# Application of Broadband Microwave Near-Field Sensors for Glucose Monitoring in Biological Media
## 3 Experiments and Results  
From the results of the numerical simulation it followed that the signal changes caused by changes in the electromagnetic parameters of blood, skin, and muscles could be detected with the probe placed in immediate proximity to the surface of the examined biological tissues. Since changes in the blood sugar level lead to changes of the dielectric permittivity, the maximum changes of the signal level are expected for radiation that has penetrated through blood vessels. Thus, the results of the numerical simulation confirmed the theoretical possibility of non-invasive near-field monitoring of blood sugar level in a patient.  
In this case, the question arises: is it possible to detect minor changes in the sugar content in blood in an actual experiment? To answer it, we performed additional experimental investigations. As an object of research, a sodium chloride (physiological) solution with different contents of dextrose (glucose) was chosen. These two liquids, when mixed, allow sufficiently adequate experimental models of biological tissues and liquids to be obtained (more complicated phantoms of biological tissues were proposed, for example, in [23, 24]). The employed experimental setup included the following main parts: (1) N5230C PNA-L Network Analyzer (Agilent Technologies) with working frequencies in the range 10 MHz-40 GHz; (2) coaxial line with variable cross section shaped like a coaxial conic probe; and (3) plastic container filled with the examined liquid located in immediate proximity to the probe aperture (Figure 4).  
Figure 3: Attenuation of the near-field flux power density of the probe radiation in biological tissues.  
During the experiments, the complex reflection coefficient from the probe input was measured depending on the frequency of arriving oscillations. The experiments were organized as follows. First, the reflection coefficient was measured for the sensing of solutions containing dextrose. Then the same procedure was repeated for the sensing of a pure physiological solution. Then the measured values of the reflection coefficients were digitized and subsequently normalized (in so doing, we fully took into account recommendations [25] concerning the reduction of the negative influence of various factors). As a result, we obtained data convenient not only for perception, but also for their substantial analysis. The spectral behavior of the reflection coefficient (the parameter S11) was investigated at frequencies in the range 0.1-10 GHz. We studied solutions with the following concentrations: 1 mmol/1 (18.02 mg/dL), 3 mmol/1 (54.05 mg/dL), 5 mmol/1 (90.09 mg/dL), 7 mmol/1 (126.13 mg/dL), and 25 mmol/1 (450.45 mg/dL). Results of the investigations are shown in the figures below in red, dark blue, green, violet, and blue (Figure 5).  
It can be seen that at frequencies higher than 3.5 GHz it became impossible to highlight the differences between signals for different concentrations due to the signals being superimposed on each other and having intersections. The same picture was observed at  
Figure 4: Experimental setup.  
Figure 5: Dependence of the modulus of the complex reflection coefficient on the indicated dextrose concentrations in the entire measuring frequency range.  
low frequencies up to 1.5 GHz. Of particular interest was the signal in the frequency range 1.4-1.7 GHz. In this range of frequencies, a clear correlation was observed between the signal amplitudes and the solution concentrations. The signal received from the solution with the least dextrose concentration--1 mmol/L--had the least modulus of the complex reflection coefficient (Figure 6).