Patent Publication Number: US-2015066113-A1

Title: Radiolucent Metal Case Plate to Facilitate Communications in an Implantable Medical Device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a non-provisional of U.S. Provisional Ser. No. 61/874,186, filed Sep. 5, 2013, which is incorporated herein by reference in its entirety, and to which priority is. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to improving wireless communications in an implantable medical device such as an implantable pulse generator. 
     BACKGROUND 
     Implantable stimulation devices deliver electrical stimuli to 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 subluxation, etc. 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. However, the present invention may find applicability with any implantable medical device or in any implantable medical device system. 
     As shown in  FIG. 1 , a SCS system includes an Implantable Pulse Generator (IPG)  10 , whose construction is further described in a Provisional Patent Application entitled “Construction for an Implantable Medical Device Employing an Internal Support Structure,” Ser. No. 61/874,194, which is concurrently filed herewith, and which is incorporated herein by reference in its entirety. IPG  10  includes a biocompatible device case  30  that holds the circuitry and battery  34  ( FIG. 2 ) necessary for the IPG to function. The IPG  10  is coupled to electrodes  16  via one or more electrode leads (two such leads  14   a  and  14   b  are shown), such that the electrodes  16  form an electrode array  12 . The electrodes  16  are carried on a flexible body  18 , which also houses the individual signal wires  20  coupled to each electrode. The signal wires  20  are connected to the IPG  10  at one or more lead connectors  24   a  and  24   b  fixed in a header  28 , which can comprise epoxy for example. In the illustrated embodiment, there are eight electrodes on lead  14   a , labeled E1-E8, and eight electrodes on lead  14   b , labeled E9-E16, although the number of leads and electrodes is application specific and therefore can vary. In a SCS application, electrode leads  14   a  and  14   b  are typically implanted on the right and left side of the dura within the patient&#39;s spinal cord. The proximal ends  22   a  and  22   b  of leads  14   a  and  14   b  are then tunneled through the patient&#39;s flesh to a distant location, such as the buttocks, where the IPG case  30  is implanted, at which point they are coupled to the lead connector(s)  24   a  and  24   b.    
       FIG. 2  shows IPG  10  with case  30  removed so that internal components can be seen. Of particular importance is the communication coil (antenna)  40 , which enables communication between the IPG  10  and a device external to the patient, such as a hand-holdable portable external controller  100 . External controller  100  itself contains a communication coil (antenna; not shown), thus allowing bidirectional communication  102  to occur by magnetic induction between the two devices. This is useful as it allows a user to interface with the external controller  100  to adjust the therapy that the IPG  10  is providing (e.g., to increase or decrease the stimulation being provided, to change which electrodes are providing the stimulation, etc.), and also to review status information reported by the IPG. Further details concerning an external controller and how it communicates with an IPG can be found in U.S. Provisional Patent Application Ser. No. 61/773,476, filed Mar. 6, 2013, which is incorporated herein by reference in its entirety, and with which the reader is assumed familiar. 
       FIG. 3  shows IPG  10  in cross section, and shows further details of construction. The majority of the room inside the case  30  is taken up by the battery  34 , which in this example is a permanent, non-wirelessly-rechargeable battery. (Battery  34  could also be rechargeable, in which case either coil  40  or another recharging coil would be used to wirelessly receive a charging field that is rectified to charge the battery  34 ). The remainder of the room in the case is taken up by circuitry, including a printed circuit board (PCB  42 ) that contains various electronic components and receives leads from the battery  34 , the communication coil  40 , and the feedthrough pins  23  that ultimately couple to the electrodes  16  via the lead connectors  24 . Communication coil  40  is coupled to modulation and/or demodulation circuitry  27  on the PCB to prepare data to be transmitted and decode data that is received. A support structure  38 , made of plastic for example, is used to hold the communication coil  40  and the PCB  42  and to provide a mechanically-stable arrangement of the components inside the case  30 , as further discussed in the above-incorporated application. Case  30  is formed in two portions  30   a  and  30   b , each having substantially parallel top  31   a  and bottom  31   b  sides (with 5%). The case portions surround the electronics, and are laser welded together as well as laser welded to a feedthrough  32 . Thereafter, the epoxy header  26  is affixed to the case  30  and formed around the lead connectors  24 . 
     The inventor has noticed a concern with traditional IPG design, and in particular with communications to and from an IPG. As is known, wireless communications to and from communication coil  40  can be attenuated by the conductive material of the case  30 . When magnetic induction is used as the means for establishing communication link  102  for example, the generated AC magnetic field will create eddy currents in the case  30 , essentially acting as an unwanted sink for the energy in the field, and thus reducing the distance at which communication between the IPG  10  and external controller  100  can reliably occur. See, e.g., U.S. Pat. No. 8,457,756. Such unwanted coupling to the case is increased when the conductivity of the material used for the case  30  is increased. 
