CAPACITANCE DETECTION IN ELECTROCHEMICAL ASSAYS
A method and system are provided to determine fill sufficiency of a biosensor test chamber by determining capacitance of the test chamber.
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This application claims the benefits of priority under 35 USC§119 and/or §120 from prior filed U.S. Provisional Application Ser. No. 61/308,167 filed on Feb. 25, 2010, which applications are incorporated by reference in their entirety into this application.
BACKGROUNDAnalyte detection in physiological fluids, e.g. blood or blood derived products, is of ever increasing importance to today's society. Analyte detection assays find use in a variety of applications, including clinical laboratory testing, home testing, etc., where the results of such testing play a prominent role in diagnosis and management in a variety of disease conditions. Analytes of interest include glucose for diabetes management, cholesterol, and the like. In response to this growing importance of analyte detection, a variety of analyte detection protocols and devices for both clinical and home use have been developed.
One type of method that is employed for analyte detection is an electrochemical method. In such methods, an aqueous liquid sample is placed into a sample-receiving chamber in an electrochemical cell that includes two electrodes, e.g., a counter and working electrode. The analyte is allowed to react with a redox reagent to form an oxidizable (or reducible) substance in an amount corresponding to the analyte concentration. The quantity of the oxidizable (or reducible) substance present is then estimated electrochemically and related to the amount of analyte present in the initial sample.
Such systems are susceptible to various modes of inefficiency and/or error. For example, variations in temperatures can affect the results of the method. This is especially relevant when the method is carried out in an uncontrolled environment, as is often the case in home applications or in third world countries. Errors can also occur when the sample size is insufficient to get an accurate result. Partially filled test strips can potentially give an inaccurate result because the measured test currents are proportional to the area of the working electrode that is wetted with sample. Thus, partially filled test strips can under certain conditions provide a glucose concentration that is negatively biased.
SUMMARY OF THE DISCLOSUREApplicants believe that effects of parallel strip resistance in determining filled biosensor test strips have been ignored, leading to inaccurate high measurement of capacitance in a test strip, especially when lower parallel resistance is encountered. Exemplary embodiments of applicants' invention take into consideration this effect and at the same time obviate the need to determine the resistance in a biosensor test chamber.
In one aspect, a method of determining capacitance of a biosensor is provided. The biosensor includes a chamber having two electrodes disposed in the chamber and coupled to a microcontroller. The method can be achieved by: initiating an electrochemical reaction in the biosensor chamber; applying an oscillating voltage of a predetermined frequency to the chamber; determining a phase angle between a current output and the oscillating voltage from the chamber; and calculating a capacitance of the chamber based on a product of the current output and a sine of the phase angle divided by a product of two times pi times the frequency and the voltage.
In a further aspect, an analyte measurement system is provided that includes an analyte test strip and analyte test meter. The analyte test strip includes a substrate having a reagent disposed thereon, and at least two electrodes proximate the reagent in test chamber. The analyte meter includes a strip port connector disposed to connect to the two electrodes, a power supply, and a microcontroller electrically coupled to the strip port connector and the power supply. The microcontroller is programmed to: initiate an electrochemical reaction in the biosensor chamber; apply an oscillating voltage of a predetermined frequency to the chamber; determine a phase angle between a current output and the oscillating voltage from the chamber; and calculate a capacitance of the chamber based on a product of the current output and a sine of the phase angle divided by a product of two times pi times the frequency and the voltage.
In yet another aspect, analyte measurement system is provided that includes an analyte test strip and analyte test meter. The test strip includes a substrate having a reagent disposed thereon, and at least two electrodes proximate the reagent in test chamber. The analyte meter includes a strip port connector disposed to connect to the two electrodes, a power supply, and a microcontroller electrically coupled to the strip port connector and the power supply such that a percent error in capacitance measurement of the test strip across a range of capacitance as compared to a referential parallel R-C circuit is less than about 3%.
