Patent Publication Number: US-11020600-B2

Title: Implantable medical device with radiopaque ID tag

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of co-pending U.S. patent application Ser. No. 15/018,379, filed Feb. 8, 2016 and entitled “Implantable Medical Device With Radiopaque ID Tag”, which claims the benefit of U.S. Provisional Patent Application Ser. No. 62/113,827 filed on Feb. 9, 2015, and also claims the benefit of U.S. Provisional Patent Ser. No. 62/138,799 filed on Mar. 26, 2015, all of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure generally relates to implantable medical devices, and more particularly, to implantable medical devices that include a radiopaque ID tag that provides identifying information regarding the implantable medical device during an imaging process such as an x-ray. 
     BACKGROUND 
     Implantable medical devices are commonly used today to monitor a patient and/or deliver therapy to a patient. For example, implantable sensors are often used to monitor one or more physiological parameters of a patient, such as heart beats, heart sounds, ECG, respiration, etc. In another example, implantable neurostimulators are used to provide neurostimulation therapy to a patient. In yet another example, pacing devices are used to treat patients suffering from various heart conditions that may result in a reduced ability of the heart to deliver sufficient amounts of blood to a patient&#39;s body. Such heart conditions may lead to slow, rapid, irregular, and/or inefficient heart contractions. To help alleviate some of these conditions, various devices (e.g., pacemakers, defibrillators, etc.) are often implanted in a patient&#39;s body. Such devices may monitor and provide electrical stimulation to the heart to help the heart operate in a more normal, efficient and/or safe manner. In some applications, it may be beneficial for the implantable medical devices to include a radiopaque ID tag that permits identification of the implantable medical device during an imaging process such as an x-ray. 
     SUMMARY 
     The present disclosure generally relates to implantable medical devices, and more particularly, to implantable medical devices including one or more radiopaque ID tags that provide identifying information regarding the implantable medical device during an imaging process such as an x-ray. 
     An example implantable medical device may include: a housing, a first ID tag secured relative to the housing at a first position, wherein the first ID tag defines a first radiopaque manufacturer code section that visually identifies a manufacturer of the implantable medical device, and a second ID tag secured relative to the housing at a second position, wherein the second position is offset from the first position in at least one dimension. The second ID tag may define a second radiopaque manufacturer code section that also visually identifies the manufacturer of the implantable medical device. 
     Alternatively or additionally to the embodiments above, the first radiopaque manufacturer code section and the second radiopaque manufacturer code section may each define one or more radiopaque alphanumeric characters. 
     Alternatively or additionally to any of the embodiments above, the one or more radiopaque alphanumeric characters may be configured to be human readable in an x-ray or other image of the implantable medical device. 
     Alternatively or additionally to any of the embodiments above, the first radiopaque manufacturer code section of the first ID tag may be structured to define a first radiopaque manufacturer code as well as a reverse image of the first radiopaque manufacturer code. 
     Alternatively or additionally to any of the embodiments above, at least one of the first ID tag and the second ID tag are disposed on an outer surface of the housing. 
     Alternatively or additionally to any of the embodiments above, at least one of the first ID tag and the second ID tag are disposed on an internal component located within the housing. 
     Alternatively or additionally to any of the embodiments above, the second position is offset from the first position in at least two dimensions. 
     Alternatively or additionally to any of the embodiments above, the housing has a cylinder along at least part of its length that includes the first position and the second position, and the second position is axially offset and radially offset from the first position. 
     Alternatively or additionally to any of the embodiments above, the first ID tag and the second ID tag may comprise portions of a helix structure that traverses along at least part of a length of the housing. 
     Alternatively or additionally to any of the embodiments above, the implantable medical device further comprises a battery, wherein at least one of the first ID tag and the second ID are disposed on or within a component of the battery. 
     Alternatively or additionally to any of the embodiments above, the implantable medical device further comprises a circuit board, wherein at least one of the first ID tag and the second ID tag is secured to the circuit board. 
     Alternatively or additionally to any of the embodiments above, the circuit board comprises at least two layers, and wherein at least one of the first ID tag and the second ID tag is positioned between two of the layers of the circuit board. 
     In one example, the implantable medical device may be a leadless cardiac pacemaker. In some instances, the leadless cardiac pacemaker may comprise: an elongated housing defining an energy storage section and a circuit section, an energy source disposed within the energy storage section, a circuit board disposed within the circuit section and operably coupled to the energy source, and an ID tag secured relative to the elongated housing, wherein the ID tag is configured to define a radiopaque manufacturer code that visually identifies a manufacturer of the leadless cardiac pacemaker. In some cases, two or more individual ID tags may be secured relative to the elongated housing, sometimes offset from one another in at least two dimensions. 
     Alternatively or additionally to any of the embodiments above, the leadless cardiac pacemaker may further comprise an insulative coating disposed over the elongated housing, and an ID tag may be covered by the insulative coating. 
     Alternatively or additionally to any of the embodiments above, an ID tag may be secured to the elongated housing. 
     Alternatively or additionally to any of the embodiments above, the energy source may comprise a battery with a battery liner, an anode disposed within the battery liner, and a cathode disposed within the anode. An ID tag may be disposed on or in one of the battery liner, the anode and the cathode. 
     Alternatively or additionally to any of the embodiments above, the leadless cardiac pacemaker may further comprise a battery pin extending from cathode of the energy source, wherein an ID tag may be disposed on or in the battery pin. 
     Alternatively or additionally to any of the embodiments above, the leadless cardiac pacemaker may further comprise a desiccant, wherein an ID tag may be disposed on or in the desiccant. 
     Alternatively or additionally to any of the embodiments above, the leadless cardiac pacemaker may further comprise an overmolding, wherein an ID tag may be disposed on or in the overmolding. 
     Alternatively or additionally to any of the embodiments above, the leadless cardiac pacemaker may further comprises a drug collar, wherein an ID tag may be disposed on or in the drug collar. 
     Alternatively or additionally to any of the embodiments above, the elongated housing may comprise a proximal end feature for retrieval of the leadless cardiac pacemaker, and an ID tag may be secured to the proximal end feature. 
     Alternatively or additionally to any of the embodiments above, the leadless cardiac pacemaker may further comprise an axial rotation marker, and an ID tag may be disposed within or by a cutout formed in the axial rotation marker. 
     Alternatively or additionally to any of the embodiments above, an ID tag may be formed from a platinum wire disposed within a slot formed in the housing or other component of the leadless cardiac pacemaker. 
     Alternatively or additionally to any of the embodiments above, an ID tag may define an alphanumeric code that is readable by an individual during an imaging process. 
     Alternatively or additionally to any of the embodiments above, an ID tag may comprises an etched, machined, cut, or sintered ID tag. 
     Alternatively or additionally to any of the embodiments above, an ID tag may comprise a molded ID tag. 
     Alternatively or additionally to any of the embodiments above, an ID tag may comprise a radiopaque ink. 
     In another example, a leadless cardiac pacemaker may comprise: an elongated housing extending along a central axis, and an ID tag system secured relative to the elongated housing. The ID tag system may comprise a first radiopaque manufacturer code section that visually identifies a manufacturer of the implantable medical device and a second radiopaque manufacturer code section that also visually identifies the manufacturer of the implantable medical device. In some cases, the first radiopaque manufacturer code section and the second radiopaque manufacturer code section may face different radial directions relative to the central axis of the elongated housing of the leadless cardiac pacemaker. 
     Alternatively or additionally to any of the embodiments above, the first radiopaque manufacturer code section is the same as the second radiopaque manufacturer code section. 
     Alternatively or additionally to any of the embodiments above, the first radiopaque manufacturer code section may be a mirror image of the second radiopaque manufacturer code section. 
     Alternatively or additionally to any of the embodiments above, the first radiopaque manufacturer code section and the second radiopaque manufacturer code section may be part of a common piece. 
     Alternatively or additionally to any of the embodiments above, the first radiopaque manufacturer code section may be a separate piece from the second radiopaque manufacturer code section. 
     Alternatively or additionally to any of the embodiments above, the first radiopaque manufacturer code section may be mechanically connected to the second radiopaque manufacturer code section before and after being secured relative to the elongated housing. 
