(12) United States Patent
`Darrow et al.
`
`USOO62O198OB1
`(10) Patent No.:
`US 6,201,980 B1
`(45) Date of Patent:
`Mar. 13, 2001
`
`(54) IMPLANTABLE MEDICAL SENSOR SYSTEM
`(75) Inventors: Christopher B. Darrow, Pleasanton;
`Joe H. Satcher, Jr., Modesto; Stephen
`M. Lane, Oakland; Abraham P. Lee,
`Walnut Creek; Amy W. Wang,
`Berkeley, all of CA (US)
`(73) Assignee: The Regents of the University of
`California, Oakland, CA (US)
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(*) Notice:
`
`(21) Appl. No.: 09/166,236
`(22) Filed:
`Oct. 5, 1998
`(51) Int. Cl."
`
`O
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`-1 - O
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`- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
`
`A61B 5/05
`
`OTHER PUBLICATIONS
`Data Transmission from an Implantable Biotelemeter by
`Load-Shift Keying Using Circuit Configuration Modulator,
`Zhengnian Tang et al., 1995 IEEE, pp. 524-528.
`Glucose-Sensing Electrode Coated With Polymer Complex
`Gel Containing Phenylboronic Acid, Akihiko Kikuchi et al.,
`1996 American Chemical Society, pp. 823-828.
`* cited by examiner
`Primary Examiner-Cary O'Connor
`ASSistant Examiner Michael AStorino
`(74) Attorney, Agent, or Firm-Alan H. Thompson; Daryl
`S. Grzybicki
`ABSTRACT
`(57)
`An implantable chemical Sensor System for medical appli
`cations is described which permits Selective recognition of
`
`an analyte using an expandable biocompatible Sensor, Such
`
`S. Fa is - - - - - - - - h - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - otes as a polymer, that undergoes a dimensional change in the
`led O SeaC .....................................
`JVL,
`600/316, 319, 323,332, 339, 348, 361,
`364-365,345, 347; 128/903, 904, 897–899
`References Cited
`
`(56)
`
`U.S. PATENT DOCUMENTS
`9/1978 Seylar.
`4,114,606
`3/1989 Cohen et al..
`4,815,469
`5,704,352 * 1/1998 Tremblay et al. ................... 600/300
`5,709,225
`1/1998 Budgifvars et al..
`5,711,861
`1/1998 Ward et al. .......................... 600/347
`
`presence of the analyte. The expandable polymer is incor
`porated into an electronic circuit component that changes its
`properties (e.g., frequency) when the polymer changes
`dimension. AS the circuit changes its characteristics, an
`external interrogator transmits a signal transdermally to the
`transducer, and the concentration of the analyte is deter
`mined from the measured changes in the circuit. This
`invention may be used for minimally invasive monitoring of
`blood glucose levels in diabetic patients.
`
`27 Claims, 5 Drawing Sheets
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`TeleNMeRS
`YEVVCee
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`Abbott
`Exhibit 1018
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`U.S. Patent
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`Mar. 13, 2001
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`Sheet 1 of 5
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`US 6,201,980 B1
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`U.S. Patent
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`Mar. 13, 2001
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`Sheet 2 of 5
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`U.S. Patent
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`Sheet 3 of 5
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`U.S. Patent
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`Mar. 13, 2001
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`Sheet 4 of 5
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`U.S. Patent
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`Mar. 13, 2001
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`Sheet 5 of 5
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`Exhibit 1018
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`1
`IMPLANTABLE MEDICAL SENSOR SYSTEM
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`US 6,201,980 B1
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`2
`The present invention addresses the need for a
`convenient, minimally invasive medical Sensor that provides
`continuous measurements of an analyte of interest using an
`expandable, biocompatible material incorporated into an
`electronic circuit element. More specifically, this invention
`will help diabetic patients in monitoring blood glucose
`levels and achieving tighter blood glucose control without
`requiring blood Samples to be drawn.
