Remotely interrogated diagnostic implant device with electrically passive sensor

An implant device is provided which is responsive to an external interrogation circuit. The implant device includes a structure implantable within a living animal and operatively configured to carry out or assist in carrying out a function within the living animal. The device further includes an electrically passive sensing circuit integral with the structure for sensing a parameter associated with the function. In particular, the sensing circuit includes an inductive element wherein the sensing circuit has a frequency dependent variable impedance loading effect on the interrogation circuit in response to an interrogation signal provided by the exciter/interrogator element, the impedance loading effect varying in relation to the sensed parameter.

TECHNICAL FIELD
 The present invention relates generally to medical implant devices, and
 more particularly to devices which may be interrogated remotely from
 outside the body.
 BACKGROUND OF THE INVENTION
 Various types of medical implant devices have been developed over the
 years. In many instances, such devices enable humans to live longer, more
 comfortable lives. Implant devices such as pacemakers, artificial joints,
 valves, grafts, stents, etc. provide a patient with the opportunity to
 lead a normal life even in the face of major heart, reconstructive, or
 other type surgery, for example.
 It has been found, however, that the introduction of such implant devices
 can sometimes lead to complications. For example, the human body may
 reject the implant device which can ultimately lead to infection or other
 types of complications. Alternatively, the implant device may malfunction
 or become inoperative. Therefore, it is desirable to be able to monitor
 the condition of the implant device. On the other hand, it is highly
 undesirable to have to perform invasive surgery in order to evaluate the
 condition of the device.
 Still further, it is desirable to be able to monitor conditions related to
 the use of implant devices. For example, in heart patients it may be
 helpful to know the amount of blood flowing through a stent or graft in
 order to evaluate the health of the patient. Again, however, it is
 undesirable to have to perform invasive surgery in order to evaluate such
 conditions.
 Techniques have been developed which enable the function of an implant
 device to be monitored remotely from outside the body of the patient.
 These techniques involve including one or more sensors in the device for
 sensing the condition of the device. The device further includes a small
 transceiver for processing the output of the sensors and transmitting a
 signal based on the output. Such signal typically is a radio frequency
 signal which is received by a receiver from outside the body of the
 patient. The receiver then processes the signal in order to monitor the
 function of the device.
 While such conventional techniques may be effective in avoiding the need to
 perform invasive surgery, there are however several drawbacks associated
 therewith. For example, the transceiver included in the implant device
 typically includes complex electrical circuitry such as mixers,
 amplifiers, microprocessors, etc. for receiving an interrogation signal
 and for transmitting a response signal based on the output of the sensors.
 Such complex circuitry has a relatively high cost associated therewith. In
 addition, the complexity of the circuitry increases the likelihood that
 the device itself may be defective. This would then require further
 invasive surgery and could even result in physical harm to the patient.
 Still another shortcoming with conventional implant devices with sensors
 included therein is power concerns. Some type of circuit for providing
 power to the transceiver is necessary. The circuit may be a built-in power
 source such as a battery, or a circuit which derives operating power from
 an external excitation signal. In either case, again the complexity of the
 circuit and/or the need to replace the battery periodically adds to the
 cost of the device and increases the opportunity for failure or defects.
 In view of the aforementioned shortcomings associated with conventional
 implant devices, there is a strong need in the art for a medical implant
 device which can be remotely interrogated but which does not require
 complex electrical circuitry such as mixers, amplifiers, microprocessors,
 etc. There is a strong need for a medical implant device which carries out
 a function within a human or other living animal, and can be remotely
 interrogated simply and reliably. There is a strong need for such an
 implant device which permits most or all of the sensor circuitry to be
 embedded directly within the device. Moreover, there is a strong need for
 a medical implant device which does not rely on batteries or other complex
 energy conversion circuits in order to operate.
 SUMMARY OF THE INVENTION
 The present invention is responsive to the aforementioned shortcomings with
 conventional devices, and is directed towards an implant device to be
 implanted within a living animal and responsive to an interrogation
 circuit having an exciter/interrogator element which is located outside
 the living animal. The implant device includes a structure implantable
 within the living animal and operatively configured to carry out or assist
 in carrying out a function within the living animal. The implant device
 further includes an electrically passive sensing circuit integral with the
 structure for sensing a parameter associated with the function, the
 sensing circuit including an inductive element wherein the sensing circuit
 has a frequency dependent variable impedance loading effect on the
 interrogation circuit in response to an interrogation signal provided by
 the exciter/interrogator element, the impedance loading effect varying in
 relation to the sensed parameter.