     Alternatively or additionally to any of the embodiments above, the ID tag system further comprises a first radiopaque MRI code section and a second radiopaque MM code section, wherein the first radiopaque MRI code section and the second radiopaque MRI code section face different radial directions relative to the central axis of the elongated housing. 
     The above summary is not intended to describe each embodiment or every implementation of the present disclosure. Advantages and attainments, together with a more complete understanding of the disclosure, will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure may be more completely understood in consideration of the following description of various illustrative embodiments in connection with the accompanying drawings, in which: 
         FIG. 1  is a schematic illustration of an implantable medical device in accordance with an illustrative embodiment of the present disclosure; 
         FIG. 1A  is a schematic illustration of a portion of the implantable medical device of  FIG. 1 ; 
         FIG. 2  is a schematic illustration of an implantable medical device in accordance with an illustrative embodiment of the present disclosure; 
         FIG. 3  is a perspective view of a leadless cardiac pacemaker in accordance with an illustrative embodiment of the present disclosure; 
         FIG. 4  is an exploded view of the illustrative leadless cardiac pacemaker of  FIG. 3 ; 
         FIG. 5  is a perspective view of a housing useable as part of the illustrative leadless cardiac pacemaker of  FIG. 3 ; 
         FIG. 6  is a perspective view of a battery liner useable as part of the illustrative leadless cardiac pacemaker of  FIG. 3 ; 
         FIG. 7  is a perspective view of an anode useable as part of the illustrative leadless cardiac pacemaker of  FIG. 3 ; 
         FIG. 8  is a perspective view of a cathode useable as part of the illustrative leadless cardiac pacemaker of  FIG. 3 ; 
         FIG. 9  is a perspective view of a battery pin useable as part of the illustrative leadless cardiac pacemaker of  FIG. 3 ; 
         FIG. 10  is a perspective view of an overmolding useable as part of the illustrative leadless cardiac pacemaker of  FIG. 3 ; 
         FIG. 11  is a perspective view of a drug collar useable as part of the illustrative leadless cardiac pacemaker of  FIG. 3 ; 
         FIG. 12  is a perspective view of a desiccant useable as part of the illustrative leadless cardiac pacemaker of  FIG. 3 ; 
         FIG. 13  is a perspective view of a liner useable as part of the illustrative leadless cardiac pacemaker of  FIG. 3 ; 
         FIG. 14  is a schematic block diagram of the illustrative leadless cardiac pacemaker of  FIG. 3 ; 
         FIG. 15  is a schematic illustration of an example electrical circuit useable as part of the illustrative leadless cardiac pacemaker of  FIG. 3 ; 
         FIG. 16  is a schematic illustration of an example electrical circuit, including an ID tag, in accordance with an example of the present disclosure; 
         FIG. 17A  is a perspective view of a illustrative leadless cardiac pacemaker, which includes an ID tag secured relative to a proximal end feature; 
         FIG. 17B  is a perspective view of an illustrative leadless cardiac pacemaker, which includes an ID tag forming a proximal end feature; 
         FIG. 18  is a schematic view of a chevron that includes an ID tag formed therein; 
         FIGS. 19 and 20  illustrate a method of forming a radiopaque ID tag in accordance with an example of the present disclosure; 
         FIG. 21  is a schematic cutaway view of an illustrative leadless cardiac pacemaker, including an ID tag on a printed circuit board; 
         FIG. 22  is a schematic cross-sectional side view of an illustrative radiopaque ID tag; 
         FIG. 23  is a schematic cross-sectional side view of another illustrative radiopaque ID tag; 
         FIG. 24  is a schematic top view of the radiopaque ID tags of  FIGS. 22 and 23 ; and 
         FIG. 25  is a perspective view of an illustrative battery cathode incorporating radiopaque ID tags formed therein. 
     
    
    
     While the disclosure is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit aspects of the disclosure to the particular illustrative embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the disclosure. 
     DESCRIPTION 
     The following description should be read with reference to the drawings in which similar elements in different drawings are numbered the same. The description and the drawings, which are not necessarily to scale, depict illustrative embodiments and are not intended to limit the scope of the disclosure. 
       FIG. 1  is a schematic illustration of an illustrative implantable medical device  10 . The implantable medical device  10  may generically represent any variety of implantable medical devices, including but not limited to sensing devices, neurostimulators, pacing devices, defibrillation devices and the like. In some embodiments, the implantable medical device  10  may be a leaded or leadless pressure sensor, for example. While illustrated as having an elongated housing  12 , it will be appreciated that the housing  12  may have other shapes, depending on where and how the implantable medical device  10  is delivered and deployed. For example, in some cases, the housing  12  may have a rectilinear shape, or may be generally cylindrical in shape. In some cases, the housing  12  may have a round or ovoid shape, depending on the application. In some cases, the implantable medical device  10  may be considered as having a longitudinal axis  14  extending lengthwise through the implantable medical device  10  from a first end  16  to a second  18 . A radial axis  20  is shown perpendicular to the longitudinal axis  14 . 
     In some instances, the implantable medical device  10  may include one or more ID tags that can be used to identify the implantable medical device  10  during imaging processes such as x-ray. As illustrated, the implantable medical device  10  includes a first ID tag  22 , a second ID tag  24  and a third ID tag  26 , shown in phantom as the third ID tag  26  is, in the illustrated orientation, on a back side of the implantable medical device  10 . While three ID tags  22 ,  24 ,  26  are shown, in some cases there may be only one or two ID tags, or there may be four or more ID tags. While schematically illustrated on the housing  12 , in some cases one or more of the ID tags  22 ,  24 ,  26 , if present, may be located internally of the housing  12 . If the first ID tag  22  is considered as being located at a first position, it can be seen that the second ID tag  24  is at a second position that is offset from the first position in at least one dimension. As illustrated, the second ID tag  24  is offset axially, along the direction of the longitudinal axis  14 , as well as being offset radially, along the direction of the radial axis  20 . As illustrated, the third ID tag  26  is at a third position that is offset both axially and radially from each of the first ID tag  22  and the second ID tag  24 . 
     In some embodiments, the ID tags  22 ,  24 ,  26  may include a radiopaque identifier using a symbol and/or 1, 2 or 3 alphanumeric characters to identify a manufacturer and may include 1, 2, 3 or 4 alphanumeric characters to identify a model. The ID tags  22 ,  24 ,  26  may, for example, be used in a leadless pacemaker, a leadless pacemaker inside the heart, a dual chamber leadless pacemaker, an epicardial pacemaker, a leadless epicardial pacemaker or an implantable cardiac diagnostic device, among others. In some cases, a bar code such as a two dimensional bar code may be used. In some embodiments, the radiopaque identifier may include a two or four digit year identifier. In some cases, the radiopaque identifier may include a “B” to identify a company and a two digit model #, although this is merely illustrative. 
     The ID tags  22 ,  24 ,  26  are configured to be visible during imaging processes such as x-ray. With the implantable medical device  10 , and thus the ID tags  22 ,  24  and  26 , implanted within the body, the ID tags are configured to be visible and readable by an imaging process instituted from outside of the body. The imaging process may use x-rays, or any other suitable penetrating wave or particle such as neutron beams or gamma rays, as desired. In some cases, the ID tags  22 ,  24 ,  26 , or portions thereof, are radiopaque. In some instances, the first ID tag  22  defines a first radiopaque manufacturer code section  28  that visually identifies a manufacturer of the implantable medical device  10 , and the second ID tag  24  defines a second radiopaque manufacturer code section  30  that also visually identifies the manufacturer of the implantable medical device  10 . In some cases, the first ID tag  22  and/or the second ID tag  24  may include a non-radiopaque substrate or carrier, and only the first radiopaque manufacturer code section  28  and/or the second radiopaque manufacturer code  30  is/are radiopaque. In some cases, the substrate or carrier forming the first ID tag  22  and/or the second ID tag  24  are radiopaque, and the first radiopaque manufacturer code section  28  and/or the second radiopaque manufacturer code section  30  represents an absence of radiopaque material. In some instances, the first ID tag  22  and/or the second ID tag  24  may be formed by printing alphanumeric characters or other identifying symbols onto a substrate or carrier using a radiopaque ink. In some cases, an ID tag  22 ,  24 ,  26  may be formed as a label or sticker that may be adhesively secured to a component within the implantable medical device  10 . An ID tag  22 ,  24 ,  26  may, for example, include a high atomic weight foil. In some cases, an ID tag  22 ,  24 ,  26  may include a platinum foil that is enclosed in heat shrink tubing around an internal component such as a battery. 
