Scientists at the Stanford University School of Medicine are developing a new type of retinal prosthesis which aims to simplify the complex surgery associated with existing, bulkier implants. The prosthetic comprises a pair of goggles and an implanted retinal sensor made up of an array of light-sensitive photodiodes.
The goggles incorporate a miniature camera, a pocket computer for processing the camera data, and an LCD screen embedded into the goggles for displaying the data. The LCD screen beams images using laser pulses of near-infrared light to a photovoltaic ultra thin silicon chip implanted beneath the retina. The chip, in turn, translates the infra-red pulses to neural pulses in the retina which can be processed as images in the brain. The key selling point for the new implant is the elimination of wires and cables and the relative simplicity of implantation.
In order to test the retinal implant, the research team implanted the array of photodiodes in the retinas of normal and blind rats. They then compared the ganglion activity of both groups in response to visible light and the infra-red light pulses generated by the goggles, as explained in the press release:
For this study, Palanker [Prof. Daniel Palanker, the senior investigator involved in the study] and his team fabricated a chip about the size of a pencil point that contains hundreds of these light-sensitive diodes. To test how these chips responded, the researchers used retinas from both normal rats and blind rats that serve as models of retinal degenerative disease. The scientists placed an array of photodiodes beneath the retinas and placed a multi-electrode array above the layer of ganglion cells to gauge their activity. The scientists then sent pulses of light, both normal and near-infrared, to produce electric current in the photodiodes and measured the response in the outer layer of the retinas.
In the normal rats, the ganglions were stimulated, as expected, by the normal visible light, but they also presented a similar response to the near-infrared light: That’s confirmation that the diodes were triggering neural activity.
In the degenerative rat retinas, the normal light elicited little response, but the near-infrared light prompted strong spikes in activity roughly similar to what occurred in the normal rat retinas.
The results from this study have been published online in the journal Nature Photonics. The team is currently undertaking a longitudinal study on the efficacy of the implant over a number of months in rat models.
Abstract in Nature Photonics: Photovoltaic retinal prosthesis with high pixel density