Patent Publication Number: US-2010109966-A1

Title: Multi-Layer Miniature Antenna For Implantable Medical Devices and Method for Forming the Same

Description:
RELATED APPLICATION 
     This application claims the benefit of and priority to U.S. Provisional Application Ser. No. 61/110,536, filed Oct. 31, 2008, entitled, “Multi-layer Miniature Antenna for Implantable Medical Devices and Method for Forming the Same,” the contents of which are incorporated by reference herein in its entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to implantable medical devices (IMDs) and, more particularly, the present invention relates to telemetry antennas suitable for deployment in IMDs. 
     BACKGROUND 
     Various types of devices have been developed for implantation into the human body to provide various types of health-related therapies, diagnostics and/or monitoring. Examples of such devices, generally known as implantable medical devices (IMDs), include cardiac pacemakers, cardioverter/defibrillators, cardiomyostimulators, cardiac event monitors, various physiological stimulators including nerve, muscle, and deep brain stimulators, various types of physiological monitors and sensors, and drug delivery systems, just to name a few. IMDs typically include functional components contained within a hermetically sealed enclosure or housing, which is sometimes referred to as a “can.” In some IMDs, a connector header or connector block is attached to the housing, and the connector block facilitates interconnection with one or more elongated electrical medical leads. The header block is typically molded from a relatively hard, dielectric, non-conductive polymer. The header block includes a mounting surface that conforms to, and is mechanically affixed against, a mating sidewall surface of the housing. 
     It has become common to provide a communication link between the hermetically sealed electronic circuitry of the IMD and an external programmer, monitor, or other external medical device (“EMD”) in order to provide for downlink telemetry transmission of commands from the EMD to the IMD and to allow for uplink telemetry transmission of stored information and/or sensed physiological parameters from the IMD to the EMD, Conventionally, the communication link between the IMD and the EMD is realized by encoded radio frequency (“RF”) transmissions between an IMD telemetry antenna and transceiver and an EMD telemetry antenna and transceiver. Generally, the IMD antenna is disposed within the hermetically sealed housing. However, the typically conductive housing can limit the radiation efficiency of the IMD RF telemetry antenna, thereby traditionally limiting the data transfer distance between the programmer head and the IMD RF telemetry antenna to a few inches. This type of system may be referred to as a “near field” telemetry system. In order to provide for “far field” telemetry, or telemetry over distances of a few to many meters from an IMD or even greater distances, attempts have been made to provide antennas outside of the hermetically sealed housing and within the header block. Many of such attempts of positioning an RF telemetry antenna outside of the hermetically sealed housing and in the header block have utilized wire antennas or planar, serpentine antennas, such as the antennas described in U.S. Pat. No. 7,317,946, which is hereby incorporated by reference in its entirety. The volume associated with the antenna and header block conventionally required for the implementation of distance telemetry in implanted therapy and diagnostic devices has been a significant contributor to the size of the IMD. 
     SUMMARY 
     In one or more embodiments, an antenna structure for an implantable medical device (IMD) is provided that includes at least one antenna conductor formed on a dielectric layer and a plurality of discrete dielectric layers positioned above the antenna conductor serving as superstrates and below the antenna conductor serving as substrates. In one or more embodiments, the superstrate dielectric layers include respective dielectric constants that gradually change in value with each superstrate layer moving away from the antenna conductor to values more closely matching the environment (e.g., body tissue) surrounding the antenna structure, such that the superstrate dielectric layers provide a matching gradient between the antenna conductor and the surrounding environment to mitigate energy reflection effects at the transition from the antenna structure to the surrounding environment. 
     In one or more embodiments, the antenna structure includes a biocompatible layer positioned as the outermost layer serving as an interface between the antenna structure and the surrounding environment, where the biocompatible layer may comprise one of the superstrate dielectric layers or another biocompatible layer positioned over the superstrate dielectric layers. 
     In one or more embodiments, the antenna structure includes a shielding layer formed from a metalized material positioned under the antenna conductor that provides electromagnetic shielding for device circuitry inside of a hermetically sealed housing to which the antenna structure is attached. In some embodiments, the shielding layer may be positioned under the substrate dielectric layers as the innermost layer of the antenna structure. In one or more embodiments, the substrate dielectric layers may include respective dielectric constants that gradually change in value with each substrate layer moving away from the antenna conductor to values more closely matching the hermetically sealed housing to the antenna structure is attached. In one or more embodiments, at least one of the substrate dielectric layers or another substrate layer may comprise an electromagnetic bandgap positioned between the antenna conductor and the shielding layer (i.e., ground plane) to prevent or minimize a reduction in antenna radiation efficiency from occurring as a result of effects from the ground plane shielding layer. 
     In one or more embodiments, the antenna structure may be formed as a monolithic structure derived from the plurality of discrete dielectric layers (superstrates and substrates) having an antenna conductor embedded within multiple layers of the plurality of dielectric layers. By forming a monolithic antenna structure derived from the plurality of dielectric layers, the dielectric constants of the plurality of dielectric layers can be selected or controlled to provide desired gradient matching and the dimensions of the overall antenna structure can be minimized to provide a miniature antenna structure. 
     In one or more embodiments, a plurality of different antenna conductor segments having different antenna characteristics may be embedded within the antenna structure, such that different antenna conductor segments or combinations of antenna conductor segments can be selected and/or switched for use in order to provide a tunable antenna to suit the needs of the particular IMD and/or the particular implant location. In some embodiments, a plurality of different antenna conductors may be formed on the same dielectric layer. In some embodiments, the antenna structure may include a plurality of discrete dielectric layers with at least one antenna conductor respectively positioned on each discrete dielectric layers with an outermost biocompatible layer and an innermost shielding (or grounding) layer, such that the effective dielectric between the antenna conductor and both the surrounding environment and the shielding/grounding plane can be switched to suit the needs of the particular IMD and/or the particular implant location. 
