Abstract:
An improved structure for an implantable medical device, such as an implantable pulse generator, is disclosed. The improved device includes a charging coil for wirelessly receiving energy via induction from an external charger. The charging coil in the device is located substantially equidistantly from the two planar sides of the device case. Because the coil is substantially equidistant within the thickness of the case of the device, the device&#39;s orientation within the patient is irrelevant, at least from the standpoint of the efficiency of charging the device using the external charger. Accordingly, charging is not adversely affected if the device is implanted in the patient with the wrong orientation, or if the device flips within the patient after implantation. Moreover, because the central portion of the device naturally corresponds to the largest lateral extent within the case due to the case&#39;s curved edges, the charging coil can be made larger in area, which improves its gain vis-à-vis the external charger.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to implantable devices, and more particularly, to a fully implantable device or system for stimulating living tissue of a patient, e.g., a pulse generator used in a Spinal Cord Stimulation (SCS) system or other type of neural stimulation system. 
     BACKGROUND 
     Implantable stimulation devices are devices that generate and deliver electrical stimuli to body nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder sublaxation, etc. The present invention may find applicability in all such applications, although the description that follows will generally focus on the use of the invention within a Spinal Cord Stimulation (SCS) system, such as that disclosed in U.S. Pat. No. 6,516,227 (“the &#39;227 patent”), issued Feb. 4, 2003 in the name of Paul Meadows et al., which is incorporated herein by reference in its entirety. 
     Spinal cord stimulation is a well-accepted clinical method for reducing pain in certain populations of patients. As shown in  FIGS. 1A and 1B , a SCS system typically includes an Implantable Pulse Generator (IPG)  100 , which includes a biocompatible case  30  formed of  titanium for example. The case  30  holds the circuitry and power source or battery necessary for the IPG to function. The IPG  100  is coupled to electrodes  106  via one or more electrode leads (two such leads  102  and  104  are shown), such that the electrodes  106  form an electrode array  1   10 . The electrodes  106  are carried on a flexible body  108 , which also houses the individual signal wires  112 ,  114 , coupled to each electrode. The signal wires  112  and  114  are connected to the IPG  100  by way of an interface  115 , which may be any suitable device that allows the leads  102  and  104  (or a lead extension, not shown) to be removably connected to the IPG  100 . Interface  115  may comprise, for example, an electromechanical connector arrangement including lead connectors  38   a  and  38   b  configured to mate with corresponding connectors  119   a  and  119   b  on the leads  102  and  104 . In the illustrated embodiment, there are eight electrodes on lead  102 , labeled E 1 -E 8 , and eight electrodes on lead  104 , labeled E 9 -E 16 , although the number of leads and electrodes is application specific and therefore can vary. 
     The electrode array  110  is typically implanted along the dura of the spinal cord, and the IPG  100  generates electrical pulses that are delivered through the electrodes  106  to the nerve fibers within the spinal column. The IPG  100  itself is then typically implanted somewhat distantly in the buttocks of the patient. 
     Further details concerning the structure and function of typical IPGs and IPG systems are disclosed in U.S. patent application Ser. No. 11/305,898, filed Dec. 14, 2005, which is filed herewith via an information disclosure statement and which is incorporated herein by reference. 
     IPGs are active devices requiring energy for operation, such as is typically provided by a battery. It is often desirable or necessary to recharge the battery within an IPG via an external charger, so that a surgical procedure to replace a power-depleted implantable pulse generator can be avoided. To wirelessly convey energy between the external charger  12  and the IPG  100 , and as shown in  FIG. 2A , the charger  12  typically includes an energized alternating current (AC) coil  17  that supplies energy  29  to a similar charging coil  18  located in or on the IPG  100  via inductive coupling. In this regard, the coil  17  within the external charger  12  is wrapped so as to lie substantially parallel to the plane of the coil  18  within the implantable medical device during charging. As shown, and as is well known, such a means of energy  29  transfer can occur transcutaneously, i.e., through the patients tissue  25 . The energy  29  received by the IPG&#39;s coil  18  can then be stored in a rechargeable battery  26  within the IPG  100 , which can then be used to  power the electronic circuitry that runs the IPG  100 . Alternatively, the energy  29  received can be used to directly power the IPG&#39;s electronic circuitry, which may lack a battery altogether. 
