Patent Publication Number: US-2022226659-A1

Title: Wireless power transfer and heat mitigation circuit for a rechargeable implantable pulse generator

Description:
PRIORITY 
     This application claims priority to U.S. Provisional Application No. 63/139,162, filed Jan. 19, 2021, the contents of which are hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure generally relates generally to implantable medical devices, and more particularly to a system and method for controlling charging energy delivered to an implantable medical device using wireless power transfer. 
     BACKGROUND 
     Neurostimulation systems are devices that generate electrical pulses and deliver the pulses to nerve tissue to treat a variety of disorders. Spinal cord stimulation (SCS) is an example of neurostimulation in which electrical pulses are delivered to nerve tissue in the spine for the purpose of chronic pain control. Other examples include deep brain stimulation, cortical stimulation, cochlear nerve stimulation, peripheral nerve stimulation, vagal nerve stimulation, sacral nerve stimulation, and the like. 
     In addition to neurostimulation (NS) systems, numerous other medical devices exist today, including but not limited to electrocardiographs (ECGs), electroencephalographs (EEGs), squid magnetometers, implantable pacemakers, implantable cardioverter-defibrillators (ICOs), electrophysiology (EP) mapping and radio frequency (RF) ablation systems, and the like, that may be implanted within a patient for facilitating therapy and/or diagnostics. 
     In general, implantable medical devices (“IMDs”) are configured to be implanted within patient anatomy and commonly employ one or more electrical leads with electrodes that either receive or deliver voltage, current or other electromagnetic pulses from or to an organ or tissue for diagnostic or therapeutic purposes. 
     In order to provide consistent therapy and reliable operation over a substantial duration of time, IMDs are often provided with one or more rechargeable batteries that may be charged and recharged to store energy, which may supply power to the rest of the IMD circuitry and associated lead systems. 
     Because IMDs are implanted within patients, the IMDs are typically charged by an external charger that transmits energy wirelessly into the IMDs, such as through radio frequency (RF) signals. It is desirable that an IMD is generally charged as quickly and safely as possible within certain ranges depending upon the therapy application. However, if charging energy is input into the IMD too quickly and/or without proper regulation, the temperature of the IMD may increase to dangerous or uncomfortable levels causing tissue damage and other deleterious effects. It is further desired that wireless energy transfer between the external charger and the IMD&#39;s charging circuitry be performed as efficiently as possible. 
     Accordingly, there is a need to provide rapid, efficient and safe battery charging capabilities for IMDs. 
     SUMMARY 
     In one embodiment, an implantable medical device (IMD) is configured to provide stimulation therapy to a patient. The IMD comprises a rechargeable battery, pulse generating circuitry powered by the rechargeable battery, an inductive coupling element including at least one inductor operative to accept radio frequency (RF) power from an external charger and generate a charging voltage or charging current, recharging circuitry configured to generate a recharge current for recharging the rechargeable battery, the recharge current based on the charging voltage or charging current generated from the inductive coupling element, the recharging circuitry operable to detect a recharging level of the rechargeable battery, a temperature sensor configured to measure a temperature of at least a part of the IMD and output measured temperature data, and a control circuit configured to control the charging voltage or the charging current received at the recharging circuitry to limit the charging voltage or the charging current based upon the recharging level and to limit the charging voltage or charging current for a period of time based upon the measured temperature data to communicate a temperature level to the external charger. 
     In another embodiment, a charging system for an implantable medical device includes an IMD comprising: a rechargeable battery, pulse generating circuitry powered by the rechargeable battery; an inductive coupling element including at least one inductor operative to accept radio frequency (RF) power from an external charger and generate a charging voltage or charging current, a temperature sensor configured to measure a temperature of at least a part of the IMD and output measured temperature data; and regulation circuitry operative to regulate a level of the charging voltage or current based upon the outputted measured temperature data and to limit the charging voltage or charging current for a period of time based upon the outputted measured temperature data to communicate a temperature level to an external charger unit. The external charging unit is configured to provide the RF power to the inductive coupling element. 
     In yet another embodiment, a method of charging an implantable medical device (IMD) is disclosed. The IMD comprises a rechargeable battery, an inductive coupling element including at least one inductor operative to accept radio frequency (RF) power from an external charger and generate a charging voltage or charging current, a temperature sensor configured to measure a temperature of at least a part of the IMD and output measured temperature data; and regulation circuitry operative to regulate a level of the charging voltage or charging current. The method comprises analyzing, by the regulation circuitry, the measured temperature data, and controlling, by the regulation circuitry, the charging voltage or charging current based upon the measured temperature data, the charging current or charging voltage being controlled to communicate a temperature level to an external charger unit. 
     Additional/alternative features, variations and/or advantages of the embodiments will be apparent in view of the following description and accompanying Figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present disclosure are illustrated by way of example, and not by way of limitation, in the Figures of the accompanying drawings in which like references indicate similar elements. It should be noted that different references to “an” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and such references may mean at least one. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to effectuate such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. 
       The accompanying drawings are incorporated into and form a part of the specification to illustrate one or more exemplary embodiments of the present disclosure. Various advantages and features of the disclosure will be understood from the following Detailed Description taken in connection with the appended claims and with reference to the attached drawing Figures in which: 
         FIG. 1  depicts block diagrams of an external charging system and an implantable medical device (IMD) having wireless power transfer circuitry according to an embodiment; 
         FIG. 2  depicts a block diagram illustrating additional details of charge control and communications circuitry of an example IMD according to an embodiment; 
