Abstract:
Analysis equipment is provided, which is capable of fulfilling a demand for miniaturization and ensuring high sensitivity, and which can be produced easily. The present invention relates to a method of continuously measuring a substrate concentration based on a response when a voltage is applied to a sensor. The present invention includes a response voltage application step of applying a response voltage E2 at which a response attributed to a substrate is obtained and a non-response voltage application step of applying a non-response voltage E1 at which the response attributed to the substrate is not obtained or is not substantially obtained. Preferably, the response voltage application step and the non-response voltage application step are repeated alternately.

Description:
TECHNICAL FIELD  
       [0001]    The present invention relates to a method of continuously measuring a substrate concentration, such as glucose. In particular, the present invention relates to a method of continuously measuring a blood sugar level, using a glucose sensor implanted in a body. 
       BACKGROUND ART 
       [0002]    As a method of measuring a blood sugar level, there is a method of continuously measuring a concentration of glucose in interstitial fluid, using a glucose sensor, for example, implanted in the abdomen or the arm of a human body. As the measurement principle, an electrochemical method is mainly adopted. The glucose sensor in this case has at least a working electrode and a counter electrode. At the working electrode, for example, glucose oxidase (GOD) is immobilized. The concentration of glucose is continuously measured based on a response current (see  FIG. 12(   b )) obtained when a constant voltage of about 0.3 to 0.6 V is continuously applied across the working electrode and the counter electrode (see  FIG. 12(   a )). 
         [0003]    The value of the response current can be obtained, for example, by electrochemically oxidizing hydrogen peroxide generated by a catalytic reaction of GOD (for example, Patent documents 1 and 2), or by using electrons extracted from glucose via GOD with an Os polymer being a mediator (see, for example, Patent document 3). 
         [0004]    On the other hand, the glucose concentration is calculated based on a sampling current obtained by sampling a current periodically from the continuously obtained response current values. 
         [0005]    However, the response current contains background components attributed to coexisting substances such as ascorbic acid in interstitial fluid or attributed to an external environment such as electromagnetic noise. Therefore, for example, even if measurement data tends to increase (or decrease), it is difficult to ascertain specifically from the response current whether the increase (or decrease) in the measurement data is caused by an increase (or decrease) in the glucose concentration, or whether the increase (or decrease) in the measurement data is caused by an increase (or decrease) in the background components. Therefore, under the environment that is likely to be influenced by the background components, the precision of a signal of the response current obtained from the glucose sensor may be degraded. 
         [0006]    Further, it is understood that a glucose oxidoreductase fixed at the working electrode causes protein to be denatured under the application of a voltage, which degrades the stability of a sensor. It is understood that such a degradation in stability depends upon the magnitude of the voltage and the application time of the voltage, as illustrated in  FIG. 13  illustrating residual activity in the case of using glucose dehydrogenase (GDH) as the glucose oxidoreductase. More specifically, the stability of the glucose oxidoreductase such as GDH tends to be degraded as the magnitude of the voltage to be applied is larger or the application time of the voltage is longer. 
       [Citation List] 
     [Patent Literature] 
       [0007]    [PTL 1] JP 2007-516814 A 
         [0008]    [PTL 2] JP 2007-535991 A 
         [0009]    [PTL 3] JP 10-505421 A 
       SUMMARY OF INVENTION  
     Technical Problem  
       [0010]    It is an object of the present invention to improve the measurement precision and the stability of a sensor to be used for measurement in continuous measurement of a substrate such as glucose. 
       Solution to Problem  
       [0011]    The present invention provides a method of continuously measuring a substrate concentration based on a response when a voltage is applied to a substrate, including: a response voltage application step of applying a response voltage at which a response attributed to the substrate is obtained; and a non-response voltage application step of applying a non-response voltage at which the response attributed to the substrate is not obtained or is not substantially obtained. 
         [0012]    It is preferred that the response voltage application step and the non-response voltage application step be repeated alternately. Each of the response voltage application step and the non-response voltage application step does not necessarily need to be conducted by applying a single pulse at a constant voltage, and may be conducted, for example, by applying a stepped pulse composed of a combination of a plurality of pulses of a constant voltage with different potentials. 
         [0013]    In the present invention, for example, the substrate is calculated based on a difference between the response obtained in the response voltage application step and the response obtained in the non-response voltage application step. In this case, when a voltage to be applied to a sensor is increased, the voltage applied in the non-response voltage application step is set in a range of −0.5 to +0.5 V, preferably in a range of −25 to +25 mV with respect to an applied voltage (reaction potential) at which the response attributed to the substrate starts being obtained. 
         [0014]    The present invention may further include a standby step of applying a voltage smaller in absolute value than the non-response voltage after performing the response voltage application step and the non-response voltage application step alternately a predetermined number of times. The standby step may be performed by setting a circuit including the sensor to become an open circuit. 
         [0015]    The sensor is implanted, for example, in a body for use. The sensor may include an enzyme-immobilized electrode. Needless to say, the present invention may be applied to the case of measuring the concentration of a substrate such as glucose in blood or interstitial fluid extracted out of a body or the case of measuring a substrate in substrate-containing fluid other than blood and interstitial fluid or fluid outside of a human body. 
         [0016]    The substrate is, for example, glucose, and the sensor is, for example, a glucose sensor. The glucose sensor may include an electrode with a glucose oxidoreductase immobilized thereon. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS  
         [0017]    [ FIG. 1 ]  FIG. 1  is a cross-sectional view illustrating an example of a glucose continuous measurement apparatus for carrying out a continuous measurement method of glucose. 
