Abstract:
The present invention provides a flexible circuit electrode array adapted for neural stimulation, comprising: a polymer base layer; metal traces deposited on said polymer base layer, including electrodes suitable to stimulate neural tissue; a polymer top layer deposited on said polymer base layer and said metal traces at least one tack opening; wherein said polymer base layer, said metal traces and said polymer top layer are thermoformed in a three dimensional shape. 
     The present invention provides further a method of making a flexible circuit electrode array comprising depositing a polymer base layer; depositing metal on said polymer base layer; patterning said metal to form metal traces; depositing a polymer top layer on said polymer base layer and said metal traces; preparing at least one tack opening; and heating said flexible circuit electrode array in a mold to form a three dimensional shape in said flexible circuit electrode array.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a divisional of application Ser. No. 11/821,328, entitled “Flexible Circuit Electrode Array with at Least One Tack Opening”, filed Jun. 21, 2007, which claims the benefit of U.S. Provisional Application No. 60/815,311, “Flexible Circuit Electrode Array with at least one Tack Opening”, filed Jun. 21, 2006 and which is a Continuation-In-Part of U.S. application Ser. No. 11/413,689, “Flexible Circuit Electrode Array”, filed Apr. 28, 2006, which is a Continuation-In-Part of U.S. application Ser. No. 11/207,644, filed Aug. 19, 2005 which claims the benefit of U.S. Provisional Application No. 60/676,008, “Thin Film Electrode Array”, filed Apr. 28, 2005, the disclosures of all are incorporated herein by reference. 

   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 electrode array for neural stimulation. 
   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 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. 
   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 visual neuronal membranes, which can initiate visual neuron 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. 
   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. 
   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 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). 
   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. 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. 
   SUMMARY OF THE INVENTION 
   Polymer materials are useful as electrode array bodies for neural stimulation. They are particularly useful for retinal stimulation to create artificial vision, cochlear stimulation to create artificial hearing, or cortical stimulation for many purposes. Regardless of which polymer is used, the basic construction method is the same. A layer of polymer is laid down, commonly by some form of chemical vapor deposition, spinning, meniscus coating or casting. A layer of metal, preferably platinum, is applied to the polymer and patterned to create electrodes and leads for those electrodes. Patterning is commonly done by photolithographic methods. A second layer of polymer is applied over the metal layer and patterned to leave openings for the electrodes, or openings are created later by means such as laser ablation. Hence the array and its supply cable are formed of a single body. Alternatively, multiple alternating layers of metal and polymer may be applied to obtain more metal traces within a given width. 
   The pressure applied against the retina, or other neural tissue, by an electrode array is critical. Too little pressure causes increased electrical resistance between the array and retina, along with electric field dispersion. Too much pressure may block blood flow causing retinal ischemia and hemorrhage. Pressure on the neural retina may also block axonal flow or cause neuronal atrophy leading to optic atrophy. Common flexible circuit fabrication techniques such as photolithography generally require that a flexible circuit electrode array be made flat. Since the retina is spherical, a flat array will necessarily apply more pressure near its edges, than at its center. Further, the edges of a flexible circuit polymer array may be quite sharp and cut the delicate retinal tissue. With most polymers, it is possible to curve them when heated in a mold. By applying the right amount of heat to a completed array, a curve can be induced that matches the curve of the retina. With a thermoplastic polymer such as liquid crystal polymer, it may be further advantageous to repeatedly heat the flexible circuit in multiple molds, each with a decreasing radius. Further, it is advantageous to add material along the edges of a flexible circuit array. Particularly, it is advantageous to add material that is more compliant than the polymer used for the flexible circuit array. 
   It is further advantageous to provide a fold or twist in the flexible circuit array at the point where it passes through the sclera. Additional material may be added inside and outside the fold to promote a good seal with the sclera tissue. 
   The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of the implanted portion of the preferred retinal prosthesis. 
       FIG. 2  is a side view of the implanted portion of the preferred retinal prosthesis showing the fan tail in more detail. 
       FIG. 3A-3E  depict molds for forming the flexible circuit array in a curve. 
       FIG. 4  depicts an alternate view of the invention with ribs to help maintain curvature and prevent retinal damage. 
