Abstract:
The present invention is an electrode array for neural stimulation. In particular it is an electrode array for use with a visual prosthesis with the electrode array suitable to be positioned on the retina. The array includes multiple attachment points to provide for even pressure across the electrode array surface. The attachment points are arranged so as to not damage retinal tissue stimulated by the electrode array.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of U.S. Provisional Patent Application 60/982,357, filed Oct. 24, 2007, for Electrode Array for Even Retinal Pressure. This application is related to and incorporates by reference, U.S. patent application Ser. No. 12/163,658, filed Jun. 27, 2008, for Flexible Circuit Electrode Array. 
    
    
     GOVERNMENT RIGHTS NOTICE 
     This invention was made with government support under grant No. R24EY12893-01, awarded by the National Institutes of Health. The government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention is generally directed to neural stimulation and more specifically to an improved method of improving resolution by selectively stimulating smaller cells. 
     BACKGROUND OF THE INVENTION 
     In 1755 LeRoy passed the discharge of a Leyden jar through the orbit of a man who was blind from cataract and the patient saw “flames passing rapidly downwards.” Ever since, there has been a fascination with electrically elicited visual perception. The general concept of electrical stimulation of retinal cells to produce these flashes of light or phosphenes has been known for quite some time. Based on these general principles, some early attempts at devising a prosthesis for aiding the visually impaired have included attaching electrodes to the head or eyelids of patients. While some of these early attempts met with some limited success, these early prosthetic devices were large, bulky and could not produce adequate simulated vision to truly aid the visually impaired. 
     In the early 1930&#39;s, Foerster investigated the effect of electrically stimulating the exposed occipital pole of one cerebral hemisphere. He found that, when a point at the extreme occipital pole was stimulated, the patient perceived a small spot of light directly in front and motionless (a phosphene). Subsequently, Brindley and Lewin (1968) thoroughly studied electrical stimulation of the human occipital (visual) cortex. By varying the stimulation parameters, these investigators described in detail the location of the phosphenes produced relative to the specific region of the occipital cortex stimulated. These experiments demonstrated: (1) the consistent shape and position of phosphenes; (2) that increased stimulation pulse duration made phosphenes brighter; and (3) that there was no detectable interaction between neighboring electrodes which were as close as 2.4 mm apart. 
     As intraocular surgical techniques have advanced, it has become possible to apply stimulation on small groups and even on individual retinal cells to generate focused phosphenes through devices implanted within the eye itself. This has sparked renewed interest in developing methods and apparati to aid the visually impaired. Specifically, great effort has been expended in the area of intraocular retinal prosthesis devices in an effort to restore vision in cases where blindness is caused by photoreceptor degenerative retinal diseases such as retinitis pigmentosa and age related macular degeneration which affect millions of people worldwide. 
     Neural tissue can be artificially stimulated and activated by prosthetic devices that pass pulses of electrical current through electrodes on such a device. The passage of current causes changes in electrical potentials across retinal neuronal cell membranes, which can initiate retinal neuronal action potentials, which are the means of information transfer in the nervous system. 
     Based on this mechanism, it is possible to input information into the nervous system by coding the sensory information as a sequence of electrical pulses which are relayed to the nervous system via the prosthetic device. In this way, it is possible to provide artificial sensations including vision. 
     Some forms of blindness involve selective loss of the light sensitive transducers of the retina. Other retinal neurons remain viable, however, and may be activated in the manner described above by placement of a prosthetic electrode device on the inner (toward the vitreous) retinal surface (epiretinal). This placement must be mechanically stable, minimize the distance between the device electrodes and the retinal neurons, and avoid undue compression of the retinal neurons. 
     In 1986, Bullara (U.S. Pat. No. 4,573,481) patented an electrode assembly for surgical implantation on a nerve. The matrix was silicone with embedded iridium electrodes. The assembly fit around a nerve to stimulate it. 
     Dawson and Radtke stimulated a cat&#39;s retina by direct electrical stimulation of the retinal ganglion cell layer. These experimenters placed nine and then fourteen electrodes upon the inner retinal layer (i.e., primarily the ganglion cell layer) of two cats. Their experiments suggested that electrical stimulation of the retina with 30 to 100 uA current resulted in visual cortical responses. These experiments were carried out with needle-shaped electrodes that penetrated the surface of the retina (see also U.S. Pat. No. 4,628,933 to Michelson). 
