Abstract:
An electrode array attached to neural tissue, such as the retina, necessarily has graded pressure exerted on the tissue, with higher pressure near the attachment point. Greater pressure improves contact between the electrodes and neural tissue while too much pressure may damage neural tissue. Hence it is advantageous to obtain equal pressure across the array field. In the present invention multiple and selective attachment points are provided on an electrode array allowing a surgeon to select the attachment points providing the best electrode tissue contact.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of U.S. application Ser. No. 12/258,296, filed Oct. 24, 2008 for Electrode Array for Even Neural Pressure. This application further claims the benefit of U.S. Provisional Application No. 61/033,723, “Attachment Arrangement for a Neural Stimulation Electrode Array”, filed Mar. 4, 2008. This application is related to and incorporates by reference US patent applications 20030069603 for “Medical Tack with a Variable Effective Length”; 20080288037 for “Flexible Circuit Electrode Array”; and 20020111658, for “Implantable retinal electrode array configuration for minimal retinal damage and method of reducing retinal stress” 
     
    
     GOVERNMENT RIGHTS NOTICE 
       [0002]    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 
       [0003]    The present invention is generally directed to neural stimulation and more specifically to an improved electrode array and means of attachment for a neural stimulation electrode array. The present invention is more specifically directed to a method of obtaining even pressure between an electrode array and a retina by attaching the electrode array at multiple points. 
       BACKGROUND OF THE INVENTION 
       [0004]    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 prostheses 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. 
         [0005]    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. 
         [0006]    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 apparatus 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. 
         [0007]    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 visual neuronal membranes, which can initiate visual neuron action potentials, which are the means of information transfer in the nervous system. 
         [0008]    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. 
         [0009]    One typical application of neural tissue stimulation is in the rehabilitation of the blind. 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 visual neurons, control the electronic field distribution and avoid undue compression of the visual neurons. 
         [0010]    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. 
         [0011]    Dawson and Radtke stimulated 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 μA 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). 
         [0012]    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. 
         [0013]    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. Ophthalmol. 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. 
         [0014]    In U.S. Pat. No. 6,743,345 “Method of Metallizing a Substrate” to Christian Belouet et al. a process for metallizing a substrate is disclosed, comprising coating the part with a precursor composite material layer consisting of a polymer matrix doped with photoreducer material dielectric particles; irradiating the surface of the substrate with a light beam emitted by a laser; and immersing the irradiated part in an autocatalytic bath containing metal ions, with deposition of the metal ions in a layer on the irradiated surface, and wherein the dimension of the dielectric particles is less than or equal to 0.5 μm. The process includes three steps. The first step is to coat the substrate part with a precursor composite material layer consisting of a polymer matrix doped with photoreducer material dielectric particles. The second step is to irradiate the surface of the substrate with a light beam emitted by a laser. The third step is to immerse the irradiated part in an autocatalytic bath containing metal ions, with deposition of the metal ions in a layer on the irradiated surface, wherein the dimension of the dielectric particles is less than or equal to 0.5 μm. 
         [0015]    In U.S. Pat. No. 5,599,592 “Process for the Metallization of Polymer Materials and Products Thereto Obtained” to Lucien D. Laude a positive metallization process for metallizing a polymer composite piece containing a polymer material and oxide particles is disclosed, the oxide particles being made of one or more oxides, comprising three successive steps. The first step consists of the irradiation of a surface area of a polymer piece to be metallized with a light beam emitted by an excimer laser. The polymer piece is made from a polymer material and oxide particles. The oxide particles are made from one or more oxides. The second step consists of immersing the irradiated polymer piece in at least one autocatalytic bath containing metal ions. The immersion induces the deposit of the metal ions onto the irradiated surface area to form a metal film on the surface area, resulting in the selective metallization of the surface area of the polymer piece. The third step consists of thermally processing the metallized polymer piece to induce diffusion of the deposited metal film into the polymer material of the polymer piece. The disclosure of U.S. Pat. No. 5,599,592 is incorporated herein by reference. 