These and other embodiments, features and advantages will become apparent to those skilled in the art when taken with reference to the following more detailed description of various exemplary embodiments of the invention in conjunction with the accompanying drawings that are first briefly described.
The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate presently preferred embodiments of the invention, and, together with the general description given above and the detailed description given below, serve to explain features of the invention (wherein like numerals represent like elements).
The following detailed description should be read with reference to the drawings, in which like elements in different drawings are identically numbered. The drawings, which are not necessarily to scale, depict selected embodiments and are not intended to limit the scope of the invention. The detailed description illustrates by way of example, not by way of limitation, the principles of the invention. This description will clearly enable one skilled in the art to make and use the invention, and describes several embodiments, adaptations, variations, alternatives and uses of the invention, including what is presently believed to be the best mode of carrying out the invention.
As used herein, the terms “about” or “approximately” for any numerical values or ranges indicate a suitable dimensional tolerance that allows the part or collection of components to function for its intended purpose as described herein. In addition, as used herein, the terms “patient,” “host,” “user,” and “subject” refer to any human or animal subject and are not intended to limit the systems or methods to human use, although use of the subject invention in a human patient represents a preferred embodiment.
The subject systems and methods are suitable for use in the determination of a wide variety of analytes in a wide variety of samples, and are particularly suited for use in the determination of analytes in whole blood, plasma, serum, interstitial fluid, or derivatives thereof. In an exemplary embodiment, a glucose test system based on a thin-layer cell design with opposing electrodes and tri-pulse electrochemical detection that is fast (e.g., about 5 second analysis time), requires a small sample (e.g., about 0.4 μL(microliter)), and can provide improved reliability and accuracy of blood glucose measurements. In the reaction cell, glucose in the sample can be oxidized to gluconolactone using glucose dehydrogenase and an electrochemically active mediator can be used to shuttle electrons from the enzyme to a working electrode. A potentiostat can be utilized to apply a tri-pulse potential waveform to the working and counter electrodes, resulting in test current transients used to calculate the glucose concentration. Further, additional information gained from the test current transients may be used to discriminate between sample matrices and correct for variability in blood samples due to hematocrit, temperature variation, electrochemically active components, and identify possible system errors.
The subject methods can be used, in principle, with any type of electrochemical cell having spaced apart first and second electrodes and a reagent layer. For example, an electrochemical cell can be in the form of a test strip. In one aspect, the test strip may include two opposing electrodes separated by a thin spacer for defining a sample-receiving chamber or zone in which a reagent layer is located. One skilled in the art will appreciate that other types of test strips, including, for example, test strips with co-planar electrodes may also be used with the methods described herein.
Referring back to
The electronic components of meter 10 can be disposed on a circuit board 34 that is within housing 11.
Referring back to
Strip port connector 308 can be located proximate the strip port opening 22 and configured to form an electrical connection to the test strip. Display 14 can be in the form of a liquid crystal display for reporting measured glucose levels, and for facilitating entry of lifestyle related information. Display 14 can optionally include a backlight. Data port 13 can accept a suitable connector attached to a connecting lead, thereby allowing glucose meter 10 to be linked to an external device such as a personal computer. Data port 13 can be any port that allows for transmission of data such as, for example, a serial, USB, or a parallel port.
Real time clock 42 can be configured to keep current time related to the geographic region in which the user is located and also for measuring time. Real time clock 42 may include a clock circuit 45, a crystal 44, and a super capacitor 43. The DMU can be configured to be electrically connected to a power supply such as, for example, a battery. The super capacitor 43 can be configured to provide power for a prolonged period of time to power real time clock 42 in case there is an interruption in the power supply. Thus, when a battery discharges or is replaced, real time clock does not have to be re-set by the user to a proper time. The use of real time clock 42 with super capacitor 43 can mitigate the risk that a user may re-set real time clock 42 incorrectly.
