US20090264718A1 - Implanted sensor processing system and method for processing implanted sensor output - Google Patents

Abstract

A quantitative measurement system includes an external unit and an internal unit are provided for obtaining quantitative analyte measurements, such as within the body. In one example of an application of the system, the internal unit would be implanted either subcutaneously or otherwise within the body of a subject. The internal unit contains optoelectronics circuitry, a component of which may be comprised of a fluorescence sensing device. The optoelectronics circuitry obtains quantitative measurement information and modifies a load as a function of the obtained information. The load in turn varies the amount of current through coil, which is coupled to a coil of the external unit. A demodulator detects the current variations induced in the external coil by the internal coil coupled thereto, and applies the detected signal to processing circuitry, such as a pulse counter and computer interface, for processing the signal into computer-readable format for inputting to a computer.

Description

CROSS REFERENCE TO RELATED APPLICATIONS
• This application is a divisional of U.S. patent application Ser. No. 10/332,619, filed Oct. 21, 2003, which is a national stage application filed under 35 U.S.C. §371 of PCT/US01/20390, filed Jun. 27, 2001, which is a continuation-in-part of U.S. patent application Ser. No. 09/605,706, now U.S. Pat. No. 6,400,974, issued Jun. 4, 2002.
BACKGROUND OF THE INVENTION
• 1. Field of the Invention
• This invention relates to a circuit and method for processing the output of an implanted sensing device for detecting the presence or concentration of an analyte in a liquid or gaseous medium, such as, for example, the human body. More particularly, the invention relates to a circuit and method for processing the output of an implanted fluorescence sensor which indicates analyte concentration as a function of the fluorescent intensity of a fluorescent indicator. The implanted fluorescence sensor is a passive device, and contains no power source. The processing circuit powers the sensor through inductively coupled RF energy emitted by the processing circuit. The processing circuit receives information from the implanted sensor as variations in the load on the processing circuit.
• 2. Background Art
• U.S. Pat. No. 5,517,313, the disclosure of which is incorporated herein by reference, describes a fluorescence sensing device comprising a layered array of a fluorescent indicator molecule-containing matrix (hereafter “fluorescent matrix”), a high-pass filter and a photodetector. In this device, a light source, preferably a light-emitting diode (“LED”), is located at least partially within the indicator material, such that incident light from the light source causes the indicator molecules to fluoresce. The high-pass filter allows emitted light to reach the photodetector, while filtering out scattered incident light from the light source. An analyte is allowed to permeate the fluorescent matrix, changing the fluorescent properties of the indicator material in proportion to the amount of analyte present. The fluorescent emission is then detected and measured by the photodetector, thus providing a measure of the amount or concentration of analyte present within the environment of interest.
• One advantageous application of a sensor device of the type disclosed in the '313 patent is to implant the device in the body, either subcutaneously or intravenously or otherwise, to allow instantaneous measurements of analytes to be taken at any desired time. For example, it is desirable to measure the concentration of oxygen in the blood of patients under anesthesia, or of glucose in the blood of diabetic patients.
• In order for the measurement information obtained to be used, it has to be retrieved from the sensing device. Because of the size and accessibility constraints on a sensor device implanted in the body, there are shortcomings associated with providing the sensing device with data transmission circuitry and/or a power supply. Therefore, there is a need in the art for an improved sensor device implanted in the body and system for retrieving data from the implanted sensor device.
SUMMARY OF THE INVENTION
• In accordance with the present invention, an apparatus is provided for retrieving information from a sensor device, comprising an internal sensor unit for taking quantitative analyte measurements, including a first coil forming part of a power supply for said sensor unit, a load coupled to said first coil, and a sensor circuit for modifying said load in accordance with sensor measurement information obtained by said sensor circuit; an external unit including a second coil which is mutually inductively coupled to said first coil upon said second coil coming into a predetermined proximity distance from said first coil, an oscillator for driving said second coil to induce a charging current in said first coil, and a detector for detecting variations in a load on said second coil induced by changes to said load in said internal sensor unit and for providing information signals corresponding to said load changes; and a processor for receiving and processing said information signals.
BRIEF DESCRIPTION OF THE DRAWINGS
• The invention will be more fully understood with reference to the following detailed description of a preferred embodiment in conjunction with the accompanying drawings, which are given by way of illustration only and thus are not limitative of the present invention, and wherein:
• FIG. 1 is a block diagram of one preferred embodiment according to the present invention;
• FIG. 2 is a schematic diagram of an internal sensor device unit according to one preferred embodiment of the invention;
• FIGS. 3 and 4 are waveform diagrams illustrating signal waveforms at various points in the sensor device circuit;
• FIGS. 5A-5eare diagrams of signals produced by the external data receiving unit;
• FIG. 6 is a schematic, section view of an implantable fluorescence-based sensor according to the invention;
• FIG. 7 is a schematic diagram of the fluorescence-based sensor shown inFIG. 6 illustrating the wave guide properties of the sensor;
• FIG. 8 is a detailed view of the circled portion ofFIG. 6 demonstrating internal reflection within the body of the sensor and a preferred construction of the sensor/tissue interface layer;
• FIG. 9 is a schematic diagram of an internal sensor device unit according to a second preferred embodiment of the invention; and
• FIG. 10 is a timing diagram illustrating voltage levels of various terminals of the comparator ofFIG. 9 as the detector circuit cycles through its operation.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
• FIG. 1 shows a block diagram of one preferred embodiment of an implanted fluorescence sensor processing system according to the present invention.
• The system includes anexternal unit101 and aninternal unit102. In one example of an application of the system, theinternal unit102 would be implanted either subcutaneously or otherwise within the body of a subject. The internal unit containsoptoelectronics circuitry102b,a component of which may be comprised of a fluorescence sensing device as described more fully hereinafter with reference toFIGS. 6-8. Theoptoelectronics circuitry102bobtains quantitative measurement information and modifies aload102cas a function of the obtained information. Theload102cin turn varies the amount of current throughcoil102d,which is coupled to coil101fof the external unit. An amplitude modulation (AM)demodulator101bdetects the current variations induced incoil101fbycoil102dcoupled thereto, and applies the detected signal to processing circuitry, such as apulse counter101candcomputer interface101d,for processing the signal into computer-readable format for inputting to acomputer101e.
• Avariable RF oscillator101aprovides an RF signal to coil101f,which in turn provides electromagnetic energy to coil102d,when thecoils101fand102dare within close enough proximity to each other to allow sufficient inductive coupling between the coils. The energy from the RF signal provides operating power for theinternal unit102 to obtain quantitative measurements, which are used to vary theload102cand in turn provide a load variation to thecoil101fthat is detected by the external unit and decoded into information. The load variations are coupled from the internal unit to the external unit through the mutual coupling between thecoils101fand102d.The loading can be improved by tuning both the internal coil and the external coil to approximately the same frequency, and increasing the Q factor of the resonant circuits by appropriate construction techniques. Because of their mutual coupling, a current change in one coil induces a current in the other coil. The induced current is detected and decoded into corresponding information.
• RF oscillator101adrives coil101f,which induces a current incoil102d.The induced current is rectified by arectifier circuit102aand used to power theoptoelectronics102b.Data is generated by the optoelectronics in the form of a pulse train having a frequency varying as a function of the intensity of light emitted by a fluorescence sensor, such as described in the aforementioned '313 patent. The pulse train modulates theload102cin a manner so as to temporarily short the rectifier output terminal to ground. This change in load causes a corresponding change in the current through theinternal coil102d,thereby causing a change in the magnetic field surroundingexternal coil101f.This change in magnetic field causes a proportional change in the voltage acrosscoil101f,which is observable as an amplitude modulation. The following equation describes the voltage seen on the external coil:
• 
  V=I[Z+((ωM)²)/Zs]  (1)
• where
• • V=voltage across the external coil
  • I=current in the external coil
  • Z=impedance of the primary coil
  • ω=frequency (rad/sec)
  • M=mutual inductance between the coils
  • Zs=impedance of the sensor equivalent circuit
• As shown by equation (1), there is a direct relationship between the voltage across the external coil and the impedance presented by the internal sensor circuit. While the impedance Zs is a complex number having both a real and imaginary part, which corresponds respectively to changes in amplitude and frequency of the oscillation signal, the system according to the present embodiment deals only with the real part of the interaction. It will be recognized by those skilled in the art that both types of interaction may be detected by appropriately modifying the external circuit, to improve the signal-to-noise ratio.