         FIG. 3  is a block diagram of a wireless power transfer system for purposes of an example embodiment of the present invention; 
         FIG. 4  is a circuit diagram of a frontend portion of a rechargeable IMD/IPG for facilitating wireless power transfer according to an embodiment of the present invention; 
         FIG. 5  depicts a flowchart of blocks, steps and/or acts that may be (re)combined in one or more arrangements with or without additional flowcharts of the present disclosure for facilitating charging operations according to some embodiments of the present disclosure; 
         FIG. 6  is a block diagram illustrating an external charging system and an implantable medical device (IMD) having wireless power transfer circuitry in use according to an embodiment; 
         FIG. 7  depicts an example IMD/IPG having a header portion and a body portion wherein an embodiment of the present invention may be practiced; 
         FIG. 8  illustrate exemplary waveforms and a temperature control table of an exemplary charging system of the present disclosure; and 
         FIG. 9  depicts an IMD charging system having wireless transfer circuitry according to an example embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     In the description herein for embodiments of the present disclosure, numerous specific details are provided, such as examples of circuits, devices, components, and/or methods, etc., to provide a thorough understanding of embodiments of the present disclosure. One skilled in the relevant art will recognize, however, that an embodiment of the disclosure can be practiced without one or more of the specific details, or with other apparatuses, systems, assemblies, methods, components, materials, parts, and/or the like. In other instances, well-known structures, materials, or operations are not specifically shown or described in detail to avoid obscuring aspects of embodiments of the present disclosure. Accordingly, it will be appreciated by one skilled in the art that the embodiments of the present disclosure may be practiced without such specific components. It should be further recognized that those of ordinary skill in the art, with the aid of the Detailed Description set forth herein and taking reference to the accompanying drawings, will be able to make and use one or more embodiments without undue experimentation. 
     Additionally, terms such as “coupled” and “connected,” along with their derivatives, may be used in the following description, claims, or both. It should be understood that these terms are not necessarily intended as synonyms for each other. “Coupled” may be used to indicate that two or more elements, which may or may not be in direct physical or electrical contact with each other, co-operate or interact with each other. “Connected” may be used to indicate the establishment of communication, i.e., a communicative relationship, between two or more elements that are coupled with each other. Further, in one or more example embodiments set forth herein, generally speaking, an electrical element, component or module may be configured to perform a function if the element may be programmed for performing or otherwise structurally arranged to perform that function. 
     Some embodiments described herein may be particularly set forth with respect to an implantable pulse generator (IPG) configured for generating electrical stimulation for application to a desired area of a body or tissue based on a suitable stimulation therapy application, such as a spinal cord stimulation (SCS) system. However, it should be understood that example wireless power transfer circuitry and methods of operation disclosed herein are not limited thereto, but have broad applicability, including but not limited to different types of implantable devices such as neuromuscular stimulators and sensors, dorsal root ganglion (DRG) stimulators, deep brain stimulator (DBS) devices, cochlear stimulators, retinal implanters, drug delivery systems, muscle stimulators, tissue stimulators, cardiac stimulators, gastric stimulators, and the like, including other bioelectrical sensors and sensing systems, which may be broadly referred to as “biostimulation” applications and/or implantable medical devices (IMDs) for purposes of the present disclosure. Moreover, example circuitry and methods of operation disclosed herein are not limited to use with respect to an IPG or any particular form of IPG or IMD. For example, some embodiments may be implemented with respect to a fully implantable pulse generator, a radio frequency (RF) pulse generator, an external pulse generator, a micro-implantable pulse generator, inter alia. 
     Referring to  FIG. 9  in particular, depicted therein is a biostimulation system  900  wherein one or more embodiments the present disclosure may be practiced in association with an IPG/IMD for achieving optimized wireless power transfer from an external charging system according to the teachings herein. By way of illustration, system  900  may be adapted to stimulate spinal cord tissue, peripheral nerve tissue, deep brain tissue, ORG tissue, cortical tissue, cardiac tissue, digestive tissue, pelvic floor tissue, or any other suitable biological tissue of interest within a patient&#39;s body, as noted above. System  900  comprises IMD  902  having a pulse generator portion that is adapted to include or otherwise interoperate with (re)chargeable battery circuitry for generating suitable stimulation pulses having adjustable target voltages that may be selectively applied for purposes of therapy. As will be set forth below in additional detail hereinbelow, IMD  902  may be implemented in one example embodiment as having a metallic housing or can that encloses a controller/processing block or module  912 , pulse generating circuitry  910 , charging voltage regulation module  911 , a charging coil  916 , a battery  918 , a far-field and/or near field communication block or module  924 , battery charging circuitry  922 , switching circuitry  920 , sensing circuitry  926 , one or more memory modules  914 , and the like. 
     Controller/processor module  912  typically includes a microcontroller or other suitable processor for controlling the various other components of IMD  902 . Software/firmware code may be stored in memory  914 , which may be integrated with the controller/processor module  912 , and/or other suitable application-specific storage components (not particularly shown in this FIG.) for execution by the microcontroller or processor  912  and/or other programmable logic blocks to control the various components of IMD  902  for purposes of an embodiment of the present patent disclosure. 
