Abstract:
Implantable medical devices such as leadless cardiac pacemakers may include a rechargeable battery, and a receiving antenna for receiving radiative energy from an external transmitter. Energy captured by the receiving antenna of the implantable medical device may be converted into electrical energy that may be used to recharge the rechargeable battery of the implantable medical device. Since the rechargeable battery does not have to initially store sufficient energy to power the implantable medical device over its entire useful life, the battery itself and thus the implanted medical device can be made smaller while still achieving device longevity expectations.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application Ser. No. 62/280,644 filed on Jan. 19, 2016, the disclosure of which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The disclosure relates generally to implantable medical devices, and more particularly, to devices and methods for wirelessly recharging a rechargeable battery of an implanted medical device. 
       BACKGROUND 
       [0003]    Cardiac pacemakers have been used to sense and pace heart rates for patients with a variety of incorrect heart rhythms, including but not limited to bradycardia, which is a slow heart rate. In some cases, a cardiac pacemaker may include a metal enclosure that contains control circuitry. Leads, which are electrically conductive, may be operatively coupled with the cardiac pacemaker and may extend into the circulatory system terminating in the heart. The leads often include pacing electrodes located at or near the distal end. 
         [0004]      FIG. 1  shows a typical pacemaker system having a pacemaker can  10  and one or more leads, where only one lead  12  is shown in  FIG. 1 . Pacemaker can  10  is shown placed in a pocket in the upper chest area. The lead  12  is shown extending from the pacemaker can  10  and through the left subclavian vein, through the superior vena cava, and down into the patient&#39;s right ventricle, terminating in an electrode  14 . In the example shown, the pacemaker can  10  is implanted external to the heart H while the lead  12  passes into the heart H, and places the electrode  14  at the ventricle wall of the heart H. In some cases, the lead  12  can be a potential weak link in the pacemaker system. Possible complications from the lead  12  may include displacement or misplacement of the lead  12 , lead fracture or perforation, and pocket infections at or near the pacemaker can  10 . In some cases, the lead  12  may impede certain surgical procedures by getting in the way. 
         [0005]    Leadless pacemakers are in development, and enjoy the potential benefit of not including or using leads. As seen in  FIG. 2 , an example leadless pacemaker  16  is shown disposed within the patient&#39;s right ventricle. In some cases, the leadless pacemaker  16  may be self-contained, including a power supply such as a battery, control circuitry and one or more electrodes that may be disposed on an outer surface of the leadless pacemaker  16 . The leadless pacemaker  16  is shown disposed entirely within the heart H, and more specifically in the right ventricle of the heart H. Because the battery life of the leadless pacemaker  16  often determines the useful life expectancy of the leadless pacemaker  16 , the battery is often made to consume a relatively large fraction of the internal volume of the leadless pacemaker  16 . There is a desire, however, to reduce the size of such leadless pacemakers, while at the same time increase their useful life expectancy. 
       SUMMARY 
       [0006]    The disclosure is directed to implantable medical devices such as leadless cardiac pacemakers that can provide a longer lasting power source within a smaller device housing. In some cases, a leadless cardiac pacemaker may include a rechargeable battery and an antenna. The antenna may be sized to be carried by the leadless cardiac pacemaker, and may be configured to capture sufficient Radio Frequency (RF) energy transmitted to the leadless cardiac pacemaker by a transmitter located outside of the patient&#39;s body to recharge the rechargeable battery faster than the rechargeable battery is depleted by powering the leadless cardiac pacemaker. Accordingly, since the rechargeable battery does not need to initially store sufficient energy to power the leadless cardiac pacemaker over its entire expected life, the battery itself and thus the leadless cardiac pacemaker may be made smaller while still meeting device longevity expectations. Alternatively, or in addition, since implanted devices may be challenging to explant, prolonging the devices&#39; longevity is desirable. In some cases, a rechargeable battery may allow the implanted device to last the patients entire life. 
         [0007]    In a first example, a charging system for remotely charging a battery of an implantable medical device is disclosed. The example charging system may include a transmitter configured to transmit Electro-Magnetic (EM) energy from outside of a patient&#39;s body to inside of the patient&#39;s body. The charging system may further include an implantable medical device configured to be implanted inside of the patient&#39;s body. The implantable medical device may be powered at least in part by a rechargeable battery and may be configured to receive at least part of the EM energy transmitted by the transmitter and to use the received EM energy to charge the rechargeable battery of the implantable medical device. 
         [0008]    In some cases, the implantable medical device of the first example may include an antenna that is tuned to receive at least part of the EM energy transmitted by the transmitter. Alternatively or additionally, the antenna may include a metamaterial-based biomimetic antenna. Alternatively or additionally, the antenna may include a Van Atta array having one or more zero-phase transmission lines, wherein at least one of the one or more zero-phase transmission lines includes a plurality of inter-digitated capacitors and a plurality of shunt inductive stubs. 
         [0009]    Alternatively or additionally to the first example discussed above, the implantable medical device may further include circuitry to convert the received EM energy into energy that can be used to charge the rechargeable battery. 
         [0010]    Alternatively or additionally to the first example, the EM energy includes RF energy, and the implantable medical device includes a leadless cardiac pacemaker. 
         [0011]    Alternatively or additionally to the first example, the transmitter is configured to provide a substantially constant power density along a path through the patient&#39;s body to the implantable medical device. 
         [0012]    Alternatively or additionally to the first example, the transmitter is configured to provide a conical shaped electromagnetic wave propagation path having a maximum width adjacent the transmitter and narrowing toward the implantable medical device. 
