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US20100169035A1 - Methods and systems for observing sensor parameters - Google Patents

Methods and systems for observing sensor parameters Download PDF

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Publication number
US20100169035A1
US20100169035A1 US12/345,354 US34535408A US2010169035A1 US 20100169035 A1 US20100169035 A1 US 20100169035A1 US 34535408 A US34535408 A US 34535408A US 2010169035 A1 US2010169035 A1 US 2010169035A1
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United States
Prior art keywords
sensor
voltage
current
sensors
electrodes
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Abandoned
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US12/345,354
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English (en)
Inventor
Bradley Chi Liang
Larry E. Tyler
Mohsen Askarinya
Charles Robert Gordon
Randal C. Schulhauser
Kenneth W. Cooper
Kris R. Holtzclaw
Brian T. Kannard
Rajiv Shah
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Medtronic Minimed Inc
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Medtronic Minimed Inc
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Application filed by Medtronic Minimed Inc filed Critical Medtronic Minimed Inc
Priority to US12/345,354 priority Critical patent/US20100169035A1/en
Assigned to MEDTRONIC MINIMED, INC. reassignment MEDTRONIC MINIMED, INC. ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: KANNARD, BRIAN T., ASKARINYA, MOHSEN, GORDON, CHARLES ROBERT, SCHULHAUSER, RANDAL C., TYLER, LARRY E., COOPER, KENNETH W., HOLTZCLAW, KRIS R., LIANG, BRADLEY CHI, SHAH, RAJIV
Priority to EP15174014.9A priority patent/EP2959829B1/fr
Priority to CA2747830A priority patent/CA2747830C/fr
Priority to PCT/US2009/069600 priority patent/WO2010078263A2/fr
Priority to DK09799839.7T priority patent/DK2369977T3/en
Priority to EP09799839.7A priority patent/EP2369977B1/fr
Publication of US20100169035A1 publication Critical patent/US20100169035A1/en
Priority to US13/151,601 priority patent/US8160834B2/en
Abandoned legal-status Critical Current

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    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/145—Measuring characteristics of blood in vivo, e.g. gas concentration or pH-value ; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid or cerebral tissue
    • A61B5/14532—Measuring characteristics of blood in vivo, e.g. gas concentration or pH-value ; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid or cerebral tissue for measuring glucose, e.g. by tissue impedance measurement
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/145—Measuring characteristics of blood in vivo, e.g. gas concentration or pH-value ; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid or cerebral tissue
    • A61B5/1468—Measuring characteristics of blood in vivo, e.g. gas concentration or pH-value ; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid or cerebral tissue using chemical or electrochemical methods, e.g. by polarographic means
    • A61B5/1486—Measuring characteristics of blood in vivo, e.g. gas concentration or pH-value ; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid or cerebral tissue using chemical or electrochemical methods, e.g. by polarographic means using enzyme electrodes, e.g. with immobilised oxidase
    • A61B5/14865—Measuring characteristics of blood in vivo, e.g. gas concentration or pH-value ; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid or cerebral tissue using chemical or electrochemical methods, e.g. by polarographic means using enzyme electrodes, e.g. with immobilised oxidase invasive, e.g. introduced into the body by a catheter or needle or using implanted sensors

Definitions

  • Embodiments of this invention provide methods and systems for determining the state of sensors, for example the sufficiency of hydration of a glucose sensor during its initial use.
  • BG measurement devices use various methods to measure the BG level of a patient, such as a sample of the patient's blood, a sensor in contact with a bodily fluid, an optical sensor, an enzymatic sensor, or a fluorescent sensor.
  • Continuous glucose measurement systems include subcutaneous (or short-term) sensors and implantable (or long-term) sensors. For each of the short-term sensors and the long-term sensors, a patient has to wait a certain amount of time in order for the continuous glucose sensor to stabilize and to provide accurate readings. In many continuous glucose sensors, the subject must wait three hours for the continuous glucose sensor to stabilize before any glucose measurements are utilized. This is an inconvenience for the patient and in some cases may cause the patient not to utilize a continuous glucose measurement system.
  • the glucose sensor when a sensor such as a glucose sensor is implanted in a patient, started up and then used to monitor glucose, the glucose sensor may not operate in a stable state.
  • the electrical readings from the sensor which optimally are directly correlated to the glucose level of the patient, can nonetheless vary and are subject to factors which confound sensor readings, for example erroneous reading that can result from phenomena such as sensor dehydration, sensor noise, sensor drift and the like.
  • Embodiments of the invention disclosed herein include methods and materials for observing and characterizing the state of a sensor for example a glucose sensor used by a diabetic patient.
  • Illustrative embodiments of the invention include a sensor system having a plurality of electrodes and a sensor electronics device which includes a connection device, a power source, an electrical circuit and a microcontroller.
  • the electrical circuit is used in methods that characterize the state of sensor elements, for example whether one or more electrodes in the sensor is hydrated and/or operating within a range of predetermined functional parameters.
  • a typical embodiments of the invention is a method of observing a state of a sensor having a plurality of electrodes, the method comprising applying voltage to the sensor, observing a peak instantaneous electrical current of the sensor, and observing a total current of the sensor over a period of time for a predetermined frequency so that the state of the sensor is observed.
  • Such methods of the invention can be used to observe a number of measurable and/or quantifiable sensor characteristics. For example, observations of the peak instantaneous electrical current and/or the total current in the sensor over a period of time for a predetermined frequency can be used to estimate sensor impedance magnitude and/or sensor capacitance.
  • observations on the states or characteristics of a sensor are used to obtain information associated with sensor operation in vivo, for example the presence or levels of sensor hydration, sensor noise, sensor offset, sensor drift or the like.
  • Embodiments of the invention include a variety of ways in which observations on the state or characteristics of a sensor can be obtained.
  • certain embodiments of the invention are designed to estimate a state of sensor capacitance, wherein an estimate of sensor capacitance comprises a voltage step analysis using a mathematical formula:
  • C comprises capacitance
  • V comprises voltage
  • dV comprises a controlled voltage step
  • dt comprises a length of time for analysis
  • t samp comprises a length of time between samples
  • dI comprises a change in current
  • Embodiments of the invention employ a variety of different methodological steps to observe sensor characteristics.
  • certain embodiments of the invention comprise applying a voltage pulse to the sensor.
  • the method comprises observing the maximum current value (counts/second) during the initial 2 seconds in response to a voltage pulse applied to the sensor, and then comparing the maximum current value to a predetermined test value.
  • the methods comprises methodological steps such as applying a plurality of voltages to the sensor and/or observing current in the sensor over multiple periods of time and/or observing current in the sensor over multiple frequencies.
  • Some embodiments of the invention are designed for use in manufacturing processes, for example to examine the uniformity of a lot of sensors manufactured according to a specific process (e.g. by performing the methods on a plurality of sensors; and then comparing the information so obtained on the state of the plurality of sensors).
  • Embodiments of the invention can be adapted for use with a wide variety of electrochemical sensors such as glucose sensors that comprise glucose oxidase.
  • the glucose sensor comprises a base layer, at least three working electrodes disposed on the base layer, a glucose oxidase layer disposed upon the working electrodes, an analyte modulating layer disposed on the glucose oxidase layer, wherein the analyte modulating layer comprises a hydrogel composition, and an adhesion promoting layer disposed between the glucose oxidase layer and the analyte modulating layer.
  • the sensor comprise a plurality of working electrodes and/or counter electrodes and/or reference electrodes (e.g.
  • the plurality of working, counter and reference electrodes are configured together as a unit and positionally distributed on the conductive layer in a repeating pattern of units.
  • Embodiment of the invention include sensor systems such as those comprising an implantable sensor having a plurality of electrodes and a sensor electronics device that is operably connected to the sensor.
  • the sensor electronics device includes a connection detection device to determine if the sensor electronics device is connected to the sensor and to transmit a connection signal, a power source to supply a regulated voltage, and a microprocessor.
  • Such systems typically include a computer-readable program code having instructions, which when executed cause the microprocessor to apply a voltage to the sensor and then record data on a peak instantaneous electrical current of the sensor in response to the applied voltage as well as record data on a total current of the sensor over a period of time for a predetermined frequency in response to the applied voltage.
  • the system further comprises a monitor for displaying the data recorded by the microprocessor, wherein the data displayed on the monitor provides information on sensor hydration, sensor noise, sensor offset, sensor drift or the like.
  • the implantable sensor is a glucose sensor comprising a base layer, at least three working electrodes disposed on the base layer, a glucose oxidase layer disposed upon the working electrodes, an analyte modulating layer disposed on the glucose oxidase layer; and an adhesion promoting layer disposed between the glucose oxidase layer and the analyte modulating layer.
  • the sensor is implantable in tissue selected from the group consisting of subcutaneous, dermal, sub-dermal, intra-peritoneal, and peritoneal tissue.
  • Yet another embodiment of the invention is a program code storage device comprising a computer-readable medium, a computer-readable program code, stored on the computer-readable medium, the computer-readable program code having instructions, which when executed cause a controller to initiate a series of voltage pulses to be applied to a sensor comprising a plurality of electrodes; and receive a signal from a detection circuit, the signal indicating a peak instantaneous electrical current of the sensor in response to the applied voltage pulses; and a total current of the sensor over a period of time for a predetermined frequency in response to the applied voltage pulses.
  • the program code storage device includes instructions, which when executed causes the controller to determine the maximum current value (counts/second) during the initial 2 seconds in response to a voltage pulse applied to the sensor, and then compare the maximum current value so determined to a predetermined range of values.
  • the program code storage device includes instructions, which when executed causes the controller to then enable the sensor to measure a physiological characteristic of a patient when the maximum current value is within the predetermined range of values.
  • FIG. 1 is a perspective view of a subcutaneous sensor insertion set and block diagram of a sensor electronics device according to an embodiment of the invention.
  • FIG. 2( a ) illustrates a substrate having two sides, a first side which contains an electrode configuration and a second side which contains electronic circuitry.