As shown in
Referring back to
Either the first electrode or the second electrode can perform the function of a working electrode depending on the magnitude and/or polarity of the applied test voltage. The working electrode may measure a limiting test current that is proportional to the reduced mediator concentration. For example, if the current limiting species is a reduced mediator (e.g., ferrocyanide), then it can be oxidized at the first electrode as long as the test voltage is sufficiently less than the redox mediator potential with respect to the second electrode. In such a situation, the first electrode performs the function of the working electrode and the second electrode performs the function of a counter/reference electrode. Note that one skilled in the art may refer to a counter/reference electrode simply as a reference electrode or a counter electrode. A limiting oxidation occurs when all reduced mediator has been depleted at the working electrode surface such that the measured oxidation current is proportional to the flux of reduced mediator diffusing from the bulk solution towards the working electrode surface. The term bulk solution refers to a portion of the solution sufficiently far away from the working electrode where the reduced mediator is not located within a depletion zone. It should be noted that unless otherwise stated for test strip 80, all potentials applied by test meter 10 will hereinafter be stated with respect to second electrode. Similarly, if the test voltage is sufficiently greater than the redox mediator potential, then the reduced mediator can be oxidized at the second electrode as a limiting current. In such a situation, the second electrode performs the function of the working electrode and the first electrode performs the function of the counter/reference electrode. Details regarding the exemplary test strip, operation of the strip and the test meter are found in U.S. Patent Application Publication No. 20090301899, which is incorporated by reference in its entirety herein, with a copy attached to the Appendix.
Referring to
Referring to
On the other hand, if current-drive signal 312a to bias driver 312 is held high, the DAC output is scaled to approximately 0 to approximately 60 mV full scale. Switch signal 312c may also be energized, causing the current path through the test strip to be diverted through a resistor in bias driver 312. The op-amp in bias driver 312 attempts to control the voltage drop across the resistor to be the same as the scaled DAC drive—producing in this case a current of approximately 600 nA. This current is used for sample detection in order to initiate a test measurement.
Bias driver 312 is also connected to a trans-impedance amplifier circuit (“TIA circuit”) 314. TIA circuit 314 converts the current flowing though the strip's electrode layer 66a (e.g., palladium) to electrode layer 64a (e.g., gold) contacts into a voltage. The overall gain is controlled by a resistor in TIA circuit 314. Because the strip 80 is a highly capacitive load, normal low-offset amplifiers tend to oscillate. For this reason a low-cost op-amp is provided in the TIA circuit 314 as a unity gain buffer and incorporated within the overall feedback loop. As a functional block, circuit 314 acts as dual op-amp system with both high drive capability and low voltage offset. The TIA circuit 314 also utilizes a virtual ground (or virtual earth) to generate the 1.024V bias on the electrode layer 64a (e.g., gold) contact of the SPC 308. Circuit 314 is also connected to a Vref amplifier circuit 316. This circuit, when in current measuring mode, uses a virtual ground rail set at Vref/2 (approximately 1.024V), allowing both positive and negative currents to be measured. This voltage feeds all of the gain amplifier stage 318. To prevent any circuit loads from ‘pulling’ this voltage, a unity gain buffer amplifier may be utilized within the Vref amplifier circuit 316.
The strip current signal 314a from the TIA circuit 314 and the virtual ground rail 316a (˜Vref/2) from the voltage reference amplifier 316 are scaled up as needed for various stages of the test measurement cycle. In the exemplary embodiment, MC 300 is provided with four channels of amplified signal sensed from the test strip with varying amplifications of the sensed current as need for different stages of the measurement cycle of the test strip during an analyte assay.
In one embodiment, the test meter 10 can apply a test voltage and/or a current between the first contact pad 47 and the second contact pad 43 of the test strip 80. Once the test meter 10 recognizes that the strip 80 has been inserted, the test meter 10 turns on and initiates a fluid detection mode. In one embodiment, the meter attempts to drive a small current (e.g. 0.2 to 1 μA) through the strip 80. When there is no sample present the resistance is greater than several Mega Ohms, so the driving voltage on the op-amp trying to apply the current goes to the rail. When a sample is introduced the resistance drops precipitously and the driving voltage follows. When the driving voltage drops below a pre-determined threshold the test sequence is initiated.