         [0013]    Alternatively or additionally to the first example, the implantable medical device includes a housing having an EM impedance and an impedance boundary layer extending over at least part of the housing. The impedance boundary layer may have an EM impedance that is between the EM impedance of the housing and the EM impedance of the patient&#39;s body adjacent the implantable medical device. 
         [0014]    Alternatively or additionally to the first example, the impedance boundary layer may include conducting particles. 
         [0015]    Alternatively or additionally to the first example, the impedance boundary layer may include two or more layers each having a different permittivity and/or permeability. 
         [0016]    Alternatively or additionally to the first example, the implantable medical device may be configured to be implanted such that the implantable medical device is fluidly coupled with flowing blood for thermal dissipation, such as inside a chamber of the heart or in a vein or artery. 
         [0017]    In a second example, an implantable medical device sized to be implanted within a patient&#39;s heart is disclosed. The example implantable medical device may include a housing and circuitry disposed within the housing. Two or more electrodes may be operatively coupled to the circuitry are exposed external to the housing. A rechargeable battery may be disposed within the housing and may be configured to power the circuitry. A receiving antenna may be operatively coupled to the circuitry and may be configured to receive transmitted Electro-Magnetic (EM) energy through the patient&#39;s body. The circuitry may be configured to use the received EM energy to charge the rechargeable battery of the implantable medical device. 
         [0018]    Alternatively or additionally to the second example, the receiving antenna may include a metamaterial-based biomimetic antenna. 
         [0019]    Alternatively or additionally to the second example, the receiving antenna may include a Van Atta array with one or more zero-phase transmission lines, and at least one of the one or more zero-phase transmission lines may include a plurality of inter-digitated capacitors and a plurality of shunt inductive stubs. 
         [0020]    Alternatively or additionally to the second example, the housing has an EM impedance and the patient&#39;s body has an EM impedance adjacent the implantable medical device, wherein the implantable medical device may further include an impedance boundary layer that has an EM impedance that is between the EM impedance of the housing and the EM impedance of the patient&#39;s body adjacent the implantable medical device. 
         [0021]    In a third example, an antenna assembly is disclosed. The example antenna assembly may include an antenna sized to be carried by an implantable medical device that is itself sized to be implanted in a heart of a human patient and that includes a rechargeable battery for powering the implantable medical device. The antenna may be tuned to a wavelength band of Electro-Magnetic (EM) energy that can be transmitted through the human patient, and may be configured to receive sufficient EM energy from the wavelength band of Electro-Magnetic (EM) to recharge the rechargeable battery of the implantable medical device at a rate faster than the rechargeable battery is depleted by powering the implantable medical device when the wavelength band of Electro-Magnetic (EM) energy is transmitted at an intensity that does not cause heat damage to the human patient. The antenna assembly may include one or more antenna output terminals for delivering the received EM energy to the implantable medical device. 
         [0022]    Alternatively or additionally to the third example, the antenna assembly may further includes one or more diodes that are operatively coupled to the antenna for rectifying the received EM energy. 
         [0023]    Alternatively or additionally to the third example, the antenna may include a metamaterial-based biomimetic antenna. 
         [0024]    Alternatively or additionally to the third example, the antenna may include a Van Atta array having one or more zero-phase transmission lines, and wherein at least one of the one or more zero-phase transmission lines includes a plurality of inter-digitated capacitors and a plurality of shunt inductive stubs disposed along an effective electrical length of the antenna. 
         [0025]    The above summary of some embodiments is not intended to describe each disclosed embodiment or every implementation of the present disclosure. The Figures, and Detailed Description, which follow, more particularly exemplify some of these embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0026]    The disclosure may be more completely understood in consideration of the following detailed description in connection with the accompanying drawings, in which: 
           [0027]      FIG. 1  is a schematic diagram of an implanted cardiac pacemaker; 
           [0028]      FIG. 2  is a schematic diagram of an implanted leadless cardiac pacemaker; 
           [0029]      FIG. 3  is a schematic diagram of an illustrative remote wireless recharging system; 
           [0030]      FIG. 4  is a schematic diagram of an illustrative implantable device usable within the illustrative remote charging system of  FIG. 3 ; 
           [0031]      FIG. 5  is a schematic diagram of an illustrative antenna with a Van Atta array; 
           [0032]      FIG. 6  is a schematic view of illustrative zero-phase transmission lines referenced in  FIG. 5 ; 
           [0033]      FIG. 7  is a schematic diagram of an illustrative metamaterial-based transmission line (MTL) element including two cascaded metamaterial circuits; 
           [0034]      FIG. 8  shows an equivalent circuit of the two cascaded metamaterial circuits of  FIG. 7 ; 
           [0035]      FIG. 9  is a planar MTL structure having a conformal topology; 
           [0036]      FIG. 10  is an equivalent circuit of a single unit cell of the planar MTL structure shown in  FIG. 9 ; 
           [0037]      FIG. 11  is an MTL embodiment incorporating a 3D structure; 
           [0038]      FIG. 12  is a schematic diagram of a wave front incident on an illustrative implanted device; 
           [0039]      FIG. 13  is a schematic diagram of a wave front incident on an implanted device that includes an impedance boundary layer; 
           [0040]      FIG. 14  is a schematic diagram of a wave front incident on an implanted device that includes an antenna with a smaller antenna aperture; 
           [0041]      FIG. 15  is a schematic diagram of a wave front incident on an implanted device that includes an antenna with a larger antenna aperture; 
           [0042]      FIG. 16  is a schematic diagram of a conical electromagnetic wave propagation path; and 
           [0043]      FIG. 17  is a schematic diagram showing heat removal around an implanted device by blood flow. 
       
