# Non-Invasive and Accurate Blood Glucose Detection Based on an Equivalent Bioimpedance Spectrum
## 4 Electrode Tests and Measurement Band Selection
In the frequency range of 20-60 MHz, the falling rate of \(S_{11}\) is the fastest in the whole process, and good linear results can be obtained in this frequency range. In the frequency range of 1-140 MHz, for ECG electrodes, the excitation signal in the frequency band of 60-70 MHz has lower return loss of the signal, stronger penetration to the skin, and contact with blood, cells, and other tissues. It is more sufficient to detect blood glucose information more accurately, so this frequency band should be included in the measurement frequency range. The above experiment shows the optimal frequency range with respect to the \(S_{11}\) parameter. The collected electrical impedance data revealed that at approximately 30 MHz, the impedance value reached its minimum, coinciding with the resonance point of the biological tissue's RC equivalent circuit. At this frequency, the impedance of the biological tissue was at its highest, while the capacitive reactance was at its lowest. Considering the above, the frequency band of the excitation signal was finally determined as 25-60 MHz. An Equivalent Bioelectrical Impedance Spectrum Acquisition Platform Based on the Electric Bridge Method  
As stated above, bioelectrical impedance technology can physiologically analyze the human body via the characteristics of electrical signals. However, blood glucose information cannot be represented by only one or several specific impedance values; thus, the bioelectrical impedance spectroscopy method is used for detection [26]. This method scans the equivalent bioelectrical impedance of the human body within a certain frequency range. In non-invasive blood glucose detection, blood glucose information can be extracted from the impedance spectrum. Moreover, by controlling the frequency band of the excitation signal, the influence of individual differences can be greatly reduced so as to enhance the accuracy and generalization ability of the measurement mode. According to the above-mentioned frequency range selection, a hardware platform for equivalent bioimpedance spectrum analysis has been designed, and its block diagram is shown in Figure 6.  
Figure 5: ECG electrode pad \(S_{11}\) parameters. The proposed hardware platform contains a microprocessor, a signal source, a phase detector, a power supply module, and other related parts. The microprocessor controls the signal source and communicates with the PC. The signal source generates excitation signals and implements frequency sweeping. The self-balance bridge module is the measurement unit for non-invasive tests. The phase detector measures the modulus and phase of the equivalent bioelectrical impedance from test signals. All the above modules are powered by the power supply module. This platform can acquire the impedance spectrum in 25-60 MHz. The next step is to use the designed hardware platform to measure the impedance of the fixed part of the human body and process the obtained impedance data to fit a model that can indirectly measure blood glucose. The test electrodes of the system are disposable ECG electrodes produced by Hangzhou Schindler Radio Equipment Co., Ltd in China Zhejiang. The electrode materials are silver/silver chloride (Ag/AgCl), and the mesh non-woven backing is used for full contact with human skin. The specific measurement point is the inner side of the human forearm. The pasting distance between the two ECG electrodes is about 10 cm. Before attaching it to the skin, wipe the measurement site with medical alcohol and dry it with a dry face to reduce the influence of skin sweat on the measurement results.