After a sample has been detected in the test strip chamber 61, the voltage between the strip electrodes is stepped to a suitable voltage in millivolts of magnitude and maintained for a set amount of time, e.g., about 1 second, then stepped to a higher voltage and held for a fixed amount of time, then a sine wave voltage is applied on top of the DC voltage for a set amount of time, then the DC voltage is applied for a further amount of time, then reversed to a negative voltage and held for a set amount of time. The voltage is then disconnected from the strip. This series of applied voltages generates a current transient such as the one shown in
In
In one embodiment for performing a sufficient volume check, a capacitance measurement is used to infer sufficient analyte fill of the chamber 61 of the test strip 80. A magnitude of the capacitance can be proportional to the area of an electrode that has been coated with sample fluid. Once the magnitude of the capacitance is measured, if the value is greater than a threshold and thus the test strip has a sufficient volume of liquid for an accurate measurement, a glucose concentration can be outputted. But if the value is not greater than a threshold, indicating that the test strip has insufficient volume of liquid for an accurate measurement, and then an error message can be outputted.
In one method for measuring capacitance, a test voltage having a constant component and an oscillating component is applied to the test strip. In such an instance, the resulting test current can be mathematically processed, as described in further detail below, to determine a capacitance value.
Applicants believe that the biosensor test chamber 61 with the electrode layers can be modeled in the form of a circuit having a parallel resistor and capacitor as shown in Table 1.
In this model in Table 1, R represents the resistance encountered by the current and C represents a capacitance resulting from the combination of the physiological fluid and reagent electrically coupled to the electrodes. To initiate a determination of capacitance of the chamber, an alternating bias voltage may be applied across the respective electrodes disposed in the chamber, and a current from the chamber is measured. The filling of the chamber 61 is believed to be generally a measure of capacitance only and thus any parasitic resistance, such as, for example, R, must not be included in any determination or calculation of capacitance. Hence, in measuring or sensing the current, any parasitic resistance is believed to affect the measured current. Applicants, however, have discovered a technique to derive capacitance without requiring utilization or knowledge of the resistance through the chamber as modeled above. In order to further explain this technique, a short discussion of the mathematical foundation underlying the technique is warranted.
According to Kirchhoff's Law, total current (iT) through the circuit of Table 1 is approximately the sum of the current flowing through the resistor (iR) and through the capacitor (iC). When an alternating voltage V (as measured as RMS) is applied, the resistor current (iR) may be expressed as:
iR=V/R Eq. 1
Capacitor current (iC) can be expressed as:
iC=jωCV Eq. 2
-
- Where:
- j is an imaginary number operator indicating that current leads voltage by about 90 degrees in a capacitor; and
- ω is the angular frequency 2πƒ where f is frequency in Hertz.
- Where:
The summation of these components is shown in the phasor diagram of Table 1. In the phasor diagram, Φ represents the phase angle of the input as compared to the output. Phase angle Φ is determined by the following trigonometric function:
tanΦ=Ic/IR Eq. 3
By Pythagoras theorem, the square of the total current iT can be calculated as:
iT2=iC2+iR2 Eq. 4
By rearranging Eq. 4 and substituting Eq. 3, the following equation is arrived at:
iC2=iT2−i
Resolving for capacitor current iC and combining with Eq. 2:
iC=√{square root over (()}iT2*(tanΦ)2/((tan Φ))2+1))=ωCV Eq. 6
Rearranging for C and expanding ω, the capacitance becomes:
C=(√{square root over (()}iT2*(tan Φ)2/((tan Φ)2+1))/2πƒV Eq. 7
Simplification of Eq. 7 leads to:
C=|(iTsinΦ)|/2πƒV Eq. 8
It can be seen that Eq. 8 does not reference to the resistor current. Consequently, if the system can drive an alternating voltage with frequency f and root-mean-squared (“RMS”) amplitude V, and measure total current iT as RMS value and phase angle Φ, capacitance C of the test chamber 61 can be accurately calculated without having to determine resistance in the biosensor test chamber. This is believed to be of substantial benefit because the resistance of the biosensor strip is difficult to measure, and varies over the 5 second assay time. Resistance is believed to arise from how many charge carriers can flow through the strip for a given electrical bias (voltage), and is therefore reaction dependent. At the 1.3 second point in the assay, the resistance is expected to be anything from 10 kΩ to perhaps 100 kΩ. Hence, by not having to determine the resistance in the biosensor chamber or even the resistance in the measuring circuit, such as a sensor resistor, applicants' invention have advanced the state of the art in improving of the entire test strip.