     It will be appreciated that by including two or more radiopaque ID tags, arranged at offset positions, it may be easier to read at least one of the ID tags during an imaging process, especially for implantable medical devices that do not have a well-defined or fixed implanted orientation. While the first radiopaque manufacturer code section  28  is illustrated as “XXX” and the second radiopaque manufacturer code section  30  is illustrated as “YYY”, it will be appreciated that this is illustrative only, as any variety of codes such as bar codes, alphanumeric characters, or any other suitable code or marking may be used, as desired. 
     In some embodiments, an ID tag may include a radiopaque manufacturer code as well as a mirror image of the radiopaque manufacturer code.  FIG. 1A  is an enlarged view of the third ID tag  26  showing a radiopaque manufacturer code  32  as well as a mirror image  34  of the radiopaque manufacturer code  32 . As illustrated, the radiopaque manufacturer code reads “AB1”, but this is of course illustrative only. Depending on the implanted orientation of the implantable medical device  10 , the radiopaque manufacturer code  32  may be legible in an x-ray. In some cases, the mirror image  34  may be more legible. Accordingly, a single ID tag may provide the benefit of having two ID tags that are offset from each other. Regardless of whether an ID tag includes a code and a mirror image thereof, or if several ID tags are offset from each other, it will be appreciated that due to the nature of imaging processes such as x-ray, it is possible to see ID tags that are at various positions, both internal and external, relative to the housing  12 . 
       FIG. 2  provides a schematic illustration of an implantable medical device  36  having a longitudinal axis  14  and a radial axis  20 . The implantable medical device  36  has a housing  38  with an outer surface  40 . In the illustrated embodiment, a helix structure  42  wraps around the outer surface  40  of the housing  38 . In other cases, the helix structure  42  may be internal to the housing  38 . The illustrative helix structure  42  includes a first ID tag  44  and a second ID tag  46 , similar to those discussed above with respect to  FIG. 1 . 
     As noted above, the implantable medical device  10  ( FIG. 1 ) or the implantable medical device  36  ( FIG. 2 ) may generally represent any number of different implantable devices. For illustrative purposes, the implantable medical device will be described with respect to a leadless cardiac pacemaker. Leadless cardiac pacemakers are often implanted within the heart and move with the heart as the heart beats. When so provided, the leadless cardiac pacemaker may not have a well-defined or fixed implanted orientation, at least relative to an imager such as an x-ray machine located outside of the body. 
       FIG. 3  provides a perspective view of an illustrative leadless cardiac pacemaker  48  extending from a proximal end  50  to a distal end  52 . The leadless cardiac pacemaker  48  may be considered as including an energy storage section  54  and a circuit section  56 . As will be discussed, the energy storage section  54  may house an energy source, such as a battery, for powering circuitry within the circuit section  56 . While not illustrated, the leadless cardiac pacemaker  48  may include a fixation mechanism such as tines or a fixation helix. 
     As seen in  FIG. 3 , the illustrative leadless cardiac pacemaker  48  includes a proximal end feature  64  that is located at the proximal end  50 . In the example shown, the proximal end feature  64  is configured to permit grasping and removal of the leadless cardiac pacemaker  48  at some point during and/or subsequent to implantation. An electrode  66  is visible at the distal end  52  of the leadless cardiac pacemaker  48 .  FIG. 4  is an exploded view of the leadless cardiac pacemaker  48 , showing some of the internal features of the leadless cardiac pacemaker  48 , including various components that can be used to create or locate an ID tag. 
     Starting with the energy storage section  54 , the illustrative leadless cardiac pacemaker  48  includes a housing  68  that includes the aforementioned proximal end feature  64 . In some cases, as illustrated, an insulative coating  70  is disposed over at least a portion of the housing  68 . The insulative coating  70  may be formed of parylene, but this is not required. The next several components form part of a battery  72 . The illustrative battery  72  includes a battery liner  74 , an anode  76  and a cathode  78 . Several components pertain to the battery  72 , including a battery feedthrough  88 , a battery lid  90  and a battery pin  92 . It will be appreciated that the battery  72  includes additional components and materials that, for simplicity, are not illustrated. The illustrative leadless cardiac pacemaker  48  includes a first ID tag  58 , a second ID tag  60  and a third ID tag  62 , each tag reading “BSC140” as an illustrative but non-limiting example. It can be seen that the second ID tag  60  is axially and radially offset from the first ID tag  58 , and that the third ID tag  62  is axially and radially offset from the first ID tag  58  and the second ID tag  60 . In the illustrated embodiment, the ID tags  58 ,  60  and  62  are located in or on the energy storage section  54  of the leadless cardiac pacemaker  48 . 
     Moving to the circuit section  56 , the illustrative leadless cardiac pacemaker  48  includes the electrode  66  and a drug collar  82  that is disposed proximate the electrode  66 . An epoxy overmolding  84  sits under the drug collar  82 . A ferrule  86  sits beneath the epoxy overmolding  84 . A stacked printed circuit board  96  sits within a liner  98 . As will be illustrated in subsequent Figures, a number of these components can be used or modified to carry or otherwise provide radiopaque ID tags such as those discussed with respect to  FIGS. 1-3 .  FIGS. 5 through 13  provide illustrative but non-limiting examples of components that can be used or modified to carry or otherwise provide radiopaque ID tags. It will be appreciated that in these Figures, for simplicity, the ID tags are represented schematically and are intended to represent ID tags such as the first ID tag  22 , including the first radiopaque manufacturer code section  28 , and/or the second ID tag  24 , including the second radiopaque manufacturer code section  30 . While the ID tags in  FIGS. 5-13  are schematically illustrated as having a particular orientation, this is not intended to be limiting in any fashion. 
       FIG. 5  illustrates the housing  68 , schematically including a first ID tag  68   a  and a second ID tag  68   b . Each of the first ID tag  68   a  and the second ID tag  68   b  include a radiopaque manufacturer code section that identifies the manufacturer of the leadless cardiac pacemaker  48  during an imaging process. The first ID tag  68   a  and the second ID tag  68   b  may each be formed in any desired manner, including but not limited to etching, machining, sputtering, cutting or sintering. In some embodiments, the first ID tag  68   a  and/or the second ID tag  68   b  may include a non-radiopaque substrate or carrier, with radiopaque characters or symbols providing the radiopaque manufacturer code information. In some instances, the substrate or carrier forming the first ID tag  68   a  and/or the second ID tag  68   b  are radiopaque, and the characters or symbols providing the manufacturer code information are either non-radiopaque or are cut out of the substrate or carrier. In some instances, the first ID tag  68   a  and/or the second ID tag  68   b  may be formed by printing alphanumeric characters or other identifying symbols onto a surface of the housing  68  using a radiopaque ink. These are just some examples. In some cases first ID tag  68   a  and/or the second ID tag  68   b  may be placed between the housing  68  and the insulative coating  70  when the insulative coating  70  is provided. 
       FIG. 6  illustrates the battery liner  74 , schematically including a first ID tag  74   a  and a second ID tag  74   b . Each of the first ID tag  74   a  and the second ID tag  74   b  include radiopaque manufacturer code sections that identify the manufacturer of the leadless cardiac pacemaker  48 . The first ID tag  74   a  and the second ID tag  74   b  may each be formed in any desired manner, including but not limited to etching, machining, sputtering, cutting or sintering. In some embodiments, the first ID tag  74   a  and/or the second ID tag  74   b  may include a non-radiopaque substrate or carrier, with radiopaque characters or symbols providing the radiopaque manufacturer code information. In some instances, the substrate or carrier forming the first ID tag  74   a  and/or the second ID tag  74   b  are radiopaque, and the characters or symbols providing the manufacturer code information are either non-radiopaque or are cut out of the substrate or carrier. In some instances, the first ID tag  74   a  and/or the second ID tag  74   b  may be formed by printing alphanumeric characters or other identifying symbols onto a surface of the battery liner  74  using a radiopaque ink. 