         [0016]    Lucien D. Laude et al. report that excimer lasers are effective tools in engraving ceramics and polymers, changing irreversibly the surface of the irradiated material, and restricting these effects to specific areas of interest. See L. D. Laude, K Kolev, Cl. Dicara and C. Dupas-Bruzek “Laser Metallization for Microelectronics for Bio-applications”, Proc. of SPIE Vol. 4977 (2003), pp 578-586. 
         [0017]    In U.S. Pat. No. 5,935,155 “Visual Prosthesis and Method of Using Same” to Mark S. Humayan et al. it is disclosed a visual prosthesis, comprising means for perceiving a visual image, said means producing a visual signal output in response thereto; retinal tissue stimulation means adapted to be operatively attached to a retina of a user; and wireless visual signal communication means for transmitting said visual signal output to said retinal tissue stimulation means. 
         [0018]    In U.S. Pat. No. 6,878,643 “Electronic Unit integrated Into a Flexible Polymer Body” to Peter a. Krulevitch et al. it is disclosed a method of fabricating an electronic apparatus, comprising the steps of providing a silicone layer on a matrix, providing a metal layer on said silicone layer, providing a second layer of silicone on said silicone layer, providing at least one electronic unit connected to said metal layer, and removing said electronic apparatus from said matrix wherein said silicone layer and said second layer of a silicone provide a spherical silicone body. 
         [0019]    J. Delbeke et al. demonstrate that silicone rubber biocompatibility is not altered by the metallization method. See V. Cince, M.-A. Thil, C. Veraart, I. M. Colin and J. Delbeke “Biocompatibility of platinum-metallized silicone rubber: in vivo and in vitro evaluation”, J. Biomater. Sci. Polymer Edn, Vol. 15, No. 2, pp. 173-188 (2004). 
         [0020]    All of these soft polymer arrays approximate the shape of neural tissue, particularly the retina. However, there is a need for an improved means for attaching an electrode array to neural tissue and, thereby, improving the array&#39;s ability to conform to the neural tissue. 
       SUMMARY OF THE INVENTION 
       [0021]    An electrode array attached to neural tissue, such as the retina, necessarily has graded pressure exerted on the tissue, with higher pressure near the attachment point. Pressure improves contact between the electrodes and neural tissue while too much pressure may damage neural tissue. Hence it is advantageous to obtain equal pressure across the array field. In the present invention multiple and selective attachment points are provided on an electrode array allowing a surgeon to select the attachment points providing the best electrode tissue contact. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  is a perspective view of the preferred electrode array with two attachment points. 
           [0023]      FIG. 2  is a perspective view of the preferred electrode array with three attachment points. 
           [0024]      FIG. 3  depicts the electrode array of the preferred embodiment. 
           [0025]      FIG. 4  depicts an electrode array of an alternate two point attachment 
           [0026]      FIG. 5  depicts an electrode array of an alternate three point attachment. 
           [0027]      FIG. 6  depicts an electrode array with another alternate three point attachment. 
           [0028]      FIG. 7  shows the whole flexible polymer array with the bond pad and the traces with holes at the edge of the electrode array. 
           [0029]      FIG. 8  shows an enlarged view of the electrode array with holes at the edge of the polyimide to improve silicone adhesion. 
           [0030]      FIG. 9  depicts the top view of the flexible circuit array being enveloped within an insulating material. 
           [0031]      FIG. 10  depicts a cross-sectional view of the flexible circuit array being enveloped within an insulating material. 
           [0032]      FIG. 11  depicts a cross-sectional view of the flexible circuit array being enveloped within an insulating material with open electrodes and the material between the electrodes. 
           [0033]      FIG. 12  depicts a cross-sectional view of the flexible circuit array being enveloped within an insulating material with open electrodes. 
           [0034]      FIG. 13  depicts a cross-sectional view of the flexible circuit array being enveloped within an insulating material with electrodes on the surface of the material. 