           [0018]    [ FIG. 2 ]  FIG. 2  is an entire perspective view illustrating a glucose sensor in the glucose continuous measurement apparatus illustrated in  FIG. 1 , together with a main portion enlarged view. 
           [0019]    [ FIG. 3 ]  FIG. 3  is a block diagram illustrating a schematic configuration of the glucose continuous measurement apparatus illustrated in  FIG. 1 . 
           [0020]    [ FIG. 4 ]  FIG. 4  is a graph illustrating an example of a voltage application pattern with respect to the glucose sensor illustrated in  FIG. 2 . 
           [0021]    [ FIG. 5 ]  FIG. 5  is a graph illustrating an example of a response current of when a voltage is applied in the pattern illustrated in  FIG. 4 . 
           [0022]    [ FIG. 6 ]  FIG. 6  is a graph illustrating measurement results of voltammetry with respect to a plurality of glucose concentrations. 
           [0023]    [ FIG. 7 ]  FIGS. 7  are graphs each illustrating another example of an application pulse of a reaction voltage and a non-reaction voltage in one step. 
           [0024]    [ FIG. 8 ]  FIG. 8  is a graph illustrating measurement results of voltammetry in Example 1. 
           [0025]    [ FIG. 9 ]  FIGS. 9(   a ) and  9 ( b ) are graphs illustrating measurement results of a change with time of a response current in Example 1. 
           [0026]    [ FIG. 10 ]  FIG. 10  is a graph illustrating measurement results of response characteristics in Example 1. 
           [0027]    [ FIG. 11 ]  FIG. 11  is a graph illustrating evaluation results of stability in Example 2. 
           [0028]    [ FIG. 12 ]  FIG. 12(   a ) is a graph illustrating a pattern of voltage application with respect to a glucose sensor in a conventional example, and  FIG. 12(   b ) is a graph illustrating an example of a response current with respect to the voltage application illustrated in  FIG. 12(   a ). 
           [0029]    [ FIG. 13 ]  FIG. 13  is a graph illustrating a change with time of residual activity of enzyme in the conventional example. 
           [0030]    [ FIG. 14 ]  FIG. 14  is a graph illustrating a change in current density in the case of applying a stepped pulse voltage as illustrated in  FIG. 7(   a ) repeatedly to the enzyme-immobilized electrode. 
           [0031]    [ FIG. 15 ]  FIG. 15  is a graph illustrating a relationship between the current density and the glucose concentration. 
           [0032]    [ FIG. 16 ]  FIG. 16  is a graph illustrating measurement results of a response current in the case of applying a pulse voltage repeatedly to the enzyme-immobilized electrode. 
           [0033]    [ FIG. 17 ]  FIG. 17  is a graph illustrating measurement results of a response current in the case of applying a constant voltage of 600 mV continuously to the enzyme-immobilized electrode. 
           [0034]    [ FIG. 18 ]  FIG. 18  is a graph illustrating evaluation results of stability in Example 4. 
           [0035]    [ FIG. 19 ]  FIG. 19  is a graph illustrating a change in response current due to an increasing glucose concentration in the case of applying a constant voltage continuously to the enzyme-immobilized electrode. 
           [0036]    [ FIG. 20 ]  FIG. 20  is a graph illustrating a change in response current due to an increasing glucose concentration in the case of applying a pulse voltage repeatedly to the enzyme-immobilized electrode. 
           [0037]    [ FIG. 21 ]  FIG. 21  is a graph illustrating the response current I 3  in the case of applying a constant voltage continuously to the enzyme-immobilized electrode, and the response current I 4  in the case of applying a pulse voltage repeatedly to the enzyme-immobilized electrode. 
       
    
    
     DESCRIPTION OF EMBODIMENTS  
       [0038]    Hereinafter, a method of substrate continuous measurement according to the present invention is described with reference to the drawings, illustrating a case of measuring a glucose concentration in body fluid. 
         [0039]    A glucose continuous measurement apparatus  1  illustrated in  FIG. 1  is capable of continuously measuring the concentration of glucose in blood or interstitial fluid, and is attached for use to the abdomen or the shoulder skin of a human body. The glucose continuous measurement apparatus  1  includes a housing  2 , a circuit board  3 , and a glucose sensor  4 . 
         [0040]    The housing  2  forms an outer shape of the glucose continuous measurement apparatus  1 , and includes a cover  20  and a base  21 . The cover  20  and the base  21  contain the circuit board  3  in a space defined by the cover  20  and the base  21 , and are fixed to each other. The housing  2  preferably has a waterproof property or water resistance. In the housing  2 , for example, at least the cover  20  (base  21  if required) is formed of a material with very low water permeability such as metal or polypropylene resin. 
         [0041]    The base  21  is a portion through which the glucose sensor  4  is inserted, and an end  40  of the glucose sensor  4  is fixed on the base  21 . An adhesive film  5  is fixed on the base  21 . The adhesive film  5  is used when the glucose continuous measurement apparatus  1  is fixed to the skin. As the adhesive film  5 , a tape having adhesion on its both surfaces is used. 
         [0042]    The circuit board  3  is equipped with electronic components required for predetermined operations (for example, the application of a voltage, the arithmetic operation of a glucose concentration, or the communication with the outside) of the glucose continuous measurement apparatus  1 . The circuit board  3  further includes a terminal  30  for coming into contact with an electrode  42  of the glucose sensor  4  (see  FIG. 2 ) described later. The terminal  30  is used for applying a voltage to the glucose sensor  4  and obtaining a response current value from the glucose sensor  4 . 