       FIG. 7  illustrates the anode  76 , schematically including a first ID tag  76   a  and a second ID tag  76   b . Each of the first ID tag  76   a  and the second ID tag  76   b  include radiopaque manufacturer code sections that identify the manufacturer of the leadless cardiac pacemaker  48 . The first ID tag  76   a  and the second ID tag  76   b  may each be formed in any desired manner, including but not limited to etching, machining, sputtering, cutting or sintering. In some embodiments, the first ID tag  76   a  and/or the second ID tag  76   b  may include a non-radiopaque substrate or carrier, with radiopaque characters or symbols providing the radiopaque manufacturer code information. In some instances, the substrate or carrier forming the first ID tag  76   a  and/or the second ID tag  76   b  are radiopaque, and the characters or symbols providing the manufacturer code information are either non-radiopaque or are cut out of the substrate or carrier. In some instances, the first ID tag  76   a  and/or the second ID tag  76   b  may be formed by printing alphanumeric characters or other identifying symbols onto a surface of the housing  68  using a radiopaque ink. 
       FIG. 8  illustrates the cathode  78 , schematically including a first ID tag  78   a  and a second ID tag  78   b . Each of the first ID tag  78   a  and the second ID tag  78   b  include radiopaque manufacturer code sections that identify the manufacturer of the leadless cardiac pacemaker  48 . The first ID tag  78   a  and the second ID tag  78   b  may each be formed in any desired manner, including but not limited to etching, machining, sputtering, cutting or sintering. In some embodiments, the first ID tag  78   a  and/or the second ID tag  78   b  may include a non-radiopaque substrate or carrier, with radiopaque characters or symbols providing the radiopaque manufacturer code information. In some instances, the substrate or carrier forming the first ID tag  78   a  and/or the second ID tag  78   b  are radiopaque, and the characters or symbols providing the manufacturer code information are either non-radiopaque or are cut out of the substrate or carrier. In some instances, the first ID tag  78   a  and/or the second ID tag  78   b  may be formed by printing alphanumeric characters or other identifying symbols onto a surface of the cathode  78  using a radiopaque ink. 
       FIG. 9  illustrates the battery pin  92 , schematically including a first ID tag  92   a  and a second ID tag  92   b . Each of the first ID tag  92   a  and the second ID tag  92   b  include radiopaque manufacturer code sections that identify the manufacturer of the leadless cardiac pacemaker  48 . The first ID tag  92   a  and the second ID tag  92   b  may each be formed in any desired manner, including but not limited to etching, machining, sputtering, cutting, or sintering. In some embodiments, the first ID tag  92   a  and/or the second ID tag  92   b  may include a non-radiopaque substrate or carrier, with radiopaque characters or symbols providing the radiopaque manufacturer code information. In some instances, the substrate or carrier forming the first ID tag  92   a  and/or the second ID tag  92   b  are radiopaque, and the characters or symbols providing the manufacturer code information are either non-radiopaque or are cut out of the substrate or carrier. In some instances, the first ID tag  92   a  and/or the second ID tag  92   b  may be formed by printing alphanumeric characters or other identifying symbols onto a surface of the battery pin  92  using a radiopaque ink. 
       FIG. 10  illustrates the epoxy overmolding  84 , schematically including a first ID tag  84   a  and a second ID tag  84   b . Each of the first ID tag  84   a  and the second ID tag  84   b  include radiopaque manufacturer code sections that identify the manufacturer of the leadless cardiac pacemaker  48 . The first ID tag  84   a  and the second ID tag  84   b  may each be formed in any desired manner, including but not limited to etching, machining, sputtering, cutting or sintering. In some embodiments, the first ID tag  84   a  and/or the second ID tag  84   b  may include a non-radiopaque substrate or carrier, with radiopaque characters or symbols providing the radiopaque manufacturer code information. In some instances, the substrate or carrier forming the first ID tag  84   a  and/or the second ID tag  84   b  are radiopaque, and the characters or symbols providing the manufacturer code information are either non-radiopaque or are cut out of the substrate or carrier. In some instances, the first ID tag  84   a  and/or the second ID tag  84   b  may be formed by printing alphanumeric characters or other identifying symbols onto a surface of the epoxy overmolding  84  using a radiopaque ink. 
       FIG. 11  illustrates the drug carrier  82 , schematically including a first ID tag  82   a  and a second ID tag  82   b . Each of the first ID tag  82   a  and the second ID tag  82   b  include radiopaque manufacturer code sections that identify the manufacturer of the leadless cardiac pacemaker  48 . The first ID tag  82   a  and the second ID tag  82   b  may each be formed in any desired manner, including but not limited to etching, machining, sputtering, cutting or sintering. In some embodiments, the first ID tag  82   a  and/or the second ID tag  82   b  may include a non-radiopaque substrate or carrier, with radiopaque characters or symbols providing the radiopaque manufacturer code information. In some instances, the substrate or carrier forming the first ID tag  82   a  and/or the second ID tag  82   b  are radiopaque, and the characters or symbols providing the manufacturer code information are either non-radiopaque or are cut out of the substrate or carrier. In some instances, the first ID tag  82   a  and/or the second ID tag  82   b  may be formed by printing alphanumeric characters or other identifying symbols onto a surface of the drug collar  82  using a radiopaque ink. 
       FIG. 12  illustrates the desiccant  94 , schematically including a first ID tag  94   a  and a second ID tag  94   b . Each of the first ID tag  94   a  and the second ID tag  94   b  include radiopaque manufacturer code sections that identify the manufacturer of the leadless cardiac pacemaker  48 . The first ID tag  94   a  and the second ID tag  94   b  may each be formed in any desired manner, including but not limited to etching, machining, sputtering, cutting or sintering. In some embodiments, the first ID tag  94   a  and/or the second ID tag  94   b  may include a non-radiopaque substrate or carrier, with radiopaque characters or symbols providing the radiopaque manufacturer code information. In some instances, the substrate or carrier forming the first ID tag  94   a  and/or the second ID tag  94   b  are radiopaque, and the characters or symbols providing the manufacturer code information are either non-radiopaque or are cut out of the substrate or carrier. In some instances, the first ID tag  94   a  and/or the second ID tag  94   b  may be formed by printing alphanumeric characters or other identifying symbols onto a surface of the desiccant  94  using a radiopaque ink. 
       FIG. 13  illustrates the liner  98 , schematically including a first ID tag  98   a  and a second ID tag  98   b . Each of the first ID tag  98   a  and the second ID tag  98   b  include radiopaque manufacturer code sections that identify the manufacturer of the leadless cardiac pacemaker  48 . The first ID tag  98   a  and the second ID tag  98   b  may each be formed in any desired manner, including but not limited to etching, machining, sputtering, cutting or sintering. In some embodiments, the first ID tag  98   a  and/or the second ID tag  98   b  may include a non-radiopaque substrate or carrier, with radiopaque characters or symbols providing the radiopaque manufacturer code information. In some instances, the substrate or carrier forming the first ID tag  98   a  and/or the second ID tag  98   b  are radiopaque, and the characters or symbols providing the manufacturer code information are either non-radiopaque or are cut out of the substrate or carrier. In some instances, the first ID tag  98   a  and/or the second ID tag  98   b  may be formed by printing alphanumeric characters or other identifying symbols onto a surface of the liner  98  using a radiopaque ink. 
     While each of  FIGS. 5-13  illustrate each component part with both a first ID tag and a second ID tag, this is not required. In some cases, some component parts will not have any ID tags. In some cases, a particular component part may have one, two or more ID tags. In some cases, a first component part may have a first ID tag and a second component part may have a second ID tag. These are just examples. 
       FIG. 14  is a conceptual drawing of an exemplary leadless cardiac pacemaker  100  that may be implanted into a patient and may operate to sense physiological signals and parameters and deliver one or more types of electrical stimulation therapy to tissues of the patient. Example electrical stimulation therapy includes anti-tachycardia pacing (ATP) therapy, cardiac resynchronization therapy (CRT), bradycardia therapy, various types of pacing therapy including rate responsive pacing therapy, and/or the like. As can be seen in  FIG. 14 , LCP  100  may be a compact device with all components housed within LCP  100  or directly on housing  120 . LCP  100  may include communication module  102 , pulse generator module  104 , electrical sensing module  106 , mechanical sensing module  108 , processing module  110 , energy storage module  112 , and electrodes  114 . 
