The challenges of restoring sight are immense, and the advances in this area seem to go hand in hand with advances in technology. As is the case with most electrical prostheses, they cross many scientific disciplines, from biophysics to electrical engineering. Of equal importance is the surgical aspect of being able to successfully implant such devices.
Two approaches are being employed. One is the subretinal implant, which is implanted at the level of the photoreceptors. Several thousand photodiodes are arrayed in a sheet and are connected to microelectrodes that in turn stimulate ganglion cells (the output neurons) in the retina. Light shining on this array generates a current that depolarizes the ganglion cells, via these microelectrodes. The other is the epiretinal implant, which is placed on the inner or opposite layer of the retina. In this, a conventional (external) camera and processing unit are connected to a readout chip that is secured to the inner retina, at the level of the ganglion cells.
Sight Disorders and the Retina
Many disorders of the eye are related to the retina; for instance age-related macular degeneration, retinitis pigmentosa, and diabetic retinopathy. They often can lead to serious disruptions in the collection of light, and may lead to blindness. Some, such as retinitis pigmentosa, in which there is a gradual deterioration of rods within the retina, eventually leaving only the central fovea intact (which is concentrated with cones), have no known cure.
Currently much of the focus is on the subretinal implant, and many groups are investing in this technology. The hurdles are huge, of course, not least of which is the surgical implantation onto the back of the retina. In general, two surgical approaches are currently employed; the first is through the cornea and into the vitreous humor, and the second is through the sclera. Both techniques are challenging. The biggest challenges involve maintaining internal pressure within the eye and contrast illumination of the surgical site during the procedure.
Once in place, there is about 50 to 100 µm between the subretinal implant array and the ganglion cells, which is sufficient to excite these cells. Even so, most of the other important neuronal elements, which refine the retinal image in normal sight within the retina, are bypassed. In animal models the subretinal implants seem to produce action potentials in the visual cortex of the brain, and the spatial resolution is around 1 degree -- a remarkable achievement. Nonetheless, the electrical stimulation of the ganglion cells is crude, and there is concurrent stimulation of their optic nerves, resulting in distorted images and cancellations of output from the photodiodes. Additionally, spatial resolution is adequate, at best, to achieve a recognizable image.
Boston Project Sees Progress
Great background information on retinal implants can be obtained at the Boston Retinal Implant Project's website, which is a collaborative effort with multiple academic, clinical and research institutions throughout the nation. They have a novel engineering solution to treat blinding diseases with retinal implants. Their current prosthesis includes an external camera that is mounted onto a pair of eyeglasses. The camera transmits images wirelessly through a coil within the glasses. The scene captured by the camera is then relayed to receiving coils on the prosthesis. The retinal stimulating array consists of a row of several hundred electrodes that are capable of stimulating ganglion cells in their immediate vicinity. The result is a pixilated array of lights that appear much like a large scoreboard image, at best. With such information, however, it is anticipated that a blind person will not require the use of a guide dog or cane.
Surgical methods have been applied in animal models and so far have shown reliable success using a combination of vitreoretinal surgery and ab externo procedures. The project's retinal implant has been tested in six humans, and several of the patients who had been legally blind for decades were able to distinguish small spots of light upon low-level stimulation of parts of the electrode array. (The original paper can be found at http://www.bostonretinalimplant.org/pdf/20031200-preceptual-thresholds.pdf.)
It is also noteworthy that close to 300 patents have been filed and granted on this topic. Over the past 12 months, 26 patents were filed or granted naming various assignees, such as the Doheny Eye Institute, Neurosystec Corporation, IMI Intelligent Medical Implants AG, Retina Implant GMBH, Second Sight Medical Products Inc., W.C.Heraeus GMBH & Co., and NewCyte, Inc., to name a few.
We've selected a few recent patent publications by Second Sight Medical Products, Inc. on retinal implants to convey some of the latest research efforts and interesting advances in this area by a company that was founded in 1998, and is devoted to creating a retinal prosthesis to provide sight to patients blinded from outer retinal degenerations.
Electrode Array for Visual Stimulation
20080275528/US-A1 USPTO Application
date filed: 2007-10-28 by Second Sight Medical Products, Inc.
The present invention is an electrode array adapted for stimulating the retina to create the perception of vision in blind patients.
This application discloses an invention for an electrode array adapted for stimulating the retina in order to create the perception of vision in blind patients. The objective is to restore color vision by electrically stimulating undamaged retinal cells, which remain in patients with lost or degraded visual function arising from various ocular disorders such as Retinitis Pigmentosa or Age-Related Macular Degeneration. Essentially, this invention is directed toward patients who have been blinded by degeneration of photoreceptors but who have sufficient bipolar cells, or other cells acting similarly, to permit electrical stimulation. It is made up of three main functional parts; electrode arrays, video camera and a power source.