`
`SUMMARY OF THE INVENTION
`The present invention provides an implantable Sensor
`System for monitoring the concentration of a chemical
`analyte of interest. The invention is used for medical
`applications, Such as implanted Sensor packages for long
`term monitoring of physiological blood or tissue analytes,
`like glucose for control of diabetes. The analyte concentra
`tion is transduced by a circuit, the characteristics (e.g.,
`resonant frequency) of which are set by at least one circuit
`component (e.g., capacitance, inductance, resistance) whose
`value can be varied by the interaction between an analyte
`Sensitive material and the analyte. For example, changing
`the distance between the plates with a glucose-SWellable
`polymer can vary the capacitance of a parallel-plate capaci
`tor. AS the electrical characteristics of the circuit vary in
`response to changes in the concentration of the analyte, an
`external interrogator transmits a signal transdermally to the
`transducer, and the concentration of the analyte is deter
`mined from the response of the transducer to that Signal.
`It is an object of the present invention to provide an
`implantable Sensor System to monitor one or more chemical
`analytes of interest, including ionic Species and molecular
`Species. It is also an object to provide an implantable
`transducer having a circuit that requires no internal Source of
`power, and which incorporates an expandable material that
`changes its dimensions in the presence of the analyte to
`influence the properties of the transducer circuit. Another
`object of the invention is to provide a Sensor System inter
`rogated transdermally by an external device to measure the
`characteristics of the circuit as the concentration of analyte
`changes. It is further an object of the invention to provide an
`implantable Sensor System for monitoring the blood glucose
`levels in diabetic patients. Other objects and advantages of
`the present invention will become apparent from the fol
`lowing description and accompanying drawings.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`The accompanying drawings, which are incorporated into
`and form part of this disclosure, illustrate embodiments of
`the invention and together with the description, Serve to
`explain the principles of the invention.
`FIG. 1 shows an implantable chemical Sensor System
`according to the present invention.
`FIG. 2 shows an embodiment of the present invention in
`which the transducer is a variable capacitor.
`FIG. 3 shows an embodiment of the present invention in
`which the transducer is a variable inductor.
`FIG. 4 shows an embodiment of the present invention in
`which the transducer is a variable capacitor formed with a
`micro-electromechanical System.
`FIG. 5 shows the results of deformation measurements on
`an embodiment of the present invention.
`FIG. 6 shows a MEMS assembly configured to form the
`transducer according to the present invention.
`FIG. 7 shows a MEMS assembly fabricated by surface
`micromachining according to the present invention.
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`The United States Government has rights in this inven
`tion pursuant to Contract No. W-7405-ENG-48 between the
`United States Department of Energy and the University of
`California for the operation of Lawrence Livermore
`National Laboratory.
`BACKGROUND OF THE INVENTION
`1. Field of the Invention
`The present invention relates to an implantable medical
`device for continuous, minimally invasive monitoring of a
`chemical analyte of interest using an expandable, biocom
`patible material incorporated into an electronic circuit com
`15
`ponent that is interrogated transdermally.
`2. Description of Related Art
`Various implantable medical monitoring devices have
`been developed to measure internal physiological conditions
`of a patient. For example, an implantable medical Sensor that
`determines the oxygen content of blood using a light
`emitting diode and a phototransistor is described in U.S. Pat.
`No. 4,815,469 to Cohen et al. U.S. Pat. No. 5,709,225 to
`Budgifvars et al. describes a medical implant with a capaci
`tive Sensor, which is coated with a magnetically Sensitive
`material that causes capacitance changes in the presence of
`a magnetic field.
`Some medical devices use Sensors coupled with fre
`quency tuned L-C circuits, where the Sensor mechanically
`translates the changes in the physiological condition to the
`inductor or capacitor of the tuned L-C circuit. An external
`transmitter detects the resulting changes in resonant fre
`quency of the circuit. For example, U.S. Pat. No. 5,704,352
`to Tremblay et al. describes an implantable passive biosen
`Sor for monitoring physiological conditions and converting
`the Signals to digital format. In particular, the Sensors are
`preSSure transducers that detect the preSSure of cerebrospinal
`fluid in the cavities of a patient's brain, which is useful for
`monitoring the operation of a cerebroSpinal fluid shunt for
`treating hydrocephalus. U.S. Pat. No. 4,114,606 to Seylar
`describes an implantable device useful for monitoring
`intracranial pressure for the treatment of hypertension. The
`resonant frequency of the passive L-C circuit implanted in
`the cranium varies with changes in intracranial pressure. An
`external monitor interrogates and detects the frequency
`changes in the pressure transducer. Neither of these patents
`describes Specific mechanisms or devices for transducing
`other physiological conditions.