         [0043]    The glucose sensor  4  is used for obtaining a response in accordance with a glucose concentration in blood or interstitial fluid. In the glucose sensor  4 , the end  40  protrudes from a skin  6  and is in contact with the terminal  30  of the circuit board  3 , and the most remaining part thereof is inserted in the skin  6 . 
         [0044]    As illustrated in  FIG. 2 , the glucose sensor  4  includes a base  41 , the electrode  42 , and an immobilized enzyme part  43 . 
         [0045]    The base  41  supports the electrode  42  and is formed in a sheet shape having insulation and flexibility. In the base  41 , an end  41 A is present in the housing  2  while an end  41 B is formed so as to be sharp. If the end  10 B is configured so as to be sharp, the glucose sensor  4  can be inserted in the skin  6  easily, which can alleviate the pain to a user. 
         [0046]    As a material for the base  40 , any material having appropriate insulation without causing any damage to a human body can be used, and for example, thermoplastic resin such as PET (Polyethylene Terephthalate), PP (polypropylene), or PE (polyethylene), or thermosetting resin such as polyimide resin or epoxy resin can be used. 
         [0047]    The electrode  42  is used for applying a voltage to the immobilized enzyme part  43  and extracting electrons from the immobilized enzyme part  43 . The electrode  42  includes a working electrode  42 A and a counter electrode  42 B. The working electrode  42 A transfers electrons with respect to glucose. The counter electrode  42 B is used for applying a voltage together with the working electrode  42 A. The electrode  42  can be formed by screen printing using carbon ink. 
         [0048]    The immobilized enzyme part  43  mediates the transfer of electrons between glucose and the working electrode  42 A. The immobilized enzyme part  43  is formed by immobilizing glucose oxidoreductase at an end  42 Aa of the working electrode  42 A. 
         [0049]    As the glucose oxidoreductase, glucose oxidase (GOD) and glucose dehydrogenase (GDH) can be used. As a method of immobilizing the glucose oxidoreductase, various known methods, for example, methods using a polymer such as polymerizable gel, polyacrylamide or phosphorus, an MPC polymer in which a silane coupling agent is introduced into a phospholipid polymer, or a protein membrane can be adopted. 
         [0050]    As illustrated in  FIG. 3 , the glucose continuous measurement apparatus  1  further includes a communication unit  10 , a power source  11 , a control unit  12 , an arithmetic operation unit  13 , and a storage unit  14 , in addition to the circuit board  3  and the glucose sensor  4 . 
         [0051]    The communication unit  10  is used for data communication between the glucose continuous measurement apparatus  1  and an external information processing terminal. The communication unit  10  includes at least a transmitting unit, and includes a receiving unit if required. 
         [0052]    For data communication, for example, wireless communication means (IrDA using infrared radiation or Bluetooth using a frequency band of 2.4 GHz) can be used. Needless to say, wired data communication may be performed by connecting the communication unit  10  of the glucose continuous measurement apparatus  1  to a communication unit of the external information processing terminal with a cable or the like. 
         [0053]    An example of the external information processing terminal includes an insulin supply apparatus for administering insulin to a human body, a simple blood sugar level measurement apparatus, a watch-type display, or a personal computer. 
         [0054]    The data communication between the glucose continuous measurement apparatus  1  and the insulin supply apparatus is performed, for example, by sending measurement results of a glucose concentration in the glucose continuous measurement apparatus  1  to the insulin supply apparatus. This enables an insulin amount to be administered to a human body to be controlled based on the measurement data from the glucose continuous measurement apparatus  1 . 
         [0055]    The data communication between the glucose continuous measurement apparatus  1  and the simple blood sugar level measurement apparatus is performed, for example, by sending blood sugar level measurement results in the simple blood sugar level measurement apparatus to the glucose continuous measurement apparatus  1 . Thus, the measurement results in the glucose continuous measurement apparatus  1  are compared with the measurement results in the simple blood sugar level measurement apparatus, and if there is a deviation by a predetermined value or more is present between those measurement results, the glucose continuous measurement apparatus  1  can be calibrated. Further, raw data (response current) measured in the glucose continuous measurement apparatus  1  may be sent to the simple blood sugar level measurement apparatus. 
         [0056]    The data communication between the glucose continuous measurement apparatus  1  and the watch-type display is performed, for example, by sending the blood sugar level measurement results in the glucose continuous measurement apparatus  1  to the watch-type display. Consequently, even when the glucose continuous measurement apparatus  1  is attached to a position such as the shoulder, which a user has difficulty in visually recognizing, the measurement results in the glucose continuous measurement apparatus  1  can be displayed on the watch-type display so that the user can grasp the measurement results. 
         [0057]    The data communication between the glucose continuous measurement apparatus  1  and a personal computer is performed, for example, by sending blood sugar level measurement results or raw data (response current) in the glucose continuous measurement apparatus  1  to the personal computer. This allows the transition of a glucose concentration to be monitored on the personal computer. Further, data for calibration from the personal computer may be sent to the glucose continuous measurement apparatus  1 . 
         [0058]    The power source  11  is a DC power source for supplying power to the circuit board  3  and the glucose sensor  4 . As the power source  11 , for example, a button battery with a power supply voltage of 1 to 3 V is used. 
         [0059]    The control unit  12  controls various operations such as the control of a timing of voltage application and an applied voltage value, the sampling of a response current, the arithmetic operation of a glucose concentration, and the communication with the external information processing terminal. 
         [0060]    The arithmetic operation unit  13  performs various arithmetic operations such as those required for the operation of the glucose continuous measurement apparatus  1 , in addition to the arithmetic operation of a glucose concentration, for example. 