    
    
       [0044]    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 the invention to the particular 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 
       [0045]    For the following defined terms, these definitions shall be applied, unless a different definition is given in the claims or elsewhere in this specification. 
         [0046]    All numeric values are herein assumed to be modified by the term “about,” whether or not explicitly indicated. The term “about” generally refers to a range of numbers that one of skill in the art would consider equivalent to the recited value (i.e., having the same function or result). In many instances, the terms “about” may include numbers that are rounded to the nearest significant figure. 
         [0047]    The recitation of numerical ranges by endpoints includes all numbers within that range (e.g. 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, and 5). 
         [0048]    As used in this specification and the appended claims, the singular forms “a”, “an”, and “the” include plural referents unless the content clearly dictates otherwise. As used in this specification and the appended claims, the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise. 
         [0049]    It is noted that references in the specification to “an embodiment”, “some embodiments”, “other embodiments”, etc., indicate that the embodiment described may include one or more particular features, structures, and/or characteristics. However, such recitations do not necessarily mean that all embodiments include the particular features, structures, and/or characteristics. Additionally, when particular features, structures, and/or characteristics are described in connection with one embodiment, it should be understood that such features, structures, and/or characteristics may also be used connection with other embodiments whether or not explicitly described unless clearly stated to the contrary. 
         [0050]    The following description should be read with reference to the drawings in which similar structures in different drawings are numbered the same. The drawings, which are not necessarily to scale, depict illustrative embodiments and are not intended to limit the scope of the disclosure. 
         [0051]    Turning to  FIG. 3 , which is a schematic diagram of an illustrative remote wireless recharging system  100 . In its broadest terms, the illustrative remote wireless recharging system  100  includes a transmitter  102  and an implantable medical device  104 . In some cases, the implantable medical device  104  may be configured to be implanted in or near the patient&#39;s heart H. In some instances, the implantable medical device  104  may be a leadless cardiac pacemaker (LCP). In some cases, the implantable medical device  104  may be configured to be implanted elsewhere within the patient&#39;s body and provide any desired functionality. 
         [0052]    As illustrated in  FIG. 3 , the implantable medical device  104  has been implanted within the patient&#39;s heart H while the transmitter  102  remains outside of the patient&#39;s body. In some cases, the transmitter  102  may be disposed on the patient&#39;s skin or may otherwise be aimed towards the patient&#39;s chest. In some cases, the transmitter  102  may include a housing (not illustrated) that enables the transmitter  102  to be suspended from a lanyard that the patient can wear about their neck when it is time to recharge the implantable medical device  104 . Overall, the transmitter may be configured to transmit electromagnetic (EM) energy from outside of the patient&#39;s body to inside the patient&#39;s body in order to transmit energy to the implantable medical device  104 . In some cases, the EM energy may include RF energy. The implantable medical device  104  may then use captured energy to recharge a rechargeable battery disposed within the implantable medical device  104 . 
         [0053]    When considering the electromagnetic regions around a transmitting antenna, there are three categories; namely, (1) reactive near-field; (2) radiated near-field and (3) radiated far-field. “Inductive” charging systems operate in the reactive near-field region. In inductive power systems, power is typically transferred over short distances by magnetic fields using inductive coupling between coils of wire, or by electric fields using capacitive coupling between electrodes. In radiative power systems (e.g. radiated near-field and radiated far-field), power is typically transmitted by beams of electromagnetic (EM) energy. Radiative power systems can often transport energy for longer distances, but the ability of a receiving antenna to capture sufficient energy can be challenging, particular for applications where the size of the receiving antenna is limited. 