     Some previous IPGs  10  have thus used a lower-conductivity material for the case  30 , such as Titanium alloys (Ti) Grade 23 (which contains within manufacturing tolerances 6% Aluminum and 4% Vanadium, and no more than 0.13% Oxygen), or Grade 5 (which contains within manufacturing tolerances 6% Aluminum and 4% Vanadium, and no more than 0.20% Oxygen). However, while these materials have preferred lower conductivity, they are also more brittle. This makes it more difficult to form such materials into a shape without cracking This is a problem for IPG designers, especially as IPGs reduce in size. As sizes reduce, the degree to which the case material must be bent also increases (i.e., the radii of curvature of the bends decrease), particularly at the edges  50  and corners  52  of the case  30 , as best seen in the isomeric view of  FIG. 1 . 
     Some previous IPGs  10  have used non-conductive ceramic materials for the case, see, e.g., U.S. Pat. No. 7,351,921, which like lower conductivity alloys will tend to reduce attenuation of communications in IPGs using internal communication coils. However, ceramic materials are also brittle and difficult to work with. Ceramic case components further require brazing to mechanically couple them together or to other metallic components, which can be difficult to perform. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an Implantable Pulse Generator (IPG) and the manner in which electrodes are affixed. 
         FIG. 2  shows the IPG in wireless communication with an external controller. 
         FIG. 3  shows the internal construction of the IPG in cross section. 
         FIG. 4  shows an improved IPG having a case with a metallic plate proximate the communication coil within the case. 
         FIGS. 5A-5C  show examples of sizing of the plate relative to the sizing of the communication coil. 
         FIG. 6  shows a cross section of the improved IPG with the plate to facilitate communications with an external controller. 
         FIG. 7  shows various manners in which the plate can be affixed to the case of the IPG. 
         FIG. 8  shows a modified IPG having two plates on either side. 
         FIG. 9  shows a cross section of an improved IPG having a charging coil and a plate to facilitate charging from an external charger. 
         FIG. 10  shows an alternative case configuration with which the disclosed plate(s) can be used. 
     
    
    
     DETAILED DESCRIPTION 
     The inventor discloses an improvement for a case for an implantable medical device having an internal communication coil in which a lower-conductivity, more-radiolucent metallic plate is provided proximate to the coil. The remainder of the case can be formed of a higher-conductivity metallic material which is easier to form and thus lends itself to the manufacture of implantable medical devices with smaller cases for example. As both the plate and the remainder of the case are metallic, they can be easily joined by reliable laser welding techniques for example. 
       FIG. 4  shows the bottom side  31   b  of the improved IPG  110  (the side proximate the communication coil  40 ) with the bottom case portion  30   b  present (left) and removed (right). As shown, the bottom case portion  30   b  includes a metallic plate  112 , a radiolucent “window” that covers a hole  116  ( FIG. 5A ) in the case  30  and essentially aligns with the location of the communication coil  40  within the IPG, as described further below. In this example, the plate  112  is flat, and thus does not encroach upon the curved surfaces of the bottom case portion  30   b , such as the edges  50  and corners  52 . This is desirable, as it is easier to affix the plate  112  to the bottom case portion  30   b , as described in detail later. That being said, it is not strictly necessary that the plate  112  be flat, and it can meet with curved case portions in other examples. 
     The plate  112  and bottom case portion  30   b  are made of two different metallic materials having different conductivities. In one non-limiting example, the bottom case portion  30   b  (and top case portion  30   a ) are formed of unalloyed TI Grades 1 (low oxygen) or 2 (standard oxygen), which consists essentially of Titanium (ignoring oxygen and other normal manufacturing impurities). This material has a higher conductivity and is less brittle than Ti Grade 23, and therefore easier to work into the three-dimensional shapes of the case portions  30   a  and  30   b . The plate  112 , by contrast, is made of a lower conductivity material such as TI Grade 23 or Grade 5. Plate  112  could also be made of other lower conductivity metallic materials as well, including other biocompatible materials not containing Titanium. Such lower-conductivity materials, as discussed earlier, are more radiolucent. When these different materials for the case portions  30   a  and  30   b  and plate  112  are used in combination, a case  30  results that is easy to form into shape, but which is less prone to attenuate wireless communications  102  to and from an external device such as external controller  100  ( FIG. 2 ). 
