Subretinal Implant



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Fig. 1: Subretinal implant

(http://www.varrd.emory.edu/techtransfer/retina.htm)


In Retinitis pigmentosa disease, the retinal pigment epithelial cells (RPE) begin to die out and the person starts loosing the vision gradually. Since the function of the retina to transduce light into biological signal is weakened, it causes blindness. Subretinal implant is used to substitute the lost RPE cells with the ones of artificial basis to restore the vision. In this implant, a microphotodiode array (MPD), a silicon micromanufactured device, or semiconductor microphotodiode array (SMA) is used. This piece of equipment is placed behind the retina between the sclera and the bipolar cells. The incident light is transformed into electrical potentials that excite the bipolar cells to form an image sensation. The general design of replacement of degenerated or lost photoreceptors by MPDA is shown in figure 2. There are specific priorities which must be taken care by subretinal implant in order to be functioning well to restore the vision: the devise has to be biocompatible so that it would be used for long-term performance. (Maynard, E. M., et al. 2001)

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Fig. 2: General design of replacement of degenerated or lost photoreceptors

by a MPDA which is positioned into the subretinal space.

(Schubert, MB, et al. "Subretinal Implants for the Recovery of Vision." Systems, Man, and Cybernetics,

1999 IEEE International Conference on 4 (1999))


The arrays can be manufactured by various silicon manufacturing procedures. MPD arrays are manufactured consistently with measurements of each stimulating unit as 20 Ám X 20 Ám, and adjacent units separated as 10 Ám. The elements are produced to be responsive to light corresponding to the visible spectrum (400-700 nm). Several thousands of the devices can be placed on a single structure of diameter of 3 mm, thickness of 100 Ám and with a density same as the replacing RPE cells. These devices have demonstrates the same electrophysiological behaviors as the healthy RPE cells. (Maynard, E. M., et al. 2001) The MPDA has to be very thin and flexible enough in order to be able to fit to the curvature of the eye ball. Figure 3 shows an example of such an ultra thin MPDA having a thickness of 1.5 micron, together with titanium substrate and silicon nitride passivation.

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Fig. 3: Ultra thin microphotodiode array

(http://nanobio.snu.ac.kr/member/workshop/1stintws/Tohru%20Yagi.pdf)


From the results of the first implantation experiments on rabbits, it was discovered that nutrition openings in the chips are needed. After that openings for nutritions have been set in MPDAs. Figure 4 indicates the scanning electron micrograph of a phototype subretinal implant, with a MPDA having a centered nutrition opening.

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Fig. 4: Scanning electron micrograph of a phototype subretinal implant,

with a MPDA having a centered nutrition opening.

(Schubert, MB, et al. "Subretinal Implants for the Recovery of Vision." Systems, Man, and Cybernetics,

1999 IEEE International Conference on 4 (1999)).


In Subretinal implant, the light-sensitive microphotodiodes with microelectrodes of gold and titanium nitride set in arrays is implanted in the subretinal space. The visible light coming from different directions is transformed into small currents by the microphotodiodes at each of hundreds of microelectrodes. These currents are then passed to the retinal network by neurons. The middle and inner retina captures current and then processes the part of vision. There are many benefits of using the subretinal prostheses. Such as, the MPD directly replaces the lost or degenerated RPE cells; the retinaĺs remaining network is still capable of processing electrical signals; ease of fixing the high density MPDA in the subretinal position; no need of any external camera or external image processing equipment; and eye movement to locate the objects is not restricted.

There are some of the limitations to the subretinal implants as well. The single MPD is not enough to stimulate enough current. So a subretinal implant is supported by an external energy source, such as transpupillary infrared illumination of receivers close to the chip or electromagnetic transfer, is currently under progress (Zrenner, 2002). Some of the additional developments in this process are movement to flexible substrates to hold the subtle nature of the retina and to decrease the light intensity. (Maynard, E. M., et al. 2001)



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BMES 212
Summer Term 2007
School of Biomedical Engineering, Science and Health Systems