  • FIG. 2( b ) illustrates a general block diagram of an electronic circuit for sensing an output of a sensor.
  • FIG. 3 illustrates a block diagram of a sensor electronics device and a sensor including a plurality of electrodes according to an embodiment of the invention.
  • FIG. 4 illustrates an alternative embodiment of the invention including a sensor and a sensor electronics device according to an embodiment of the present invention.
  • FIG. 5 illustrates an electronic block diagram of the sensor electrodes and a voltage being applied to the sensor electrodes according to an embodiment of the present invention.
  • FIG. 6A-6D provide graphs of sensor profiles under various conditions in vitro.
  • FIG. 7 provides a graph illustrating a 24-hour EIS sampling, SITS.
  • FIG. 8 provides a graph illustrating a transient 10 mV voltage pulse.
  • FIG. 9 provides a graph illustrating a current response, 10 mV pulse in SITS.
  • FIG. 10 provides a graph illustrating capacitance, 10 mV step, SITS.
  • FIG. 11 provides a graph illustrating current response estimation of complex impedance.
  • FIG. 12 provides a flow chart illustrating a proposed diagnostic/time limit feedback loop.
  • FIG. 13 provides a schematic of customized firmware executing a diagnostic test multiple times.
  • FIG. 14 provides a schematic of data storage for each diagnostic step.
  • FIGS. 15A and 15B provide graphs illustrating current/capacitance profiles of a hydrated sensor over time.
  • FIGS. 16A and 16B provide graphs illustrating current/capacitance profiles of “sleepy” sensors (e.g. those having hydration issues).
  • FIGS. 17A and 17B provide graphs illustrating a current/capacitance profile from intentional reinitialization.
  • FIG. 18 provides a figurative representation of the theory behind determination of sensor offset.
  • Lowercase v and i are representative of paired times for voltage and corresponding current.
  • a relatively long dt time can be used to confirm current that matches relatively closely with the stable-state current value for that potential.
  • the current is typically not sampled over the entire dt time, (e.g. and instead just the last 1-2 seconds).
  • FIG. 19 illustrates aspects of an embodiment of the invention where a current step is used to estimate the impedance modeled as a series R-C.
  • Vbase normally applied stable-state voltage
  • the system can measure the stable-state current, Ibase, under normal operating conditions. Then, the system can switch from forcing voltage to forcing current, using the Ibase measured previously.
  • the current can be stepped to a new value, Ibase+deltaI.
  • This will initially cause a voltage step, deltaV, due to resistance.
  • a voltage slope can then be obtained, dV/dt (dV is not the same as the initial deltaV step), one related to the charging of the capacitance by the deltaI.
  • analyte as used herein is a broad term and is used in its ordinary sense, including, without limitation, to refer to a substance or chemical constituent in a fluid such as a biological fluid (for example, blood, interstitial fluid, cerebral spinal fluid, lymph fluid or urine) that can be analyzed.
  • a biological fluid for example, blood, interstitial fluid, cerebral spinal fluid, lymph fluid or urine
  • Analytes can include naturally occurring substances, artificial substances, metabolites, and/or reaction products.
  • the analyte for measurement by the sensing regions, devices, and methods is glucose.
  • other analytes are contemplated as well, including but not limited to, lactate. Salts, sugars, protein fats, vitamins and hormones naturally occurring in blood or interstitial fluids can constitute analytes in certain embodiments.
  • the analyte can be naturally present in the biological fluid or endogenous; for example, a metabolic product, a hormone, an antigen, an antibody, and the like.
  • the analyte can be introduced into the body or exogenous, for example, a contrast agent for imaging, a radioisotope, a chemical agent, a fluorocarbon-based synthetic blood, or a drug or pharmaceutical composition, including but not limited to insulin.
  • a contrast agent for imaging for example, a radioisotope, a chemical agent, a fluorocarbon-based synthetic blood, or a drug or pharmaceutical composition, including but not limited to insulin.
  • the metabolic products of drugs and pharmaceutical compositions are also contemplated analytes.
  • Determining the state of a sensor such as an electrochemical glucose sensor has traditionally been performed by using a standard timer function.
  • a voltage pulse to solicit a current response, in which complex impedance values are derived. These then impedance values provide an indicator of the sensor's state (pre vs. post initialization).
  • Embodiments of the invention can then limit the use of a commercial sensor to its regulatory agency approved labeling standard (e.g. 3 or 6 days).
  • Embodiments of the invention analyze electrochemical properties of a sensor to determine its overall health (i.e. operability parameters).
  • Applications for embodiments of the invention include for example: optimizing sensor startup and/or to prevent the use of old sensors; the prevention of a startup/initialization sequence for non-hydrated and/or incompletely hydrated sensors; to indicate when sensors begin to provide bad data (e.g. bad data that results from noise, drift and the like.); and to provide corrective measures to changes in the state of the sensor (e.g. adjusting for drift or changes in sensor offset).
  • bad data e.g. bad data that results from noise, drift and the like.
  • corrective measures to changes in the state of the sensor e.g. adjusting for drift or changes in sensor offset.
  • Embodiments of the invention relate to Electrochemical Impedance Spectroscopy (EIS) and analogs thereof. Without being bound by a specific scientific theory, evidence indicates that a sensor operating for example in vivo undergoes changes in impedance. As impedance can be difficult to determine using conventional methods, an analogue test for the GST(R) can be useful in a variety of situations.
  • Embodiments of the invention observe: (1) capacitance to estimate the imaginary component of complex impedance; and (2) current response because for example a different current with same voltage in a sensor provides evidence that some parameter of sensor function has changed. In one typical capacitance estimation, the sum of counts over sample time provides an indication of sensor charge capacity.
  • the current analysis can then for example analyze changes in counts/sec given a specific voltage (e.g. 10 mV).
  • a specific voltage e.g. 10 mV
  • these factors can be used to provide an analogue for impedance magnitude and are further helpful in determining noise level as well.
  • embodiments of the invention combine current and capacitive analysis as a simple and elegant way to diagnose sensor health.
  • These computer program instructions may also be stored in a computer-readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instructions which implement the function specified in the flowchart block or blocks.
  • the computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart block or blocks, and/or menus presented herein.
  • FIG. 1 is a perspective view of a subcutaneous sensor insertion set and a block diagram of a sensor electronics device according to an embodiment of the invention.
  • a subcutaneous sensor set 10 is provided for subcutaneous placement of an active portion of a flexible sensor 12 (see FIG. 2 ), or the like, at a selected site in the body of a user.
  • the subcutaneous or percutaneous portion of the sensor set 10 includes a hollow, slotted insertion needle 14 , and a cannula 16 .
  • the needle 14 is used to facilitate quick and easy subcutaneous placement of the cannula 16 at the subcutaneous insertion site.
  • the one or more sensor electrodes 20 may include a counter electrode, a working electrode, and a reference electrode.
  • embodiments of the sensor comprise a plurality of working electrodes and/or counter electrodes and/or reference electrodes (e.g. 3 working electrodes, a reference electrode and a counter electrode), in order to, for example, provide redundant sensing capabilities.
  • the plurality of working, counter and reference electrodes are configured together as a unit and positionally distributed on the conductive layer in a repeating pattern of units.
  • the subcutaneous sensor set 10 facilitates accurate placement of a flexible thin film electrochemical sensor 12 of the type used for monitoring specific blood parameters (i.e. measurable and/or quantifiable characteristics) representative of a user's condition.
  • the sensor 12 monitors glucose levels in the body, and may be used in conjunction with automated or semi-automated medication infusion pumps of the external or implantable type as described in U.S. Pat. Nos. 4,562,751; 4,678,408; 4,685,903 or 4,573,994, to control delivery of insulin to a diabetic patient.
  • the flexible electrochemical sensor 12 are constructed in accordance with thin film mask techniques to include elongated thin film conductors embedded or encased between layers of a selected insulative material such as polyimide film or sheet, and membranes.
  • the sensor electrodes 20 at a tip end of the sensing portion 18 are exposed through one of the insulative layers for direct contact with patient blood or other body fluids, when the sensing portion 18 (or active portion) of the sensor 12 is subcutaneously placed at an insertion site.
  • the sensing portion 18 is joined to a connection portion 24 that terminates in conductive contact pads, or the like, which are also exposed through one of the insulative layers.
  • other types of implantable sensors such as chemical based, optical based, or the like, may be used.
  • connection portion 24 and the contact pads are generally adapted for a direct wired electrical connection to a suitable monitor or sensor electronics device 100 for monitoring a user's condition in response to signals derived from the sensor electrodes 20 .
  • a suitable monitor or sensor electronics device 100 for monitoring a user's condition in response to signals derived from the sensor electrodes 20 .
  • the connection portion 24 may be conveniently connected electrically to the monitor or sensor electronics device 100 or by a connector block 28 (or the like) as shown and described in U.S. Pat. No. 5,482,473, entitled FLEX CIRCUIT CONNECTOR, which is also herein incorporated by reference.
  • subcutaneous sensor sets 10 may be configured or formed to work with either a wired or a wireless characteristic monitor system.
  • the sensor electrodes 10 may be used in a variety of sensing applications and may be configured in a variety of ways.
  • the sensor electrodes 10 may be used in physiological parameter sensing applications in which some type of biomolecule is used as a catalytic agent.
  • the sensor electrodes 10 may be used in a glucose and oxygen sensor having a glucose oxidase enzyme catalyzing a reaction with the sensor electrodes 20 .
  • the sensor electrodes 10 along with a biomolecule or some other catalytic agent, may be placed in a human body in a vascular or non-vascular environment.
  • the sensor electrodes 20 and biomolecule may be placed in a vein and be subjected to a blood stream, or may be placed in a subcutaneous or peritoneal region of the human body.
  • the monitor 100 may also be referred to as a sensor electronics device 100 .
  • the monitor 100 may include a power source 110 , a sensor interface 122 , processing electronics 124 , and data formatting electronics 128 .