     As shown in  FIGS. 2A and 2B , an IPG  100  typically includes an electronic substrate assembly  14  including a printed circuit board (PCB)  16 , along with various electronic components  20 , such as microprocessors, integrated circuits, and capacitors, mounted to the PCB  16 , as well as the coil  18 . Ultimately, the electronic circuitry performs a therapeutic function, such as neurostimulation. The IPG  100  further includes a plastic insert  23  with a retainer  22  for holding the electronic substrate assembly  14  in place. A feedthrough assembly  24  routes the various electrode signals from the electronic substrate assembly  14  to the lead connectors  38   a ,  38   b , which are in turn coupled to the leads  102  and  104  (see  FIGS. 1A and 1B ). The IPG  100  further comprises a header connector  36 , which among other things houses the lead connectors  38   a ,  38   b . The IPG  100  can further include a telemetry antenna or coil (not shown) for receipt and transmission of data to an external device such as a hand-held or clinician programmer (not shown), which can be mounted within the header connector  36 . As noted earlier, the IPG  100  usually also includes a power source, and in particular a rechargeable battery  26 , which may be mounted in place by other retainers formed as a portion of the plastic insert  23 . 
     As also noted earlier, the IPG  100  also includes a case  30 , which serves to house all of the aforementioned components in a suitable manner. In particular, the case  30  comprises two case halves  32 ,  34  that mate with each other in a clam-shell arrangement to hermetically seal the IPG  100 . The case  30  has a top surface  40 , a bottom surface  42 , and an edge  44  between the top and bottom surfaces  40 ,  42 . 
     As can be seen from  FIG. 2A , the charging coil  18  according to conventional wisdom is situated adjacent the top surface  40  of the case  30  to minimize the attenuation of the energy  29  before it is received by the charging coil  18 . That is, assuming that the IPG  100  is implanted within the patient such that the bottom surface  42  faces away from the external charger  12  and the top surface  40  faces towards the external charger  12 , the distance D that the energy  29  must travel before it impinges on the charging coil  18  will be minimized, which in turn maximizes the efficiency of the power transmission between the two coils  17  and  18 . In addition, such top-sided placement of coil  18  requires the energy  29  only to traverse the case  30 , and no other components, before it reaches the charging coil  18 , which again minimizes energy  29  attenuation. While according to this conventional wisdom it is preferred to place the coil  18  as  near the top surface  40  as possible, in reality it can be expected that the center of the coil is within the top 25% of the thickness T of the case (i.e., ΔT &lt;25% of T). 
     However, while such top-sided placement of the coil  18  within the IPG  100  has been preferred to minimize the distance D, and hence to minimize energy  29  attenuation, such a design is met with other problems. 
     First, because the charging coil  18  is located closely adjacent the wall of the case  30 , the case  30  may electrically interact with the charging coil  18 , thereby degrading the performance of the coil  18 . 
     Second, the edge  44  of the case  30  has a curved surface  46 , resulting from manufacturing limitations as well as the clinical desire to avoid sharp edges that may otherwise irritate or damage the tissue  25  surrounding the IPG  100 . (The header connector  36  likewise has curved surfaces to avoid sharp edges). To locate the charging coil  18  adjacent the top surface  40  of the case  30 , the charging coil  18  must necessarily be placed at the curved surface  46  of the edge  44 . Unfortunately, this limits the lateral size of the coil  18 , and as best shown in  FIG. 2B , works a loss of lateral distance ΔL within the IPG case  30 . This loss of lateral distance means that each turn of the coil encompasses a smaller area, which in turn limits the gain of the coil  18 . Therefore, while placing the coil toward the top surface  40  of the case increases the gain from the perspective of minimizing the distance D from the external charger  12 , it reduces the gain from the perspective of coil  18  area. 
     Third, when implanting the IPG  100 , the physician must ensure that the top surface  40  of the case  30  faces towards the external charger  12 . If the physician accidentally flips the IPG  100  during implantation such that it is in an improper top-down configuration, the external charger  12  will not be able to as effectively communicate with the IPG, since the coil  18  will be facing in the opposite direction away from external charger  12 . (It has been suggested that the incidence of flipped IPG during implantation due to physician inadvertence may be on the order of 3 to 5%). Moreover, in some cases, the IPG  100  may be properly oriented when initially implanted, but then inadvertently flipped within the patient&#39;s tissue  25 , such as by the patient “fiddling” with the IPG through his or her skin. Regardless of the reason, if the IPG  100  is inadvertently disoriented in the patient with in an improper top-down configuration, the power transfer efficiency benefits realized from placement of the coil  18  toward the top surface  40  of the IPG  100  are lost.  
