## 2 Materials and Methods
### Exposure Simulations  
All simulations were implemented with the finite-difference time-domain (FDTD) solver of the Maxwell's equations through the approximation to finite differences. Briefly, the FDTD method involves both a spatial and temporal discretization of the electric and magnetic fields over a period of time and a specific spatial domain limited with the boundary conditions. Typically, the minimum spatially sampling is at intervals of 10-20 per wavelength, and temporal sampling is sufficiently small to maintain stability of the algorithm [16; 17]. All the simulations were performed in the platform Sim4 life (ZMT Zurich Med Tech AG, Zurich, Switzerland, www.zurichmedtech.com, accessed on 28 April 2022). The human models [PERSON], an adult male (age = 34 years old, height = 1.77 m, mass = 70.3 kg, BMI = 22.4 kg/m\({}^{2}\)) and [PERSON], an adult female (age = 26 years old, height = 1.63 m, mass = 57.3 kg, BMI = 21.5 kg/m\({}^{2}\)), from the Virtual Population [18] were used.  
The antenna was placed in several positions relative to the human model, according to both the most common and possible uses of these devices: (i) on the ankle, (ii) on the arm, (iii) on the leg, (iv) on the shoulder, (v) on the torso at the height of the heart, and, finally, (vi) on the head. In each simulation, the wearable antenna with the side of the substrate  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Values** & **Dimensions [mm]** \\ \hline L & 38,5 \\ W & 25 \\ S & 3 \\ h\({}_{1}\) & 20 \\ h\({}_{2}\) & 18 \\ h\({}_{3}\) & 14 \\ h\({}_{4}\) & 9 \\ h\({}_{5}\) & 15.5 \\ h\({}_{6}\) & 1 \\ w\({}_{1}\) & 9.75 \\ w\({}_{2}\) & 5 \\ w\({}_{3}\) & 7 \\ w\({}_{4}\) & 17 \\ w\({}_{5}\) & 4.75 \\ w\({}_{6}\) & 5.5 \\ w\({}_{7}\) & 3.5 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Sizes of the antenna tuned at f = 2.45 GHz.  
Figure 1: Simulated wearable patch antenna tuned at f = 2.45 GHz. (**a**) front view and the side view and (**b**) performance of the simulated antenna: on the upper part the radiation patterns of the antenna (on the left with phi = 0\({}^{\circ}\), on the right with theta = 90\({}^{\circ}\)); on the lower part, the reflection coefficient centered on the frequency f = 2.45 GHz.  
turned towards the human model was positioned at a distance from the human model of 2 mm except for the head's case in which the distance is almost equal to the thickness of the helmet (10 mm). Indeed, when the antenna was positioned close to the head, the antenna has been placed outside a helmet, worn by the user. The helmet was simulated made of steel (\(\sigma\) = 1.1 \(\times\) 10\({}^{6}\) S/m, \(\varepsilon_{\mathrm{r}}\) = 1). Only in the exposure scenario with the antenna near to the head, the wearable antenna has been bent in order to better adapt the shape of the antenna to the curvature of the helmet. All of these configurations have been implemented on both human models (Ella and Duke).  
The boundary conditions were set as absorbing conditions with the Perfect Matched Layer (PML) and the dielectric properties of each tissue of the human models were selected according to the literature [19; 20]. Furthermore, the mesh was set with step ranging from 0.2 mm up to 2 mm in order to discretize all model tissues correctly according to the working frequency.  
Figure 2 shows, as an example, the positions of the antenna here analyzed on Duke. Furthermore, the figure shows how for each antenna position the computational domain was limited to a specific region of interest of the human body, which was supposed to be the most exposed.