Abstract:
The present disclosure relates to an improved transcutaneous energy transfer (TET) system that generates and wirelessly transmits a sufficient amount of energy to power one or more implanted devices, including a heart pump, while maintaining the system&#39;s efficiency, safety, and overall convenience of use. The disclosure further relates one or more methods of operation for the improved system.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims the benefit of the filing date of U.S. Provisional Patent Application No. 61/979,835 filed Apr. 15, 2014, the disclosure of which is hereby incorporated herein by reference. 
     
    
     FIELD OF THE TECHNOLOGY 
       [0002]    The present invention relates to transcutaneous energy transfer (TET) systems and methods of operation for such systems. 
       BACKGROUND 
       [0003]    Transcutaneous energy transfer (TET) systems are used to supply power to devices such as pumps implanted internally within a human body. A magnetic field generated by a transmitting coil outside the body can transmit power across a cutaneous (skin) barrier to a magnetic receiving coil implanted within the body. The receiving coil can then transfer the received power to the implanted pump or other internal device and to one or more batteries implanted within the body to charge the battery. 
         [0004]    Such systems should efficiently generate and wirelessly transmit a sufficient amount of energy to power one or more implanted devices while maintaining the system&#39;s efficiency, safety, and overall convenience of use. 
         [0005]    With respect to those systems&#39; efficiency, one drawback suffered by present TET systems arises from the nature of the magnetic field generated by the transmitting coil. By its nature, the field extends from the transmitting coil in every direction. As such, much of the energy from the electromagnetic field emitted by the transmitting coil is not focused effectively or optimally at the receiving coil. This limits the efficiency (i.e., the coupling coefficient) of the wireless energy transfer. Another challenge arises from the fact that power and/or current demands of an implanted device are not constant but rather subject to vary. As such, there is a need to efficiently accommodate such changes in power and/or current demand in order to most effectively power the implanted device. 
         [0006]    With respect to convenience of the system, one challenge among present TET systems arises from the difficulty in maintaining optimal axial alignment (in proximity to the surface of the patient&#39;s skin) and radial alignment (across the surface of the patient&#39;s skin) between the transmitting and receiving coils to increase power transfer efficiency and minimize transmitting coil losses that would result in heating. Firstly, a transmitting coil worn on the exterior of the body is subject to shift in position, such as due to movement by the wearer. Moreover, once the transmitting coil is shifted out of place, repositioning the coil, such as determining in which direction to move the coil in order to reestablish alignment, may be difficult without some form of guidance. As such, there is a need for a system that assists the wearer in positioning or repositioning the transmitting coil. 
         [0007]    Further, a shift in the position of a transmitting coil worn on the exterior of the body also poses issues with respect to health and safety of the system&#39;s wearer. If the coil shifts out of its proper alignment while operating at full power, not only may the coupling coefficient of the power transfer be reduced, but it may cause unwanted overheating to the wearer, and such overheating may be harmful to the skin or surrounding tissue. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    One aspect of the present disclosure provides for a transcutaneous energy transfer system, including: an internal component having a power-consuming device and an internal coil electrically connected to the power-consuming device, the internal component being adapted for mounting within the body of an animal; an external coil adapted for mounting outside of the body; a current monitor operative to measure current flow in the external coil and to provide an indication of whether or not the external coil is electromagnetically coupled to the internal coil based on the measured current flow; and a drive circuit operative to apply a power-level alternating potential to the external coil responsive to an indication from the current monitor that the external coil is electromagnetically coupled to the internal coil. The drive circuit may also be operative to apply a test-level alternating potential less than the power-level alternating potential to the external coil when the not applying the power-level alternating potential. The drive circuit may further be operative to cease application of the power-level alternating potential to the external coil in response to an indication from the current monitor that the external coil is not electromagnetically coupled to the internal coil. The drive circuit may yet further be operative to apply the test-level alternating potential intermittently when the drive circuit is not applying the power-level alternating potential. In further examples, the current monitor may be operative to provide information representing a degree of coupling, and the drive circuit may be operative to apply the power-level alternating potential when the degree of coupling exceeds a threshold value. 
         [0009]    Another aspect of the present disclosure provides for a transcutaneous energy transfer system including an internal component adapted for mounting within the body of an animal, and an external component adapted for mounting outside of the body. The internal component includes an internal coil, an internal device electrically connected to the internal coil for receipt of power from the internal coil, and a telemetry transmitter operative to send telemetry signals representing one or more parameters relating to operation of the internal component. The external component includes an external coil, a telemetry receiver adapted to receive the telemetry signals from the telemetry transmitter, a current monitor operative to measure current flow in the external coil and to provide an indication of whether or not the external coil is electromagnetically coupled to the internal coil based on the measured current flow, and a drive circuit operative in a normal mode of operation when the telemetry receiver receives the telemetry signals, and in a safe mode of operation when the telemetry receiver does not receive the telemetry signals. The drive circuit may apply more power to the external coil in the normal mode than in the safe mode. In the safe mode, the drive circuit may apply an amount of power to the external coil sufficient to power the internal device and the telemetry transmitter. In some examples, the drive circuit may be configured to operate in the safe mode only when the telemetry receiver does not receive the telemetry signals and the current monitor indicates that the external coil is inductively coupled to the internal coil. Also, in some examples, the external coil, current monitor, and drive circuit may be disposed within a common housing. Yet further, in some examples, the drive circuit may be operative to drive the external coil so as to supply at least about 20 watts of power to the internal device. 
         [0010]    Yet another aspect of the disclosure provides for an implanted component of a wireless energy transfer system, including: a secondary coil having a secondary axis and a secondary conductor extending in a spiral around the secondary axis; a secondary shield composed of a magnetizable, electrically insulating material extending transverse to the secondary axis in proximity to the secondary coil and to the rear of the secondary coil; and a power-consuming device electrically connected to the secondary coil. The secondary conductor may have inner and outer ends disposed substantially on a common radial line perpendicular to the secondary axis. The secondary shield may have a round hole extending through it in alignment with the secondary axis. In some examples, the implanted component may further include an implantable coil housing having a biocompatible exterior surface, containing the secondary coil, and having front and rear sides. A front side of the secondary coil may face toward the front side of the coil housing. Additionally, the coil housing may include one or more visually-perceptible indicia differentiating the front and rear sides of the housing. 