       FIG. 5  depicts an alternative view of the invention with ribs to help maintain curvature and prevent retinal damage fold of the flexible circuit cable and a fold A between the circuit electrode array and the flexible circuit cable. 
       FIG. 6  depicts the flexible circuit array before it is folded and attached to the implanted portion. 
       FIG. 7  depicts the flexible circuit array folded. 
       FIG. 8  depicts a flexible circuit array with a protective skirt. 
       FIG. 9  depicts a flexible circuit array with a protective skirt bonded to the back side of the flexible circuit array. 
       FIG. 10  depicts a flexible circuit array with a protective skirt bonded to the front side of the flexible circuit array. 
       FIG. 11  depicts a flexible circuit array with a protective skirt bonded to the back side of the flexible circuit array and molded around the edges of the flexible circuit array. 
       FIG. 12  depicts a flexible circuit array with a protective skirt bonded to the back side of the flexible circuit array and molded around the edges of the flexible circuit array and flush with the front side of the array. 
       FIG. 13  is an enlarged view of a single electrode within the flexible circuit electrode array. 
       FIG. 14  shows a top view of a flexible electrode with a tack opening. 
       FIG. 15  shows a top view of a modified tack opening, which is made thinner and has an increased open angle. 
       FIG. 16  shows a cross-sectional view of a modified tack opening, which is made thinner and has an increased open angle. 
       FIG. 17  shows a top view of a modified tack opening, which can be thin height, rotate better, adjust the angle better and use softer material. 
       FIG. 18  shows a cross-sectional view of a modified tack opening, which can be thin height, rotate better, adjust the angle better and use softer material. 
       FIG. 19  shows a top view of a modified tack opening, which can adjust the diameter, and adjust thickness, can use softer materials, can be manufactured integrally or discretely, flush or protruding. 
       FIG. 20A  and  FIG. 20B  show a cross-sectional view of a modified tack opening, which can adjust the diameter, and adjust thickness, which can use softer materials, and can be manufactured integrally or discretely, flush or protruding. 
       FIG. 21A  and  FIG. 21B  show a cross-sectional view of a modified tack opening. 
       FIG. 22  shows a top view of a flexible electrode with a tack opening containing membrane material. 
       FIG. 23  shows a cross-sectional view of a flexible electrode with a tack opening containing membrane material. 
   

   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 a perspective view of the implanted portion of the preferred retinal prosthesis. A flexible circuit  1  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. 
   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 electronics package  14  and secondary inductive coil  16  are held together by a molded body  18 . 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. 
     FIG. 2  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 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. 
   It is advantageous to make this metal thicker at the electrode and bond pad to improve electrical continuity. 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 photolithograph and wet etch. Making the electrode openings in the top layer smaller than the electrodes promotes adhesion by avoiding delaminating around the electrode edges. 
   The pressure applied against the retina by the flexible circuit electrode array is critical. Too little pressure causes increased electrical resistance between the array and retina. It should be noted that while the present invention is described in terms of application to the retina, the techniques described are equally applicable to many forms of neural stimulation. Application to the retina requires a convex spherical curve. Application to the cochlea requires a constant curve in one dimension and a spiral curve in the other. Application to the cerebral cortex requires a concave spherical curve. Cortical stimulation is useful for artificial vision or hearing, touch and motor control for limb prostheses, deep brain stimulation for Parkinson&#39;s disease and multiple sclerosis, and many other applications. 
   Common flexible circuit fabrication techniques such as photolithography generally require that a flexible circuit electrode array be made flat. Since the retina is spherical, a flat array will necessarily apply more pressure near its edges, than at its center. With most polymers, it is possible to curve them when heated in a mold. By applying the right amount of heat to a completed array, a curve can be induced that matches the curve of the retina. To minimize warping, it is often advantageous to repeatedly heat the flexible circuit in multiple molds, each with a decreasing radius.  FIG. 3  illustrates a series of molds according to the preferred embodiment. Since the flexible circuit will maintain a constant length, the curvature must be slowly increased along that length. As the curvature  30  decreases in successive molds ( FIGS. 3A-3E ) the straight line length between ends  32  and  34 , must decrease to keep the length along the curvature  30  constant, where mold  3 E approximates the curvature of the retina or other desired neural tissue. The molds provide a further opening  36  for the flexible circuit cable  12  of the array to exit the mold without excessive curvature. 
