IMPLANTABLE MEDICAL DEVICE

Cross Reference to Related Applications

[0001] This application claims the benefit of U.S. Provisional Patent Application Serial No. 62/280,644 filed on January 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

[00Θ3] 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] Figure 1 shows a typical pacemaker system having a pacemaker can 10 and one or more leads, where only one lead 12 is shown in Figure 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'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 Figure 2, an example leadless pacemaker 16 is shown disposed within the patient'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'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' 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 sy stem 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's body to inside of the patient's body. The charging sy stem may further include an implantable medical device configured to be implanted inside of the patient'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.

[001Θ] 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'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'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'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'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'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'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] Figure 1 is a schematic diagram of an implanted cardiac pacemaker;

[0028] Figure 2 is a schematic diagram of an implanted leadless cardiac pacemaker;

[0029] Figure 3 is a schematic diagram of an illustrative remote wireless recharging system;

[0030] Figure 4 is a schematic diagram of an illustrative implantable device usable within the illustrative remote charging system of Figure 3;

[0031] Figure 5 is a schematic diagram of an illustrative antenna with a Van Atta array;

[0032] Figure 6 is a schematic view of illustrative zero-phase transmission lines referenced in Figure 5;

[0033] Figure 7 is a schematic diagram of an illustrative metamaterial-based transmission line (MTL) element including two cascaded metamaterial circuits;

[0034] Figure 8 shows an equivalent circuit of the two cascaded metamaterial circuits of Figure 7;

[0035] Figure 9 is a planar MTL structure having a conformal topology;

[0036] Figure 10 is an equivalent circuit of a single unit cell of the planar MTL structure shown in Figure 9;

[0037] Figure 11 is an MTL embodiment incorporating a 3D structure; [0038] Figure 12 is a schematic diagram of a wave front incident on an illustrative implanted device;

[0039] Figure 13 is a schematic diagram of a wave front incident on an implanted device that includes an impedance boundary layer;

[0040] Figure 14 is a schematic diagram of a wave front incident on an implanted device that includes an antenna with a smaller antenna aperture;

[0041] Figure 15 is a schematic diagram of a wave front incident on an implanted device that includes an antenna with a larger antenna aperture;

[0042] Figure 16 is a schematic diagram of a conical electromagnetic wave propagation path; and

[0043] Figure 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 I, 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.

[005Θ] 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 Figure 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'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's body and provide any desired functionality.

[0052] As illustrated in Figure 3, the implantable medical device 104 has been implanted within the patient's heart H while the transmitter 102 remains outside of the patient's body. In some cases, the transmitter 102 may be disposed on the patient's skin or may otherwise be aimed towards the patient'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's body to inside the patient'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'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] Figure 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'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 batter 110 to provide a voltage or current between to the electrodes 08 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 iieurostimulation 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 batter}' 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 06 and may be configured to receive transmitted EM energy from, for example, the transmitter 102 of Figure 3. The antenna 112 may be an antenna assembly with one or more antenna output terminals 113a and 113b 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 1 12 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 1 10 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.

|0057J In the example shown, a power or charging circuit 1 14 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 Figure 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 Figure 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] Figure 5 is a schematic diagram of a metamaterial-based retrodi recti ve (biomimetic) Van Atta array antenna configuration 150. The incident wave front is shown as a dashed line 156, running perpendicular to incident wave lines 1 2 and scattered waves 1 54. 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 Li and hi 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 Figure 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 Figure 6 uses a easeaded-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 metamatenal-based transmission line has a positive phase constant. Cascading two such transmission lines 172 and 174 in the appropriate manner, such as shown in Figure 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 LR, 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 metamatenal-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] Figure 7 shows a schematic diagram of an exemplary metamaterial-based transmission line (MTL) element 174 including two cascaded metamaterial circuits 190 and 192. Figure 8 shows an equivalent circuit of the two cascaded metamaterial circuits of Figure 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 Figure 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 90 and 92 is shown in Figure 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] Figure 9 shows an alternate approach to using interdigitated capacitors and shunt inductors to realize a metamaterial transmission line. Figure 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 Figure 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 Figure 10, it is appreciated that other shapes, e.g. rectangular hexagonal, etc., may be used.

[0066] Figure 11 shows a third illustrative MTL embodiment incorporating a 3D structure 220. This is a generalization of the 2D embodiment shown in Figures 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 Figure 4, in some cases, the housing 106 of the implantable medical device 04 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 simi lar to that of the tissue proximate an intended implantation site. As shown in Figures 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:

Z _w = £ (Ohms) where μ is the total permeability of the medium the EM wave is propagating in and e 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 e change, the wave impedance changes. This results in a reflection of the EM wave off of that boundary. This can be seen in Figure 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 boundar 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 Figure 3 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'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'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 04. 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:

7 — - p 7

^-'boundary ^tissue'-' device

where Z _boundarv 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 Figure 14. The effective aperture of an antenna can be computed using the following equation:

Ae = G λ ^ζ /4π where G (unit-less) is the gain of the antenna and A (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 nr. However, larger apertures can he 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 A. This can be accomplished by controlling the phase-constant over the structure of the antenna, which is denoted as ?(rad/m) and related to A by:

A = 2π/β.

As detailed above, the use of metamaterials and other techniques can reduce the phase-constant β of the antenna. This increases the value of A, and by relation increases the value of Ae, the effective aperture of the antenna. An antenna having an increased effective aperture is shown in Figure 1 5. 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 Figure 16 may be employed. Such a co ical 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'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 Figure 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's scope is, of course, defined in the language in which the appended claims are expressed.