 To the accomplishment of the foregoing and related ends, the invention,
 then, comprises the features hereinafter fully described and particularly
 pointed out in the claims. The following description and the annexed
 drawings set forth in detail certain illustrative embodiments of the
 invention. These embodiments are indicative, however, of but a few of the
 various ways in which the principles of the invention may be employed.
 Other objects, advantages and novel features of the invention will become
 apparent from the following detailed description of the invention when
 considered in conjunction with the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 The present invention will now be described with reference to the drawings,
 wherein like reference numerals are used to refer to like elements
 throughout.
 Referring initially to FIG. 1, a system for remotely interrogating a
 medical implant device in accordance with the invention is generally
 designated 30. The system 30 includes a medical implant device 32 which is
 implanted in a living animal such as a human patient 34. As is discussed
 in more detail below, the medical implant device 32 can be any of a wide
 variety of different types of devices including, for example, a stent,
 graft, artificial joint, etc.
 The device 32 is configured to carry out or assist in carrying out a
 function within the patient 34. For example, in the case of a stent the
 device 32 prevents the closing of an arterial wall and permits the flow of
 blood therethrough. In the case of a graft, the device 32 serves to couple
 blood flow between two separate ends of an artery. The device 32 may
 instead consist of an artificial hip or knee which facilitates movement of
 the leg of the patient 34. Other functions include, but are not limited
 to, a hemodialysis shunt and spinal brace, for example.
 The device 32 includes a sensing circuit (not shown in FIG. 1) which serves
 to sense a parameter associated with the function performed by the device.
 For example, in the case of a stent or graft the sensor may be used to
 detect the degree of restenosis which occurs within the device 32.
 Alternatively, for example, the sensing circuit may detect an amount of
 strain or displacement which occurs in an artificial hip or knee. Still
 further, the sensor may serve to sense the condition of the implant device
 in carrying out its intended function. For example, in the case of a
 pacemaker the sensor may detect the pulse rate.
 The system 30 further includes interrogation instrumentation 36 for
 remotely interrogating the implant device 32 in order to evaluate the
 device function. The instrumentation 36 includes an exciter/interrogator
 unit 38 which is positioned outside the patient 34 in close proximity to
 the implant device 32. As will be discussed in more detail below, the
 exciter/interrogator unit 38 serves to excite the sensing circuit within
 the device 32. The sensing circuit is designed to have a variable
 impedance loading effect on the exciter/interrogator unit 38, which varies
 in relation to the sensed parameter (e.g., blood flow, amount of
 restenosis, etc.).
 The exciter/interrogator unit 38 is coupled via an electrical cable 40 to
 the main circuitry 42 included in the interrogation instrumentation 36.
 The main circuitry 42 includes suitable circuits for driving the
 exciter/interrogator unit 38 as described below, and for processing the
 output of the exciter/interrogator unit 38 in order to provide an output
 to an operator (e.g., display 44). In particular, the variable impedance
 loading effect of the device 32 on the exciter/interrogator unit 38 is
 detected at different frequencies and processed to produce a display or
 the like indicative of the function performed using the device 32.
 As will be better understood based on the description which follows, the
 present invention preferably utilizes magnetic coupling between the
 exciter/interrogator unit 38 and the implant device 32. The sensing
 circuit in the device 32 is a passive circuit designed to have an
 impedance loading effect on the exciter/interrogator unit 38. In this
 manner, the sensing circuit can be a very simple, low cost circuit which
 is less prone to failure. The device 32 does not require an active
 transmitter, mixer, amplifier, etc. as in other conventional devices.
 Moreover, the sensing circuit can be embedded within the device structure
 to reduce the amount of obstruction which occurs in the device and, for
 example, to increase performance.