           [0035]      FIG. 14  depicts a cross-sectional view of the flexible circuit array being enveloped within an insulating material with electrodes on the surface of the material insight the eye with an angle in the fold of the flexible circuit cable and a fold between the circuit electrode array and the flexible circuit cable. 
           [0036]      FIG. 15  depicts a side view of the enlarged portion of the flexible circuit array being enveloped within an insulating material with electrodes on the surface of the material in the eye and contacting the retina. 
           [0037]      FIG. 16  is a perspective view of the implanted portion of the preferred retinal prosthesis. 
           [0038]      FIG. 17  is a side view of the implanted portion of the preferred retinal prosthesis showing the fan tail in more detail. 
           [0039]      FIG. 18  is a view of the completed package attached to an electrode array. 
           [0040]      FIG. 19  is a cross-section of the package. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0041]    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. 
         [0042]      FIG. 1  shows the preferred electrode array. The array  10  is preferably made of metal traces sandwiched between polyimide layers. The array  10  and cable  12  are a single polyimide structure. A relatively hard polymer, such as polyimide, is needed to protect delicate metal traces from breaking. A molded array body  34 , preferably silicone, is molded over the polyimide structure. Perforations  58  in the polyimide promote adhesion of the molded array body  34 . A backbone structure  38  is molded in silicone across the back of the array  10 . Attachment points  54  are provided on either side of the array field to provide even pressure across the array surface. A strain relief  56  is provided around each attachment point  54 . The strain relief  56  may be thinner or softer polymer. The strain relief  56  may also include cut out portions. In the preferred embodiment the attachment point  54  is a hole suitable to accept a retinal tack (not shown). 
         [0043]      FIG. 2  shows an alternate embodiment with three attachment points  54 . The additional attachment point is in the center of the electrode field. With either embodiment, a surgeon may decide at the time of surgery which attachment point to use. The ideal attachment may be determined through impedance. Electrodes with higher impedance have more intimate contact with neural tissue and result in lower a threshold of neural stimulation. Hence, a surgeon may place a tack in the attachment point  54  closest to the cable  12  and measure impedance across the electrode array. If acceptable impedance is found, no additional tacks are required. If not, tacks may be placed where additional force is needed to obtain good electrode contact. 
         [0044]    A retinal array is pre-curved to match the approximate curvature of the retina. However, retinas vary considerably in their curvature. In a small eye, the array may not be curved enough. Using only the center attachment point  54  would achieve the best result. In a large eye, the array may be too curved. Using the outer attachment points  54  would achieve the best result. 
         [0045]      FIG. 3  shows the flexible circuit electrode array  10  in another embodiment. A flexible circuit cable  12  connects to the flexible circuit electrode array  10 . Further, an attachment point  54  is provided near the heel of the flexible circuit electrode array  10 . A retina tack (not shown) is placed through the attachment point  54  to hold the flexible circuit electrode array  10  to the retina or other neural tissue. A stress relief  56  is provided surrounding the attachment point  54 . The stress relief  56  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  10 . A molded body  34  covers the flexible circuit electrode array  10 , and extends beyond its edges. It may be further advantageous to include wings  36  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 molded body  34 . The molded body  34  may be directly bonded through surface activation or indirectly bonded using an adhesive. The molded body  34  may be a molded completely around the electrode array  10  and cable  12 . 
         [0046]    Preferably the electrode array  10  is constructed from a hard polymer such as polyimide while the molded body  34  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 molded body  34  then protects the neural tissue from the hard polymer. 
         [0047]    Further a strap  26  may be provided over the array  10  opposite the attachment point  54  near the heel attached at either end by attachment points  54  with retinal tacks. Retinal nerve fibers and blood vessels run orbitally out from the optic nerve. It may be advantageous not to tack between the electrode array  10  and the optic nerve as you may damage the nerve fibers which are stimulated by the electrode array  10 . The strap  26  allows the attachment points  54  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  10 . 
