Abstract:
An implantable medical device advantageously utilizes low frequency (e.g., 100 kHz or below) for telemetry communication with an external control module avoiding power dissipation through eddy currents in a metallic case of an implant and/or in human tissue, thereby enabling smaller implants using a metallic case such as titanium and/or allow telemetry signals of greater strength for implantation to a greater depth.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application is related to four co-pending and commonly-owned applications filed on even date herewith, the disclosure of each being hereby incorporated by reference in their entirety, entitled respectively:  
         [0002]     “TRANSCUTANEOUS ENERGY TRANSFER PRIMARY COIL WITH A HIGH ASPECT FERRITE CORE” to James Giordano, Daniel F. Dlugos, Jr. &amp; William L. Hassler, Jr., Ser. No. ______;  
         [0003]     “MEDICAL IMPLANT HAVING CLOSED LOOP TRANSCUTANEOUS ENERGY TRANSFER (TET) POWER TRANSFER REGULATION CIRCUITRY” to William L. Hassler, Jr., Ed Bloom, Ser. No. ______;  
         [0004]     “SPATIALLY DECOUPLED TWIN SECONDARY COILS FOR OPTIMIZING TRANSCUTANEOUS ENERGY TRANSFER (TET) POWER TRANSFER CHARACTERISTICS” to Resha H. Desai, William L. Hassler, Jr., Ser. No. ______; and  
         [0005]     “LOW FREQUENCY TRANSCUTANEOUS ENERGY TRANSFER TO IMPLANTED MEDICAL DEVICE” to William L. Hassler, Jr., Daniel F. Dlugos, Jr., Ser. No. ______. 
     
    
     FIELD OF THE INVENTION  
       [0006]     The present invention pertains to a telemetry system and, in particular, to a low frequency telemetry system that can be used in conjunction with a low frequency transcutaneous energy transfer (TET) system to transmit data between an external control module and a medical implant.  
       BACKGROUND OF THE INVENTION  
       [0007]     It is known to surgically implant a medical device in a patient&#39;s body to achieve a number of beneficial results. In order to operate properly within the patient, a reliable, consistent communication link between the medical implant and an external control module is often necessary to monitor the implant&#39;s performance or certain patient parameters and/or to command certain operations by the implant. This communication link has traditionally been achieved with telemetry systems operating at frequencies from 100 kHz. to upwards of 30 MHz. These higher frequencies have been used to minimize the required coil size, thus enabling the coil to fit inside the implant case. It is also known to place a telemetry coil outside of an implant case in order to use a larger coil. Doing so, however, increases the complexity and expense of the implant since electrical leads must extend outside of the implant case to the coil, posing challenges to maintain a hermetic seal to the case and to avoid damage to the external coil.  
         [0008]     While high frequency telemetry signals reduce the required coil size, such signals also reduce the effective communication distance between the transceivers in the system. Oftentimes, the implanted transceiver must be placed just under the surface of the patient&#39;s skin in order to effectively communicate with the external transceiver. At the shorter wavelengths (i.e., higher frequencies), the signals dissipate over a shorter distance when passing through tissue.  
         [0009]     High frequency telemetry signals above 100 kHz have a greater likelihood of electromagnetic interference or compatibility issues with other communication devices, and thus additional constraints arise under federal regulations. Conformance increases the time and complexity involved in developing the implant as well as limiting transmission power.  
         [0010]     As an example of an implantable device that may benefit from use of telemetry is an artificial sphincter, in particular an adjustable gastric band that contains a hollow elastomeric balloon with fixed end points encircling a patient&#39;s stomach just inferior to the esophago-gastric junction. These balloons can expand and contract through the introduction of saline solution into the balloon. In generally known adjustable gastric bands, this saline solution must be injected into a subcutaneous port with a syringe needle to reach the port located below the skin surface. The port communicates hydraulically with the band via a catheter. While effective, it is desirable to avoid having to adjust the fluid volume with a syringe needle since an increased risk of infection may result, as well as inconvenience and discomfort to the patient.  
