The piezoelectric devices, based on micro–nano electromechanical systems, are well known nowadays due to their small features, ability for integration with the integrated circuit in a single platform, robust, and easily fabricated in bulk. The enhanced performance of piezoelectric systems, which is soft, flexible, and stretchable made them have unique opportunities to be used in bio-integrated applications as nanodevices for energy harvesting, sensing, actuation, and cell stimulation. The selection of optimized configurations depends on thin geometries, neutral mechanical plane construction, and controlled buckling, while inorganic piezoelectric materials are preferred for interfaces with human bodies. The key considerations in designs, the analytical derivations for voltage and displacement, and the effect of the voltmeter resistance on the voltage measurements are presented. Devices for energy harvesting from natural motions of internal organs, sensors, and actuators for medical applications are reviewed. The PMN-PT energy harvester that produced current of 0.22 mA is higher than the rest of the discussed harvesters. Thus, it is more suitable to be used as a sufficient source of energy in biomedical applications. The use of piezoelectric nanowires and ribbons proved successful, and the dual features of device (sensor and actuator) seem advantageous.

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Although many hybrid EH devices had been investigated by researchers, their performances at different operating resonance frequencies were not reported. Radial magnetic field was reported as the most efficient architecture to use in electromagnetic energy conversion, this was utilized in the design of a low frequency and efficient hybrid harvester comprising piezoelectric (PZT) and electromagnetic generators. FE simulation was used to obtain the magnetic field, design the coil and locate its position relative to the magnets. The electromagnetic generator consists of ring magnets which act as proof mass, with a hanging coil inside. The harvester was tested at frequency range of (34-40) Hz, produced maximum power of ( 710)μW. The maximum normalized power density and maximum efficiency of the harvester are (2.272) mW/cm 3 /g 2 and (30.1%) respectively, at frequency of 36Hz and induced acceleration of (0.25)g. The new hybrid harvester has a higher normalized power density compared with others.