         [0061]    The storage unit  14  stores programs and data (for example, data on a calibration curve, correction data, or data on a voltage application pattern) required for various arithmetic operations. The storage unit  14  may further store a response current value from the glucose sensor  4  and a glucose concentration obtained by an arithmetic operation. 
         [0062]    The control unit  12 , the arithmetic operation unit  13 , and the storage unit  14  are realized by electronic components mounted on the circuit board  3 , such as a CPU (or an MPU), a ROM, and a RAM. 
         [0063]    Here, the application of a voltage to the glucose sensor  4  in the glucose continuous measurement apparatus  1  is performed, for example, in a pattern illustrated in  FIG. 4 . In the illustrated example, assuming a combination of a state in which a non-reaction voltage E 1  is applied for a predetermined time T 2  and a state in which a reaction voltage E 2  is applied for a predetermined time T 1  as one step (one pulse), the step is performed repeatedly, and a standby time T 3  is set after a plurality of the steps. Further,  FIG. 5  illustrates an example of a response current in the case of the voltage application pattern illustrated in  FIG. 4 . 
         [0064]    The non-reaction voltage E 1  is a voltage at which glucose does not react at all or at which glucose reacts so slightly as not to react substantially (see  FIG. 6 ). More specifically, a response current obtained when the non-reaction voltage E 1  is applied to the glucose sensor  4  contains a very small response amount attributed to glucose, and is mainly attributed to a background (sensitivity) peculiar to the sensor, a background attributed to coexisting substances, and a background attributed to external environment such as electromagnetic noise. 
         [0065]    The magnitude of the non-reaction voltage E 1  is set in accordance with the specifications (for example, the amount of enzyme to be used, an immobilizing method, constituent materials for electrodes, or a reaction area) of the glucose sensor  4 . Specifically, the non-reaction voltage E 1  is set at a voltage (reaction potential) at which a response current attributed to glucose starts flowing when a voltage applied to the glucose sensor  4  is increased or in a range of −0.5 to +0.5 V (preferably −25 to +25 mV) from the reaction potential. In the illustrated glucose sensor  4 , the non-reaction voltage E 1  is set at, for example, −1 to 1 V. Further, the application time T 2  of the non-reaction voltage E 1  in one step is set at a time required for a response current to be stabilized to a constant value, e.g., 0.1 to 300 sec. 
         [0066]    The reaction voltage E 2  is a potential at which glucose reacts with sufficient resolution (see  FIG. 6 ). The magnitude of the reaction voltage E 2  is set in accordance with the specifications (for example, the amount of enzyme to be used, an immobilizing method, constituent materials for electrodes, or a reaction area) of the glucose sensor  4 , and is set at, for example, −1 to 1 V. Further, the application time T 1  of the reaction voltage E 2  in one step is set at a time required for a rapid change of a response current to disappear, for example, 0.1 to 300 micro sec. 
         [0067]    The non-reaction voltage E 1  and the reaction voltage E 2  do not necessarily need to be applied to the glucose sensor  4  as a single pulse of a constant voltage in one step as illustrated in  FIG. 4 , and may be applied to the glucose sensor  4 , for example, as a stepped pulse of a combination of a plurality of pulses of constant voltages with different potentials as illustrated in  FIGS. 7(   a ) to  7 ( d ). In the case where the non-reaction voltage E 1  and the reaction voltage E 2  are applied as a stepped pulse in one step, the number of constant voltages maybe two or four or more although the number of constant voltages is three in  FIGS. 7(   a ) to  7 ( d ). 
         [0068]    The standby time T 3  corresponds to the state in which a standby voltage E 3  is applied as a voltage applied to the glucose sensor  4 , which is further smaller than the non-reaction voltage E 1  in absolute value. The standby time T 3  is for preventing an unnecessarily large voltage from being applied to the glucose sensor  4 , and is set, for example, at 0.1 to 300 sec. The application voltage (standby voltage) E 3  during the standby time T 3  is set at 0 V or more and less than the non-reaction voltage E 1  in absolute value. Here, as a method of setting the standby voltage E 3  at 0 V, there is a method of turning off a main power source, in addition to a method of setting the voltage applied to the glucose sensor  4  at 0 V while applying a driving voltage to the circuit board  3 . Needless to say, the standby time T 3  is not necessarily required, and further, the standby time T 3  may be provided arbitrarily after one step or a plurality of steps (for example, 2 to 1,500 steps) including the non-reaction voltage E 1  and the reaction voltage E 2 . 
         [0069]    In the application voltage pattern as de scribed above, during the arithmetic operation of a glucose concentration, a response current I 2 (n) is sampled at the reaction voltage E 2 , while a response current I 1 (n), during the application of a non-reaction voltage E 1  in the same step as the step during which the response current I 2 (n) is sampled, is sampled. Respective sampling timings T′ 1  and T′ 2  for current values obtained during the application of the voltages E 1  and E 2  are provided to a state in which a fluctuation of a response current is small and a response current is relatively stabilized. 
         [0070]    It should be noted that impedance, energy, or quantity of electricity, for example, may be obtained as a response in place of sampling of a response current. 
         [0071]    On the other hand, in the arithmetic operation unit  13 , a glucose concentration is calculated considering a response current sampled during the application of the non-reaction voltage E 1 . Typically, a glucose concentration is arithmetically operated from a response current value (I2(n)−I1(n)) obtained by subtracting the response current I 1 (n) at the non-reaction voltage E 1  from the response current I 2 (n) at the reaction voltage E 2 . 
         [0072]    An glucose concentration may be calculated once in one cycle including a plurality of steps, or may be performed in one cycle selected from a plurality of cycles. Further, in one cycle, a glucose concentration may be subjected to arithmetic operation for each step to determine an average value of the arithmetic operation results as a glucose concentration of that step. 