         [0011]    Yet a further aspect of the disclosure provides for a driver for a wireless energy transfer system, including: an external coil having a primary axis and a primary conductor extending around the primary axis; a drive circuit operative to drive the external coil so that power applied to the external coil will be coupled to the internal coil; and a shield composed of a ferromagnetic or ferrimagnetic material having electrical conductivity less than about 0.3×10̂6σ and extending transverse to the primary axis, the shield including a plurality of plate-like segments arranged generally edge-to-edge with one another with gaps between edges of mutually adjacent segments. In some examples, the shield may be composed of a ferrite. Also, in some examples, at least some of the gaps may extend substantially radially with respect to the primary axis. 
         [0012]    An even further aspect of the disclosure is directed to a driver for a wireless energy transfer system including: a primary coil having a primary axis and a primary conductor extending in a spiral around the primary axis; a drive circuit operative to drive the primary coil; a main shield composed of a magnetizable, electrically insulating material extending transverse to the primary axis in proximity to the primary coil; and a shield wall composed of a magnetizable, electrically insulating material. The shield wall extends around the primary axis and projects from a rear surface of the main shield facing away from the primary coil, so that the shield wall and main shield cooperatively define a generally cup-like structure. At least a portion of the drive circuit may be disposed within the shield wall. 
         [0013]    In some examples, the drive circuit may further include one or more capacitors connected in a resonant circuit with the primary coil and one or more power semiconductors connected to the resonant circuit for supplying power to the resonant circuit. The capacitors and power semiconductors may be disposed within the shield wall. 
         [0014]    One more aspect of the disclosure provides for a driver for a wireless energy transfer system including: a primary coil having a primary axis and a primary conductor extending in a spiral around the primary axis; a drive circuit operative to drive the primary coil; and a main shield composed of a magnetizable, electrically insulating material extending transverse to the primary axis in proximity to the primary coil. The main shield may have a hole extending through it in alignment with the primary axis. The hole extending through the main shield may be square. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a schematic diagram of a transcutaneous energy transfer (TET) system in accordance with an aspect of the disclosure. 
           [0016]      FIG. 2  is a schematic diagram of the power system circuitry for the TET system of  FIG. 1  in accordance with an aspect of the disclosure. 
           [0017]      FIG. 3  is a schematic diagram of the communication system circuitry for the TET system of  FIG. 1  in accordance with an aspect of the disclosure. 
           [0018]      FIG. 4  is an exploded view of an external module of the TET system of  FIG. 1  in accordance with an aspect of the disclosure. 
           [0019]      FIGS. 5A-5C . are top-down views of a printed circuit board, a shielding element, and an external wire coil included in the external module of  FIG. 4  in accordance with an aspect of the disclosure. 
           [0020]      FIG. 6  is a schematic diagram of the implanted components of the TET system of  FIG. 1  in accordance with an aspect of the disclosure. 
           [0021]      FIG. 7  is an exploded view of an implanted coil module of the TET system of  FIG. 1  in accordance with an aspect of the disclosure. 
           [0022]      FIGS. 8A and 8B  are top down views of implementations of a circuit board included in the implanted coil module of  FIG. 7  in accordance with aspects of the disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]      FIG. 1  schematically illustrates a transcutaneous energy transfer (TET) system  100  used to supply power to an implanted therapeutic electrical device  102  in an internal cavity within the body, i.e., below the skin of a patient  104 . The implanted electrical device  102  can include a pump such as for use in pumping blood as a ventricular assist device (“VAD”), for example. The internal or implanted electrical device  102  can include controlling circuitry to control, for example, a pump. 
         [0024]    As illustrated in  FIG. 1 , the TET system  100  includes both external electronics  120  mounted outside the body of the patient  104 , as well as internal or implanted electronics  150  mounted within the body of the patient  104 . The external electronics are electrically coupled to one or more power sources, including, for example, an external battery  125  and a building power source  112  (such as AC power, or converted DC power, supplied from an electrical outlet in a building). The external power sources may supply an input voltage anywhere between about 20V and about 250V. The external electronics  120  are also electrically coupled to an external primary coil  130 , and the implanted electronics  150  are electrically coupled to an internal or implanted secondary coil  140 . The external and implanted coils  130  and  140  are inductively coupled to one another through electromagnetic induction in order to transfer energy wirelessly therebetween. In the example of  FIG. 1 , the external coil  130  is housed in a common external module  110  together with the external electronics  120 , whereas the implanted coil  140  and implanted electronics  150  are not housed together. 
         [0025]    The implanted electronics  150  are electrically coupled to an implanted battery  155  and to the implanted electrical device  102 . Energy received at the implanted coil  140  is stored in the implanted battery  155 , provided to the implanted medical device  102 , or both, via the implanted electronics  150 . Additionally, energy stored at the implanted battery may be provided to the implanted medical device  102  via the implanted electronics  150 . 
         [0026]    The external electronics  120  of the system  100  may include control circuitry  122 , radio frequency (RF) telemetry circuitry  124 , power source selection circuitry  126 , drive circuitry  128 , and a user interface  129 . The power source selection circuitry  126  is configured to select an external power source (e.g., battery  125 , wall source  112 ) from which to provide power to the external coil  130 . The drive circuit  128  is configured to drive the external coil  130  such that energy is transferred from the external coil  130  to the implanted coil through electromagnetic induction. The control circuitry  122  is configured to determine and execute instructions for controlling the power source circuitry  126  and drive circuitry  128  in order to control the wireless transfer of energy between the external and implanted coils. Such control may include setting the pulse width and/or frequency of transmission, controlling which power source is selected by the power source circuitry  126 , instructing the drive circuitry  128  to drive the external coil  130 , etc. Determinations made by the control circuitry  120  may be based on signals received from the telemetry circuitry  124 , information received from external sensors  115 , and/or inputs from the user interface  129 . 
