## 4 Techniques: Energy Harvesting for IoMT Devices
### Energy Harvesting Sources  
Efficient energy harvesting from ambient sources is very important and can extend or replace the battery-based supply [50; 51; 52]. Electrical energy can be extracted using one or a combination of different transduction mechanisms. Several principles of converters can be implemented, such as piezoelectric [53], triboelectric [54], electromagnetic [55] and electrostatic [56] converters. However, the ambient sources can fluctuate, affected by ambient conditions or aging, which makes their predictability and controllability quiet challenging (Table 7). Piezoelectric energy harvesting is an efficient technique to generate electricity from movements, vibrations, and shocks. Several piezoelectric materials can be used, including crystals, ceramics, and polymers. To enhance the piezoelectric effect, it is required to install the converter in a part of the body that is exposed to a large compressing force. Piezoelectric energy harvesters (PEHs) have been frequently implemented in shoe insoles, where the foot can apply a large amount of pressure [57; 58; 59]. Several studies focusing on kinetic energy generated by body motion to supply wearable systems, have recently been conducted. In [60] the authors proposed a piezoelectric harvester embedded in the heel of a shoe in order to extract the kinetic energy generated by human walking. Although the device can generate significant levels of power, some optimizations for the structure are \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Wearable Device** & **Voltage Rate in V** & **Power Consumption** & **Reference** \\ \hline Heart rate monitors & & & \\ \hline MAX30102 pulse oximetry & 1.8–3.3 & \(<\)1 mW & [44] \\ BH1790 GLC optical heart rate sensor & 1.7–3.6 & 720 μW & [45] \\ \hline Blood glucose monitoring system & & & \\ \hline IoT-based glucose monitoring device & 2.0 & 1 mW & [5] \\ Implantable RFID transducer for continuous glucose monitoring & 1.0–1.2 & 50 mu W & [46] \\ \hline Blood pressure sensors & & & \\ \hline CMOS tactile sensor & 5 & 11.5 mW & [47] \\
3-Axis Fully Integrated Capacitive Tactile Sensor & 1.8–3.3 & 1.2–4.6 mW & [48] \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Energy consumption of typical wearable devices. Figure 6: Energy harvesting for autonomous wearable devices. required. The mechanical specifications of the piezoelectric ceramics have to be considered in case of their implementations in areas with large stresses such as walking or running. This can consequently be used to avoid mechanical damage to the piezoelectric layers. Electromagnetic energy harvesting is based on [PERSON]'s law induction. An electric current is induced once a conductor is moving through an electric field. Electromagnetic energy harvesting systems are designed as a system of springs, magnets and coils. The output power depends mostly on the number of coils and magnetic mass. Therefore, reducing the size, weight and complexity of these energy harvesters is quiet challenging. For example, in [61], the authors demonstrated the performance of a frequency up-converted electromagnetic harvester in harvesting energy from human limbs. In this work, an average power density of 0.33 mW/cm\({}^{-3}\) was achieved via low frequency human vibration to supply wearable devices with extremely low frequency (\(\sim\)5 Hz). In [62], triboelectric nanogenerators and hybridized systems were reviewed. The authors highlighted the recent development of thermoelectric nanogenerators (TENG)-based hybrid generators (e.g., incorporation of TENG with other transducers such as piezoelectric, electromagnetic and thermoelectric generators) and hybridized systems, from the perspective of operation concepts, energy management strategies, optimizing techniques, and the system integrity. New implementations of these systems for outdoor, indoor, wearable, and implantable applications were also reviewed, with an overview of future trendy applications of hybrid energy harvesters in healthcare, robotics, and the Internet of Things (IoT).