Retinal prosthetic devices

Summary form only given. In case of retinitis pigmentosa (RP) and aged-macular de generation (AMD), the photoreceptor cells are dysfunctional, whereas most of the other retinal cells, such as ganglion cells, remain healthy, unless the disease is in the terminal stage. Consequently, by stimulating the remaining retinal cells, visual sensation or phosphene can be evoked. This is the principle of the retinal prosthesis, or artificial vision. Based on this principle, a retinal prosthesis device stimulates retinal cells with a patterned electrical signal so that a blind patient may sense a patterned phosphene, or something like an image. According to the place where the retinal stimulator is placed, the retinal prosthesis device is classified into three categories; sub-retinal, epi-retinal, and STS (supra choroid transretinal stimulation) which is recently developed. In epi-retinal implantation, a retinal stimulator is attached to the retina. In these methods, power supply and stimulus pattern data generated from input image data are transmitted by wireless technique of electromagnetic coupling of primary coil placed outside the body and secondary coil placed inside the body. These wireless transmission technology has been established in artificial cochlear system. There are several groups engaged in epi-retinal stimulation and mostly advanced in clinical trials. The disadvantage of this type is the fixation of the stimulator. In addition, the electrodes may happen to stimulate optic nerve instead of ganglion cells. The fixation of the stimulator sometimes causes this type of phosphen. STS has the same advantage of epi-retinal implantation. In addition, the surgical operation is the easiest among retinal prostheses. The eye is not needed to be open, because a stimulator is inserted into a pocket produced in a sclera. The implantation in a sclera pocket means the electrode has a distance from a retina, so that the threshold current to elicit the phosphen may be higher than the other methods. The other advantage of STS is the wider coverage of the stimulation area. Acute clinical trials have shown that the threshold is 200-1000 mA, which is a little higher of than one in the other methods and the patients suffered from RP sensed phosphens when stimulated. In sub-retinal implantation, a retinal stimulator is inserted underneath the retina, so that the device is fixed naturally. The electrodes are attached to the retina, especially photoreceptors and thus the retinotopy is probably maintained. Some sub-retinal devices are integrated with stimulus electrodes and photo-sensors in the same plane, so that they act like photoreceptors. Thus such a sub-retinal device is ideal for artificial retina. The disadvantage is that the stimulator may obstruct the flow of internal fluids including nutrition from epithelium to retinal cells. Clinical trials are executed in sub retinal implantation. In order to realize better vision through a retinal prosthesis, over 1,000 electrodes would be preferable. When increasing the number of electrodes, we are faced with problems associated with interconnection between electrodes and external lead wires with good mechanical flexibility. Specifically, the stimulator must be bent to match the curvature of the eyeball. It is a good idea to introduce a CMOS-based chip in the stimulator because scanning circuits can be integrated in order to reduce the amount of wiring. For implantation, the thin and flexible CMOS-based stimulator is preferable to be thin and flexible in order to fit the eye and to avoid damaging tissue. To solve this problem, a smart stimulator that consists of a number of CMOS-based microchips distributed on a flexible substrate is proposed and demonstrated. We have developed such a multiple microchip-based retinal stimulator and validated its effectiveness by animal experiments. We need to solve several issues to realize retinal stimulator with 1000+ stimulus electrodes; the bio-compatible capsulation of a microchip with better water-resistance, ultra-small stimulus electrodes, better contact to tissues, high efficient power and data transmission system, and so on.