Abstract:
In an implantable prosthesis system such as a cochlear implant, a pair of coils are used to exchange control and/or power signals between the external and internal components. The coils are offset or displaced laterally by a predetermined distance to reduce variations in the signal transmission due to the spacing between the external and internal components.

Description:
BACKGROUND OF THE INVENTION 
     A. Field of Invention 
     This invention pertains to transcutaneous tissue stimulating devices such as cochlear prostheses and functional electrical stimulators. Such devices are frequently of a two-part design, there being a part implanted within the body of the patient, for the purpose of providing therapy, and an external component in communication with the implanted part by means of a transcutaneous wireless link. 
     B. Description of the Prior Art 
     Implantable prosthesis devices are generally required to be small to reduce the trauma and other complications arising from the implantation and maintenance of foreign matter inside a patient&#39;s body. For example, cochlear implant systems typically include an external component and an internal component. The external component includes a microphone for sensing ambient sounds and a signal processor for generating stimulation signals corresponding to these sounds. The stimulation signals are transmitted to the internal component which applies the same to the aural nerve of the patient through an array of electrodes. Because the internal component may not have a permanent power supply, power for the component is also derived from the external component. 
     The means for providing both power and communication between the internal and external components is typically a pair of coils, one in the external component and one in the internal component. The coils are planar and are positioned parallel to each other so that energy is coupled through the skin and flesh of the patient from the external coil to the internal coil for the purpose of powering and/or controlling the internal component. Until now, the preferred arrangement for these coils was such that the voltage induced into the internal coil was very sensitive to variations in inter-coil distance. 
     Since the coils are separated by a partition comprising skin and tissue, the inter-coil distance is not constant from patient-to-patient but varies according to the anatomical characteristics of each implantee. A ramification of this variability is that in order to ensure that an adequate voltage level is induced in the internal coil, for purposes of operating the internal component of the cochlear implant, the voltage applied to the external coil must be sufficiently high to take into account patients having considerably thicker or thinner partition widths than typically encountered. As a consequence the voltage applied by the transmit coil is set to a relatively high value in order that the implanted portion will be supplied with sufficient power no matter the width of the implanteeÕs partition. Accordingly, an undesirable power wastage occurs in the majority of patients. A cochlear implant system of this kind is described in U.S. Pat. No. 4,532,930. 
     In a related known arrangement, tuned or tank circuits are used in the transmitter and receiver to transmit signals and power. In this arrangement, the critical parameter for power transmission is the coupling coefficient between the tuned circuit coils. This coupling coefficient is optimal only at a critical spacing between coils. One such system is disclosed in U.S. Pat. No. 4,654,880 to Sontag. Again, for optimal coupling, the two coils must be spaced at a critical distance from each other and must be co-axial. If the coils are not at the optimal distance, the signals are exchanged between the components at a low efficiency. 
     OBJECTIVES AND SUMMARY OF THE INVENTION 
     In view of the above, it is an objective of the present invention to provide an improved communication system for implantable devices which is less sensitive to variations in inter-coil spacing. 
     A further objective of the invention is to provide a transcutaneous tissue stimulation system such as a cochlear implant having coils positioned to obtain optimum power transfer characteristics. 
     A further objective is to provide a system which is easy and inexpensive to implement with minimal changes from the existing systems. 
     Other objectives and advantages of the invention shall become apparent from the following description. 
     Briefly, the subject invention provides a transcutaneous tissue stimulating system including: 
     a) an external portion for location on a first side of a partition, said external portion comprising a first coil for transmission of RF power, arranged to form an arc of a first radius and being mounted on a first coil platform said coil platform engaging a first magnet; and 
     b) an internal portion for location on a side opposite the first side of said partition, comprising a second coil for receiving RF power, arranged to form an arc of a second radius said second coil being mounted on a second coil platform said second coil platform engaging a second magnet for attraction to said first magnet; 
     wherein the first and second coil, platform and magnets are arranged so that upon minimising the distance between said first and second magnets the centres of the first and second arcs are radially offset. 
     According to a further aspect of the invention there is provided a transcutaneous tissue stimulating system including: 
     a) an external portion for location on a first side of a partition, said external portion including a first coil for transmission of RF power, arranged to form an arc of a first radius and being retained by means of a first coil platform said coil platform engaging a first magnet in one of a first and a second selectable positions, said two selectable positions being radially offset from each other; and 
     b) an internal portion for location on a side opposite the first side of said partition, comprising a second coil for receiving RF power, arranged to form an arc of a second radius said second coil being mounted on a second coil platform said second coil platform engaging a second magnet for attraction to said first magnet; 
     wherein the first and second coil, platform and magnets are arranged so that when said first magnet is engaged in said first selectable position then upon minimising the distance between said first and second magnets the centres of said first and second arcs are radially co-located and when said first magnet is engaged in said second selectable position then upon minimising said distance said centres are radially offset. 