Implementation of an exemplary technique to determine capacitance C based on Eq. 8 can be understood in relation
In
In this technique, a mean of all the 65 samples, referenced here as 602, in
To determine the phase angle, the system or MC, as appropriately programmed can compare the oscillating input voltage, shown here in
Once the phase angle has been derived, capacitance can be calculated using Eq. 8. In practice, however, it has been determined that the implementation of the trans-impedance amplifier 314 and the gain amplifier introduces additional phase shift into the system. This additional phase shift can be offset by introduction of a compensation value ΦCOMP by measuring the capacitance of the system without a strip in use.
C=|iTsin(Φ+ΦCOMP)|/2πƒV Eq. 9
In the preferred embodiments, the compensation phase angle ΦCOMP ranges from about 5 to about 7 degrees.
Once capacitance of the test strip 80 has been determined, a two-point calibration can be performed to normalize the capacitance value to a value that is independent of any tolerances of the analog components (e.g., resistors, capacitors, op-amps, switches and the like). Briefly, the two-point calibration is performed by: placing a 550 nF capacitor with 30 k parallel resistance across the measurement input; command the meter to measure the capacitance, and note the value produced; place a 800 nF capacitor with 30 k parallel resistance across the measurement input; command the meter to measure the capacitance, and note the value produced. These two points will give an indication of the gain and offset of the measurement capability of that particular hardware instance (not the design). A slope and offset are then calculated from the measurement errors, and stored in the meter's memory. The meter is now calibrated.
- When a strip is inserted and a sample applied, the capacitance is measured and the stored slope and offset are applied to correct the measurement.
After completion of the device calibration, an evaluation is made to determine whether the test chamber 61 has been sufficiently filled with test fluid. The evaluation can be based on a capacitance magnitude of at least 65% to 85% of an average capacitance value derived from a large sample of good filled test strips.
To test the robustness of this exemplary technique, applicants intentionally introduced noise into the system to determine the percent error as compared to referential parallel R-C circuit. In Table 2 below, despite the number of Analog-to-Digital-Converter (“ADC”) noise counts were introduced, error relating to current, phase angle and capacitance were less than 1%.
Comparison of the exemplary techniques with other techniques confirms the increased accuracy of applicants' technique. For example, in
Although the exemplary embodiments, methods, and system have been described in relation to a blood glucose strip, the principles described herein are also applicable to any analyte measurement strips that utilize a physiological fluid on a reagent disposed between at least two electrodes.
As noted earlier, the microcontroller can be programmed to generally carry out the steps of various processes described herein. The microcontroller can be part of a particular device, such as, for example, a glucose meter, an insulin pen, an insulin pump, a server, a mobile phone, personal computer, or mobile hand held device. Furthermore, the various methods described herein can be used to generate software codes using off-the-shelf software development tools such as, for example, C or variants of C such as, for example, C+, C++, or C-Sharp. The methods, however, may be transformed into other software languages depending on the requirements and the availability of new software languages for coding the methods. Additionally, the various methods described, once transformed into suitable software codes, may be embodied in any computer-readable storage medium that, when executed by a suitable microcontroller or computer, are operable to carry out the steps described in these methods along with any other necessary steps.