 FIG. 2 represents a simplified block diagram showing the positional
 relationship between the implant device 32 and the exciter/interrogator
 unit 38. The exciter/interrogator unit 38 preferably is a hand-held sized
 device which is held by a doctor, nurse or medical assistant in close
 proximity to the implant device 32. Since the system 30 is non-invasive,
 the exciter/interrogator unit 38 may be placed adjacent the implant device
 32 with the body of the patient (e.g., skin, muscle tissue, etc.),
 designated 50, disposed therebetween. The preferred embodiment of the
 present invention relies on magnetic and/or electromagnetic coupling
 (represented by field lines 52) between the exciter/interrogator unit 38
 and the implant device 32 to interrogate the device 32 non-invasively.
 More particularly, the preferred embodiment of the present invention
 introduces sensor technology developed in the aerospace industry into
 medical implant devices. Commonly owned U.S. Pat. No. 5,581,248 describes
 in detail how magnetic coupling between an interrogation circuit and a
 sensor coil, based on an impedance loading effect, can be used to
 interrogate an embedded sensor. Heretofore, however, no one has thought to
 utilize such technology in medical implant devices. The entire disclosure
 of U.S. Pat. No. 5,581,248 is incorporated herein by reference.
 FIG. 3 illustrates the electrical configuration of the exciter/interrogator
 unit 38 and implant device 32 in more detail. The exciter/interrogator
 unit 38 includes an exciter/interrogator coil 52, a voltage controlled
 oscillator 54, and a load sensing resistor 56. The oscillator 54 provides
 an excitation signal to the exciter/interrogator coil 52 and the load
 sensing resistor 56 which are coupled in series. The exciter/interrogator
 unit 38 is coupled via the cable 40 to the main circuitry 42 which
 includes signal conditioning electronics 58 and a data processing and
 control section 60. The data processing and control section 60 produces a
 control signal on line 62 for controlling the frequency and the magnitude
 of the excitation signal that the oscillator 54 applies to the
 exciter/interrogator coil 52. The exciter/interrogator coil 52, sensing
 resistor 56 and oscillator 54 provide a resonant exciter/interrogator
 circuit that is used to induce currents in a coil within the implant
 device 32 in order to perform interrogation.
 More specifically, the implant device 32 includes a sense coil 64 which is
 embedded in the structure of the implant device. As is discussed in more
 detail below in connection with FIGS. 6a, 7a, 8a, etc., the implant device
 32 may be any type of implant such as a stent or graft. The sense coil 64
 may be integrally secured to a surface of the stent or graft, for example,
 or even formed directly within the structure. The sense coil 64 is part of
 a passive resonant sensing circuit 65 which includes, for example, a
 capacitor 66 and a sensing element 68 in electrical series with the sense
 coil 64. The sensing element 68 can be any sensor which produces a
 variable impedance (e.g., resistance, capacitance or inductance), or which
 produces an output that can be converted into a variable impedance that
 can change or modulate the impedance of one or more of the resonant
 circuit components.
 As shown in FIG. 3, the sensing element 68 is represented by a variable
 resistance which varies based on a sensed parameter. In an alternative
 embodiment, the sensing element 68 may provide a capacitance, inductance
 and/or resistance which varies based on a sensed parameter. As long as the
 sensing element 68 in combination with the sense coil 64 alone or together
 with one or more elements (e.g., capacitor 66) form a resonant sensing
 circuit 65 (e.g., LC or LRC), the benefits of the invention may be
 obtained.
 The sensing element 68 can be any of a variety of known types of sensors
 which may be used to sense a functional parameter within the living body.
 Such parameters may include, but are not limited to, vascular parameters
 such as blood flow rate, blood pressure, oxygen content, cholesterol,
 restenosis, glucose level, temperature, etc.; hematology parameters such
 as blood gases, blood chemistry, hemoglobin content, etc., and
 skeletal/muscular parameters such as force, strain, displacement, etc. As
 mentioned above, the sensing element 68 itself may be characterized as an
 impedance based sensor whose resistance, capacitance and/or inductance
 varies directly with respect to frequency as a function of the sensed
 parameter, or another type sensor whose output can be converted into a
 variable impedance. Exemplary sensor types include electrical,
 piezoelectric, sonic optical, microfluidic, chemical, membrane, thermal,
 magnetohydrodynamic, an NMR varient, magnetic, magnetostrictive,
 biological, microelectromechanical sensors (MEMs), etc.