• FIG. 2 shows a schematic diagram of one embodiment of an internal sensor device unit according to the invention. Thecoil102d(L1) in conjunction with capacitor C1, diode D1 (rectifier102a) zener diode D2 and capacitor C2 constitute a power supply for theinternal unit102. Current induced in coil L1 by the RF voltage applied toexternal coil101fbyoscillator101a(seeFIG. 1) is resonated in the L-C tank formed by L1 and capacitor C1, rectified by diode D1, and filtered by capacitor C2. Zener diode D2 is provided to prevent the voltage being applied to the circuit from exceeding a maximum value, such as 5 volts. As is known by those skilled in the art, if the voltage across capacitor C2 starts to exceed the reverse breakdown voltage of the zener diode D2, diode D2 will start to conduct in its reverse breakdown region, preventing the capacitor C2 from becoming overcharged with respect to the maximum allowable voltage for the circuit.
• Voltage regulator205 receives the voltage from capacitor C2 and produces a fixed output voltage V_refto the noninverting input ofoperational amplifier201. The output terminal of theoperational amplifier201 is connected to a light-emitting diode (LED)202 connected in series with a feedback resistor R1. The inverting input terminal ofoperational amplifier201 is supplied with the voltage across R1, to thereby regulate the current throughLED202 to V_ref/R1 (ignoring small bias current). Light emitted fromLED202 is incident on the sensor device (not shown) and causes the sensor device to emit light as a function of the amount of the particular analyte being monitored. The light from the sensor device impinges on thephotosensitive resistor203, whose resistance changes as a function of the amount of light incident thereon.Photoresistor203 is connected in series with a capacitor C3, and the junction of the photoresistor and the capacitor C3 is connected to the inverting input terminal ofcomparator204. The other end ofphotoresistor203 is connected to the output terminal of thecomparator204 through a conductor V_comp. The output of thecomparator204 is also connected to a load capacitor C4 and a resistor network R2, R3 and R4. The comparator forms a variable resistance oscillator, with switching points determined by the values of R2, R3 and R4. C3 is a charge-up capacitor, which determines the base frequency of the oscillator for a given light level. This frequency is given by
• 
  f=1/(1.38*Rphoto*C3)  (2)
• 
  Rphoto=R_2fc[10^{−γlog(a/2fc)}]  (3)
Where
• • R_2fc(=24 kΩ) is the resistance ofphotoresistor203 at 2 footcandles
  • γ (=0.8) is the sensitivity of the photoresistor
  • a=the incident light level in footcandles
• Equation (3) can be inverted to determine the intensity of light for a given photoresistance; in conjunction with equation (2), the light intensity can be determined from frequency. Of course, the values given above are provided as examples only for purposes of explanation. Such values are determined on the basis of the particular photoresistor geometry and materials used.
• Thecomparator204 switches to a high output when Vtime=V/3, Vcomp=V, and Vtrip=2V/3. Capacitor C3 begins to charge with time constant Rphoto*Ctime. When Vtime reaches 2V/3 the comparator switches states to a low output, changing Vcomp to Vcomp=0, and Vtrip to Vtrip=V/3. At this point C3 will discharge through Rphoto. Therefore a 50% duty cycle is established, with the frequency being determined by equation (2). Rphoto varies as a function of incident light, given by equation (3).
• C4 is a load capacitor, which causes a voltage across C2 to decrease when the comparator switches states. C4 must be charged from 0V to Vdc whencomparator204 switches to a high output level state. The current through C4 is supplied by C2, causing the voltage across C2 to decrease. This in turn causes current to flow throughrectifier102ato begin charging capacitor C2, changing the instantaneous load on the tank circuit includinginternal coil102d.This load is reflected into the impedance of theexternal coil101fas given by equation (1).
• The sensor operation for a single pulse is illustrated inFIG. 3.Channel4 is the DC voltage on C2,channel3 shows the same pulse on theexternal coil101f,and the output of the AM demodulator is shown atchannel2.Channel1 shows the output of a comparator which converts the AM demodulator output to a square wave capable of being processed by a digital counter.FIG. 4 shows two complete operation cycles, with the same channel designations indicating the same points in the circuit.
• Theexternal unit101 uses a microprocessor to implement thepulse counter101c.When sufficient data has been received to obtain a valid reading, the processor shuts down the MF oscillator.FIGS. 5A-5E illustrate timing diagrams for a measurement reading.FIG. 5A shows the envelope of the RF voltage signal applied to the external coil;FIG. 5B shows the waveform of the internal power supply voltage;FIG. 5C shows a waveform of the intensity ofLED202;FIG. 5D shows the output of theAM demodulator101b;andFIG. 5E shows the timing of the state of circuit operations in accordance with the power supplied to the sensor unit. The internal unit power supply ramps up as the field strength increases. When the power supply output crosses the threshold voltage of the LED plus the feedback voltage, the LED turns on. The AM demodulator output contains the measurement data and digital data in the form of ID codes and other parameters specific to the subject in which the internal unit is implanted. This data is encoded on the RF voltage signal through time division multiplexing of the optoelectronic output with digital identification and parameter storage circuits (not shown). The digital circuits use the RF voltage to generate appropriate clock signals.