   It should be noted that suitable polymers include thermoplastic materials and thermoset materials. While a thermoplastic material will provide some stretch when heated a thermoset material will not. The successive molds are, therefore, advantageous only with a thermoplastic material. A thermoset material works as well in a single mold as it will with successive smaller molds. It should be noted that, particularly with a thermoset material, excessive curvature in three dimensions will cause the polymer material to wrinkle at the edges. This can cause damage to both the array and the retina. Hence, the amount of curvature is a compromise between the desired curvature, array surface area, and the properties of the material. 
   Referring to  FIG. 4 , the edges of the polymer layers are often sharp. There is a risk that the sharp edges of a flexible circuit will cut into delicate retinal tissue. It is advantageous to add a soft material, such as silicone, to the edges of a flexible circuit electrode array to round the edges and protect the retina. Silicone around the entire edge may make the flexible circuit less flexible. So, it is advantageous to provide silicone bumpers or ribs to hold the edge of the flexible circuit electrode array away from the retinal tissue. Curvature  40  fits against the retina. The leading edge  44  is most likely to cause damage and is therefore fit with molded silicone bumper. Also, edge  46 , where the array lifts off the retina can cause damage and should be fit with a bumper. Any space along the side edges of curvature  40  may cause damage and may be fit with bumpers as well. It is also possible for the flexible circuit cable  12  of the electrode array to contact the retina. It is, therefore, advantageous to add periodic bumpers along the flexible circuit cable  12 . 
   It is also advantageous to create a reverse curve or service loop in the flexible circuit cable  12  of the flexible circuit electrode array to gently lift the flexible circuit cable  12  off the retina and curve it away from the retina, before it pierces the sclera at a sclerotomy. It is not necessary to heat curve the service loop as described above, the flexible circuit electrode array can simply be bent or creased upon implantation. This service loop reduces the likelihood of any stress exerted extraocularly from being transmitted to the electrode region and retina. It also provides for accommodation of a range of eye sizes. 
   With existing technology, it is necessary to place the implanted control electronics outside of the sclera, while a retinal flexible circuit electrode array must be inside the sclera in order to contact the retina. The sclera is cut through at the pars plana, forming a sclerotomy, and the flexible circuit passed through the sclerotomy. A flexible circuit is thin but wide. The more electrode wires, the wider the flexible circuit must be. It may be difficult to seal a sclerotomy over a flexible circuit wide enough to support enough wires for a high resolution array. A narrow sclerotomy is preferable. 
     FIG. 5  depicts a further embodiment of the part of the prosthesis shown in  FIG. 4  with a fold A between the circuit electrode array  10  and the flexible circuit cable  12 . The angle in the fold A also called ankle has an angle of 1°-180°, preferably 80°-120°. The fold A is advantageous since it reduces tension and enables an effective attachment of the flexible electrode circuit array  10  to the retina. 
     FIG. 6  shows the flexible circuit electrode array prior to folding and attaching the array to the electronics package  14 . At one end of the flexible circuit cable  12  is an interconnection pad  52  for connection to the electronics package  14 . At the other end of the flexible circuit cable  12  is the flexible circuit electrode array  10 . Further, an attachment point  54  is provided near 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. 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  10 . The flexible circuit cable  12  is formed in a dog leg pattern so than when it is folded at fold  48  it effectively forms a straight flexible circuit cable  12  with a narrower portion at the fold  48  for passing through the sclerotomy. 
     FIG. 7  shows the flexible circuit electrode array after the flexible circuit cable  12  is folded at the fold  48  to form a narrowed section. The flexible circuit cable  12  may include a twist or tube shape as well. With a retinal prosthesis as shown in  FIG. 1 , the bond pad  52  for connection to the electronics package  14  and the flexible circuit electrode array  10  are on opposite side of the flexible circuit. This requires patterning, in some manner, both the base polymer layer and the top polymer layer. By folding the flexible circuit cable  12  of the flexible circuit electrode array  10 , the openings for the bond pad  52  and the electrodes are on the top polymer layer and only the top polymer layer needs to be patterned. 