 In the particular examples discussed below, the sensing element 68 may be a
 MEMs device whose impedance varies as a function of the amount or rate of
 blood flow through a stent or graft. Alternatively, the sensing element 68
 may be a surface acoustic wave (SAW) device which can detect blood flow.
 In yet another alternative, the sensing element 68 may be a piezoelectric
 device within a stent or graft for detecting blood pressure.
 According to yet another embodiment discussed below, the sensing element 68
 may be included within the sense coil 64 itself. For example, the
 embodiments of FIGS. 7a, 8a, 9a, etc. as described below incorporate the
 sense coil 64 within the tubular housing of a stent or graft. Changes in
 the amount of blood flow through the stent or graft and/or the occurrence
 of restenosis therein affect the overall inductance of the sense coil 64.
 Hence, the sense coil 64 alone or in combination with one or more other
 sensing elements 68 may be used to vary the impedance of the resonant
 sensing circuit based on the sensed parameter.
 As is explained more fully in the aforementioned '248 patent, the basic
 operation of the system 30 of FIG. 3 according to the invention is as
 follows. The sensing circuit 65 exhibits a resonant frequency which is
 defined as the frequency which is the point of maximum sensitivity to
 changes in the excitation current I.sub.P for a given change in the
 impedance of the sensing element 68. The resonant frequency f.sub.s is
 determined by the sum total of the reactive elements of the circuit which
 includes the inductance of the sense coil 64 and the exciter/interrogator
 coil 52, as well as the capacitance 66 (and parasitic capacitances
 C.sub.P1 and C.sub.P2 shown in FIG. 4) and the value of a coupling
 constant K. The amplitude of the current through the coil 64 is also a
 function of the sensing element 68, particularly at the resonant frequency
 of the sensing circuit 65. When the exciter/interrogator coil 52 has an AC
 signal applied, current in the primary or exciter/interrogator coil 52
 induces current in the secondary or sense coil 64, as in an air gap
 transformer. This current in the sense coil 64, however, is reflected back
 to the exciter/interrogator coil 52 by the mutual coupling of the two
 coils. The sensing resistor 56 is used to detect the current in the
 exciter/interrogator coil 52.
 When the excitation frequency is approximately at the resonant frequency of
 the sensing circuit 65, the current in the exciter/interrogator coil 52
 changes maximally in relation to the value of the sensing element 68.
 Thus, the condition of the sensing element 68 can be determined as a
 function of the detected current in the exciter/interrogator coil 52.
 Using an amplifier 72, the signal conditioning electronics 58 amplifies
 the voltage developed across the sensing resistor 56 by the
 exciter/interrogator circuit current I.sub.P. This amplified voltage is
 then rectified and low pass filtered via a rectifier and low pass filter
 circuit 74 to provide a DC voltage output V.sub.dc. The control circuit 60
 then uses the DC value to determine the state or output of the sensing
 element 68.
 FIG. 4 provides a more detailed circuit model of an exciter/interrogator
 unit 38 and the implant device 32. As shown, the exciter/interrogator unit
 38 includes the exciter/interrogator coil 52 that has a determinable
 inductance L.sub.P. The coil 52 and associated components of the
 exciter/interrogator unit 38 also will exhibit an overall parasitic
 capacitance, C.sub.P1, that appears in parallel with the coil inductance.
 The exciter/interrogator unit 38 further includes the variable frequency
 oscillator 54 and the sensing resistor 56 used to sense the primary or
 excitation current I.sub.P. Thus, all components in the
 exciter/interrogator unit 38 are known quantities for each application.
 The resonant sensing circuit 65 includes the sense coil 64 which has a
 determinable inductance, L.sub.S, in one embodiment; or in another
 embodiment an inductance which varies in relation to the sensed parameter.