     In one or more embodiments, at least one of the plurality of dielectric layers used to form the antenna structure may include metamaterials to produce an effective permittivity and/or permeability having a negative value. The metamaterials may be epsilon-negative (ENG), mu-negative (MNG) or double negative (DNG). An antenna structure including at least one dielectric layer including metamaterials can be used to create effective permittivities and/or permeabilities that result in a desired impedance match condition for the metamaterial antenna structure having improved radiation efficiencies compared to similar antenna structures including natural double-positive (DPS) dielectric materials. 
     In one or more embodiments, the dielectric layers comprise at least one of a low temperature co-fire ceramic (LTCC) material and/or a high temperature co-fire ceramic (HTCC) material, where the ceramic dielectric layers, the antenna conductor(s), the biocompatible outermost layer, and the innermost shielding layer can be co-fired together to form a monolithic antenna structure. 
    
    
     
       DRAWINGS 
       The above-mentioned features and objects of the present disclosure will become more apparent with reference to the following description taken in conjunction with the accompanying drawings wherein like reference numerals denote like elements and in which: 
         FIG. 1  illustrates an implantable medical device implanted in a human body in accordance with one or more embodiments of the present disclosure. 
         FIG. 2  is a schematic block diagram illustration of exemplary implantable medical device in accordance with one or more embodiments of the present disclosure. 
         FIG. 3  is a perspective, exploded view of an antenna structure for an implantable medical device formed in accordance with one or more embodiments of the present disclosure. 
         FIG. 4  is a cross-sectional side view of an antenna structure for an implantable medical device formed in accordance with one or more embodiments of the present disclosure. 
         FIG. 5  is a cross-sectional side view of a co-fired monolithic antenna structure for an implantable medical device formed in accordance with one or more embodiments of the present disclosure. 
         FIG. 6  is a schematic block diagram illustration of an antenna structure connected to implantable medical device in accordance with one or more embodiments of the present disclosure. 
         FIG. 7  is a perspective, exploded view of an antenna structure for an implantable medical device formed in accordance with one or more embodiments of the present disclosure. 
         FIG. 8  is a partial top view of a layer of an antenna structure for an implantable medical device formed in accordance with one or more embodiments of the present disclosure. 
         FIGS. 9A-9F  are schematic illustrations of different antenna conductor configurations in accordance with one or more embodiments of the present disclosure. 
         FIG. 10  is an enlarged, partial cutaway, perspective view of an anodized antenna conductor in accordance with one or more embodiments of the present disclosure. 
         FIG. 11  is an exploded perspective view of an anodized antenna conductor having a superstrate radome in accordance with one or more embodiments of the present disclosure. 
         FIG. 12  is a cross-sectional side view of an antenna structure for an implantable medical device formed in accordance with one or more embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely illustrative and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. 
     The following description refers to components or features being “connected” or “coupled” together. As used herein, unless expressly stated otherwise, “connected” means that one component/feature is directly or indirectly connected to another component/feature, and not necessarily mechanically. Likewise, unless expressly stated otherwise, “coupled” means that one component/feature is directly or indirectly coupled to another component/feature, and not necessarily mechanically. Thus, although the figures may depict example arrangements of elements, additional intervening elements, devices, features, or components may be present in an actual embodiment (assuming that the functionality of the IMDs are not adversely affected). 
     In one or more embodiments, an IMD having a monolithic antenna structure derived from a plurality of discrete dielectric layers is provided. For the sake of brevity, conventional techniques and aspects related to RF antenna design, IMD telemetry, RF data transmission, signaling, IMD operation, connectors for IMD leads, and other functional aspects of the systems (and the individual operating components of the systems) may not be described in detail herein. Furthermore, the connecting lines shown in the various figures contained herein are intended to represent example functional relationships and/or physical couplings between the various elements. It should be noted that many alternative or additional functional relationships or physical connections may be present in a practical embodiment. 
     An IMD antenna generally has two functions: to convert the electromagnetic power of a downlink telemetry transmission of an EMD telemetry antenna propagated through the atmosphere (and then through body tissues) into a signal (e.g., a UHF signal or the like) that can be processed by the IMD transceiver into commands and data that are intelligible to the IMD electronic operating system; and to convert the uplink telemetry signals (e.g., a UHF signal or the like) of the IMD transceiver electronics into electromagnetic power propagated through the body tissue and the atmosphere so that the EMD telemetry antenna or antennas can receive the signals. 
       FIG. 1  is a perspective view of an IMD  10  implanted within a human body  12  in which one or more embodiments of the invention may be implemented. IMD  10  comprises a hermetically sealed housing  14  (or “can”) and connector header or block module  16  for coupling IMD  10  to electrical leads and other physiological sensors arranged within body  12 , such as pacing and sensing leads  18  connected to portions of a heart  20  for delivery of pacing pulses to a patient&#39;s heart  20  and sensing of heart  20  conditions in a manner well known in the art. For example, such leads may enter at an end of header block  16  and be physically and electrically connected to conductive receptacles, terminals, or other conductive features located within header block  16 . IMD  10  may be adapted to be implanted subcutaneously in the body of a patient such that it becomes encased within body tissue and fluids, which may include epidermal layers, subcutaneous fat layers, and/or muscle layers. While IMD  10  is depicted in  FIG. 1  in an ICD configuration, it is understood that this is for purposes of illustration only and IMD  10  may comprise any type of medical device requiring a telemetry antenna. 