         [0027]    The implanted electronics of the system  100  may include implanted control circuitry  152  and RF telemetry  154 , as well as a rectifier circuit  156 , a voltage regulator circuitry  158 , and power source selection circuitry  159 . The rectifier circuit  156  may be configured to convert AC power generated at the implanted coil  140  to DC power. The voltage regulator circuit is configured to adjust the voltage level of the converted DC power and power from the implanted battery  155  before being provided to the implanted medical device  102 . The implanted power switching circuitry  159  is configured to control whether the implanted medical device  102  is powered from the implanted battery  155 , the implanted coil  140 , or both. Similar to the purpose of the external control circuitry  122 , the implanted control circuitry  152  may be used to determine and execute instructions for controlling the voltage regulation settings of the voltage regulator circuitry  158 , power source selections made by the implanted power switching circuitry  159 , and overall delivery of power to the implanted medical device  102 . In some examples, the implanted control circuitry  152  may further control an efficiency of the inductive coupling between the external and implanted coils  130  and  140 , such as by instructing an adjustment in the resonant frequency of resonant circuit components  145  in the implanted coil  140 . As with the external circuitry  120 , such determinations at the implanted circuitry may be based on RF telemetry  154  signals as well as other information received from internal sensors  165 . 
         [0028]    The TET system  100  may optionally include a clinical monitor  160  for collecting system parameters (e.g., implanted battery life, charge stored in implanted battery, alarms, etc.) to be monitored, such as by the patient  104  or by a hospital clinical staff. The clinical monitor may include a memory, internal or external, for storing the collected parameters, as well as for logging an event history of the patient  104  (e.g., a low flow condition, a no-flow condition, an interrupt, etc.). The clinical monitor  160  may further be coupled to and receive/transmit information to and from units other than the TET system, such as to and from the patient&#39;s watch or smartphone, or to and from a hospital computer database. The clinical monitor  160  may also be powered by its own dedicated power source or battery  170 . 
         [0029]    In some examples, the clinical monitor  160 , aside from receiving and monitoring data from the other components of the TET system  100 , may deliver set points or parameters (e.g., a flow rate) pertaining to the desired operation of the system  100 . Such set points may be communicated to the external electronics  120 , implanted electronics  150 , or both as an instruction for operating the system  100 , and thereby utilized in setting further parameters of the system&#39;s operation, such as a pulse width and/or frequency for driving the wireless energy transmission to power the implanted medical device  102 . 
         [0030]      FIG. 2  schematically illustrates the power system circuitry of the TET system  100  of  FIG. 1  for supplying power to the implanted medical device  102 . As shown in  FIG. 2 , the power source selection circuitry  126  of the external electronics  120  includes two inputs electrically coupled to the external battery  125  and building power source  112 , respectively. Based on instructions from the control circuitry  122 , the power source selection circuitry  126  outputs power from one of the external power sources to an input of the drive circuit  128 . The drive circuit  128  amplifies the outputted power. The amplified power is then provided to the external coil  130 . The external coil is coupled to additional circuitry such as one or more capacitors  135  that form a resonant circuit with the external coil. The capacitance may be between about 50 nF and 200 nF. The external coil  130  generates a magnetic field which inductively couples to the implanted coil  140  at the resonant frequency of the resonant circuits. 
         [0031]    As described above, the external power source selection circuitry  126  may be controlled by the external control circuitry  122 . For example, if the external control circuitry  122  determines that the external electronics  120  are not connected to a building power source  112 , the external control circuitry  122  may instruct the external power source selection circuitry  126  to provide power to the external coil  130  from the external battery power source  125 . For further example, if the external control circuitry  122  determines that the external electronics  120  are connected to a building power source  112 , the external control circuitry  122  may instruct the external power source selection circuitry  126  to provide power to the external coil  130  from the building power source  112  instead. 
         [0032]    The driver circuitry  128  may also be controlled by the external control circuitry  122 . For example, the external control circuitry  122  may determine an appropriate setting (e.g., voltage, current, pulse width) at which the external coil  130  should be driven so as to inductively generate enough power at the implanted coil  140  that the implanted medical device  102  may be supplied with a sufficient amount of power. The power requirements of the implanted device will depend on the nature of the device and also may vary during operation of the device. For example, systems for use with a typical VAD may be arranged to transmit at least 5 watts, at least 10 watts, at least 15 watts, or at least 20 watts of continuous power to the implanted device  102 . 
         [0033]    At the implanted electronics  150 , the rectifier circuitry  156  receives the AC power generated at the implanted coil  140 , and rectifies the AC power to provide DC power. The rectifier circuitry  156  may include a diode bridge, synchronous rectifier or other components known in the art for AC-to-DC rectification. The DC output of the rectifier circuitry  156  is then input to the voltage regulator circuitry  158 , where it is capped to a predefined voltage limit or threshold (e.g., 60V) by a voltage limiter, e.g., breakdown diodes. The voltage is further conditioned using a step-down DC to DC (DC-DC) converter  252 , such as a buck switching controller, single-ended primary-inductor converter (SEPIC), or other components known in the art, to a voltage and current level required for powering the implanted medical device  102  (e.g., about 18V). Optionally, in some systems, the order of the rectifier circuitry and the voltage regulator may be reversed. For instance, the DC-DC converter may be replaced with a transformer used to convert the voltage level of the AC power, and the converted AC power may then be converted to DC power by the rectifier circuitry. The output of the voltage regulator circuitry  158  is provided to one of the inputs of the implanted power source selection circuitry  159 . A second input of the implanted power source selection circuitry  159  is electrically coupled to the implanted battery  155 . In the example of  FIG. 2 , the implanted battery  155  outputs a direct current that is coupled to an input of a DC-DC step-up or boost converter  254 . The step-up converter  254  conditions the voltage and current level of the power output by the implanted battery  155  to a level required for powering the implanted medical device  102 . For example, the step-up converter  254  may raise the voltage of the power output by the implanted battery  155  from about 12V to about 18V. The implanted power source selection circuitry  159  includes an output electrically coupled to the implanted medical device  102 . 
         [0034]    The implanted power source selection circuitry  159  is configured to switch between providing power to the implanted medical device  102  from one of an implanted battery  155  and the implanted coil  140 . In similar fashion to switching regulation of the external circuitry  120 , such internal switching may be determined based on inputs provided to the implanted control circuitry  152 . Inputs to the implanted control circuitry  152  may also indicate an amount of voltage received at the implanted coil  140 , and a temperature of the implanted electronics  150 . For instance, if the implanted control circuitry  152  determines that not enough energy is received at the implanted coil  140 , or that the temperature of one or more internal components is too high to safely operate, then the implanted control circuitry  152  may instruct the implanted power source selection circuitry  159  to supply power to the implanted medical device  102  from the implanted battery  155 . 