 In such embodiment, the sense coil 64 itself forms part of the sensing
 element 68. The sense coil 64 also has an associated parasitic
 capacitance, which parasitic capacitance is in effect part of the
 capacitance C.sub.P2 which is a discrete capacitor selected to optimize
 the sensitivity of the device 32 to changes in the value of the sensing
 element 68. In other words, the value of C.sub.P2 can be selected, such as
 based on experimental data for specific circuits, to maximize the current
 I.sub.P induced in the exciter/interrogator unit 38 as a function of
 changes in the resistance of the sensing element 68. The sensing circuit
 65 also includes the additional discrete capacitor 66 which is selected to
 adjust the frequency at which the change in current vs. change in sensing
 element resistance ratio is optimized.
 Thus, for the sensing circuit 65, all of the component parameters are known
 quantities except the coupling constant, K, and the value of the sensing
 element 68 output. Accounting for the coupling constant K as described
 more fully in the '248 patent, the DC output of the signal conditioning
 electronics 58 is indicative of the sensed parameter of the implant device
 32.
 FIG. 5 is a graph showing in a representative manner a typical frequency
 response characteristic of the circuit of FIG. 4. By comparing a family of
 curves determined by monitoring the primary current I.sub.P vs. excitation
 frequency for different K values (in this example for K=0.1, K=0.5 and
 K=0.9) and different resistance values for the sensing element 68, the
 sensed parameter (e.g., blood flow rate, degree of restenosis, etc.) may
 be determined.
 FIG. 6a presents a first embodiment of the present invention in which the
 medical implant device 32 is a stent. As is known, a stent is a round,
 spring-like device that provides mechanical support to the wall of a blood
 vessel such as an artery. As is shown in FIG. 6a, the stent 32 is inserted
 within a blood vessel 80. The stent 32 is tube shaped structure made up of
 a generally helical formed wall 82. The stent 32 prevents the walls of the
 blood vessel 80 from collapsing while providing a path 84 through which
 blood may flow.
 The wall 82 typically is formed of stainless steel or some other material
 (e.g., a composite and/or plastic material) which is biocompatible within
 the body. Depending on the embodiment, the wall 82 preferably is made of a
 non-conductive material or materials in one case, or a conductive material
 in another case. In this particular embodiment, the wall 82 preferably is
 made of a non-conductive material such as plastic. The sense coil 64 is
 formed on an outer (or inner surface) of the tube shaped structure.
 Alternatively, the sense coil 64 may be embedded within the wall 82. The
 sense coil 64 is coupled via electrical conductors 86 and one or more
 through holes 87 to the remainder of the sensing circuit 65 which is
 formed on an inner surface of the wall structure 82. The sensing element
 68 in such an embodiment may be a MEMs device whose capacitance and/or
 resistance varies as a function of the amount of restenosis which forms on
 the element 68 within the stent 32. Alternatively, the sensing element 68
 may be a piezoelectric device which produces an impedance output which
 varies as a function of the pressure of the blood flowing within the stent
 32. If desirable, the sense coil 64 and all or part of the remainder of
 the sensing circuit 65 may be covered with a protective coating material
 to avoid corrosion or other related problems.
 Upon being implanted within the vessel 80, the exciter/interrogator unit 38
 (FIG. 3) can be positioned outside the body of the patient in close
 proximity to the stent 32. The exciter/interrogator unit 38 serves to
 excite the sense coil 64 which in turn induces a current in the load
 resistor 56 which varies as a result of the variable impedance loading
 effect of the sensing circuit 65 with respect to frequency. Thus, as the
 output of the sensing element 68 varies based on the build up of
 restenosis, change in blood pressure, or other desired parameter, such
 variation may be detected remotely.
 FIG. 6b illustrates the equivalent circuit for the sensing circuit 65 in an
 embodiment where the sensing element 68 provides a resistance which varies
 in response to a sensed parameter. FIG. 6c illustrates an equivalent
 circuit for the sensing circuit 65 in an embodiment where the sensing
 element 68' produces an output which varies in capacitance based on the
 sensed parameter. In each case, the impedance loading effect of the
 sensing circuit 65 varies in accordance with the sensed parameter by
 virtue of the resonance of the circuit being affected.
 An alternative embodiment for a stent 32 is shown in FIG. 7a. In this
 particular embodiment, the helical shaped wall 82 preferably is made of a
 molded plastic. The sense coil 64 is made up of a conductive wire 92
 embedded through several turns in the wall of the helix 82 as shown in
 cross-section in FIG. 7b. Return wires 94 embedded in and traversing the
 helix 82 are provided to connect the respective ends of the coil 64 to the
 remainder of the resonant sensing circuit 65 mounted on the helix 82 as in
 the previous embodiment. During manufacture, the sense coil 64 may serve
 as the frame about which the molded plastic helix 82 is formed.