`The application of a transducer in an implantable medical
`device that reliably monitors changes in Specific chemical
`analytes, Such as blood glucose, would be advantageous.
`Blood glucose levels are of particular concern because
`diabetes is a chronic illness that affects more than 110
`million people worldwide. Conventional therapy for the
`most Severe form of diabetes, insulin-dependent diabetes
`mellitus (Type I), is to administer one or more injections per
`day of various forms of insulin, while monitoring blood
`glucose levels two or three times daily with commercial
`glucometers that require the withdrawal of blood Samples. In
`practice, near normal blood Sugar levels are difficult to
`maintain with this type of therapy due to the enormous
`inconvenience and day-to-day burden of conventional
`home-monitoring techniques. The resulting large fluctua
`tions in blood glucose levels may be responsible for a
`number of Serious Secondary ailments commonly associated
`with diabetes, including Stroke, liver and kidney damage,
`and loSS of eyesight.
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`FIG. 8 shows a MEMS assembly fabricated by surface
`micromachining according to the present invention.
`FIG. 9 shows a MEMS assembly fabricated by surface
`micromachining according to the present invention.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`The present invention is an implantable chemical Sensor
`System for medical applications. The Sensor System permits
`Selective recognition of an analyte using an expandable
`biocompatible material, Such as a polymer, which undergoes
`a dimensional change in response to changes in the concen
`tration of the analyte of interest. The expandable polymer is
`incorporated into or mechanically connected to an electronic
`circuit element (e.g., capacitor, inductor, resistor) to cause a
`change in the value of that component, thereby altering the
`electrical characteristics of the circuit in a measurable way
`in response to changes in the analyte concentration. For
`example, changes induced in the value of a capacitor or
`inductor when the polymer changes dimension alter the
`resonant frequency of an L-C resonator circuit. Similarly,
`varying the value of a resistor leads to a measurable change
`in the R-C time constant of a discharging capacitor circuit.
`Changes in the electrical characteristics of the circuit are
`detected transdermally from outside the body using an
`electronic interrogation device and then analyzed to deter
`mine the concentration of the analyte. Examples of trans
`dermal interrogation methods include (1) frequency depen
`dent electromagnetic loading of an interrogator antenna by
`a passive resonator Sensor circuit, and (2) audio monitoring
`of a tone chirped by the Voltage of discharging R-C circuit.
`In medical applications, it is generally desirable to have the
`implanted Sensor circuits powered externally by the inter
`rogator circuit, although the Sensor circuits may also be
`designed for passive interrogation.
`FIG. 1 shows a basic chemical Sensor System according to
`the present invention. An implantable transducer package 10
`includes a transducer circuit 12, which incorporates a circuit
`component 14 and a Sensor 16 operably connected to the
`circuit component 14. A telemetry device 18 above the
`Surface of the Skin interrogates the implanted transducer
`circuit 12. The transducer package 10 is implanted
`Subcutaneously, typically 2–4 mm below the Surface of the
`skin, via a simple Surgical procedure. The package 10 is
`miniaturized, typically about one centimeter in diameter.
`The implant comprises a biocompatible material that forms
`a hermetic (airtight) Seal between the physiological envi
`ronment and the electronic environment. The Sensor circuit
`12 may be mounted on a monolithic circuit board and
`contained within the hermetically Sealed package 10.
`Transduction of the analyte concentration is performed by
`the Sensor 16, which maintains physical contact with the
`physiological environment while maintaining mechanical
`contact with at least one mechanically actuated circuit
`component 14 within the hermetically Sealed package 10.
`The Sensor 16 comprises a polymer that is designed (e.g.,
`chemically altered) to undergo a (reversible) dimensional
`change (i.e., Swelling, contraction) as the concentration of a
`Selected analyte changeS. Transduction occurs as a result of
`the mechanical action of the dimensional change of the
`Sensor 16 being transferred to the mechanically actuated
`circuit component 14, thereby leading to a change in the
`electrical characteristics of the circuit 12.