         [0035]    In addition to the circuitry for supplying power to the implanted medical device, the implanted electronics  150  also includes charging circuitry  256  for charging the implanted battery  155  using the generated wireless energy. The charging circuitry may be arranged so as to permit charging the implanted battery  155  even while wireless energy is supplied to the implanted medical device  102 . The charging circuitry  256  may include one or more switches controlled by the implanted control circuitry  152 . 
         [0036]    In some examples, power provided to the implanted battery  155  may be controlled so as to avoid constant discharging and recharging of the implanted battery, (commonly referred to as “micro disconnects”) which affect the battery life of TET powered VAD systems, for instance due to fluctuations in power demands from the implanted medical device  102 . For example, commonly owned U.S. Pat. No. 8,608,635, the disclosure of which is hereby incorporated herein in its entirety, describes a TET system that dynamically adjusts the energy emitted by a transmitting coil based on power demands of an implanted VAD. 
         [0037]      FIG. 3  schematically illustrates communication circuitry for enabling communication among the electronic components of the TET system  100 . Each of the dotted lines  312 ,  314  and  316  represents a wireless communication channel between two of the components. Each of the solid lines  322 ,  324  and  326  represents a wired communication channel. 
         [0038]    Beginning with the external electronics  120 , the external electronics are communicatively coupled to each of the external coil  130  (via channel  322 ), external battery  125  (via channel  324 ), clinical monitor  160  (via channel  312 ), and implanted electronics  150  (via channel  314 ). The external electronics  120  may be wired to those components with which it shares a housing (e.g., in the present example, the external battery  125 , housed together in module  110 ), and are wirelessly coupled to the separately housed components (e.g., in the present example, the separately housed clinical monitor  160 ). Communication between the external electronics  120  and any implanted component (e.g., the implanted electronics  150 ) is wireless. 
         [0039]    In the example of  FIG. 3 , the sensors  115  associated with the external electronics are configured to measure each of the supply voltage and supply current for the connected power sources, including the wall power source  112  and the external battery power source  125 . Additional sensors are configured to measure an amount of current supplied to the external power source selection circuitry ( 126  in  FIGS. 1 and 2 ), as well as the temperature of the external coil  130  and associated electronics. In addition to these sensed values, the external electronics  120  may receive information signals from the implanted electronics  150  indicating other values associated with the TET system  100 , such as the voltage and current at a load of the implanted coil  140 , the voltage at the implanted rectifier circuitry  156 , etc. 
         [0040]    Beyond accumulating data from communicatively coupled components and sensors  115 / 165 , the external electronics  120  may also share gathered data with other components of the TET system  100 , such as with the clinical monitor  160  and implanted electronics  150 . For example, the external electronics  120  may transmit all received and measured values to the clinical monitor  160  for further monitoring, logging, processing and/or analysis. Communication to the clinical monitor may be intermittent. 
         [0041]    The implanted electronics  150  are responsible for gathering measured sensor values and data of the implanted components of the TET system  100 . For instance, the implanted electronics  150  may receive information regarding the voltage and current at a load of the implanted coil  140 . As described above, this data may be relayed to the external electronics  150  and/or clinical monitor  160  to further coordinate control and optimize efficiency between the transmitter (external) and receiver (implanted) sides of the system  100 . 
         [0042]    The external electronics  120 , implanted electronics  150 , and clinical monitor  160  may all communicate by radio frequency telemetry modules having RF transmitters and/or receivers, such as those modules described in commonly owned U.S. Pat. No. 8,608,635. For example, the external electronics may communicate with the clinical monitor (via channel  312 ) using a medical Bluetooth communication channel. The implanted electronics may communicate with the external electronics (via channel  314 ) and clinical monitor (via channel  316 ) using a medical implant communication service (MICS). 
         [0043]    One configuration of an external module  110  such as the module is depicted in FIGS.  4  and  5 A- 5 C.  FIG. 4  illustrates an exploded view of the external module  110 . The external module  110  contains each of the external electronics  120  and a primary coil (the external coil  130 ) disposed entirely within a carrying system or housing  405 . Efficiency of the external module is improved by integrating the power electronics and primary coil within a common housing. In TET systems having a separately housed primary coil and drive electronics, the distance between the coil and drive electronics (often 1 meter) can result in cable losses and overall weakness in the system. Co-locating the drive electronics and primary coil eliminates such cable losses, and enables a high Q and higher efficiency to be achieved. 
         [0044]    In the example of  FIG. 4 , the housing  405  is made of a durable non-conductive material, such as a plastic. The housing includes each of an “outward-facing” cap  407  which faces away from the patient  104  and an “inward-facing” base  406  which faces towards the patient  104  when the module  110  is in use. The cap  407  and base  406  may fasten to one another by any suitable fastening modality as e.g., press fitting, spin welding, ultrasonic welding, adhesive, etc. In the example of  FIG. 4 , the module  110  is circular, although modules may take a different shape such as, e.g., square, oblong, etc. A thermal isolation layer  409  is integrated into the base  406  of the housing  405 , or added as an additional layer on the surface of the inward facing side of the housing  405  to provide an additional thermal barrier between the primary coil and the patient&#39;s skin. The thermal isolation may be made of a polymer material (e.g., silicone), and may provide a breathable surface for the skin pores of the patient. 
         [0045]    The external electronics  120  are arranged on a printed circuit board  420  (PCB) disposed near the “outward-facing” end of the module (e.g., within the cap  407 ) and extending transverse or perpendicular to a primary axis A of the module  110 . The primary axis A extends in the outward direction, i.e., from the center of the base  406  to the center of the cap  407 . The primary coil  430  is disposed near the opposite “inward-facing” end of the module (e.g., within the base  406 ). Such an arrangement ensures that the electronic components of the module do not interfere with the inductive coupling between the external and implanted coils  130  and  140  of the TET system  100 . 