     In one arrangement, IMD  902  may be configured to couple to one or more stimulation leads  909 - 1  to  909 -M using an implantable multi-lead connector  908  operative to receive corresponding stimulation leads  909 - 1  to  909 -M at their respective proximal ends for securely engaging and providing electrical connectivity with respect to each stimulation lead&#39;s distal end having a plurality of stimulation electrodes. By way of illustration, stimulation lead  909 -M is exemplified with stimulation electrodes  904 - 1  to  904 -N, which may be implanted near or adjacent to the patient&#39;s target tissue. Stimulation leads  909 - 1  to  909 -M may comprise percutaneous leads, paddle leads, etc., wherein the electrodes may comprise ring electrodes, segmented or split electrodes, planar electrodes, and the like, that may be energized by the pulse generating circuitry  910  according to applicable therapy protocols/regimes. Preferably, a single lead cable  906  may be provided for electrically connecting the multi-lead connector  908  to IPG  902  via a suitable connector interface or socket  903  that may be mated to an interface receptacle or header portion  905  of IMD  902 . In general operation, electrical pulses may generated by the pulse generating circuitry  910  under the control of processing block  912 , which may be provided to the switching circuitry  920  that is operative to selectively connect to the electrical outputs of the IMD, which are ultimately coupled to one or more electrodes of any combination of leads  904 - 1  to  904 -M at a distal end of the lead system via respective electrical conductive traces. 
     An external device  930  may be implemented to charge/recharge the battery  918  of IMD  902 , to access memory  914 , and/or to program or reprogram IMD  902  with respect to the stimulation set parameters including pulsing specifications while implanted within the patient (although a separate recharging device could alternatively be employed). In alternative embodiments, accordingly, separate programmer and charger devices may be employed for charging and/or programming IMD  902  and/or any programmable components thereof. Regardless of whether charging functionalities and communication/programming functionalities are integrated, an example embodiment of the external device  930  may be a processor-based system that possesses wireline and/or wireless communication capabilities, near field magnetic/RF coupling capabilities, etc. Software may be stored within a non-transitory memory of the external device  930 , which may be executed by the processor  936  to control the various operations of the external device  930 . A connector or “wand”  934  may be electrically coupled to the external device  930  through suitable electrical connectors (not specifically shown), which may be electrically connected to a telemetry/charging component  932  (e.g., inductor coil, RF transceiver, etc.) at the distal end of wand  934  through respective links that allow bi-directional communication with IMD  902 . Optionally, in some embodiments, wand  934  may comprise one or more temperature sensors for use during charging operations. 
     Turning attention now to  FIG. 1 , depicted therein is a block diagram of charging system  100  comprising an external charger  102  and an IPG device  162  that includes an embodiment of wireless power transfer circuitry according to the teachings herein. For purposes of the present patent disclosure, example IPG  162  may comprise any of the IMDs having any number or type of lead systems set forth above. Accordingly, the terms “IMD”, “IPG”, or related terms of similar import will be somewhat synonymously used herein. In one arrangement, charger  102  may include a controller or processor  104  (e.g., any suitable commercially available microcontroller) for controlling the operations of charger  102  according to instructions stored in non-volatile (non-transitory) memory  106 . In one arrangement, charger  102  may be powered by a battery  110  having a suitable output voltage range. In some embodiments, battery  110  may comprise a rechargeable Lithium (Li) ion battery although other battery types or chemistries may be used. In some further embodiments, inductive step-up converters may be used in conjunction with a battery to obtain a suitable coil drive voltage. External charger  102  also comprises charging and communication circuitry  108 , which may be adapted or otherwise configured in some embodiments to electrically couple to a coil  107  operating as a charging energy source. In some embodiments, coil  107  may be disposed in an external wand (not shown in this FIG.) that may be held, during charging, by a patient or an authorized healthcare professional about the patient&#39;s body adjacent to an implant site of IMD  162 . Alternatively, the charger&#39;s coil  107  (which may be referred to as a primary coil) may be integrated in the same device package with the circuitry of charger  102 . Preferably, charging and communication circuitry  108  may be configured to drive the primary coil  107  using a suitable RF signal for charging purposes. In some arrangements, charging and communication circuitry  108  may also drive the coil  107  using a suitable modulated RF signal to communicate/receive data to/from IMD  162 . In still further embodiments, charger  102  may also be adapted for use as a controller to control the operations of IMD  162  by communicating suitable control parameters using communication circuitry  108 , as noted above. 
     Example IMD  162 , which is another representation of IMD  902  described above, is illustrated herein as comprising controller  164  (e.g., any suitable commercially available microcontroller that may be specially programmed for use as described herein) for controlling the pulse generation functionalities and other operations of IMD  162  according to instructions stored in non-volatile memory  166 . IMD  162  comprises pulse generating circuitry  172  for generating stimulation pulses for delivery to tissue of the patient. It should be appreciated that any suitable existing or later developed pulse generating circuitry may be employed. An example of pulse generating circuitry is described in U.S. Patent Application Publication No. 2006/0259098, entitled “SYSTEMS AND METHODS FOR USE IN PULSE GENERATION,” which is incorporated herein by reference. Pulse generating circuitry  172  may comprise one or multiple pulse sources. Also, pulse generating circuitry  172  may operate according to constant voltage stimulation, constant current stimulation, or any other suitable mode of operation. 
     The various components of IMD  162  are powered by one or more internal batteries  170  (e.g., Li-ion rechargeable batteries, NiCad or the like). Battery  170  may be recharged by converting electromagnetic, such as RF, power radiated or received from external charger  102 . Charging and communication circuitry  168  of IMD  162  is operative to couple to a coil  167  (referred to as a secondary coil) for effectuating near field coupling  150  with the coil  107  of external charger  102 . When external charger  102  radiates RF power using its coil  107 , the inductive coupling between the coil  107  of charger  102  with the coil  167  of IMD  162  causes an alternating current to be induced in the coil  167  of IMD  162 . As will be set forth in detail further below, at least a portion of circuitry  168  may be configured to utilize the induced current in order to provide a charging voltage to battery  170  in a controllable manner. Also, in some embodiments, circuitry  168  may optionally use the same coil  167  to effectuate control communications signaling with charger  102 . Further, it will be seen that an embodiment of the present disclosure advantageously uses only two feedthrough connections for connecting a coil-based frontend portion disposed in the header portion of IMD  162  to the rest of the internal circuitry of IMD  162 . As skilled artisans will appreciate, the pulse generation circuitry  172  may be coupled to one or more stimulation leads through electrical connections provided in the header portion of the IMD&#39;s housing (i.e., feedthroughs), and by minimizing the number of feedthroughs used for connecting electrical conductors for other purposes (e.g., charging/communications), the number of leads that may be deployed in a stimulation therapy system may be advantageously maximized. 