  • the monitor 100 may be coupled to the sensor set 10 by a cable 102 through a connector that is electrically coupled to the connector block 28 of the connection portion 24 . In an alternative embodiment, the cable may be omitted.
  • the monitor 100 may include an appropriate connector for direct connection to the connection portion 104 of the sensor set 10 .
  • the sensor set 10 may be modified to have the connector portion 104 positioned at a different location, e.g., on top of the sensor set to facilitate placement of the monitor 100 over the sensor set.
  • the sensor interface 122 the processing electronics 124 , and the data formatting electronics 128 are formed as separate semiconductor chips, however alternative embodiments may combine the various semiconductor chips into a single or multiple customized semiconductor chips.
  • the sensor interface 122 connects with the cable 102 that is connected with the sensor set 10 .
  • the power source 110 may be a battery.
  • the battery can include three series silver oxide 357 battery cells. In alternative embodiments, different battery chemistries may be utilized, such as lithium based chemistries, alkaline batteries, nickel metalhydride, or the like, and different number of batteries may used.
  • the monitor 100 provides power, through the power source 110 , provides power, through the cable 102 and cable connector 104 to the sensor set. In an embodiment of the invention, the power is a voltage provided to the sensor set 10 . In an embodiment of the invention, the power is a current provided to the sensor set 10 . In an embodiment of the invention, the power is a voltage provided at a specific voltage to the sensor set 10 .
  • FIGS. 2( a ) and 2 ( b ) illustrates an implantable sensor and electronics for driving the implantable sensor according to an embodiment of the present invention.
  • FIG. 2( a ) shows a substrate 220 having two sides, a first side 222 of which contains an electrode configuration and a second side 224 of which contains electronic circuitry.
  • a first side 222 of the substrate comprises two counter electrode-working electrode pairs 240 , 242 , 244 , 246 on opposite sides of a reference electrode 248 .
  • a second side 224 of the substrate comprises electronic circuitry.
  • the electronic circuitry may be enclosed in a hermetically sealed casing 226 , providing a protective housing for the electronic circuitry.
  • the sensor substrate 220 may be inserted into a vascular environment or other environment, which may subject the electronic circuitry to fluids.
  • the electronic circuitry may operate without risk of short circuiting by the surrounding fluids.
  • pads 228 are also shown in FIG. 2( a ) to which the input and output lines of the electronic circuitry may be connected.
  • the electronic circuitry itself may be fabricated in a variety of ways. According to an embodiment of the present invention, the electronic circuitry may be fabricated as an integrated circuit using techniques common in the industry.
  • FIG. 2( b ) illustrates a general block diagram of an electronic circuit for sensing an output of a sensor according to an embodiment of the present invention.
  • At least one pair of sensor electrodes 310 may interface to a data converter 312 , the output of which may interface to a counter 314 .
  • the counter 314 may be controlled by control logic 316 .
  • the output of the counter 314 may connect to a line interface 318 .
  • the line interface 318 may be connected to input and output lines 320 and may also connect to the control logic 316 .
  • the input and output lines 320 may also be connected to a power rectifier 322 .
  • the sensor electrodes 310 may be used in a variety of sensing applications and may be configured in a variety of ways.
  • the sensor electrodes 310 may be used in physiological parameter sensing applications in which some type of biomolecule is used as a catalytic agent.
  • the sensor electrodes 310 may be used in a glucose and oxygen sensor having a glucose oxidase enzyme catalyzing a reaction with the sensor electrodes 310 .
  • the sensor electrodes 310 along with a biomolecule or some other catalytic agent, may be placed in a human body in a vascular or non-vascular environment.
  • the sensor electrodes 310 and biomolecule may be placed in a vein and be subjected to a blood stream.
  • FIG. 3 illustrates a block diagram of a sensor electronics device and a sensor including a plurality of electrodes according to an embodiment of the invention.
  • the sensor set or system 350 includes a sensor 355 and a sensor electronics device 360 .
  • the sensor 355 includes a counter electrode 365 , a reference electrode 370 , and a working electrode 375 .
  • the sensor electronics device 360 includes a power supply 380 , a regulator 385 , a signal processor 390 , a measurement processor 395 , and a display/transmission module 397 .
  • the power supply 380 provides power (in the form of either a voltage, a current, or a voltage including a current) to the regulator 385 .
  • the regulator 385 transmits a regulated voltage to the sensor 355 . In an embodiment of the invention, the regulator 385 transmits a voltage to the counter electrode 365 of the sensor 355 .
  • the sensor 355 creates a sensor signal indicative of a concentration of a physiological characteristic being measured.
  • the sensor signal may be indicative of a blood glucose reading.
  • the sensor signal may represent a level of hydrogen peroxide in a subject.
  • the amount of oxygen is being measured by the sensor and is represented by the sensor signal.
  • the sensor signal may represent a level of oxygen in the subject.
  • the sensor signal is measured at the working electrode 375 .
  • the sensor signal may be a current measured at the working electrode.
  • the sensor signal may be a voltage measured at the working electrode.
  • the signal processor 390 receives the sensor signal (e.g., a measured current or voltage) after the sensor signal is measured at the sensor 355 (e.g., the working electrode).
  • the signal processor 390 processes the sensor signal and generates a processed sensor signal.
  • the measurement processor 395 receives the processed sensor signal and calibrates the processed sensor signal utilizing reference values. In an embodiment of the invention, the reference values are stored in a reference memory and provided to the measurement processor 395 .
  • the measurement processor 395 generates sensor measurements.
  • the sensor measurements may be stored in a measurement memory (not pictured).
  • the sensor measurements may be sent to a display/transmission device to be either displayed on a display in a housing with the sensor electronics or to be transmitted to an external device.
  • the sensor electronics device 350 may be a monitor which includes a display to display physiological characteristics readings.
  • the sensor electronics device 350 may also be installed in a desktop computer, a pager, a television including communications capabilities, a laptop computer, a server, a network computer, a personal digital assistant (PDA), a portable telephone including computer functions, an infusion pump including a display, a glucose sensor including a display, and or a combination infusion pump/glucose sensor.
  • PDA personal digital assistant
  • the sensor electronics device 350 may be housed in a blackberry, a network device, a home network device, or an appliance connected to a home network.
  • FIG. 4 illustrates an alternative embodiment of the invention including a sensor and a sensor electronics device according to an embodiment of the present invention.
  • the sensor set or sensor system 400 includes a sensor electronics device 360 and a sensor 355 .
  • the sensor includes a counter electrode 365 , a reference electrode 370 , and a working electrode 375 .
  • the sensor electronics device 360 includes a microcontroller 410 and a digital-to-analog converter (DAC) 420 .
  • DAC digital-to-analog converter
  • the sensor electronics device 360 may also include a current-to-frequency converter (I/F converter) 430 .
  • I/F converter current-to-frequency converter
  • the microcontroller 410 includes software program code, which when executed, or programmable logic which, causes the microcontroller 410 to transmit a signal to the DAC 420 , where the signal is representative of a voltage level or value that is to be applied to the sensor 355 .
  • the DAC 420 receives the signal and generates the voltage value at the level instructed by the microcontroller 410 .
  • the microcontroller 410 may change the representation of the voltage level in the signal frequently or infrequently.
  • the signal from the microcontroller 410 may instruct the DAC 420 to apply a first voltage value for one second and a second voltage value for two seconds.
  • the sensor 355 may receive the voltage level or value.
  • the counter electrode 365 may receive the output of an operational amplifier which has as inputs the reference voltage and the voltage value from the DAC 420 .
  • the application of the voltage level causes the sensor 355 to create a sensor signal indicative of a concentration of a physiological characteristic being measured.
  • the microcontroller 410 may measure the sensor signal (e.g., a current value) from the working electrode.
  • a sensor signal measurement circuit 431 may measure the sensor signal.
  • the sensor signal measurement circuit 431 may include a resistor and the current may be passed through the resistor to measure the value of the sensor signal.
  • the sensor signal may be a current level signal and the sensor signal measurement circuit 431 may be a current-to-frequency (I/F) converter 430 .
  • the current-to-frequency converter 430 may measure the sensor signal in terms of a current reading, convert it to a frequency-based sensor signal, and transmit the frequency-based sensor signal to the microcontroller 410 .
  • the microcontroller 410 may be able to receive frequency-based sensor signals easier than non-frequency-based sensor signals. The microcontroller 410 receives the sensor signal, whether frequency-based or non frequency-based, and determines a value for the physiological characteristic of a subject, such as a blood glucose level.
  • the microcontroller 410 may include program code, which when executed or run, is able to receive the sensor signal and convert the sensor signal to a physiological characteristic value. In an embodiment of the invention, the microcontroller 410 may convert the sensor signal to a blood glucose level. In an embodiment of the invention, the microcontroller 410 may utilize measurements stored within an internal memory in order to determine the blood glucose level of the subject. In an embodiment of the invention, the microcontroller 410 may utilize measurements stored within a memory external to the microcontroller 410 to assist in determining the blood glucose level of the subject.
  • the microcontroller 410 may store measurements of the physiological characteristic values for a number of time periods. For example, a blood glucose value may be sent to the microcontroller 410 from the sensor every second or five seconds, and the microcontroller may save sensor measurements for five minutes or ten minutes of BG readings.
  • the microcontroller 410 may transfer the measurements of the physiological characteristic values to a display on the sensor electronics device 450 .
  • the sensor electronics device 450 may be a monitor which includes a display that provides a blood glucose reading for a subject.
  • the microcontroller 410 may transfer the measurements of the physiological characteristic values to an output interface of the microcontroller 410 .
  • the output interface of the microcontroller 410 may transfer the measurements of the physiological characteristic values, e.g., blood glucose values, to an external device, e.g., such as an infusion pump, a combined infusion pump/glucose meter, a computer, a personal digital assistant, a pager, a network appliance, a server, a cellular phone, or any computing device.