         [0073]    Further, a threshold value of a measured value or a threshold value regarding the difference from that at the previous measurement may be set regarding the response current I 2 (n) at the reaction voltage E 2 , the response current I 1 (n) at the non-reaction voltage E 1 , or a difference value (I2(n)−I1(n)) of the response current values, and comparison may be made with respect to the threshold value before the arithmetic operation of a glucose concentration. This makes it possible to determine whether each parameter is an appropriate value to be used for an arithmetic operation or determine whether or not to perform an arithmetic operation of a glucose concentration (measurement of glucose). Consequently, in the glucose continuous measurement apparatus  1 , the quantification of glucose with higher precision can be performed. 
         [0074]    The glucose concentration obtained by the arithmetic operation is used mainly for determining the amount of administration of insulin and calibrating the glucose continuous measurement apparatus  1  if required. For the arithmetic operation of a glucose concentration for calibration, it is preferred to adopt an average value of a glucose concentration calculated for each step selected arbitrarily from each of a plurality of cycles. 
         [0075]    As described above, a response current with respect to the non-reaction voltage E 1  is not related to or not substantially related to a glucose reaction, and is mainly attributed to background components. Therefore, a glucose concentration can be calculated while minimizing the influence of the background components if a response current corresponding to the non-reaction voltage E 1  is considered, to thereby enhance measurement precision. 
         [0076]    Further, in a pattern in which the non-reaction voltage E 1  and the reaction voltage E 2  are applied to the glucose sensor  4  repeatedly as a voltage application pattern, compared with the case where a constant voltage is applied continuously, a voltage (energy) to be applied to the glucose sensor  4  per unit time is decreased. Therefore, the degradation caused by the denaturation and the like of glucose oxidoreductase can be suppressed, and hence, the sensitivity of the glucose sensor  4  can be stabilized over a long period of time and the life of the glucose sensor can be prolonged. Consequently, the replacement frequency of the glucose sensor  4  and the calibration frequency of the glucose continuous measurement apparatus  1  can be reduced, which alleviates an operational burden and an economical burden on a user. Further, if a voltage (energy) to be applied to the glucose sensor  4  can be decreased, the power consumption in the case of measuring glucose continuously can be reduced, and hence, the life of the battery can be prolonged. As a result, in the case of using a disposable battery such as a button battery as a battery, the economical burden on the user can be alleviated. In particular, if the standby time T 3  is provided and the power source is turned off during the standby time T 3 , the life of the battery can be prolonged securely. 
         [0077]    According to the glucose continuous measurement apparatus  1  described above, the arithmetic operation function of a glucose concentration is incorporated in the glucose continuous measurement apparatus  1 , but the external information processing terminal may be allowed to calculate a glucose concentration while a response value such as a response current value is measured in the glucose continuous measurement apparatus  1 . 
         [0078]    Further, in the above-mentioned embodiment, the case where the reaction voltage E 2  is applied after the non-reaction voltage E 1  is applied is defined as one cycle (one pulse). However, the case where the non-reaction voltage E 1  is applied after the reaction voltage E 2  is applied may be defined as one cycle (one pulse). 
         [0079]    The present invention can be applied to the case of using a substrate other than glucose or substrate-containing fluid other than body fluid, as well as to the case of measuring a glucose concentration in body fluid of a human body. 
         [0080]    Next, the effects of the present invention are studied as examples. 
       Example 1  
       [0081]    In this example, response characteristics of when a pulse voltage is applied repeatedly to an enzyme-immobilized electrode were studied. 
         [0082]    (Production of Enzyme-Immobilized Electrode) 
         [0083]    An enzyme-immobilized electrode was produced by immobilizing GDH on the surface of a carbon electrode via a phospholipid polymer. For producing the carbon electrode, first, 100 micro liter of liquid paraffin was added to powder mixed thoroughly with 120 mg of Ketjen Black (produced by Lion Corporation), followed by mixing well to form a material paste. The material paste was packed into a base electrode for producing a paste electrode with an area of 3 phi (produced by BAS Inc.), and compressed with a JURAN connecting rod to obtain a carbon electrode. As the phospholipid polymer, an MPC polymer (trade name: “LIPIDURE”, produced by NOF Corporation) with tetraethoxysilane introduced thereinto as a silane coupling agent was used. Enzyme immobilization was performed by soaking the carbon electrode modified with the phospholipid in 1,250 U/mL of a GDH solution. 
         [0084]    (Setting of a Voltage to be Applied) 
         [0085]    A voltage to be applied was determined based on results obtained by measuring voltage dependency of a glucose response by voltammetry, using an enzyme-immobilized electrode produced previously. 
         [0086]    The glucose concentration was set at 600 mg/dL in voltammetry. 
         [0087]    As an electrode system in voltammetry, the enzyme-immobilized electrode produced previously as a working electrode, a platinum electrode as a counter electrode, and a silver/silver chloride electrode as a reference electrode were used. 
         [0088]    The sweep in voltammetry was conducted in a positive direction and a negative direction, respectively, setting a sweep speed at 0.1 V/sec in a range of 0 to 0.6 V.  FIG. 8  illustrates measurement results of voltammetry. 
         [0089]    As seen from  FIG. 8 , it is understood that an oxidation current responding to glucose is detected at an applied voltage of around 75 mV. From this result, regarding the non-response voltage E 1  that does not cause a glucose reaction or does not substantially cause a glucose reaction, a constant voltage of 100 mV was set considering a margin in the following study. On the other hand, regarding the reaction voltage E 2  exhibiting a glucose response, a constant voltage of 700 mV was set, assuming that the potential difference from the non-response voltage E 1  was set at 600 mV. 