     The Michelson &#39;933 apparatus includes an array of photosensitive devices on its surface that are connected to a plurality of electrodes positioned on the opposite surface of the device to stimulate the retina. These electrodes are disposed to form an array similar to a “bed of nails” having conductors which impinge directly on the retina to stimulate the retinal cells. U.S. Pat. No. 4,837,049 to Byers describes spike electrodes for neural stimulation. Each spike electrode pierces neural tissue for better electrical contact. U.S. Pat. No. 5,215,088 to Norman describes an array of spike electrodes for cortical stimulation. Each spike pierces cortical tissue for better electrical contact. 
     The art of implanting an intraocular prosthetic device to electrically stimulate the retina was advanced with the introduction of retinal tacks in retinal surgery. De Juan, et al. at Duke University Eye Center inserted retinal tacks into retinas in an effort to reattach retinas that had detached from the underlying choroid, which is the source of blood supply for the outer retina and thus the photoreceptors. See, e.g., E. de Juan, et al., 99 Am. J. Opthalmol. 272 (1985). These retinal tacks have proved to be biocompatible and remain embedded in the retina, and choroid/sclera, effectively pinning the retina against the choroid and the posterior aspects of the globe. Retinal tacks are one way to attach a retinal electrode array to the retina. U.S. Pat. No. 5,109,844 to de Juan describes a flat electrode array placed against the retina for visual stimulation. U.S. Pat. No. 5,935,155 to Humayun describes a retinal prosthesis for use with the flat retinal array described in de Juan. 
     SUMMARY OF THE INVENTION 
     The present invention is a visual prosthesis having an electrode array suitable to be positioned on the retina. The array includes multiple attachment points to provide for even pressure across the electrode array surface. The attachment points are arranged so as to not damage retinal tissue stimulated by the electrode array. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts the electrode array of the preferred embodiment. 
         FIG. 2  depicts an electrode array of an alternate two point attachment 
         FIG. 3  depicts an electrode array of an alternate three point attachment. 
         FIG. 4  depicts an electrode array with another alternate three point attachment. 
         FIG. 5  is a perspective view of the implanted portion of the preferred visual prosthesis. 
         FIG. 6  is a side view of the implanted portion of the preferred visual prosthesis showing the fan tail in more detail. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description is of the best mode presently contemplated for carrying out the invention. This description is not to be taken in a limiting sense, but is made merely for the purpose of describing the general principles of the invention. The scope of the invention should be determined with reference to the claims. 
       FIG. 1  shows the flexible circuit electrode array of the current invention. A flexible circuit cable  112  connects to the flexible circuit electrode array  110 . Further, a primary attachment point  54  is provided near the heel of the flexible circuit electrode array  110 . A retina tack (not shown) is placed through the primary attachment point  54  to hold the flexible circuit electrode array  110  to the retina or other neural tissue. A stress relief  55  is provided surrounding the attachment point  54 . The stress relief  55  may be made of a softer polymer than the flexible circuit, or it may include cutouts or thinning of the polymer to reduce the stress transmitted from the retina tack to the flexible circuit electrode array  110 . A skirt or molded body  60  covers the flexible circuit electrode array  10 , and extends beyond its edges. It is further advantageous to include wings  62  adjacent to the attachment point  54  to spread any stress of attachment over a larger area of the retina or other neural tissue. There are several ways of forming and bonding the skirt  60 . The skirt  60  may be directly bonded through surface activation or indirectly bonded using an adhesive. The skirt  60  may be a molded body from completely around the electrode array  110  and cable  112 . 
     Preferably the electrode array  110  is constructed from a hard polymer such as polyimide while the skirt  60  is constructed from a softer polymer such as silicone. Traces and electrodes can be laid out on a hard polymer by photolithography and the hard polymer protects the delicate traces. A soft polymer skirt or molded body  60  then protects the neural tissue from the hard polymer. 