     In some embodiments, hermetically sealed housing  14  is generally circular, elliptical, prismatic, or rectilinear, with substantially planar major sides joined by perimeter sidewalls. Housing  14  is typically formed from pieces of a thin-walled biocompatible metal such as titanium. Two half sections of housing  14  may be laser seam welded together using conventional techniques to form a seam extending around the perimeter sidewalls. Housing  14  and header block  16  are often manufactured as two separate assemblies that are subsequently physically and electrically coupled together. Housing  14  may contain a number of functional elements, components, and features, including (without limitation): a battery; a high voltage capacitor; integrated circuit (“IC”) devices; a processor; memory elements; a therapy module or circuitry; an RF module or circuitry; and an antenna matching circuit. These components may be assembled in spacers and disposed within the interior cavity of housing  14  prior to seam welding of the housing halves. During the manufacturing process, electrical connections are established between components located within housing  14  and elements located within header block  16 . For example, housing  14  and header block  16  may be suitably configured with IC connector pads, terminals, feedthrough elements, and other features for establishing electrical connections between the internal therapy module and the therapy lead connectors within header block  16  and for establishing connections between the internal RF module and a portion of a telemetry antenna located within header block  16 . Structures and techniques for establishing such electrical (and physical) feedthrough connections are known to those skilled in the art and, therefore, will not be described in detail herein. For example, U.S. Pat. No. 6,414,835 describes a capacitive filtered feedthrough array for an implantable medical device, the contents of which are hereby incorporated by reference. 
     Header block  16  is preferably formed from a suitable dielectric material, such as a biocompatible synthetic polymer. In some embodiments, the dielectric material of header block  16  may be selected to enable the passage of RF energy that is either radiated or received by a telemetry antenna (not shown in  FIG. 1 ) encapsulated within header block  16 . The specific material for header block  16  may be chosen in response to the intended application of IMD  10 , the electrical characteristics of the environment surrounding the implant location, the desired operating frequency range, the desired RF antenna range, and other practical considerations. 
       FIG. 2  is a simplified schematic representation of an IMD  10  and several functional elements associated therewith. IMD  10  generally includes hermetically sealed housing  14  and header block  16  coupled to housing  14 , a therapy module  22  contained within housing  14 , and an RF module  24  contained within housing  14 . In practice, IMD  10  will also include a number of conventional components and features necessary to support the functionality of IMD  10  as known in the art. Such conventional elements will not be described herein. 
     Therapy module  22  may include any number of components, including, without limitation: electrical devices, ICs, microprocessors, controllers, memories, power supplies, and the like. Briefly, therapy module  22  is configured to provide the desired functionality associated with the IMD  10 , e.g., defibrillation pulses, pacing stimulation, patient monitoring, or the like. In this regard, therapy module  22  may be coupled to one or more sensing or therapy leads  18 . In practice, the connection ends of therapy leads  18  are inserted into header block  16 , where they establish electrical contact with conductive elements coupled to therapy module  22 . Therapy leads  18  may be inserted into suitably configured lead bores formed within header block  16 . In the example embodiment, IMD  10  includes a feedthrough element  26  that bridges the transition between housing  14  and header block  16 . Therapy leads  18  extend from header block  16  for routing and placement within the patient. 
     RF module  24  may include any number of components, including, without limitation: electrical devices, ICs, amplifiers, signal generators, a receiver and a transmitter (or a transceiver), modulators, microprocessors, controllers, memories, power supplies, and the like. RF module  24  may further include a matching circuit or a matching circuit may be positioned between RF module  24  and antenna  28 . Matching circuit may include any number of components, including, without limitation: electrical components such as capacitors, resistors, or inductors; filters; baluns; tuning elements; varactors; limiter diodes; or the like, that are all suitably configured to provide impedance matching between antenna  28  and RF module  24 , thus improving the efficiency of antenna  28 . Briefly, RF module  24  supports RF telemetry communication for IMD  10 , including, without limitation: generating RF transmit energy; providing RF transmit signals to antenna  28 ; processing RF telemetry signals received by antenna  28 , and the like. In practice, RF module  24  may be designed to leverage the conductive material used for housing  14  as an RF ground plane (for some applications), and RF module  24  may be designed in accordance with the intended application of IMD  10 , the electrical characteristics of the environment surrounding the implant location, the desired operating frequency range, the desired RF antenna range, and other practical considerations. 
     Antenna  28  is coupled to RF module  24  to facilitate RF telemetry between IMD  10  and an EMD (not shown). Generally, antenna  28  is suitably configured for RF operation (e.g., UHF or VHF operation, 401 to 406 MHz for the MICS/MEDS bands, 900 MHz/2.4 GHz and other ISM bands, etc.). In the example embodiment shown in  FIG. 2 , antenna  28  is located within header block  16  and outside of housing  14 . However, the volume associated with the antenna  28  and the volume within the header block  16  required for the implementation of distance telemetry in implanted therapy and diagnostic devices can be a significant contributor to the size of the IMD  10 . Antenna  28  may have characteristics resembling a monopole antenna, characteristics resembling a dipole antenna, characteristics resembling a coplanar waveguide antenna characteristics resembling a stripline antenna, characteristics resembling a microstrip antenna, and/or characteristics resembling a transmission line antenna. Antenna  28  may also have any number of radiating elements, which may be driven by any number of distinct RF signal sources. In this regard, antenna  28  may have a plurality of radiating elements configured to provide spatial, pattern, or polarization diversity 
     In one or more embodiments, antenna  28  is coupled to RF module  24  via an RF feedthrough in feedthrough  26 , which bridges housing  14  and header block  16 . Antenna  28  may include a connection end that is coupled to RF feedthrough in feedthrough  26  via a conductive terminal or feature located within header block  16 . Briefly, a practical feedthrough  26  includes a ferrule supporting a non-conductive glass or ceramic insulator. The insulator supports and electrically isolates a feedthrough pin from the ferrule. During assembly of housing  14 , the ferrule is welded to a suitably sized hole or opening formed in housing  14 . RF module  24  is then electrically connected to the inner end of the feedthrough pin. The connection to the inner end of the feedthrough pin can be made by welding the inner end to a substrate pad, or by clipping the inner end to a cable or flex wire connector that extends to a substrate pad or connector. The outer end of the feedthrough pin serves as a connection point for antenna  28 , or as a connection point for an internal connection socket, terminal, or feature that receives the connection end of antenna  28 . The feedthrough  26  for antenna  28  may be located on any desired portion of housing  14  suitable for a particular design. 