• The internal storage circuits can store ID codes and parametric values such as calibration constants. This information is returned along with each reading or quantitative measurement. The signals are clocked out by switching from analog pulse train loading to digitally controlled loading at a predefined point in the measurement sequence. This point is detected in the external unit by detecting a predefined bit synchronization pattern in the output data stream. The ID number is used to identify a particular subject and to prevent data corruption when two or more subjects are in the vicinity of the external unit. The calibration factors are applied to the measurement information to obtain analyte levels in clinical units.
• Asensor10 according to one aspect of the invention, which operates based on the fluorescence of fluorescent indicator molecules, is shown inFIG. 6. Thesensor10 is composed of asensor body12; amatrix layer14 coated over the exterior surface of thesensor body12, withfluorescent indicator molecules16 distributed throughout the layer; aradiation source18, e.g. an LED, that emits radiation, including radiation over a wavelength or range of wavelengths which interact with the indicator molecules, i.e., in the case of a fluorescence-based sensor, a wavelength or range of wavelengths which cause theindicator molecules16 to fluoresce; and aphotosensitive element20, e.g. a photodetector, which, in the case of a fluorescence-based sensor, is sensitive to fluorescent light emitted by theindicator molecules16 such that a signal is generated in response thereto that is indicative of the level of fluorescence of the indicator molecules. The sensor110 further includes a module orhousing66 containing electronic circuitry, and atemperature sensor64 for providing a temperature reading. In the simplest embodiments,indicator molecules16 could simply be coated on the surface of the sensor body. In preferred embodiments, however, the indicator molecules are contained within thematrix layer14, which comprises a biocompatible polymer matrix that is prepared according to methods known in the art and coated on the surface of the sensor body. Suitable biocompatible matrix materials, which must be permeable to the analyte, include methacrylates and hydrogels which advantageously can be made selectively permeable to the analyte.
• Thesensor body12 advantageously is formed from a suitable, optically transmissive polymer material which has a refractive index sufficiently different from that of the medium in which the sensor will be used such that the polymer will act as an optical wave guide. Preferred materials are acrylic polymers such as polymethylmethacrylate, polyhydroxypropylmethacrylate and the like, and polycarbonates such as those sold under the trademark Lexan®. The material allows radiation generated by the radiation source18 (e.g., light at an appropriate wavelength in embodiments in which the radiation source is an LED) and, in the case of a fluorescence-based embodiment, fluorescent light emitted by the indicator molecules, to travel through it. Radiation source orLED18 corresponds toLED202 shown inFIG. 2.
• As shown inFIG. 7, radiation (e.g., light) is emitted by theradiation source18 and at least some of this radiation is reflected internally at the surface of thesensor body12, e.g., as atlocation22, thereby “bouncing” back-and-forth throughout the interior of thesensor body12.
• It has been found that light reflected from the interface of the sensor body and the surrounding medium is capable of interacting with indicator molecules coated on the surface (whether coated directly thereon or contained within a matrix), e.g., exciting fluorescence in fluorescent indicator molecules coated on the surface. In addition, light which strikes the interface at angles (measured relative to a direction normal to the interface) too small to be reflected passes through the interface and also excites fluorescence in fluorescent indicator molecules. Other modes of interaction between the light (or other radiation) and the interface and the indicator molecules have also been found to be useful depending on the construction of and application for the sensor. Such other modes include evanescent excitation and surface plasma resonance type excitation.
• As demonstrated byFIG. 8, at least some of the light emitted by thefluorescent indicator molecules16 enters thesensor body12, either directly or after being reflected by the outermost surface (with respect to the sensor body12) of thematrix layer14, as illustrated inregion30. Suchfluorescent light28 is then reflected internally throughout thesensor body12, much like the radiation emitted by theradiation source18 is, and, like the radiation emitted by the radiation source, some will strike the interface between the sensor body and the surrounding medium at angles too small to be reflected and will pass back out of the sensor body.
• As further illustrated inFIG. 6, thesensor10 may also includereflective coatings32 formed on the ends of thesensor body12, between the exterior surface of the sensor body and thematrix layer14, to maximize or enhance the internal reflection of the radiation and/or light emitted by fluorescent indicator molecules. The reflective coatings may be formed, for example, from paint or from a metallized material.
• Anoptical filter34 preferably is provided on the light-sensitive surface of thephotodetector20, which is manufactured of a photosensitive material.Photodetector20 corresponds to photodetector203 shown inFIG. 2.Filter34, as is known from the prior art, prevents or substantially reduces the amount of radiation generated by thesource18 from impinging on the photosensitive surface of thephotosensitive element20. At the same time, the filter allows fluorescent light emitted by fluorescent indicator molecules to pass through it to strike the photosensitive region of the detector. This significantly reduces “noise” in the photodetector signal that is attributable to incident radiation from thesource18.