`Variations in the electrical properties (e.g., resonant
`frequency) of the circuit 12 induced by changes in the
`analyte concentration are detected and may be processed by
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`the telemetry device 18, which is positioned near the
`implanted transducer package 10. The telemetry device 18
`could be connected to a drug delivery System, So that when
`the measured analyte concentration reaches a pre
`determined level, the System responds to a signal from the
`device 18 by delivering a pre-determined dosage of medi
`cation to the patient. The Sensor System of the present
`invention can also be expanded to incorporate multiple
`circuits and multiple Sensors/transducers in a single package
`to detect more than one analyte Simultaneously or Sequen
`tially.
`Polymers that are responsive to certain analytes are
`known in the art. The analytes may be molecular Species
`Such as SugarS (glucose), urea, ammonia, enzymes, or nar
`cotic Substances, or ionic species (electrolytes) Such as
`hydrogen ions (pH), alkaline earth ions, alkali metalions, or
`transition metal ions. For example, Kikuchi et al. (Anal.
`Chem. Vol. 68, No. 5, March 1996), describe a glucose
`Swellable hydrogel (poly(DMAA-co-MAPB-co-DMAPAA
`co-BMA)-PVA) that undergoes a reversible volumetric
`expansion in response to a change in glucose concentration.
`A volumetric expansion as high as AV/V-30% occurs in
`response to a change of glucose concentration of 200 mg/dL.
`This corresponds to a linear dimensional Swelling of Ar/r=
`10%.
`Referring to FIG. 2, one embodiment of the invention is
`a Subcutaneously implanted, biocompatible, hermetically
`Sealed transducer package 20 containing a Sensor circuit 22
`that incorporates and is in electrical contact with a mechani
`cally variable plate capacitor 24. The package 20 is formed
`to enclose the circuit 22 including the capacitor 24 and to
`create a housing for the Sensor. In this embodiment, the
`housing is a pleated, expansible, bellows-shaped indentation
`or cavity 26 with a movable or deformable base 32. The
`bellows acts like a Spring, with a reversible and predictable
`displacement for a given force. The axis of the bellows is
`normal to the Surface of the package 20, and the cavity 26
`may be several millimeters in diameter and approximately 1
`mm deep. The cavity 26 is filled with an expandable polymer
`28 and capped with a rigid, fine-pitch biocompatible mate
`rial 30, such as a mesh, that allows perfusion of the polymer
`28 by extracellular fluid, while mechanically constraining
`the analyte-Sensitive polymer 28. AS a result of this design,
`the Swelling of the polymer 28 causes a measurable deflec
`tion of the base 32 of the bellows.
`A planar conducting plate 34 is affixed to the deflectable
`or deformable base 32 of the bellows, and forms a parallel
`plate capacitor with a Second fixed planar conducting plate
`36. Deflections of the base 32 of the bellows cause changes
`in the distance between the plates 34.36 of the capacitor,
`which results in an attendant change in capacitance. Some
`form of electrical connection, Such as wire microbonds 38,
`connect the capacitor plates 34.36 to the circuit 22, So
`changes in the electrical properties of the circuit 22 are
`measured in response to changes in concentration of the
`analyte. Other configurations of the capacitor plates 34.36 in
`the transducer are possible, as long as the capacitance
`changes in response to the analyte-Sensitive polymer. For
`example, the capacitor may comprise two fixed plates and a
`movable dielectric, where the Sensor and dielectric are
`connected Such that the changes in dimensions of the Sensor
`cause relative motion of the dielectric and the plates, causing
`the capacitance of the capacitor to change.
`Referring to FIG. 2, as the bellows 26 extends or
`contracts, the hermetic Seal between the circuitry and the
`physiological environment is maintained. Variations in the
`electrical properties of the circuit 22 induced by changes in
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`Page 008
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`the analyte concentration are detected by a telemetry device
`39 that is positioned outside the body, but near the implanted
`transducer package 20. The device 39 determines the analyte
`concentration by remotely determining the properties of the
`circuit 22 by evaluating a response of the transducer circuit
`obtained by electromagnetic, acoustic, or other means, to the
`applied interrogation Signal. The results, having been read
`out transdermally, are presented on a display and archiving
`device, which can provide a continuous readout of the
`analyte concentration (e.g., mg/dL); the data may be
`recorded if desired (digital or analog format). The device 39
`is preferably compact and portable, and may be a hand held
`device or worn on the body, like a wristwatch. Processing
`electronics within the device 39 may execute a stored
`program to evaluate and characterize the analyte-dependent
`data using known parameters of the transducer to determine
`the analyte concentration.