         [0046]    The PCB  420  may be shaped to fit the housing  405  of the module  110 . In the example of the circular module  110 , the PCB  420  may be circular or annular in shape.  FIG. 5A  depicts a top down view of an annular shaped PCB  420  with a gap having a diameter between about 20 mm and about 35 mm in the center of the PCB  420 , which lies on the primary axis A. The electronic circuit components, which may include one or more capacitors  135  and other components coupled to the external coil  130  to form a resonant circuit, are arranged around the gap. The gap in the center of the PCB  420  permits or at least simplifies connection of the electronic circuit components to the primary coil  130 , although the gap may be omitted, such as from a circular PCB, and the primary coil  130  may be connected via a different path. Also, as described in greater detail below, the PCB  420  includes connection points  436  and  438  to facilitate connecting the primary coil  130  to the other electronic circuit components. 
         [0047]    The housing  405  of the module  110  may be wide enough to contain a primary coil  130  with a diameter 70 mm or greater. For instance, the housing of  FIG. 4  has an outer diameter of about 90 mm or greater. As such, the PCB  420  may be wide enough to fit inside the housing  405  without having to stack the capacitors physically above, or below, other components disposed on the PCB. As shown in  FIG. 5A , the capacitors  135  may be disposed alongside the other circuitry on the PCB. In turn, the housing of  FIG. 4  may be made thinner (i.e., along the primary axis), relative to a smaller diameter housing of similar design. In the example of  FIG. 4 , the housing  405  may have a thickness (at the primary axis A) of between about 10 mm and 20 mm (e.g., 15 mm). 
         [0048]    The primary coil  430  is a substantially planar coil comprised of a single continuous conductor wire (e.g., Litz wire) wrapped in a planar spiral pattern around the primary axis A. As used in the present disclosure, the term “spiral” should be understood to include both curves that begin at the primary axis and wrap around the axis, as well as curves that wrap around the axis beginning at a location radially apart from the axis, thereby leaving a gap or opening at the center of the coil. The coil  130  may be wrapped anywhere between 5 and 15 turns. Based on the given value ranges, and based the formula for calculating air-core inductors L=(d̂2*n̂2)/(18*d+40*l) (where d is the coil diameter, l is the coil length, and n is the number of turns in the coil), the coil  130  may have an inductance anywhere between 15 pH and 25 pH. 
         [0049]      FIG. 5C  depicts a top-down view of the primary coil  430 . The conductor wire of the primary coil has an inner end  432  and an outer end  434 . In the example of  FIG. 5C , each of the wire ends  432  and  434  is disposed substantially at a common radial axis B extending radially from the primary axis A. As shown in  FIG. 4 , each of the wire ends  432  and  434  may curl upward and away from the plane of the coil  430  and towards the PCB  420 . Each wire end may be soldered or otherwise connected to the respective connection points  436  and  438  on the PCB  420 . 
         [0050]    In order to shield the electronics of the PCB  420  from the magnetic field generated by the primary coil  130 , the module  110  includes a shield  450  disposed between the PCB  420  and the primary coil  130 . The shield  450  includes an annular disc  453  centered at and extending transverse to the primary axis A, and pair of concentric rings  457  and  458  defining a wall having a surface of revolution about the primary axis A and extending parallel to the primary axis A in the outward direction from the inner edge and outer edges of the annular disc  453 , respectively. 
         [0051]    The rings  457  and  458  may extend along the primary axis A for a length equal or greater than the height of the PCB  420  electronics such that the electronics (including the capacitors) are completely disposed within the semi-toroidal cavity formed by the shield  450 . 
         [0052]    Both the disc  453  and rings  457  and  458  are composed of a ferromagnetic or ferrimagnetic material (e.g., a ferrite) having an electrical conductivity less than about 0.3×10̂6σ and a relative permeability of between about 2000 and about 12000. The disc  453  may be a rigid plate having a thickness (in the primary axis A direction) between about 1 mm and about 2 mm, and the rings  457 / 458  may be made of a flexible foil, each having a thickness (in the radial axis B direction) between about 0.5 mm and about 1 mm. Other example modules (e.g., a module having a circular PCB with no gap) may include a circular shield with no hole in the center and a single ring extending from the outer edge of the disc. In such an example, the PCB  420  electronics (including the capacitors) may be completely disposed within the regular shaped cavity formed by the shield  450 . Yet further examples may include a shield that is made from a single piece of ferromagnetic or ferrimagnetic material and molded into a regular or semi-toroidal shape, depending on whether the module  110  includes a circular or annular PCB, respectively. 
         [0053]    The shield  450  is disposed between the PCB  420  and the external coil  430  along the primary axis A. The disc  453  of the shield  450  redirects or focuses the magnetic field emitted from primary coil towards the secondary coil  140  implanted within the patient. This focusing increases the coupling coefficient of the TET system  100 , and further protects the electronics of the PCB  420  from unwanted inductive coupling. The inner and outer rings  457  and  458  provide further protection, effectively guiding the magnetic field around (instead of through) the annular PCB  420 . 
         [0054]    The disc  453  may be made up of multiple segments or sections.  FIG. 5B  illustrates a top-down view of a disc  453  having quarter segments  502 - 508 , although other discs may have a different number of segments (e.g., 2-8 segments). Each segment has a radius of between about 20 mm and about 35 mm. Gaps  512 - 518  are present between edges of mutually adjacent segments. The gaps  512 - 518  may be formed by cutting the disc during assembly, and may extend substantially radially from the primary axis A at the center of the disc  453 . The gaps range from 0 mm to 0.5 mm. In the example of  FIG. 5B , each segment is about 1.5 mm thick (i.e., along the primary axis A). Sectioning the disc  453  in the above manner is believed to improve efficiency of the TET system. At the center of the disc  453  is an internal hole  525 . In the example of  FIG. 5B , the internal hole  525  is square, as such shape is believed to achieve an optimal scatter field characteristic for coupling the primary and secondary coils  130  and  140 . The size of the internal hole  525  may range from 20 mm to 35 mm, and in some examples may be shaped differently (e.g., circular, rectangular, triangular, etc.). 
         [0055]    Each of the rings  457  and  458  may include a small slit (not shown) to permit passage of the primary coil wire through the rings in order to connect the conductor wire ends  432  and  434  of the primary coil  430  to the respective connection points  436  and  438  of the PCB  420 . The inner wire end  432  at the inner perimeter of the primary coil  130  may pass through the slit of the inner ring  457  to the inner connection point  436 , and the outer wire end  434  at the outer perimeter of the primary coil  130  may pass through the slit of the outer ring  458  to the outer connection point  438 . The slits may be radially aligned with one another such that the wire ends connect to the PCB  420  at substantially the same area of the PCB  420 . In an alternative example, the rings  457  and  458  may not include slits and each wire end  432  and  434  may curl over and around a respective ring on order to connect to the connection points  436  and  438  of the PCB  420 . 