• The application for which thesensor10 according to one aspect of the invention was developed in particular—although by no means the only application for which it is suitable—is measuring various biological analytes in the human body, e.g., glucose, oxygen, toxins, pharmaceuticals or other drugs, hormones, and other metabolic analytes. The specific composition of thematrix layer14 and theindicator molecules16 may vary depending on the particular analyte the sensor is to be used to detect and/or where the sensor is to be used to detect the analyte (i.e., in the blood or in subcutaneous tissues). Two constant requirements, however, are that thematrix layer14 facilitate exposure of the indicator molecules to the analyte and that the optical characteristics of the indicator molecules (e.g., the level of fluorescence of fluorescent indicator molecules) are a function of the concentration of the specific analyte to which the indicator molecules are exposed.
• To facilitate use in-situ in the human body, thesensor10 is formed, preferably, in a smooth, oblong or rounded shape. Advantageously, it has the approximate size and shape of a bean or a pharmaceutical gelatin capsule, i.e., it is on the order of approximately 300-500 microns to approximately 0.5 inch in length L and on the order of approximately 300 microns to approximately 0.3 inch in depth D, with generally smooth, rounded surfaces throughout. The device of course could be larger or smaller depending on the materials used and upon the intended uses of the device. This configuration permits thesensor10 to be implanted into the human body, i.e., dermally or into underlying tissues (including into organs or blood vessels) without the sensor interfering with essential bodily functions or causing excessive pain or discomfort.
• Moreover, it will be appreciated that any implant placed within the human (or any other animal's) body—even an implant that is comprised of “biocompatible” materials—will cause, to some extent, a “foreign body response” within the organism into which the implant is inserted, simply by virtue of the fact that the implant presents a stimulus. In the case of asensor10 that is implanted within the human body, the “foreign body response” is most often fibrotic encapsulation, i.e., the formation of scar tissue. Glucose—a primary analyte which sensors according to the invention are expected to be used to detect—may have its rate of diffusion or transport hindered by such fibrotic encapsulation. Even molecular oxygen (O2), which is very small, may have its rate of diffusion or transport hindered by such fibrotic encapsulation as well. This is simply because the cells forming the fibrotic encapsulation (scar tissue) can be quite dense in nature or have metabolic characteristics different from that of normal tissue.
• To overcome this potential hindrance to or delay in exposing the indicator molecules to biological analytes, two primary approaches are contemplated. According to one approach, which is perhaps the simplest approach, a sensor/tissue interface layer—overlying the surface of thesensor body12 and/or the indicator molecules themselves when the indicator molecules are immobilized directly on the surface of the sensor body, or overlying the surface of thematrix layer14 when the indicator molecules are contained therein—is prepared from a material which causes little or acceptable levels of fibrotic encapsulation to form. Two examples of such materials described in the literature as having this characteristic are Preclude™ Periocardial Membrane, available from W.L. Gore, and polyisobutylene covalently combined with hydrophiles as described in Kennedy, “Tailoring Polymers for Biological Uses,” Chemtech, February 1994, pp. 24-31.
• Alternatively, a sensor/tissue interface layer that is composed of several layers of specialized biocompatible materials can be provided over the sensor. As shown inFIG. 8, for example, the sensor/tissue interface layer36 may include threesublayers36a,36b,and36c.Thesublayer36a,a layer which promotes tissue ingrowth, preferably is made from a biocompatible material that permits the penetration ofcapillaries37 into it, even as fibrotic cells39 (scar tissue) accumulate on it. Gore-Tex® Vascular Graft material (ePTFE), Dacron® (PET) Vascular Graft materials which have been in use for many years, and MEDPOR Biomaterial produced from high-density polyethylene (available from PORYX Surgical Inc.) are examples of materials whose basic composition, pore size, and pore architecture promote tissue and vascular ingrowth into the tissue ingrowth layer.
• Thesublayer36b,on the other hand, preferably is a biocompatible layer with a pore size (less than 5 micrometers) that is significantly smaller than the pore size of thetissue ingrowth sublayer36aso as to prevent tissue ingrowth. A presently preferred material from which thesublayer36bis to be made is the Preclude Periocardial Membrane (formerly called GORE-TEX Surgical Membrane), available from W.L. Gore, Inc., which consists of expanded polytetra-fluoroethylene (ePTFE).
• Thethird sublayer36cacts as a molecular sieve, i.e., it provides a molecular weight cut-off function, excluding molecules such as immunoglobulins, proteins, and glycoproteins while allowing the analyte or analytes of interest to pass through it to the indicator molecules (either coated directly on thesensor body12 or immobilized within a matrix layer14). Many well known cellulose-type membranes, e.g., of the sort used in kidney dialysis filtration cartridges, may be used for the molecular weight cut-off layer36c.
