link
• Stimulator device

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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

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Microfabrication (IV)
The most important point in epiretinal implant is
the microfabrication of polymide film
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Subretinal Microimplant (I)
The device resembles the degenerated
photoreceptors, therefore the retina must be
only partially damaged to apply this approach
Final device
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Microfabrication (II)

• Oxidation (TEOS)

• Photoresist layer

• Etching of contact hole

• Titanium nitride deposited and
• micropatterned by lift off

• Grooves for chip separation

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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 SIH, monocrystalline SI,
  
• metal induced crystallization (high
  perpendicular
• conductivity and low lateral parasitic loss)

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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

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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

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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
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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 natures sophistication.
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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.

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Thank you!