       FIG. 2  depicts a block diagram of charging circuitry  200 , which is a further representation of circuitry  168  of  FIG. 1 , illustrating additional components thereof according to one example embodiment. Circuitry  200  comprises coil and bridge rectifier circuitry  206 , wherein a coil thereof (e.g., secondary coil  167  shown in  FIG. 1 ) may be used for charging operations as well as communications with an external charger (e.g., charger  102 ) in some embodiments. In some other embodiments, the secondary coil may be used only for charging, with alterative links being available for communication purposes as previously noted. A near field receiver  202  is coupled to the coil, e.g., through a suitable capacitive arrangement as will be set forth further below. In one arrangement, receiver  202  may be configured to demodulate data when a carrier at an appropriate frequency is detected, whereupon a data stream may be communicated to controller  164 . In similar fashion, near field transmitter  204  may be configured in one arrangement to receive a data stream from controller  164  for generating a modulated RF signal therefor, which may be applied to the secondary coil to communicate data via NFC to charger  102 . Signal modulation and demodulation may, alternatively, be implemented in software executing on controller  164 . Further, in some example embodiments, near field receiver  202  and transmitter  204  may be configured to not operate (e.g., disabled) when charging operations are taking place. Accordingly, a separate charger transmitter  214  may be employed to provide charging status messages to charger  102  when charging/discharging operations are being effectuated. 
     In one example arrangement, sensor circuitry  210  may be provided to measure certain aspects of the IMD, such as temperature of the device or an individual component thereof, such as the temperature of the battery  170 , the metal can (housing) of the IMD, the coil  107 , controller  104 , voltage rectifier  206 , or any other component of charging circuitry  200 , for control of charging operations. In one embodiment, regulatory circuitry  216  is configured to control charging operations in response to one or more feedback/measurement signals (e.g., from sensor circuitry  210 ). 
     In one embodiment, charge control circuitry  208  may be provided to control the charging of battery  170 . In one embodiment, charge control circuitry  208  may be configured to use the measurement functionality of battery measurement circuitry  212  to detect the state of battery  170 . By way of illustration, charge control circuitry  208  may be operable to be battery measurement circuitry to measure the battery voltage, charging current, battery discharge current, temperature and/or the like. In some example embodiments, charge control circuitry  208  may prevent battery charging when an end-of-life (EOL) state has been reached for battery  170 , which may be determined responsive to measurements provided by battery measurement circuitry  212 . In further embodiments, charge control circuitry  208  may be configured to use a number of measurements to conduct fast charging operations as disclosed in greater detail in U.S. Patent Application Publication No. 2006/0259098, entitled “SYSTEMS AND METHODS FOR USE IN PULSE GENERATION,” incorporated by reference hereinabove. In still further embodiments, charge control circuitry  208  may also be configured to monitor one or more output signals from sensor circuitry  210  to further regulate the output voltage from rectifier circuitry  206 . 
       FIG. 3  is a high level circuit block diagram of a wireless charging system  300  for purposes of an example embodiment of the present disclosure. Broadly, a power sender block  302  is operative as an external charger  102  that supplies RF energy to a power receiver block  350  (e.g., an IMD  162 ,  900 ) through respective series resonant coils that operate as a loosely coupled transformer (i.e., via magnetic coupling). A DC voltage input (V1N) having a suitably configurable voltage range is provided to the power sender block  302 , which includes a DC-to-AC converter  304  coupled to a sender-side tuning circuit comprising a primary coil  306  and a capacitor  308  connected in series. A clamp detector/monitor  310  may be included in the power sender block  302  for sensing the state of input current ( 11 N). In one example embodiment, clamp detector/monitor  310  may be configured to generate a control signal  303  to DC-to-AC converter  304  in response to the input current status. It should be appreciated that DC-to-AC converter  304  is operative as a coil driver in order to supply adequate RF power to the power receiver block  350 . When power receiver block  350  is not accepting power during a charging cycle (e.g., due to internal voltage/charging regulation and/or other internal ambient and status control signals), current flow through the sender-side tuning circuit is negligible (i.e., turned off), which condition may be sensed as a status change in the input current by the clamp detector/monitor circuitry  310  to generate control signal  303  operative to deactivate the power sender circuitry during the off state, thereby saving power. 
     To effectuate near field inductive RF power transfer, the power receiver block  350  is provided with a receiver-side tuning circuit comprising at least a secondary coil  352  coupled to at least a capacitor  354  in series (e.g., similar to the sender-side tuning circuit arrangement). An induced AC signal from the receiver-side tuning circuit is rectified by a rectifier  358 , whose output may be optionally and/or suitably conditioned to apply power to a load, i.e., a battery  368  having terminals  366 A,  3668 . In an example arrangement, battery  368  may be disposed between output nodes  364 A,  3648  of conditioning circuitry having an output capacitor arrangement (C OUT )  362  for providing a suitable DC output voltage (V CHG  or V OUT ). In one example embodiment, voltage regulation control circuitry  360  may be coupled between the rectifier/conditioning portion  358  and battery load  368 , which may be configured to generate one or more control signals for controlling a series switch arrangement  356  connected to the receiver-side tuning circuit arrangement. 