• As will be recognized, the sensor as shown inFIG. 6 is wholly self-contained such that no electrical leads extend into or out of the sensor body, either to supply power to the sensor (e.g., for driving the source18) or to transmit signals from the sensor. All of the electronics illustrated inFIG. 2 may be housed in amodule66 as shown inFIG. 6.
• A second preferred embodiment of the invention is shown inFIG. 9, in which two detectors are employed, asignal channel detector901 and areference channel detector902. In the first embodiment as shown inFIG. 2, asingle detector203 is used to detect radiation from the fluorescent indicator sensor device. While this system works well, it is possible that various disturbances to the system will occur that may affect the accuracy of the sensor output as originally calibrated.
• Examples of such disturbances include: changes or drift in the component operation intrinsic to the sensor make-up; environmental conditions external to the sensor; or combinations thereof. Internal variables may be introduced by, among other things: aging of the sensor's radiation source; changes affecting the performance or sensitivity of the photosensitive element; deterioration of the indicator molecules; changes in the radiation transmissivity of the sensor body, of the indicator matrix layer, etc.; and changes in other sensor components; etc. In other examples, the optical reference channel could also be used to compensate or correct for environmental factors (e.g., factors external to the sensor) which could affect the optical characteristics or apparent optical characteristics of the indicator molecule irrespective of the presence or concentration of the analyte. In this regard, exemplary external factors could include, among other things: the temperature level; the pH level; the ambient light present; the reflectivity or the turbidity of the medium that the sensor is applied in; etc. The optical reference channel can be used to compensate for such variations in the operating conditions of the sensor. The reference channel is identical to the signal channel in all respects except that the reference channel is not responsive to the analyte being measured.
• Use of reference channels in optical measurement is generally known in the art. For example, U.S. Pat. No. 3,612,866, the entire disclosure of which is incorporated herein by reference, describes a fluorescent oxygen sensor having a reference channel containing the same indicator chemistry as the measuring channel, except that the reference channel is coated with varnish to render it impermeable to oxygen.
• U.S. Pat. Nos. 4,861,727 and 5,190,729, the entire disclosures of which are incorporated herein by reference, describe oxygen sensors employing two different lanthanide-based indicator chemistries that emit at two different wavelengths, a terbium-based indicator being quenched by oxygen and a europium-based indicator being largely unaffected by oxygen. U.S. Pat. No. 5,094,959, the entire disclosure of which is also incorporated herein by reference, describes an oxygen sensor in which a single indicator molecule is irradiated at a certain wavelength and the fluorescence emitted by the molecule is measured over two different emission spectra having two different sensitivities to oxygen. Specifically, the emission spectra which is less sensitive to oxygen is used as a reference to ratio the two emission intensities. U.S. Pat. Nos. 5,462,880 and 5,728,422, the entire disclosures of which are also incorporated herein by reference, describe a ratiometric fluorescence oxygen sensing method employing a reference molecule that is substantially unaffected by oxygen and has a photodecomposition rate similar to the indicator molecule. Additionally, Muller, B., et al, ANALYST, Vol. 121, pp. 339-343 (March 1996), the entire disclosure of which is incorporated herein by reference, describes a fluorescence sensor for dissolved CO₂, in which a blue LED light source is directed through a fiber optic coupler to an indicator channel and to a separate reference photodetector which detects changes in the LED light intensity.
• In addition, U.S. Pat. No. 4,580,059, the entire disclosure of which is incorporated herein by reference, describes a fluorescent-based sensor containing a reference light measuring cell for measuring changes in the intensity of the excitation light source—see, e.g.,column 10,lines 1, et seq.
• As shown inFIG. 9, the signal and reference channel detectors are back-to-back photodiodes901 and902. While photodiodes are shown, many other types of photodetectors also could be used, such as photoresistors, phototransistors, and the like.LED903 corresponds tolight source202 inFIG. 2. In operation,comparator904 is set to trigger at ⅓ and ⅔ of the supply voltage Vss, as biased byresistors905,906, and907. The trigger voltages forcomparator904 could be modified, if desired, by changing the values of the resistors. Capacitor C2 is a timing element, the value of which is adjusted for the magnitude of the signal and reference channels. The current through each photodiode is a function of the intensity or power of incident light entering it, as represented by the equation I=RP, where
• • I=current
  • R=responsivity (Amp/Watt) and
  • P=light power in watts.