       FIG. 5A  describes further details of the size of hole  116  and/or plate  112  in the bottom case portion  30   b  relative to the communication coil  40  for the example IPG  110  depicted, with the size of the hole  116  and the plate  112  being discussed together because they are essentially equal in this example. As shown the communication coil  40 &#39;s windings comprise outer dimensions X 2  and Y 2  generally defining an outer area A 2 , and inner dimensions X 1  and Y 1  generally defining an inner area A 1 . The hole  116 /plate  112  likewise comprises dimensions X and Y generally defining an outer area A. It is preferable that the hole  116 /plate  112 &#39;s area A be at least equal to or larger than and overlaps the coil  40 &#39;s inner area A 1  (e.g., X&gt;X 1  and Y&gt;Y 1 ), and further are centered around an axis  101 . Particularly if magnetic induction is used as the communication means for link  102 , it is desired that the magnetic flux created or received inside the coil  40  be attenuated as little as possible, which the lower-conductivity plate  112  promotes, and so having A be larger than and overlapping A 1  assists in this regard. 
       FIG. 5B  shows another example for a different IPG in which the communication coil  40  has a smaller area inside the case  30 , and as a result the area A of the hole  116 /plate  112  can be made much larger. In particular, note that in this example the hole  116 /plate  112 &#39;s area A is equal to or larger than and overlaps the coil  40 &#39;s outer area A 2  (e.g., X&gt;X 2  and Y&gt;Y 2 ), and further are centered around axis  101 . This provides the largest benefit, as some of the magnetic flux implicated in inductive communications appears outside of the outer area A 2  of the communication coil  40 , as one skilled in the art would understand. 
     Having as large of a hole  116 /plate  112  as possible forces eddy currents  41  through the more-conductive bottom case portion  30   b  to take a longer more-resistive paths. As this path becomes longer, the less-conductive plate  112  material may become the path of least resistance for such eddy currents. Regardless whether eddy currents travel the longer path through the case portion or through the plate portion, the effect is to increase the resistance met by such eddy currents  41  in the case  30 , thus reducing the attenuation of wireless communications. 
     Despite the foregoing examples, any overlap between the hole  116 /plate  112  and the communication coil  40  will assist in reducing attenuation, even if such overlap is only partial, and even if the hole  116 /plate  112  and coil  40  are not centered around the same axis  101 , as shown in the different examples depicted in  FIG. 5C . In other examples, the hole  116 /plate  112  might comprise the entirety of the flat side  31   b  of the bottom case portion  30   b , or the plate  112  might comprise the entirety of the bottom case portion  30   b , although this isn&#39;t depicted. 
       FIG. 6  shows the improved IPG  110  with the plate  112  in cross section. Note that the IPG  110  is implanted in the patient&#39;s tissue  25  with the bottom side  31   b  of the case out, which is preferred as communication coil  40  is placed closer to the bottom side  31   b  than to the top side  31   a , and is thus in closer proximity to the external controller  100 . This orientation of the IPG  110  in the patient also tends to prevent the IPG&#39;s PCB  42 , which is proximate to the top case portion  30   a , from interfering with communications  102  between the IPG  110  and the external controller  100 . 
       FIG. 7  shows different manners in which the plate  112  can be affixed to the bottom case portion  30   b  to cover hole  116 . In the first example, the plate  112  is sized to match the hole  116  in the bottom case portion  30   b , such that the outer surfaces of the bottom case portion  30   b  and the plate  112  are substantially flush (e.g., less than 10 mils difference). To prevent the plate  112  from falling through the hole  116  during its attachment, it is supported by a chuck  114 , which may be made of high-temperature plastic for example and thus not affected by the laser welding to follow. Once the plate  112  is held in place within the hole  116  by the chuck  114 , the seam between the plate  112  and the case portion  30   b  can be laser welded to form one or more welds  118 , thus allowing the plate  112  to cover the hole  116  with a good hermetic seal. Laser welding is easily accomplished, especially considering that the materials for the plate  112  and the case portion  30   b , while different, are similar enough to facilitate adhesion thought melting. 
     In the second example, the edges of the plate  112  and bottom case portion have been formed with interlocking steps  120 . This holds the plate  112  in place during welding  118  without the need for chuck  114 . 
     In the third example, plate  112  is slightly larger than hole  116 , and thus sits on top of the hole during laser welding  118 , again without falling through and without the need for chuck  114 . 
     In the fourth example, plate  112  comprises a lower portion  112   a  design to fit the size of the hole  116 , and an upper portion  112   b  which is slightly bigger than the hole  112   b , which prevents it from falling though. 
     In the fifth example, the same plate  112  is used as in the fourth example, but is inserted from the inside of the bottom case portion  30 . As a result, the plate  112  is substantially flush with the outside portion of the bottom case portion, which can then be laser welded  118 . Chuck  114  is again helpful in holding the plate  112  in place during welding. 
     The last example shows a portion  122  of the bottom case portion  30   b  that has been thinned (t′) compared to the bulk thickness (t) of the case portion. As there is no hole  116  in the bottom case portion, the plate  112  will not fall through the bottom case portion  30   b , and again no chuck  114  is needed. In this example, although some amount of higher-conductivity case portion material is present in portion  122 , thinning of this material will still reduce attenuation of communications to some degree. 