     As depicted in  FIG. 14 , LCP  100  may include electrodes  114 , which can be secured relative to housing  120  but exposed to the tissue and/or blood surrounding LCP  100 . Electrodes  114  may generally conduct electrical signals to and from LCP  100  and the surrounding tissue and/or blood. Such electrical signals can include communication pulses, electrical stimulation pulses, and intrinsic cardiac electrical signals. Intrinsic cardiac electrical signals may consist of the electrical signals generated by the heart and may be represented by an electrocardiogram (ECG). Electrodes  114  can be made up of one or more biocompatible conductive materials such as various metals or alloys that are known to be safe for implantation within a human body. In some instances, electrodes  114  may be generally disposed on either end of LCP  100  and may be in electrical communication with one or more of modules  102 ,  104 ,  106 ,  108 , and  110 . In examples where electrodes  114  are secured directly to housing  120 , electrodes  114  may have an insulative portion that electrically isolates electrodes  114  from adjacent electrodes, housing  120 , and/or other portions of LCP  100 . Some or all of electrodes  114  may be spaced from housing  120  and connected to housing  120  and/or other components of LCP  100  through connecting wires. In such embodiments, the electrodes  114  may be placed on a on a tail that extends from the housing  120 . As shown in  FIG. 14 , in some examples, LCP  100  may additionally include electrodes  114 ′. Electrodes  114 ′ are similar to electrodes  114  except that electrodes  114 ′ are disposed on the sides of LCP  100  and increase the number of electrodes by which LCP  100  may deliver communication pulses and electrical stimulation pulses and/or sense for intrinsic cardiac electrical signals, communication pulses, and/or electrical stimulation pulses. 
     Electrodes  114  and/or  114 ′ may have any of a variety of sizes and/or shapes, and may be spaced at any of a variety of distances. For example, electrodes  114  may have a diameter of two to twenty millimeters (mm). However, in other examples, electrodes  114  and/or  114 ′ may have a diameter of two, three, five, seven millimeters (mm), or any other suitable diameter, dimension and shape. Example lengths for electrodes  114  and/or  114 ′ include a length of zero, one, three, five, ten millimeters (mm), or any other suitable length. As used herein, the length is a dimension of electrodes  114  and/or  114 ′ that extends outward from housing  120 . Additionally, at least some of electrodes  114  and/or  114 ′ may be spaced from one another by a distance of twenty, thirty, forty, fifty millimeters (mm), or any other suitable distance. The electrodes  114  and/or  114 ′ of a single device may have different sizes with respect to each other, and the spacing of the electrodes on the device may not be uniform. 
     Communication module  102  may be electrically coupled to electrodes  114  and/or  114 ′ and configured to deliver communication pulses to tissues of the patient for communicating with other devices such as sensors, programmers, other medical devices, and the like. Communication pulses, as used herein, may be any modulated signal that conveys information to another device, either by itself or in conjunction with one or more other modulated signals. In some examples, communication pulses are limited to only including sub-threshold signals which convey information. Other devices that communication module  102  may be configured to communicate with may be located either external or internal to the patient&#39;s body. Communication module  102  may additionally be configured to sense for communication pulses delivered by the other devices, which are located externally to LCP  100 . Irrespective of the location, LCP and the other devices may communicate with each other via communication module  102  to accomplish one or more desired functions. Some example functions include storing communicated data, using communicated data for determining occurrences of arrhythmias, coordinating delivery of electrical stimulation therapy, and/or other functions. 
     LCP  100  and the other devices may use the delivered communication pulses to communicate raw information, processed information, messages, and/or other data. Raw information may include information such as sensed electrical signals (e.g. a sensed ECG), signals gathered from coupled sensors, and the like. In some examples, the raw information may include signals that have been filtered using one or more signal processing techniques. Processed information may include any information that has been determined by LCP  100 . For example, processed information may include a determined heart rate, timings of determined heartbeats, timings of other determined events, determinations of threshold crossings, expirations of monitored time periods, and determined parameters such as activity parameters, blood-oxygen parameters, blood pressure parameters, heart sound parameters, and the like. Messages may include instructions directing another device to take action, notifications of imminent actions of the sending device, requests for reading from the receiving device or writing data to the receiving device. 
     In at least some examples, communication module  102  (or LCP  100 ) may further include switching circuitry to selectively connect one or more of electrodes  114  and/or  114 ′ to communication module  102  in order to select via which electrodes  114  and/or  114 ′ communication module  102  delivers the communication pulses. Additionally, communication module  102  may be configured to use one or more methods for communicating with other devices. For example, communication module  102  may communicate via conducted signals, radiofrequency (RF) signals, optical signals, acoustic signals, inductive coupling, and/or any other signals or methods suitable for communication. 
     Pulse generator module  104  of LCP  100  may also be electrically connected to one or more of electrodes  114  and/or  114 ′. Pulse generator module  104  may be configured to generate electrical stimulation pulses and deliver the electrical stimulation pulses to tissues of a patient via electrodes  114  and/or  114 ′ electrodes in order to effectuate one or more electrical stimulation therapies. Electrical stimulation pulses as used herein are meant to encompass any electrical signals that may be delivered to tissue of a patient for purposes of treatment of any type of disease or abnormality. When used to treat heart diseases or abnormalities, the electrical stimulation pulses may generally be configured so as to capture the heart of the patient—cause the heart to contract in response to the delivered electrical stimulation pulse. In at least examples where pulse generator  104  is configured to generate specific types of electrical stimulation pulses termed defibrillation/cardioversion pulses, pulse generator module  104  may include one or more capacitor elements. 
     Pulse generator module  104  may include capability to modify the electrical stimulation pulses, such as by adjusting a pulse width or amplitude of the electrical stimulation pulses, in order to ensure that the delivered electrical stimulation pulses consistently capture the heart. Pulse generator module  104  may use energy stored in energy storage module  112  to generate the electrical stimulation pulses. In at least some examples, pulse generator module  104  (or LCP  100 ) may further include switching circuitry to selectively connect one or more of electrodes  114  and/or  114 ′ to pulse generator module  104  in order to select via which electrodes  114  and/or  114 ′ pulse generator  104  delivers the electrical stimulation pulses. 
     In some examples, LCP  100  may include electrical sensing module  106  and mechanical sensing module  108 . Electrical sensing module  106  may be configured to sense intrinsic cardiac electrical signals conducted from electrodes  114  and/or  114 ′ to electrical sensing module  106 . For example, electrical sensing module  106  may be electrically connected to one or more electrodes  114  and/or  114 ′ and electrical sensing module  106  may be configured to receive cardiac electrical signals conducted through electrodes  114  and/or  114 ′. In some examples, the cardiac electrical signals may represent local information from the chamber in which LCP  100  is implanted. For instance, if LCP  100  is implanted within a ventricle of the heart, cardiac electrical signals sensed by LCP  100  through electrodes  114  and/or  114 ′ may represent ventricular cardiac electrical signals. Mechanical sensing module  108  may include, or be electrically connected to, various sensors, such as accelerometers, blood pressure sensors, heart sound sensors, blood-oxygen sensors, and/or other sensors which measure one or more physiological parameters of the heart and/or patient. Mechanical sensing module  108  may gather signals from the sensors indicative of the various physiological parameters. Both electrical sensing module  106  and mechanical sensing module  108  may be further connected to processing module  110  and may provide signals representative of the sensed cardiac electrical signals and/or physiological signals to processing module  110 . Although described with respect to  FIG. 1  as separate sensing modules, in some examples, electrical sensing module  106  and mechanical sensing module  108  may be combined into a single module. 