         [0056]    Also shown in  FIG. 4  are spacers  440 , disposed between the disc  453  and the PCB  420 . The spacers  440  provide sufficient distance between the PCB  420  and the disc  453  in order to prevent possible shorting due to the conductivity of the disc  453 . The spacers are preferably made from a non-conductive, non-magnetic, material such as plastic, and may have a thickness between about 1 millimeter and about 10 millimeters (e.g., about 6 millimeters thick). The example module of  FIG. 4  depicts four spacers, each spacer displaced over a respective segment  502 - 508  of the disc  453 . Other examples may include more or fewer spacers (e.g., 2 spacers, 8 spacers, etc.). 
         [0057]    Also shown in  FIG. 4  at the outward facing side of the cap  407  is a visual indicator  480  including a plurality of light emitting diodes (LEDs)  481 - 486 . As described below, the LEDs  481 - 486  are configured to indicate the position of the external primary coil  130  relative to an implanted secondary coil  140  and to further indicate a direction and/or distance that the implanted coil  140  must be moved in order to better align with the implanted coil  140 . The example module of  FIG. 4  depicts a row of six LEDs, but other examples may other display technologies known in the art but also include more or fewer lights (e.g., 5 LEDs, 8 LEDs, etc.), and the lights may be arranged in other configurations (e.g., a grid, a circle, etc.). 
         [0058]    Turning now to the implanted components of the TET system  100 ,  FIG. 6  illustrates a schematic view of an example arrangement of the components implanted within the patient  140 . As shown in  FIG. 6 , each of the implanted coil  140 , the implanted battery  155 , the implanted medical device  102 , and the implanted electronics  150  is disposed in a separate housing and dispersed throughout the patient&#39;s body in order to accommodate the anatomy of the patient. Each of the implanted coil  140 , battery  155  and medical device  102  is electrically coupled to the implanted electronics  150  via a separate electrical power cable. 
         [0059]    As discussed above, the secondary coil  140  is inductively coupleable to a primary coil  130 . Positioning of the secondary coil  140  within the patient may be done in such a manner that makes mounting the external module  110  in proximity to the secondary coil  140  easy for the patient. For instance, the secondary coil  140  may be positioned close to the skin of the patient. Moreover, the secondary coil  140  may be positioned close to a relatively flat part of the patient&#39;s body to make mounting the external module  110  easier. In the example of  FIG. 6 , the secondary coil  140  is positioned close to the front of the patient&#39;s chest, such that mounting the external module  110  to the patient&#39;s chest is easy and puts the external module  110  in close proximity to the secondary coil  140 . 
         [0060]      FIG. 7  illustrates an exploded view of an implanted coil module  700  containing the secondary coil  140 . As shown in  FIG. 7 , the secondary coil  140  is disposed within a housing  705  of the module  700  having a cap  710  and base  720  that fit together. Fitting the cap  710  and base  720  together may be accomplished in any suitable manner known in the art, such as those described above in connection with the external module  110 , and may be the same or different as fitting the cap  408  and base  406  of the external module  110 . The housing  705  may be made of a biocompatible material with a dissipation factor suitable to avoid overheating the module  700  or surrounding tissue. Preferably, the housing does not increase more than about two degrees (° C.) due to heat generated from inductive charging between the primary coil  130  and secondary coil  140 . 
         [0061]    Each of a circuit board  740  holding one or more capacitors  745  (e.g., collectively acting as a high-voltage bulk capacitor), a shield  730 , and a secondary wire coil  140  are disposed entirely within the housing  705 , and extend transverse or perpendicular to a secondary axis C of the module  700 . The secondary axis C extends in the inward direction, i.e., from the center of the base  710  to the center of the cap  720 . The secondary coil  140  is preferably disposed proximate the base  720  of the housing  705 , which is adapted to be implanted closer to the skin of the patient (and therefore closer to the external module  110 ), and the board  740  with the capacitors  745  is preferably disposed proximate the cap  710  of the housing  705  farther from the patient&#39;s skin. Additionally, the cap  710  and/or base  720  of the housing  705  may include one or more visually perceptible indicia to indicate or differentiate which side of the housing  705  is front facing (i.e., the secondary coil  140  being disposed at that side) and which side of the housing  705  is rear facing (i.e., opposite the front facing side). The indicia aid implantation of the secondary coil module  800  in its proper orientation to maximize the coupling coefficient between the external and secondary coils  130  and  140 . 
         [0062]    The capacitors  745  are evenly distributed around the circuit board  740  in order to distribute heat losses over a larger, more unified area.  FIGS. 8A and 8B  illustrate alternative arrangements of the circuit board and capacitors. In  FIG. 8A , the capacitors  745  are positioned at the outer perimeter  810  of a ring shaped circuit board  740  having an opening  820  at the center. Each of the capacitors is electrically connected to the ring via pins (e.g.,  812 ,  814 ). In  FIG. 8B , the capacitors  745  are positioned in a circular pattern on a solid (no opening in the center) circuit board  740 . Both arrangements permit for heat losses to be evenly distributed due to the even distribution of the capacitors. 
         [0063]    The shield  730  is disposed between the board  740  and the secondary coil  140 . As with the shielding  450  of the external module  110 , the shield  730  is beneficial both for shielding the board  740  from inductive coupling, as well as improving the focusing of the magnetic field generated at the primary coil  130 , thereby increasing the coupling coefficient between the primary and secondary coils  130  and  140 . 
         [0064]    In the example of  FIG. 7 , the implanted coil  140  is a substantially planar coil comprised of a single continuous conductor wire (e.g., Litz wire) wrapped in a spiral pattern around the secondary axis C. The coil  140  may be wrapped anywhere between 5 and 15 turns, and may have a diameter substantially equal to the diameter of the primary coil  130 , for example about 80 mm or more. The conductor wire may be electrically coupled to the capacitors  745  at each of an inner wire end  742  and an outer wire end  744 . In order to connect the wire ends  742  and  744  to the capacitors  745 , the ends may be curled upward and away from the plane of the coil  740  (which is transverse to the secondary axis C) and generally axially towards the board  740 . Electrical connection between the wire ends  742  and  744  and the capacitors  745  may be established by soldering each wire end to a respective connection point  746  and  748  on the board  740  holding the capacitors  745 . As shown in  FIG. 7 , each of the wire ends  742  and  744  and connection points  746  and  748  may be disposed substantially at a common axis D extending radially from the secondary axis C. 