         [0090]    (Pattern of an Applied Voltage) 
         [0091]    The application times T 1  and T 2  of the non-response voltage E 1  and the response voltage E 2  were set at 10 seconds. 
         [0092]    The standby time T 3  was set at 40 seconds. It should be noted that the standby time T 3  was set so that an electrode system including an enzyme-immobilized electrode became an open circuit (0 V) every four steps (four pulses). 
         [0093]    (Measurement of Response Characteristics of Glucose) 
         [0094]    The response characteristics were measured by adding glucose at a particular timing while rotating a stirring bar with a stirrer under the condition that an enzyme-immobilized electrode (working electrode), a platinum electrode (counter electrode), and a silver/silver chloride electrode (reference electrode) were soaked in a container containing the stirring bar together with water. 
         [0095]    Glucose was added during the standby time T 3  in the fourth cycle, with four steps (four pulses) being one cycle. Glucose was added three times, and the concentration of glucose was changed successively from 0 mg/dL, to 50 mg/dL, 100 mg/dL, and 600 mg/dL. 
         [0096]    A response current was measured, setting the non-reaction voltage E 1  and the reaction voltage E 2  at constant voltages of 0 mV and 600 mV, respectively.  FIG. 9(   a ) illustrates measurement results of the response current. For reference,  FIG. 9(   b ) illustrates the case where a response current was measured, setting the non-reaction voltage E 1  and the reaction voltage E 2  at 100 mV and 700 mV, respectively. 
         [0097]    Further, the current values I 1  and I 2  after 10 seconds of the application of the non-response voltage E 1  and the response voltage E 2  were sampled, respectively, and  FIG. 10  illustrates a relationship between a difference value between the current values I 1  and I 2  and the glucose concentration. In  FIG. 10 , the difference value of the response currents is illustrated as a value (current density) obtained by dividing by the area of the enzyme-immobilized electrode. 
         [0098]    As seen from  FIGS. 9(   a ) and  9 ( b ), it was confirmed that a current value increased due to the addition of glucose. More specifically, it was confirmed that glucose reacted to the response voltage E 2  to thereby generate a response current. 
         [0099]    Further, in the case where the non-response voltage E 1  was set at 100 mV when an applied voltage was decreased from the response voltage E 2  to the non-response voltage E 1 , a fluctuation amount (background) of a response current became remarkably smaller, compared with the case of setting the non-response voltage E 1  at 0 mV. 
         [0100]    On the other hand, as illustrated in  FIG. 10 , in the case where the non-response voltage E 1  was set at 100 mV, compared with the case of setting the non-response voltage E 2  at 0 mV, the difference (I 2 −I 1 ) between the response current I 2  during the application of the response voltage E 2  and the response current I 1  during the application of the non-response voltage E 1  decreases irrespective of the glucose concentration. It was also understood from this point that, in the case of setting the non-response voltage E 1  at 100 mV, i.e., in the case of setting the non-response voltage E 1  to be larger than 0V, the influence by background is small and response sensitivity is high. 
       Example 2  
       [0101]    In this example, stability of an enzyme-immobilized electrode when a pulse voltage is applied repeatedly to the enzyme-immobilized electrode was evaluated. 
         [0102]    (Production of Enzyme-Immobilized Electrode) 
         [0103]    The enzyme-immobilized electrode was produced by immobilizing GDH on the surface of a carbon electrode via a phospholipid polymer. The carbon electrode was formed on the surface of a polyimide base material. As a material for the electrode, printing ink containing a mixture of 40 wt % of Ketjen Black (produced by Lion Corporation), 40 wt % of polyester resin as a binder, and 20 wt % of isophorone as a solvent was used. The printing ink was printed onto the surface of the polyimide base material to a thickness of 10 micro meters, and dried under environment of 150 deg C. for 30 minutes to obtain an electrode. As the phospholipid polymer, an MPC polymer (trade name: “LIPIDURE”, produced by NOF Corporation) with tetraethoxysilane introduced thereinto as a silane coupling agent was used. Enzyme immobilization was performed by soaking the carbon electrode modified with the phospholipid in 1,250 U/mL of a GDH solution. 
         [0104]    (Evaluation of Stability) 
         [0105]    The stability was evaluated as the sensitivity of the enzyme-immobilized electrode after a response current was measured continuously using 600 mg/dL of a glucose solution. 
         [0106]    The electrode system and the applied voltage pattern were set to be the same as those in Example 1. It should be noted that the non-response voltage E 1  and the response voltage E 2  were set at 100 mV and 700 mV, respectively. 
         [0107]    The voltage application time was set at 18 hours in total, and the sensitivity of the enzyme-immobilized electrode was measured every 6 hours as a response current. For comparison, the sensitivity in the case of applying a constant voltage of 600 mV continuously was evaluated.  FIG. 11  illustrates the sensitivity in these measurement methods as relative values of response currents with respect to initial sensitivity (0-hour response current). 
         [0108]    As seen from  FIG. 11 , according to the method of applying a constant voltage continuously as a comparative example, a response current was decreased with time, and thus, the degradation in an enzyme electrode was observed. In contrast, according to a method of applying a pulse voltage repeatedly, the decrease in a response current with time was not observed, and the stability of the enzyme-immobilized electrode was confirmed to be satisfactory. In this respect, GDH is denatured and deteriorated by application of the voltage, but a response voltage application time in the method of applying a pulse voltage repeatedly totally becomes shorter as compared to the method of applying a constant voltage continuously in the same measurement time. Accordingly, it is understood that the shorter response voltage application time suppresses GDH from being deteriorated, and maintains the stability of GDH. 