     The invention operates to locate the paired coils relative to each other so as to reduce the sensitivity of the inter-coil voltage transfer ratio to variations in inter-coil distance as, will now be explained in more detail. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A shows a perspective schematic view of an arrangement of two coils for a first prior art implantable prosthesis system; 
     FIG. 1B shows a plan view of the arrangement of FIG. 1A; 
     FIG. 2A shows the geometry of the coil of the internal component of the prosthesis of FIGS. 1A,  1 B; 
     FIG. 2B shows the geometry of the coil of the external component of the prosthesis of FIGS. 1A,  1 B; 
     FIG. 3 shows an equivalent circuit for a prior art implantable prosthesis system; 
     FIG. 4 shows the relationship between the coupling coefficient and distance for the system of FIG. 3; 
     FIGS. 5A and 5B show respectively the relationships between a voltage transfer ratio A and the coupling coefficient, and the voltage transfer ratio A and distance for the system of FIG. 2 using the prior art coil positioning arrangement of FIGS. 1A and 1B; 
     FIG. 6 depicts a coil arrangement according to the present invention; 
     FIG. 7 shows a comparison between the coupling coefficients of concentric and offset coils as a function of distance; 
     FIG. 8 shows a comparison between transfer ratios of concentric and offset coils as a function of distance; 
     FIG. 9 shows a further embodiment of the invention which facilitates selection between two inter-coil positions. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1A and 1B depict the arrangement of a typical prior art transcutaneous tissue stimulating system, being a cochlear implant prosthesis operatively positioned. While the present invention will be described with reference to a two-part cochlear prosthesis it will be understood that the invention is applicable to any device which makes use of a transcutaneous inductive link featuring electromagnetically coupled coils. 
     The cochlear implant prosthesis comprises an internal component  1  located on a first side of partition  5 , said partition being formed by the skin and flesh of an implanted patient. For the sake of clarity the width of partition  5  has been greatly exaggerated in FIG.  1 A. Internal component  1  includes an electronics housing  7  to which is electrically connected receiver coil  17 . Coil  17  is retained by a pliable plastic coil platform  23 , which is mechanically connected to electronics housing  7 , for the purpose of forming the coil into a shape that will shortly be described. Also mounted to coil platform  23  is magnet  15 . It will be realised that post-implantation (as shown in FIGS. 1A,  1 B) internal component  1  of the cochlear implant prosthesis is fixed relative to partition  5 . 
     Located on the opposite (external) side of partition  5  is external component  2  comprising a circular coil  19  mounted in coil platform  25  and magnet  13  which is engaged by the coil platform. Coil platform  25  is comprised of a circular coil enclosure  14  having three spokes  16  which hold the magnet  13  in a central position. Other parts of external component  2  of the cochlear implant prosthesis are microphone  9 , behind-the-ear sound processor  11 , which is tucked behind the outer ear  4  of the implanted patient, and cable  27 . 
     In operation internal magnet  15  attracts external magnet  13  forcing coil platform  25  against the external side of partition  5 , i.e. the side of the implanteeÕs head. Consequently external platform  25  is held in place by static friction between it and partition  5 . It will be understood that there is a strong tendency for external platform  25  to be placed so that the distance between internal magnet  15  and external magnet  13  is reduced to a minimum. Consequently the magnets typically come to be positioned so that they are lined up over each other as shown in FIGS. 1A and 1B and indicated by the line X—X. 
     With reference to FIG. 2A, it will be noted that in the prior art arrangement internal coil  17  is not completely circular but rather describes an arc  31 , limited by points  31  A and  31 B, having a centre  33  and radius R2. With reference to FIG. 2B it is seen that the prior art external component has a circular transmitter coil  19  of radius R1. Typically R2 is approximately 75% of R1. Magnets  15  and  13  are located at the centres of arc  31  and the circle formed by coil  19  respectively. 