     These are merely examples of how the plate  112  can be affixed to the bottom case portion  30   b , and other manners are possible. For example, the plates  112  can be affixed from the inside of the case portion  30   b  (as in the fifth example), even though affixing from the outside has largely been depicted. As some of the examples show, the plate  112  can have a thickness equal to the thickness of the bottom case portion (t), although again this is not strictly necessary as some of the examples make clear. It should be noted that attenuation of communications will be benefitted by having the thinnest plate  112  possible (that still provides suitable strength and hermeticity) as a thinner plate  112  will lower plate conductivity even further. In this regard, it may be beneficial in other examples that the plate  112  be thinner than the bottom case portion (t). Further, while the area of the hole  116  and plate  112  are substantially equal in the depicted embodiments (varying by no more than 1% for example), the plate  112  can be substantially larger than the hole  116  in other examples. 
     Furthermore, while laser welding is the preferred manner for affixing the plate to the case portion, this also is not strictly necessary. Instead, the plate  112  may be affixed by brazing, or using a biocompatible and hermitic glue or other adhesive. 
       FIG. 8  shows another example of an improved IPG  110 ′ having two plates  112   a  and  112   b  of the type described earlier, one in the bottom case portion  30   b  and one in the top case portion  30   a . As shown, the two plates  112   a  and  112   b  are centered along axis  101  and communication coil  40  as noted before. In this example, the additional plate assists in promoting flux through both sides  31   a  and  31   b  of the IPG  110 ′. This can be beneficial given the generally circular nature of flux lines with respect to the communication coil  40 . Providing plates  112   a  and  112   b  on each side of the IPG  110 ′ can also allow either side of IPG  110 ′ to be oriented outward of the patient. 
     Although the addition of a metallic plate  112  to the case  30  of an IPG  110  has been developed with the primary goal of improving communications, plate  112  can also assist in wireless charging of the battery in the IPG. This is shown in  FIG. 9 , in which an external charger  140  is used to produce an AC magnetic field  142  to charge a battery  132  in IPG  130 . This field  142  is received by a charging coil  134 , and the AC current induced in this coil  134  is coupled to rectifier circuitry  135  that rectifies the current to a DC level that is used to charge the battery  132 . See, e.g., U.S. Patent Application Publication 2013/0096651, discussing external chargers and battery charging in more detail. The AC magnetic charging field  142  is subject to the same concerns, namely attenuation in the case  30 . As such, a plate  112   a  is positioned proximate to the charging coil  134 . Because the charging coil  134  is wound around the entirety of the inside of the case, the plate  112   a  effectively comprises the entire flat surface of the bottom case portion  30   b  in this example. 
     IPG  130  additionally contains a separate communication coil  40  such as that described earlier. Even though it is not proximate to the plate  112   a  (it is proximate the top case portion  30   a  rather than the bottom case portion  30   b  to which the plate  112   a  is affixed), the communication coil  40  will still benefit from reduced attenuation in plate  112   a . A secondary plate  112   b  used primarily for the benefit of the communication coil  40  could also be used, as shown in dotted lines. If communication coil  40  was proximate to the same side of the IPG as the charging coil  134 , that side  31  or case portion might contain two holes  116  covered by two plates  112 , although this is not depicted. Alternatively, although not depicted, either of the coils  40  or  134  could comprise a combined communication/charging coil coupleable to both rectifier circuitry  135  and modulation/demodulation  27  circuitry and thus capable of performing both functions. 
     It should be noted that plates  112  have been described as being particularly helpful when near-field magnetic induction is used as the means for wireless communications of data or a charging field, plates  112  are not limited in their utility to such means of communication. Indeed, plates  112 , by virtue of their lower conductivities, will assist in reducing attenuation of far-field Radio Frequency (RF) means of communication, such as Bluetooth, Zigbee, Wifi, etc. 
     Although the case  30  is described as comprising two case portions  30   a  and  30   b , one skilled will understand that the case  30 , and use of the disclosed plate(s)  112 , are not so limited. For example, and referring to  FIG. 10 , case  30  can also comprise a uniform structure generally resembling a “cup”  150  into which the internal components are positioned during assembly. A cap  152  can then be used to seal (e.g., weld) the open end of the cup. Although not shown, the cap  152  would likely have a feedthrough  32  surrounded by a header  26  as illustrated earlier. The disclosed plate(s)  112  can be used with such a case, or any other case construction, regardless of the number of pieces such case may comprise during assembly. 
     The following claims at times recite “a” structure, but this should not be construed as limiting scope to devices that only contain a singular one of such structures. For example, while the claims recite a case having “a” hole, and “a” plate, a case having two holes and corresponding plates would still be within the scope of the claims by virtue of any hole and its corresponding plates. 
     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.