     Referring now to  FIG. 3 , a perspective, exploded view of an antenna structure  100  formed in accordance with one or more embodiments is respectively illustrated. Certain features and aspects of antenna structure  100  are similar to those described above in connection with antenna  28 , and shared features and aspects will not be redundantly described in the context of antenna structure  100 . Antenna structure  100  includes at least one antenna conductor  106  formed on a dielectric layer  104 . A plurality of discrete dielectric layers  108  are positioned above the antenna conductor  106  serving as superstrates, and a plurality of discrete dielectric layers  112  are positioned below the antenna conductor  106  serving as substrates. In one or more embodiments, the antenna structure  100  includes a biocompatible layer  110  positioned as the outermost layer over the superstrate dielectric layers  108  serving as an interface between the antenna structure  100  and the surrounding environment. In some embodiments, the biocompatible layer  110  may comprise the outermost of the superstrate dielectric layers  108 . Different types of biocompatible materials can be selected based on the intended use of antenna structure  100  and IMD  10  and the intended surrounding environment. For example, outermost layer  110  may comprise inorganic materials, such as Alumina (Al 2 O 3 ), zirconium oxide (ZrO 2 ), mixtures thereof, or bone-like systems [hydroxyapatite—Ca 5 (POH)(PO 4 ) 3 ], organic materials, such as silicone and its derivatives, and other traditionally implantable biocompatible materials. 
     In one or more embodiments, antenna structure  100  may include an shielding layer  114  positioned in a layer under the antenna conductor  106  formed from a metalized material that provides electromagnetic shielding of device circuitry inside of the hermetically sealed housing  14  to which the antenna structure  100  is attached through a feedthrough via  116 . In some embodiments, the shielding layer  114  is positioned as the innermost layer of the antenna structure  100 , while it is understood that shielding layer  114  can also be positioned within another intermediate substrate layer  112  positioned under the antenna conductor  106 . 
     In one or more embodiments, at least one of the substrate dielectric layers  112  or an electromagnetic bandgap layer  115  positioned under antenna conductor  106  may be selected from a material so as to function as an electromagnetic bandgap between antenna conductor  106  and shielding layer  114  (i.e., ground plane), as illustrated in  FIG. 3  and further in the cross-sectional side view of antenna structure  100  in  FIG. 4 . Typically, when a radiating antenna element is placed above and in parallel with a ground plane, the field radiated by the antenna element and the field reflected by the ground plane are 180° out of phase due to the reflection coefficient presented by the ground plane short circuit. As a result, when the separation distance between the antenna element and the ground plane is reduced, the total antenna radiated fields tend to zero as the field radiated from the antenna element and its ground plane reflection will tend to completely cancel each other. An electromagnetic bandgap layer  115  prevents this reduction in antenna radiation efficiency by introducing a ground perturbation known as an electromagnetic bandgap, or high impedance surface, between antenna conductor  106  and ground plane shielding layer  114 . The electromagnetic bandgap layer  115  prevents or minimizes a reduction in antenna radiation efficiency from occurring as a result of the close proximity of the antenna conductor  106  to the ground plane  114 . In one aspect, the electromagnetic bandgap layer  115  at resonance appears as an open circuit with a reflection coefficient in phase with the incident field. For instance, the electromagnetic bandgap layer  115  will cause the field radiated from antenna conductor  106  and the field radiated by its ground plane image to be co-directed thus maintaining the same orientation and not canceling each other out. The electromagnetic bandgap layer  115  further provides a high electromagnetic surface impedance that allows the antenna conductor  106  to lie directly adjacent to the ground plane  114  without being shorted out. This allows compact antenna designs where radiating elements are confined to limited spaces Thus, the electromagnetic bandgap layer  115  assists in miniaturization of the device by allowing the distance between antenna conductor  106  and ground plane shielding layer  114  to be reduced to a small distance. In one or more embodiments, electromagnetic bandgap layer  115  may be vacuum deposited on the surface of one of the layers of the device  100  or adhered via epoxy after ceramic densification in order to minimize material alterations induced by thermal excursion of the firing process. 
     In one or more embodiments, the electromagnetic bandgap layer  115  may comprise a high impedance ground plane (e.g., artificial perfect magnetic conductor or PMC) that has the property of isolating the radiating elements from nearby electromagnetic surroundings. The high impendence surface of the electromagnetic bandgap layer  115  further provides the benefit of directing radiated energy away from ground plane shielding layer  114  and improves the antenna radiated front-to-back ratio resulting in improved antenna efficiency. In one or more embodiments, the electromagnetic bandgap layer  115  is made of a periodic structure, such as a plurality of discrete metal areas or a plurality of periodic lattice cells that are connected electrically to neighboring lattice cells, where such an interconnected bandgap structure topology conducts DC currents but not AC currents within a forbidden band. In one or more embodiments, the physical geometry the electromagnetic bandgap layer  115  may comprise a metal sheet, textured with a 2D lattice of resonant elements which act as a 2D filter to prevent the propagation of electric currents, such as described in the paper, “A High Impedance Ground Plane Applied to a Cellphone Handset Geometry,” by Sievenpiper et al., IEEE MTT Vol. 49 No. 7 July 2001 Pg 1262-1265, the contents of which are hereby incorporated by reference in its entirety. 