         [0065]    The board  740  may be annular shaped having a circular inner hole between about 30 mm and about 70 mm in diameter (e.g., 17.5 mm), and a thickness (in the secondary axis C direction) of about 1 mm. As described above, the board  740  may include one or more capacitors  745  that are coupled to the secondary coil  140 , and having a capacitance of between about 100 nF and 150 nF. Together, the secondary coil  140  and capacitors  745  form a resonant circuit. The resonant circuit has a pair of load terminals (which may be the connection points  846  and  848 ) disposed within the housing  705 . In some examples, the board may optionally include additional circuitry for adjusting the resonant frequency of the resonant circuit, for instance through selective coupling of the capacitors, and may also optionally include one or more temperature sensors for monitoring the temperature of the implanted coil module  700 . The board  740  of  FIG. 7  is shown holding 9 capacitors in a ring, but other example boards, such as those of similar shape and size, may fit more (e.g., 10) or fewer (e.g., 2 or 3) capacitors, and the capacitors may be arranged differently (e.g., in a grid). 
         [0066]    Additionally shown in  FIG. 7  is a port  715  built into both the cap  710  and base  720  of the housing  705 . The port is adapted to permit one or more power cables or wires (not shown) to pass therethrough such that the cables or wires electrically connect the components disposed within the housing  705  to the implanted electronics  150 . For instance, a cable having conductors may pass through the port  715  in order to electrically connect the load terminals disposed in the housing  705  to the implanted electronics  150 . It is preferable to include the capacitors  755  on the implanted coil  140  side of the cable (i.e., in the implanted coil module  700 ) to reduce the distance from the implanted coil  140  and the load terminals. This in turn minimizes any power losses over the cable. Returning to  FIG. 6 , the implanted electronics  150  are electrically coupled to, but housed separately from, the implanted coil  140 . The implanted electronics  150  may separated between two or more circuit boards, such as a voltage rectifier board and control board. The voltage rectifier board would include the voltage rectifier circuit  156  described above in connection with  FIGS. 1 and 2 , which rectifies AC power generated at the implanted coil into DC power. The voltage rectifier board also would include the voltage regulator circuitry  158  described above, which conditions the voltage supplied to the implanted medical device  102  to a required level, as well as the implanted power source selection circuitry  159  for switching between providing power to the implanted medical device  102  from the implanted battery  155  and the implanted coil  140 . 
         [0067]    The control board would include circuitry, such as a MOSFET inverter, responsible for driving the implanted medical device  102 , as well as the control circuitry  152  responsible for instructing a power source selection of the implanted power source selection circuitry  159 . The control circuitry  152  may determine proper operation parameters of the implanted coil  140  (e.g., a resonant frequency), and whether to power the implanted medical device  102  using energy from the implanted coil  140 , from the implanted battery  155 , or both. The control board may additionally collect and communicate various data about the TET system  100 . For example, the control board may be configured to receive, interpret, store and/or relay information regarding the temperature of the power source selection circuitry  159 . For further example, where the implanted medical device  102  is an implantable pump, such as the VAD of  FIG. 7 , the control board may be configured to handle information transmitted from sensors  165  at the pump, such as back-EMF exhibited by the pump, and electrical current at the pump&#39;s stators. Storage of such information may be done on a memory included on the control board, and the information may be communicated to other components of the TET system  100 , such as the external electronics  120  and the clinical monitor  160 , using the RF telemetry circuitry  154  discussed above. 
         [0068]    In an alternative embodiment, the voltage rectifier board and control board may be housed separately. In such examples, the cable extending from the housing  705  of the implanted coil module  700  (described above in connection to  FIG. 7 ) electrically connects to an input terminal of the rectifier housing, and from there connects to an input terminal of the rectifier circuitry  156 . As such, the rectifier circuit is electrically coupled between the implanted coil  140  and the implanted medical device  102  such that only load current passing from the capacitors  845  passes along the conductors of the cable to the rectifier circuitry  156  to the implanted medical device  102 . In other examples, the voltage rectifier board and control board may be housed together, with the cable extending from the housing  705  of the implanted coil  140  electrically connecting to an input terminal of the common housing. 
         [0069]    The implanted battery  155  may be a cell lithium ion cell/battery housed within a titanium or medical grade plastic casing. In the case of powering a VAD, the battery may be configured to store charge between about 12 volts and 16.8 volts. As stated above, the implanted battery is coupled to the implanted medical device  102  in order to power the implanted medical device  102  in response based on a determination by the implanted control circuitry  152 . The implanted battery  155  may also be electrically coupled to the implanted coil  140  through the voltage rectifier board of the implanted circuitry  150  in order to temporarily store power generated at the implanted coil  140  in excess of the power needed at the implanted medical device  102 . That excess power may be used to charge the implanted battery  155  for later use in operating the implanted medical device  102 . 
         [0070]    In an alternative embodiment to the example arrangement of  FIG. 6 , the implanted coil may be disposed in a housing that is mounted to the implanted medical device. For instance,  FIG. 9  illustrates an perspective view of the implanted medical device  102  (which is in this example a ventricular assist device, or VAD, for assisting cardiac function of the patient) having an implanted coil housing  905  mounted to a flat end  902  of the VAD  102 . The flat end  902  of the VAD  102  is preferably positioned facing away from the heart and towards the chest of the patient, such that the implanted coil is positioned close to the patient&#39;s skin. Further, the implanted coil housing  905  is preferably mounted such that the implanted coil  140  disposed therein faces towards the chest of the patient so that the coil shield is positioned between the implanted coil and the VAD  102 . This permits the implanted coil  140  to be positioned proximate to an external module  110  mounted to the chest of the patient, maximizing coupling between the external and implanted coils, while further shielding the magnetic components and conductive surfaces of the VAD from the electromagnetic TET field. The alternative arrangement of  FIG. 9  is also advantageous for providing a heat sink for the VAD. The implanted electronics are also mounted to VAD  102 . This makes implantation of the VAD and TET system significantly simpler, since there are no additional device pockets and no cabling between the implanted coil housing  905 , implanted electronics  907  and VAD  102 . 
