Improving vision with electronic retinal implants
Instead, the implant relies on the optics of the eye to provide a focussed image on the retina and this also means that the image is scanned with the ocular saccades. This could result in a more natural interpretation of the visual scene, according to MacLaren.
A complementary metal-oxide-semiconductor (CMOS) camera-like chip with 1500 pixels about 3 x 3 mm in diameter makes up the light-capturing component of the device.
Each pixel is 70 x 70 µm in diameter and comprises a light-sensitive photodiode and an electrode that stimulates the overlying retina. The photodiode acts like the outer segment of the photoreceptor, while the electrode acts like the photoreceptor synapse, transmitting signals to the inner retina.
This incorporation of the sensory and stimulatory functions in a single chip also sets the device apart from the Argus II, which consists of a stimulatory electrode inside the eye picking up the signal from an external camera.
The Alpha-IMS retinal implant spans a visual field of about 12 degrees, depending on the size of the eye. Its pixel density could yield a theoretical best-corrected visual acuity of 6/75. This would assume perfect contact of the individual 70 µm electrodes with overlying functional bipolar/horizontal cells. In the trial, 2 patients came close to this theoretical maximum resolution.
In the absence of photoreceptors to amplify energy from photons entering the eye, the device relies on an induction loop connection through the skin behind the ear. This uses a magnetic interface resembling that of a power cochlear implant.
A cochlear implant surgeon, and an anaesthetist who is comfortable with relatively long periods of general anaesthesia, must both participate in this surgery.
In 2011, MacLaren and colleagues implanted the first generation of Alpha-IMS retinal implant as part of a multicentre clinical trial to assess the retinal implant in clinical practice. The trial made up part of a clinical investigation led by Retina Implant AG, that led to a CE Mark of approval.
In the multicenter trial, patients reported improvements in routine visually guided tasks, recognition tasks and mobility. Twenty-five of the 29 participants regained some visual acuity, light perception or object recognition.
After about 12 months, however, the implants started failing. After explantation, the researchers traced the problems to cable breakage and other deterioration related to “using a complex electronic device in a saline environment,” MacLaren wrote.
The researchers are now testing a new version of the device, the Alpha AMS, in which they have attempted to address these weaknesses. The new trial, funded by the National Institute for Health Research Invention for Innovation award, began in 2015. The first patient was still using the device after 16 months as of January 6, 2017.