# Non-Invasive and Accurate Blood Glucose Detection Based on an Equivalent Bioimpedance Spectrum
## 4 Electrode Tests and Measurement Band Selection  
In the process of impedance spectrum measurement, once the signal is applied, a reverse electromotive force will arise between the test electrode and the skin, and it will prevent the current from entering organic tissues. This phenomenon is called the polarization of the electrode, and the reverse electromotive force is called the polarization potential, as shown in Figure 3. In non-invasive blood glucose detection based on bioelectrical impedance, the polarization of the electrode has different influences among persons under test, and that will make the excitation signal unable to effectively penetrate the skin and bodily fluids, eventually leading to inaccurate results. This chapter conducts experiments on the scattering parameter S of the test electrode, analyzes the return loss of the electrode  
Figure 2: Dispersion curve of biological tissue. in different frequency bands, and determines the frequency range of the test excitation signal in the hardware acquisition platform. In order to reduce the polarization of the electrodes, experiments were carried out on the electrodes in different concentrations of blood glucose solutions to simulate the conditions of human bodily fluids with different blood glucose levels. Electrocardiogram electrodes are utilized in the tests. Different amounts of pure glucose are added to 0.9% normal saline to obtain blood-like solutions with different blood glucose concentrations. According to the physiological significance, the normal blood glucose concentration is in the range of 0.68\(\sim\)1.3 g/L, so five different glucose ratios are selected: 0.3 g/L, 0.6 g/L, 0.9 g/L, 1.2 g/L, and 1.5 g/L. The concentration of blood-like solution required to simulate changes in the blood glucose varies. The instrument used is Keysight Technology's E5061B vector network analyzer with a 10 kHz-50 GHz frequency range, which meets the experimental requirements. When bioelectrical impedance technology is used for blood glucose detection, the selected excitation signal is a high-frequency signal, so the concept of a two-port network needs to be used for circuit analysis. As shown in Figure 4, if \(a_{1}\) is represented as the signal incident wave, then \(a_{2}\) can be represented as the outgoing wave after \(a_{1}\) penetrates the network, and \(b_{1}\) is represented as the signal reflected wave of \(a_{1}\). Then, the return loss degree \(S_{11}\) is as follows:  
\[S_{11}=20*\log(\sfrac{b_{1}}{a_{1}}). \tag{3}\]  
Due to impedance mismatch and other factors, the signal will be partially reflected on the electrode, and that will cause energy loss of the signal. In addition, according to the principle of radio frequency, a smaller \(S_{11}\) means a smaller return loss of the signal, and thus, more energy of the signal can penetrate the skin in non-invasive blood glucose detection. In that case, the electrical characteristics of human tissues, as well as the blood glucose concentration, can be acquired with better reliability. Physiological parameters  
Figure 4: Two-port network diagram. Figure 3: Schematic diagram of electrode polarization. are primarily acquired using biomedical electrode pads. However, biological signals are inherently weak and prone to interference. The material and design of the electrode pads significantly influence return loss and polarization voltage. While this aspect is not the primary focus of the article, it is important to note that we have opted for well-established traditional silver/silver chloride electrode pads. The relationship between parameter \(S_{11}\) and the frequency obtained through the experiment is shown in Figure 5. It was observed that the glucose concentration in the blood-like solution increases, and the amplitude of the \(S_{11}\) parameter decreases. With the increase in the test signal frequency, the \(S_{11}\) parameter shows a downward trend and reaches the lowest point at 60-70 MHz.