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 . fig1 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 . fig2 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 . fig3 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 ( fig3 a - 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 fig4 , 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 . fig5 depicts a further embodiment of the part of the prosthesis shown in fig4 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 . fig6 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 . fig7 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 fig1 , 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 . fig7 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 . fig8 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 fig9 . 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 io . 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 . fig1 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 . fig1 shows a flexible circuit electrode array 10 similar to fig1 , 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 fig8 . it should be noted that fig9 - 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 fig1 , 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 ( fig6 - 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 . fig1 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 . fig1 shows a top view of a modified tack opening 54 , which is made thinner and has an increased open angle 542 . fig1 shows a cross - sectional view of a modified tack opening , which is made thinner and has an increased open angle 542 . fig1 and 16 further explain the modification shown in fig1 . by making the material 121 thinner between the tack hole 54 and the cut out 541 it becomes more flexible . fig1 shows a top view of a modified tack opening , which can be thin height , rotate better , adjust the angle better and use softer material . fig1 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 . fig1 shows an alternative embodiment of the embodiment shown in fig1 . the c shaped cut outs may decouple the forces in a different way as discussed before for one c shaped cutout in fig1 . fig1 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 . fig2 shows 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 . fig1 and fig2 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 . fig2 shows a cross - sectional view of a modified tack opening 54 , which applies to the modifications in fig1 - 18 , which can be flat ( disk ) or curved ( hemispherical ), and can be fabricated with part or added separately . the fig . 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 . fig2 shows a top view and fig2 shows a cross sectional view of a flexible electrode 12 with a tack opening 54 containing membrane material 543 and a silicone coating 60 . fig2 is similar to fig1 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 .
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 .