• In the fluorescence embodiment, the incident light power impinging upon the photodiode detectors changes with analyte concentration.
• FIG. 10 is a timing diagram showing the voltage levels of theterminals904a,904b,and904cof thecomparator904. At the cycle start, the voltage level ofoutput terminal904cis at ground (low output state), the voltage level of capacitor C2 (which corresponds to the voltage level atinput terminal904b)is at ⅔ Vss, and the voltage level ofinput terminal904ais at ⅓ Vss. In this instance,photodiode901 is forward-biased andphotodiode902 is reverse-biased. The voltage drop across the forward-biasedphotodiode901 is simply its threshold voltage, while the reverse-biasedphotodiode902 exhibits a current flow proportional to the incident light impinging upon it. This current discharges the capacitor C2 at a rate of dV/dt=I902/C2, until it reaches a voltage level of ⅓ Vss as shown inFIG. 10. Inserting the above equation for photodiode current results in the equation dV/dt=RP/C2. Solving for P, P=(dV*C2)/(dt*R), where
• • dV=difference between comparator trigger points (in the example ⅓ Vss)
  • C2=value of capacitor C2 in farads
  • dt=time to charge or discharge (as measured by the external unit) and
  • R=responsivity (in amps/watts) of the photodetector
• At this time, thecomparator904 switches to a high output state Vss onoutput terminal904c.The trigger point (input terminal904a) is now at ⅔ Vss, and the polarity of thephotodiodes901 and902 is now reversed. That is,photodiode901 is now reverse-biased andphotodiode902 is now forward-biased.
• Photodiode901 now controls the charging of capacitor C2 at a rate of dV/dt=I901/C2 until the voltage of capacitor C2 reaches ⅔ Vss. When the voltage across capacitor C2 reaches ⅔ Vss, the output of thecomparator904 again switches to the low output state. So long as the system is powered and incident light is present on the photodiodes, the cycle will continue to repeat as shown inFIG. 10.
• If the incident light intensity on eachphotodiode detector901 and902 is equal, then the comparator output will be a 50% duty cycle. If the incident light on each photodiode detector is not equal, then the capacitor charge current will be different than the capacitor discharge current. This is the case shown inFIG. 10, wherein the capacitor charge current is higher than the capacitor discharge current. Because the same capacitor is charged and discharged, the different charge and discharge times are a function only of the difference between the incident light levels on the two photodiode detectors. Consequently, the duty cycle of the squarewave produced by thecomparator904 is indicative of changes between incident light on the signal channel photodiode and incident light on the reference channel photodiode. Suitable algorithms for taking into account changes in duty cycle of the squarewave from the comparator in determining analyte concentration are generally known in the art (see prior art references discussed supra) and will not be further discussed herein.
• Once the squarewave is established, it must be transferred to the external unit. This is done by loading theinternal coil908, and then detecting the change in load on the external coil inductively coupled to the internal coil. The loading is provided by resistor910, which is connected to theoutput terminal904cof thecomparator904. When the comparator is in a high output state, an additional current Vss/R910 is drawn from thevoltage regulator909. When the comparator is in a low output state, this additional current is not present. Consequently, resistor910 acts as a load that is switched into and out of the circuit at a rate determined by the concentration of analyte and the output of the reference channel. Because the current through resistor910 is provided by the internal tuned tankcircuit including coil908, the switching of the resistor load also switches the load on the tank includinginternal coil908. The change in impedance of the tank caused by the changing load is detected by a corresponding change in load on the inductively coupled external coil, as described above. Thevoltage regulator909 removes any effects caused by coil placement in the field. TheLED903 emits the excitation light for the indicator molecule sensor. Power for theLED903 is provided by the voltage regulator. It is important to keep the intensity of the LED constant during an analyte measurement reading. Once the output of the voltage regulator is in regulation, the LED intensity will be constant. The step recovery time of the regulator is very fast, with the transition between loading states being rapid enough to permit differentiation and AC coupling in the external unit.
• As also will be recognized, the fluorescence-based sensor embodiments described inFIGS. 6-8 are just examples to which the disclosed invention may be applied. The present invention may also be applied in a number of other applications such as, for example, an absorbance-based sensor or a refractive-index-based sensor as described in U.S. patent application Ser. No. 09/383,148, filed Aug. 28, 1999, incorporated herein by reference.
• The invention having been thus described, it will be apparent to those skilled in the art that the same may be varied in many ways without departing from the spirit and scope of the invention. For example, while the invention has been described with reference to an analog circuit, the principles of the invention may be carried out equivalently through the use of an appropriately programmed digital signal processor. Any and all such modifications are intended to be encompassed by the following claims.