     It should be appreciated that the relationships between the sender-side coil voltage and current and the receiver-side coil voltage and current may be determined in an example implementation by the series tuning of the respective coils. For instance, such relationships may depend upon the operating frequency, tuning accuracy, coil separation, coil geometries, and the like. Accordingly, power transfer in an example arrangement involving wireless charging system  300  may in general depend on coupling between coils  306 ,  352 , which in turn may depend on the distance between coils  306 ,  352 , alignment, coil dimensions, coil materials, respective number of turns, magnetic shielding, impedance matching, applicable power band and associated resonant frequency, duty cycle, etc. Skilled artisans will recognize that at least some of these parameters may be selected in the design of an embodiment in order to comply with known or heretofore unknown wireless power transfer standards and specifications (e.g., Wireless Power Consortium WPC 1.1 Standard). Further, the voltage regulation control circuitry  360  may be appropriately configured in an example embodiment such that the time spent in the ON and OFF states may be suitably designed depending on the IMD application. In one embodiment, the time spent in the ON and OFF states is used as a machine readable code for indicating the temperature of one or more components of the IMD  162 , as further described below. 
     In another embodiment, the time spent in the ON and OFF states may be determined based on an applicable voltage hysteresis band (V HIGH -V LOW ), the rectifier output current IR and the load current I OUT . In one example embodiment, an upper output threshold voltage V HIGH  that begins clamping may be selected to be at 4.5V and a lower threshold voltage V LOW  that ends clamping may be selected to be at 4.1 V, resulting in a nominal voltage hysteresis voltage of 0.13 V. In embodiments, the DC voltage should be within a range of from 4.1V to 4.5V. In an example embodiment, the power sender block  302  may be configured to continually adjust its RF output power to maintain at least substantially constant power transfer to the power receiver block  350  across a range of distances. Further, certain additional design criteria may be implemented in order to achieve maximum power transfer efficiency in an implementation. For example, one requirement may be that the charger, i.e., power sender block  302 , should deliver a select battery charging current suitable for a use case or application scenario. In an example use case, such a requirement may comprise a charging current of 50 mA. Another design requirement may be that the charger should deactivate during the OFF states to conserve power. 
     Accordingly, in one arrangement, the clamp detector/monitor circuit  310  of the power sender block  302  may be configured to sense the time periods between clamping events of the power receiver block  350  in order to modulate the output power, as previously noted. Related details with respect to utilizing a clamp detection signal in a charging system may be found in U.S. Pat. No. 8,731,682, entitled “EXTERNAL CHARGING DEVICE FOR CHARGING AN IMPLANTABLE MEDICAL DEVICE AND METHODS OF REGULATING DUTY CYCLE OF AN EXTERNAL CHARGING DEVICE,” incorporated by herein. 
     In one embodiment, the wireless charging system  300  may be configured such that it involves only two feedthrough connections for connecting the receiver-side tuning circuit comprising coil  352  and capacitor  354  to the rest of the IMD internal circuitry. Moreover, the series switch arrangement  356  may be configured such that the receiver-side tuning circuit may be detuned or otherwise disabled during the OFF condition, thereby advantageously eliminating a high voltage condition that can develop during the time when the power receiver block  350  is in the clamped state because the receiver-side tuning circuit may be in resonance. As one skilled in the art will appreciate, the voltage in the secondary coil  352  can reach significantly high levels in the clamped state in some implementations (e.g., as high as 300 V), which is highly undesirable in an IMD application. It is noted that the secondary coil  352  may be the same as secondary coil  167  in some embodiments. 
       FIG. 4  depicts a circuit diagram of a frontend portion  400  of a rechargeable IMD/IPG device operating as a power receiver for facilitating wireless power transfer according to an example embodiment of the present disclosure. An inductive coupling element  406  comprising at least one inductor or coil  402  connected with at least one capacitor  404  in a series LC circuit configuration is operative as a receiver-side tuning circuit wherein the at least one capacitor  404  may be configured to be tunable over a range of frequencies. In one example implementation, coil  402  may comprise an inductor or its equivalent having an inductance of about 350-500 microhenries (pH) and tuning capacitor  404  may comprise a capacitance of about 500-1000 picofarads (pF) or its equivalent. Regardless of the actual number and/or type of inductors and/or tuning capacitors used in a particular implementation, a lumped-element model of the series LC circuit configuration of RF coupling element  406  may preferably be connected in an arrangement that defines a first electrical node  408 A at a terminal of at least one inductor  402  and a second electrical node  4088  at a terminal of at least one capacitor  404 . Where the LC circuit configuration forming the inductive coupling element  406  is disposed in the IMD&#39;s header, nodes  408 A/ 408 B are operative to be electrically connected to the remainder of the frontend circuitry  400  via respective feedthroughs in accordance with the teachings herein. A series switch  412  is disposed between the first electrical node  408 A and a trace  410 A coupled to an input terminal of a bridge rectifier (not shown in this FIG.) for detuning the LC circuit element  406  during the OFF state of the power receiver. In one embodiment, switch  412  may comprise an N-channel metal oxide semiconductor field-effect transistor (NMOSFET) that may be opened when the charging is OFF. During the ON period, switch  412  may be configured to be automatically closed by deriving a gate drive voltage from the LC circuit element  406 . On the other hand, switch  412  may be configured to be opened in the OFF state responsive to a gate control signal derived from a clamp signal using appropriate logic circuitry. Skilled artisans will recognize upon reference hereto that suitable switch protection circuitry and/or ON-state gate control circuitry may be provided using appropriate electrical/electronic components including but not limited to, inter alia, capacitors, transistors, FETs, diodes, etc., in various combinations to control and condition power transfer operations depending on a particular wireless power transfer application. 
     In one implementation, a Zener diode  420  may be connected between drain and source nodes/terminals of switch  412  in order to provide protection therefor against inductive spikes. For example, a Zener diode of appropriate electrical characteristics may be disposed for providing clamping protection against inductive spikes at around 30 V to 60 V. A pair of Schottky diodes  416 A,  416 B coupled in a configuration such that respective cathodes thereof are commonly connected to a resistor  415 , which in turn is connected to a gate of switch FET  412 . A capacitor  418  may be disposed between the gate and one of the terminals of switch FET  412  (e.g., source node coupled to bridge rectifier trace  410 A). Anode terminal of Schottky diode  416 A is coupled to a resistor  414 , which in turn is commonly connected to the cathode terminal of Zener diode  420 , first electrical node  408 A and a terminal of switch FET  412  (e.g., drain). On the other hand, anode terminal of Schottky diode  416 B may be directly coupled to bridge rectifier trace  410 A. In one implementation, resistor  414  may have a resistance of about 5-15 kOhms and resistor  415  may have a resistance of about 0.5-1.5 kOhms. In one implementation, capacitor  418  may comprise a capacitor rated to about 50 V±10% and having a capacitance of about 500-1500 pF. 