`Referring to FIG. 3, a second embodiment of the inven
`tion shows a Subcutaneously implanted, biocompatible, her
`metically Sealed transducer package 40 containing a circuit
`42 that incorporates and is in electrical contact with a
`mechanically variable inductor 44. The package 40 is
`formed to enclose the circuit 42 including the inductor 44
`and, as in FIG. 2, create a bellows-shaped cavity 46 with a
`deformable base 48. The axis of the bellows is normal to the
`Surface of the package 40, and the cavity 46 may be Several
`millimeters in diameter and approximately 1 mm deep. The
`cavity 46 is filled with an expandable polymer 50 and
`capped with a rigid, fine-pitch biocompatible plate or mesh
`52 that allows perfusion of the sensor polymer 50 by
`extracellular fluid, while mechanically constraining the
`analyte-Sensitive polymer 50. The expansion and contrac
`tion of the polymer 50 gives rise to a deflection of the base
`48 of the bellows.
`A disk 54 of magnetic material (e.g., ferrite) is affixed or
`in Some manner connected to the deflectable or deformable
`base 48 of the bellows. The disk 54 forms an inductor 44
`with a planar monolithic variable inductor coil 56, which is
`parallel to and in close proximity with the ferrite disk 54.
`Deflections of the base 48 of the bellows lead to changes in
`the distance between the disk 54 and the coil 56, which
`results in a change in the inductance of the inductor 44. The
`inductor 44 is electrically connected to the rest of the circuit
`42, for example, by wire microbonds 58; so changes in the
`electrical properties of the circuit 42 are measured in
`response to changes in concentration of the analyte. Other
`configurations of the magnetic disk 54 and inductor coil 56
`in the transducer are possible, as long as the inductance
`changes in response to the analyte-Sensitive polymer.
`AS the Sensor 50 changes dimensions, the hermetic Seal
`between the circuitry and the physiological environment is
`maintained. Variations in the electrical properties of the
`circuit 42 induced by changes in the analyte concentration
`are detected by a telemetry device 59, such as that described
`for FIG. 2. The device 59 is positioned outside the body, but
`near the implanted transducer 40, and determines the analyte
`concentration by remotely determining the properties of the
`circuit by evaluating a response of the transducer circuit
`obtained by electromagnetic, acoustic, or other means, to the
`applied interrogation Signal. The results, having been read
`out transdermally, are presented on a display and archiving
`device, which can provide a continuous readout of the
`analyte concentration and record these data if desired. The
`telemetry device 59 may also be connected to a drug
`delivery System.
`Referring to FIG. 4, another embodiment of the invention
`is a Subcutaneously implanted, biocompatible, hermetically
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`Sealed transducer package 60 containing a Sensor circuit 62
`that incorporates and is in electrical contact with a mechani
`cally variable plate capacitor 64. The package 60 is formed
`to hermetically enclose the circuit 62 including a micro
`electro-mechanical system (MEMS) transducer device 66.
`The MEMS device comprises a silicon micromachined well
`68 filled with an expandable polymer 70 that Swells or
`shrinks in the presence of the analyte of interest. The well 68
`is bonded to and capped by a rigid, analyte-permeable grid
`chip 72 or retaining plate to mechanically constrain the
`polymer 70 in the well 68.
`The well 68 may be formed by etching a silicon wafer 84,
`the opposite Side of which has been deposited with a layer
`74 of Super-elastic conductive material, for example, a metal
`Such as Ni,Ti. The etching process results in a well with a
`depth equal to the thickness of the silicon wafer. The
`conductive (metallic) layer 74 that remains after etching
`forms an elastic, deformable membrane 76 or diaphragm at
`the bottom of the well, which further serves as a hermetic
`Seal between the physiological environment outside the
`package (on the polymer side of the membrane) and the
`electronic environment inside the package. Expansion (or
`contraction) of the Sensor 70 gives rise to displacement or
`deflection of the membrane 76. The conductive layer 74
`forming the deformable membrane 76 serves as one plate of
`the capacitor 64 and can extend only the length of the
`membrane 76, or beyond (as shown) if desired.
