Abstract:
Communication systems and methods for dynamically controlling the power wirelessly delivered by a remote reader unit to separate sensing device, such as a device adapted to monitor a physiological parameter within a living body, including but not limited to intraocular pressure, intracranial pressure (ICP), and cardiovascular pressures that can be measured to assist in diagnosing and monitoring various diseases. The communication method entails electromagnetically delivering power from at least one telemetry antenna within the reader unit to at least one telemetry antenna within the sensing device, and controlling the power supplied to the sensing device within a predetermined operating power level range of the sensing device.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application Nos. 61/203,400 and 61/203,401, both filed Dec. 22, 2008, and U.S. Provisional Application No. 61/268,731 filed Jun. 17, 2009. The contents of these prior patent applications are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention generally relates to implantable medical devices and to communication schemes and medical procedures performed therewith. More particularly, this invention relates to systems and methods for dynamically controlling power wirelessly delivered to such devices. 
         [0003]    Wireless devices such as pressure sensors have been implanted and used to monitor various physiological parameters of humans and animals, including but not limited to heart, brain, bladder and ocular function. With this technology, capacitive pressure sensors are often used, by which changes in pressure cause a corresponding change in the capacitance of an implanted capacitor. The change in capacitance can be sensed, for example, by sensing a change in the resonant frequency of a tank or other circuit coupled to the implanted capacitor. 
         [0004]    Telemetric implantable sensors that have been proposed include batteryless pressure sensors developed by CardioMEMS, Inc., Remon Medical, and the assignee of the present invention, Integrated Sensing Systems, Inc. (ISSYS). For example, see commonly-assigned U.S. Pat. Nos. 6,926,670 and 6,968,734 to Rich et al., and N. Najafi and A. Ludomirsky, “Initial Animal Studies of a Wireless, Batteryless, MEMS Implant for Cardiovascular Applications,” Biomedical Microdevices, 6:1, p. 61-65 (2004). With such technologies, pressure changes are typically sensed with an implant equipped with a mechanical (tuning) capacitor having a fixed electrode and a moving electrode, for example, on a diaphragm that deflects in response to pressure changes. The implant is further equipped with an inductor in the form of a fixed coil that serves as an antenna for the implant, such that the implant is able to receive a radio frequency (RF) signal transmitted from outside the patient to power the circuit, and also transmit the resonant frequency as an output of the circuit that can be sensed by an interrogator/reader unit outside the patient. Tele-powered implants of this type, as well as RFID (radio frequency identification) transponders, require an interrogator/reader unit equipped with an antenna to generate a sufficiently strong electromagnetic field capable of being received by the antenna of the implant. In the USA, the FCC (Federal Communications Commission) allows radio frequency devices to transmit in specific industrial, scientific, and medical (ISM) frequency bands ranging from 125 kHz to 2.4 GHz. The higher frequencies (greater than 100 MHz) suffer from tissue absorption and cannot easily be used for deeply implanted devices. Of the lower frequencies (less than 100 MHz), the 13.56 MHz ISM band is often used due to its compatibility with the desire to minimize the size of the coil and resonant capacitor of an implant. 
         [0005]    For certain applications, the implant may be placed just below the skin or otherwise in proximity to an accessible external location, for example, within the eye to monitor intraocular pressure in the treatment of glaucoma disease. However, in order to monitor certain other parameters, including cardiovascular pressures to diagnose and monitor cardiovascular diseases such as chronic heart failure (CHF) and congenital heart disease (CHD) and intracranial pressure (ICP) to diagnose and monitor intracranial hypertension (ICH), the implant is typically placed farther from an accessible external location, for example, directly within a heart chamber whose pressure is to be monitored or in an intermediary structure, for example, the atrial or ventricular septum of the heart. Consequently, while communication distances of a few centimeters are sufficient for some applications, greater communication distances, for example, fifteen centimeters or more, would be desirable for others. 
         [0006]    A complication of greater communication distances is that, for the lower communication frequencies (including the 13.56 MHz ISM band), the electromagnetic field generated by the reader appears nearly purely magnetic, and its level largely varies in inverse proportion to the distance between the reader and implant antennas. Consequently, the power coupled into an implant can vary by a factor of one hundred or more, depending on the location of the implant relative to the reader. In a typical RFID application, excess power supplied to an RFID device can be dissipated as heat since digital data typically read from RFID devices are typically not prone to erroneous measurements due to heat or temperature gradients. However, physiological parameters such as temperature and pressure can be distorted by excessive power delivered to a tele-powered implant. Accordingly, to promote the performance of a tele-powered implant device, power delivery and/or absorption should be compensated for or regulated in some manner. Implants equipped with a MEMS (microelectromechanical system) pressure transducer typically require a temperature sensor to provide for temperature compensation. Though systematic errors attributable to constant temperature gradients or peculiar transfer characteristics can be overcome by calibration, attempts to regulate and dissipate excess absorbed power within an implant will often result in localized heating and temperature gradients within the implant, including the temperature sensor, contributing to erroneous temperature measurements and, therefore, erroneous pressure measurements. As such, varying power dissipation levels within an implant can cause uncertainty due to the effects on the operation of the temperature sensor. 
         [0007]    Excess power dissipation can also be detrimental to the transducer parameter extraction circuit used in implants. In the example of a MEMS pressure transducer, the extraction circuitry may be a capacitance-controlled relaxation oscillator (CCO) that transforms the MEMS capacitance into a frequency tone. Such circuitry depends on an on-chip ploy-resistor that has a temperature dependant resistance (for example, Tc=3500 ppm/° C.). Temperature uncertainty resulting from localized heating is reflected in the relaxation time and hence the oscillator frequency. Because the frequency tolerance of CCO relaxation oscillators demands a very low temperature variation or uncertainty (for example, less than 0.03° C.), even a small amount of excess power cannot be tolerated in the implant, necessitating some type of management scheme. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    The present invention provides communication systems and methods for dynamically controlling the power wirelessly delivered by a remote reader unit to a separate sensing device, such as a device adapted to monitor a physiological parameter within a living body, including but not limited to intraocular pressure, intracranial pressure (ICP), and cardiovascular pressures that can be measured to assist in diagnosing and monitoring various diseases. According to a particular aspect of the invention, such a communication system can be adapted to provide enhanced functionality and data rate transfers by combining digital and analog communication between the sensing device and reader unit. 
