Chronically implantable retinal prosthesis

Details Chronically implantable retinal prosthesis Chronically implantable retinal prosthesis

1999-05-07

2001-11-27

Bockelman, Mark (Department: 3762)

Surgery: light, thermal, and electrical application

Light, thermal, and electrical application

Electrical therapeutic systems

C623S006630

Reexamination Certificate

active

06324429

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to visual prostheses, and more specifically, the invention relates to retinal implants which employ controlled electrical current pulses to create the impression of the presence of light in patients whose photoreceptor cells (rods and cones) are not functioning by stimulating the ganglion cells at the retina surface.
BACKGROUND OF THE INVENTION
Some important and common forms of blindness result from degeneration of the rods and cones of the eye and in these cases the sole output cells (i.e., ganglion cells) are relatively spared. The development of a fully functioning intraocular prosthesis integrated into a single unit has been a long time goal.
In the human eye, the ganglion cell layer of the retina becomes a monolayer at a distance of 2.5-2.75 mm from the foveola center. Since the cells are no longer stacked in this outer region, this is the preferred location for stimulation with an epiretinal electrode array. The feasibility of a visual prosthesis operating on such a principle has been demonstrated by Humayun et al. in an experiment in which the retinas of patients with retinitis pigmentosa, age-related macular degeneration, or similar degenerative diseases of the eye were stimulated using bundles of insulated platinum wire. The patients were under local anesthesia, and they described seeing points of light which correctly corresponded with the region of the retina in which the stimulus was applied (Humayun, M., et al.,
Archiv. Ophthalmol.,
114: 40-46, 1996). The form of the stimulating device was, however, not suited for chronic implantation. The threshold for perception was reported to be in the range of 0.16-70 mC/cm
2
. This confirmed the results of earlier experiments on animal subjects by the instant inventors and others which indicated that strong evoked cortical potentials could be observed when rabbit retinas were stimulated using passive microfabricated electrode arrays similar in some respects to the ones proposed in the current invention (Rizzo, J. F., et al., ARVO Poster Session Abstract,
Investigative Ophthalmology and Visual Science,
37: S707, 1996; Walter, P., et al.
Investigative Ophthalmology and Visual Science,
39: S990, 1998). The instant inventors have, with others, performed three surgical procedures using microfabricated electrode arrays and similar in technique to those described by Humayun and confirmed that a consistent response to input electrical stimuli could be noted by the patient.
The task of creating a retinal implant has been addressed by Chow, in U.S. Pat. No. 5,016,633, who proposed a subretinal implant based on a microphotodiode array. The procedure involved in its implantation is so biologically intrusive, however, that successful implementation of such a device in human subjects has not been reported. Furthermore, an entirely passive array will be rather insensitive under normal lighting conditions, and an array powered from outside the body by means of a direct electrical connection will likely lead to infections and again, be so intrusive as to be objectionable.
Earlier designs of the present inventors placed all components of the prosthesis on the retinal surface (U.S. patent application Ser. No. 19/074,196, filed May 7, 1998, and U.S. Pat. No. 5,800,530, both of which are incorporated herein by reference). It became quickly apparent that the delicate retina could not withstand the mechanical burden which was at least partially the result of the relatively thick profile of the microelectronic components. A later prototype included one significant change in design—the bulky microelectronic components were moved anteriorly within the eye, off of the retinal surface. In this configuration, the microelectronics are held in a custom-designed intraocular lens, and only a thin ribbon containing the microelectrodes extends rearwardly to the retinal surface.
In summary, the present invention is a contribution to an ongoing area of development of visual prostheses which embodies the state of the art in both silicon processing technology and our understanding of the mechanisms underlying phosphene generation in human retinas by surface electrical stimulation. The feasibility of creating a chronically implantable retinal prosthesis capable of restoring some useful vision to blind patients over at least several degrees of their former field of view has been demonstrated.
SUMMARY OF THE INVENTION
The current invention is an improvement of prior art by some of the instant inventors in which a flexible, cantilevered epiretinal implant structure was first disclosed (U.S. Pat. Nos. 5,411,540 and 5,476,494, both of which are incorporated herein by reference). The primary improvement in the current invention is the monolithic integration of the signal processing, power conversion, control functions, and stimulating current sources on an ultrathin silicon membrane which is part of the implant itself and not physically removed from the retinal area to be stimulated. By making the electronics as flexible as the remainder of the implant, the biocompatibility of the overall device is greatly improved. Furthermore, the absence of micro-cables in a hybrid assembly makes the surgical procedure considerably simpler. While a polyimide-based extension of the essential silicon structure is not ruled out, it can be limited to a region extending not more than several mm beyond the silicon region's periphery.
The biocompatibility of the materials which are proposed to be employed in the fabrication of a chronically implantable device has been addressed by others, and in the current invention we propose a hermetic seal on the silicon-based portion of the device made possible by plasma enhanced deposition of amorphous silicon carbide (S. Bayliss, et al.
Thin Solid Films,
297, 308-310, 1997; R. Hauert, et al.
Thin Solid Films,
308-309, 191-194, 1997; R. Richardson, et al.
Biomaterials,
14: 627-635, 1993). While it is of course essential for the implant to have a benign effect on the surrounding tissue in terms of cytotoxicity, cell adhesion and the absence of toxic electrochemical reactions at the electrode-electrolyte interface, it is also important that metal ions be kept away from the sensitive MOS circuitry. The multiple layer coating structure proposed in the current invention serves both these needs.
One object of the invention is to process and retransmit to the ganglion cells on the retina surface images which cover at least several degrees of a blind patient's former field of view. Since the precise mechanism by which the perception of light is elicited by electrical current injection to these cells is not fully understood, the circuitry used to perform the signal processing functions is envisioned to operate primarily in a slave mode, with control functions for the raster presentation of incoming video data performed externally where possible. The power dissipation of the implanted silicon circuit must be minimized to limit the requirements on the external power supply. In a preferred embodiment, the external circuitry could be mounted on a pair of eyeglasses or in the patient's pocket.
Another object of the invention is to provide an implantable device suitable for chronic use. That is, its components should be biocompatible; its mass and profile should be minimized; it should be epiretinal in placement; it should have a long working life in the eye environment; it should be highly flexible; and surgically tractable; and it should allow for the continued health of tissue which it covers.
These objects, together with other objects, features, and advantages of the invention will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings.


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