# Towards Bio-Hybrid Energy Harvesting in the Real-World:
## 5 Research Progress in Biomechanical Energy-Harvesting Technologies
### Biomechanical Energy-Harvesting Mechanisms  
According to [PERSON] et al. [286], the four major bio-mechanical energy harvesters are piezoelectric, electromagnetic, electrostatic, and triboelectric mechanisms. In most cases, these mechanisms fall either within the vibration-based energy-harvesting technologies or motion-based energy-harvesting technologies. Piezoelectric materials with cantilever geometry have been the classic choice [287] for mechanical energy harvesting. These materials are a subset of ferroelectric materials and produce an electrical charge when being mechanically deformed [288]. [PERSON] et al. [14] reported a micro-spiral piezoelectric energy harvester for extracting energy from regular blood pressure variation, [PERSON] et al. [289] reported an energy-harvesting system from the wings of flying birds and bats that could be used to power environmental monitoring systems. Vibration-based piezoelectric energy-harvesting mechanism is more applicable for insects or animals with flapping wings than from human joint movements. Currently with piezoelectric technology, it is possible to generate around 20 watts from foot strikes as well as from movement of the body's center of mass, 60 watts from ankle movements, 30 watts from both knee and heap movements, 2 watts from both elbow and shoulder movements [290]. A generalized piezoelectric-based energy-harvesting system is depicted in Figure 14.  
An electromagnetic induction principle has also been utilized to extract biomechanical energy. [PERSON] et al. [292] reported their prototype for harvesting energy from endocardial heart motion by electromagnetic coupling and the system has been tested in vivo in domestic pigs. The proposed energy-harvesting device consists of serially aligned copper coils, surrounded by a linear arrangement of permanent magnets are suspended between two spiral springs. The permanent magnet stack oscillates in the case of motion, which in this case is the heart's endocardial motion. [PERSON] et al. [293] developed an elec  
Figure 14: Piezoelectric based biomechanical electricity generation principle, redrawn from the schematic shown in [291]. tromagnetic generator, suitable for inserting in the abdominal cavity of the birds and tested their performance in chickens and pheasants and obtained a power average of 0.47 mW. The objective of this work was to power up a biosensor to test the antigen-antibody reaction of avian influenza. Powering up the sensor was performed by a tiny generator with an electromagnetic induction coil implanted in the bird's abdominal cavity. The generator supplied power when chickens walk and pheasants flew. [PERSON] et al. [294] efficiently used a magneto-acoustic resonator for tracking aquatic animals. The authors developed a system that converts low-frequency fish motion to excite high-frequency acoustic pulses. The prototype was able to generate an average acoustic sound of 55 dB sound pressure level at 1 m of distance with a resonant frequency of 15 kHz. Electrostatic energy harvesters transform energy from changing in capacitance according to Coulomb's law for two parallel plate capacitors [295]. Although some examples can be found [296; 297], these components did not gain much popularity for biomechanical energy harvesting. Triboelectric nano generators (TENG) were first introduced by [PERSON] et al. [298] and rely on the principle of electric charge separation at the friction of two surfaces, different in terms of nano-scale roughness and thus creating an electric charge layer, by imposing variation in the capacitance electric energy that can be obtained from such small, flexible systems. This tribo-electrification is the working principle creating the natural phenomena of amber effect and lightning [299]. Combining the principle of tribo-electrification and electrostatic induction, TENG were developed and have gained huge popularity for harvesting biomechanical energy.