  • an external device e.g., such as an infusion pump, a combined infusion pump/glucose meter, a computer, a personal digital assistant, a pager, a network appliance, a server, a cellular phone, or any computing device.
  • FIG. 5 illustrates an electronic block diagram of the sensor electrodes and a voltage being applied to the sensor electrodes according to an embodiment of the present invention.
  • an op amp 530 or other servo controlled device may connect to sensor electrodes 510 through a circuit/electrode interface 538 .
  • the op amp 530 utilizing feedback through the sensor electrodes, attempts to maintain a prescribed voltage (what the DAC may desire the applied voltage to be) between a reference electrode 532 and a working electrode 534 by adjusting the voltage at a counter electrode 536 . Current may then flow from a counter electrode 536 to a working electrode 534 .
  • Such current may be measured to ascertain the electrochemical reaction between the sensor electrodes 510 and the biomolecule of a sensor that has been placed in the vicinity of the sensor electrodes 510 and used as a catalyzing agent.
  • the circuitry disclosed in FIG. 5 may be utilized in a long-term or implantable sensor or may be utilized in a short-term or subcutaneous sensor.
  • a glucose oxidase enzyme is used as a catalytic agent in a sensor
  • current may flow from the counter electrode 536 to a working electrode 534 only if there is oxygen in the vicinity of the enzyme and the sensor electrodes 10 .
  • the voltage set at the reference electrode 532 is maintained at about 0.5 volts
  • the amount of current flowing from a counter electrode 536 to a working electrode 534 has a fairly linear relationship with unity slope to the amount of oxygen present in the area surrounding the enzyme and the electrodes.
  • increased accuracy in determining an amount of oxygen in the blood may be achieved by maintaining the reference electrode 532 at about 0.5 volts and utilizing this region of the current-voltage curve for varying levels of blood oxygen.
  • Different embodiments of the present invention may utilize different sensors having biomolecules other than a glucose oxidase enzyme and may, therefore, have voltages other than 0.5 volts set at the reference electrode.
  • a sensor 510 may provide inaccurate readings due to the adjusting of the subject to the sensor and also electrochemical byproducts caused by the catalyst utilized in the sensor.
  • a stabilization period is needed for many sensors in order for the sensor 510 to provide accurate readings of the physiological parameter of the subject. During the stabilization period, the sensor 510 does not provide accurate blood glucose measurements. Users and manufacturers of the sensors may desire to improve the stabilization timeframe for the sensor so that the sensors can be utilized quickly after insertion into the subject's body or a subcutaneous layer of the subject.
  • a typical embodiment of the invention is a method of observing a “state” of a sensor (e.g. a measurable and/or quantifiable characteristic of the sensor, for example one or more functional parameters of the sensor, one or more conditions or characteristics of the sensor etc.), the method comprising applying voltage to the sensor, observing a peak instantaneous electrical current of the sensor (e.g. the excitation voltage, namely the nominal voltage required for excitation of a circuit), and observing a total current of the sensor over a period of time for a predetermined frequency so that the state of the sensor is observed.
  • Such methods of the invention can be used to observe a number of sensor parameters.
  • observations of the peak instantaneous electrical current and/or the total current in the sensor over a period of time for a predetermined frequency can be used to estimate sensor impedance magnitude and/or sensor capacitance.
  • Certain embodiments of the invention can be used in manufacturing processes, for example to examine the uniformity of sensors manufactured according to a specific process (e.g. by performing the methods on a plurality of sensors; and then comparing the information so obtained on the state of the plurality of sensors).
  • Such methods can employ those disclosed herein as well as those known in the art and described for example in U.S. Pat. Nos. 7,185,300, 5,751,284, and 5,491,416, the contents of which are incorporated by reference.
  • Some embodiments of the invention can observe a single phenomena to characterize a state of a sensor such as a peak instantaneous electrical current of the sensor; or alternatively, a total current in the sensor over a period of time for a predetermined frequency.
  • Other embodiments of the invention can observe a combination of such phenomena to characterize a state of a sensor such as a peak instantaneous electrical current of the sensor and in addition a total current in the sensor over a period of time for a predetermined frequency. For example, if an observation of a first phenomena provides in an ambiguous result (e.g.
  • a peak instantaneous electrical current calculated value is too close to the threshold level, or the value indicates a previously used sensor
  • a second phenomena such as a maximum current value (counts/second) over the first 2 second of the pulse (e.g. and use a threshold to determine if the sensor is old or has hydration issues).
  • a threshold to determine if the sensor is old or has hydration issues.
  • an estimate of sensor capacitance comprises a voltage step analysis using a mathematical formula:
  • C comprises capacitance
  • V comprises voltage
  • dV comprises a controlled voltage step
  • dt comprises a length of time for analysis
  • t samp comprises a length of time between samples
  • dI comprises a change in current.
  • This formula can be used to provide a “capacitance” estimate, which also includes charge passing through the sensor for an estimation of “impedance magnitude”.
  • Such observations on the state or characteristics of the sensor are used to obtain information associated with sensor function in vivo, for example the amount or state of sensor hydration, sensor noise, sensor offset, sensor drift or the like.
  • embodiments of the invention can be used to observe sensor offset (i.e. the current not generated in response to glucose).
  • the stable-state current response of multiple controlled voltage steps can be used to calculate (by linear regression) the sensor's offset value.
  • one can observe/sample “stable current” e.g. capture current at the last second of each step
  • voltages such as 535, 545, 555, and 565 mV.
  • One can then calculate a linear regression on the points (y_regression mx+b).
  • the offset value is what the current value would read if the sensor did not detect any glucose. Consequently, one can subtract this calculated offset from a calibration signal to provide greater accuracy in displaying glucose values.
  • calculations of linear regression are well known in the art.
  • one simple way to calculate linear regression involves the use of least-squares estimation for linearity, in which the sum of squared error between theoretical and experimental points (current corresponding to each voltage level) is minimized using a set of partial derivatives.
  • such computations of linear regression is handled by a mathematical computational program such as MATLAB.
  • Embodiments of the invention can use a variety of different methodological steps.
  • the method comprises applying a voltage pulse to the sensor.
  • the method comprises observing the maximum current value (counts/second) during the initial 2 seconds in response to a voltage pulse applied to the sensor, and then comparing the maximum current value to a predetermined test value.
  • Other embodiments comprise methodological steps such as applying a plurality of voltages and/or currents to the sensor and/or observing current and/or voltage in the sensor over multiple periods of time and/or observing current and/or voltage in the sensor over multiple frequencies.
  • a voltage and/or a current applied to the sensor can be in a particular waveform known in the art, for example, a ramped waveform, a sinusoid-shaped waveform, a stepped waveform, a rectangular waveform, a triangular waveform, a trapezoidal waveform, a sawtooth waveform, a logarithmic waveform, a exponential waveform and the like (e.g. other waveforms known in the art).
  • a voltage step or the like
  • a current step or the like
  • R-C series resistor-capacitor
  • One exemplary embodiment of the invention that involves the application and manipulation of sensor current is a method of observing a state of a sensor having a plurality of electrodes, the method comprising: applying a voltage to the sensor; and then measuring a stable-state current (Ibase) produced in response to the voltage applied to the sensor.
  • Ibase stable-state current
  • deltaI comprises the difference between the Ibase and the second current.
  • one can then measure sensor voltage during the application of this second current. Following this measurement, one can then observe a first voltage step between the first voltage measurement and the second voltage measurement, a step that results from electrical resistance (R) in the sensor. In this way, the state of the sensor can observed using the application and manipulation of sensor current.
  • Certain embodiments of the invention that involve the manipulation of sensor current comprise further methodological steps. For example, after the methodological steps discussed in the paragraph above, one can then change applied current applied to the sensor to a third current comprising Ibase+deltaII, wherein deltaII comprises the difference between the Ibase and the third current.
  • a voltage slope so calculated can then be correlated to a change in sensor capacitance (C) that results from the different currents applied to the sensor.
  • one can calculate resistance (R) in the sensor using a formula R deltaV/deltaI.
  • one can calculate capacitance (C) in the sensor using a formula C deltaI/(dV/dt).
  • a user can start with a sensor having a normally applied stable-state voltage, Vbase (535 mV, for example).
  • Vbase normally applied stable-state voltage
  • the sensor system can be used to measure the stable-state current, Ibase, under normal operating conditions.
  • the system can switch from forcing voltage to forcing current, using the Ibase measured in the previous step.
  • this step can keep the sensor at the same voltage, while in other embodiments the sensor may stabilize at a slightly different voltage (and one can take into account any settling time required for this stabilization, if necessary).
  • the current can then be stepped to a new value, Ibase+deltaI.
  • This step will initially result in a voltage step, deltaV, due to resistance.
  • deltaV voltage step
  • dV/dt voltage slope
  • the above-noted embodiments of the invention that involve the application and manipulation of sensor current can be used to examine a variety of sensors in a variety of contexts.
  • the sensor is an electrochemical glucose sensor implanted in vivo and the observations on the state of the sensor provide information on sensor hydration, sensor noise, sensor offset, or sensor drift (e.g. to assess sensor start-up/initialization).
  • Other embodiments of the invention can be used to examine, for example, process variations between batches or lots of sensors made according to identical or non-identical manufacturing processes (e.g. by performing the method on a plurality of sensors made by differing manufacturing processes; and then comparing the information so obtained on the state of the plurality of sensors).
  • the embodiments of the invention that involve the application and manipulation of sensor current also include a sensor system, comprising an implantable sensor, the sensor including a plurality of electrodes; a sensor electronics device, the sensor electronics device capable of being operably connected to the sensor, and the sensor electronics device including: a connection detection device to determine if the sensor electronics device is connected to the sensor and to transmit a connection signal; a power source to supply a regulated voltage; a microprocessor; and a computer-readable program code having instructions, which when executed cause the microprocessor to apply and/or manipulate and/or measure sensor current and/or voltage as disclosed herein.