     Appropriate control signaling for the LC circuit configuration of inductive coupling element  406  and as well as gate control for switch  412  may be effectuated by way of a frontend control signaling portion  423  (referred to herein as a “clamp circuit” or “clamp control circuitry”) that is driven by a clamp control signal  424  generated by a voltage regulation control block  426  operative to provide clamping that in one embodiment is used to provide temperature data, as well as optional over-voltage protection in another embodiment. In one implementation, clamp control signaling portion  423  comprises a pair of FETs  422 A/ 4228  whose respective gates are driven by clamp control signal  424 , wherein source terminals thereof are commonly tied to a reference potential, e.g., ground. Whereas a drain terminal of FET  4228  is connected to the common cathode connection of Schottky diodes  416 A,  416 B, a drain terminal of FET  422 A is connected the second electrical node  408 B formed at a terminal of at least one capacitor  404  of the LC circuit configuration. Further, the second electrical node  408 B is also coupled to a trace  410 B extending to a second input terminal of the bridge rectifier (shown in  FIG. 5 ). In one implementation, when clamp control signal  424  is asserted (e.g., a logic high) during the OFF state, gate voltages of FETs  422 A and  422 B are driven high, thereby causing FETs  422 A and  422 B to be turned on. As FET  422 A is turned on, the second electrical node  408 B connected to bridge rectifier trace  410 B is pulled to ground. At the same time, as FET  422 B is turned on, it causes the gate terminal of series switch FET  412  to be pulled to ground. Accordingly, series switch FET  412  is turned off, thereby opening the series connection path between the first electrical node  408 A and bridge rectifier trace  410 A. As a result, the series LC circuit is opened during the OFF state, whereby it is caused to be detuned with respect to a primary coil in the external charger. Since it is detuned, there is no resonance-caused high voltage condition developed in the receiver-side circuitry of an IMD. As noted above, series switch FET  412  is automatically closed in the ON state (e.g., clamp control signal  424  is deasserted), wherein a suitable gate drive voltage is derived from the LC circuit component  406  whose output is conditioned through the Schottky diode arrangement  416 A/ 416 B. 
     A process for charging an IMD according to the present disclosure is now further described with reference to  FIG. 5 . At operation  500 , the charger  102  is activated, for example by pressing a button or powering on the charger  102  using a user interface  600  ( FIG. 6 ). The user interface is electronically coupled to charger  102  and may include one or more of a display, such as an electronic display using LCD, LED or the like, and a user input, such as one or more buttons, keyboard, or touchscreen (which may be the display). After the charger  102  has been activated, the charging parameters are set at operation  502 . During the setting of parameters  502 , one or more of the charging frequency, maximum current, minimum current, normal current and the like may be set for the charger  102 . At this point, the charger  102  should be within close enough proximity to IMD  162  to enable the electric charging field (e.g., RF emissions) of charger  102  to be received by coil  167 . At operation  504 , the charger  102  detects, using charging and comm circuitry  108 , whether coil  107  is electromagnetically coupled to secondary coil  167  of the IMD  162 . In another embodiment, the IMD  162 , using charging and comm circuitry  168  detects whether coil  107  is electromagnetically coupled to secondary coil  167  of the IMD  162 . Once such electromagnetic coupling of coil  107  to secondary coil  167  has occurred, mutual stabilization of the magnetic charging is facilitated, and the RF field emitted from coil  107  is received by secondary coil  167  such that a current is generated by coil  167  converted to a DC current by charging and comms circuitry  168  for charging battery  170 . 
     Subsequently, a charging cycle  506  may begin. During the charging cycle  506  the typical converted DC voltage V CHG  (e.g., from the rectifier  358 ) is from about 4.1V to 4.5V, but may be any voltage that allows the charger to function as described herein. The converted DC voltage is used both to charge the battery  368  (which may be the same as battery  170 ) as well as the capacitor  362 . The charging cycle  506  may cause the temperature of the IMD  162  to rise. However, in some embodiments, one or more of the IMD housing, coil  167 , battery  368 , secondary coil  352 , charging and comm circuitry  168  or any other component of IMD  162  may rise during the charging cycle  506 . The temperature of one or more of the components is measured by a temperature sensor of sensor circuitry  210 . 
     Just prior to the beginning of the charging cycle  506 , the temperature of the IMD  162  or one or more of its components is approximately within the range of from 36° C. to about 37° C. Once the charging cycle  506  begins, the temperature of the IMD  162  or one or more of its components starts to increase to within the range of from 37° C. to about 39° C. In one embodiment, the sensor circuitry outputs temperature data to the charge control circuitry  208 . In one embodiment, the voltage regulation control block  426  utilizes the temperature data, which is received by the voltage regulation control block  426  to clamp the circuit, such that no load (OFF state) or no charging of the battery  368  occurs. Such clamping of the circuit, and the associated time spent in the ON and OFF states is used in one embodiment to provide a machine readable code for determining the temperature of the IMD  162  or one or more of its components. 
     With reference to  FIG. 8 , the coding of the temperature data is further described. During charging  506  of the battery  368 , in a normal (non-temperature regulated) charging state  800  the charging voltage V CHG  follows a predetermined, or scheduled, amount of time in the ON state (unclamped state) in which the battery  368  is being charged. The ON state  802  is represented in normal charging state  800  by the upper portion of the line, while the clamped OFF state is represented by the troughs  804 , which have a pulse width  806 . The pulse width  806  representing a time spent in the OFF state. In one embodiment, as described above, the voltage regulation control block  426  is programmed such that the pulse width  806  (i.e., the time spent in the clamped, OFF state) is based upon the temperature data output by the sensor circuitry  210 . 
     In one exemplary embodiment, a temperature coding database  810  (also referred to as a temperature coding map) is accessed and utilized by voltage regulation control block  426  to determine the pulse width  806 . An exemplary temperature coding map is shown below at Table 1. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Temperature Coding Map 
               
