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 . an electrode array 10 is mounted by a retinal tack or similar means to the epiretinal surface . the electrode array 10 is electrically coupled by a cable 12 which pierces the sclera and is electrically coupled to an electronics package 14 , external to the sclera . it is advantageous to encase the electronics within a hermetic package . this can be accomplished by use of a metal , ceramic polymer or a combination of these materials case , or by applying a thin film hermetic coating such as described in u . s . patent application 20020038134 package for an implantable medical device and 20020120296 , implantable device using ultra - nanocrystalline diamond . an electronics package with a coil mounted on the lateral surface of the retina is described in us patent application attorney docket s293 - usa . all of the above applications are incorporated herein by reference . it is advantageous to mount electronics external to the sclera , as the fatty tissue there is less heat sensitive , and blood flow rapidly dissipates excess heat . the vitreous within the eye is does not change often and the retina is very heat sensitive . 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 may be made from a thin film polymer sandwich with wire traces deposited between layers of thin film 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 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 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 . it is an advantage of the present design , that the entire implanted portion of the prosthesis is attached to and supported by the sclera . by placing the device under the rectus muscles with the electronics package in an area of fatty issue between the rectus muscles , eye motion does not cause any flexing which might fatigue , and eventually damage , the device . as we improve the resolution of retinal prostheses , the number of electrodes increases . as the number of electrodes increases the number of wires between the electronics package and the electrode array must increase . this increase requires a wider thin film cable piercing the sclera . if the thin film cable is too wide , the sclerotomy may not heal properly . fig2 - 5 present embodiments to provide for a narrower thin film cable between the electronics , external to the sclera , and the electrode array within the sclera . a common method of reducing the conductor count in cables , is the use of a multiplexer and demultiplexer . however , multiplexers , like any electronic circuit , present unique problems when implanted within the human body . all electronics must be sealed to prevent the saline body fluids from harming the electronics and to prevent the electronics from harming the body . each electrical wire entering and exiting the electronics package must also be sealed . therefore it is advantageous the limit the number of such wire electronics package interconnects . while it is easy to include a multiplexer within the electronics package , it is more difficult to house a demultiplexer within the sclera . fig2 , depicts the preferred embodiment where a computer chip 110 , including a demultiplexer is placed directly on the retina . the demultiplexer interconnects with the thin film cable 112 on one side and includes electrode openings , for contacting the retina on the other side . this is best accomplished by create feedthroughs in the silicon chip to allows electrodes on one side of the chip and connection to the flexible cable on the other side of the chip ( described in more detail in fig7 . cable 112 pierces the sclera and attaches to an electronics package 114 . the electronics package connects to a coil 116 . the chip 110 must be coated with a thin film hermetic coating as described above . this is the simplest most cost effective method of providing a demultiplexer within the sclera . silicon chips such as chip 110 are necessarily flat . it is possible through polishing to slightly curve one side , but the curvature is limited as the electrical circuit on the chip must be flat . it is possible that chip 110 may be a series of silicon chips bonded to a flexible membrane to approximate the curvature of the retina . this allows for a larger electrode array than would be possible with a single chip 110 . however , the retina is extremely delicate and can be damaged by either the weight of such a demultiplexer chip or chips , or by the heat generated by the demultiplexer chip . if the chip can not be made light enough or cool enough to attach to the sclera , other solutions are needed . fig3 , depicts an alternate embodiment where a demultiplexer chip 224 is positioned within the eye , but not on the retina . electrode array 210 is connected to the demultiplexer 224 by a wide cable 226 , and demultiplexer 224 is connected to the electronics package 214 by a narrow cable 212 . narrow cable 212 pierces the sclera to attach to an electronics package 214 . the electronics package 214 also connects to a coil 216 . this embodiment requires two interconnects on the external electronics package 214 , one for the coil , and a low density interconnect to a cable piercing the sclera . this cable piercing the sclera is connected to another low density interconnect on the demultiplexer chip within the sclera . finally there must be a high density interconnect on the demultiplexer connected to a thin film electrode array . while this solution is more complex , it moves the weight and heat of the demultiplexer off the retina . fig4 , depicts yet another embodiment where the demultiplexer is not a silicon based integrated circuit , but an integrated circuit deposited directly on the thin film electrode array 310 . the demultiplexer interconnects with the thin film cable 312 on one side and includes electrode openings , for contacting the retina on the other side . cable 312 pierces the sclera and attaches to an electronics package 314 . the electronics package connects to a coil 316 . several integrated circuit manufacturers are building integrated circuits on mylar thin films . the same technique can be used to build integrated circuits on polyimide or other biocompatible thin film polymers . by depositing the demultiplexer circuitry directly on the thin film , one avoids the weight of a silicon based integrated circuit and the possible damage to the retina cause by the weight of a silicon based demultiplexer . further , building the demultiplexer circuit directly on the thin films avoids the need for a hermetic interconnection to a demultiplexer chip or a hermetic coating on the chip . if the heat output of a thin film based demultiplexer is sufficiently small , the demultiplexer can be built directly into the electrode array , completely eliminating the need for a thin film wide enough to include one connector for each electrode . if the heat output of a thin film demultiplexer is likely to cause damage to the retina , another embodiment , shown in fig5 , provides a thin film demultiplexer between the electrode array and electronics package , but sufficiently distant from the retina , to avoid heat damage . fig5 , depicts an alternate embodiment where a thin film demultiplexer 424 is positioned within the eye , but not on the retina . electrode array 410 is connected to the demultiplexer 424 by a wide cable 426 , and demultiplexer 424 is connected to the electronics package 414 by a narrow cable 412 . narrow cable 412 pierces the sclera to attach to an electronics package 414 . the electronics package 414 connects to a coil 416 . it is possible to form the electrode 410 , wide cable , 426 , demultiplexer 424 , and thin cable 412 on a single thin film substrate . while this solution is more complex , it moves the heat of the demultiplexer off the retina . fig6 depicts the profile of a user wearing the external portion of the retinal prosthesis . the entire device may be built into the temple of a pair of glasses . a camera 530 collects a video image and transmits data to an external electronics package 532 . a battery 534 powers the camera 530 , external electronics package 532 , and provides power to a primary inductive coil 536 . the primary inductive coil 536 sends power and data through the skin and skull to the secondary inductive coil 16 . maximum efficiency is obtained when the primary inductive coil 536 and secondary inductive coil 16 are the same size , shape and as close together as possible . fig7 , depicts the preferred demultiplexer chip 110 . a silicon substrate 610 has a demultiplexer 612 applied by conventional means . micromachining techniques are used to create voids in the silicon substrate 610 , which are filled with conductive feedthroughs 616 . electrodes 614 , for contact with the retina , are applied to the conductive feedthroughs 616 by electroplating or other means . a thin film hermetic coating 618 is applied to the silicon substrate 610 allowing voids for electrodes 614 and contacts 620 . finally a thin film cable 622 is attached to the contacts 620 to supply power and signal to the demultiplexer chip . accordingly , what has been shown is an improved retinal prosthesis . 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 invention is a retinal prosthesis with an improved configuration mounting necessary components within and surrounding the eye . the present invention better allows for the implantation of electronics within the delicate eye structure . the invention further limits the necessary width of a thin film conductor passing through the sclera by use of a multiplexer external to the sclera and a demultiplexer internal to the sclera .