## 4 Techniques: Energy Harvesting for IoMT Devices
### Energy Harvesting Solutions for Medical Wearable Devices  
Several solutions have been proposed to realize energy scavenging from the human body [43; 76]. This can be from everyday activities, without performing a specific workout, such as breathing, arm motion, walking, running, or pedaling. Mechanical energy is available from the movements of different body zones like the elbow, the knee, the ankle or heel (Figure 6). In [77], the performance of three different vibration generators was  
Figure 7: Flexible coil misalignment (**a**) ideal, (**b**) bended transmitter coil, (**c**) bending of both coils, (**d**) bended receiver coil, and (**e**) coil stretching.  
investigated at nine positions on the body of a person walking on a treadmill. The results show that at lower body locations (hip, knee and ankle), the amount of energy generated is four times higher than the amount generated at upper locations.  
Body heat provides also interesting possibilities for supplying wearable systems. In [78], a flexible thermoelectric generator (TEG) was able to generate 4.95 mW of power from body heat based on the Seeback effect and was then used to power a wearable multi-sensing bracelet. The self-powered multi-sensing bracelet can work sustainably at various conditions including human motion. In such systems, the amount of energy is highly dependent on the temperature difference between the human body and the ambient temperature [79].  
In some investigations, involuntary activities like cardiac motion, blood pressure and breathing have been used to produce biomechanical energy, which can regularly provide energy for wearable devices. In [80] cardiac contractions are used as a source of energy to power low-power pacemakers. The developed harvester delivers 11.1 uJ of electrical energy when powered by a constant 90 bpm heartbeat. The extraction of energy from the human body is in general much more complex than energy harvesting from machines [81] due to the requirement for small size and weight. The available energy is often weak and not easily usable, e.g., human body kinetic energy has often a low frequency and a low amplitude.  
In [82] the efficient use of thermoelectric nanogenerators (TENG) in healthcare as a convenience technique for patient rehabilitation was recently demonstrated. A wearable TENG-based rehabilitation device (Rehab-TENG) was implemented as an exercise gaming device while extracting energy. First, they successfully controlled a game on a laptop with the device using arm flexion and extension. This can be an effective technique for testing the motor function of the patients' impaired arm. Second, the Rehab-TENG device was implemented as an energy harvester in an exercise system where the patient moved their impaired arm to store energy in a capacitor. Based on the charging rate of the storage capacitor, the level of deficiency could be evaluated, which can thereby improve patient compliance since it motivates them to do more repetitive motions of the impaired body zone and this feature can eventually speed up recovery. Finally, authors discussed the possibility of using the Rehab-TENG device as an autonomous home-based exercise and monitoring system which is particularly useful in the current pandemic situation, avoiding hospital visits for rehabilitation treatment and monitoring.  
The main trend in energy harvesting technologies for IoT biomedical applications [49] is towards the development of biocompatible wearable energy harvesters such us textiles, footwear or watches to extract energy from the human body, which are lightweight and easy integrable in textiles and provide important possibilities to extend their sizes for a better energy output.