Visibly better: More details on the powered-up retinal prosthesis

As we get older the photoreceptors in our eyes start to get tired and stop working. For some people, age related macular degeneration  (AMD), as it’s called, actually means they start to go blind.

But although the photoreceptors are dead and gone, the optical nerves remain intact, offering a tantalising hope that one day a prosthetic retina could start sending visual signals through them again.

In fact, it has been possible to stimulate these nerves artificially for a long time, but to do this is a way which people could live on a day to day basis is far from easy [1].

The crux of the problem is that visible light is not very energetic at all. Any artificial retina implanted into the back of the eye would have to be made from some kind of photodiode array. Research has shown that, to generate enough current from a PDA to excite the nerve cells, something more energetic than visible light is needed.

I recently reported for Chemistry World on the news that Daniel Palanker’s group from Stanford University have developed a completely wireless retinal prosthesis which uses near infra-red (NIR) light to excite the photodiodes [2]. I thought the idea of bringing sight to the blind was so interesting that I’d unpack the news a bit here.

The reason this is big news is that in the past, retinal prostheses had to include a power coil along with the PDA inside the eye to keep them powered up. This was pretty unsatisfactory. Putting anything inside an eye and leaving the organ in good working order is always going to be challenging – but having to include a cm-scale power coil is really pushing the limits.

The sensible thing to do, you would think, would be to move to a more energetic wavelength, like infra-red, so that the light alone would be powerful enough to excite the nerve cells. However, most experts had thought that using IR light would burn the retina, and so had discounted it. Palanker’s new work shows conclusively that using NIR laser pulses, it’s possible to excite the diodes without doing any damage to the eye. In fact, the heat generated was 100 times lower than the accepted safety limit for ocular heating [3].

Image Credit: Nature Photonics and James Loudin

[Click to enlarge the image above]

However, the way someone sees with one of these devices fitted is not like a sighted person sees, as Konstantin Nikolic, a biomedical engineer at Imperial College London told me. “The problem is biology, not really the technology,” he said. To recreate proper vision, we would need to stimulate each nerve cell independently – a huge challenge which is not within reach at present.

The current photodiode array – just a few millimeters across stimulates ‘thousands’ of neurones, according to Loudin. That’s a huge improvement on existing technology, but still not the sort of resolution people experience normally.

So what is it actually like to see again with one of these devices implanted and wired in? Is there a moment of epiphany, when the machine is switched on, when all suddenly becomes visible again?

Well, no. But already, people have begun to ‘see’ again through the medium of what are called ‘phosphenes’; the flashes of light that sighted people see when they rub their closed eyes.

“It is possible to create phosphenes with this sort of technology,” said Loudin. “And a few people have started to show that it’s possible to form these flashes into shapes and letters.”

At the moment the medical procedures to install retinal prostheses are complex and difficult, and the resolution is extremely poor. But with this new development, huge improvements could be made on both fronts. In future, it seems a lot less impossible that blind people could end up with artificial vision akin to that which Geordi LaForge experiences in Star Trek.

Notes and references

1. I was initially surprised to think that this sort of thing is possible at all. But, as Nature News reported, at least 66 people have already had operations to fit such prosthetics.

2. K. Mathieson et al., N. Photon., 2012 DOI: 10.1038/NPHOTON.2012.104

3. The figure is between 0.2 and 10 mW per square mm.

It’s also worth checking out the website of Mark Humayun (a leader in the field) for more information on retinal prostheses.

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One Response to Visibly better: More details on the powered-up retinal prosthesis

  1. Audrey says:

    Never heard of phosphenes but always wondered about the interesting flashes of light behind the eyelids.

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