     In one or more embodiments, the electromagnetic bandgap layer  115  may comprise a reactive impedance substrate. PMC surfaces are usually constructed from resonant structures operating at resonance. By utilizing a reactive impedance substrate design, the adverse effects of the antenna interaction with the substrate are minimized such as the mutual coupling between the antenna conductor  106  and its image. The electromagnetic bandgap layer  115  can be engineered to exhibit normalized substrate impedance (image impedance) that could compensate for the stored energy in the source itself (antenna conductor  106 ). If the antenna conductor  106  shows a capacitive load and its image can store magnetic energy, a resonance can be achieved at a frequency much lower than the resonant frequency of the antenna conductor  106  in free space. An example of a reactive impedance substrate is set forth in the paper, “Antenna Miniaturization and Bandwidth Enhancement using a Reactive Impedance Substrate,” by Mosallaei et al, IEEE APS vol. 52 No. 9 September 2004 pg 2403-2414, the contents of which are hereby incorporated by reference in its entirety. 
     In one or more embodiments, at least one of the plurality of dielectric layers  104 ,  108 , or  112  may be formed to include metamaterials to produce an effective permittivity and/or permeability having a negative value for the particular dielectric layers  104 ,  108 , or  112  including the metamaterials. Metamaterials are artificial materials that exhibit electromagnetic properties that are not generally found in nature. For example, naturally occurring dielectric materials found in substrates are referred to as double-positive (DPS) as both epsilon (ε) and mu (μ) are positive. However, to the contrary, metamaterials may be epsilon-negative (ENG), mu-negative (MNG) or double negative (DNG) in which both epsilon and mu are negative. An antenna structure  100  including at least one dielectric layer  104 ,  108 , or  112  including metamaterials can be used to create effective permittivities and/or permeabilities for antenna structure  100  that result in a desired impedance match condition for the antenna structure  100 . Typically, electrically small antennas (i.e., those that are much shorter than a wavelength) are known to be very inefficient radiators as they possess a low resistive component and a large capacitive reactance component in their measure input impedance, thereby typically causing a poor impedance match condition. By using a metamaterial based antenna structure  100 , the periodic inclusions in the metamaterial, which are located in the extreme near field of antenna conductor  106 , can be adjusted to create effective permittivities and/or permeabilities that result in the desired impedance match condition for the antenna structure  100 . This provides improved radiation efficiencies compared to similar antenna structures including natural double-positive (DPS) dielectric materials. For example, in some embodiments, an optimized metamaterial antenna structure  100  can demonstrate radiation efficiency improvements in excess of 35 dB when compared to the same antenna structure with natural DPS dielectric materials. An example of a metamaterial used formed using frequency selective surfaces (FSS) of gangbuster dipoles is set forth in the paper, “A Metamaterial Surface for Compact Cavity Resonators,” by Maci et al., IEEE AP Letters vol. 3 2004, pages 261-264, the contents of which are hereby incorporated by reference in its entirety. Further, metamaterial period cells include, 1-D Split-Ring Structure, Symmetrical-Ring Structure, Omega Structure, Unit S Cell Structure, as described in the paper, “A Study Using Metamaterials As Antenna Substrate To Enhance Gain,” by Grzegorczyk et al., PIER 51 2005, pages 295-328, the contents of which are hereby incorporated by reference in its entirety. 
     With further reference to the cross-sectional side view of antenna structure  100  illustrated in  FIG. 4 , in one or more embodiments, the edges  118  of the various layers of the antenna structure  100  (i.e., dielectric layers  104 ,  108  and  112 , outermost biocompatible layer  110 , electromagnetic bandgap layer  115 , and shielding layer  114 ) may be brazed or otherwise sealed to hermetically seal the edges  118  of antenna structure  100  to a ferrule or body that would enable integration of antenna structure  100  to the housing  14 . Generally, brazing involves melting and flowing a brazing material (e.g., a metal such as gold) around the portions of the desired surfaces to be brazed (e.g., the edges  118  of the layers of antenna structure  100  and housing  14 ). 
     In one or more embodiments, superstrate dielectric layers  108  can be selected to possess respective dielectric constants that gradually change in value with each superstrate layer  108  moving away from antenna conductor  106  to values more closely matching the dielectric constant of the environment (e.g., body tissue) surrounding the antenna structure  100 . For instance, Alumina (Al 2 O 3 ) has a dielectric constant k=9. In this manner, superstrate dielectric layers  108  provide a matching gradient between antenna conductor  106  and the surrounding environment to mitigate energy reflection effects at the transition from the antenna structure  100  to the surrounding environment. The change in dielectric constants in the various superstrate layers  108  can be achieved by incorporating materials that are cofireable, compatible and possess dielectric constants that differ from the other of the superstrate layers  108 . In conventional antenna structures possessing abrupt transitions and differences in dielectric constants at the boundary between the antenna structures and the surrounding environment, there can be large energy reflection effects. The effects are reduced by the matching gradient provided by the superstrate dielectric layers  108 , where the gradual change in dielectric values between the various superstrate dielectric layers  108  further helps to mitigate energy reflection effects between superstrate dielectric layers  108 . 
     In one or more embodiments, various biocompatible layers formed for the superstrate dielectric layers  108  may comprise polymers that are loaded with high dielectric constant powders so as to produce an antenna structure  100  that contains a graded dielectric constant extending from one portion of the antenna structure  100  to another portion. For example, powders with different dielectric constants can be loaded on the different polymer layers, different concentrations of powder loading can be performed on the different polymer layers, or the dielectric constant of each polymer layer can otherwise have its powder loading adjusted to produce a structure having a graded dielectric constant between various superstrate dielectric layers  108 . High dielectric loading may also modify the radio pattern of the antenna conductor  106  to reduce the power directly dissipated into the human body surrounding IMD  10 . 
