Tuesday, 25 March 2014
Subretinal prosthesis Alpha IMS
This post I would like to dedicate to the subretinal prosthesis Alpha IMS produced by Retina Implant AG, Reutlingen, Germany [company's web-site]. The scientific article of Prof. Eberhart Zrenner, one of the developers of subretinal prosthesis gives quite clear picture of what this prosthesis is [article's link].
Subretinal prothesis has the microchip which senses light and generates stimulation signals simultaneously at many pixel locations, using microphotodiode arrays. The Subretinal prothesis seeks to replace the function of degenerated photoreceptors directly by translating the light of the image falling onto the retina point by point into small currents that are proportional to the light stimulus. It is the only approach where the photodiode–amplifier–electrode set is contained within a single pixel of the MPDA such that each electrode provides an electrical stimulus to the remaining neurons nearby, thereby reflecting the visual signal that would normally be received via the corresponding, degenerated photoreceptor.
In summer 2013 Alpha IMS received a CE Mark.
Price around 100,000 EUROs (as of April, 2013).
The picture is taken from the article "Subretinal electronic chips allow blind patients to read letters and combine them to words", E. Zrenner et al.Essentially, an image is captured several times per second simultaneously by all photodiodes. Each element (‘pixel’) generates monophasic anodic voltage pulses at its electrode. Thus, pixelized repetitive stimulation is delivered simultaneously by all electrodes to adjacent groups of bipolar cells, the amount of current provided by each electrode being dependent on the brightness at each photodiode. Light is converted to charge pulses by each pixel. The chip is estimated to cover a visual angle of approximately 11º by 11º (1º approx. 288 mkm on the retina). The distance between two MPDA electrodes corresponds to a visual angle of 15 min of arc. Although small, it is sufficient for orientation and object localization, as is well established in patients with peripheral retinal dystrophies. Reading requires a field of 3 by 5 degrees.
The picture is taken from the article "Subretinal electronic chips allow blind patients to read letters and combine them to words", E. Zrenner et al.Because Alpha IMS microchip receives the image not from the external camera, but via eye, it is the only one retinal implant so far, where the image receiver array moves exactly with the eye. This has practical implications, as natural eye movements can be used to find and fixate a target.