     Processing module  110  may be configured to control the operation of LCP  100 . For example, processing module  110  may be configured to receive cardiac electrical signals from electrical sensing module  106  and/or physiological signals from mechanical sensing module  108 . Based on the received signals, processing module  110  may determine occurrences and types of arrhythmias. Processing module  110  may further receive information from communication module  102 . In some examples, processing module  110  may additionally use such received information to determine occurrences and types of arrhythmias. However, in other examples, LCP  100  may use the received information instead of the signals received from electrical sensing module  106  and/or mechanical sensing module  108 —for instance if the received information is more accurate than the signals received from electrical sensing module  106  and/or mechanical sensing module  108  or if electrical sensing module  106  and/or mechanical sensing module  108  have been disabled or omitted from LCP  100 . 
     Based on any determined arrhythmias, processing module  110  may then control pulse generator module  104  to generate electrical stimulation pulses in accordance with one or more electrical stimulation therapies to treat the determined arrhythmias. For example, processing module  110  may control pulse generator module  104  to generate pacing pulses with varying parameters and in different sequences to effectuate one or more electrical stimulation therapies. In controlling pulse generator module  104  to deliver bradycardia pacing therapy, processing module  110  may control pulse generator module  104  to deliver pacing pulses designed to capture the heart of the patient at a regular interval to prevent the heart of a patient from falling below a predetermined threshold. For ATP therapy, processing module  110  may control pulse generator module  104  to deliver pacing pulses at a rate faster than an intrinsic heart rate of a patient in attempt to force the heart to beat in response to the delivered pacing pulses rather than in response to intrinsic cardiac electrical signals. Processing module  110  may then control pulse generator module  104  to reduce the rate of delivered pacing pulses down to a safe level. In CRT, processing module  110  may control pulse generator module  104  to deliver pacing pulses in coordination with another device to cause the heart to contract more efficiently. Additionally, in cases where pulse generator module  104  is capable of generating defibrillation and/or cardioversion pulses for defibrillation/cardioversion therapy, processing module  110  may control pulse generator module  104  to generate such defibrillation and/or cardioversion pulses. In other examples, processing module  110  may control pulse generator module  104  to generate electrical stimulation pulses to provide electrical stimulation therapies different than those described herein to treat one or more detected cardiac arrhythmias. 
     Aside from controlling pulse generator module  104  to generate different types of electrical stimulation pulses and in different sequences, in some examples, processing module  110  may also control pulse generator module  104  to generate the various electrical stimulation pulses with varying pulse parameters. For example, each electrical stimulation pulse may have a pulse width and a pulse amplitude. Processing module  110  may control pulse generator module  104  to generate the various electrical stimulation pulses with specific pulse widths and pulse amplitudes. For example, processing module  110  may cause pulse generator module  104  to adjust the pulse width and/or the pulse amplitude of electrical stimulation pulses if the electrical stimulation pulses are not effectively capturing the heart. Such control of the specific parameters of the various electrical stimulation pulses may ensure that LCP  100  is able to provide effective delivery of electrical stimulation therapy. 
     In some examples, processing module  110  may further control communication module  102  to send information to other devices. For example, processing module  110  may control communication module  102  to generate one or more communication pulses for communicating with other devices of a system of devices. For instance, processing module  110  may control communication module  102  to generate communication pulses in particular sequences, where the specific sequences convey different data to other devices. Communication module  102  may also conduct any received communication signals to processing module  110  for potential action by processing module  110 . 
     In further examples, processing module  110  may additionally control switching circuitry by which communication module  102  and pulse generator module  104  deliver communication pulses and electrical stimulation pulses to tissue of the patient. As described above, both communication module  102  and pulse generator module  104  may include circuitry for connecting one or more electrodes  114  and/ 114 ′ to communication module  102  and pulse generator module  104  so those modules may deliver the communication pulses and electrical stimulation pulses to tissue of the patient. The specific combination of one or more electrodes by which communication module  102  and pulse generator module  104  deliver communication pulses and electrical stimulation pulses influence the reception of communication pulses and/or the effectiveness of electrical stimulation pulses. Although it was described that each of communication module  102  and pulse generator module  104  may include switching circuitry, in some examples LCP  100  may have a single switching module connected to all of communication module  102 , pulse generator module  104 , and electrodes  114  and/or  114 ′. In such examples, processing module  110  may control the single switching module to connect modules  102 / 104  and electrodes  114 / 114 ′. 
     In still additional examples, processing module  110  may control pulse generator module  104  to generate the communication pulses for communicating with external devices. In such examples, communication module  102  may not include the capability to generate communication pulses. In some even additional examples, electrical sensing module  106  may further include the capability to sense communication pulses. In such examples, electrical sensing module  106  may communicate any received communication pulses to processing module  110 . In such examples, LCP  100  may not include communication module  102 , as the functions of communication module  102  are subsumed within pulse generator module  104  and electrical sensing module  106 . However, in such examples, LCP  100  may not be able to simultaneously generate both communication pulses and electrical stimulation pulses. 
     In some examples, processing module  110  may include a pre-programmed chip, such as a very-large-scale integration (VLSI) chip or an application specific integrated circuit (ASIC). In such embodiments, the chip may be pre-programmed with control logic in order to control the operation of LCP  100 . By using a pre-programmed chip, processing module  110  may use less power than other programmable circuits while able to maintain basic functionality, thereby increasing the battery life of LCP  100 . In other examples, processing module  110  may include a programmable microprocessor or the like. Such a programmable microprocessor may allow a user to adjust the control logic of LCP  100  after manufacture, thereby allowing for greater flexibility of LCP  100  than when using a pre-programmed chip. 
     Processing module  110 , in additional examples, may further include a memory circuit and processing module  110  may store information on and read information from the memory circuit. In other examples, LCP  100  may include a separate memory circuit (not shown) that is in communication with processing module  110 , such that processing module  110  may read and write information to and from the separate memory circuit. The memory circuit, whether part of processing module  110  or separate from processing module  110  may have address lengths of, for example, eight bits. However, in other examples, the memory circuit may have address lengths of sixteen, thirty-two, or sixty-four bits, or any other bit length that is suitable. Additionally, the memory circuit may be volatile memory, non-volatile memory, or a combination of both volatile memory and non-volatile memory. 
     Energy storage module  112  may provide a power source to LCP  100  for its operations. In some examples, energy storage module  112  may be a non-rechargeable lithium-based battery. In other examples, the non-rechargeable battery may be made from other suitable materials known in the art. Because LCP  100  is an implantable device, access to LCP  100  may be limited. In such circumstances, it is necessary to have sufficient energy capacity to deliver therapy over an extended period of treatment such as days, weeks, months, or years. In some examples, energy storage module  112  may a rechargeable battery in order to facilitate increasing the useable lifespan of LCP  100 . In still other examples, energy storage module  112  may be other types of energy storage devices such as capacitors. 
     To implant LCP  100  inside a patient&#39;s body, an operator (e.g., a physician, clinician, etc.), may fix LCP  100  to the cardiac tissue of the patient&#39;s heart. To facilitate fixation, LCP  100  may include one or more anchors  116 . Anchor  116  may include any number of fixation or anchoring mechanisms. For example, anchor  116  may include one or more pins, staples, threads, screws, helix, tines, and/or the like. In some examples, although not shown, anchor  116  may include threads on its external surface that may run along at least a partial length of anchor  116 . The threads may provide friction between the cardiac tissue and the anchor to help fix anchor  116  within the cardiac tissue. In other examples, anchor  116  may include other structures such as barbs, spikes, or the like to facilitate engagement with the surrounding cardiac tissue. 
     The modules shown in  FIG. 14  may be manifested in circuitry that is disposed within the circuit section  56  ( FIG. 3 ).  FIG. 15  provides an illustrative but non-limiting example of the stacked printed circuit board  96  ( FIG. 4 ). In the example of  FIG. 15 , example circuit  400  has three separate island sections including first island section  401 , second island section  403 , and third island section  405 . Island sections  401 ,  403 , and  405  are shown separated by first ribbon section  406  and second ribbon section  407 . Each of island sections  401 ,  403 , and  405  may include first major opposing surfaces  412 A,  414 A, and  416 A and second major opposing surfaces  412 B,  414 B, and  416 B. Second island section  403  and third island section  405  may also include feedthroughs (not visible) that may be electrically connected to electrodes  114 / 114 ′, an electrical common reference, and/or an energy storage device. 