         [0054]    In some cases, the transmitter  102  and implantable medical device  104  of the present disclosure may operate at or above about 400 MHz within the patient&#39;s body. When so provided, the system does not operate in the reactive near-field (as in inductive charging system), but rather operates in either the radiated near-field or radiated far-field regions (depending on the placement of the implanted device and band of usage). For example, when the EM energy is transmitted at 400 MHz, the system is in the radiated near-field region and at 2.45 GHz the system is in the radiated far-field region. In some cases, the present system may operate at a frequency that is between, for example, about 400 MHz and 3 GHz. In some cases, more than one frequency within this range may be used. When so provided, multiple implanted devices may be simultaneously or sequentially charged using both the radiated near-field and radiated far-field regions. 
         [0055]      FIG. 4  is a schematic diagram of the implantable medical device  104 , illustrating some of the components that may be included. In some instances, the implantable medical device  104  may be sized to be implanted within a patient&#39;s heart H. The implantable medical device  104  may include a housing  106 . A first electrode  108  and a second electrode  109  may be disposed relative to the housing  106 . In some cases, the electrodes  108  and  109  may be exposed external to the housing  106 , and in some cases, may be disposed on an outer surface of the housing  106 . While two electrodes  108  and  109  are illustrated, in some cases there may be three or more distinct electrodes. The illustrative implantable medical device  104  also includes a rechargeable battery  110  that is disposed within the housing  106 , and is used to power the operation of the implantable medical device  104 . In some cases, the implantable medical device uses power from the rechargeable battery  110  to provide a voltage or current between to the electrodes  108  and  109  in order to pace the heart H, for example. In some cases, the electrodes  108  and  109  may be used as sense electrodes, and power from the rechargeable battery  110  may be used to process and/or communicate the signal(s) sensed by electrodes  108  and  109 . In some cases, the implantable medical device  104  may provide a therapy such as a pacing therapy, a neurostimulation therapy, and/or any other suitable therapy. In some cases, the implantable medical device  104  may be merely a diagnostic device (e.g. no therapy provided), which collects diagnostic data and communicates the diagnostic data to a remote device. When so provided, the rechargeable battery  110  may provide energy to communicate the diagnostic data to a remote device. 
         [0056]    The illustrative implantable medical device  104  also includes a receiving antenna  112 , which may be disposed relative to the housing  106  and may be configured to receive transmitted EM energy from, for example, the transmitter  102  of  FIG. 3 . The antenna  112  may be an antenna assembly with one or more antenna output terminals  113   a  and  113   b  for delivering the received EM energy to the implantable medical device. In some cases, the antenna  112  may be sized to be carried by an implantable medical device  104  that is sized to be implanted in a heart of a human patient. The antenna  112  may be tuned to a wavelength band of Electro-Magnetic (EM) energy that can be transmitted through a human patient. The antenna may be configured to receive sufficient EM energy from the wavelength band of Electro-Magnetic (EM) to recharge the rechargeable battery  110  of the implantable medical device  104  at a rate faster than the rechargeable battery  110  is depleted by powering the implantable medical device  104 , even when the wavelength band of Electro-Magnetic (EM) energy is transmitted at an intensity that is low enough to not cause heat damage to the human patient. 
         [0057]    In the example shown, a power or charging circuit  114  may be configured to receive the transmitted EM energy received by the antenna  112  and convert at least some of the energy into a form (e.g. voltage and/or current) that can be used to recharge the rechargeable battery  110 . In some cases, the antenna assembly  112  and/or the charging circuit  114  may include one or more diodes that are configured to half-wave or full-wave rectify the received signal to produce a power source for circuity (e.g. charging circuit  114 , control circuit  116 , and/or communication circuitry  117 ) of the implantable medical device  104 . In the example shown, control circuit  116  may control the operation of the implantable medical device  104 . 