         [0011]     Unlike the previously mentioned medical implants, an infuser device for an artificial sphincter is typically implanted below a thicker dermal layer of skin and adipose tissue. This is particularly true for patients that typically receive an adjustable gastric band as a treatment for morbid obesity. Moreover, being more deeply implanted may allow for greater client comfort. However, the thickness of tissue presents difficulties for effective communication.  
         [0012]     Consequently, in order to provide for a larger effective communication range between the primary and secondary transceivers, and also to minimize the issue of FCC conformance, a significant need exists for enhancing telemetry with a deeply implanted medical device at a lower frequency than commonly used.  
       BRIEF DESCRIPTION OF THE FIGURES  
       [0013]     The invention overcomes the above-noted and other deficiencies of the prior art by providing a telemetry system for an implantable medical device that operates at a frequency less than 100 kHz, advantageously minimizes eddy current losses and allow uses of metallic cases to achieve smaller implant sizes. In instances where the telemetry carries significant power, the lower frequency avoids heating human tissue. Moreover, the low frequency telemetry system includes a telemetry coil encompassed within a hermetically sealed implantable device, ensuring the integrity of the device.  
         [0014]     In one aspect of the invention, telemetry circuitry communicates across a physical boundary between primary and secondary resonant tank circuits having an inductance and capacitance combination selected for resonance within a range of 25 to 100 kHz. Thereby, an implantable medical device may be deeply implanted with an integral secondary telemetry coil yet achieve reliable telemetry.  
         [0015]     These and other objects and advantages of the present invention shall be made apparent from the accompanying drawings and the description thereof. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0016]     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention, and, together with the general description of the invention given above, and the detailed description of the embodiments given below, serve to explain the principles of the present invention.  
         [0017]      FIG. 1  is a block diagram illustrating a remote control system including low frequency power and telemetry systems of an implantable medical device system in accordance with the present invention;  
         [0018]      FIG. 2  is a schematic diagram illustrating the low frequency TET power system and telemetry system of the present invention;  
         [0019]      FIG. 3  is a more detailed schematic of an exemplary version of the telemetry transceiver including signal filtering circuitry;  
         [0020]      FIG. 4   a  is a diagram illustrating magnetic fields between primary and secondary power and telemetry coils of the remote control system of  FIG. 1 ; and  
         [0021]      FIG. 4   b  is a diagram illustrating the magnetic fields between the primary and secondary coils of the power and telemetry systems of  FIG. 1  for an alternative embodiment in which the primary power and telemetry coils are placed around a ferrite core. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]     Referring now to the drawings in detail, wherein like numerals indicate the same elements throughout the views, in  FIG. 1 , a remotely controlled implantable medical device system  10  includes a remote control system  12  that advantageously performs both transcutaneous energy transfer (TET) through a TET power system  14  and telemetry through a Telemetry system  16 . Internal portions  18 ,  20  of the TET power system  14  and the telemetry system  16  respectively reside in an implantable medical device (“implant”)  22  and external portions  24 ,  26  of both respectively reside in an external control module  28 . The implant  22  and external control module  28  are spaced apart by a physical boundary  30 , which in the illustrative version is composed of dermal tissue typically including a thick layer of adipose tissue.  
         [0023]     Implantable, bi-directional infusing devices that would benefit from enhanced TET powering and telemetry are disclosed in four co-pending and co-owned patent applications filed on May 28, 2004, the disclosure of which are hereby incorporated by reference in their entirety, entitled 1) “PIEZO ELECTRICALLY DRIVEN BELLOWS INFUSER FOR HYDRAULICALLY CONTROLLING AN ADJUSTABLE GASTRIC BAND” to William L. Hassler, Jr., Ser. No. 10/857,762; (2) “METAL BELLOWS POSITION FEED BACK FOR HYDRAULIC CONTROL OF AN ADJUSTABLE GASTRIC BAND” to William L. Hassler, Jr., Daniel F. Dlugos, Jr., Rocco Crivelli, Ser. No. 10/856,971; (3) “THERMODYNAMICALLY DRIVEN REVERSIBLE INFUSER PUMP FOR USE AS A REMOTELY CONTROLLED GASTRIC BAND” to William L. Hassler, Jr., Daniel F. Dlugos, Jr., Ser. No. 10/857,315 ; and (4) “BI-DIRECTIONAL INFUSER PUMP WITH VOLUME BRAKING FOR HYDRAULICALLY CONTROLLING AN ADJUSTABLE GASTRIC BAND” to William L. Hassler, Jr., Daniel F. Dlugos, Jr., Ser. No. 10/857,763.  