     In one or more embodiments, the substrate dielectric layers  112  under antenna conductor  106  may comprise materials with higher dielectric values than dielectric layer  104  on which antenna conductor  106  is formed, such that the higher dielectric values associated with substrate dielectric layers  112  allow the distance between antenna conductor  106  and ground plane shielding layer  114  to be minimized, thereby allowing a reduction in size of antenna structure  100  to be achieved. The high dielectric constant K of each layer may be achieved by incorporating cofireable materials having high dielectric constants K (e.g., capacitive materials). Depending upon the materials used to form substrate dielectric layers  112  and electromagnetic bandgap layer  115 , dielectric constant values can vary anywhere from k=5-6 for the LTCC layer itself to at least 1-2 orders of magnitude higher with the use of capacitive pastes that are LTCC compatible. In addition, a ceramic loaded printed wiring board (PWB) is another embodiment to the LTCC based structure. LTCC materials offer the ability to embed passive components to spatially and functionally tailor the dielectric constant or capacitance to optimize packaging efficiency and/or performance. Since materials with high dielectric constants are typically not biocompatible, substrate dielectric layers  112  and electromagnetic bandgap layer  115  may be separated and isolated from potential contact with body environment surrounding IMD  10  by the biocompatible materials used to form outermost biocompatible layer  110  or other superstrate dielectric layers  108 . The isolation of substrate layers  112  and electromagnetic bandgap layer  115  from the body environment surrounding IMD  10  allows the possible selection of materials for superstrate dielectric layers  108  to be wide ranging. For example, dielectric oxide (e.g., barium titanium oxide (BaTiO 3 )) based systems with dielectric constants k in the hundreds to thousands are possible. 
     In one or more embodiments, the various layers used to form antenna structure  100  may be formed using any material layer deposition technique known in the art, including but not limited to depositing, spraying, screening, dipping, plating, etc. In some embodiments, molecular beam epitaxy (MBE), atomic layer deposition (ALD) or other thin film, vacuum deposited processes may be used to deposit the various layers building them on top of one another, such that ALD allows thin high dielectric materials to be used in forming substrate dielectric layers  112  and thin lower dielectric materials to be used in forming superstrate dielectric layers  108 , thereby achieving size reduction and miniaturization of overall antenna structure  100  while still improving performing of antenna structure  100 . The metal layers can be stacked to form a stacked plate capacitor structure to increase the dielectric constant of the area surrounding the antenna conductor  106 . 
     In one or more embodiments, after the various layers of antenna structure  100  and formed or otherwise deposited with respect to one another, as illustrated in  FIG. 4 , the various layers may be co-fired to a monolithic structure derived from the various layers, as illustrated in  FIG. 5 , having antenna conductor  106  embedded within the resulting monolithic structure  102 . Feedthrough via  116  extends through monolithic structure  102  and may be used to connect antenna conductor  106  to housing  14 , such as through a feedthrough. By forming a monolithic antenna structure  102  derived from the plurality of dielectric layers  104 ,  108  and  112 , the dielectric constants of the plurality of dielectric layers  104 ,  108  and  112  can be selected or controlled to provide desired gradient matching and the dimensions of the overall antenna structure can be minimized to provide a miniature antenna structure. For example, in one or more embodiments, the plurality of dielectric layers  104 ,  108  and  112  can be selected such that they each possess gradually changing dielectric constants in the direction of arrows  120 , such that the gradual changes can occur in either direction. 
     In one or more embodiments, at least one interlayer metal material having a high dielectric constant may be positioned at one or more locations between layers of high temperature co-fired ceramic (HTCC) material when forming the dielectric layers  104 ,  108  or  112  in order to increase the effective dielectric constant of such layers without requiring changes to the materials in forming such layers. In some embodiments, the metal interlayers can be patterned to provide the high dielectric values only where desired or needed, which can be useful in reducing cofire issues when the materials are cofired together. In some embodiments, the metal interlayers can be deposited through the use of vacuum deposition, ALD, screen printed thick film processes or other deposition techniques. 
     In one or more embodiments, after the antenna structure  100  has been formed as a co-fired monolithic structure  102 , the edges  118  or side surfaces of the various layers of the antenna structure  100  (i.e., dielectric layers  104 ,  108  and  112 , electromagnetic bandgap layer  115 , outermost biocompatible layer  110  and innermost shielding layer  114 ) may be brazed or otherwise sealed to hermetically seal the edges  118  of antenna structure  100 . The brazed side edges  118  along with the outermost biocompatible layer  110  of antenna structure  100  provide a hermetic seal for antenna structure  100  so that it can be connected directly to housing  14  without requiring a header to enclose and seal the antenna conductor  106 , as typically required with conventional far field telemetry antennas for IMDs. As illustrated in  FIG. 6 , antenna structure  100  may be coupled to housing  14  using brazing, glassing, diffusion bonding or other suitable bonding techniques that will provide a hermetic seal, as known to those skilled in the art. The antenna structure  100  thus reduces the overall volume and physical dimension required for antenna conductor  106  for adequate radiation. In some embodiments, a header block  16  having reduced dimensions may still be utilized for connecting external leads to therapy module  16 . In some embodiments, portions of the antenna structure  100  may be hermetically sealed to the housing  14  prior to overall formation of the co-fired monolithic structure  102 , such that various layers used to form the co-fired monolithic structure  102  could be formed on one another after certain portions of the antenna structure  100  have been hermetically sealed to the housing  14 . 