   Also, since the narrowed portion of the flexible circuit cable  12  pierces the sclera, shoulders formed by opposite ends of the narrowed portion help prevent the flexible circuit cable  12  from moving through the sclera. It may be further advantageous to add ribs or bumps of silicone or similar material to the shoulders to further prevent the flexible circuit cable  12  from moving through the sclera. 
   Further it is advantageous to provide a suture tab  56  in the flexible circuit body near the electronics package to prevent any movement in the electronics package from being transmitted to the flexible circuit electrode array  10 . Alternatively, a segment of the flexible circuit cable  12  can be reinforced to permit it to be secured directly with a suture. 
     FIG. 7  shows that it is advantageous to provide a sleeve or coating  50  that promotes healing of the sclerotomy. Polymers such as polyimide, which may be used to form the flexible circuit cable  12  and flexible circuit electrode array  10 , are generally very smooth and do not promote a good bond between the flexible circuit cable  12  and scleral tissue. A sleeve or coating of polyester, collagen, silicone, Gore-Tex or similar material would bond with scleral tissue and promote healing. In particular, a porous material will allow scleral tissue to grow into the pores promoting a good bond. 
     FIG. 8  shows that the flexible circuit electrode array  10  may be inserted through the sclera, behind the retina and placed between the retina and choroid to stimulate the retina subretinally. In this case, it is advantageous to provide a widened portion, or stop, of the flexible circuit cable  12  to limit how far the flexible circuit electrode array is inserted and to limit the transmission of stress through the sclera. The stop may be widening of the flexible circuit  1  or it may be added material such as a bumper or sleeve. 
   A skirt  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. 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. 
   Alternatively, a flexible circuit electrode array  10  may be layered using different polymers for each layer. Using too soft of a polymer may allow too much stretch and break the metal traces. Too hard of a polymer may cause damage to delicate neural tissue. Hence a relatively hard polymer, such a polyimide may be used for the bottom layer and a relatively softer polymer such a silicone may be used for the top layer including an integral skirt to protect delicate neural tissue. 
   The simplest solution is to bond the skirt  60  to the back side (away from the retina) of the flexible circuit electrode array  10  as shown in  FIG. 9 . While this is the simplest mechanical solution, sharp edges of the flexible circuit electrode array  10  may contact the delicate retina tissue. Bonding the skirt to the front side (toward the retina) of the flexible circuit electrode array  10  will protect the retina from sharp edges of the flexible circuit electrode array  10 . However, a window  62  must be cut in the skirt  60  around the electrodes. Further, it is more difficult to reliably bond the skirt  60  to the flexible circuit electrode array  10  with such a small contact area. This method also creates a space between the electrodes and the retina which will reduce efficiency and broaden the electrical field distribution of each electrode. Broadening the electric field distribution will limit the possible resolution of the flexible circuit electrode array  10 . 
     FIG. 11  shows another structure where the skirt  60  is bonded to the back side of the flexible circuit electrode array  10 , but curves around any sharp edges of the flexible circuit electrode array  10  to protect the retina. This gives a strong bond and protects the flexible circuit electrode array  10  edges. Because it is bonded to the back side and molded around the edges, rather than bonded to the front side, of the flexible circuit electrode array  10 , the portion extending beyond the front side of the flexible circuit electrode array  10  can be much smaller. This limits any additional spacing between the electrodes and the retinal tissue. 
     FIG. 12  shows a flexible circuit electrode array  10  similar to  FIG. 11 , with the skirt  60 , flush with the front side of the flexible circuit electrode array  10  rather than extending beyond the front side. While this is more difficult to manufacture, it does not lift the electrodes off the retinal surface as with the array in  FIG. 8 . It should be noted that  FIGS. 9-12  show skirt  60  material along the back of the flexible circuit electrode array  10  that is not necessary other than for bonding purposes. If there is sufficient bond with the flexible circuit electrode array  10 , it may advantageous to thin or remove portions of the skirt  60  material for weight reduction. 
