# Monitoring of Surgically Treated Upper Arm Fracture by Implanted Antenna at 402 MHz
## 3 The Method
### Operating Frequency and Biological Effect  
Many parameters have major effects on the amount of absorbed energy in human tissues, such as the EM wave strength, operating frequency, and the tissue's composition as its water and salt contents. At high frequencies, the absorption of the EM field by heat conversion at the tissue becomes stronger, which shortens the distance that the EM wave can penetrate into the body. However, the penetration depth also depends on the initial field strength. At the same operating frequency, the initial field strength must be increased to obtain a longer penetration into the body. The dielectric properties of the tissue, such as permittivity and conductivity, are governed by the tissue structure and distribution of water molecules, ions, and other molecules. This influences the level of absorption of the field in different tissues. A higher conductivity, due to higher water or salt content, increases the absorption of the field within the tissue and thus the thermal effect. Hence, there are areas in the body with higher local heating and areas with lower heating compared to the surrounding tissues. Bone and fatty tissue, for example, are heated less than other tissues because of their low conductivity due to low water content. The depth of penetration is associated with the tissue-depending thermal effect. It is higher in fatty and bone tissues than in muscle tissues, which absorb the field to a greater extent. Thus, the field energy is converted into heat efficiently. Most of the loss occurs in the muscles and skin due to the largest conductivity and the thickness of the muscle [25]. For the electric field intensity \(E_{i}\) (V/m) and conductivity \(\sigma_{i}\) (S/m), the average power dissipation \(P_{i}\) (W/m\({}^{3}\)) at a position (i) in the tissue is expressed as:  
\[P_{i}=\frac{|E_{i}|^{2}\sigma_{i}}{2}\;\mathrm{W/m^{3}} \tag{1}\]  
Figure 3 shows the penetration of the EM wave through a model composed of three consecutive layers of muscle, fat, and skin at the common operating frequencies of the MICS and SIM bands. The x-axis represents the distance travelled by the wave through the consecutive tissue layers. The value of the APD, along with the tissues, shows better penetration at lower frequencies. The electric and magnetic fields' equations are taken from [26]. Power density values S\({}_{\mathrm{av}}\) at any position (i), along with the model in Equation (2), are calculated by MATLAB codes. \[\mathrm{S_{av}}=\frac{1}{2}\;Re\;(E_{i}\;\times\;H_{i}^{*}) \tag{2}\]  
A universal radio frequency (RF) band of 401-406 MHz, for which the core band is 402-405 MHz, has been proposed for the medical implant communication systems (MICSs). The EM waves corresponding to this band have good penetration in the human body, a higher data rate, and a longer communication range [27; 28]. Table 3 illustrates the dielectric properties of human tissues at 402 MHz [29]. \begin{table}
\begin{tabular}{l l l l l l} \hline
**Reference** & **Antenna Type** & **Operating Frequency** & **Size in mm\({}^{3}\)** & **Substrate** & **Gain** \\ \hline [18] & Microstrip & 2.4 GHz & 482 & Polymide & \(-18.8\) dB \\ [20] & Multilayer helical & 401–406 MHz & 301 & Rogers TMM10 & \(-28.8\) dB \\ [21] & Microstrip and meandered & 434 MHz & 119 & FR4 & \(-33\) dB \\ [22] & Microstrip & 402 MHz & 240 & Rogers RT/Duroid 5882 & \(-29.64\) dB \\ [23] & Asymmetric dipole fed & 402 MHz & 264 & Rogers 3010 & \(-37\) dB \\ [24] & Asymmetric dipole & 401–406 MHz & 75 & Polymide & \(-25\) dB \\ \hline \end{tabular}
\end{table} 
Table 2: Biocompatible antennas. The specific absorption rate (SAR) quantifies the EM radiation absorption by tissues and represents the amount of energy or power absorption per unit mass of biological tissue.