         [0009]    The communication system includes at least one telemetry antenna within the reader unit and adapted for electromagnetically delivering power to the sensing device, at least one sensing element within the sensing device for sensing a parameter of the fluid and producing an output based on the parameter, electronic components within the sensing device for processing the output of the sensing element and generating therefrom a processed data signal of the sensing device, and at least one telemetry antenna within the sensing device for receiving the power electromagnetically delivered by the reader unit and communicating the processed data signal to the reader unit. The electronic components are adapted to be powered at an operating power level. The communication further includes means for preventing the power supplied to the electronic components from exceeding the operating power level. 
         [0010]    The communication method generally entails a reader unit and sensing device that can be of the type described above, and involves electromagnetically delivering power from a telemetry antenna within the reader unit to a telemetry antenna within the sensing device, and preventing the power supplied to electronic components of the sensing device from exceeding the operating power level. 
         [0011]    The communication scheme and method are particularly intended for use with wireless implantable medical devices that obtain all of their power from a reader unit located outside the body, enabling safe, detailed, real-time, and continuous monitoring of a physiological parameter. According to a preferred aspect of the invention, excess power supplied to the device can be avoided, thereby eliminating the requirement to dissipate heat, avoiding potential measurement errors arising from localized heating or temperature gradients within the device, and avoiding unnecessary heating of tissue that surrounds the device when implanted in a body. 
         [0012]    Other aspects and advantages of this invention will be better appreciated from the following detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIGS. 1 and 2  schematically represent implantable devices of types that can be employed in the present invention. 
           [0014]      FIG. 3  is a block diagram of a wireless pressure monitoring system utilizing a passive sensing scheme that can be utilized by the present invention. 
           [0015]      FIGS. 4 through 6  schematically represent communication schemes for dynamically controlling power that is wirelessly delivered to an implantable device, for example, of the types depicted in  FIGS. 1 and 2 , in accordance with three embodiments of this invention. 
           [0016]      FIG. 7  is a graph representing an encoding scheme that can be used with the invention to transmit sampled data from an implantable device to a remote reader unit. 
           [0017]      FIG. 8  is a block diagram representing a communication protocol that can be used with the invention to transmit information between an implantable device and a remote reader unit. 
           [0018]      FIG. 9  is a graph representing a reader-to-sensor protocol that can be used with the invention to transmit information from an implantable sensing device to a remote reader unit. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]      FIG. 1  schematically depicts one example of an implantable sensing device  10  of a type that can be used with the present invention. The device  10  is represented as having a cylindrical housing  12 , which is convenient for placing the sensing device  10  within certain types of anchors adapted to secure the sensing device  10  to or within a wall-like structure, for example, the skull or the atrial or ventricular septum of the heart. Other exterior shapes for the housing  12  are also possible to the extent that the exterior shape permits placement of the sensing device  10  in a desired location or assembly of the sensing device  10  with an anchor. The cylindrical-shaped housing  12  of  FIG. 1  includes a flat distal face  14 , though other shapes are also possible, for example, a torpedo-shape in which the peripheral face  16  of the housing  12  immediately adjacent the distal face  14  is tapered or conical (not shown). The housing  12  can be formed of glass, for example, a borosilicate glass such as Pyrex Glass Brand No 7740 or another suitable material capable of forming a hermetically-sealed enclosure for the electrical components of the sensing device  10 . A biocompatible coating, such as a layer of a hydrogel, titanium, nitride, oxide, carbide, silicide, silicone, parylene and/or other polymers, can be deposited on the housing  12  to provide a non-thrombogenic exterior for the biologic environment in which the sensing device  10  will be placed. A nonlimiting example of an overall size for the housing  12  is about 3.7 mm in diameter and about 16.5 mm in length. 
         [0020]    As schematically depicted in  FIG. 1 , the sensing device  10  includes a transducer  18  located at the flat distal face  14 , and the housing  12  contains electronics  20  and an antenna  22 , the latter of which occupies most of the internal volume of the housing  12 . The transducer  18  can be adapted to sense a variety of parameters, including but not limited to pressure. The transducer  18  is preferably a MEMS device, more particularly a micromachine fabricated by additive and subtractive processes performed on a substrate. The substrate can be rigid, flexible, or a combination of rigid and flexible materials. Notable examples of rigid substrate materials include glass, semiconductors, silicon, ceramics, carbides, metals, hard polymers, and TEFLON. Notable flexible substrate materials include various polymers such as parylene and silicone, or other biocompatible flexible materials. A particular but nonlimiting example of the transducer  18  is a MEMS capacitive pressure sensor for sensing pressure, such as bariatric pressure, blood pressure, or intracranial pressure (ICP) of cerebrospinal fluid. A nonlimiting example of a preferred MEMS capacitor has a gauge pressure range of about −100 to about +300 mmHg, an absolute pressure range of about 300 mmHg to 1500 mmHg, and an accuracy of about 1 mmHg. A variety of additional or other sensing elements could be incorporated into the sensing device  10 , for example, inductive, resistive, and piezoelectric sensing elements could be used. Furthermore, the transducer  18  could be configured to sense temperature, flow, acceleration, vibration, pH, conductivity, dielectric constant, and chemical composition, including the composition and/or contents of a sensed fluid. Though the transducer  18  is shown located on the flat distal face  14  of the cylindrical housing  12 , the transducer  18  can be located at various locations near the distal end of the sensing device  10 , for example, on the peripheral face  16  of the housing  12  immediately adjacent the distal face  14 . The distal face  14  can be defined by a biocompatible semiconductor material, such as a heavily boron-doped single-crystalline silicon, in whose outer surface the transducer  18  (for example, a pressure-sensitive diaphragm of a capacitor) is formed. In this manner, only the distal face  14  of the housing  12  need be in contact with the media being sensed, such as blood, cerebrospinal fluid, etc., whose physiological parameter is to be monitored. 