   Referring to  FIG. 13 , the flexible circuit electrode array  10  is manufactured in layers. A base layer of polymer  70  is laid down, commonly by some form of chemical vapor deposition, spinning, meniscus coating or casting. A layer of metal  72  (preferably platinum) is applied to the polymer base layer  70  and patterned to create electrodes  74  and traces for those electrodes. Patterning is commonly done by photolithographic methods. The electrodes  74  may be built up by electroplating or similar method to increase the surface area of the electrode  74  and to allow for some reduction in the electrodes  74  over time. Similar plating may also be applied to the bond pads  52  ( FIG. 6-8 ). A top polymer layer  76  is applied over the metal layer  72  and patterned to leave openings for the electrodes  74 , or openings are created later by means such as laser ablation. It is advantageous to allow an overlap of the top polymer layer  76  over the electrodes  74  to promote better adhesion between the layers, and to avoid increased electrode reduction along their edges. The overlapping top layer promotes adhesion by forming a clamp to hold the metal electrode between the two polymer layers. Alternatively, multiple alternating layers of metal and polymer may be applied to obtain more metal traces within a given width. 
     FIG. 14  shows a perspective view of a flexible electrode with a tack opening  54 . The tack opening  54  is in the vicinity of defined, here c shape cut out  541 . The cut out  541  decouples force from the tack opening  54  to portions of the flexible electrode  12 . The cutout  541  allows independent deflection of different regions. The cut out  541  as well as the opening  54  can be manufactured by different process, such as laser, assorted mechanical means, or molding process. 
     FIG. 15  shows a top view of a modified tack opening  54 , which is made thinner and has an increased open angle  542 .  FIG. 16  shows a cross-sectional view of a modified tack opening, which is made thinner and has an increased open angle  542 .  FIGS. 15 and 16  further explain the modification shown in  FIG. 14 . By making the material  121  thinner between the tack hole  54  and the cut out  541  it becomes more flexible. 
     FIG. 17  shows a top view of a modified tack opening, which can be thin height, rotate better, adjust the angle better and use softer material.  FIG. 18  shows a cross-sectional view of a modified tack opening, which can be thin height, rotate better, adjust the angle better and use softer material.  FIG. 17  shows an alternative embodiment of the embodiment shown in  FIG. 15 . The c shaped cut outs may decouple the forces in a different way as discussed before for one c shaped cutout in  FIG. 15 . 
     FIG. 19  shows a top view of a modified tack opening, which can adjust the diameter, and adjust thickness, can use softer materials, can be manufactured integrally or discretely, flush or protruding. 
     FIG. 20A  and  FIG. 20B  show a cross-sectional view of a modified tack opening, which can adjust the diameter, and adjust thickness, can use softer materials, can be manufactured integrally or discretely, flush or protruding.  FIG. 19  and  FIG. 20  show a membrane  543  made of a soft polymer, such as silicone or mixtures thereof with other soft polymers. The membrane  543  contains the tack opening  54 . This embodiment does not require a gap  541  and presents a more continues surface. The membrane  543  and the opening  54  can be located on the top or bottom to the tack opening. 
     FIG. 21A  and  FIG. 21B  show a cross-sectional view of a modified tack opening  54 , which applies to the modifications in  FIGS. 14-18 , which can be flat (disk) or curved (hemispherical), and can be fabricated with part or added separately. The figure shows in particular a pedestal  544  feature. The potential benefit lies in lifting global electrode region off retina by a small amount and localizing high pressure on tissue to tack site.  FIG. 21A  shows a shape cut out  541 .  FIG. 21B  shows an open angle opening  542 . 
     FIG. 22  shows a top view and  FIG. 23  shows a cross sectional view of a flexible electrode  12  with a tack opening  54  containing membrane material  543  and a silicone coating  60 .  FIG. 22  is similar to  FIGS. 19 and 20  except that this embodiment is more flat. It could be manufactured in faster and easier method as the previous variation. Due to the material properties of the membrane, small static forces are transferred to the electrode array to maintain contact proximity but large, transient forces are not transferred to reduce the likelihood of array and/or retinal damage. 
   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.