            
           
           
               
               
               
            
               
                   
                 Temperature 
                 Coded Pulse Width 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                   
                 36° C. and below 
                 0 
                 ms 
               
               
                   
                 From 36° C. to 37° C. 
                 2-3 
                 ms 
               
               
                   
                 From 37° C. to 38° C. 
                 3-4 
                 ms 
               
               
                   
                 From 38° C. to 39° C. 
                 4-5 
                 ms 
               
               
                   
                 From 39° C. to 40° C. 
                 5-6 
                 ms 
               
               
                   
                 From 40° C. to 41° C. 
                 6-7 
                 ms 
               
               
                   
                 From 41° C. to 42° C. 
                 7-8 
                 ms 
               
               
                   
                 From 42° C. to 43° C. 
                 8-9 
                 ms 
               
            
           
           
               
               
               
            
               
                   
                 From 43° C. or greater 
                 Full Window 
               
               
                   
                   
               
            
           
         
       
     
     As shown in Table 1, the coded pulse with is based upon the temperature of the IMD  162  or one of its components for which the temperature data from the sensor circuit  210  is associated with. In an exemplary embodiment, a temperature coded charging waveform  850  includes a coded pulse width  860 . It should be noted that the temperature coding map is exemplary, and in other embodiments the temperatures and/or the coded pulse widths may be essentially any values that allow the charger to function as described herein. In one embodiment, the Full Window value is 40 ms, but in other embodiments the Full Window value may be more or less than 40 ms and essentially any time value that allows the charge to function as described herein. 
     In one embodiment, the clamp detector/monitor circuit  310  of the power sender block  302  ( FIG. 3 ) may be configured to sense the pulse widths  806 . The pulse widths  806  are monitored at operation  508  ( FIG. 5 ) in order to modulate the output power of the power sender block  302 . The monitored pulse widths  806  are evaluated  510  by the clamp detector/monitor circuit  310 , using the temperature coding map to determine an associated temperature of the IMD  162 . If the determined temperature parameter falls within a predetermined threshold, such as from 38° C. to 39° C., (e.g., low risk) the output power of the power sender block  302  may be maintained at a specified value. However, if the determined temperature parameter falls within a predetermined threshold indicated to be of increased risk (such as a medium risk or high risk) based upon the determined temperature, such as any temperature above 39° C., one or more output values of power sender block  302  may be adjusted  512 . For example, if the determined temperature presents an increased risk, one or more of the charging gate timing or magnetic load (e.g., the output of the primary coil  306 ) may be reduced until the determined temperature value is reduced to a within the predetermined threshold or below. In one embodiment, the clamp detector/monitor circuit  310  monitors the pulse widths on a continuous (e.g., real time) basis. In the embodiments, the clamp detector/monitor circuit  310  monitors the pulse widths  806  on a periodic basis according to a set timing of predetermined intervals. In some embodiments, the clamp detector/monitor circuit  310  monitors a rate of change of the determined temperatures, and if the determined temperatures indicate an increasing temperature change, it may trigger the increased risk state, and conversely, if the determined temperatures indicate a decreasing temperature change, it may indicate a low risk state. 
     In some embodiments, if there is determined to be a medium risk or high risk state, the power sender block  302  may be controlled to automatically turn off for a predetermined period of time, such as 5 ms. It should be noted, that in some embodiments, the battery  368  may continue to charge for a period of time due to the discharge of capacitor  362 . In some embodiments, the amount of time that the power sender block  302  is controlled to be in the off state may be longer than, or the same as, the period of time the battery continues to charge from the capacitor  362 . After the predetermined period of time, the power sender block  302  is controlled to turn back on to continue the charging cycle. In some embodiment, after the power sender block  302  is turned back on, it may be set to output at a normal or reduced power output level, depending on the determined temperature, or risk level, of the IMD  162 , or a component thereof. 
     At operation  514 , the level of charge of the battery  368  is determined by the charger  102 . The level of charge may be determined, in some embodiments, by using the determined temperature level of the battery  368 . In other embodiments, the IMD  162  may send a signal via charging and comm circuitry  168  of the IMD  162  to the charging and comm circuitry  108  of the charger  102  indicating a charge level of the battery. If the charge level of the battery  368  is full, the charger  102  is controlled to turn off to end the charging cycle. However, if the charging level is indicated to be less than full, the charging cycle  506  is continued. 
       FIG. 7  depicts an example IMD/IPG housing  700  having a header portion  702  and a body portion  704  wherein an embodiment of the present invention may be practiced. Regardless of any particular form factor, header portion  702  may preferably be configured to operate as a housing portion for an inductive coupling component or circuit that may comprise one or more inductors and one or more tuning capacitors in a series LC configuration  706  having two feedthrough terminals. Likewise, body portion  704  may be configured to house an IPG circuit portion  708  that may include various pieces of the circuitry described in detail hereinabove, e.g., including frontend circuitry portion, bridge circuitry portion, voltage regulation circuitry portion, battery, etc., as exemplified by various blocks  710 ,  712 ,  714 , in addition to one or more other blocks or functionalities set forth in reference to  FIG. 9 . As previously noted, electrical connectivity between LC configuration circuit  706  and IPG circuit portion  708  may be accomplished using only two feedthrough paths controlled by a series detuning switch in accordance with the described above, whereby the availability of remaining feedthroughs may be maximized for other purposes (e.g., for supporting additional lead systems). 
     In the above-description of various embodiments of the present disclosure, it is to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and may not be interpreted in an idealized or overly formal sense expressly so defined herein. 
     At least some example embodiments are described herein with reference to one or more circuit diagrams/schematics, block diagrams and/or flowchart illustrations. It is understood that such diagrams and/or flowchart illustrations, and combinations of blocks in the block diagrams and/or flowchart illustrations, can be implemented by any appropriate circuitry configured to achieve the desired functionalities. Accordingly, example embodiments of the present disclosure may be embodied in hardware and/or in software (including firmware, resident software, micro-code, etc.) operating in conjunction with suitable processing units or microcontrollers, which may collectively be referred to as “circuitry,” “a module” or variants thereof. An example processing unit or a module may include, by way of illustration, a general purpose processor, a special purpose processor, a conventional processor, a digital signal processor (DSP), a plurality of microprocessors, one or more microprocessors in association with a DSP core, a controller, a microcontroller, Application Specific Integrated Circuits (ASICs), Field Programmable Gate Array (FPGA) circuits, any other type of integrated circuit (IC), and/or a state machine, as well as programmable system devices (PSDs) employing system-on-chip (SoC) architectures that combine memory functions with programmable logic on a chip that is designed to work with a standard microcontroller. Example memory modules or storage circuitry may include volatile and/or nonvolatile memories such as, e.g., random access memory (RAM), electrically erasable/programmable read-only memories (EEPROMs) or UV-EPROMS, one-time programmable (OTP) memories, Flash memories, static RAM (SRAM), etc. 
     Skilled artisans will recognize upon reference hereto that various switching components of one or more circuits described herein may be implemented using a variety of monolithic or integrated semiconductor devices known in the electrical arts, e.g., including but not limited to bipolar junction transistors (BJTs), metal oxide semiconductor field effect transistors (MOSFETS), junction gate FETs (JFETs), n-channel MOSFET (NMOS) devices, p-channel MOSFET (PMOS) devices, depletion-mode or enhancement-mode devices, and the like, as well as any logic gates built therefrom. Likewise, various types of comparators, e.g., inverting and/or non-inverting comparators, latched comparators, single ended comparators, differential op amp circuits and the like may be implemented in an example embodiment. It will be further understood that the sizing (e.g., channel width and length) and biasing of the switching devices used in any of the components can be highly configurable, depending on the voltage/current ratings, application requirements, and the like. 