       FIG. 3  illustrates another example of an IPG  50  capable of wirelessly receiving energy from an external charger  12  via inductive coupling. The IPG  50  is similar to the previously described IPG  100 , with the exception that the charging coil  52  resides on the top surface of the case  54 . To so mount the charging coil  52 , the case  54  is encapsulated with a suitable biocompatible material  56  (e.g., epoxy), which holds the charging coil  52  in place. Thus, it can be appreciated that the charging coil  52  can be located even closer to the external charger  12 , and the attenuation effect of the case  54  can be eliminated, thereby making energy  29  transfer between the external charger  12  and pulse generator  50  more efficient. Moreover, in this embodiment, because the coil  52  is on the outside of the case  54 , its location is more readily apparent, making it less likely that an implanting physician would inadvertently implant in an improper top-down configuration. 
     There are, however, drawbacks to the design of  FIG. 3 . In particular, placement of the charging coil  52  on the exterior surface of the case  54  and the addition of the encapsulating material  56  increases the overall thickness of the pulse generator  50 , thereby making the implanted pulse generator  50  more noticeable to the patient. Also, additional feedthrough holes must be made through the case  54  to connect the charging coil  52  to the electronic circuitry contained within the case  54 , thereby increasing the design complexity and cost of the pulse generator  50 . 
     SUMMARY 
     An improved structure for an implantable medical device, such as an implantable pulse generator, is disclosed. The improved device includes a charging coil for wirelessly receiving energy via induction from an external charger. The charging coil in the device is located substantially equidistantly from the two planar sides of the device case. Because the coil is substantially equidistant within the thickness of the case of the device, the device&#39;s orientation within the patient is irrelevant, at least from the standpoint of the efficiency of charging the device using the external charger. Accordingly, charging is not adversely affected if the device is implanted in the patient with the wrong orientation, or if the device flips within the patient after implantation. Moreover, because the central portion of the device naturally corresponds to the largest lateral extent within the case due to the case&#39;s curved edges, the charging coil can be made larger in area, which improves its gain vis-a-vis the external charger.  
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects of the present invention will be more apparent from the following more particular description thereof, presented in conjunction with the following drawings wherein: 
         FIGS. 1A and 1B  show an implantable pulse generator (IPG), and the manner in which an electrode array is coupled to the IPG in accordance with the prior art. 
         FIGS. 2A and 2B  respectively show cross-sectional and top-down views of a prior art implantable pulse generator, particularly showing one way of configuring a charger coil within the interior of a case for the IPG; 
         FIG. 3  shows a side view of another prior art implantable generator, particularly showing another way of configuring a charger coil exterior to the case of the IPG; 
         FIG. 4  shows an exploded perspective view of an improved IPG structure in accordance with the prior art in which the charging coil is substantially equidistant within the thickness of the IPG case; and 
         FIGS. 5A and 5B  respectively show top-down and cross-sectional views of the improved IPG structure of  FIG. 4 . 
     
    
    
     Corresponding reference characters indicate corresponding components throughout the several views of the drawings. 
     DETAILED DESCRIPTION 
     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 and their equivalents. 
     At the outset, it is noted that the present invention may be used with an implantable pulse generator (IPG), or similar electrical stimulator and/or electrical sensor, that may be used as a component of numerous different types of stimulation systems. The description that follows relates to use of the invention within a spinal cord stimulation (SCS) system. However, it is to be understood that the invention is not so limited. Rather, the invention may be used with any type of implantable electrical circuitry that could benefit from an improved structure for  positioning of the charging coil. For example, the present invention may be used as part of a pacemaker, a defibrillator, a cochlear stimulator, a retinal stimulator, a stimulator configured to produce coordinated limb movement, a cortical and deep brain stimulator, or in any other neural stimulator configured to treat urinary incontinence, sleep apnea, shoulder sublaxation, etc. 
