As a common method of measuring a concentration of a specific component in a body fluid (such as glucose in blood), an oxidation-reduction reaction is used. On the other hand, for handy measurement of the blood glucose levels at home and elsewhere, palm-size, portable blood glucose level testers are used widely. These handy-type blood glucose level testers make use of a disposable biosensor which is attached to the tester and serves as an enzyme reaction field. The blood glucose level measurement is made by supplying the blood to the biosensor (See JP-B 8-10208 for example.)
The biosensor includes, for example, a substrate provided with a pair of electrodes and a reagent pad bridging the electrodes. The reagent pad includes, for example, an oxidation-reduction enzyme and an electron transfer material in an oxidized form. When the reagent pad is supplied with a sample fluid, the reagent pad and the sample fluid constitute a liquid phase reaction system. In this liquid phase reaction system, the oxidation-reduction enzyme promotes oxidation of blood glucose and release of electrons while the electron transfer material is supplied with the electrons and thereby reduced.
The blood glucose level tester, on the other hand, includes a voltage source and an electric current measurer for example. When the biosensor is attached to the blood glucose level tester, an electric circuit is formed by the reagent pad (liquid phase reaction system), the voltage source and the electric current measurer. The voltage source applies a voltage between the pair of electrodes of the biosensor. When the voltage is applied between the electrodes, a chemical reaction reduces the electron transfer material, which releases electrons to one of the electrodes, causing oxidation. The amount of electrons released and the current passed through the circuit at this time are measured by the electric current measurer.
In the measurement of the blood glucose level, the voltage applied to the reagent pad (liquid phase reaction system) and the current measured are generally in a relationship shown in FIG. 4.
Specifically, from the time when blood is supplied to the reagent pad of the biosensor which is attached to the blood glucose level tester to the time when the supply of the blood to the reagent pad is detected (FIG. 4, t1→t2), a constant voltage V is applied to the electrodes of the biosensor. During this time, the supply of the blood to the reagent pad increases the measured current, and at a point (FIG. 4, t2) when the measured current has reached a predetermined value (FIG. 4, I1), the blood glucose level tester determines that the sample fluid has been supplied. On the other hand, upon the detection of the blood supply, the application of the voltage is stopped for a predetermined duration of time (FIG. 4, t2→t3). Upon passage of the predetermined duration of time (FIG. 4, t3) since the detection of blood supply, application of the constant voltage V is resumed between the electrodes of the biosensor. During this time, the electron transfer material which has been reduced releases electrons to one of the electrodes of the biosensor. The amount of the released electrons is measured as a response current I2 at a time point (FIG. 4, t4) when a predetermined amount of time has been passed since the reapplication of the voltage, and calculations are made for the concentration of the specific component based on the response current I2.
When the biosensor is attached to the blood glucose level tester, a circuit is formed as described earlier, by the biosensor electrodes, the source of voltage and the electric current measurer. In order to turn off the voltage to the reagent pad (the liquid phase reaction system), the electric circuit may simply be opened, and opening the circuit is easily be made by providing a mechanical switch and turning it off in the circuit. Disadvantages of this method however, include a high cost incurred by the mechanical switch, and low reliability due to potential failure of the mechanical switch. For these reasons, when cost and reliability are factors of concern, a method is used in which the voltage supplied from the voltage source is turned to zero, without using a mechanical switch. In this method however, the circuit is not opened as completely as by a mechanical switch. Therefore, if there is a difference in electric potential in the circuit, electromotive force in the reagent pad (liquid phase reaction system) can cause a small amount of electric current to pass through the circuit.
Once such an electric potential difference occurs in the reagent pad (liquid phase reaction system), an amount of the electron transfer material in the reduced form for example is oxidized to release electrons, and these electrons will move to one of the biosensor electrodes. In another occasion, the electron transfer material in the oxidized form is supplied with electrons from one of the biosensor electrodes to become the reduced form. Under such an occasion, as shown in FIG. 5, the measurement of the response current may give a lower value (I2′) or a higher value (I2″) than the real value (I2). As a result, the blood glucose level which is the final target value will be different from the real blood glucose level, and each measurement will give an inconsistent value.