         [0021]    The size and location of the antenna  22  are governed by the need to couple to a magnetic field to enable tele-powering of the sensing device  10  when implanted within the body using a remote interrogator/reader unit located outside the body, as will be discussed in more detail below. The antenna  22  generally comprises a coil assembly that can be made using any method known in the art, such as winding a conductor around a ferrite core, depositing (electroplating, sputtering, evaporating, screen printing, etc.) a conductive coil (preferably made from a highly conductive metal such as silver, copper, gold, etc.) on a rigid or flexible substrate), or any other method known to those skilled in the art. As such, the antenna  22  can be flat or three-dimensional such as cylindrical (as represented in  FIG. 1 ), cubic, etc. 
         [0022]    An advantage of a flat configuration is that it can be easily implanted under the skin, such as between the scalp and skull so that the antenna  22  lies flat against the skull. Such an embodiment is represented in  FIG. 2 , which represents an implantable sensing device  30  configured to have a housing  32  that contains a transducer  38  located adjacent a distal end  34  of the housing  32  and electronics  40 , and is coupled to an external flexible antenna  42 . This type of device  30  is adapted for deep implantation of the housing  32  within the body, for example, the brain, while permitting the antenna  22  to be located remote from the device  30 . The antenna  42  can be fabricated by forming a coil  44  on a flexible or rigid film  46 , which can be formed of any suitable biocompatible material. The antenna  42  is shown as physically and electrically interconnected with the housing  32  by a cable  36 , which may be flexible, rigid, or combination of flexible and rigid. The cable  36  may be coated, potted or covered with a biocompatible material. 
         [0023]      FIG. 3  schematically illustrates a monitoring system  50  and components thereof capable of implementing the implantable sensing devices  10  and  30  of  FIGS. 1 and 2 , as well as various other implantable sensing devices within the scope of the invention. An implantable sensing device and its companion interrogator/reader unit (hereinafter, reader unit) are identified by reference numbers  60  and  80  in  FIG. 3 . The reader unit  80  is adapted to wirelessly communicate with the sensing device  60  while the sensing device  60  is implanted at a desired location within a body. Because the sensing device  60  and reader unit  80  wirelessly communicate with each other, the monitoring system  50  lacks a wire, cable, tether, or other physical component that conducts the output of the sensing device  60  to the reader unit  80 . As such, the sensing device  60  defines the only implanted portion of the monitoring system  50 . 
         [0024]      FIG. 3  represents the sensing device  60  and reader unit  80  as configured to perform a wireless pressure sensing scheme disclosed in U.S. Pat. Nos. 6,926,670 and 6,968,734 to Rich et al. A wireless telemetry link is established between the sensing device  60  and reader unit  80  using a passive, magnetically-coupled scheme, in which onboard circuitry of the sensing device  60  receives power from the reader unit  80 .  FIG. 3  depicts the sensing device  60  as containing a transducer  62  and an antenna  64  represented as an inductor coil. The transducer  62  is represented in  FIG. 3  as being in the form of a pressure sensor, and more specifically a mechanical capacitor adapted to sense pressure as a physiological parameter of interest. In addition to sensing physiological parameters, the sensing device  60  can be configured to include various actuation functions, including but not limited to thermal generators, voltage and/or current sources, probes, and/or electrodes, drug delivery pumps, valves, and/or meters, microtools for localized surgical procedures; radiation-emitting sources, defibrillators, muscle stimulators, pacing stimulators, etc. 
         [0025]    As a passive communication scheme, the sensing device  60  lacks any internal means to power itself lies and therefore lies passive in the absence of the reader unit  80 . When a pressure reading is desired, the reader unit  80  is brought within range of the antenna  64  of the sensing device  60  to enable magnetic coupling between the antenna  64  and a second antenna  82  associated with the reader unit  80 . The antenna  82  is adapted to transmit an alternating electromagnetic field to the antenna  64  of the sensing device  60  and induce a sinusoidal voltage across the coil of the antenna  64 . When sufficient voltage has been induced, a supply regulator  66  within the sensing device  60  converts the alternating voltage on the antenna  64  into a direct voltage that can be used by electronics  68  as a power supply for signal conversion and communication. At this point the sensing device  60  can be considered alert and ready for commands from the reader unit  80 . To minimize the size of the sensing device  60 , the antenna  64  may be employed for both reception and transmission, or the sensing device  60  may utilize the antenna  64  solely for receiving power from the reader unit  80  and employ a second antenna (not shown) for transmitting signals to the reader unit  80 . 
         [0026]    The supply regulator  66  contains rectification circuitry that preferably outputs a constant voltage level for the other electronics from the alternating voltage input from the antenna  64 . The rectification circuitry can be of any suitable type, including but not limited to full-bridge diode rectifiers, half-bridge diode rectifiers, and synchronous rectifiers. The rectification circuitry may further include a capacitor for transient energy storage to reduce the noise ripple on the output supply voltage. The supply regulator  66  is represented as implemented on the same integrated circuit die as other components of the sensing device electronics  68 , for example, an application-specific integrated circuit, or ASIC. As represented in  FIG. 3 , the device electronics  68  include signal transmission circuitry  70  that receives an encoded signal generated by signal conditioning circuitry  72  based on the output of the transducer  62 , and then generates a signal that is propagated to the reader unit  80  with the antenna  64 . 
         [0027]    A benefit of configuring the sensing device  60  without a battery is that the device  60  and its operation do not require replacement or charging of a battery, and the size of the device  60  is not dictated by the need to accommodate a battery. However, the sensing device  60  of  FIG. 3  could be modified to use one or more batteries or other power storage devices to power the sensing device  60  when the reader unit  80  is not sufficiently close to induce a voltage in the sensing device  60 . Furthermore, it is also within the scope of the invention that such power storage devices may be rechargeable and capable of being recharged with the reader unit  80 . 