         [0058]    While not required, in some instances, the receiving antenna  112  may be a metamaterial-based biomimetic antenna. In some cases, the receiving antenna  112  may include a Van Atta array having at least one zero-phase transmission line. In some instances, the receiving antenna  112  may be a conformal antenna. In some cases, the receiving antenna  112  may include one or more antenna components that are printed on a substrate. In some cases, and as better shown in  FIG. 7 , the receiving antenna  112  may include a plurality of inter-digitated capacitors embedded within the receiving antenna  112 . In some instances, and as also better shown in  FIG. 7 , the receiving antenna  112  may include shunt inductive stubs disposed along an effective electrical length of the receiving antenna  112 . 
         [0059]    For purposes of this disclosure, the phrase “metamaterial and biomimetic antenna” is herein defined as an antenna which uses metamaterials and nature inspired structures to increase the performance of miniaturized (electrically small) antenna systems. The term “metamaterials” is herein defined as materials engineered with microscopic structures to produce unusual physical properties. A “biomimetic antenna” is herein defined as an antenna that uses designs based on natural structures to improve the performance. By incorporating metamaterials and biomimetic sub-structures, these antennas can step up the receiving power of the antenna sometimes by increases the effective antenna aperture, while still being confined to a smaller form factor that is driven by the size of the implantable medical device for a given application. 
         [0060]      FIG. 5  is a schematic diagram of a metamaterial-based retrodirective (biomimetic) Van Atta array antenna configuration  150 . The incident wave front is shown as a dashed line  156 , running perpendicular to incident wave lines  152  and scattered waves  154 . A Van Atta array  150  can receive electromagnetic waves from arbitrary directions and scatter the electromagnetic wave in the direction of the source automatically without the addition of additional circuitry (i.e. passive steering). This is achieved by connecting the antenna elements  160  shown in pairs symmetrically about the middle of the array. The inner antenna elements  160  are coupled by a first transmission line  168  to form the receive phase  164 , and the outer antenna elements  160  are coupled by a second transmission line  166  to form the transmit phase  162 . It will be appreciated that by choosing the lengths L 1  and L 2  of the connecting transmission lines  168  and  166  to be multiples of the guided wavelength of the source energy, the array is able to passively steer the beam in the direction of the source. This passive steering may be useful in supporting backscatter communication. However, when backscatter communication is not desired, the use of a Van Atta array antenna configuration  150  may be less compelling. 
         [0061]    In some cases, diodes (not shown) may be inserted along each connecting transmission line  166 ,  168 . When so provided, the array  150  may passively scan the region for EM energy and then use the connected diodes to full-wave rectify the signal to provide a DC power signal for powering digital and/or analog circuitry of an implantable medical device connected to the transmission lines  166 ,  168 . 
         [0062]    With reference to  FIG. 6 , connecting transmission lines  166 ,  168  essentially act as zero-phase transmission lines, meaning that the voltage and phase at both ends  176  and  178  of the transmission lines  166 ,  168  have both equal magnitude and phase. A transmission line is considered to be a zero-phase transmission line if the phase at both ends  176  and  178  is substantially the same (i.e. less than 30 degrees out of phase). 
         [0063]    In some cases, a traditional transmission line has a negative phase constant introduced by the transmission line being a space-filling antenna construction. The illustrative array  150  of  FIG. 6  uses a cascaded-hybrid metamaterial-based transmission line segment  174  and traditional transmission line segment  172  to significantly reduce the length of the transmission lines. A traditional space-filled transmission line has a negative phase constant, while a metamaterial-based transmission line has a positive phase constant. Cascading two such transmission lines  172  and  174  in the appropriate manner, such as shown in  FIG. 6 , results in a zero-phase transmission line (i.e., the phase introduced by the traditional transmission line is substantially removed by the metamaterial-based transmission line). This hybrid transmission line serves to reduce the effective transmission line length L R , sometimes to about 0.25 to about 0.50 of the EM wavelength λ, less than 1 cm in length, and/or any other suitable length as desired). Therefore, by using metamaterial-based antenna elements in the cascaded transmission line  166 ,  168 , an array for backscatter communications and power harvesting can be implemented on the embedded electrodes  18 ,  58 , many times with a physical size of less than 1 cm, less than 0.5 cm, less than 0.1 cm, depending on the desired wavelength of the transmitted EM energy through the body. 