  • a sensor system comprising an implantable sensor, the sensor including a plurality of electrodes; a sensor electronics device, the sensor electronics device capable of being operably connected to the sensor, and the sensor electronics device including: a connection detection device to determine if the sensor electronics device is connected to the sensor and to transmit a connection signal; a power source to supply a regulated voltage; a microprocessor; and a computer-readable program code
  • the embodiments of the invention that involve the application and manipulation of sensor current also include a program code storage device, comprising: a computer-readable medium; a computer-readable program code, stored on the computer-readable medium, the computer-readable program code having instructions, which when executed cause a controller to apply and/or manipulate and/or measure sensor current and/or voltage as disclosed herein.
  • Embodiments of the invention can be adapted for use with a wide variety of electrochemical sensors such as glucose sensors that comprise glucose oxidase.
  • the glucose sensor comprises a base layer, at least three working electrodes disposed on the base layer, a glucose oxidase layer disposed upon the working electrodes, an analyte modulating layer disposed on the glucose oxidase layer, wherein the analyte modulating layer comprises a hydrogel composition, and an adhesion promoting layer disposed between the glucose oxidase layer and the analyte modulating layer.
  • the sensor comprise a plurality of working electrodes and/or counter electrodes and/or reference electrodes (e.g.
  • the plurality of working, counter and reference electrodes are configured together as a unit and positionally distributed on the conductive layer in a repeating pattern of units.
  • an element of the sensor such as an electrode or an aperture is designed to have a specific configuration and/or is made from a specific material and/or is positioned relative to the other elements so as to facilitate a function of the sensor.
  • a working electrode, a counter electrode and a reference electrode are positionally distributed on the base and/or the conductive layer in a configuration that facilitates sensor start up and/or maintains the hydration of the working electrode, the counter electrode and/or the reference electrode when the sensor is placed in contact with a fluid comprising the analyte (e.g. by inhibiting shadowing of an electrode, a phenomena which can inhibit hydration and capacitive start-up of a sensor circuit).
  • a fluid comprising the analyte e.g. by inhibiting shadowing of an electrode, a phenomena which can inhibit hydration and capacitive start-up of a sensor circuit.
  • Such embodiments of the invention facilitate sensor start-up and/or initialization.
  • embodiments of the sensor comprise a plurality of working electrodes and/or counter electrodes and/or reference electrodes (e.g. 3 working electrodes, a reference electrode and a counter electrode), in order to, for example, provide redundant sensing capabilities.
  • Certain embodiments of the invention comprise a single sensor.
  • Other embodiments of the invention comprise multiple sensors.
  • the plurality of working, counter and reference electrodes are configured together as a unit and positionally distributed on the conductive layer in a repeating pattern of units.
  • the elongated base layer is made from a flexible material that allows the sensor to twist and bend when implanted in vivo; and the electrodes are grouped in a configuration that facilitates an in vivo fluid contacting at least one of working electrode as the sensor twists and bends when implanted in vivo (e.g. so as to facilitate hydration).
  • the electrodes are grouped in a configuration that allows the sensor to continue to function if a portion of the sensor having one or more electrodes is dislodged from an in vivo environment and exposed to an ex vivo environment.
  • the working electrode, the counter electrode and the reference electrode are positionally distributed on conductive layer in a parallel configuration arranged so that a first electrode is disposed in a region on a first edge of the elongated base layer; a second electrode is disposed in a region on an opposite edge of the elongated base layer; and a third is disposed in a region of the elongated base layer that between the first electrode and the second electrode.
  • the working electrode, the counter electrode and the reference electrode are positionally distributed on conductive layer in a configuration arranged so that the working electrode is disposed in a region on a first edge of the elongated base layer; the counter electrode is disposed in a region on an opposite edge of the elongated base layer; and the reference electrode is disposed in a region of the elongated base layer that between the working electrode and the counter electrode.
  • an edge or center of a reference electrode is lined up with an edge or center of the working or counter electrode.
  • an edge or center of a reference electrode is offset with an edge or center of the working or counter electrode.
  • an electrode matrix is formed in the sensor so as to have no side walls in a manner that further improve hydration of the sensor electrodes.
  • Related embodiments of the invention include methods for using a distributed electrode configuration to facilitate and maintain the hydration and/or initialization properties of various sensor embodiments of the invention.
  • an element of the sensor such as an electrode or an aperture is designed to have a specific configuration and/or is made from a specific material and/or is positioned relative to the other elements so as to facilitate a function of the sensor.
  • sensor embodiments e.g. simple three electrode embodiments
  • sensor embodiments may be more susceptible to local environment changes around a single electrode. For example, a gas bubble on top of or close to a reference or another electrode, and/or a stagnating or semi-stagnating pool of fluid on top of or close to a reference or another electrode may consequently compromises sensor performance.
  • a distributed electrode configuration appears be advantageous because the distribution of the electrode area allows the sensor to compensate for signal lost to a small local area (e.g. as can occur due to lack of hydration, fluid stagnation, a patient's immune response, or the like).
  • a system which combines such sensor configurations and the methods for observing sensor parameters provides an optimized system that remedies a number of problems associated with, for example, imperfect sensor hydration.
  • distributed electrode configurations are used in methods designed to overcome problems with sensors and sensor systems that occur due to lack of hydration (e.g. slow start-up initialization times), fluid stagnation, a patient's immune response, or the like.
  • embodiments of the invention having distributed electrode configurations can be combined with certain complementary elements disclosed herein so as to further overcome problems that result from a lack of hydration, fluid stagnation, a patient's immune response, or the like (e.g. multiple electrode sensors, voltage pulsing methods etc.).
  • Various elements of the sensor can be disposed at a certain location in the sensor and/or configured in a certain shape and/or be constructed from a specific material so as to facilitate strength and/or function of the sensor.
  • One embodiment of the invention includes an elongated base comprised of a polyimmide or dielectric ceramic material that facilitates the strength and durability of the sensor.
  • the structural features and/or relative position of the working and/or counter and/or reference electrodes is designed to influence sensor manufacture, use and/or function.
  • the sensor is operatively coupled to a constellation of elements that comprise a flex-circuit (e.g. electrodes, electrical conduits, contact pads and the like).
  • One embodiment of the invention includes electrodes having one or more rounded edges so as to inhibit delamination of a layer disposed on the electrode (e.g. an analyte sensing layer comprising glucose oxidase).
  • Related embodiments of the invention include methods for inhibiting delamination of a sensor layer using a sensor embodiments of the invention (e.g. one having one or more electrodes having one or more rounded edges).
  • Embodiments of the invention include sensor systems such as those comprising an implantable sensor having a plurality of electrodes, a sensor electronics device that is capable of being operably connected to the sensor.
  • the sensor electronics device includes a connection detection device to determine if the sensor electronics device is connected to the sensor and to transmit a connection signal, a power source to supply a regulated voltage and a microprocessor.
  • Such systems typically include a computer-readable program code having instructions, which when executed cause the microprocessor to apply a voltage to the sensor and then record data on a peak instantaneous electrical current of the sensor in response to the applied voltage as well as record data on a total current of the sensor over a period of time for a predetermined frequency in response to the applied voltage.
  • one or more steps controlled by the microprocessor include applying a plurality of voltages to the sensor and/or applying a voltage pulse to the sensor and/or recording data on a current in the sensor over multiple periods of time and/or recording data on a current in the sensor over multiple frequencies.
  • the system further comprises a monitor for displaying the data recorded by the microprocessor, wherein the data displayed on the monitor provides information on sensor hydration, sensor noise, sensor offset, sensor drift or the like.
  • a test sensor e.g. a sensor phenomena noted herein such as peak instantaneous current, maximum current value, some combination of these phenomena or the like
  • a comparative reading from a control sensor know to be functioning within desired operating parameters and/or a predetermined range of values associated with such desired operating parameters.
  • the implantable sensor is a glucose sensor comprising a base layer, at least three working electrodes disposed on the base layer, a glucose oxidase layer disposed upon the working electrodes, an analyte modulating layer disposed on the glucose oxidase layer; and an adhesion promoting layer disposed between the glucose oxidase layer and the analyte modulating layer.
  • the sensor is implantable in tissue selected from the group consisting of subcutaneous, dermal, sub-dermal, intra-peritoneal, and peritoneal tissue.
  • Yet another embodiment of the invention is a program code storage device comprising a computer-readable medium, a computer-readable program code, stored on the computer-readable medium, the computer-readable program code having instructions, which when executed cause a controller to initiate a series of voltage pulses to be applied to a sensor comprising a plurality of electrodes; and receive a signal from a detection circuit, the signal indicating a peak instantaneous electrical current of the sensor in response to the applied voltage pulses; and a total current of the sensor over a period of time for a predetermined frequency in response to the applied voltage pulses.
  • the program code storage device includes instructions, which when executed causes the controller to determine the maximum current value (counts/second) during the initial 2 seconds in response to a voltage pulse applied to the sensor, and then compare the maximum current value so determined to a predetermined range of values.
  • the program code storage device includes instructions, which when executed causes the controller to then enable the sensor to measure a physiological characteristic of a patient when the maximum current value is within the predetermined range of values.
  • the program codes storage device includes instructions, which when executed cause a controller to initiate a sensor observation routine; and transmit a first signal to a digital-to-analog converter (DAC), the DAC being coupled to an electrode of a sensor, the first signal representative of a observation sequence of voltages that the DAC is to output to the electrode of the sensor, wherein the observation sequence of voltages includes: a first voltage applied for a first time frame; a second voltage applied for a second time frame; and a repeating of the application of the first voltage and the second voltage to the electrodes.
  • the program code storage device includes instructions, when executed cause the controller to repeat the application of the first voltage and the second voltage for a number of iterations.
  • the program code storage device includes instructions, which when executed cause the controller to: change a duration of the first amount of time and a duration of the second amount of time for at least one of the number of iterations.
  • the program code storage device includes instructions, which when executed cause the controller to: instruct the DAC to change a magnitude of the first voltage to be applied to the electrode of the sensor at least once during the repeating of the application of the first voltage; and/or instruct the DAC to change a magnitude of the second voltage to be applied to the electrode of the sensor at least once during the repeating of the application of the second voltage.
