# Application of Broadband Microwave Near-Field Sensors for Glucose Monitoring in Biological Media
## 2 Numerical Simulations  
The main components of the biological tissue are blood, fat, muscles, and bones. From the viewpoint of monitoring the blood glucose level in patients, the simplest and most convenient places for sensor location are the hands and feet, in particular, the wrists. They conditionally have the following layers: the skin subdivided into epidermis and dermis (the main component of this layer); the hypodermis that forms the cell space with fatty deposits and blood vessels; the biological layer of muscles occupying the greatest part of the feet or hands; and the bone in the center of all layers. As is well known, microwave monitoring is based on the established interrelation between changes in the dielectric permittivity of the propagation medium and the sensing signal parameters. In theoretical investigations, the dielectric permittivity is usually approximated by the Debye model or its slightly modified variant--the Cole-Cole model. Exactly the latter model was used to calculate the dielectric permittivity of the above tissues in a wide frequency range [18,19]. We calculated independently the dependences of the real part of the dielectric permittivity \(\varepsilon^{\prime}\) of blood, fat, muscles, and bones at frequencies in the range from 10 MHz to 10 GHz [20]. As expected, the tissues comprising aqueous solution (blood, muscles, and skin) have high dielectric permittivity in a wide frequency range. The same special feature is also peculiar to the imaginary part of the dielectric permittivity.  
In problems of near-field microwave sensing, the quality of the employed active probes is important. Therefore, we calculated in CST Microwave Studio the electric field distribution for the probe shaped like a conical horn with a conic wire placed inside that expanded toward the horn aperture (Figure 1a). Such a probe has a much wider band in comparison with the open end of a waveguide of the simplest narrow-band probe. The probe geometry provides formation of an extended near zone at its aperture with high electric field strength (Figure 1b).  
The special features of the near-field probe interaction with different biological tissues were studied by analyzing the behavior of the real part of the radiation flux power density (the Poynting vector). The formation of such flux is a characteristic feature of interference interaction of overlapping evanescent fields, irrespective of their sources (in this case, the counter-propagating probe field and the field reflected from the phantom of the tissue were overlapped) [21]. Results of the calculations allowed us to estimate the depth of radiation penetration into the examined phantoms of tissues. The probe was placed at a distance of 1 mm from the examined phantom of the biological tissue (Figure 2), and the Poynting vector was calculated at a frequency of 4 GHz at points of the segment of the straight line orthogonal to the probe aperture (Figure 2).  
Figure 1: Simulation of the cone probe.  
Figure 2: Scheme of simulation of the probe radiation interaction with phantoms of biological tissues.  
Results of the numerical simulation of the attenuation of the near field of the probe in different biological tissues are illustrated by Figure 3. It can be seen that the radiation attenuation by blood, muscles, and skin had an analogous exponential character. Such behavior was due to the dielectric properties of these tissues, namely, the presence of a considerable amount of liquid in their structure (86.3% of water in muscles; blood consists of plasma with 85% of water and platelets; the skin contains sebaceous glands composed of two-thirds water) [22]. As can be seen, the radiation flux power density almost vanished for radiation that penetrated through blood, muscles, and skin at a depth of 30 mm, through bone at a depth of 60 mm, and through fat at a depth of more than 100 mm.