         [0028]    In addition to the antenna  82  for communicating with and powering the sensing device  60 , the reader unit  80  is represented in  FIG. 3  as including a separate antenna  84  for receiving the signals transmitted by the antenna  64  of the sensing device  60 , and front-end electronics  86  for processing the signal of the sensing device  60  as well as generating the alternating electromagnetic field sent by the antenna  82  to the sensing device  60 . For purposes of compactness, the functions of the antennas  82  and  84  could be performed by a single antenna. The front-end electronics  86  include field generation circuitry  88  for generating the alternating electromagnetic field generated by the antenna  82 , signal detection circuitry  90  for receiving data transmitted by the antenna  64  of the sensing device  60 , and a processing unit  92  that processes the data received through the detection circuitry  90 , relays the processed data to a user interface  94 , and enables control of the field generation circuitry  88 . The fabrication and operation of the front-end electronics  88  and its components are well known in the art and therefore will not be discussed in any detail here. The user interface  94  may be a display, computer, or other data logging devices that can be physically incorporated into the reader unit  80  or separate and coupled to the unit  80  through a cable or wirelessly. 
         [0029]    As alternatives to the sensing scheme of  FIG. 3 , wireless telemetry links can be established using other schemes, such as a resonant scheme also disclosed in U.S. Pat. Nos. 6,926,670 and 6,968,734 to Rich et al. or a fully or partially active scheme in which the sensing device  60  may contain batteries and/or rechargeable power storage devices. In a resonant scheme, the sensing device contains a packaged inductor coil (similar to the antenna  64  of  FIG. 3 ) and a pressure sensor in the form of a mechanical capacitor (similar to the capacitor  62  of  FIG. 3 ), which together form an LC (inductor-capacitor) tank resonator circuit that has a specific resonant frequency, expressed as 1/(LC) 1/2 , that can be detected from the impedance of the circuit. At the resonant frequency, the circuit presents a measurable change in magnetically-coupled impedance load to an external antenna associated with a separate reader unit (similar to the antenna  82  and reader unit  80  of  FIG. 3 ). Because the resonant frequency is a function of the capacitance of the capacitor within the sensing device, the resonant frequency of the LC circuit changes in response to pressure changes that alter the capacitance of the capacitor. Because the coil within the sensing device has a fixed inductance value, the reader unit is able to determine the pressure sensed by the sensing device by monitoring the resonant frequency of the circuit. 
         [0030]    A wireless communication platform implemented with the monitoring system  50  should take into consideration a number of important aspects. Regarding data sample bandwidth, the sampling rate should be greater than 200 Hz for some applications to achieve high resolution and clinically useful data when monitoring many biologic parameters, such as cardiovascular and intracranial pressures. As an example, AAMI standards for blood pressure monitoring specify a 200 Hz cutoff frequency. The sensing devices (e.g.,  10 ,  30  and  60  in  FIGS. 1 ,  2  and  3 ) and their reader units (e.g.,  80  in  FIG. 3 ) should also be capable of communicating distances as required for communication between internal organs intended to be monitored and the nearest accessible locations outside of the body. As previously noted, while a few centimeters of communication can be sufficient for some applications, a communication distance of fifteen centimeters or more will be desirable or necessary for others. Finally, the sensing devices  10 ,  30  and  60  should ideally be capable of being delivered to the site of implantation with a catheter not larger than French 15 size (about 5 mm in diameter), and preferably French 11 (about 3.7 mm in diameter), which establishes limitations on the type and size of electronics within the housing (e.g.,  12  and  32 ) of the sensing device  10 ,  30  and  60 . On the other hand, greater coil size corresponds to longer communication distances. Therefore, for the sensing device  10  of  FIG. 1  (and other designs with an enclosed antenna), the antenna  22  should be as large as possible, necessitating that the electronics within the housing  12  be as small as possible to meet a desired package size. As an example, the coil of the antenna may have a maximum size of a few millimeters in diameter and a length of about ten to fifteen millimeters, and an ASIC die carrying the electronics may have a maximum width and length of about 2 mm. A wireless sensing device meeting these dimensional goals should be capable of delivery using minimally invasive procedures, have minimal impact on the body in which it is implanted, and be more readily accepted for research and clinical use. 
         [0031]      FIGS. 4 through 6  represent further aspects of the monitoring system  50  of  FIG. 3  for achieving dynamic control of power delivered to the sensing device  60 . Dynamic power control is provided for the purpose of compensating for potentially very large variations in the power level delivered to the sensing device  60  as a result of the likelihood that the transmission distance between the antennas  64  and  82  of the sensing device  60  and reader unit  80  will vary widely, depending on the location and use of the sensing device  60 . The maximum achievable transmission distance between the antennas  64  and  82  (and, if present, the separate reception antenna  84 ) will be limited by various factors, including the magnetic field strength generated by the reader unit  80  and the quality and size of the antenna coil of the sensing device  60 . As the transmission distance is reduced, more power is transmitted to the sensing device  60  and, if excessive, can lead to damage to the device  60 , damage to body tissue surrounding the device  60 , and sensor output errors. In the embodiments of  FIGS. 4 through 6 , power delivery is dynamically controlled to avoid the delivery of excess power to the sensing device  60 , instead of relying on power dissipation within the device  60 . As such, damage to the sensing device  60  and surrounding body tissue is avoided, as well as errors that can occur in the output of the sensing device  60  and its transducer  62  as a result of power oversupply and heating of the device  60 . As a result, the embodiments of the monitoring system  50  represented in  FIGS. 4 through 6  are capable of improving the accuracy and stability of the signal generated by the sensing device  60 , and thereby provides a more accurate indication of the physiological parameter being monitored. 