     In some examples, each island section may be circular in shape, but this is not required. In some cases, each island section has a diameter that is less than an inner diameter of a cross section of an implantable medical device housing (such as LCP  100 ) so that the circuit  400  may fit once folded into a stacked configuration. Example diameters range from 3.8 to 12.7 millimeters (mm). The island sections may be triangular, square, ovoid or any other desired shape. In some cases, the flexible ribbon sections may range from 3.8 to 12.7 mm. 
     Processing module  410  and circuit elements  408 A-H may be examples of circuit elements that may implement the functions of communication module  102 , pulse generator module  104 , electrical sensing module  106 , mechanical sensing module  108 , and/or processing module  110 . Processing module  410  may include any of the circuit elements or components described with respect to processing module  110 , such as a pre-programmed logic chip or a programmable microprocessor. Circuit elements  408 A-H may represent capacitors, resistors, diodes, ASICS, and/or any other suitable circuit elements or components. 
     In some examples, at least one island section may have one or more components affixed to both major opposing surfaces of that island section. In the specific example of  FIG. 15 , island section  401  includes processing module  410  affixed to first major opposing surface  412 A and circuit elements  408 A-B (shown in dashed) on second major opposing surface  412 B. In examples where island sections  401 ,  403 , and  405  include PCBs, the PCBs may include conductive traces that electrically connect processing module  410  and circuit elements  408 A-H to produce the desired circuit functionality. Alternatively, in examples where circuit  400  includes one common substrate, any processing module  410  and/or circuit element  408 A-H connected to an island section may be connected to one or more internal conductive trace layers, thereby electrically connecting the processing module  410  and/or the various circuit elements  408 A-H to produce the desired circuit functionality. 
     Ribbon sections  406 ,  407  may include traces, such as trace  422  in first ribbon section  406  and trace  423  in second ribbon section  407 . Traces  422 ,  423  may be conductive and thereby electrically connect certain components on island sections  401 ,  403 , and  405 . First and second ribbon sections  406 ,  407  may be relatively more flexible than island sections  401 ,  403 , and  405 . For example, first and second ribbon sections  406 ,  407  may be made from a flexible substrate, such as a polymer, with traces  422 ,  423  embedded within the flexible substrate while island sections  401 ,  403 , and  405  include more rigid PCBs. Alternatively, where island sections  401 ,  403 , and  405  and first and second ribbon sections  406 ,  407  share a common substrate, first and second ribbon sections  406 ,  407  may be relatively thinner than island sections  401 ,  403 , and  405 . 
     Additionally, in at least some examples, first ribbon section  406  and second ribbon section  407  may have differing lengths. As depicted in  FIG. 15 , first ribbon section  406  has a shorter length than second ribbon section  407 , however, in other examples, the lengths may be reversed and, of course, the lengths may be the same. Island sections  401 ,  403 , and  405  are stacked with first major opposing surfaces  412 A and  414 A of island sections  401  and  403  facing each other and with second major opposing surface of island section  401  and first major opposing surface  416 A facing each other, thereby creating spaces  431  and  432  between island sections  405 ,  401  and island sections  401 ,  403 , respectively. 
     In some embodiments, the island sections  401 ,  402 ,  403  may include rigid printed circuit boards, with metal or other traces electrically connecting each of the components on each of the island sections  401 ,  402 ,  403 . Ribbon sections  406 ,  407  may include a flexible substrate, such as a polymer including a polyimide. Traces may be embedded within the ribbon sections  406 ,  407  to provide electrical communication therethrough. In some cases, a common substrate may instead extend through the island sections  401 ,  402 ,  403  and through the ribbon sections  406 ,  407 . The island sections  401 ,  402 ,  403  may include a multi-layered substrate that includes alternating conductive substrates and non-conductive substrates while the ribbon sections  406 ,  407  are thinner and thus more flexible. In at least some examples, the conductive substrate may be metal, or other suitable conductive material, and the non-conductive substrate may be a type of polymer, such as a polyamide or other suitable non-conductive material. Further details regarding the construction of the circuit  400  may be found in U.S. Provisional Application No. 62/086,015 filed Dec. 1, 2014, which application is incorporated by reference herein in its entirety. 
     In some examples, a filler material may be disposed within spaces  431  and  432  in order isolate processing module  410  and circuit elements  408 A-H disposed on different island sections. In at least some examples, the filler material may be formed such that when the filler material is disposed within spaces  431  and/or  432 , the filler material folds around the processing module  410  and/or circuit elements  408 A-H to isolate even the components on the same island section. In some examples, the isolation that the filler material provides may be electrical isolation. For instance, the filler material may prevent the components on islands  401 ,  403 , and/or  405  from contacting each other and causing a short circuit. In other examples, the filler material may instead, or additionally, provide mechanical isolation between the components of islands  401 ,  403 , and/or  405 . For instance, the device housing islands  401 ,  403 , and/or  405  may be subjected to motion, and the filler material may prevent the components of islands  401 ,  403 , and/or  405  from striking each other and causing damage. In at least some examples, the filler material may be a desiccant. Some example filler materials include silicone or other inert compounds. 
       FIG. 16  provides a genericized view of a circuit such as circuit  400  ( FIG. 15 ), but provides additional details regarding the possible inclusion of one or more ID tags. In  FIG. 15 , a circuit  500  includes a first island section  502  and a second island section  504 , operably coupled together via a flexible ribbon section  506 . While two island sections  502 ,  504  are illustrated, it will be appreciated that in other embodiments the circuit  500  may include only a single island section or may include three or more island sections. In some embodiments, an ID tag  508  may be secured relative to the flexible ribbon section  506 . The ID tag  508  may be printed directly onto the flexible ribbon section  506  using radiopaque ink, for example. In some instances, the ID tag  508  may be separately formed on a substrate or carrier that is subsequently attached to the flexible ribbon section  506 . While a single ID tag  508  is illustrated on the flexible ribbon section  506 , in some instances there may be multiple ID tags, or a single ID tag  508  may include manufacture identification information portrayed twice, once in mirror fashion. 
     In some embodiments, the island sections  502 ,  504  may include one or more ID tags. For example, in some cases, an ID tag  510  may be printed or otherwise formed on an outer surface of an island section such as the island section  502 . In some cases, an island section such as the island section  504  may include several layers  504   a  and  504   b , for example, and an ID tag  512  may be disposed between the layers  504   a  and  504   b . In some cases, an ID tag may be disposed on the back of the ASIC and/or adhesively secured to any of the electronic components present in the circuit  400  ( FIG. 15 ). 
       FIG. 17A  is a view of an illustrative leadless cardiac pacemaker  48   a , including a proximal end feature  64   a  that enables the leadless cardiac pacemaker  48   a  to be grasped during initial delivery and deployment and/or during subsequent removal. In some cases, as illustrated, an ID tag  65  may be crimped or otherwise secured to the proximal end feature  64   a  in order to provide identifying information during an imaging process such as x-ray. While the ID tag  65  is shown schematically, it will be appreciated that the ID tag  65  may include a radiopaque manufacturer code section, and may optionally also include a mirror image thereof. In some cases, the ID tag  65  may be inside or embedded in the proximal end feature  64   a.    
       FIG. 17B  is a view of an illustrative leadless cardiac pacemaker  48   b , including an ID tag  65   a  that is secured to the leadless cardiac pacemaker  48   b  via one or more (two are illustrated) cables  65   b , or other flexible or rigid attachment mechanisms. While the ID tag  65   a  is shown schematically, it will be appreciated that the ID tag  65   a  may include a radiopaque manufacturer code section, and may optionally also include a mirror image thereof. In some cases, the leadless cardiac pacemaker  48   b  may be retrieved by grabbing the ID tag  65   a  with a snare or similar tool. 
       FIG. 18  is a schematic view of a chevron  200  that may be used within an implantable medical device such as the implantable medical device  10  in order to provide an indication during delivery as to whether the implantable medical device  10  is twisting or otherwise moving/rotating. The illustrative chevron  200  is formed of a radiopaque material. In some instances, as illustrated, a first ID tag  202   a  and a second ID tag  202   b  may be formed by cutting, etching or otherwise removing radiopaque material to form characters  204   a  and  204   b . When so provided, the characters  204   a  and  204   b  will show up in an x-ray as relatively darker than the rest of the radiopaque chevron  200 . 
