# A Survey of NFC Sensors Based on Energy Harvesting for IoT Applications
## 2 NFC Energy Harvesting
### NFC for Wearable Applications  
One of the most interesting applications for NFC sensors is wearable applications for which the tag is on the body. With wearable devices, the effects of the body on the antenna must be taken into account. At this point, it is important to note that inductance is not affected by the dielectric substrate and is, therefore, unaltered by the body in applications where the tag is attached to the skin. However, parasitic capacitance (\(C_{p}\)) increases due to the high permittivity of bodily matter. The tag's resonance frequency must, therefore, take into account the body's presence. The coupling coefficient is also essentially unaltered by the body's presence. This panorama is dramatically different at UHF or microwave frequencies, where the high losses and the detuning of the antennas due to the body reduce the efficiency of the antennas, thus noticeably reducing the read range. Therefore, as we describe below, NFC technology is highly compatible with wearable applications. The dielectric losses can be modeled by adding a resistance in parallel to the antenna (\(R_{pa}\) in Figure 3b). To quantify the effect of the body on the antenna, several simulations were performed. The same antenna as in Figure 6, printed on a 0.8-mm-high FR4 substrate was simulated in the air and on the body. It was assumed that the tag was on the arm, which was simulated using a planar stack of different dielectrics, as shown in Table 3. For the sake of simplicity, the curvature of the arm was ignored. The data for relative permittivity and conductivity were taken from Reference [49]. However, there was a large variation between individuals depending on the water content of their tissues. The results are shown in Table 4. Apart from the increase in capacity due to the high permittivity of the body, much deterioration in the quality factor was observed due to the losses. One solution is to isolate the body with a ferrite foil, as in the case of metal tags. A ferrite foil with a thickness of 100 \(\upmu\)m (\(\mu_{r}=120-j5\)) and an adhesive layer of a thickness of 100 \(\upmu\)m (\(\epsilon_{r}=2.0\), \(tan\delta=0.002\)) was inserted below the antenna. An increase in antenna inductance  
Figure 10: (**a**) Magnetic field on the surface of a metal sheet. Coupling between antennas (**b**) in free space, (**c**) with metal, and (**d**) with ferrite foil. was observed due to the permeability of the ferrite foil and a smooth increase in losses compared to in the air. Another improvement when ferrite foil was inserted was that the design was insensitive to changes in individuals or parts of the body. One drawback, however, is that sintered ferrite sheets are expensive. Another simple solution to mitigate the effects of the body consists of introducing a spacer made with a low-permittivity material (such as a plastic \(\varepsilon_{r}\approx 2\) or foam \(\varepsilon_{r}\approx 1\)) between the skin and the tag substrate. The separation of the antenna from the body significantly reduces the effects of the latter. In Table 4, simulated results for a spacer of foam (thickness: 1 mm) and plastic (thickness: 1 mm and 2 mm) are shown. In the case of plastic, to obtain results closer to the foam case (or air), it is necessary to double the thickness. The low-permittivity spacer allows for reducing the effective permittivity. Consequently, the parasitic capacitance decreases and the antenna resonance frequency increases, approaching the values expected for the air case. This solution is often implemented on wristbands [50; 51] where the spacer is integrated into the belt, usually made with a biocompatible material such as silicone. In other cases, such as on body patch or tattoo tags, the thickness required is not allowed. The reduction of tag quality factor \(Q_{T}\) due to the higher losses on the antenna caused by the presence of the body and the detuning due to the change of parasitic antenna capacitance increases the value of \(H_{min}\).