         [0032]      FIGS. 4 through 6  generally represent communication schemes that incorporate dynamic power control in accordance with three embodiments of the present invention. In  FIG. 4 , the reader unit  80  is adapted to control the power level delivered to the sensing device  60  using one or more feedback signals that are transmitted by the sensing device  60  and then received and processed by the reader unit  80 . Such feedback signals may be based on signal strength, signal-to-noise ratio, signal-to-carrier ratio, etc., of the data transmission signal generated by the sensing device  60 . In  FIG. 5 , power level control is accomplished using an interactive signal between the reader unit  80  and the sensing device  60 . Finally, power level control is accomplished in  FIG. 6  by varying the tank load resistance and/or reactance of the coil of the antenna  64  of the sensing device  60 . For convenience,  FIGS. 4 through 6  depict only those components of the sensing device  60  and the reader unit  80  that are particularly relevant to the description of the dynamic power control scheme, while others (including components represented in  FIG. 3 ) are omitted. Furthermore, reference numbers used in  FIG. 3  are also used in  FIGS. 4 through 6  to identify the same or functionally equivalent components, and reference numbers used in  FIGS. 4 through 6  to identify additional components are consistently used throughout  FIGS. 4 through 6  to identify the same or functionally equivalent components employed in the embodiments. 
         [0033]    With reference to  FIG. 4 , powering of the sensing device  60  does not contain any means for providing direct feedback/communication from the sensing unit  60  to the reader system  80 , and there are no direct means of assessing the power level delivered by the reader unit  80  to the sensing device  60  or providing feedback of the power level to the reader unit  80  to the sensing device  60 . Instead, the sensing device  60  relies entirely on the reader unit  80  to determine the appropriate power level delivered to the sensing device  60 . The reader unit  80  contains components for evaluating an internal receiver signal characteristic of the sensing device  60 , including but not limited to receive signal strength indicator (RSSI), signal-to-noise ratio (S/N), signal-to-carrier ratio (S/C), minimum (or desired) detectable signal strength, etc., to determine what power level should be delivered to the device  60 .  FIG. 4 . depicts the sensing device  60  as containing the antenna  64  and electronics  68 , corresponding to the components represented in  FIG. 3 . Similarly, the reader unit  80  is shown in  FIG. 4  as containing the antenna  82  corresponding to the antenna  82  represented in  FIG. 3  and, as such, the antenna  82  creates a magnetic (electromagnetic) field that powers the antenna  64  of the sensing device  60 . (In  FIGS. 4 through 6 , the second antenna  84  is omitted and its reception function merged into the antenna  82 .) The reader unit  80  further includes an oscillator  96  which sets the carrier frequency and drives a power amplifier (PA)  98 . According to a preferred aspect of this embodiment, the power amplifier  98  has a variable gain and hence a variable output signal amplitude. The amplified signal drives the antenna  82  through a directional coupler  100 . Signals returning from the sensing device  60  via the antenna  82  are sampled by the directional coupler  100  and processed by a receiver (RX) chain  102 . In this embodiment, one or more signal parameters  104  characteristic of the communication link between the sensing device  60  and reader unit  80  are examined to assess and control the output signal amplitude (power level) transmitted by the antenna  82 . A power control  106  uses the signal parameters  104  to assess the power level being received by the sensing device  60  and then, if necessary, adjusts the output signal amplitude of the power amplifier  98  to a level that will avoid overpowering the sensing device  60 . 
         [0034]    Nonlimiting examples of signal parameters  104  of particular interest are represented in  FIG. 4  as including RSSI, S/N, S/C and combinations thereof, which can be used individually or in combination to provide an indication as to the proximity of the sensing device  60  to the reader unit  80  or the distance between the antennas  64  and  82  of the device  60  and reader unit  80  based on information sent by the sensing device  60  to the reader unit  80 . For example, RSSI can be used by the reader unit  80  to estimate the strength, quality or amount of power received by the sensing device  60 , and therefore an indication of the distance between the sensing device  60  to the reader unit  80 , which is then used by the reader unit  80  to enable the power control  106  to adjust the output signal amplitude of the power amplifier  98  as needed. 
         [0035]    In contrast to the embodiment of  FIG. 4 ,  FIG. 5  represents an embodiment that relies on a feedback signal from the sensing device  60  to adjust the power level transmitted by the reader unit  80  to the device  60 . In this case, the sensing device  60  requires power level detection, modulator control, and antenna modulation circuitry to sense and transmit information regarding the power level back to the reader unit  80 , which then determines whether the power level being received by the sensing device  60  is adequate (within a predetermined range) or above or below a predetermined threshold, and if necessary adjusts the power level transmitted to the sensing device  60  until a targeted power level is achieved. 
         [0036]    Similar to  FIG. 4 , the reader unit  80  is represented in  FIG. 5  as comprising an antenna  82 , oscillator  96 , power amplifier (PA)  98 , directional coupler  100 , receiver (RX) chain  102 , and power control  106 . Unless otherwise indicated, these components perform the same operations as described for  FIG. 4 . In contrast to  FIG. 4 , the sensing device  60  contains a power detector  74  adapted to assess the power level received by the antenna  64  of the sensing device  60 , and then provide such information to a power level encoder  76 . The power level encoder  76  dictates information that is encoded by a modulator  77  onto the antenna  64 . In particular, in addition to the signal pertaining to the measurements performed by the sensing device  60 , the power level encoder  76  drives the modulator  77  to encode information pertaining to the power level received by the sensing device  60 , and specifically whether the power level is within or outside a predetermined range for the sensing device  60 . When this information is received by the reader unit  80 , the information is sampled by the directional coupler  100  and processed by the RX chain  102 . In this embodiment, the power level signal  108  is extracted by the RX chain  102  and directly used by the power control  106  to adjust, if necessary, the output signal amplitude of the power amplifier  98  to ensure that the sensing device  60  is continuously receiving an appropriate power level. 