     The improved structure and arrangement of an IPG  190  will now be described in further detail with reference to  FIG. 4 ,  5 A and  5 B, which respectively show the improved structure in exploded perspective, top-down, and cross-sectional views. To the extent structures in the improved IPG  190  are similar in function to those discussed earlier in the Background section, such functionality is not repeated again here. 
     The IPG  190  generally comprises a case  200  comprising a header connector  202 , an electronic substrate assembly  204 , a feedthrough assembly  223  for coupling the electronic substrate assembly  204  with the header connector  202  and ultimately with the lead connectors  205 a,  205 b, and an AC charging coil  208 . The electronic substrate assembly  204  includes a printed circuit board (PCB)  210  on which various electronic components  212  are mounted or otherwise carried. The electronic components  212  can be, e.g., the electronic components described above with respect to  FIGS. 2A and 2B . The charging coil  208  may be any suitable coil capable of creating a current in response to magnetic energy, but in the illustrated embodiment, takes the form of double-layer copper coil. Although not strictly required in all embodiments of the invention, the IPG  190  also preferably includes a power source  206 , such as a rechargeable battery. 
     The IPG  190  preferably includes a unitary plastic insert  250  having the various retaining mechanism to hold the various components of the IPG  190  in place. Such an insert  250  in a preferred embodiment resides only in the bottom half  220  of the case, but this is not strictly necessary. In the embodiment shown, the insert  250  comprises first retainers  214  for supporting the coil  208  within the case  200 , and second retainers  216  for supporting the power source  206  within the case  200 . The battery  206  and charging coil  208  serve to provide a means for continuously supplying the IPG  190  with renewable energy, such as was described earlier with respect to power source  26  and coil  18  (see  FIG. 2A ). As compared to the prior art, the battery  206  is located at the above the PCB  210 , and on the same side of the PCB  210  as the electronic components  212 .  
     In the illustrated embodiment, the case  200  is a single hermetically-sealed rounded case having, for instance, a diameter X ( FIG. 5A ) of less than 55 mm and a maximum thickness T ( FIG. 5B ) of 10 mm. Preferably, the thickness of the case  200  is 7 mm or less to make the IPG  190 , when implanted under the skin, more inconspicuous to the user. In any event, the exact dimensions of the case  200  are not critical to the invention and can vary in size, and would be expected to shrink as technology progresses. In the illustrated embodiment, the case  200  has two case halves  218 ,  220  that mate together in a clam-shell arrangement to house the components. Alternatively, the case  200  may have a unibody construction that includes a closed end and an open end through which the various components are loaded during assembly. 
     The case  200  is preferably composed of a biologically compatible material having a relatively high resistivity to reduce heat generated by eddy currents induced in the case  200  during recharging of the IPG  190  via the external charger  12  (not shown in  FIGS. 4 ,  5 A and  5 B). The wall thickness of the case  200  is also minimized as much as structurally possible to further increase the resistance in the case  200 . Titanium 6-4 (6% aluminum, 4% vanadium), which has a resistivity of 177 micro-ohms centimeter (60 times the resistivity of copper), is a suitable material from which the case  200  can be manufactured. Alternatively, the case  200  may be fabricated from another metal, such as Titanium 8-1-1 (8% aluminum, 1% molybdenum, 1% vanadium), Titanium 3-2.5 (3% aluminum, 2.5% vanadium), Haynes® 25, or from a ceramic material, such as alumina (Al 2 O 3 ) or zirconia (ZrO 2 ). If the case  200  is fabricated from a ceramic material, the case  200  can be filled with a potting material, such as that described in U.S. Pat. No. 6,411,854, which is incorporated herein by reference. 
     The header connector  202  is mounted to the case  200  using suitable means, such as welding. As briefly mentioned earlier, the header connector  202  includes a feedthrough assembly  223  which mates with corresponding pins extending from the electronic substrate assembly  204 . A data telemetry coil (not shown) may also be located in the header connector  202  and coupled to the input of the electronic substrate assembly  204  via the feedthrough assembly  223  or by other means. 