Claims (13)

1. In a system comprising (a) an internal sensor unit having a circuit and (b) an external sensor unit, a method for determining the concentration of an analyte, comprising:

using the external sensor unit to provide power to the internal sensor;

while the external sensor unit is providing power to the internal sensor unit,

communicating to the external sensor unit information from which the concentration of the analyte can be determined, wherein the communicating step comprises changing an impedance of the circuit as a function of the concentration of the analyte, wherein a change in the impedance of the circuit causes a voltage in the external sensor unit to change; and

obtaining measurements of the voltage; and

generating information from the voltage measurements, wherein the information includes information from which the concentration of the analyte can be determined.

2. The method ofclaim 1, wherein the communicating step consists essentially of changing the impedance of the circuit as a function of the concentration of the analyte.

3. The method ofclaim 2, wherein the communicating step consists of changing the impedance of the circuit as a function of the concentration of the analyte.

4. The method ofclaim 1, wherein the communicating step comprises changing a resistance of the circuit as a function of the concentration of the analyte.

5. The method ofclaim 1, wherein the internal sensor unit comprises a signal channel detector to detect the presence or concentration of the analyte and a reference channel detector not responsive to the analyte, and the information communicated to the external unit comprises information that is a function of the output of the signal detector and the output of the reference channel.

6. The method ofclaim 5, further comprising generating a signal having a duty cycle, wherein the duty cycle of the signal is a function of the output of the signal channel detector and the output of the reference channel detector.

7. The method ofclaim 6, comprising using the signal to change the impedance of the circuit.

8. Apparatus for retrieving information from a sensor device, comprising:

an internal sensor unit for taking quantitative analyte measurements, including a first coil forming part of a power supply for said sensor unit, a load coupled to said first coil, and a sensor circuit for modifying said load in accordance with sensor measurement information obtained by said sensor circuit, said sensor circuit including a signal channel detector responsive to analyte measurement information, and a reference channel detector responsive to reference measurement information, outputs of said signal channel detector and said reference channel detector being combined in said sensor information for modifying said load, wherein said signal channel detector is configured to produce an output determinative of a positive transition time of said load modification, and said reference channel detector is configured to produce an output determinative of a negative transition time of said load modification;

an external unit including a second coil which is mutually inductively coupled to said first coil upon said second coil being placed within a predetermined proximal distance from said first coil, an oscillator for driving said second coil to induce a charging current in said first coil, and a detector for detecting variations in a load on said second coil induced by changes to said load in said internal sensor unit and for providing information signals corresponding to said load changes; and

a processor for receiving and processing said information signals;

wherein said reference channel detector is configured to produce an output determinative of a positive transition time of said load modification, and said signal channel detector is configured to produce an output determinative of a negative transition time of said load modification.

9. A sensor system, comprising:

an internal unit, including:

a tank circuit comprising a first coil and a capacitor, and

a optoelectronics circuitry coupled to the tank circuit, said optoelectronics circuitry being configured to (1) obtain quantitative measurement information and (2) change the impedance of a circuit connected to the tank circuit as a function of the obtained quantitative measurement information; and

an external unit, including:

a second coil,

an oscillator coupled to the second coil, and

a detector coupled to the second coil, said detector being configured to detect a change in said impedance when the second coil is inductively coupled to the first coil.

10. The sensor system ofclaim 9, wherein the optoelectronics circuitry includes a light source, a signal channel photodetector and a reference channel photodetector, said first and second photodetectors being arranged in a back-to-back manner.

11. The sensor system ofclaim 10, wherein the optoelectronics circuitry further includes a comparator, wherein an input terminal of the comparator is connected to a terminal of one of said photodetectors.

12. The sensor system ofclaim 11, wherein the optoelectronics circuitry is configured to output a signal having a duty cycle that is indicative of changes between incident light on the signal channel photodetector and incident light on the reference channel photodetector.

13. The sensor ofclaim 12, wherein a resistor is electrically connected between an output terminal of the comparator and a terminal of the first coil.

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