         [0037]    Alternatively, in  FIG. 5 . the sensing device  60  may be equipped to produce a signal that offers a much wider spectrum, for example, analog or higher numbers of digital values. The specific indicator signal may be digital or analog or a combination thereof. In one embodiment, if the power level is too low or is decreasing beyond a certain level the sensing device  60  can be configured to drop its transmission frequency to a another value (for example, 30% below the normal operating frequency or to a specific pre-determined frequency outside the normal operation range), and if the power level is too high or is increasing above a certain level the sensing device  60  may push its transmission frequency to a another value (for example, 30% above the normal operating frequency or to a specific pre-determined frequency outside the normal operation range). Finally, the sensing device  60  may be configured to simply control an indicator on the reader unit  80  that allows the operator to manually select the power level generated by the reader unit  80 . In addition, either the sensing device  60  or reader unit  80 , or both may incorporate other means for indicating the proximity of the sensing device  60  to the reader unit  80 , such as a proximity sensor, for example, a capacitive or ultrasonic sensor that determines the distance between the reader unit  80  and the sensing device  60 . The sensing device  60  may include various other components capable of generating a specific indicator signal to indicate whether the power received by the sensing device  60  is within an acceptable range. Such a component may generate a signal indicating low power and another for excess power. 
         [0038]    The third embodiment of  FIG. 6  simplifies the reader unit  80  by transferring the entire dynamic power control function to the sensing device  60 . In this case, the power level is detected and fed into a power control circuit within the sensing device  60 , which itself controls the power level that can be coupled into the device  60  by the antenna  64 . In a preferred aspect of this embodiment, the power level transmitted by the reader unit  80  is detected and controlled via antenna tank load de-tuning within the sensing device  60 . Similar to  FIGS. 4 and 5 , the reader unit  80  is represented in  FIG. 6  as comprising an antenna  82 , oscillator  96 , power amplifier (PA)  98 , directional coupler  100 , receiver (RX) chain  102 , and power control  106 . Unless otherwise indicated, these components perform the same operations as described for  FIGS. 4 and 5 . 
         [0039]    As with the prior embodiments, the oscillator  96  sets the carrier frequency and drives the power amplifier  98 , the output signal of the power amplifier  98  drives the antenna  82  through the directional coupler  100 , and the antenna  82  generates a magnetic (electromagnetic) field for powering the sensing device  60 . In contrast to the prior embodiments, the power amplifier  98  can have a fixed gain and hence a fixed output signal amplitude level. The antenna  64  of the sensing device  60  couples to the magnetic field generated by the reader unit  80  for powering the sensing device  60 . As in the embodiment of  FIG. 5 , the sensing device  60  includes a power detector  74  for assessing the power level transmitted by the reader unit  80  and received by the antenna  64 , and provides that information to a power control  78  that dictates the state that an antenna de-tuner  79  applies to the antenna  64 . The de-tuner  79  controls the tank mismatch or load circuit of the antenna  64 . If the power level received by the antenna  64  is within a predetermined range for the sensing device  60 , the power control  74  drives the antenna de-tuner  79  to maintain the operation of the antenna  64 . If the power level is above or below the predetermined range, the power control  78  drives the antenna de-tuner  79  to increase or decrease, respectively, the tank load resistance and/or reactance, thereby adjusting the power absorbed by the antenna  64 . If the power level transmitted by the reader unit  80  is above a predetermined threshold, the antenna mismatch load is increased to reject the extra power transmitted by the reader unit  80 . Conversely, if the internal power level of the sensing device  60  is below a predetermined threshold, the antenna mismatch load is reduced to increase the power coupled into the device  60  by the antenna  64 . 
         [0040]    In contrast to the embodiments of  FIGS. 4 and 5 , no information related to the power level at the sensing device  60  needs to be communicated back to the reader unit  80  in the embodiment of  FIG. 6 . Nonetheless, features of the first and second embodiments can be incorporated into the embodiment of  FIG. 6  to provide coarse power setting or provide further indicators of power level for reasons other than power control, such as signal indication. For example, at the extremes of the power control range, the embodiment of  FIG. 6  can be modified to provide a feedback signal that may be used as described for the embodiment of  FIG. 5 , or can simply be used as a range indicator. 
         [0041]    It is foreseeable that a combination or combinations of the three embodiments described above could be used, in which both the sensing device  60  and the reader unit  80  manage the dynamic power control. In such embodiments, the output of the power amplifier  98  is controlled as well as antenna de-tuning performed by the de-tuner  79  of sensing unit  60 . 
         [0042]    In view of the above, each of the embodiments of  FIGS. 4 ,  5  and  6  provides a power control technique in the sensing device  60  to mitigate excess powering of the device  60 . As such, the invention can prevent damage to the device  60 , prevent heating and damage to surrounding body tissue, enable more accurate and stable sensing information, as well as other benefits as a result of avoiding incidences of the sensing device  60  receiving excessive power from the reader unit  80 . In medical-related implants, a more significant effect is the avoidance or at least a significant reduction in measurement errors resulting from excessive power supplied to the components of the sensing device  60  and/or localized heating of the components attributable to receiving excessive power levels. For example, the invention avoids or at least mitigates sensing errors that can occur as a result of excessive powering and/or localized heating of a temperature sensor used to compensate the output of the transducer  62  for variations in temperature, and/or avoids or at least mitigates output errors that can occur in the output of the transducer  62  itself as a result of the transducer  62  receiving excess power and/or localized heating of the transducer  62  attributable to receiving excess power. 
         [0043]    The embodiments of the invention described above, as well as a variety of other monitoring systems, can be modified to make use of a wireless communication platform that transmits both digital and analog data. As will become apparent from the following description, the mixed analog and digital communication is capable of both enhanced functionality via digital communication while allowing higher sensor data rates (or other information) via analog communication. Furthermore, the analog communication can eliminate the need for an analog-to-digital convertor in a sensing device (such as one of the sensing devices  60  described above), which is advantageous since such converters can consume considerable power and may add noise to the signal transmitted by the sensing device. Additional potential advantages include the ability to reduce the size of the sensing device and increase transmission distances and the potential for longer sensor life when monitoring physiological parameters of the human body. In addition, the wireless communication platform can enable bi-directional communication that could allow for actively responding to individual needs, such as closed-loop drug delivery. 
