## 2 Piezolectric Harvesters (PEHs)
### Micro-Electro-Mechanical System (MEMS) Piezoelectric Energy Harvesting Elements  
The MEMSs have revolutionized the field of energy harvesting, providing highly efficient and compact solutions for powering small electronic devices [32; 33]. When compared to traditional flexible-foil-based wearable piezoelectric generators, MEMS piezoelectric energy harvesting elements offer several distinct advantages. Specifically, silicon-based MEMS piezoelectric energy harvesting devices excel in their miniaturization and precise control over dimensions, allowing for enhanced energy conversion efficiencies and sensitivity to mechanical stimuli. These devices can be tailored to resonate at specific frequencies, optimizing energy extraction from various mechanical inputs, such as vibrations. This level of customization sets MEMS piezoelectric harvesters apart from their flexible foil counterparts, enabling them to operate in a wider range of conditions and environments (Figure 5). The versatility of MEMS piezoelectric energy harvesters extends to their ability to generate electrical power from diverse mechanical sources, including ambient vibrations, structural deformations, and even human motion. This broad spectrum of energy sources makes MEMS devices well suited for applications where continuous but low-magnitude vibrations are present, offering a reliable power source for a variety of portable electronics and sensors. While specific ranges for each parameter of silicon piezoelectric MEMS harvesters can vary based on the design, application, and fabrication process, here are typical ranges for some key performance parameters [35,36]:  
* _Energy conversion efficiency:_ This is a key factor in determining the overall performance of the harvester in converting mechanical vibrations into electrical energy, which is very often expressed by the output power or by power density for an activated volume of material. In this regard, typical values for the output power are in the range of 800 nW to 14 W [32], according to the cantilever shape and the material's crystal structure and thickness, and for the power density, they are in the range of 3 uW/mm\({}^{3}\)-3 mW/cm\({}^{3}\)[37]. * _Size/dimensions_: These devices can be miniaturized to dimensions ranging from a few millimeters down to sub-millimeter scales, with thicknesses typically falling in the range of tens to hundreds of micrometers. These compact sizes enable integration into small-scale systems and applications where space is limited, offering versatility and flexibility in deployment. With the silicon MEMS harvesters, it is possible to maintain high aspect ratios, where the thickness of the device is significantly smaller than its lateral dimensions. This characteristic enables the harvester to effectively convert mechanical vibrations into electrical energy while maintaining structural integrity and mechanical robustness. The high-aspect-ratio design also enhances the harvester's sensitivity to vibrations, optimizing its energy harvesting performance across a wide range of frequencies. * _Frequency range:_ Silicon MEMS harvesters can be tuned to resonate at frequencies ranging from a few hertz (Hz) to several kilohertz (kHz), allowing for efficient energy extraction from a broad spectrum of mechanical vibrations, such as for structural health monitoring, wearable technologies, transportation systems, and industrial machinery. By adjusting the dimensions, material properties, and structural configurations of the harvester, engineers can tailor its resonant frequency to match the dominant frequencies present in the ambient vibration source. * _Reliability:_ Silicon-based MEMS devices are known for their high reliability, with the mean time between failures (MTBF) exceeding tens of thousands of operational hours.