Development of a portable intelligent blood sugar tester

Blood sugar test refers to the measurement of glucose concentration in human blood. For diabetic patients, blood sugar concentration is an important indicator of the condition, and blood sugar measurement is required regularly to monitor the development of the disease. Therefore, portable blood sugar testers have become a rapidly developing type of home medical instrument, which allows patients to monitor their blood sugar concentration by themselves when their condition is relatively stable. This article will introduce a portable intelligent blood sugar tester with a single-chip microcomputer as the core.

2. Principles of blood glucose measurement

1. Glucose oxidase printed electrodes

The portable intelligent blood glucose tester uses a disposable glucose oxidase printed electrode (hereinafter referred to as enzyme electrode) as a sensor. The structure of the enzyme electrode is shown in Figure 1.

Figure 1 Structure of enzyme electrode

A constant working voltage V is applied to the enzyme electrode, and the blood sample to be measured is dropped on the measuring point. The oxidase on the electrode reacts with the glucose in the blood sample. The characteristic is that after a period of time (about 30s), the current of the enzyme electrode and the glucose concentration in the blood sample show a certain linear relationship. Figure 2 shows the current change curve of the enzyme electrode during the measurement process. The experiment shows that at a working voltage of 0.4V, corresponding to a blood glucose concentration of 2 to 25mmol/L, the current response of the enzyme electrode is about 3 to 50μA.

Figure 2 Response curve of enzyme electrode

2. Reference measurement method

Since enzyme electrodes have a certain degree of dispersion, the reference electrode measurement method is used in actual measurement. The reference electrode is determined as follows: a number of enzyme electrodes are sampled from the same batch of enzyme electrodes to measure samples of known concentration (C0), and the average value of their response current I0 is the characteristic parameter reflecting the performance of this batch of enzyme electrodes. The reference electrode is a fixed resistor, and its resistance is equal to the working voltage V divided by the average response current I0.

The measurement steps using the reference electrode are: first insert the reference electrode into the electrode socket of the tester, and the instrument measures the current I0. This process is called calibration. Then use the enzyme electrode to measure the blood sample normally and measure the current I. The blood glucose concentration is calculated according to the following formula:

C = ( C0·I) / I0

3. Temperature compensation

Glucose oxidase is sensitive to temperature changes. In the experiment, the response value of 25 ℃ is used as the benchmark. In the temperature range of 0-40 ℃, the temperature coefficient Kt of glucose oxidase response is 0.7-1.2. The higher the temperature, the greater the response value. It can be seen that temperature compensation must be performed to ensure the accuracy of the measurement. The formula for calculating blood glucose concentration considering temperature compensation is as follows:

C = ( C0·I) / ( I0·Kt)

In this tester, the ambient temperature is measured using a semiconductor thermistor, whose resistance is inversely proportional to the temperature in the range of 0 to 50°C. By supplying constant voltage to the thermistor and measuring its response current, the current resistance of the thermistor can be measured and the ambient temperature can be calculated.

3. Hardware Design

The hardware of the portable intelligent blood glucose tester consists of 89C51 single-chip microcomputer chip, LCD, V/F converter, constant potential measurement circuit, multi-way conversion switch, temperature resistor, standard resistor, power supply voltage monitoring and battery. Figure 3 shows the schematic diagram of the constant potential measurement circuit, Rx represents the enzyme electrode. The 2.5V voltage provided by the voltage reference source LM336 is divided to obtain a 0.4V working voltage, and the enzyme electrode is excited by the voltage follower; according to the characteristics of the operational amplifier, If ≈ Io≈0, then the response current Ix ≈ Io; the current signal is obtained by the sampling resistor R1, and is differentially amplified to a voltage signal of 0~2.5V and sent to the V/F converter.

Figure 3 Constant potential measurement circuit

The multi-way switch 4051 can switch the measurement channel between the measurement socket (enzyme electrode or reference electrode), thermistor, 10kΩ standard resistor and no load. When switched to thermistor, the system measures the ambient temperature and performs temperature compensation. When switched to no load, the system measures the zero drift of the measurement channel; when the system switches to 10kΩ standard resistor, the system can measure the drift of the channel gain, and the system can perform self-correction calculations based on these data.

The system is powered by three AA 1.5V batteries. When the voltage is lower than 4V, it indicates that the voltage is too low. The whole machine consumes less than 0.125mW. The system shuts down at a fixed time under software control. After shutdown, the 89C51 is in power-down mode. The measurement results and calibration data are stored in its on-chip RAM, and the current is maintained at about 50μA. The whole machine has only one button. Pressing this button will turn on or reset the system.

4. Software Design

Figure 4 shows the system program flow chart of the portable blood glucose meter. Timer 0 of 89C51 works in the timer interrupt mode, and the interrupt service program completes the system clock, LCD drive, reading V/F results, etc. Timer 1 works in the counting mode, and measures the frequency signal of V/F output under the control of timer 0. The main program implements the entire measurement process according to the methods and steps described in the first section of this article, which uses some basic intelligent technologies, such as automatic compensation, automatic identification of information, and reliability evaluation of measurement.

Figure 4 Program flow chart of portable intelligent blood glucose tester

1. Automatic compensation

The measuring instrument performs a self-calibration measurement and temperature compensation measurement each time it is turned on or reset. The self-calibration coefficient and temperature compensation coefficient obtained will be used in the following measurement calculations. The specific steps are as described above.

2. Automatic identification of information

Since the measuring instrument has only one operating button, the process information of the measurement should be automatically judged according to the characteristics of the measurement signal. During the 30-second countdown, the measuring instrument continuously measures the signal of the socket to monitor whether its response value exceeds an agreed threshold (slightly greater than the no-load value). If it exceeds the threshold, it is judged that the user has dropped the blood sample on the enzyme electrode or inserted the reference electrode, and the program enters the measurement phase of the countdown. If the measured signal is still no-load when the 30-second countdown is completed, it is judged that the user is no longer measuring and the system is shut down.

After the measuring instrument enters the positive countdown measurement state, it determines whether the inserted electrode is the enzyme electrode or the reference electrode based on the response characteristics of the signal in the first 10 seconds: the response signal of the enzyme electrode is a changing process (as shown in Figure 2); while the response signal of the reference electrode is constant.

3. Evaluation of measurement reliability

When performing self-correction and calibration measurements, since the signal variation range is known, the measurement data is judged, and signals exceeding the upper and lower limit values ​​are judged as system failures or operational errors.

After each blood sample measurement, the current measurement result will be displayed for 10 seconds, and then the previous measurement result will be displayed alternately for 8 seconds before shutting down, reminding the user to compare the two measurements. If the difference is large, it should be determined whether the condition has improved, worsened, or is due to operational errors.

V. Conclusion

The portable intelligent blood glucose tester introduced in this article has been identified by relevant departments. The measurement performance of the electrical part of the tester is: current measurement range 0 ~ 50μA, resolution 0.1μA, accuracy 0.25%, temperature measurement range 0 ~ 50 ℃, resolution 0.1℃, accuracy 0.4%; blood glucose measurement accuracy is 3% when used with glucose enzyme electrode. The instrument itself has the advantages of low cost, small size, stable performance, simple and reliable operation, etc. Obviously, these characteristics are largely due to the use of single-chip microcomputers and the application of intelligent technology.