       Example 3  
       [0109]    In Example 3, studies were made on a performance evaluation of a stepped pulse voltage. The production of an enzyme-immobilized electrode, and an electrode system are the same as those in Example 1.  FIG. 14  is a graph illustrating a change in current density in the case of applying a stepped pulse voltage as illustrated in  FIG. 7(   a ) repeatedly to the enzyme-immobilized electrode. The current density is a value calculated by dividing a response current value by the area of the enzyme-immobilized electrode. In  FIG. 14 , the stepped pulse voltage is represented by a dashed line, and the current density is represented by a solid line. One pulse is assumed to be a combination of a state where the non-response voltage E 1  is applied, a state where the response voltage E 2  is applied, and a state where the intermediate voltage is applied. For the voltage to be applied, the non-response voltage E 1  was set at a constant voltage of 300 mV, the response voltage E 2  was set at a constant voltage of 600 mV, and the intermediate voltage was set at a constant voltage of 450 mV. 
         [0110]      FIG. 15  is a graph illustrating a relationship between the current density and the glucose concentration. Each of the response current I 1  obtained after application of the non-response voltage E 1  and the response current I 2  obtained after application of the response voltage E 2  was sampled to calculate a difference value between the response current I 1  and the response current I 2 . The current density is a value calculated by dividing the difference value between the response currents by the area of the enzyme-immobilized electrode. 
         [0111]      FIG. 15  illustrates, as the stepped pulse voltage, a pulse voltage assuming a combination of a state where the non-response voltage E 1  is applied, a state where the response voltage E 2  is applied, and a state where the intermediate voltage is applied as one pulse. For the voltage to be applied, the non-response voltage E 1  was set at a constant voltage of 300 mV, the response voltage E 2  was set at a constant voltage of 600 mV, and the intermediate voltage was set at a constant voltage of 450 mV.  FIG. 15  also illustrates, as a normal pulse voltage, a pulse voltage assuming a combination of a state where the non-response voltage E 1  is applied and a state where the response voltage E 2  is applied as one pulse. For the voltage to be applied, the non-response voltage E 1  was set at a constant voltage of 300 mV, and the response voltage E 2  was set at a constant voltage of 600 mV. 
         [0112]    As illustrated in  FIG. 15 , both the stepped pulse voltage and the normal pulse voltage exhibit an increased glucose concentration and an increased current density. Thus, it was confirmed that the stepped pulse voltage had almost the same performance as compared to the normal pulse voltage. 
       Example 4  
       [0113]    In Example 4, in the case of using GOD as a glucose oxidoreductase, studies were made on response characteristics in the case of applying a pulse voltage repeatedly to the enzyme-immobilized electrode. 
         [0114]    (Production of Enzyme-Immobilized Electrode) 
         [0115]    Platinum was deposited on the surface of a polyimide base material by a sputtering method to a thickness of 30 nm, to thereby form a conductive layer on the surface of the polyimide base material. Then, the conductive layer was soaked in a GOD solution (1 mg/mL GOD aqueous solution) to form a GOD layer on the conductive layer. Thus, an enzyme-immobilized electrode was produced. The concentration of GOD in the GOD solution was 1,100 U/mL in terms of activity. 
         [0116]    (Measurement of Response Characteristics of Glucose) 
         [0117]    Response characteristics of glucose were measured by adding glucose while allowing a stirring bar to rotate with a stirrer in a state where an enzyme-immobilized electrode (working electrode), a platinum electrode (counter electrode), and a silver/silver chloride electrode (reference electrode) were soaked in a container containing a phosphate buffer (100 mM, pH=7.0) and the stirring bar. The concentration of glucose was changed from 0 mg/dL to 600 mg/dL by adding glucose once. 
         [0118]      FIG. 16  is a graph illustrating measurement results of a response current in the case of applying a pulse voltage repeatedly to the enzyme-immobilized electrode. For the voltage to be applied, the non-response voltage E 1  was set at a constant voltage of 300 mV, and the response voltage E 2  was set at a constant voltage of 600 mV. For the applied voltage pattern, the application time T 2  of the non-response voltage E 1  was set at 30 seconds, and the application time T 1  of the response voltage E 2  was set at 30 seconds. The standby time T 3  was set at 0 second. As illustrated in FIG.  16 , it was confirmed that, in the case of applying a pulse voltage repeatedly to the enzyme-immobilized electrode, the response current was kept at an almost constant value. 
         [0119]    For comparison,  FIG. 17  illustrates measurement results of a response current in the case of applying a constant voltage of 600 mV continuously to the enzyme-immobilized electrode. As illustrated in  FIG. 17 , it was confirmed that, in the case of applying a constant voltage continuously to the enzyme-immobilized electrode, the response current was increased with time. GOD exhibits a high Km value and a slow reaction speed. Therefore, when a constant voltage is continuously applied, GOD and unreacted glucose are accumulated on the electrode surface with time. It is understood that, when a constant voltage is subsequently applied continuously in this state, glucose accumulated on the electrode surface reacts with GOD late, and as a result, the response current is increased with time. In contrast, when a pulse voltage is applied repeatedly to the enzyme-immobilized electrode using GOD, a response current obtained during application of the pulse voltage is measured, and hence, the response current comes to an almost constant value. In the pulse method, response voltages for individual pulses are each independently applied for only a predetermined application time. Accordingly, only a response current corresponding to a reaction of GOD and glucose within the predetermined application time is obtained. Because no response voltage is applied before the time at which glucose accumulated on the electrode surface reacts with GOD late, it is conceivable that the response current should come to an almost constant value. It should be noted that GDH exhibits a low Km value and a high reaction speed. Therefore, even if a constant voltage is applied continuously to the enzyme-immobilized electrode using GDH, the response current is not increased. The denaturation and deterioration of GDH through application of the voltage causes a decrease in response current depending on the continuous application time of the constant voltage. 