     In one or more embodiments, antenna conductor  106  is formed from a biocompatible conductive material, such as but not limited to at least one of the following materials: Platinum, Iridium, Platinum-Iridium alloys, Alumina, Silver, Gold, Palladium, Silver-Palladium or mixtures thereof, or Niobium, Molybdenum and/or Moly-manganese or other suitable materials. In one or more embodiments, dielectric layers  104 ,  108  and  112  may be comprise at least one of a ceramic material, a semiconductor material, and/or a thin film dielectric material. In some embodiments in which the dielectric layers  104  include at least one ceramic material, the dielectric layers  104 ,  108  and  112  may include at least one of a low temperature co-fired ceramic (LTCC) material or a high temperature co-fired ceramic (HTCC) material or a PWB material that enable the incorporation of materials having desired dielectric constant values. Generally, a LTCC material has a melting point between about 850° C. and 1150°C., while a HTCC material has a melting point between about 1100° C. and 1700° C. The ceramic dielectric layers  104 ,  108  and  112 , antenna conductor  106 , electromagnetic bandgap layer  115 , outermost biocompatible layer  110  and innermost shielding layer  114  and via  116  are sintered or co-fired together to form a monolithic antenna structure  102  including an embedded antenna conductor  106 , as illustrated in  FIG. 5 . Methods for co-firing layers of ceramic materials together to form monolithic structures for use in IMDs are described, for example, in U.S. Pat. No. 6,414,835 and U.S. Pat. No. 7,164,572, the contents of both of which are hereby incorporated by reference in their entireties. 
     According to one or more embodiments, the use of a co-firing technique to form a monolithic antenna structure  102  including an embedded antenna  106  allows for the manufacture of low-cost, miniaturized, hermetically sealed antenna structures  100  suitable for implantation within tissue and/or in direct or indirect contact with diverse body fluids. The monolithic antenna structure  102  can be hermetically connected directly to a portion of housing  14  of an IMD  10  or alternatively sealed within a header block  16 . 
     In one or more embodiments, the plurality of different individual discrete layers or sheets of materials (or segments of tape) that comprise the various ceramic dielectric layers  104 ,  108  and  112 , antenna conductor  106 , electromagnetic bandgap layer  115 , outermost biocompatible layer  110  and innermost shielding layer  114  may be printed with a metalized paste and other circuit patterns, stacked on each other, laminated together and subjected to a predetermined temperature and pressure regimen, and then fired at an elevated temperature(s) during which the majority of binder material(s) (present in the ceramic) and solvent(s) (present in the metalized paste) vaporizes and/or is incinerated while the remaining material fuses or sinters. The number of dielectric layers  104 ,  108  and  112  may be variably selected based on the desired antenna characteristics. In some embodiments, the materials suitable for use as cofireable conductors for forming the antenna conductor  106  are the biocompatible metal materials described herein or other materials suitable for the metalized paste. In one or more embodiments, the stacked laminates are then co-fired together at temperatures between about 850° C. and 1150° C. for LTCC materials and between about 1100° C. and 1700° C. for HTCC materials. 
     In one or more embodiments, the dielectric layers  104 ,  108  and  112  include a plurality of planar ceramic layers. Each ceramic layer may be shaped in a green state to have a desired layer thickness. In general, the formation of planar ceramic layers starts with a ceramic slurry formed by mixing a ceramic particulate, a thermoplastic polymer and solvents. This slurry is spread into ceramic sheets of predetermined thickness, from which the solvents are volitized, leaving self-supporting flexible green sheets. Holes in certain dielectric layers  104  and  112  that will be filled with conductive material to form via  116  are made, using any conventional technique, such as drilling, punching, laser cutting, etc., through the green sheets from which the ceramic layers  104  and  112  are formed. The materials suitable for use as cofireable ceramics include alumina (Al 2 O 3 ), aluminum nitride, beryllium oxide, Silica (SiO 2 ), Zirconia (ZrO 2 ), glass-ceramic materials, glass suspended in an organic (polymer) binder, or mixtures thereof. 
     Referring now to  FIG. 7 , a perspective, exploded view of an antenna structure  200  formed in accordance with one or more embodiments is illustrated in which a plurality of different antenna conductors  206   a - 206   g  having different antenna characteristics may be embedded within antenna structure  200 . Certain features and aspects of antenna structure  200  are similar to those described above in connection with antenna  100 , and shared features and aspects will not be redundantly described in the context of antenna structure  200 . Antenna structure  200  may include a plurality of discrete dielectric layers  204   a - 204   g  with at least one antenna conductor  206  respectively positioned on each discrete dielectric layer  204 . An outermost biocompatible layer  110  and an innermost ground shielding layer  114  are respectively arranged as the upper and lower surfaces of antenna structure  200 . Each of the antenna conductors  206   a - 206   g  may possess the same antenna configuration or different antenna configurations from the other antenna conductors  206   a - 206   g  arranged on different dielectric layers  204   a - 204   g.  Further, each of the dielectric layers  204   a - 204   g  may have the same or different dielectric values from the other dielectric layers  204   a - 204   g.  At least one switch is provided in order to allow different respective antenna conductors  206   a - 206   g  to be selectively switched in or out based the desired operating characteristics for antenna structure  100 . In this manner, antenna structure  100  can adapt to provide a specific desired radiation polarization, such that antenna structure  200  can be controlled to provide x-polarized, y-polarized and/or even circular polarizations with the simple toggling of switches to reconfigure antenna structure  200  to provide the desired performance. Similarly, antenna conductors  206   a - 206   g  may be selectively switched in or out to provide a specific desired radiation pattern. In this manner, the structure can be adapted to provide directivity so as to optimize the reception of a signal from a specific EMD or, alternatively, to optimize the transmission of a signal to a specific EMD. In one or more embodiments, MEMS switches may be utilized and located on respective layers of antenna structure  200  in order to maintain the miniaturization of antenna structure  100 . Antenna structure  200  is thus able to change frequencies by selectively switching the particular antenna conductors  206   a - 206   g  to utilize in order to increase or decrease the resultant antenna length. In some embodiments, multiple ones of antenna conductors  206   a - 206   g  may be switched to be connected and used together (e.g., through vias interconnecting antenna conductors  206   a - 206   g ). Further, the effective dielectric between the selected antenna conductor  206   a - 206   g  and both the surrounding environment and the ground shielding layer  114  can be switched to suit the needs of the particular IMD  10  and/or the particular implant location. 