     In the illustrated embodiment, the case  200  has a standard shape. As best shown in  FIG. 5B , the case  200  has substantially flat, external, opposing surfaces  224  and  226  respectively on the top and bottom case halves  218  and  220 , with an edge  228  located between them. The edge  228  preferably has upper and lower curved surfaces  230  and  232  in the top and  bottom case halves  218  and  220  so that the case  200  does not have any sharp edges. (The header connector  202  likewise lacks sharp edges). The edge  228  further has a flat surface  234  between the curved surfaces  230  and  232 . Alternatively, the edge  228  can comprise a single continuous curved surface without an intervening flat surface  234 . In any event, these various curved surfaces define various case regions, i.e., an upper region  240  corresponding to upper curved surface  230 , a lower region  242  corresponding to lower curved surface  232 , and a center region  244  corresponding to the flat surface (or center “point” if no flat surface  234  is present). 
     In accordance with embodiments of the invention, and as shown in  FIG. 5B , the charging coil  208  in the improved IPG  190  is wound in a plane within or at the center region  244  (or center point as the case may be). This means the charging coil  208  is preferably mounted such that its plane lies substantially parallel to, and substantially equidistant between, the opposing surfaces  224  and  226  of the case  200 . In other words, the center of the coil  208  is located at the center point of the thickness T of the case (i.e., ΔT≈50% of T), which defines a maximum lateral area within the case. This serves many useful purposes when compared to the prior art designs of  FIGS. 1A through 3 . 
     First, because the charging coil  208  is not located closely adjacent the wall of the case  200 , the case  200  is less prone to electrically interacting with the coil  208 , thereby improving its performance. 
     Second, because of the curved surfaces  230  and  232 , the center region  244  has a greater lateral extent than either of the upper or lower regions  240  or  242 . This allows each turn of the charging coil  208  to encompass a larger area, which increases the gain of the coil  208 , which in turn improves efficiency of energy transfer between the external charger  12  and the IPG  190 . In other words, and in comparison to the prior art illustration of  FIG. 2B , the loss of lateral distance within the IPG case  200  (ΔL) is reduced or eliminated. From a charging perspective, the increased size of the charging coil  208  compensates for the increased distance between it and the top surface  224  of the case  200 . 
     Third, and perhaps most significantly, when the charging coil  208  is mounted at the center region  244  of the case  200 , the efficient of energy transfer from the external charger  12  to the coil  208  is made essentially independent of the orientation of the IPG  190  when implanted in a patient. In other words, it is basically irrelevant (at least from a charging standpoint) whether the IPG  190  is oriented within the patient with its top surface  224  toward the external charger  12 ,  or with its bottom surface  226  toward the charger. Because the coil  208  is located substantially equidistantly within the case  200 , the charger-to-IPG distance D (see  FIG. 2A ) is the same, meaning that the amount of charging energy received is substantially equivalent for either orientation. In other words, the magnitude of the energy received by the coil  208  is substantially the same regardless of which of the surfaces  224  or  226  the energy traverses. Thus, a physician need not be concerned with whether the IPG  190  is facing up or facing down when implanting it within a patient, and it is of little concern (to charging efficiency at least) should the IPG  190  flip inside the patient after implantation. 
     While the charging coil  208  has been described as useful in charging a rechargeable power source, such as a rechargeable battery  206 , it should be recognized that use of a rechargeable battery or other power source (e.g., a capacitor) is not strictly necessary. For example, the charging coil  208  can be used to provide power directly to the electronic components within the IPG  190 . In such a case, the IPG  190  may or may not have an on-board power source, whether or not rechargeable. Of course, if no power source is present, the energy  29  transmitted by the external charger  12  may need to be continuous, or at least periodic. 
     Of course, one skilled in the art will recognize that embodiments of the invention will still have utility even if the charging coil  208  is not positioned exactly at the center of the case (i.e., ΔT≈50% of T), and even if the energy transfer from the external charger  12  to the coil  208  is not exactly equivalent from both sides of the IPG. In this regard, the a coil “substantially equidistant within the thickness of the case” or “substantially equidistant between opposing surfaces of the case” should be understood as encompassing a plus or minus 10% deviation from the center of the case (i.e., 40% of T≦ΔT≦60% of T). 
     Although particular embodiments of the present invention have been shown and described, it should be understood that the above discussion is not intended to limit the present invention to these embodiments. It will be obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the present invention. Thus, the present invention is intended to cover alternatives, modifications, and equivalents that may fall within the spirit and scope of the present invention as defined by the claims.