         [0064]      FIG. 7  shows a schematic diagram of an exemplary metamaterial-based transmission line (MTL) element  174  including two cascaded metamaterial circuits  190  and  192 .  FIG. 8  shows an equivalent circuit of the two cascaded metamaterial circuits of  FIG. 7 . The illustrative MTL element  174  generally includes a series capacitance and a shunt inductance, which is intended to introduce a positive phase constant. As such, MTL element  174  includes one or more printed conductors  190 ,  192  on a conformal planar surface  188 . While two circuits  190 ,  192  are shown in the embodiment of  FIG. 7 , it is appreciated that MTL element  174  may include one, two or more circuits. In some cases, the printed conductors  190 ,  192  are printed on a different layer than the printed reference plane  188 . The wave from each antenna element in a Van Atta array is guided between the printed conductors  190 ,  192  via port  173  (coupled to the space-filled segment  172 ) and port  178 . In the example shown, capacitance segment  180  includes interdigital capacitor fingers  184  that are used to introduce series capacitance. Each side of the interdigital capacitor has a printed conductor connected to a Via  186  that extends down to the reference plane  188  to introduce a shunt inductance  182 . The equivalent circuit of the two cascaded metamaterial circuits  190  and  192  is shown in  FIG. 8 . In the example shown, a signal can be injected in port  173  and arrive at port  178 , or a signal can be injected into port  178  and arrive at port  173  (a linear system). 
         [0065]      FIG. 9  shows an alternate approach to using interdigitated capacitors and shunt inductors to realize a metamaterial transmission line.  FIG. 9  shows a planar MTL structure  200  having a conformal topology. This topology has a single printed conducting plane  188  printed on a conformal surface with small apertures  202 ,  204  and  206  of the conductor removed. The wave from each antenna element in the Van Atta array is guided on this structure  200 . By removing conducting material in a particular manner, a series capacitance and shunt inductance can be introduced for a guided wave between ports  173  and  178 . The equivalent circuit  210  of a single unit cell of structure  200  is shown in  FIG. 10 . Each region with the removed conductor  188  (circle) can be modeled as a capacitor  180  in parallel with an inductor  182 . By choosing the appropriate aperture size and spacing (e.g. large apertures  202 , medium apertures  204 , and small apertures  206 ), specific inductance and capacitance values can be introduced at various frequencies of interest. Therefore, if a wave is launched from port  173 , it will immediately see conducting regions (e.g. large apertures  202 ) with a dominant capacitance (this introduces the required series capacitance for the positive phase advancement) and by using other regions with specific conducting regions removed near the end of the conducting plane  188 , a shunt inductance (smaller apertures  206 ) can be introduced. Overall, the structure  200  will have the behavior of a metamaterial transmission line. While apertures  202 ,  204  and  206  are shown as circular openings in  FIG. 10 , it is appreciated that other shapes, e.g. rectangular hexagonal, etc., may be used. 
         [0066]      FIG. 11  shows a third illustrative MTL embodiment incorporating a 3D structure  220 . This is a generalization of the 2D embodiment shown in  FIGS. 9 and 10 , except spheres  224  and  226  are used to control the series capacitance and inductance instead of circles in the plane of the conductor. In particular, the spherical material  224  and  226  may be embedded into a 3D host material  222 . The spacing and spherical sizes are adapted to determine if the region is capacitively dominant or inductively dominant. This can then be used to synthesize a 3D metamaterial transmission line that shows phase advancement and zero phase properties. 
         [0067]    Returning briefly to  FIG. 4 , in some cases, the housing  106  of the implantable medical device  104  may have an outer surface  118 . In some cases, the outer surface  118  of the housing  106  may be configured to have an impedance that is similar to that of the tissue proximate an intended implantation site. As shown in  FIGS. 12 and 13 , the impedance of the outer surface  118  of the housing  106  may impact how effectively energy may be transmitted to the implantable medical device  104 . An electromagnetic wave (EM) propagating in a medium has a wave impedance. In air this impedance is 377 Ohms for example. This wave impedance can be computed in a medium using the following equation: 
         [0000]    
       