     Further, in at least some additional and/or alternative implementations, the functions/acts described in the blocks may occur out of the order shown in the flowcharts. For example, two blocks shown in succession may in fact be executed substantially concurrently or the blocks may sometimes be executed in the reverse order, depending upon the functionality/acts involved. Moreover, the functionality of a given block of the flowcharts and/or block diagrams may be separated into multiple blocks and/or the functionality of two or more blocks of the flowcharts and/or block diagrams may be at least partially integrated. Furthermore, although some of the diagrams include arrows on communication paths to show a primary direction of communication, it is to be understood that communication may occur in the opposite direction relative to the depicted arrows. Finally, other blocks may be added/inserted between the blocks that are illustrated. 
     It should therefore be clearly understood that the order or sequence of the acts, steps, functions, components or blocks illustrated in any of the flowcharts depicted in the drawing Figures of the present disclosure may be modified, altered, replaced, customized or otherwise rearranged within a particular flowchart, including deletion or omission of a particular act, step, function, component or block. Moreover, the acts, steps, functions, components or blocks illustrated in a particular flowchart may be inter-mixed or otherwise inter-arranged or rearranged with the acts, steps, functions, components or blocks illustrated in another flowchart in order to effectuate additional variations, modifications and configurations with respect to one or more processes for purposes of practicing the teachings of the present patent disclosure. 
     The following embodiments are provided to illustrate aspects of the disclosure, although the embodiments are not intended to be limiting and other aspects and/or embodiments may also be provided. 
     Embodiment 1. An implantable medical device (IMD) configured to provide stimulation therapy to a patient, the IMD comprising: a rechargeable battery; pulse generating circuitry powered by the rechargeable battery; an inductive coupling element including at least one inductor operative to accept radio frequency (RF) power from an external charger and generate a charging voltage or charging current; recharging circuitry configured to generate a recharge current for recharging the rechargeable battery, the recharge current based on the charging voltage or charging current generated from the inductive coupling element, the recharging circuitry operable to detect a recharging level of the rechargeable battery; a temperature sensor configured to measure a temperature of at least a part of the IMD and output measured temperature data; and a control circuit configured to control the charging voltage or the charging current received at the recharging circuitry to limit the charging voltage or the charging current based upon the recharging level and to limit the charging voltage or charging current for a period of time based upon the measured temperature data to communicate a temperature level to the external charger. 
     Embodiment 2. The IMD of Embodiment 1, wherein the control circuit is operative to clamp the output voltage. 
     Embodiment 3. The IMD of any prior Embodiment, wherein the control circuit is operative to regulate a level of the charging voltage or charging current for a predetermined period of time based upon the measured temperature data. 
     Embodiment 4. The IMD of any prior Embodiment, wherein the predetermined period of time is based on a lookup table accessed by the control circuit. 
     Embodiment 5. The IMD of any prior Embodiment, wherein the temperature sensor measures the temperature of the rechargeable battery. 
     Embodiment 6. The IMD of any prior Embodiment, wherein the temperature sensor measures the temperature of at least one of the rechargeable battery, a housing of the IMD, the inductive coupling element or the pulse generating circuitry. 
     Embodiment 7. A charging system for an implantable medical device, the system comprising: an IMD comprising: a rechargeable battery, pulse generating circuitry powered by the rechargeable battery; an inductive coupling element including at least one inductor operative to accept radio frequency (RF) power from an external charger and generate a charging voltage or charging current, a temperature sensor configured to measure a temperature of at least a part of the IMD and output measured temperature data; and regulation circuitry operative to regulate a level of the charging voltage or current based upon the outputted measured temperature data and to limit the charging voltage or charging current for a period of time based upon the outputted measured temperature data to communicate a temperature level to an external charger unit; and the external charging unit, wherein the external charging unit is configured to provide the RF power to the inductive coupling element. 
     Embodiment 8. The charging system according to Embodiment 7, wherein the regulation circuitry is operative to clamp the charging voltage. 
     Embodiment 9. The charging system of any prior Embodiment, wherein the regulation circuitry is programmed to clamp the charging voltage for a predetermined period of time based upon the outputted measured temperature data, the predetermined period of time defining a pulse width for communicating the temperature level to the external charger unit. 
     Embodiment 10. The charging system of any prior Embodiment, wherein the external charging unit comprises a clamping detection circuit operative to detect a waveform that is output from the inductive coupling element based upon the pulse width. 
     Embodiment 11. The charging system of any prior Embodiment, wherein the external charging unit is configured to determine a temperature based risk level of the IMD based upon the detected waveform. 
     Embodiment 12. The charging system of any prior Embodiment, wherein the RF power outputted from the external charging unit is adjusted based upon the determined temperature based risk level. 
     Embodiment 13. The charging system of any prior Embodiment, wherein the RF power is reduced if the determined temperature based risk level is a medium level or a high level. 
     Embodiment 14. The charging system of any prior Embodiment, wherein the RF power is adjusted for a predetermined period of time based upon the determined temperature based risk level. 
     Embodiment 15. A method of charging an implantable medical device (IMD), the IMD comprising a rechargeable battery, an inductive coupling element including at least one inductor operative to accept radio frequency (RF) power from an external charger and generate a charging voltage or charging current, a temperature sensor configured to measure a temperature of at least a part of the IMD and output measured temperature data; and regulation circuitry operative to regulate a level of the charging voltage or charging current, the method comprising: analyzing, by the regulation circuitry, the measured temperature data, and controlling, by the regulation circuitry, the charging voltage or charging current based upon the measured temperature data, the charging current or charging voltage being controlled to communicate a temperature level to an external charger unit. 
     Embodiment 16. The method according to Embodiment 15, further comprising: clamping the charging voltage for a predetermined period of time based upon the measured temperature data. 
     Embodiment 17. The method of any prior Embodiment, wherein the clamping the charging voltage for the predetermined period of time defines a waveform output by the inductor for communicating the temperature level, the method further comprising adjusting the RF power from the external charger based upon the waveform. 
     Embodiment 18. The method of any prior Embodiment, wherein the external charger determines a risk level associated with the temperature level and reduces the RF power if the determined risk level is a medium level or a high level. 
     Embodiment 19. The method of any prior Embodiment, wherein the external charger is controlled to determine the risk level based upon a temperature coding map. 
     Embodiment 20. The method of any prior Embodiment, wherein the temperature coding map comprises at least a plurality of temperatures and a plurality of pulse widths associated with the plurality of temperatures. 
     Although various embodiments have been shown and described in detail, the claims are not limited to any particular embodiment or example. None of the above Detailed Description should be read as implying that any particular component, element, step, act, or function is essential such that it must be included in the scope of the claims. Reference to an element in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather “one or more.” All structural and functional equivalents to the elements of the above-described embodiments that are known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present claims. Accordingly, those skilled in the art will recognize that the exemplary embodiments described herein can be practiced with various modifications and alterations within the spirit and scope of the claims appended below.