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.

Figure 1. Subretinal implant. 

(a) The microphotodiode array (MPDA) is a light sensitive 3.0 x 3.1 mm CMOS-chip with 1500 pixel-generating elements on a 20 mkm thick polyimide foil carrying an additional test field with 16 electrodes for direct electrical stimulation (DS test field). 

(b) The foil exits approximately 25 mm away from the tip at the equator of the eyeball and is attached to the sclera by means of a small fixation pad looping through the orbit to a subcutaneous silicone cable that connects via a plug behind the ear to a power control unit. 

(c) Magnification of the DS electrode array showing the 16 quadruple electrodes and their dimensions. 

(d) Pattern stimulation via DS array (e.g. ‘U’). 

(e,f ) switching from a triangle to a square by shifting stimulation of a single electrode. 

(g) Magnification of four of the 1500 elements (‘pixels’), showing the rectangular photodiodes above each squared electrode and its contact hole that connects it to the amplifier circuit (overlaid sketch).
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.

Figure 2. Implant position in the body. 

(a) The cable from the implanted chip in the eye leads under the temporal muscle to the exit behind the ear, and connects with a wirelessly operated power control unit. 

(b) Position of the implant under the transparent retina. 

(c) MPDA photodiodes, amplifiers and electrodes in relation to retinal neurons and pigment epithelium. 

(d) Patient with wireless control unit attached to a neckband. 

(e) Route of the polyimide foil (red) and cable (green) in the orbit in a three-dimensional reconstruction 

of CT scans. 

(f) Photograph of the subretinal implant’s tip at the posterior eye pole through a patient’s pupil.
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.

In summer 2013 Alpha IMS received a CE Mark.

Price around 100,000 EUROs (as of April, 2013). 

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