         
           
             
               Z 
               w 
             
             = 
             
               
                 
                   μ 
                   ε 
                 
               
                
               
                 ( 
                 Ohms 
                 ) 
               
             
           
         
       
     
         [0000]    where μ is the total permeability of the medium the EM wave is propagating in and ∈ is the total permittivity of the medium the EM wave is propagating in. This expression holds for many different dielectrics and is a good estimate of the wave impedance within the body for the far-field. Next, when a wave is propagating in a medium and the constitutive parameters of either μ or ∈ change, the wave impedance changes. This results in a reflection of the EM wave off of that boundary. This can be seen in  FIG. 12 , in which an incident wave front  122  reflects off the implantable medical device  104  as a reflected EM wave  124 . This can be detrimental to wireless recharging because there is energy in the reflected wave that does not propagate to the implantable medical device  104 . This also occurs at the boundary between the tissue in which the implantable medical device  104  is embedded in and the materials forming the implantable medical device  104 . 
         [0068]    To help improve the recharging capabilities of this system, it may be useful to provide an impedance boundary layer to minimize this reflection. Such a boundary  120  is shown in  FIG. 13  and can take on many different embodiments. As a result of including the boundary  120 , it can be seen that the incident EM wave front  122  is only partially reflected as a partially reflected EM wave front  126 . 
         [0069]    In some cases, the housing  106  itself may be formed from a material(s) that have an EM impedance that substantially matches (e.g. within 10 percent) the EM impedance of the patient&#39;s body adjacent the implantable medical device  104 . In other cases, an impedance boundary layer(s)  120  may be provided over at least part of the housing  106 . The impedance boundary layer  120  may have an impedance value that is between the EM impedance of the housing  106  and an EM impedance of the patient&#39;s body adjacent the implantable medical device  104 . There are several techniques for designing the impedance boundary layer(s)  120 . For example, microscopic conducting particles with various shapes in a regular or irregular manner may be embedded within a thin layer on the surface of the implantable medical device  104 . Alternatively, or in addition, the implantable medical device  104  may be coated with thin layers having different permittivities and/or permeabilities. As another example, conducting patterns may be printed on the surface of the housing  106  or on a thin insulating layer on the housing  106 . 
         [0070]    For example, by placing appropriate particles/elements in an impedance boundary layer(s), the wave impedance at the boundary can be tailored to be closer to the impedance of the tissue around the implantable medical device thereby reducing the amount of energy that is reflected by this boundary. In some cases, the wave impedance at the boundary may be tailored to be above, below or the same as the impedance of the tissue around the implantable medical device. In some cases, the wave impedance at the boundary is tailed to be: 
         [0000]      Z boundary =√{square root over (Z tissue Z device )}
 