`FIG. 5 shows the results of deformation measurements on
`a mechanical prototype of the embodiment shown in FIG. 4.
`In this example, optical profilometry was used to demon
`strate the extent of deflection (about 30 um) of the mem
`brane (4.0 mmx4.0 mm) in response to Swelling of a
`P-HEMA hydrogel polymer by an ionic solution to which
`the polymer was exposed.
`To complete the capacitor 64, a Second planar conductive
`material is needed. FIG. 4 shows a possible configuration; a
`second silicon or glass wafer 80 is etched to form a well
`defined recess, and a conductive (metallic) coating 82 is
`deposited on the bottom of the recess. The wafers 82.84 can
`be bonded together to form a parallel plate capacitor with a
`defined capacitor gap, where the capacitance is determined
`by the separation of the flexible membrane 76 and the
`conductive coating 82. The capacitor 64 is connected to the
`circuit 62, such as by wire microbonds 86 or by direct
`integration of the circuit. In an alternative embodiment of
`the invention, the deformable membrane MEMS actuator
`device can be configured to form a variable planar inductor
`(as in FIG. 3), instead of a capacitor.
`AS the Sensor 70 changes dimensions and displaces the
`membrane 76, the seal formed by the membrane 76 between
`the circuit 62 and the physiological environment is main
`tained. Variations in the electrical properties of the circuit 62
`induced by changes in the analyte concentration are detected
`by a telemetry device 88 that is positioned outside the body,
`but near the implanted transducer 60. The interrogation
`device 88 determines the analyte concentration by remotely
`determining the properties of the circuit by evaluating a
`response of the transducer circuit obtained by
`electromagnetic, acoustic, or other means, to the applied
`interrogation Signal. Processing electronics within the
`device 88 may execute a Stored program to evaluate and
`characterize the analyte-dependent data using known param
`eters of the transducer to determine the analyte concentra
`tion. The results are read out transdermally and presented on
`a display and archiving device, which can provide a con
`tinuous readout of the analyte concentration and record these
`data if desired.
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`Page 009
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`Further integration of the variable circuit component with
`the other required circuit elements for a variable capacitor
`MEMS sensor, such as depicted in FIG. 6, may be advan
`tageous from a manufacturing standpoint. The MEMS-based
`capacitor (or inductor) can be directly fabricated on a wafer
`(or wafers) with other required circuit components, to form
`an integrated, MEMS-based implantable transducer circuit.
`FIG. 6 illustrates an embodiment of an integrated MEMS
`assembly formed from two subassemblies 101,102. The first
`(lower) subassembly 101 comprises a well 104 filled with an
`analyte-Sensitive polymer that expands and contracts in
`response to the analyte. A rigid retaining mesh 103 allows
`permeation of the polymer by the analyte-containing fluid. A
`flexible, deformable conductive membrane 105 is patterned
`on the top Surface 112 facing the Second (upper) Subassem
`bly 102. Also patterned on this top surface 112 is a planar
`inductor coil 106 that is electrically connected to the mov
`able capacitor membrane 105 on the first Subassembly 101
`and to a fixed capacitor plate 107 on the second subassembly
`102. Electrical contact from the coil 106 to the movable
`capacitor membrane 105 can be made through a patterned
`trace 108. Electrical contact to the fixed plate 107 can be
`made by a conductive via 109 that connects a first (lower)
`subcircuit pad 110 and a second (upper) subcircuit pad 111.
`The pads 110,111 are connected upon assembly of the first
`and second subassemblies 101,102, thereby completing the
`L-C circuit of this embodiment.
`FIG. 7 illustrates an alternative configuration of FIG. 4,
`where the opposing electrode (capacitor plate) is Surface
`micromachined instead of bulk micromachined and bonded.
`By polysilicon Surface micromachining, an opposing elec
`trode can be integrated on the opposite Side of the flexible
`membrane through a batch process, resulting in lower cost
`and smaller size of the overall device. This type of design
`will also provide a much smaller gap (<1000 A) for higher
`measurement Sensitivity.