  • a voltage application device applies a first voltage to an electrode for a first time or time period.
  • the first voltage may be a DC constant voltage. This results in an anodic current being generated.
  • a digital-to-analog converter or another voltage source may supply the voltage to the electrode for a first time period.
  • the anodic current means that electrons are being driven away from electrode to which the voltage is applied.
  • an application device may apply a current instead of a voltage.
  • the voltage regulator may not apply a voltage for a second time, timeframe, or time period.
  • the voltage application device waits until a second time period elapses.
  • the non-application of voltage results in a cathodic current, which results in the gaining of electrons by the electrode to which the voltage is not applied.
  • the application of the first voltage to the electrode for a first time period followed by the non-application of voltage for a second time period is repeated for a number of iterations. This may be referred to as an anodic and cathodic cycle.
  • the number of total iterations of the methodological embodiment is three, i.e., three applications of the voltage for the first time period, each followed by no application of the voltage three times for the second time period.
  • the first voltage may be 1.07 volts.
  • the first voltage may be 0.535 volts.
  • the first voltage may be approximately 0.7 volts.
  • the result of the repeated application of the voltage and the non-application of the voltage results in the sensor (and thus the electrodes) being subjected to an anodic-cathodic cycle.
  • the anodic-cathodic cycle results in the reduction of electrochemical byproducts, which are generated by a patient's body reacting to the insertion of the sensor or the implanting of the sensor.
  • the electrochemical byproducts cause generation of a background current, which results in inaccurate measurements of the physiological parameter of the subject.
  • the electrochemical byproduct may be eliminated. Under other operating conditions, the electrochemical byproducts may be reduced or significantly reduced.
  • the first voltage being applied to the electrode of the sensor may be a positive voltage. In an embodiment of the invention, the first voltage being applied may be a negative voltage. In an embodiment of the invention, the first voltage may be applied to a working electrode. In an embodiment of the invention, the first voltage may be applied to the counter electrode or the reference electrode.
  • the duration of the voltage pulse and the no application of voltage may be equal, e.g., such as three minutes each.
  • the duration of the voltage application or voltage pulse may be different values, e.g., the first time and the second time may be different.
  • the first time period may be five minutes and the waiting period may be two minutes.
  • the first time period may be two minutes and the waiting period (or second timeframe) may be five minutes.
  • the duration for the application of the first voltage may be two minutes and there may be no voltage applied for five minutes. This timeframe is only meant to be illustrative and should not be limiting.
  • a first timeframe may be two, three, five or ten minutes and the second timeframe may be five minutes, ten minutes, twenty minutes, or the like.
  • the timeframes (e.g., the first time and the second time) may depend on unique characteristics of different electrodes, the sensors, and/or the patient's physiological characteristics.
  • more or less than three pulses may be utilized to stabilize the glucose sensor.
  • the number of iterations may be greater than 3 or less than three.
  • four voltage pulses e.g., a high voltage followed by no voltage
  • six voltage pulses may be applied to one of the electrodes.
  • three consecutive pulses of 1.07 volts may be sufficient for a sensor implanted subcutaneously.
  • three consecutive voltage pulses of 0.7 volts may be utilized.
  • the three consecutive pulses may have a higher or lower voltage value, either negative or positive, for a sensor implanted in blood or cranial fluid, e.g., the long-term or permanent sensors.
  • more than three pulses e.g., five, eight, twelve
  • the first voltage may be 0.535 volts applied for five minutes
  • the second voltage may be 1.070 volts applied for two minutes
  • the first voltage of 0.535 volts may be applied for five minutes
  • the second voltage of 1.070 volts may be applied for two minutes
  • the first voltage of 0.535 volts may be applied for five minutes
  • the second voltage of 1.070 volts may be applied for two minutes.
  • the pulsing methodology may be changed in that the second timeframe, e.g., the timeframe of the application of the second voltage may be lengthened from two minutes to five minutes, ten minutes, fifteen minutes, or twenty minutes.
  • a nominal working voltage of 0.535 volts may be applied after the three iterations are applied in this embodiment of the invention.
  • the 1.08 and 0.535 volts are illustrative values. Other voltage values may be selected based on a variety of factors. These factors may include the type of enzyme utilized in the sensor, the membranes utilized in the sensor, the operating period of the sensor, the length of the pulse, and/or the magnitude of the pulse. Under certain operating conditions, the first voltage may be in a range of 1.00 to 1.09 volts and the second voltage may be in a range of 0.510 to 0.565 volts. In other operating embodiments, the ranges that bracket the first voltage and the second voltage may have a higher range, e.g., 0.3 volts, 0.6 volts, 0.9 volts, depending on the voltage sensitivity of the electrode in the sensor.
  • the voltage may be in a range of 0.8 volts to 1.34 volts and the other voltage may be in a range of 0.335 to 0.735.
  • the range of the higher voltage may be smaller than the range of the lower voltage.
  • the higher voltage may be in a range of 0.9 to 1.09 volts and the lower voltage may be in a range of 0.235 to 0.835.
  • the first voltage and the second voltage may be positive voltages, or alternatively in other embodiments of the invention, negative voltages.
  • the first voltage may be positive and the second voltage may be negative, or alternatively, the first voltage may be negative and the second voltage may be positive.
  • the first voltage may be different voltage levels for each of the iterations.
  • the first voltage may be a D.C. constant voltage.
  • the first voltage may be a ramp voltage, a sinusoid-shaped voltage, a stepped voltage, or other commonly utilized voltage waveforms.
  • the second voltage may be a D.C.
  • the first voltage or the second voltage may be an AC signal riding on a DC waveform.
  • the first voltage may be one type of voltage, e.g., a ramp voltage
  • the second voltage may be a second type of voltage, e.g., a sinusoid-shaped voltage.
  • the first voltage (or the second voltage) may have different waveform shapes for each of the iterations.
  • the first voltage in a first cycle, may be a ramp voltage, in the second cycle, the first voltage may be a constant voltage, and in the third cycle, the first voltage may be a sinusoidal voltage.
  • a duration of the first timeframe and a duration of the second timeframe may have the same value, or alternatively, the duration of the first timeframe and the second timeframe may have different values.
  • the duration of the first timeframe may be two minutes and the duration of the second timeframe may be five minutes and the number of iterations may be three.
  • the methodological steps of various embodiments of the invention may include a number of iterations.
  • the duration of each of the first timeframes may change and the duration of each of the second timeframes may change.
  • the first timeframe may be 2 minutes and the second timeframe may be 5 minutes.
  • the first timeframe may be 1 minute and the second timeframe may be 3 minutes.
  • the first timeframe may be 3 minutes and the second timeframe may be 10 minutes.
  • a first voltage of 0.535 volts is applied to an electrode in a sensor for two minutes to initiate an anodic cycle, then a second voltage of 1.07 volts is applied to the electrode to the sensor for five minutes to initiate a cathodic cycle.
  • the first voltage of 0.535 volts is then applied again for two minutes to initiate the anodic cycle and a second voltage of 1.07 volts is applied to the sensor for five minutes.
  • 0.535 volts is applied for two minutes to initiate the anodic cycle and then 1.07 volts is applied for five minutes.
  • the voltage applied to the sensor is then 0.535 during the actual working timeframe of the sensor, e.g., when the sensor provides readings of a physiological characteristic of a subject.
  • Shorter duration voltage pulses may be utilized.
  • the shorter duration voltage pulses may be utilized to apply the first voltage, the second voltage, or both.
  • the magnitude of the shorter duration voltage pulse for the first voltage is ⁇ 1.07 volts and the magnitude of the shorter duration voltage pulse for the second voltage is approximately half of the high magnitude, e.g., ⁇ 0.535 volts.
  • the magnitude of the shorter duration pulse for the first voltage may be 0.535 volts and the magnitude of the shorter duration pulse for the second voltage is 1.07 volts.
  • the voltage may not be applied continuously for the entire first time period. Instead, in the first time period, the voltage application device may transmit a number of short duration pulses during the first time period. In other words, a number of mini-width or short duration voltage pulses may be applied to the electrodes of the sensors over the first time period. Each mini-width or short duration pulse may a width of a number of milliseconds. Illustratively, this pulse width may be 30 milliseconds, 50 milliseconds, 70 milliseconds or 200 milliseconds. These values are meant to be illustrative and not limiting. In an embodiment of the invention, these short duration pulses are applied to the sensor (electrode) for the first time period and then no voltage is applied for the second time period.
  • each short duration pulse may have the same time duration within the first time period.
  • each short duration voltage pulse may have a time width of 50 milliseconds and each pulse delay between the pulses may be 950 milliseconds. In this example, if two minutes is the measured time for the first timeframe, then 120 short duration voltage pulses may be applied to the sensor.
  • each of the short duration voltage pulses may have different time durations.
  • each of the short duration voltage pulses may have the same amplitude values. In an embodiment of the invention, each of the short duration voltage pulses may have different amplitude values.
  • short duration voltage pulses By utilizing short duration voltage pulses rather than a continuous application of voltage to the sensors, the same anodic and cathodic cycling may occur and the sensor (e.g., electrodes) is subjected to less total energy or charge over time.
  • the use of short duration voltage pulses utilizes less power as compared to the application of continuous voltage to the electrodes because there is less energy applied to the sensors (and thus the electrodes).
  • embodiments of the innovation include a voltage generation device.
  • the voltage generation or application device typically includes electronics, logic, or circuits, which generate voltage pulses.
  • the sensor electronics device may also include a input device to receive reference values and other useful data.
  • the sensor electronics device may include a measurement memory to store sensor measurements.
  • the power supply may supply power to the sensor electronics device.
  • the power supply may supply power to a regulator, which supplies a regulated voltage to the voltage generation or application device.
  • the voltage generation or application device supplies a voltage, e.g., the first voltage or the second voltage, to an input terminal of an operational amplifier.
