Interaction between wearable devices and human tissues: electromagnetic characterization of a real heart rate monitor using high-resolution body models and FDTD technique

In the last decade, the market of wearable devices assists a rapid growth in daily activity, mainly due to the development of new technologies like smartwatches or smart glasses useful to monitor several physiological parameters. A specific type of wearables is the Heart Rate Monitor (HRM), a personal monitoring device that allows to measure and display in real-time heart rate. As external sources, these devices require the transmitting and receiving of electromagnetic waves near the body, which, at high enough exposure levels, may damage proximate tissues. The Specific Absorption Rate (SAR) is the quantity commonly used to evaluate exposure levels, used to verify if their exposure-induced biological effects are within the safety standard regulations. For this reason, the purpose of this study is the characterization, through Finite-different Time-Domain (FDTD) simulation, of the electromagnetic interaction between a real heart rate monitor and human tissues. The use of the FDTD method allows to simulate the field distribution generated by the device’s antenna and to determine the dissipated power on the high-resolution body model. In order to achieve this goal a real device has been simulated in the 3D space and its interaction has been visualized in terms of generated field magnitude and power distribution. As reported in the results, the antenna generates a hot spot of the electric field in the feeding point of the monopole, and the power dissipates reaching not so high depth in the body. The tissues that absorbed the majority of the power are the skin, the muscle and subcutaneous fat, even if this quantity is a portion of non-reflected power, lowered by the dispersive material present in the device. Another aspect that emerged is the fact that the electric field and the distributed power are sensitive to the distance from the tissues and the difference in body masses, which suggests the need for statistical analysis to fully evaluate the exposure condition due to the device. In conclusion, the field distribution and SAR values obtained through FDTD simulations are plausible and could be used to quantify the electromagnetic interaction of the real device under study. Further studies could consider various exposure configurations, due to the strong dependence of the field distribution on the posture