         [0044]    The wireless communication platform is particularly well suited for the magnetic telemetry technique described above for the sensing device  60  and reader unit  80 , though other technologies (including but not limited to ultrasonic telemetry techniques) could be employed. In a preferred application of this platform, a passive communication scheme as described above for the reader unit  80  and the sensing device  60  is employed, meaning that the sensing device  60  does not contain a battery and receives all of its operating power from the reader unit  80 , though an active scheme utilizing a power storage device (e.g., a battery) could also be used. In addition, the communication platform makes advantageous use of the second antenna  84  shown for the reader unit  80  of  FIG. 3 . Accordingly, the communication platform will be described in reference to the monitoring systems  50 , sensing devices  10 ,  30  and  60 , and reader unit  80  of  FIGS. 1 through 6 , though it should be understood that the communication platform is not limited to the particular embodiments disclosed and described for these figures. 
         [0045]    Magnetic telemetry schemes of the type previously described for the sensing devices  10 ,  30  and  60  and reader unit  80  of  FIGS. 1 through 6  have been proven and used extensively in the identification and tracking industry, for example, RFID tags. However, a number of modifications are desirable in order to implement the strictly digital identification technology employed by RFID tags to sensing applications suitable for medical implants. RFID technologies to do not employ an analog interface, and their protocols are not intended for sensors and other implants (such as actuators). Furthermore, traditional RFID magnetic telemetry schemes employ a single coil on the RFID tag to both receive power from a reader unit and also transmit information back to the reader unit. While convenient from a packaging perspective and minimizing costs, this approach may compromise the effectiveness of both the receiver and the transmitter coils in some applications. With this in mind, the following will describe a wireless communication platform that divides the functions of transmitting and receiving performed by the reader unit  80  between two separate coils, such as the antennas  82  and  84  in  FIG. 3 . In this way, the transmitting coil ( 82 ) can be optimized for communication with the sensing device  60 , while simultaneously optimizing the receiving coil ( 84 ) for efficient capture of digital and analog signals from the sensing device  60 . However, as with the embodiments of  FIGS. 4 through 6 , the transmission and reception functions could be merged onto a single antenna (e.g.,  82  in  FIGS. 4 to 6 ). 
         [0046]    Modulation of sampled data onto the subharmonic carrier for transmission from the sensing device  10 ,  30  or  60  to the reader unit  80  can be accomplished with many schemes including analog modulation such as amplitude modulation (AM) frequency modulation (FM), and digital modulation such as phase shift keying (PSK) and frequency shift keying (FSK). For example, FSK modulation can be used to map two distinct frequencies to the digital bits  1  and  0 . This particular coding scheme is very robust to interference, has adequate bandwidth, and is technologically mature. The FSK signal is then Manchester encoded to ensure proper timing synchronization between the sensing device  10 ,  30  or  60  and reader unit  80 .  FIG. 7  is illustrative of a suitable Manchester encoding scheme, which represents a bit transition from 0 to 1 or vice versa as occurring during the middle of the bit interval. This modulation/coding scheme is believed to offer a high level of immunity to noise and other interferences. 
         [0047]    Because higher radio frequencies (above 100 MHz) suffer from tissue absorption, lower frequencies are preferred by the invention for the sensing devices  10 ,  30  and  60  when deeply implanted into the human body, such as within the heart. Of the lower frequencies, the 13.56 MHz ISM band is most attractive as the power transmission frequency from the reader unit  80  to the sensing device  10 ,  30  or  60  due to the minimal size required for the coil of the sensing device  10 ,  30  or  60  and its associated resonant capacitor. Both power transmission frequency from the reader unit  80  and the data transmission frequency from the sensing device  10 ,  30  or  60  should be optimized for optimum performance of the monitoring system  50 . To select the FSK carriers and modulation rates, one will evaluate bandwidth capacity and noise immunity of all subharmonic bands of 13.56 MHZ down to 423.8 kHz. Tradeoffs for different frequencies may include signal-to-noise immunity, circuit size, power consumption, and transmitter antenna efficiency. The rate of FSK modulation should also be chosen in view of the direct tradeoff between bandwidth and noise immunity. The data transmission frequency from the sensing device  10 ,  30  and  60  to the reader unit  80  can be the same frequency or different from the power transmission frequency. A preferred subharmonic for FSK modulation of the data transmission frequency is believed to be 3.39 MHz, for reasons including a sufficiently high frequency to maintain transmission efficiency and transmit the required bandwidth, and sufficiently far enough from 13.56 MHz to allow for bandstop filters. In addition, this data transmission frequency allows for the use of a single coil for both reception and transmission of RF signals (digital and analog) with the sensing device  10 ,  30  or  60 , thereby minimizing the required internal volume of the sensing device  10 ,  30  or  60 . 
         [0048]    In view of the above, a preferred modulation scheme between the reader unit  80  and the sensing device  10 ,  30  or  60  is believed to be digital transmission using a 13.56 MHz carrier frequency. For simultaneous transmission of both analog and digital information between the sensing device  10 ,  30  or  60  and the reader unit  80 , a preferred modulation scheme is believed to include the following: 20-200 kHz modulation bandwidth, digital transmission using FSK modulation of an AM frequency (for example, Logic 0: AM frequency equal to 75.625 kHz, and Logic 1: AM frequency equal to 105.94 kHz), and analog transmission using frequency modulation (FM) of an AM frequency (for example, the analog signal is proportional to the AM frequency). In view of the foregoing, specific electronics for achieving these modulation schemes will be evident to those skilled in the art, and therefore will not be described in any detail here. 