While the invention has been described in terms of particular variations and illustrative figures, those of ordinary skill in the art will recognize that the invention is not limited to the variations or figures described. In addition, where methods and steps described above indicate certain events occurring in certain order, those of ordinary skill in the art will recognize that the ordering of certain steps may be modified and that such modifications are in accordance with the variations of the invention. Additionally, certain of the steps may be performed concurrently in a parallel process when possible, as well as performed sequentially as described above. Therefore, to the extent there are variations of the invention, which are within the spirit of the disclosure or equivalent to the inventions found in the claims, it is the intent that this patent will cover those variations as well.
Claims
1. A method of determining capacitance of a biosensor chamber having a two electrodes disposed in the chamber and coupled to a microcontroller, the method comprising:
- initiating an electrochemical reaction in the biosensor chamber;
- applying an oscillating voltage of a predetermined frequency to the chamber;
- determining a phase angle between a current output and the oscillating voltage from the chamber; and
- calculating a capacitance of the chamber based on a product of the current output and a sine of the phase angle divided by a product of two times pi times the frequency and the voltage.
2. The method of claim 1, in which the calculating comprises calculating capacitance with an equation of the form:
- C=|(iTsinΦ)|÷2πƒV
- where: C≈capacitance; iT≈total current; Φ≈phase angle between total current and resistor current; ƒ≈frequency; and V≈voltage.
3. The method of claim 2, in which the calculating comprises:
- sampling a plurality of current outputs from the chamber over one cycle of the frequency;
- obtaining a mean of sampled current output;
- subtracting the mean from each sampled current of the plurality of current outputs; and
- extracting root-mean-squared value of all negative values from the subtracting to provide for the total current output.
4. The method of claim 3, in which the calculating comprises:
- determining from the sampling, at least one cross-over point of the current from negative to positive values; and
- interpolating proximate the at least one cross-over point of the current to determine a first angle at which the current changes from positive to negative or negative to positive.
5. The method of claim 4, in which the interpolating the at least one cross-over point of the current comprises:
- interpolating another cross-over point from the sampling to determine another angle at which the current changes from positive to negative or negative to positive; and
- subtracting from the another angle approximately 180 degrees to provide for a second angle.
6. The method of claim 5, in which the subtracting further comprises calculating an average of the first and second angles.
7. The method of claim 5, in which the calculating comprises determining a difference in the angle between the oscillating input current and the output current as the phase angle.
8. An analyte measurement system comprising:
- An analyte test strip including: a substrate having a reagent disposed thereon; at least two electrodes proximate the reagent in test chamber;
- an analyte meter including: a strip port connector disposed to connect to the two electrodes; a power supply; and a microcontroller electrically coupled to the strip port connector and the power supply, the microcontroller being programmed to: (a) initiate an electrochemical reaction in the biosensor chamber; apply an oscillating voltage of a predetermined frequency to the chamber; (b) determine a phase angle between a current output and the oscillating voltage from the chamber; and (c) calculate a capacitance of the chamber based on a product of the current output and a sine of the phase angle divided by a product of two times pi times the frequency and the voltage.
9. An analyte measurement system comprising:
- An analyte test strip including: a substrate having a reagent disposed thereon; at least two electrodes proximate the reagent in test chamber;
- an analyte meter including: a strip port connector disposed to connect to the two electrodes; a power supply; and
- a microcontroller electrically coupled to the strip port connector and the power supply such that a percent error in capacitance measurement of the test strip across a range of capacitance as compared to a referential parallel R-C circuit is less than about 3%.
Type: Application
Filed: Feb 24, 2011
Publication Date: Aug 25, 2011
Applicant: LifeScan Scotland Ltd. (Inverness-shire)
Inventors: David ELDER (Inverness), Sven RIPPEL (Zwingenberg)
Application Number: 13/034,281
International Classification: G06F 19/00 (20110101); G01N 31/00 (20060101); G01N 33/50 (20060101); G01N 27/22 (20060101);