     FIG. 3 depicts an electrical circuit model of the arrangement of the cochlear implant prosthesis shown in FIGS. 1A and 1B. In the model source  35  represents the signal generating electronics housed in behind-the-ear speech processor  11  with the processing and stimulating electronics housed in receiver casing  7  being represented by load  37 . Transmit coil  19  forms part of a tuned circuit  28  with capacitor  41 . Similarly coil  17  and tuning capacitor  43  form a second tuned circuit  30 . Each of the tuned circuits has a natural frequency of oscillation, said circuits  28 ,  30  being tuned so that their natural frequencies are close to each other. Moreover the two coils  19 ,  17  are positioned relative to each other as has previously been explained with reference to FIGS. 1A and 1B. Energy from coil  19  is transferred to coil  17  by the mutual electromagnetic or inductive coupling between the coils, said coupling being quantitatively indicated by a coefficient k. More specifically, the coupling coefficient k is indicative of the proportion of the magnetic fields linking the coils. When the coils are close the coefficient k takes a large value. As the distance d, shown in FIGS.1A and 1B, increases the value of k decreases as shown in FIG.  4 . 
     The ratio A of the RMS voltage induced in coil  17  in response to an RMS voltage in coil  19  varies with coefficient k and distance d as illustrated in FIGS. 5A and 5B. It will be realised that the distance d corresponds to the width of partition  5  of FIG.  1 . The partition width varies from patient to patient and consequently the voltage transfer ratio A also differs from patient to patient, being less than optimal in patients having either a considerably thicker or thinner partition width than usual. A ramification of this variability is that in order to ensure that an adequate voltage level is induced in coil  17 , for purposes of operating the internal component of the cochlear prosthesis, the voltage applied to coil  19  must be sufficiently high to take into account patients having considerably thicker or thinner partition widths than typically encountered. As a consequence while the voltage induced in coil  17  is sufficient in patients having an atypical partition width, it is higher than required in most patients who have a typical partition and so leads to an undesirable power wastage. 
     Referring now to FIG. 6 there is illustrated an embodiment of the present invention which addresses this problem. It will be noted that the centre  47  of arc  31  of receive coil  17  has been radially offset from the centre  49  of circular transmit coil  19  by a distance L so that arc  31  and circular coil  19  are no longer concentric. The radial offsetting of the transmit and receive coil centres is accomplished by locating magnet  13  at an eccentric position relative to circular coil  19  by means of magnet platform members  51  and  53 . A similar offsetting of the transmit and receive coils could also be achieved by eccentrically locating magnet  15  (not visible in FIG. 6) or indeed by eccentrically locating both magnets to some extent. 
     The result of offsetting the receive and transmit coils as depicted in FIG. 6 is that the coefficient k decreases at a lower rate as inter-coil distance d increases than was the case in the prior art arrangement, as illustrated in FIG.  7 . It will be appreciated from FIG. 7 that in the case of the offset coils of FIG. 6, coefficient k decreases at a lower rate with d than in the prior art arrangement and that this effect is most pronounced for smaller values of d. 
     As shown in FIG. 8, the voltage transfer A for smaller values of d is not only greater than for concentric coils but is also less variable. Thus FIGS. 7 and 8 demonstrate that the inventive intercoil arrangement reduces the voltage transfer ratioÕs sensitivity to variations in the intercoil distance d as desired. 
     The value of the offset L depends on the radii R1 and R2. In the examples given above, where R2 is about 75% of R1, the offset L is about 40% of R1, though it may be varied over the range of 20%-100% of R1 while still achieving the desired effect. 
     Again with reference to FIG. 8, it will be noted that for large values of d, such as may occur in patients having very thick skin, the voltage transfer ratio associated with the inventive arrangement (shown by the dashed curve) falls off markedly. Consequently it would be advantageous if, when applied to a patient having very thick skin, the relative coil position could be adjusted from the off set position to the prior art concentric position. A configuration which allows selection between the two positions is shown in FIG.  9 . 
     In the embodiment depicted therein coil platform  25  is provided with an elongated capsule  120 A configured to locate magnet  13  in at least two possible locations, shown as  134 A,  134 B. When occupying the  134 B position the external magnet aligns the centre of the transmit coil with the centre of the receive coil arc. Consequently the ÒconcentricÓ voltage transfer ratio of FIG. 8 is selected, which is appropriate for patients having a considerably thicker-than-typical skin. Alternatively when the prosthesis is used applied to a patient having a partition of lesser thickness the eccentric magnet position  134 A is preferred as in that position the more appropriate offset transfer ratio of FIG. 8 is selected. 
     Obviously numerous modifications can be made to the invention without departing from its scope as defined in the appended claims. For example, and not exhaustively, the shape of the coils or coil platforms may be varied without affecting the operation of the invention.