  • the voltage generation or application device may also supply the voltage to a working electrode of the sensor.
  • Another input terminal of the operational amplifier is coupled to the reference electrode of the sensor.
  • the application of the voltage from the voltage generation or application device to the operational amplifier drives a voltage measured at the counter electrode to be close to or equal the voltage applied at the working electrode.
  • the voltage generation or application device could be utilized to apply the desired voltage between the counter electrode and the working electrode. This may occur by the application of the fixed voltage to the counter electrode directly.
  • the voltage generation device generates a first voltage that is to be applied to the sensor during a first timeframe.
  • the voltage generation device then transmits this first voltage to an op amp, which drives the voltage at a counter electrode of the sensor to the first voltage.
  • the voltage generation device also could transmit the first voltage directly to the counter electrode of the sensor.
  • the voltage generation device does not transmit the first voltage to the sensor for a second timeframe. In other words, the voltage generation device is turned off or switched off.
  • the voltage generation device may be programmed to continue cycling between applying the first voltage and not applying a voltage for either a number of iterations or for a predetermined timeframe, e.g., for twenty minutes.
  • the voltage regulator transfers the regulated voltage to the voltage generation device.
  • a control circuit controls the closing and opening of a switch. If the switch is closed, the voltage is applied. If the switch is opened, the voltage is not applied.
  • the timer provides a signal to the control circuit to instruct the control circuit to turn on and off the switch.
  • the control circuit includes logic which can instruct the circuit to open and close the switch a number of times (to match the necessary iterations).
  • the timer may also transmit a signal to identify that the sequence is completed, i.e. that a predetermined timeframe has elapsed.
  • the voltage generation device generates a first voltage for a first timeframe and generates a second voltage for a second timeframe for example by using a two-voltage switch.
  • the voltage generation device generates a first voltage for the first timeframe.
  • timer sends a signal to the control circuit indicating the first timeframe has elapsed and the control circuit directs the switch to move to the second position.
  • the switch is at the second position, the regulated voltage is directed to a voltage step-down or buck converter to reduce the regulated voltage to a lesser value. The lesser value is then delivered to the op amp for the second timeframe.
  • the control circuit moves the switch back to the first position.
  • the voltage application device may include a control device, a switch, a sinusoid generation device, a ramp voltage generation device, and a constant voltage generation device.
  • the voltage application may generate an AC wave on top of a DC signal or other various voltage pulse waveforms.
  • the control device may cause the switch to move to one of the three voltage generation systems (sinusoid, ramp, and/or constant DC). This results in each of the voltage regulation systems generating the identified voltage waveform.
  • the control device may cause the switch to connect the voltage from the voltage regulator to the sinusoid voltage generator in order for the voltage application device to generate a sinusoidal voltage.
  • the control device may cause the switch, during the first timeframes in the anodic/cathodic cycles, to move between connecting the voltage from the voltage generation or application device to the ramp voltage generation system, then to the sinusoidal voltage generation system, and then to the constant DC voltage generation system.
  • the control device may also be directing or controlling the switch to connect certain ones of the voltage generation subsystems to the voltage from the regulator 385 during the second timeframe, e.g., during application of the second voltage.
  • Embodiments of the invention may include a microcontroller, a digital-to-analog converter (DAC), an op amp, and a sensor signal measurement circuit.
  • the sensor signal measurement circuit may be a current-to-frequency I/F) converter.
  • software or programmable logic in the microcontroller provides instructions to transmit signals to the DAC, which in turn instructs the DAC to output a specific voltage to the operational amplifier.
  • the microcontroller may also be instructed to output a specific voltage to the working electrode. As discussed above, the application of the specific voltage to operational amplifier and the working electrode may drive the voltage measured at the counter electrode to the specific voltage magnitude.
  • the microcontroller outputs a signal, which is indicative of a voltage or a voltage waveform that is to be applied to the sensor (e.g., the operational amplifier coupled to the sensor).
  • a fixed voltage may be set by applying a voltage directly from the DAC between the reference electrode and the working electrode.
  • a similar result may also be obtained by applying voltages to each of the electrodes with the difference equal to the fixed voltage applied between the reference and working electrode.
  • the fixed voltage may be set by applying a voltage between the reference and the counter electrode.
  • the microcontroller may generates a pulse of a specific magnitude which the DAC understands represents that a voltage of a specific magnitude is to be applied to the sensor.
  • the microcontroller After a first timeframe, the microcontroller (via the program or programmable logic) outputs a second signal, which either instructs the DAC to output no voltage or to output a second voltage.
  • the microcontroller after the second timeframe has elapsed, then repeats the cycle of sending the signal indicative of a first voltage to apply, (for the first timeframe) and then sending the signal to instruct no voltage is to be applied or that a second voltage is to be applied (for the second timeframe).
  • the microcontroller may generate a signal to the DAC, which instructs the DAC to output a ramp voltage. Under other operating conditions, the microcontroller may generate a signal to the DAC, which instructs the DAC to output a voltage simulating a sinusoidal voltage. These signals could be incorporated into any of the pulsing methodologies discussed above in the preceding paragraph or earlier in the application. In an embodiment of the invention, the microcontroller may generate a sequence of instructions and/or pulses, which the DAC receives and understands to mean that a certain sequence of pulses is to be applied.
  • the microcontroller may transmit a sequence of instructions (via signals and/or pulses) that instruct the DAC to generate a constant voltage for a first iteration of a first timeframe, a ramp voltage for a first iteration of a second timeframe, a sinusoidal voltage for a second iteration of a first timeframe, and a squarewave having two values for a second iteration of the second timeframe.
  • the microcontroller may include programmable logic or a program to continue this cycling for a timeframe or for a number of iterations.
  • the microcontroller may include counting logic to identify when the first timeframe or the second timeframe has elapsed. Additionally, the microcontroller may include counting logic to identify that a timeframe has elapsed. After any of the preceding timeframes have elapsed, the counting logic may instruct the microcontroller to either send a new signal or to stop transmission of a signal to the DAC.
  • the microcontroller allows a variety of voltage magnitudes to be applied in a number of sequences for a number of time durations.
  • the microcontroller may include control logic or a program to instruct the digital-to-analog converter to transmit a voltage pulse having a magnitude of approximately 1.0 volt for a first time period of 1 minute, to then transmit a voltage pulse having a magnitude of approximately 0.5 volts for a second time period of 4 minutes, and to repeat this cycle for four iterations.
  • the microcontroller may be programmed to transmit a signal to cause the DAC to apply the same magnitude voltage pulse for each first voltage in each of the iterations.
  • the microcontroller may be programmed to transmit a signal to cause the DAC to apply a different magnitude voltage pulse for each first voltage in each of the iterations. In this embodiment of the invention, the microcontroller may also be programmed to transmit a signal to cause the DAC to apply a different magnitude voltage pulse for each second voltage in each of the iterations.
  • the microcontroller may be programmed to transmit a signal to cause the DAC to apply a first voltage pulse of approximately one volt in the first iteration, to apply a second voltage pulse of approximately 0.5 volts in the first iteration, to apply a first voltage of 0.7 volts and a second voltage of 0.4 volts in the second iteration, and to apply a first voltage of 1.2 and a second voltage of 0.8 in the third iteration.
  • the microcontroller may also be programmed to instruct the DAC to provide a number of short duration voltage pulses for a first timeframe. In this embodiment of the invention, rather than one voltage being applied for the entire first timeframe (e.g., two minutes), a number of shorter duration pulses may be applied to the sensor. In this embodiment, the microcontroller may also be programmed to program the DAC to provide a number of short duration voltage pulses for the second timeframe to the sensor. Illustratively, the microcontroller may send a signal to cause the DAC to apply a number of short duration voltage pulses where the short duration is 50 milliseconds or 100 milliseconds. In between these short duration pulses the DAC may apply no voltage or the DAC may apply a minimal voltage.
  • the DAC may cause the microcontroller to apply the short duration voltage pulses for the first timeframe, e.g., two minutes.
  • the microcontroller may then send a signal to cause the DAC to either not apply any voltage or to apply the short duration voltage pulses at a magnitude of a second voltage for a second timeframe to the sensor, e.g., the second voltage may be 0.75 volts and the second timeframe may be 5 minutes.
  • the microcontroller may send a signal to the DAC to cause the DAC to apply a different magnitude voltage for each of short duration pulses in the first timeframe and/or in the second timeframe.
  • the microcontroller may send a signal to the DAC to cause the DAC to apply a pattern of voltage magnitudes to the short durations voltage pulses for the first timeframe or the second timeframe.
  • the microcontroller may transmit a signal or pulses instructing the DAC to apply thirty 20 millisecond pulses to the sensor during the first timeframe. Each of the thirty 20 millisecond pulses may have the same magnitude or may have a different magnitude.
  • the microcontroller may instruct the DAC to apply short duration pulses during the second timeframe or may instruct the DAC to apply another voltage waveform during the second timeframe.
  • a current may also be applied to the sensor in certain embodiments of the invention.
  • a first current may be applied during a first timeframe to initiate an anodic or cathodic response and a second current may be applied during a second timeframe to initiate the opposite anodic or cathodic response.
  • the application of the first current and the second current may continue for a number of iterations or may continue for a predetermined timeframe.
  • a first current may be applied during a first timeframe and a first voltage may be applied during a second timeframe.
  • one of the anodic or cathodic cycles may be triggered by a current being applied to the sensor and the other of the anodic or cathodic cycles may be triggered by a voltage being applied to the sensor.
  • a current applied may be a constant current, a ramp current, a stepped pulse current, or a sinusoidal current. Under certain operating conditions, the current may be applied as a sequence of short duration pulses during the first timeframe.
  • spectroscopic analysis a spectrum of wavelengths applied to a sample generates a corresponding response specific to its material properties. Data from an unknown sample can be analyzed with respect to calibrated response profiles in an attempt to guess at its material properties.