         [0120]    Further, in Example 4, in the case of using GOD as a glucose oxidoreductase, an evaluation was made on the stability of the enzyme-immobilized electrode in the case of applying a pulse voltage repeatedly to the enzyme-immobilized electrode. The stability was evaluated as the sensitivity of the enzyme-immobilized electrode after continuous measurement of a response current by using a 600 mg/dL glucose solution. The setting of the voltage to be applied, and the applied voltage pattern are the same as those in the measurement of response characteristics of glucose. 
         [0121]    The voltage application time was set at 105 minutes, and the sensitivity of the enzyme-immobilized electrode was measured as a response current every 15 minutes. For comparison, an evaluation was made on the sensitivity in the case of applying a constant voltage of 600 mV continuously to the enzyme-immobilized electrode.  FIG. 18  illustrates the sensitivity in those measurement methods expressed as a relative value (%) of the response current to the initial sensitivity (response current at 0 hour). 
         [0122]    As illustrated in  FIG. 18 , in the method of applying a constant voltage continuously as a comparative example, the response current was increased with time. GOD exhibits a high Km value and a slow reaction speed. Therefore, when a constant voltage is continuously applied, GOD and unreacted glucose are accumulated on the electrode surface with time. It is understood that, when a constant voltage is subsequently applied continuously in this state, glucose accumulated on the electrode surface reacts with GOD late, and as a result, the response current is increased with time. In contrast, in the method of applying a pulse voltage repeatedly, an increase in response current was not observed even after a lapse of time, and it was revealed that the enzyme-immobilized electrode had satisfactory stability. In the pulse method, response voltages for individual pulses are each independently applied for only a predetermined application time. Accordingly, only a response current corresponding to a reaction of GOD and glucose within the predetermined application time is obtained. Because no response voltage is applied before the time at which glucose accumulated on the electrode surface reacts with GOD late, it is conceivable that the response current should come to an almost constant value. 
       Example 5  
       [0123]    In Example 5, studies were made on the dependence of the glucose concentration in the case of applying a constant voltage continuously to the enzyme-immobilized electrode, and the dependence of the glucose concentration in the case of applying a pulse voltage repeatedly to the enzyme-immobilized electrode. More specifically, in the case of applying a constant voltage continuously to the enzyme-immobilized electrode, a measurement was made on a response current in the case of changing the glucose concentration. Further, in the case of applying a pulse voltage repeatedly to the enzyme-immobilized electrode, a measurement was made on a response current in case of changing the glucose concentration. The response current was measured by adding glucose at a particular timing while allowing a stirring bar to rotate with a stirrer in a state where an enzyme-immobilized electrode (working electrode), a platinum electrode (counter electrode), and a silver/silver chloride electrode (reference electrode) were soaked in a container containing a phosphate buffer (100 mM, pH=7.0) and the stirring bar. 
         [0124]    (Production of Enzyme-Immobilized Electrode) 
         [0125]    Platinum was deposited on the surface of a polyimide base material by a sputtering method to a thickness of 30 nm, to thereby form a conductive layer on the surface of the polyimide base material. Then, the conductive layer was soaked in a GOD solution (1 mg/mL GOD aqueous solution) to form a GOD layer on the conductive layer. Thus, an enzyme-immobilized electrode was produced. The concentration of GOD in the GOD solution was 1,100 U/mL in terms of activity. 
         [0126]      FIG. 19  is a graph illustrating a change in response current due to an increasing glucose concentration in the case of applying a constant voltage continuously to the enzyme-immobilized electrode. The addition of glucose was performed twice, and the glucose concentration was successively changed from 0 mg/dL to 100 mg/dL to 600 mg/dL. The response current was measured by applying a constant voltage of 600 mV continuously to the enzyme-immobilized electrode. 
         [0127]      FIG. 20  is a graph illustrating a change in response current due to an increasing glucose concentration in the case of applying a pulse voltage repeatedly to the enzyme-immobilized electrode. A pulse voltage was applied twice and then glucose was added once, a pulse voltage was further applied twice and then glucose was added once, and a pulse voltage was still further applied five times. The glucose concentration was successively changed from 0 mg/dL to 100 mg/dL to 600 mg/dL by adding glucose twice. The response current was measured by setting the non-response voltage E 1  to a constant voltage of 300 mV, and setting the response voltage E 2  to a constant voltage of 600 mV. Further, the application time T 1  of the non-response voltage E 1  was set at 30 seconds, and the application time of the response voltage E 2  was set at 30 seconds. 
         [0128]      FIG. 21  is a graph illustrating the response current I 3  in the case of applying a constant voltage continuously to the enzyme-immobilized electrode, and the response current I 4  in the case of applying a pulse voltage repeatedly to the enzyme-immobilized electrode. As illustrated in  FIG. 21 , the response current I 3  and the response current I 4  were increased along with an increasing glucose concentration. Thus, it was confirmed that, even in the case of using GOD as a glucose oxidoreductase, the response current was increased through an increase in glucose concentration. 
       REFERENCE SIGNS LIST  
       [0000]    
       
           1  glucose continuous measurement apparatus 
           4  glucose sensor 
         E 1  non-reaction voltage (non-response voltage) 
         E 2  reaction voltage (response voltage) 
         E 3  standby voltage