The electrode arrays (with electrodes that are about 100 to 500 microns in length) are placed at the level of the photoreceptors. The array is powered inductively via an external power source. The actual visual prosthesis, which collects the image, is placed outside the eye, and is a video camera (such as a CCD or CMOS video camera). It is composed of a number of subsystems, including an external imager, an eye-motion compensation system, a head-motion compensation system, a video data-processing unit, a patient's controller, a physician's local controller, a physician's remote controller, and a telemetry unit. The camera sends an image in the form of electrical signals to the video data-processing unit that in turn formats a grid-like or pixel-like pattern to be sent to electronic circuitry (part of the internal part) within the eye, and which drives the electrodes. The electrodes replicate the incoming pattern in a usable form for stimulation of the retina so as to reproduce a facsimile of the external scene. The camera acquires color information and this data is processed in the video data processing unit. The video data processing unit consists of microprocessor CPU's and associated processing chips including high-speed data signal processing (DSP) chips. Part of the novelty of the invention is this aspect and indeed the electrical stimulation of the retina is intended to produce phosphenes and to produce induced color vision.
Also, part of the present invention relates to electronic image stabilization techniques based on tracking the movements of the eye as well as to telemetry in and out of the eye for uses such as remote diagnostics and recording from the retinal surface.
An additional advantageous feature of this invention is that it allows a physician to perform these diagnostics at a remote location, e.g., from his office. In other words, a patient could be traveling distantly and obtain physician monitoring and control of the retinal color prosthetic parameters. Additionally, the patient has a lot of control in setting the parameters of the device, as outlined in the specification. For instance, the image brightness may be increased or decreased by the patient at any time, under normal circumstances. A system of these components would itself constitute part of a visual prosthetic to form images in real time within the eye of a person with a damaged retina. In the process of giving back sight to those who are unable to see, it would be advantageous to supply artificial colors in this process of reconstructing sight so that the patient would be able to enjoy a much fuller version of the visual world.
Trans-Retinal Flexible Circuit Electrode Array
20080086183/US- USPTO Application
date filed: 2007-10-25 by Second Sight Medical Products, Inc.
This is an interesting patent that seems to relate to the invention described above. Essentially, the applicant has proposed an improved electrode array for subretinal stimulation that incorporates a polymer such as polyimide that is biocompatible and strong for supporting an electrode array and supporting traces in a thin flexible circuit array. In the present invention applicant takes advantage of the precise nature of thin polyimide making a point on the end of the electrode array. This allows the flexible circuit electrode array to be both electrode array and surgical tool to cut the retinal and slide the array under the retina in a single action.
The novel features of the invention are set forth with particularity in the appended claims and one of the salient features is that while the cutting point aids in passing the flexible circuit through the sclerotomy, it is particularly useful for passing the flexible circuit through the retina. The point can cut the retina (retinotomy) as it is inserted and this minimizes the size of the retinotomy, reduces the chances of hemorrhage or retina detachment, and simplifies the implant surgery. The electronics package is electrically coupled to a secondary inductive coil. Additionally, the secondary inductive coil may be made from a flexible circuit polymer sandwich with wire traces deposited between layers of the flexible circuit polymer.
Power Scheme for Implant Stimulators on the Human or Animal Body
2008060343/WO-A1 WIPO Application
date filed: 2007-08-15 by Second Sight Medical Products, Inc.
This invention describes a power control system for a retinal implant. It comprises a charging circuit to provide power to deliver controlled stimulation currents to electrodes that are in turn used to stimulate nerve cells within the retina, by means of a retinal implant array. The invention also embodies a capacitive storage element connected to the charging circuit and charged by the charging circuit; a shunting arrangement to limit voltage on the capacitive storage arrangement; a driver array configured to transfer charges from the capacitive storage arrangement to the tissue; and an electrode array connected with the driver array and the tissue. The implant comprises a capacitive storage element, and the shunting circuit comprises a current sensor with terminals connected in parallel with the capacitive storage arrangement, and the current-sensor sinking current from the capacitive storage arrangement when the voltage of the capacitive storage arrangement reaches a critical voltage value.
Also incorporated is a constant current-type electrode driver that is comprised of a driver controller, used to generate anodic and cathodic stimulation switching signals as well as generate a defined output current suitable for neuronal stimulation at the level of the retina. A method to control power in an implanted electrode array is also shown in the invention. Essentially current is supplied to the electrodes via a capacitative storage unit that in turn is monitored so that they can be controlled when the electric charges are above a high value or below a low value; and controlling the electric charges when above the high value or below the low value.
About the Analyst
Nerac Analyst Richard Hendriks, Ph.D., partners with pharmaceutical companies to discover the most effective ways of expanding their business goals. This encompasses a range of solutions from innovations in biotechnology to analytical analyses of recent trends in disease treatments. Qualitative analyses of citation and patent literature is the foundation of such endeavors, and Dr. Hendriks has specialized in this area for almost 10 years with Nerac. He has a Ph.D. in neuroscience from University of Melbourne, Australia, and a background in the area of neuronal electrophysiology. During his research career, Dr. Hendriks received several grants and fellowships from the National Institutes of Health and the National Science Foundation. Dr. Hendriks has authored dozens of published articles and abstracts on such topics as physiology and electrophysiology. Besides neuroscience and electrophysiology, his areas of expertise include neurophysiology, pharmacology, receptors and ion channels, biophysics, developmental neuroscience, drug development, electrophysiology, bioengineering and neurotransmitters.
Nerac analysts deliver custom assessments in the following areas:
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