# Non-Invasive and Accurate Blood Glucose Detection Based on an Equivalent Bioimpedance Spectrum
## 3 Bioelectrical Impedance Technology  
The basic units of the human body are cells, the intracellular fluid inside cells, and the extracellular fluid between cells, all of which are components of bodily fluids. Since human bodily fluid contains a number of ions, when an AC excitation signal enters the human body, the current will propagate in the bodily fluid with lower impedance and stronger conductivity. As the frequency of the excitation signal increases, the propagation of the current will also be enhanced, as shown in Figure 1. This implies that the current-carrying human body may be equivalent to an impedance model. According to the classic Cole-Cole model, the human body shows resistance and capacitance characteristics when driven by an AC signal. Therefore, the human body can be equivalent to an R-C (resistance-capacitance) model in such cases [24], which is the basis of bioelectrical impedance technology.  
Bioelectrical impedance technology can reveal the characteristics of different human body tissues by measuring their impedances. According to [PERSON]'s frequency dispersion theory, there are three different frequency bands [25], namely the \(\alpha\) frequency area, the \(\beta\) frequency area, and the \(\gamma\) frequency area, as shown in Figure 2. The \(\alpha\) band mainly reflects the characteristics of the ionic environment in biological fluids and is mostly used in biological component analysis and organ inspection. In the \(\beta\) frequency region, with the  
Figure 1: Schematic diagram of an AC current flowing through cells in the human body. (**a**) Low-frequency current flowing through the cells; (**b**) high-frequency current flowing through the cells.  
increase in the excitation signal, the equivalent capacitance of the cell membrane remains relatively stable, so a relatively stable impedance characteristic can be obtained. The \(\beta\) band is used in blood glucose detection and blood-related information detection, and the \(\gamma\) band reflects some characteristics related to water molecules in cells. The frequency band characteristics appear because the applied electric field causes the dipole moment of protein binding water and protein molecules to rotate. Currently, the \(\gamma\) band cannot be effectively applied to equivalent biimpedance research, mainly because the interior of the human body is very complicated. For the non-invasive blood glucose detection model in this paper, band \(\beta\) is selected for the excitation signal on the hardware platform.  
In summary, when conducting research on bioelectrical impedance, the \(\alpha\) frequency band and the \(\beta\) frequency band are concentrated. But, the study found that it is difficult for a current to enter the cell in the \(\alpha\) frequency region. As the frequency band of the test signal rises, an electric current will penetrate the cell membrane and flow into the intracellular fluid. In the \(\beta\) band, the conductivity of the extracellular fluid increases with the frequency, and the relative dielectric constant value of the cell membrane decreases with the frequency. Within the \(\beta\) dispersion frequency, the dielectric properties of biological tissues can effectively reflect the internal state of the cell, especially for the equivalent impedance change caused by the difference in blood glucose concentration inside and outside the cell. For the non-invasive blood glucose detection model designed in this paper, the \(\beta\) band is selected as the signal frequency band of the excitation signal to design the hardware detection platform.