     In some embodiments, a radiopaque ID tag may be formed by first etching alphanumeric characters or other symbols into a substrate, then filling the etching with a radiopaque material.  FIGS. 19 and 20  illustrate an embodiment in which characters  208  are outlined on a substrate  206  by etching out the shape of the characters  208 . A radiopaque material  210  is placed within the etched shapes to provide radiopaque characters. In some instances, a platinum wire may be used to provide the radiopaque material. 
       FIG. 21  is a schematic cutaway view of a leadless pacemaker  300 , including a circuit section  302  and an energy storage section  304 . A printed circuit board  306  is disposed within the circuit section  302  and in some instances may be a planar circuit board that is axially aligned within the circuit section  302 . One or more (one is illustrated) ID tags  308  may be disposed on the printed circuit board  306  and may include radiopaque characters  310 . The ID tag(s)  308  may be formed of any desired materials and using any particular techniques as described herein. An energy storage device  312  may be disposed within the energy storage section  304  and provides power to the printed circuit board  306 . 
     The previous Figures illustrate various parts of an implantable medical device, such as a leadless pacemaker, that include one or more radiopaque ID tags that are secured relative to the part, or formed within the part.  FIGS. 22-24  provide illustrative but non-limiting examples of the construction of a radiopaque ID tag. 
       FIG. 22  is a schematic cross-sectional side view of an illustrative radiopaque ID tag  400 . The illustrative ID tag  400  is formed on a substrate  402 . The substrate  402  may, for example, be alumina, silicon or glass. In the example shown, a polyimide layer  404  is formed on the substrate  402  in any suitable manner, such as spin coating. In some cases, the polyimide layer  404  may have a thickness that is about 7 to 10 microns, but this is not required. A radiopaque layer  406  is formed on the polyimide layer  404 . The radiopaque layer  406  may be formed of any desired material, but in some cases may be tantalum, platinum or gold. In some cases, the radiopaque layer  406  may be formed via sputtering, gravure printing, screen printing, ink jet printing, vacuum evaporation, electron-assisted evaporation (EBPVD), thermal vapor evaporation, atomic layer deposition, and/or any other suitable process or technique. In some cases, the radiopaque layer  406  may have a thickness sufficient to provide adequate visibility during imaging processes such as x-ray. In some cases, the thickness may vary depending upon the specific material used for the radiopaque layer  406 . Once the radiopaque layer  406  has been formed, the radiopaque layer  406  may be patterned and etched to form an identifiable character or characters that are visible under x-ray. If the radiopaque layer  406  is printed or otherwise patterned when formed, this patterning step may not be needed. In some cases, a polyimide layer  408  may be formed on top of the radiopaque layer  406 . The radiopaque ID tag  400  may be removed from the substrate  402 . 
       FIG. 23  is a schematic cross-sectional side view of another illustrative radiopaque ID tag  400 . The ID tag  410  is formed on a substrate  402 . The substrate  402  may, for example, be alumina, silicon or glass. In the example shown, a polyimide layer  404  is formed on the substrate  402  in any suitable manner, such as spin coating. In some cases, the polyimide layer  404  may have a thickness that is about 7 to 10 microns, but this is not required. An adhesion layer  412  may be applied over the polyimide layer  404 . In some cases, the adhesion layer  412  is titanium, but this is not required. The adhesion layer  412  may be used to help improve adhesion between the radiopaque layer  414  and the polyimide layer  404 . In the example shown, a radiopaque layer  414  is sputtered or otherwise applied over the adhesion layer  412 . In some cases, the radiopaque layer  414  is tantalum, platinum or gold. However, it is contemplated that the radiopaque layer  414  may be any suitable radiopaque material. Once the radiopaque layer  414  has been formed, the radiopaque layer  414  may be patterned and etched to form an identifiable character or characters that are visible under x-ray. If the radiopaque layer  414  is printed or otherwise patterned when formed, this patterning step may not be needed. In some cases, a polyimide layer  408  may be formed on top of the radiopaque layer  414 . When desired, a second adhesion layer (not shown) may be provided over the radiopaque layer  414  and under the polyimide layer  408  to help improve adhesion between the radiopaque layer  414  and the polyimide layer  408 . The radiopaque ID tag  410  may be removed from the substrate  402 . While polyimide is used for layers  404 ,  408  in the illustrative radiopaque ID tag  400  and  410 , it is contemplated that any suitable material may be used for these layers. Moreover, it is contemplated that additional layers may be provided if desired. 
       FIG. 24  is a schematic top view of the illustrative radiopaque ID tag  400  (or  410 , as it will be appreciated that these tags would appear the same from the top). The illustrative radiopaque ID tag  400 ,  410  has a top surface  416 . A set of characters  418  are visible via x-ray when viewed from the top surface  416 . As illustrated, the set of characters  418  reads “XX1”, but this is merely illustrative and is not intended to be limiting in any fashion. In some cases, the set of characters  418  may include alphanumeric characters, a one or two dimensional bar code, and/or any other suitable marking as desired. In some cases, the set of characters  418  may identify a manufacturer and/or model number of an implanted device or component. 
       FIG. 25  is a perspective view of an illustrative battery cathode  476  in accordance with an embodiment of the present disclosure. Unlike the cathode  76  ( FIG. 7 ), which bears a first ID tag  76   a  and/or a second ID tag  76   b  that are secured to the cathode  76 , or formed on or in a surface of the cathode  76 , the cathode  476  is formed of a radiopaque material that has been twisted into several distinct planes. In the example shown in  FIG. 25 , the battery cathode  476  includes a first plane  478 , a second plane  480  and a third plane  482 , each rotated about 60 degrees from the adjacent plane, although the relative rotational position of each plane may be different depending on the application. In some embodiments, the battery cathode  476  may have just two planes, or could have four or more planes. 
     In the example shown, each plane  478 ,  480  and  482  includes one or more characters. As illustrated, the first plane  478  includes several characters  484 , the second plane  480  includes several characters  486  and the third plane  482  includes several characters  488 . As shown, each of the characters  484 ,  486  and  488  spell out “BSC 140”, but this is merely illustrative. In some cases, the characters  484 ,  486  and  488  may all be different. In the example shown, the characters  484 ,  486  and  488  are each formed by cutting, etching or otherwise removing radiopaque material to form the characters  484 ,  486  and  488 . In some cases, the characters  484 ,  486  and  488  are apertures that extends all the way through the battery cathode  476  as shown. In other cases, the characters  484 ,  486  and  488  are depressions that extends only part way through the battery cathode  476 . In either cases, the characters  484 ,  486  and  488  may show up in an x-ray as relatively darker than the rest of the battery cathode  476 . It will be appreciated that by creating each plane in a different orientation, it may be easier to read in an x-ray, regardless of the position of the device that includes the battery cathode  476 . 
     In some instances, the battery cathode  476  may be made from a solid piece of radiopaque material. In other cases, the battery cathode  476  may be made from a non-radiopaque substrate that is coated with a radiopaque material. When the battery cathode  476  is made from a non-radiopaque substrate that is coated with a radiopaque material, the characters  484 ,  486  and  488  may be formed by etching through the radiopaque material to form an image of the characters  484 ,  486  and  488 , or a reverse image of the characters  484 ,  486  and  488 , under x-ray radiation, as desired. 
     While  FIG. 25  shows an illustrative battery cathode  476 , it is contemplated that a similar structure may function as a battery anode. In some cases, a similar structure may not form part of a battery at all. In some cases, a similar structure may serve as a conductor, a support structure or some other function within an implantable medical device. In some case, a similar structure may not perform any other function other than a radiopaque ID tag for an implantable medical device. 
     Those skilled in the art will recognize that the present disclosure may be manifested in a variety of forms other than the specific examples described and contemplated herein. For instance, as described herein, various examples include one or more modules described as performing various functions. However, other examples may include additional modules that split the described functions up over more modules than that described herein. Additionally, other examples may consolidate the described functions into fewer modules. Accordingly, departure in form and detail may be made without departing from the scope and spirit of the present disclosure as described in the appended claims.