         [0000]    where Z boundary  is the wave impedance at the boundary, Z tissue  is the wave impedance in the tissue, and Z device  is the wave impedance created by the material of the implantable medical device that is between the transmitter  102  and the antenna. 
         [0071]    In some cases, if the power collection area of an antenna can be made larger than its actual physical size, then more power can be collected so that the transmitted power density in the surrounding tissue can be lowered. This can be accomplished by increasing the effective aperture Ae of the antenna, which is illustrated in  FIG. 14 . The effective aperture of an antenna can be computed using the following equation: 
         [0000]        Ae=Gλ   2 /4π
 
         [0000]    where G (unit-less) is the gain of the antenna and λ (m) is the wavelength of the EM wave in the material around the antenna. Typically, the effective aperture is equal to or less than the physical size of the antenna, hence the units are in m 2 . However, larger apertures can be achieved using antenna arrays or volumetric configurations. Also, while many antenna are designed to have a physical dimension that is comparable to the wavelength λ, which is defined as the electrical length of the antenna, we here disclose an antenna that is designed to have an electrical length that is larger than the wavelength of the EM wave λ. This can be accomplished by controlling the phase-constant over the structure of the antenna, which is denoted as β(rad/m) and related to λ by: 
         [0000]      λ=2π/β.
 
         [0000]    As detailed above, the use of metamaterials and other techniques can reduce the phase-constant β of the antenna. This increases the value of λ, and by relation increases the value of Ae, the effective aperture of the antenna. An antenna having an increased effective aperture is shown in  FIG. 15 . Having an increased effective aperture increases the energy harvesting capabilities of the antenna, and thus the recharging capability of the implantable medical device  104 . As detailed above, illustrative methods to control the value of β include: (1) embedding inter-digitated capacitors into the design of the antenna; and (2) using shunt inductive stubs in a periodic manner along the effective electrical length of the antenna. 
         [0072]    For devices to be implanted within the body, the intensity of the energy transmitted by transmitter  102  is limited to prevent tissue damage in the patient (often through heating). Also, there is power loss as the EM wave passes through the body. As such, the amount of energy that can delivered to an implanted medical device is limited. To deliver sufficient energy to recharge a rechargeable battery of small implantable medical device in a reasonable amount of time (e.g. less than 1 day, less than 8 hours, less than 4 hours, less than 2 hours, less than 1 hour), the energy delivery must be relatively efficient. 
         [0073]    To help improve the energy delivery efficiency, a conical wave propagation scheme as illustrated in  FIG. 16  may be employed. Such a conical shape may be used to focus the transmitted energy on the implanted device and improve the recharging capabilities of the system. In some cases, the angle defining the cone may be chosen such that the power density at each layer from 1 to N along the length of the cone is substantially the same (within plus or minus 20 percent). Such an angle may be accomplished by active and/or passive beam steering by the transmitter  102 . In some cases, the transmitter  102  may use a phased-array. Alternatively, or in addition, parasitic elements may be embedded in the body between the transmitter  102  on the surface and the implantable medical device  104  that help guide the transmitted energy to the implantable medical device  104 . 
         [0074]    Without such a system, as an EM wave propagates from a transmitter  102  and into the body, the energy radiates out in a radial manner spreading the energy density of the wave across an ever larger volume of tissue as the wave progresses into the body. By the time the wave arrives at the implantable medical device  104 , the power density may be low, possibly too low for re-charging the rechargeable battery of the implantable medical device  104 . The transmit power could be increased, but the energy density at the skin may exceed regulatory levels and possibly cause damage to the patient&#39;s tissue. When a conical propagation path is used, the power density along the path from the transmitter  102  to the implantable medical device  104  may be more controlled and focused, and in some cases, may be substantially constant (within plus or minus 20 percent) along the path. 
         [0075]    In some cases, as shown for example in  FIG. 17 , it may be useful to consider implanting the implantable medical device  104  at a location in the body in which the implantable medical device  104  is thermally coupled with a flowing fluid that can help dissipate heat that could otherwise build up as a result of EM energy impinging on the implantable medical device  104 . For example, in some cases the implantable medical device  104  may be implanted in a location, such as within the heart H, wherein the implantable medical device  104  may be subjected to substantial blood flow. 
         [0076]    It should be understood that this disclosure is, in many respects, only illustrative. Changes may be made in details, particularly in matters of shape, size, and arrangement of steps without exceeding the scope of the disclosure. This may include, to the extent that it is appropriate, the use of any of the features of one example embodiment being used in other embodiments. The invention&#39;s scope is, of course, defined in the language in which the appended claims are expressed.