         [0071]    The TET system collectively described above may include additional features further improving upon several aspects of the system&#39;s operation. One such feature is the implementation of normal, start-up, and safe mode routines for operation, as well as testing routines for determining In which mode to operate. The testing routines provide for the TET system  100  to drive the external coil  130  using different amounts of current. Under normal mode operation, when the external components of the TET system  100  are in proper communication with the implanted components, the drive circuitry  128  applies a power level alternating potential (e.g., a maximum amount of current) to drive the external coil  130 . As described above, under normal operation, the TET system may generate at least 5 watts, at least 10 watts, at least 15 watts, or at least 20 watts of continuous power. This power may be used to operate all power demands of the implanted medical device, RF telemetry needs, primary and back-up electronic system requirements, and further to power to recharge the implanted battery. If, however, one or more of the external components, such as the wireless energy transfer coils or RF telemetry coils, are not in properly coupled with one or more corresponding implanted components, less current may be applied to drive the external coil  130 . The amount of reduction of the current may be based on the particular component or components that are not properly coupled. 
         [0072]    The start-up routine may determine between operating the TET system  100  in one of the start-up and normal modes, and may be controlled by the external control circuitry  122 . In the start-up routine, the TET system  100  may begin in start-up mode by applying a test-level alternating potential to drive the external coil  130  in order to test the degree of coupling between the external coil  130  and the implanted coil  140 . The test-level alternating potential generates enough power to sense an implanted system or coil, but not enough power to operate the implanted device. For example, the test-level alternating potential may generate about 250 mW or less. The sensors  115  of the external control circuitry  122  may include a coupling detection circuit operative to detect the degree of inductive coupling between the external coil  130  and the implanted coil  140 . This detection may be performed at least in part using a current monitor to measure the current flow in the external coil  130 . Information regarding the detected coupling may then provided from the coupling detection circuit to the external control circuitry  122 . The external control circuitry  122  may then determine, based on the provided coupling information, whether to continue in start-up mode or transition to normal mode. 
         [0073]    If the external control circuitry  122  is in normal mode and does not receive an indication (or otherwise determines) that the external and implanted coils  130  and  140  are properly coupled, the external control circuitry  122  may instruct the drive circuitry  128  to cease application of the power-level alternating potential to drive the external coil  130 , and may further transition to start-up mode and apply the test-level alternating potential to the external coil  130 . The test-level alternating potential may be applied intermittently to determine whether the external and implanted coils  130  and  140  are properly or sufficiently coupled. The test-level alternating potential may provide sufficient current to determine the presence of inductive coupling without generating a magnetic field strong enough to harm the patient (such as overheating the skin or tissue of the patient) despite the lack of inductive coupling between the external and implanted coils  130  and  140 . Additionally, the test-level alternating potential avoids unnecessary expenditure of power, while still enabling the external control circuitry  122  to continue monitoring and evaluating the coupling between the coils  130  and  140 . 
         [0074]    In the safe-mode routine, the level of wireless power transmitted may be determined based on whether the RF telemetry circuits of the external and implanted electronics  124  and  154  are properly communicating with one another. For example, if the external control circuitry  122  determines that a receiver of the external RF telemetry circuitry  124  is not receiving RF telemetry signals from a transmitter of the implanted RF telemetry circuitry  154 , then the external control circuitry  122  may instruct the drive circuitry  128  to apply a relatively low power-level alternating potential to the external coil  130 . In other words, the drive circuitry  128  would apply less current (a shorter pulse width) to the external coil  130  in the safe mode as compared to a normal mode of operation. The low power-level alternating potential would be strong enough to drive the external coil  130  to generate enough power to operate the implanted medical device  102 . For example, with respect to a VAD, the power needs of the VAD may be defined by the blood flow needs of the patient (which in turn may be programmed by clinical staff). Such power needs can range from about 2 watts to about 5 watts. 
         [0075]    The external control circuitry may be configured to implement both start-up and safe mode routines. Under such conditions, the drive circuit  128  may be operative to apply the low power-level alternating potential to the external coil  130  only if the coupling detection circuitry determines that the coils are properly coupled, and the external control circuitry determines that the external RF telemetry circuitry  124  is not receiving RF telemetry signal from the implanted electronics. 
         [0076]    Another feature of the TET system is an alignment protocol for aiding a user in properly aligning the external and implanted coils in order to maximize efficiency of energy transfer therebetween. 
         [0077]    The external control circuitry  122  may determine the then-present degree of coupling between the external and implanted coils  130  and  140  based on received information from the sensors  115 . The information may be received in the form of input signals. One such signal may be provided by a voltage or current monitor coupled to the external coil  130 , and may indicate an amount of voltage and/or amount of current at the external coil  130 . Another such signal may be provided by the external RF telemetry circuitry  124  and may be indicative of power transfer (e.g., a coupling coefficient, or a current efficiency) between the coils. The telemetry signal may be received from the implanted RF telemetry circuitry  154 , which itself is coupled to an implanted sensor  165  that measures current in the implanted coil  140 . 
         [0078]    The external control circuitry  122  alerts the patient as to the degree of coupling between the external and implanted coils  130  and  140 . 
         [0079]    Such alerts may be conveyed visually, such as by activating a human perceptible signal, such as with visual or aural indicator. In the example of the visual indicator, the indicator may include a number of lights or LEDs (e.g., the LEDs  481 - 486  on the outward facing cap  407  of the external module  420  of  FIG. 4 ). For example, the number of lights activated may indicate the degree of coupling. The number of lights activated for any given degree of coupling may be preconfigured, for instance such that the greater the degree of coupling, the more (or alternatively, the fewer) lights that are activated. 
         [0080]    The above disclosure generally describes a TET system for use in a user having an implanted VAD. Nonetheless, the disclosure is similarly applicable to any system having a transcutaneous stage of wireless power delivery. As such, the disclosure is similarly applicable for driving any power-consuming device implanted in any human or other animal (e.g., hearing aids, pacemakers, artificial hearts, etc.). 
         [0081]    Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.