         [0048]    Alternatively,  FIG. 4  show a central secondary attachment point  54 , with a stress relief  56 . If the array is not aligned with the nerve fibers a central secondary attachment point may be preferable.  FIG. 4  varies from  FIG. 2  in that the attachment point  54  near the toe is within the flexible body  34  but outside the array  10 . This provides additional stress relief from attachment. 
         [0049]      FIG. 5  shows another alternate embodiment. It this case the array may be place in the preferred orientation or an opposite orientation, with the cable passing over the optic nerve. The attachment points  54  includes stress reliefs  56 . Attachment points  54 , with stress relief  56 , are included in the wings  62 . An additional advantage of this embodiment is that any rotational torque from the array cable is transmitted to the electrode field portion of the flexible body. 
         [0050]      FIG. 6  shows another alternate embodiment similar to the embodiment shown in  FIG. 3 , but with attachment points  54  integral to the array body rather than on a separate strap. As with the embodiment of  FIG. 3 , the attachment points are outside of the area of the nerve fibers and blood vessels supplying the areas to be stimulated. 
         [0051]      FIG. 7  shows the preferred electrode array for a visual prosthesis. The structure is a single polyimide sandwich with metal traces. The array  10  is at one end. Bond pads  92  are at the other end and the cable  12  is in the middle. One trace connects each electrode with a bond pad. The flexible circuit  1  is a made by the following process. First, a layer of polymer (such as polyimide, fluoro-polymers, silicone or other polymers) is applied to a support substrate (not part of the array) such as glass. Layers may be applied by spinning, meniscus coating, casting, sputtering or other physical or chemical vapor deposition, or similar process. Subsequently, a metal layer is applied to the polymer. The metal is patterned by photolithographic process. Preferably, a photo-resist is applied and patterned by photolithography followed by a wet etch of the unprotected metal. Alternatively, the metal can be patterned by lift-off technique, laser ablation or direct write techniques. 
         [0052]    It is advantageous to make this metal thicker at the electrode to improve contact with neural tissue, and at the bond pad to improve contact with the package. This can be accomplished through any of the above methods or electroplating. Then, the top layer of polymer is applied over the metal. Openings in the top layer for electrical contact to the electronics package  14  and the electrodes may be accomplished by laser ablation or reactive ion etching (RIE) or photolithography and wet etch. Making the electrode openings in the top layer smaller than the electrodes promotes adhesion by avoiding delamination around the electrode edges. 
         [0053]      FIG. 8  is an enlarged view of the electrode array  10 . Traces must be routed around the attachment point  54  and stress relief  56 . The electrode field  9 , shown by a doffed line, is that portion of the electrode array having electrodes and stimulating neural tissue. 
         [0054]      FIG. 9  depicts the top view of the flexible circuit array  10  being enveloped within a molded body  34 . The electrode array  10  is encased within the oval-shaped molded body  34 , a plurality of electrodes  13  made of a conductive material, such as platinum or one of its alloys, but that can be made of any conductive biocompatible material such as iridium, iridium oxide or titanium nitride. The electrode array  10  is enveloped within a molded body  34  that is preferably silicone. “Oval-shaped” electrode array body means that the body may approximate either a square or a rectangle shape, but where the corners are rounded. This shape of an electrode array is described in the U.S. Patent Application No. 20020111658, entitled “Implantable retinal electrode array configuration for minimal retinal damage and method of reducing retinal stress” and No. 20020188282, entitled “Implantable drug delivery device” to Robert J. Greenberg et al., the disclosures of both are incorporated herein by reference. 
         [0055]    The molded body  34  is made of a soft material that is compatible with the electrode array  10 . In a preferred embodiment the molded body  34  made of silicone having hardness of about 50 or less on the Shore A scale as measured with a durometer. In an alternate embodiment the hardness is about 25 or less on the Shore A scale as measured with a durometer. 
         [0056]      FIG. 10  depicts a cross-sectional view of the flexible circuit array  10  being enveloped within the molded body  34 . It shows how the edges of the molded body  34  are lifted off due to the contracted radius at the edges. The electrode array  10  preferably also contains a fold A between the cable  12  and the electrode array  10 . The angle of the fold A secures a relief of the implanted material. 