         [0024]     The external portion  26  of the telemetry system  16  includes a primary transceiver  32  for transmitting interrogation commands to and receiving response data from implant  22 . Primary transceiver  32  is electrically connected to a primary controller  34  for inputting and receiving command data signals from a user or automated programmer. In particular, the primary controller  34  is in communication with a primary telemetry arbitrator  36  that is responsible for deconflicting and buffering downlink telemetry communication via primary telemetry output interface logic  38  to the primary transceiver  32  and uplink telemetry communication from the primary transceiver  32  via primary telemetry interface differential amplifier-based input logic  40 . Primary transceiver  32  resonates at a selected radio frequency (RF) communication frequency to generate a downlink alternating magnetic field  42  that transmits command data to implant  22 .  
         [0025]     The internal portion  18  of the telemetry system  16  also includes a secondary transceiver  44  in a spaced relationship from primary transceiver  32  and is located on the opposite side of boundary  30  within the casing (not shown) of implant  22 . In the present invention, secondary transceiver  38  is electrically connected to a secondary controller  46 . In particular, the secondary controller  46  is in communication with a secondary telemetry arbitrator  48  that is responsible for deconflicting and buffering uplink telemetry communication via secondary telemetry output interface logic  50  to the secondary transceiver  44  and downlink telemetry communication from the secondary transceiver  44  via secondary telemetry interface differential amplifier-based input logic  52 . Secondary transceiver  44  is magnetically coupled to primary transceiver  32  via alternating magnetic field  36  for downlink communication and via alternating magnetic field  54  for uplink communication. Magnetic flux from primary transceiver  32  generates an electrical command signal in secondary transceiver  44 . The command signal is applied to a secondary controller  46  in implant  22  to direct operation of implant  22 . Similarly, secondary transceiver  44  is electrically connected to controller  46  to transmit command response data from implant  22  to the external portion  26  of the telemetry system  16 . When data transmission is requested, transceiver  44  resonates at the selected RF frequency to generate the uplink alternating magnetic field  54 . Uplink magnetic field  54  is coupled into primary transceiver  32 , which generates an electrical signal that is input to the primary controller  34 .  
         [0026]     Still referring to  FIG. 1 , the external portion  24  of the TET power system  14  also includes a primary power circuit  56  that is electrically coupled to a power supply  58  via a power amplifier  60  to resonate at a selected power signal RF frequency. An alternating magnetic field  62  is generated by primary circuit  56  in response to an electrical signal provided by power supply  58 . The internal portion  18  of the TET power system  14  includes a secondary power circuit  64  in a spaced relationship from primary power circuit  56 . Secondary power circuit  64  is located on the opposite side of boundary  30  from primary power circuit  56  within implant  22 . Secondary power circuit  64  is electrically coupled to primary power circuit  56  via alternating magnetic field  62 . Secondary power circuit  64  generates an electrical power signal  66  from magnetic field  62 . Power signal  66  is rectified and regulated by a power conditioning circuit  68  and applied to an implant driver  70  to power various active components of the implant  22 .  
