## 4 Techniques: Energy Harvesting for IoMT Devices
### Energy Transfer  
An interesting alternative for supplying medical wearable devices is energy transfer. Both Radio-Frequency energy transfer (RF) and Inductive Power Transfer (IPT) are interesting for supplying WSN for e-health. Typically, an IPT system consists of an AC signal generator supplying the transmitter coil to generate magnetic flux, which induces voltage in the receiver coil situated in the proximity of the transmitter coil. To increase the transmission efficiency and the received power, LC resonance on both IPT system sides is required, which consists of additional compensation capacitors connected to the transmitter coil and the receiver coil in series or in parallel [63; 64]. The connected loads are generally supplied by a DC voltage within specific voltage and current levels. For that, additional energy management stages includes \begin{table}
\begin{tabular}{c c c c} \hline
**Energy Sources** & **Power Density** & **Advantages** & **Disadvantages** \\ \hline Ambient light & 100 mW/cm\({}^{2}\) (direct sun) & High power density & • Intermittent \\  & 100 μW/cm\({}^{2}\) (indoor) & & • Determined by \\  & & & weather/lighting conditions \\ \hline Thermoelectric & 40 μW/cm\({}^{2}\) & Widely available & • Limited power density \\ \hline Radio frequency & 1 μW/cm\({}^{2}\) (ambient) & Widely available & • Power output depends on \\  & & & distance between \\  & & & harvester and RF source \\ \hline Vibration & 300 mW/cm\({}^{3}\) & High power density & • Dependent on the vibration \\  & (electrostatic-triboelectric & & source properties \\  & conversion) & & • Rectifying the interface is \\ \hline Human & 200 μW/cm\({}^{3}\) & Light weight & • Power output dependent \\  & (biomechanical piezoelectric) & & on activity \\ \hline \end{tabular}
\end{table} 
Table 7: Energy sources for supplying wearable devices (literature survey). an AC-DC rectifier and an DC-DC converter becomes primordial of an IPT system. On the other hand, the IPT system works properly within an ideal load impedance, which depends on the current consumption, the output voltage. For that, an impedance matching between both sides is recommended by the uses of controllable gain in the transmitter side as well as controllable DC-DC converter in the receiver side [65]. Moreover, an IPT system works properly with the highest performance when the transmitter and the receiver coils are in resonance conditions as well as concentric position with minimal separation distance [52; 66]. The misalignment between the coils situated in transmitter and the receiver sides influences the coupling factor of the IPT systems and then the transmission efficiency and the received power. In the case of flexible coils, additional deviations can occur caused by the bending or stretching of the transmitter or the receiver coils. The bending can be in concave or convex directions. However, when both coils are bent in the same direction and with the same angle, the flexible coil performances become like rigid flat coils (Figure 7). To overcome the bending issues, the selection of a proper coil in terms of the parameters [67] and geometry [68] to tolerate the bending are investigated. The fabrication of the coil for wearable applications can be generally based on the rigid coil [69], flexible coils [68; 70; 71] or fabric-based coils [67; 72; 73] associated with clothes. Rigid coils show the highest efficiency comparing to the other types of coils, but they are not preferred due to their weight and lack of flexibility. On the other hand, flexible coils can be a good alternative with which design a printed coil with different material properties like copper [68] or silver [74] and with various possible geometries and sizes. However, flexible coils are difficult to associate with the clothes, especially where the fabrics are stretchable and a specific fabrication process for the coil is required [73]. For that, many studies work on fabric-based coils and show more and more potential.