ARTIFICIAL THIN-FILM TRANSISTOR RETINA

By,Charu Lakshmi T R15PH62R07M.Tech SSTIIT KHARAGPUR

SOURCE : http://www.secondsight.com

Restoring vision Restoring independence

This light-sensing implant developed by Germany’s Retina Implant contains 1,500 photodiodes.

• The first application of an implantable stimulator for vision restoration was

developed by Dr S. Brindley and Lewin in 1968.

Source : http://retinatoday.com/2015/03/the-trajectory-of-innovation-in-retinal-implants.

Retinal Implant Technology

It uses microelectronics and microchip electrodes surgically implanted into the back of the eye (retina) to restore the function of the damaged light-activated cells.

These photoreceptor cells respond to light and convert it to an electrical signal which is passed to nerve cells in the eye, and then ultimately to the brain where it is perceived as vision.

A surprising finding is that in patients with retinitis pigmentosa and age-related macular degeneration where the photoreceptors have been damaged, often the nerve cells that relay the signal to the brain (the ganglion cells) are still intact after many years.

Therefore, it is possible to directly stimulate these retinal nerve cells in these patients and produce signals that will be received by the brain and perceived as vision.

What is it Like to See Again with an Artificial Retina ?

“I can see if you are in front of me, and if you try to go away,” he says. “Or, if I look at a big tree with the system on I can maybe see some darkness and if it’s bright outside and I move my head to the left or right I can see different shadows that tell me there is something there. There’s no way to tell what it is,” says Konstantopoulos.

Some people with artificial retinas can read large letters, see slow-moving cars, or identify tableware. Other patients experience no benefit. How well people can learn to use the device and retrain their brain is also important.

http://www.fightingblindness.ie

There are several parts to IRIS, including an implant placed inside the eye and attached to the surface of the retina.

As the company describes, a patient would then wear a special pair of glasses built with a minicamera and wireless transmitter, which is connected to a pocket computer.

The computer transforms camera image into a signal transferred back to the glasses then to the retinal implant, where it stimulates the optic nerve and generates ages. Then through a special rehabilitation program, the brain learns how to interpret the images.

What is it ? An artificial retina using thin-film

photodiodes (TFPDs) and poly-Si TFTs achieves edge enhancement, one of the functions of living retinas.

Artificial retinas direct the stimulus signal in to the neuron cells, so that living retina cells are not seriously damaged.

Source : Google images

Implantation of Artificial Retina

Source : Google images

ARTIFICIAL RETINA working The retina array includes matrix-like multiple retina pixels.

The retina pixel consists of a photo transistor, current mirror, and load resistance.

The photo transistor is optimized to achieve high efficiency, and the current mirror and load resistance are designed accordingly.

The photosensitivity of the reverse-biased p/i/n poly-Si phototransistor is 150 pA at 1000 lx for white light.

First, the photo transistors perceive the irradiated light and induce the photo-induced current .

Next, the current mirror amplifies to the mirror current .

Finally, the load resistance converts mirror current to the output voltage.

Consequently, the retina pixels irradiated with bright light output a higher output voltage, whereas the retina pixels irradiated with darker light output a lower output voltage.

Fabrication essentials It is fabricated on transparent and flexible substrates like a

plastic substrate. Glass substrate can be used too.

Since the human eyeballs are curved, the flexible substrate is also preferable.

It uses the same fabrication processes as conventional poly-Si TFTs and encapsulated using SiO2, in order to perform in corrosive environments.

As a result, the artificial retina using TFTs are suitable for the epi-retinal implant on the curved human eyeballs.

Fabrication of poly Si-TFTs’ Firstly, a 50nm thick undoped amorphous Si (a-Si-H) and

20nm thick n-type phosphorous doped amorphous Si layer were deposited by PECVD on oxidised Si substrate.

During deposition, the substrate temperature, RF power density and chamber pressure are fixed at 250 degree celscius,0.11 W/cm2 and 0.45 torr, respectively.

Second the channel region was defined and samples were sent to furnace to expel hydrogen from the film for 10 minutes at 500 degree celcius under Nitrogen environment.

After annealing by the excimer laser (KrF, duration time in ns), multiple nanowires were patterned by electron beam lithography and transferred by reactive ion etching.

Then, a 100 nm thick Sio2 was deposited by PECVD at 250 degree celcius as a gate insulator’.

Followed by oxygen plasma treatment of the dangling bonds in the laser crystallised poly-Si films for 30 seconds .

The contact windows for source and drain were opened. The samples were treated by NH3 plasma to passivate the grain boundary for three hours with pressure of 2 torr.

References

http://www.fightingblindness.ie /cure/retinal-implant-technology.

https://www.technologyreview.com /s/514081/can-artificial-retinas-restore-natural-sight/.

Artificial Retina Using Thin-Film Transistors Driven by Wireless Power Supply. - Yuta Miura (IEEE SENSORS JOURNAL, VOL. 11, NO. 7, JULY 2011)

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