 The embodiment of FIG. 7c varies slightly from that shown in FIGS. 7a and
 7b. In this particular embodiment, the return wires 94 are formed on the
 inner surface of the helix 82. Such embodiment simplifies the
 manufacturing process by allowing the helix 82 to be formed without the
 return wires 94 traversing the helical turns in an embedded manner.
 FIG. 7d illustrates generally the equivalent circuit for the stent 32 shown
 in FIGS. 7a thru 7c. As will be appreciated, the sensing element 68 may be
 a resistive device as before, or some other type of sensor. In each case,
 the sense coil 64 provides a means for magnetic coupling between the
 exciter/interrogator coil 52 and the resonant sensing circuit 65. As blood
 flow, restenosis, etc. varies within the stent 32, the impact of such
 variation on the impedance loading effect of the resonant sensing circuit
 65 on the exciter/interrogator unit 38 may be detected with respect to
 frequency. Such information can then be utilized in ascertaining the
 precise rate of blood flow, degree of restenosis, etc. via the data
 processing and control 60. As will be appreciated, in each of the
 embodiments discussed herein the particular type of sensing element 68
 will be dictated, of course, by the particular parameter of interest and
 the manner in which the output of the exciter/interrogator unit 38 is
 processed.
 FIG. 8a illustrates another embodiment of a stent 32 which utilizes the
 conductive properties of a metal-type helix wall 82. The helix wall 82 is
 made of metal and therefore can itself form the sense coil 64. The metal
 helix is electrically isolated via a non-conductive coating, for example.
 Each end 96 of the helix is connected to the remainder of the resonant
 sensing circuit 65 via return wires 94 as shown in phantom in FIG. 8a. As
 in the previous embodiments, the resonant sensing circuit with the sensing
 element 68 may be mounted on the inner surface of the stent 32. FIG. 8b
 diagrammatically represents the electrical circuit of this particular
 embodiment.
 In each of the embodiments which utilize the body 82 of the stent 32 to
 form the sense coil 64, e.g., the embodiments of FIGS. 7a, 7c and 8a, it
 will be appreciated the inductance of the sense coil 64 may itself vary as
 a function of the sensed parameter. In such instance, the sense coil 64
 serves as a sensing element in addition and/or in place a discrete sensing
 element 68. More particularly, the sense coil 64 formed within the helix
 may be considered an inductive element. It is combined with a discrete
 capacitor 66 and resistance 68 to form an LRC resonant sensing circuit 65.
 The inductance of the sense coil 64 depends directly on the magnetic
 permeability of the material inside it. Since iron strongly affects
 permeability, the amount of blood in the stent 32 as a fraction of the
 available volume (reduced by restenosis) will modulate the permeability
 and hence the resonant frequency of the sensing circuit 65. The resonant
 frequency can be determined by inductively coupling the stent 32 to the
 exciter/interrogator unit 38 via the externally generated swept frequency
 magnetic field. Knowledge of the resonant frequency then allows a
 determination of the inductance of the coil 64. Since the value of
 inductance depends on the degree of restenosis, an estimate of its
 occlusion of the stent 32 can be made.
 The embodiments of FIGS. 7c and 8a each include some type of direct linear
 connection via the return wires 94 between the sense coil 64 and the
 remainder of the resonant sensing circuit 65. Such design may not be
 optimum from a biocompatibility standpoint or manufacturing standpoint.
 FIGS. 9a and 9b represent an embodiment which eliminates the need for such
 return wires 94. In this case, a double helix configuration is used to
 complete the resonant circuit.
 As is shown more clearly in FIG. 9b, the helix wall 82 is made of
 conductive metal and from one end to the other forms part of the coil 64.
 The return wire 94 is a second helix with the same pitch as the helix 82
 but having an axial direction which is reversed relative to the helix 82.