     Further a strap  20  may be provided over the array  110  opposite the primary attachment point  54  attached at either end by secondary attachment points  22  with retinal tacks. The secondary attachment points  22  include stress relief  24  like the stress relief  55  described above. Retinal nerve fibers and blood vessels run orbitally out from the optic nerve. It is advantageous not to tack between the electrode array  110  and the optic nerve as you may damage the nerve fibers which are stimulated by the electrode array  110 . The strap  20  allows the secondary attachment points  22  to be out of the line of the stimulated nerve fibers. The optic nerve  30  is the central access point for both nerve fibers and blood vessels.  32 . A tack through either a nerve fiber or blood vessel may cause damage to the area to be stimulated by the electrode array  110 . 
     Alternatively,  FIG. 2  show a central secondary attachment point  26 , with a stress relief  28 . If the array is not aligned with the nerve fibers a central secondary attachment point may be preferable. 
       FIG. 3  show a second alternate embodiment. It this case the array may be place in the opposite orientation, with the cable passing over the optic nerve. The primary attachment point  40  includes a stress relief  42 . The secondary attachment points  44 , with stress relief  48 , are included in the wings  62 . 
       FIG. 4  shows another alternate embodiment similar to the embodiment shown in  FIG. 1 , but with the secondary attachment points  26  integral to the array body rather than on a separate strap. As with the embodiment of  FIG. 1 , the secondary attachment points are outside of the area of the nerve fibers and blood vessels supplying the areas to be stimulated. 
       FIG. 5  shows a perspective view of the implanted portion of the preferred retinal prosthesis. An electrode array  110  is mounted by a retinal tack or similar means to the epiretinal surface. The electrode array  110  is electrically coupled by a cable  112 , which pierces the sclera and is electrically coupled to an electronics package  114 , external to the sclera. 
     The electronics package  114  is electrically coupled to a secondary inductive coil  116 . Preferably the secondary inductive coil  116  is made from wound wire. Alternatively, the secondary inductive coil may be made from a thin film polymer sandwich with wire traces deposited between layers of thin film polymer. The electronics package  114  and secondary inductive coil  116  are held together by a molded body  118 . The molded body  118  may also include suture tabs  120 . The molded body narrows to form a strap  122  which surrounds the sclera and holds the molded body  118 , secondary inductive coil  116 , and electronics package  114  in place. The molded body  118 , suture tabs  120  and strap  122  are preferably an integrated unit made of silicone elastomer. Silicone elastomer can be formed in a pre-curved shape to match the curvature of a typical sclera. However, silicone remains flexible enough to accommodate implantation and to adapt to variations in the curvature of an individual sclera. The secondary inductive coil  116  and molded body  118  are preferably oval shaped. A strap can better support an oval shaped coil. 
     It should be noted that the entire implant is attached to and supported by the sclera. An eye moves constantly. The eye moves to scan a scene and also has a jitter motion to prevent image stabilization. Even though such motion is useless in the blind, it often continues long after a person has lost their sight. It is an advantage of the present design, that the entire implanted portion of the prosthesis is attached to and supported by the sclera. By placing the device under the rectus muscles with the electronics package in an area of fatty issue between the rectus muscles, eye motion does not cause any flexing which might fatigue, and eventually damage, the device. 
       FIG. 6  shows a side view of the implanted portion of the retinal prosthesis, in particular, emphasizing the fan tail  124 . When implanting the retinal prosthesis, it is necessary to pass the strap  122  under the eye muscles to surround the sclera. The secondary inductive coil  116  and molded body  118  must also follow the strap under the lateral rectus muscle on the side of the sclera. The implanted portion of the retinal prosthesis is very delicate. It is easy to tear the molded body  118  or break wires in the secondary inductive coil  116 . In order to allow the molded body  118  to slide smoothly under the lateral rectus muscle, the molded body is shaped in the form of a fan tail  124  on the end opposite the electronics package  114 . 
     Accordingly, what has been shown is an improved method of making a neural prosthesis and an improved method of stimulating neural tissue. While the invention has been described by means of specific embodiments and applications thereof, it is understood that numerous modifications and variations could be made thereto by those skilled in the art without departing from the spirit and scope of the invention. In particular, the preferred embodiment describes a retinal prosthesis for artificial vision. It should be obvious to one skilled in the art that the invention has broad applicability to other types of neural stimulation. It is therefore to be understood that within the scope of the claims, the invention may be practiced otherwise than as specifically described herein.