         [0049]    The protocol for communication between the sensing device  10 ,  30  or  60  and the reader unit  80  specifies an agreed order and content for transmitting information, and is an important aspect of a wireless communication platform used in the monitoring system  50  because it determines the complexity of electronics needed in the instrument. Particularly suitable protocols should allow simple electronics to perform basic operations while allowing for expanded capabilities, including communication between the reader unit  80  and a number of different sensing devices  10 ,  30  or  60  adapted to sense a variety of physiological parameters, in which case the protocol should also include a code that identifies the individual sensing devices, for example, by family and serial number. The protocol should also preferably identify a checksum for data integrity, along with potentially additional features including, but not limited to, calibration information, addressing capability, programming, and multiple parameters such as temperature, pressure, flow, pH, etc. Start and stop patterns are defined as well as the transmission rate and bit order for encoding, which will determine the signal to noise immunity vs. bandwidth tradeoff. 
         [0050]    Using the IEC15693 standard for contactless vicinity ID cards as starting point, a communication protocol suitable for using in the monitoring system  50  may include the following features. The reader unit  80  initially requests the sensing device  10 ,  30  or  60  to respond, there is a start and end of frame for each communication direction, the digital data rate may be changed to ascertain distance, provisions for analog modulation are included to simplify implant electronics, and identification information is transmitted for responses from each sensing device (if the system  50  contains multiple sensing devices).  FIG. 8  represents a suitable sequence, which begins with a start-of-frame (SOF) and is followed by parameter information that describes the data it precedes. The sequence finishes with an end-of-frame (EOF). The same basic sequence can be used for power and data transmission between reader unit and sensing device. 
         [0051]    Communication from the reader unit  80  to the sensing device  10 ,  30  or  60  can be accomplished by suppressing the RF power from the reader unit  80  for short periods of time (reset).  FIG. 9  represents an exemplary timing for this protocol. The reader unit  80  is the first to communicate, so that multiple sensing devices (if present) do not interfere with each other and corrupt the signal the reader unit  80  is attempting to read. A simplified version of the full protocol may include the following: only one 4-bit word (16 options) for parameters (a parameter selects which sensing device is to respond, no data transmission follows the parameters, the sensing device responds after the selection is made), no EOF, and all sensing devices respond unless asked not to. 
         [0052]    As previously stated, the communication from the sensing device  10 ,  30  or  60  to the reader unit  80  can take place on a subharmonic carrier (3.39 MHz) of the power RF signal (13.56 MHz). The 3.39 MHz can be 100% amplitude modulated at various rates to determine the logic values and the framing. The protocol is preferably comprehensive, in that it allows for both digital and analog signal transmission and allows for future design flexibility in assigning codes, data types, and data bandwidth. As noted above, framing can be the same as discussed above in reference to  FIG. 8  (SOF, Parameters, Data, EOF). A nonlimiting example of a suitable modulation for the digital portion of the transmission is as follows: data is 32 bits wide (parameters may include calibration, sensor identification, CRC (cyclic redundancy check), and/or data rate); logic 0 (nominal data rate)—48 cycles of 70.625 kHz (3.39 MHz/48); logic 1 (nominal data rate)—72 cycles of 105.9375 kHz (3.39 MHz/32), SOF—108 cycles of 105.9375 kHz followed directly by 72 cycles of 70.625 kHz followed directly by logic 1 followed directly by logic 0; and EOF—logic 0 followed directly by logic 1 followed directly by 72 cycles of 70.625 kHz followed directly by 108 cycles of 105.9375 kHz. 
         [0053]    In addition to advantages associated with the transmission of both digital and analog data, such as improved accuracy and greater communication distance by allowing optimization of the antennas  64 ,  82  and  84 , the wireless communication platform outlined above provides a comprehensive communication platform (including modulation scheme and modulation protocol) capable of addressing and communicating with a large number of different sensing devices  10 ,  30  or  60 . In particular, the platform as described allows for communication with up to 256 sensing devices, with greater numbers achievable with appropriate modifications. In addition, the communication protocol can achieve the following: bi-directional communication, simultaneous and continuous tele-powering and tele-communication, high-speed communication (for example, greater than two hundred samples per second), greater insensitivity to the implant orientation in regards to the readout unit, ease of hardware implementation in an ASIC within the sensing device  10 ,  30  or  60 , and minimal size of the sensing device  10 ,  30  or  60 . 
         [0054]    A wide variety of potential applications exist for the monitoring system, implantable sensing devices, and reader units of the types described above. Commercial applications include those in the medical field, and particularly applications that entail chronic or continuous measurements of physiological parameters, for example, in support of the trend toward home health monitoring. Particular examples include the diagnosis and/or monitoring of significant disease conditions, including congestive heart failure (CHF), hydrocephalus disease, and glaucoma disease. Other commercial applications encompass virtually any area that is in need of wireless sensing, for example, monitoring fluids in aerospace, automotive and industrial applications, including the monitoring of such physical and chemical parameters as pressure, flow, density, pH, and chemical composition of fluids, temperature, humidity, oxygen concentration, acceleration, radiation, etc. Military and governmental applications also exist that involve sensing of the above-noted physiological, physical and chemical parameters. As particular but nonlimiting examples, potential applications within the National Aeronautics and Space Administration (NASA) of the USA include implantable sensors for monitoring biological pressures in space and centrifuge-based systems, supporting animal studies of fundamental biological processes in cardiovascular, neurological, urological, and gastroenterological systems, monitoring effect of gravity or high accelerations on biological pressures, sensors requiring minimal power that can non-invasively measure pressure in environments with different gravity ranges, wireless sensors for remotely monitoring physical or chemical parameters in sealed containers, wireless telemetry communication for micro-biochemical and physical instruments and sensors, miniaturization of instruments through integration with MEMS-based sensors, in situ measurement and real time control of biological and physical phenomena, capability for automated acquisition, processing, and communication of biological data, miniature bio-processing systems that allow for precise measurement and closed loop control of multiple environmental parameters such as temperature, pH, oxygen, etc., and multiple intelligent implanted sensors that are addressable by a readout unit in a single or multiple animals in one or more environments. 
         [0055]    While the invention has been described in terms of specific embodiments, it is apparent that other forms could be adopted by one skilled in the art. Therefore, the scope of the invention is to be limited only by the following claims.