Q: Could you please describe the subretinal implant device?

The subretinal implant device is a small chip, similar to the camera chip in a mobile phone. It has a size of 3 × 3 mm² and is 0.1 mm thick. On its surface there are 1500 tiny light sensitive photodiodes, each of which has an amplifier that amplifies the light-evoked current and forwards it to the adjacent electrode.

The subretinal chip is placed beneath the fovea and connects its 1500 electrodes to the bipolar cell layer. (Image courtesy of Retina Implant.)

This electronic chip is placed on the tip of a very thin foil that carries gold wires for power supply of the amplifiers. At the other end of the 25 mm long foil there is a connection pad with a thin cable that leads to a small coil, which is comparable in size to a €2-coin.

The complete implant is situated within the body. During the implantation the surgeon makes a tiny window into the globe of the eye within the orbit and advances the light sensitive array under the retina in our new study towards the place of the best vision, called fovea. The implant's thin power connection foil, which is also under the retina, leaves the globe through the little window that is closed thereafter. It is connected to the cable that makes a loop in the orbit and — under the skin — leads back to a position behind the ear where the coil is placed under the skin. When being operated on, another coil is placed on top of the subdermal coil and kept there by a small magnet behind the ear sending power and signals through the skin.

The subretinal implant: The chip on the implant’s tip (left), magnified below is under the fovea. The cable leads through the orbit to the subdermal coil on the right. Four of the 1500 ‘pixels’ are shown on the lower right. (Image courtesy of Retina Implant.)

To control the implant the patient has a battery pack of the size of a mobile phone in his pocket and has two control knobs, one for contrast and one for brightness so that the sensitivity of the subretinal micro photodiode array can be adjusted according to the ambient light.

Q: What are microphotodiode arrays and how do they differ to other approaches?

One advantage this approach offers is that there is no camera outside the body. The light sensitive chip is in the eye right at the place where the photoreceptors had been before they had degenerated. Therefore, the chip with its array of 1500 light sensitive photodiodes moves with the gaze of the eye and the patient directly sees objects that he is looking at. Also, the image is refreshed constantly by involuntary eye movements, the so called microsaccades.

The retina with its vessels is lying on top of the chip that has five wires connected for power and signal transfer. In the ongoing study it is placed beneath the fovea. (Image courtesy of Retina Implant.)

Moreover, this implant has 1500 'pixels' in comparison to all other approaches where the maximum of pixels is 60. This high number of points improves the spatial resolution; for instance, patients can see individual teeth, when a person is laughing.

The entire implant is within the body and nothing is visible from outside and finally, the training period is very short, in the range of days, as the shape of objects can be perceived very naturally.

Q: Could you further explain the training period?

Professor Zrenner showing the principle of subretinal implants in an eight-times magnitude model. (Image courtesy of Bild der wissenschaft.)

The training period in our study is very short. Many patients can differentiate objects such as a knife or a plate after very few days of testing. They improve thereafter more slowly although we do not perform formal training.

What we have found is that after implantation patients have the ability to see shapes of objects, for example the familiar shape of a banana or their own hand, as they had seen it before going blind. In our approach there is no need to spatially rearrange the information transmitted to the electrodes because our chip simply replaces natural photoreceptors and the brain sees the input pixel by pixel in the shape the image falls through the pupil and the lens onto the subretinal camera chip.

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However, after several weeks patients learn to better interpret the new visual characteristics of objects such as sunflowers, spectacle frames, headlights of cars during night and so on. Therefore, training — like in a new born baby — is simply performed by doing and seeing and understanding the characteristics of the image (such as a window, a door handle etc.) that are visualized in black and white with several shades of grey.