         [0057]      FIG. 11  depicts a cross-sectional view of the flexible circuit array  10  being enveloped within a molded body  34  with open electrodes  13  and the molded body  34  between the electrodes  13 . 
         [0058]      FIG. 12  depicts a cross-sectional view of the flexible circuit array  10  being enveloped within the molded body  34  with open electrodes  13 . This is another embodiment wherein the electrodes  13  are not separated by the molded body  34 . This may allow closer contact with the neural tissue. 
         [0059]      FIG. 13  depicts a cross-sectional view of the flexible circuit array  10  being enveloped within the molded body  34  with electrodes  13  on the surface of the molded body  34 . This is a further embodiment with the electrode  13  on the surface of the molded body, preferably silicone. The embodiments shown in 
         [0060]      FIG. 14  depicts a cross-sectional view of the flexible circuit array  10  being enveloped within the molded body  34  with electrodes  13  on the surface of the molded body  34  insight the eye with an angle K in the fold of the flexible circuit cable  12  and a fold A between the circuit electrode array  10  and the flexible circuit cable  12 . The molded body  34  and electrode array body  10  are in intimate contact with retina R. The surface of electrode array body  10  in contact with retina R is a curved surface with a matched radius compared to the spherical curvature of retina R to minimize pressure concentrations therein. Further, the decreasing radius of spherical curvature of the molded body  34  near its edge forms edge relief that causes the edges of the molded body  34  to lift off the surface of retina R eliminating pressure concentrations at the edges. The edge of molded body  34  is rounded to reduce pressure and cutting of retina R. 
         [0061]      FIG. 15  shows a part of the  FIG. 14  enlarged showing the electrode array  10  and the electrodes  13  enveloped by the molded body  34 , preferably silicone in intimate contact with the retina R. 
         [0062]    The electrode array  10  embedded in or enveloped by the molded body  34  can be preferably produced through curing the silicone in a mold around the polyimide array  10 . The molded body  34  has a shape with a decreasing radius at the edges so that the edges of the molded body  34  lift off from the retina R. 
         [0063]      FIG. 16  shows a perspective view of the implanted portion of the preferred retinal prosthesis. A flexible circuit I includes a flexible circuit electrode array  10  which is mounted by a retinal tack (not shown) or similar means to the epiretinal surface. The flexible circuit electrode array  10  is electrically coupled by a flexible circuit cable  12 , which pierces the sclera and is electrically coupled to an electronics package  14 , external to the sclera. 
         [0064]    The electronics package  14  is electrically coupled to a secondary inductive coil  16 . Preferably the secondary inductive coil  16  is made from wound wire. Alternatively, the secondary inductive coil  16  may be made from a flexible circuit polymer sandwich with wire traces deposited between layers of flexible circuit polymer. The secondary inductive coil receives power and data from a primary inductive coil  17 , which is external to the body. The electronics package  14  and secondary inductive coil  16  are held together by the molded body  18 . The molded body  18  holds the electronics package  14  and secondary inductive coil  16  end to end. The secondary inductive coil  16  is placed around the electronics package  14  in the molded body  18 . The molded body  18  holds the secondary inductive coil  16  and electronics package  14  in the end to end orientation and minimizes the thickness or height above the sclera of the entire device. The molded body  18  may also include suture tabs  20 . The molded body  18  narrows to form a strap  22  which surrounds the sclera and holds the molded body  18 , secondary inductive coil  16 , and electronics package  14  in place. The molded body  18 , suture tabs  20  and strap  22  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  16  and molded body  18  are preferably oval shaped. A strap  22  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 improve acuity. Even though such motion is useless in the blind, it often continues long after a person has lost their sight. By placing the device under the rectus muscles with the electronics package in an area of fatty tissue between the rectus muscles, eye motion does not cause any flexing which might fatigue, and eventually damage, the device. 