     Referring now to  FIG. 8 , in one or more embodiments, a plurality of different antenna conductors  306   a - 306   c  may be formed on the same dielectric layer  304 , as illustrated by the partial schematic illustrate of a single dielectric layer  304  of antenna  100 . Certain features and aspects of dielectric layer  304  and antenna conductors  306   a - 306   c  are similar to those described above in connection with dielectric layer  104  and antenna conductor  106 , and shared features and aspects will not be redundantly described in the context of dielectric layer  304  and antenna conductors  306   a - 306   c.  A switch  302  may interconnect antenna conductors  306   a - 306   c  to via  116 , such that particular antenna conductors  306   a - 306   c  may be selectively switched to be used to reconfigure antenna structure  200  to provide the desired performance (e.g., desired antenna length, desired radiation polarization, desired radiation pattern, to account for particular IMD  10 , particular implant location, and/or particular EMD location, etc.). Each of the antenna conductors  306   a - 306   c  may possess the same or different antenna configurations as the other antenna conductors  306   a - 306   c.  In some embodiments, multiple antenna conductors  306   a - 306   c  on the same dielectric layer  304  may be connected and used together. In some embodiments, a plurality of different antenna conductors  306   a - 306   c  may be formed on a plurality of different dielectric layers, such as illustrated in  FIG. 7 , where specific dielectric layers may be selected and specific antenna conductors  306   a - 306   c  on a selected dielectric layer may be selected based on the desired antenna characteristics. 
     Referring now to  FIGS. 9A-9F , multiple different possible types of antenna arrangements for any of the antenna conductors  106 ,  206   a - 206   g,    306   a - 306   c  are illustrated in accordance with one or more embodiments. 
     The use of a multi-layer ceramic antenna structure  100  comprised of co-fired materials provide for reduced antenna volume, increased device density and functionality, and the ability to provide embedded antenna functionality, all in a hermetically-sealed monolithic antenna structure  102 . For example, in one embodiment, a multi-layer ceramic antenna structure  100  having structural dimensions of 50 mm×12.5 mm×1.0 mm can be produced, while in another embodiment, a multi-layer ceramic antenna structure  100  having structural dimensions of 20 mm×5 mm×0.4 mm can be produced. 
     In one or more embodiments, rather than forming a monolothic, multi-layer ceramic antenna structure  100  comprised of co-fired materials, the antenna conductor  106  may simply be coated with a high dielectric constant superstrate  108  coating, as illustrated in  FIG. 10 . The superstrate coating  108  may comprise one or more coatings of high dielectric constant material that are formed on the antenna conductor  106  by an anodization process. Anodization processes tend to be low in cost and highly reliable. It is also possible to deposit or form the high dielectric constant superstrate  108  coating on the antenna conductor  106  using other deposition techniques known to those skilled in the art. In this manner, an anodized antenna conductor  106  having a high dielectric constant superstrate coating  108  is provided. Coating the antenna conductor  106  with the high dielectric constant superstrate  108  provides a simple manner of improving antenna performance with a minimal change to existing device configurations while providing a matching gradient of dielectric constant between the antenna conductor  106  and the surrounding environment. The matching gradient reinforces the energy transition from the header  16  (e.g., ε=4) to the surrounding environment (e.g., ε=80) using the high dielectric constant superstrate  108  (e.g., ε≈10≈80). High dielectric loading may also modify the radiation pattern to reduce the power directly dissipated into the human body. In one or more embodiments, the high dielectric constant superstrate  108  coating may comprise silicone doped with high dielectric constant materials, such as titanium dioxide or barium strontium titanate (BST). 
     In accordance with one or more embodiments, the antenna conductor  106  (either anodized as described with reference to  FIG. 10  or non-anodized) may further be situated within the header  16  such that the superstrates  108  are formed as an antenna radome having a controlled dielectric gradient that encloses the antenna conductor  106  within the header  16 , as illustrated in the exploded perspective view of  FIG. 11 . In other embodiments, the superstrates  108  may simply be formed within the header  16  between the antenna conductor  106  and a surface of the header  16 . 
     In one or more of the embodiments described with reference to  FIGS. 10 and 11 , a layer of high electromagnetic impedance material (e.g., similar to electromagnetic bandgap layer  115 ) may be positioned below the antenna conductor  106  capable of suppressing the propagation of surface current in the ground (e.g., housing  14 ), thereby isolating the radiating elements from the nearby surroundings in order to further improve the radiation efficiency of the antenna conductor  106 , as illustrated in  FIG. 12 . 
     In one or more embodiments, when a multi-layer ceramic antenna structure  100  is formed from the various layers described herein in connection with  FIGS. 1-9 , one or more of the layers of the multi-layer ceramic antenna structure  100  may be patterned to possess a desired shape with respect to the antenna conductor  106 . For example, one or more of the layers of the multi-layer ceramic antenna structure  100  could be patterned to possess a substantially similar shape as the antenna conductor  106  such that the multi-layer ceramic antenna structure  100  could be formed as described herein in connection with  FIGS. 1-9  while having an overall shape that substantially mimics the shape of the antenna conductor  106  (e.g., such as the shape illustrated in  FIG. 10 ). In other embodiments, some of the layers (e.g., superstrate layers  108 ) of the multi-layer ceramic antenna structure  100  may be patterned to mimic the shape of the antenna conductor  106  while other layers in the multi-layer ceramic antenna structure  100  may be formed having different shapes. In still further embodiments, the various layers of the multi-layer ceramic antenna structure  100  could be patterned to possess other shapes to provide desired operational characteristics for the multi-layer ceramic antenna structure  100 . 
     While the system and method have been described in terms of what are presently considered to be specific embodiments, the disclosure need not be limited to the disclosed embodiments. It is intended to cover various modifications and similar arrangements included within the spirit and scope of the claims, the scope of which should be accorded the broadest interpretation so as to encompass all such modifications and similar structures. The present disclosure includes any and all embodiments of the following claims.