 The return wire 94 is connected to one end of the helix 82 and returns to
 the other end where the resonant sensing circuit 65 can be closed with the
 capacitance 66 and resistance 68. Electrically, such configuration doubles
 the inductance L of the coil 64, and currents in the two helical sections
 82 and 94 will produce magnetic fields which add rather than cancel. In
 the presence of a changing magnetic field, conversely, the current in the
 circuit 65 is doubled.
 Other embodiments may include a stent 32 which has a uniform wall rather
 than a helix shaped wall. In such case, the sense coil 64 may be formed on
 a surface as in the embodiment of FIG. 6a. Alternatively, the sense coil
 64 may be embedded in the structure as in the embodiments of FIGS. 7b and
 7c, for example.
 FIG. 10 illustrates an embodiment of the invention wherein the implant
 device 32 comprises a graft for joining separate ends 100 of a blood
 vessel. The graft 32 is a tube shaped structure 102 made up of metal such
 as stainless steel, or a composite and/or plastic material. Using known
 techniques, the graft 32 is implanted within the patient by securing
 respective ends 100 of a blood vessel to corresponding ends of the graft
 32. Consequently, blood will flow through the interior of the graft 32 as
 represented by arrow 84.
 As in the case of the stent described above, the resonant sensing circuit
 65 can be any combination of a sense coil 64, a capacitor 66, a resistor
 68, etc. One or more of these components presents an impedance which
 varies as a function of the parameter to be sensed. Similar to the stent,
 it is desirable with the graft 32 to sense remotely the degree of
 restenosis and/or blood flow in the device. By using impedance-based
 sensing devices, the frequency dependent impedance loading effect of the
 sensing circuit may be detected externally using the exciter/interrogator
 unit 38 as previously described.
 The embodiment of FIG. 10 is similar to that of FIG. 6a where the sense
 coil 64 is mounted on a surface of the tube structure 100. The sensing
 element 68 and capacitor 66, for example, are mounted on an interior
 surface of the structure 100. Electrical connections to the coil 64 are
 provided by conductors 86 and vias 87. Operation is fundamentally the same
 as described above in relation the stent embodiment.
 FIGS. 11a thru 11c illustrate an embodiment of a graft 32 analogous to the
 stent of FIGS. 7a thru 7c. The structure 100 is made of a non-conductive
 material and the windings of the coil 64 are embedded directly within the
 tube. Again, for example, the structure 100 may be molded plastic or the
 like with the coil 64 serving as a skeletal support.
 FIG. 12 represents an embodiment of a graft 32 which uses a double helix
 structure similar to the stent in FIG. 9a. In this case, however, since
 the structure 100 is uniform rather than helical, two separate helical
 wires 104 and 106 are embedded along the length of the tube 102.
 Electrically speaking, the circuit is identical to that shown in FIG. 9b.
 As the amount of blood/restenosis varies in the graft 32, the inductance
 of the helical wires 104 varies which changes the impedance loading effect
 on the exciter/interrogator unit 38.
 FIG. 13 illustrates yet another embodiment of a graft 32 (or stent) which
 is remotely interrogated in accordance with the present invention. In the
 case of a tube shaped structure 102 serving as the body of the graft or
 stent, a conventional device may be modified by placing a desired number
 of windings around the outer surface of the structure 102 to form the
 sense coil 64. The capacitor 66 or other fixed components may similarly be
 mounted on the outer surface. The sensing element 68 is mounted on the
 inside surface and connected through vias 87 to the coil 64 and capacitor
 66 to form the LRC resonant sensing circuit 65. Alternatively, the sensing
 element 68 may be mounted on the outer surface also, provided the sensing
 element is capable of sensing the desired parameter through the structure
 102.
 Subsequently, a laminate sheath 110 is applied over the outer surface of
 the structure 102 and heated to form an integrated graft 32. The sensing
 circuit 65 can then be interrogated in the same manner described above in
 connection with the other embodiments.
 Although the invention has been shown and described with respect to certain
 preferred embodiments, it is obvious that equivalents and modifications
 will occur to others skilled in the art upon the reading and understanding
 of the specification. For example, various other types of implant devices
 can benefit from the present invention and the invention is not intended
 to be limited only to stents and grafts in its broadest application. The
 present invention includes all such equivalents and modifications, and is
 limited only by the scope of the following claims.