         [0065]      FIG. 17  shows a side view of the implanted portion of the retinal prosthesis, in particular, emphasizing the fan tail  24 . When implanting the retinal prosthesis, it is necessary to pass the strap  22  under the eye muscles to surround the sclera. The secondary inductive coil  16  and molded body  18  must also follow the strap  22  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  18  or break wires in the secondary inductive coil  16 . In order to allow the molded body  18  to slide smoothly under the lateral rectus muscle, the molded body  18  is shaped in the form of a fan tail  24  on the end opposite the electronics package  14 . The strap  22  further includes a hook  28  the aids the surgeon in passing the strap under the rectus muscles. 
         [0066]    Referring to  FIG. 18 , the flexible circuit  1 , includes platinum conductors  94  insulated from each other and the external environment by a biocompatible dielectric polymer  96 , preferably polyimide. One end of the array contains exposed electrode sites that are placed in close proximity to the retinal surface  10 . The other end contains bond pads  92  that permit electrical connection to the electronics package  14 . The electronic package  14  is attached to the flexible circuit  1  using a flip-chip bumping process, and epoxy underfilled. In the flip-chip bumping process, bumps containing conductive adhesive placed on bond pads  92  and bumps containing conductive adhesive placed on the electronic package  14  are aligned and melted to build a conductive connection between the bond pads  92  and the electronic package  14 . Leads  76  for the secondary inductive coil  16  are attached to gold pads  78  on the ceramic substrate  60  using thermal compression bonding, and are then covered in epoxy. The electrode array cable  12  is laser welded to the assembly junction and underfilled with epoxy. The junction of the secondary inductive coil  16 , array  1 , and electronic package  14  are encapsulated with a silicone overmold  90  that connects them together mechanically. When assembled, the hermetic electronics package  14  sits about 3 mm away from the end of the secondary inductive coil. 
         [0067]    Since the implant device is implanted just under the conjunctiva it is possible to irritate or even erode through the conjunctiva. Eroding through the conjunctiva leaves the body open to infection. We can do several things to lessen the likelihood of conjunctiva irritation or erosion. First, it is important to keep the over all thickness of the implant to a minimum. Even though it is advantageous to mount both the electronics package  14  and the secondary inductive coil  16  on the lateral side of the sclera, the electronics package  14  is mounted higher than, but not covering, the secondary inductive coil  16 . In other words the thickness of the secondary inductive coil  16  and electronics package should not be cumulative. 
         [0068]    It is also advantageous to place protective material between the implant device and the conjunctiva. This is particularly important at the scleratomy, where the thin film electrode array cable  12  penetrates the sclera. The thin film electrode array cable  12  must penetrate the sclera through the pars plana, not the retina. The scleratomy is, therefore, the point where the device comes closest to the conjunctiva. The protective material can be provided as a flap attached to the implant device or a separate piece placed by the surgeon at the time of implantation. Further material over the scleratomy will promote healing and sealing of the scleratomy. Suitable materials include DACRON®, TEFLON®, GORETEX® (ePTFE), TUTOPLAST® (sterilized sclera), MERSILENE® (polyester) or silicone. 
         [0069]    Referring to  FIG. 19 , the package  14  contains a ceramic substrate  60 , with metallized vias  65  and thin-film metallization  66 . The package  14  contains a metal case wall  62  which is connected to the ceramic substrate  60  by braze joint  61 . On the ceramic substrate  60  an underfill  69  is applied. On the underfill  69  an integrated circuit chip  64  is positioned. On the integrated circuit chip  64  a ceramic hybrid substrate  68  is positioned. On the ceramic hybrid substrate  68  passives  70  are placed. Wirebonds  67  are leading from the ceramic substrate  60  to the ceramic hybrid substrate  68 . A metal lid  84  is connected to the metal case wall  62  by laser welded joint  63  whereby the package  14  is sealed. 
         [0070]    Accordingly, what has been shown is an improved method making a neural electrode array and 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. It is therefore to be understood that within the scope of the claims, the invention may be practiced otherwise than as specifically described herein.