         [0027]     In  FIG. 2 , resonant circuitry portions are shown of the TET power system  14  and Telemetry system  16  of the remote control system  12 . In particular, the primary transceiver  32 , comprises a parallel tuned tank circuit  72  having a capacitance made up of one or more capacitors  74  connected in parallel with an inductive coil  76 . Capacitance  74  and coil  76  are tuned to resonate at a particular frequency when a voltage is applied by controller  34 . Similarly, secondary transceiver  44  comprises a parallel tuned tank circuit  78  having a capacitance  80  and inductive coil  62  tuned to resonate at the same frequency as primary telemetry tank circuit  72 . Also as shown in  FIG. 2 , primary power circuit  56  comprises a parallel tuned tank circuit with a capacitance  86  and coil  66  tuned to a low power frequency. Secondary power circuit  64  comprises a series tuned tank circuit with a capacitance  92  and coil  94  that are also tuned to a low frequency level. In an illustrative version of the TET system, primary power circuit  56  transmits approximately one Watt of power at a resonant frequency under 10 kHz, and particularly under 5 kHz, by matching a high Q, low impedance primary tuned tank circuit  84  with a lower Q, low impedance secondary tuned tank circuit  90 .  
         [0028]     The TET power system  14  is described in further detail in the above-identified commonly assigned co-pending U.S. patent application Ser. No. ______ entitled ““LOW FREQUENCY TRANSCUTANEOUS ENERGY TRANSFER TO IMPLANTED MEDICAL DEVICE” filed on even date herewith and previously incorporated by reference. In the present invention, primary power circuit  56  operates at low frequency levels in order to effectively communicate with secondary power circuit  64  through the implant casing, as well as multiple layers of body tissue. For purposes of this discussion, the terms “low frequency” and “low frequency level” refer to frequencies below 100 kilohertz (kHz). As mentioned above, power coils  88 ,  94  also resonate at a low frequency to enable secondary power coil  94  to be encased within the sealed implant enclosure.  
         [0029]     To transmit both power and telemetry magnetic fields  62 ,  42 ,  54  at low frequency levels, signal filter  96  filters the electrical signals received on the secondary transceiver  44 , specifically from tank circuit  78 . Filter  96  decouples the lower energy telemetry magnetic field  42  from the higher energy power field  64 . Filters  96  may be any type of filter scheme selected to block frequencies other than the telemetry resonant frequency.  
         [0030]      FIG. 3  illustrates one exemplary version of a filter  96  suitable for use in the present invention. In this version, the command signal from either the primary or secondary telemetry coil  82  is applied to a series of single pole low and high pass filter stages that isolate the telemetry signal from the TET power signal. For the single pole embodiment shown in  FIG. 3 , AC magnetic fields  62 ,  42 ,  54  are transmitted in alternate intervals to decouple the high Q of the power field  62  from the telemetry signals  42 ,  54 . In another embodiment, filter  74  comprises one or more 2 pole filters such as, for example, a Chebyshev filter. The 2-pole filters provide more effective filtering of the high Q power signal, and enable AC magnetic fields  62 ,  42 ,  54  to be transmitted simultaneously. In order to effectively filter the lower energy telemetry signal from the higher energy power signal, the resonant frequencies of the two signals are separated by at least one decade of frequency.  
         [0031]      FIG. 4A and 4B  illustrate magnetic fields  62  and  42 / 54  respectively radiating from primary power coil  88  and primary transceiver coil  76  to subcutaneous secondary TET coil  94  and telemetry coil  82 . In the version illustrated in  FIG. 4A , magnetic fields  62  and  42 / 54  both have a double circular toroidal shape that only penetrates in a shallow manner cross physical boundary  30  to respective secondary TET power and telemetry coils  94 ,  82 , thereby reducing the energy transfer between the coils and necessitating corresponding shallow placement of the implant device  22 .  FIG. 4B  illustrates an alternative embodiment for the invention, described in greater detail in the previously referenced patent application entitled “TRANSCUTANEOUS ENERGY TRANSFER PRIMARY COIL WITH A HIGH ASPECT FERRITE CORE”, in which the primary power and transceiver coils  56 ,  66  are placed around a magnetically conductive ferrite core  98 . As shown in  FIG. 4   b,  the addition of ferrite core  98  causes the magnetic flux  62 ,  42 / 54  from primary coils  88 ,  76  to be drawn towards the core  98 . Magnetic fields  62 ,  42 / 54  thus collapse radially into core  98  and change from a circular shape to an elliptical shape. The elliptical shape of fields  62 ,  42 / 54  increases the coupling efficiency between both the primary and secondary telemetry coils  76 ,  82  and the primary and secondary power coils  88 ,  94 . The increased coupling efficiency with ferrite core  98  provides improved telemetry between transceivers  32 ,  44  at increased physical distances or at a lower power level.  