  • electrochemical impedance spectroscopy the complex impedance of an object at different frequencies will vary depending on its size (i.e. low vs. high surface area) and material content (i.e. platinum vs. gold).
  • potentiometric EIS two or more leads are connected across a sample while the potential at one end is cycled through various frequencies with a set peak-to-peak voltage. The resulting current between two leads is recorded. Using excitation voltage and reading current over a full period (or multiple periods) for a particular frequency, one is able to extract the complex impedance (magnitude and phase).
  • a voltage step is performed for 60 seconds during which dV is controlled (10 mV), dt is controlled (1-60 sec), and dI is calculated from the difference between Isig and Ibase (instantaneous current vs. pre-step current).
  • the sensor current will track with any change in interstitial glucose/hydrogen peroxide concentration (as well as interfering compounds, i.e. acetaminophen). As a result, any changes in voltage will scale current values depending on the transient systemic glucose concentration at the time.
  • the capacitance measurement obtained in 100 mg/dl (standard) glucose concentration can be smaller than one obtained in 300 mg/dl (elevated glucose level).
  • a scale factor for the capacitance measurement based on the current values obtained before a pulse is initiated can solve this problem. Effectively, analyzing a properly scaled, differential capacitance measurement can overcome any possible errors due to varying glucose concentration (assume constant glucose concentration over one sample). It may be possible to avoid using a scale factor and rely on differential current to calculate our stored charge variable.
  • a second concern is in the identification of sensor failures.
  • a sensor enters failure mode with either delamination or a general loss of sensitivity. Delamination is characterized by a significant increase in signal noise (may be easier to identify based on current stability), whereas loss of sensitivity refers to instances in which the sensor no longer follows with glucose concentration.
  • impedance magnitude will change with membrane delamination.
  • changes in impedance may be difficult to detect: bad sensors may be indistinguishable from the low capacitance of initialized, working sensor. Sensors that lose their sensitivity may or may not have changes in their complex impedance—it remains to be seen the degree (if at all) of difference in capacitance between working and failed sensors.
  • sensors are inserted within a flow system simulating interstitial fluid in the body and left unpowered until glucose concentration is confirmed (via YSI) to be stable at approximately 100 mg/dt.
  • Individual sensors will be powered by independent potentiostats of a Gamry E-Chem system. Further modifications to glucose concentration, temperature, or gas content will be performed depending on experimental requirements.
  • Capacitance was calculated as described in sections 7.3 and 9.2.3. The results are uniform across multiple sensors: there is a significant difference in measured capacitance between pre- and post-initialized sensors. Additionally, the post-initialization samples show very little drift (system noise) between measurements over time.
  • the capacitance reading at 10 sec is marked on the left and right edges in red. The lone high mark represents capacitance at pre-initialization levels. The other grouping comes from the various samples taken at different time points after initialization. A lack of failing sensor data limits what one can determine from such demonstrations, but alternative embodiments can be used to examine capacitance in sensors that have become non-sensitive to changes in glucose concentration—whether by membrane delamination or in our case, enzyme degradation.
  • sensors could potentially be improperly identified as initialized sensors.
  • a mostly dry sensor would have high impedance (low current), indicative of a sensor that has already gone through initialization. This misrepresentation of sensor state may be overlooked for our purposes: sensors should not be initialized until they are fully hydrated. However, it may be difficult to categorize or separate initialized sensors from those that are only partially hydrated.
  • glucose concentration will drift anywhere between 40 mg/dl and 400 mg/dl.
  • the resulting sensor response would drive changes in Isig and capacitance, possibly pushing post-nit pulse sampling into the range of pre-init samples. EIS sampling at different glucose concentrations has not indicated any significant impedance drift.
  • sensors inserted into PTS were initialized in two separate concentrations of glucose: 107.3 mg/dl and 289.7 mg/dl (100 or 300 mg/dl, respectively).
  • Pre-initialized samples were taken after running the sensor at 535 mV for 5 minutes; initialized data was sampled 9 hours after the completion of the initialization sequence.
  • the experimental sequence taken here was the same as in 10.1: running over-potential of 535 mV with intermediate 60 sec pulsing of 10 mV to 545 mV.
  • the data provides evidence that the concentration of glucose does not affect the results of the differential pulse analysis technique.
  • sensors were inserted in PTS, initialized, and run for several hours in the 300 mg/dl glucose solution used in the concentration dependence test. After 24 hours, the glucose concentration was increased to 397 mg/dl (400 mg/dl) and the oxygen content of the solution was displaced using nitrogen gas. The sensors were then run for a 36-hour period in which pulse testing and EIS were conducted side-by-side every 3 or 6 hours.
  • failed sensors can have the following properties: 1) low sensitivity to glucose (high CF), and 2) no significant change in impedance magnitude. Given these assumptions, it may be difficult to analyze sensor differences using the pulse test method.
  • Working sensors have a high impedance (can be the same with failed sensors), but are sensitive to changes in glucose concentration. Ideally, a pulse test would be able to identify and differentiate between newly inserted, working (post-init), and failing sensors.
  • EIS sampling at 0.1 Hz performed in sensors with CF>50 showed impedance magnitude and phase measurements that could be mischaracterized as either non-intialized or initialized sensors.
  • An alternative may be to choose specific excitation frequencies for different diagnostic checks ⁇ 0.1 Hz excitation to differentiate between initialized sensors and non-initialized ones, 100 Hz to differentiate between new sensors and failing ones. Combining this information with other tests (different excitation frequencies) may allow the system to quickly diagnose the state of the sensor.
  • This example illustrates test methods, data extraction, and analysis involved in testing the current response to small voltage pulse as a method to differentiate between new and used sensors.
  • the complex impedance of a sensor could be used as a diagnostic identifier for sensor life and age (via electrochemical impedance spectroscopy, EIS).
  • EIS electrochemical impedance spectroscopy
  • One alternative to traditional impedance spectroscopy is to analyze the response of current to a small voltage pulse.
  • the sensor transmitter may be able to differentiate between sensors that are inserted and ready, those that are inserted but not completely hydrated, and those that have been inserted and initialized.
  • the voltage pulse test can provide a close estimation of the imaginary and real components of impedance across an electrode.
  • this technique can be implemented within existing hardware to set limits on sensor use and help to identify whether sensors can be initialized (prevents re-initialization damage on accidental disconnects).
  • the diagnostic test can also allow for hydration detection, determining not only whether sensors can be initialized but when (see, e.g. FIGS. 11 and 12 ).
  • sensors with modified firmware were connected for 3 days each to 174 sensors on 18 dogs, spread out over a two-week period.
  • the firmware was modified to add in the pulse-test functionality, and all analysis was conducted post-download using MATLAB.
  • the pulse test was hard-coded to run before initialization, 30 min after initialization, and every three hours following, as shown for example in FIGS. 13 and 14 .
  • the sensor was extracted and the data from the sensor was downloaded in HTML format.
  • the sensor memory was cleared and the unit was placed on the charger in preparation for reuse with a new sensor.
  • the script takes into account the number of true positives and false positives, assigning a 400% relative weight to true positives.
  • the relative scores for combinations of thresholds are tabulated and the highest scores are annotated in the output.
  • the goal of threshold modification is to minimize the amount of false hits (both negative and positive). Strong emphasis is placed on reducing false negatives (a new sensor marked as used is a larger problem than an old sensor marked as new). All analyzed scores are saved to a comma-separated file in a specified directory as threshold_data.csv.
  • the thresholds were set arbitrarily, with the “current amplitude” threshold. Once the dataset was extracted, the threshold analysis script was run on the full dataset. The high and low thresholds with the highest score are listed below (they provided identical results). Deviation from these thresholds would result in either a larger number of false positives or an increase in false negatives.
  • each line corresponds to a different pulse test (in time) on the same sensor.
  • the first plot displays the change in counts over the 20 sec-time period in A+B (instantaneous current response).
  • the second plot outputs the differential sum of counts over 10 sec in region B (count amplitude). Note the lack of y-axis scaling control between sensors—use the blue line (second check) as a reference in comparison to the red line (first check) in the following figures.
  • thresholds When moving to clinical trials and human data, thresholds will more than likely need to be revised. With enough human data, the threshold selection script should be able to accurately supply thresholds that will maximize our used-sensor detection (true negative) while minimizing our misidentified new sensors (false positive).
  • One consideration may be to increase the number of allowed false positives (new sensors recognized as old ones) in order to reduce the number of false negatives (used sensors that are recognized as new ones).
  • the working theory related to false negatives is that they are a direct result of sensor hydration issues. In fact, if the analysis script is modified to ignore Instantaneous Current and only check Total Charge (section 8.2, Individual Lines), resulting failed sensors exhibit a consistent “sleepy” sensor profile (verified by running through the TGMS2 algorithm)—slow run-in time and low sensitivity to glucose concentration for the first 12 hours. If “sleepy sensors” can be eliminated (either by use of revised tubing designs or by the feedback check detailed in FIG. 12 ), then the number of false negatives should decrease significantly. At the moment, full hydration remains a difficult problem in our analysis of the voltage pulse test.
  • thresholds may be determined from which a system will be able to clearly differentiate between newly inserted and initialized sensors. To differentiate between working sensors and those which have lost sensitivity (or high-amplitude noise) will be substantially higher, and should depend on current-sensing resolution.
  • Thresholds may require modification depending on electrode design, substrate makeup, and process changes. Any change in the sensor design (narrow electrode vs. Distributed electrodes vs. Standard production layout) will result in a change in capacitive response. To reconcile changes in design, the system will need to modify either 1) the thresholds for different sensor states, or 2) the stimulus amplitude to maintain comparable responses.

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US8160834B2 (en) 2012-04-17
WO2010078263A2 (fr) 2010-07-08
EP2959829B1 (fr) 2020-05-06
EP2959829A1 (fr) 2015-12-30
WO2010078263A3 (fr) 2010-12-02
CA2747830A1 (fr) 2010-07-08
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US20110230741A1 (en) 2011-09-22
EP2369977B1 (fr) 2015-08-12

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