`The fabrication proceSS includes the low pressure vapor
`deposition of a Silicon nitride layer 122 and patterning of
`grounding feedthroughs 124. A Silicon nitride membrane
`126 is then formed by patterning and anisotropially etching
`from the backside of the silicon Substrate 120. A thin film
`128 shape memory alloy (SMA) (e.g., Ti-Ni) is deposited
`on the front Side to enhance toughness of the Silicon nitride
`membrane 126. Electrical feedthrough from the SMA film
`128 to the ground substrate 120 is established through the
`Silicon nitride opening 124. A Second Silicon nitride layer
`130 with a thickness of 100 A-1000A is deposited on top
`of the SMA film 128 for insulation between the opposing
`electrode layers 128 and 132. A polycrystalline silicon layer
`132 is deposited over a sacrificial SiO layer 134 to form the
`top electrode. The Sacrificial layer 134 (e.g., SiO2 or glass),
`with a thickness of about 0.5 um to 2 um, is deposited on top
`of the silicon nitride layer 130 and patterned to form the gap
`134 between the electrodes 128,132. The sacrificial layer
`134 is later selectively etched away with etchants such as
`buffered hydrofluoride.
`FIG. 8 is an embodiment of the present invention that
`relies on inductance change rather than capacitance change
`and utilizes the same fabrication techniques as in FIG. 7. In
`this embodiment, a spiral thin film metal inductor 140 is
`patterned on the top electrode 142. A hole 144 is patterned
`in the top electrode 142, and a pedestal 146 made of a
`magnetic material is patterned on the insulating nitride layer
`148. The insulating layer 148 overlays the flexible mem
`brane 150 and its support layer 152. When the membrane
`65
`150 moves in response to dimensional changes of the
`analyte-sensitive polymer (not shown), the pedestal 146
`
`35
`
`40
`
`45
`
`8
`patterned on top of the moving membrane moves through
`the inductor coil 140, causing an inductance change. The
`change in inductance is then detected by an external telem
`etry device, as described previously.
`FIG. 9 is another capacitive Sensing configuration fabri
`cated using Surface micromachining techniques. This
`embodiment includes moving comb pedestals 160 that are
`patterned on top of the moving membrane 162. The pedes
`tals 160 are situated to move through static comb fingers 164
`that are isolated from the movement of the membrane 162.
`As the pedestals 160 move through the fingers 164, the
`capacitive change (and therefore displacement of the mov
`ing membrane 162) is detected. The pedestals 160 and
`fingers 164 are made of conductive material, Such as poly
`Silicon or a metal (e.g., Al, Au). The advantage of this
`configuration is that the capacitance change is proportional
`to displacement and independent of the position. This Sim
`plifies the circuitry design from the parallel plate configu
`ration of FIG. 7.
`The foregoing description of preferred embodiments of
`the invention is presented for purposes of illustration and
`description and is not intended to be exhaustive or to limit
`the invention to the precise form disclosed. Many modifi
`cations and variations are possible in light of the above
`teaching.
`What is claimed is:
`1. An implantable chemical Sensor System for measuring
`the concentration of an analyte of interest, comprising:
`a Sensor, comprising a material that Selectively responds
`to an analyte of interest by changing its dimensions,
`an implantable transducer, comprising an electronic cir
`cuit having at least one variable electrical characteristic
`that changes in response to dimensional changes of the
`Sensor; and
`a telemetry device that interrogates the transducer trans
`dermally to measure the changes in the electrical char
`acteristic of the circuit as the concentration of analyte
`changes.
`2. The Sensor System as recited in claim 1, wherein the
`circuit comprises at least one mechanically actuated circuit
`component, wherein the dimensional changes of the Sensor
`are used to mechanically actuate changes in the circuit
`component.
`3. The Sensor System as recited in claim 2, wherein the
`circuit component comprises a variable capacitor.
`4. The Sensor System as recited in claim 3, wherein the
`capacitor comprises two plates, wherein at least one plate
`moves relative to the other in response to the Sensor chang
`ing its dimensions.
`5. The sensor system as recited in claim 4, wherein the
`transducer further comprises an area that moves in response
`to the Sensor changing dimensions and that is connected to
`at least one plate.
`6. The Sensor System as recited in claim 1, wherein the
`transducer further comprises a housing for the Sensor,
`including an area that moves in response to the Sensor
`changing dimensions, and including a rigid, analyte
`permeable material that constr