         [0032]     In an experimental embodiment of the present invention, primary and secondary transceiver coils  76 ,  82  were each formed of 220 turns of 36 gauge magnet wire. Coils  76 ,  82  were each placed in parallel with a capacitance that resulted in a resonant frequency for the tank circuit of approximately 25 kHz. The primary power coil  88  was formed of 102 turns of litz wire made up of 100 individually insulated 30-gauge magnet wire. The magnet wires were connected in parallel with 9.2 microfarads of capacitance, which created a parallel tuned tank circuit with a high Q and a resonant frequency under 10 kHz, and particularly under 5 kHz. Both the primary power coil  88  and primary telemetry coil  76  were placed around a ferrite core  98  having a length of 3 inches and a diameter of 0.75 inches. With these parameters and resonant frequencies, the primary coil  88  transmitted approximately one watt of power and the primary telemetry coil  76  transmitted power in the milliwatt range. The power and telemetry coils  88 ,  76  alternated transmission intervals, with the telemetry system  16  transmitting data at a baud rate of 1 kHz. In this experimental embodiment, a distance of 3 inches separated the primary and secondary coils.  
         [0033]     In designing a low frequency telemetry system for a deeply implanted medical device, it is desirous to make the Qs of the two magnetically coupled telemetry coils in their parallel tuned tank circuit to be within a range of 10 to 20. If the Qs of the two tank circuits are below this range, it will be difficult to achieve any significant deep penetration telemetry range. If the Qs were above this range, it would be difficult to manufacture the system in high quantities without individually tuning each pair of parallel tuned tank circuits.  
         [0034]     It is also possible to have the primary (or external) telemetry tank circuit be of very Q (greater than 100) while having a lower Q (around 10) in the implant. An advantage of doing this as opposed to having the high Q circuit in the implant is that a higher Q usually requires a larger and heavier coil, and inductance. This arrangement would still allow for the natural frequency of the high Q circuit to fall within the effective frequency range of the low Q circuit without the need for individual circuit tuning or matching.  
         [0035]     The coils in the deep implantation telemetry system may have their number of coil turns maximized to couple better with and better generate the AC magnetic field that is the telemetry medium. This needs to be done without creating a significantly high impedance at resonance in the parallel tuned tank circuits. The open cross sectional area within the perimeter of the coil also needs to be maximized in order to improve the magnetic coupling between the tank circuits. The coils used had 220 turns of 36-gauge magnet wire which when put in parallel with 5600 pF of capacitance, created a resonant frequency of 25 kHz, with a calculated Q of 19, and a calculated impedance of around 20 kilo-Ohms at resonance. The actual Q is always around 10% to 30% lower than the calculated value due to parasitic losses, and other non-linear effects.  
         [0036]     While the present invention has been illustrated by description of several embodiments and while the illustrative embodiments have been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications may readily appear to those skilled in the art.  
         [0037]     For example, while the telemetry system  16  described has particular advantages for an implantable medical device system  10 , aspects consistent with the present invention have application to other scientific and engineering scenarios including inanimate physical boundaries. For instance, in a processing apparatus it may be desirable to monitor and/or control an actuator that is contained within a vessel without compromising the integrity of the vessel with wires or conduits passing therethrough.  
         [0038]     Furthermore, telemetry system  16  may be used in the absence of a TET power system  14 . As yet another alternative, telemetry system  16  may provide a one-way communication channel rather than a two-way channel.