## 2 Piezolectric Harvesters (PEHs)
### Micro-Electro-Mechanical System (MEMS) Piezoelectric Energy Harvesting Elements
Figure 5: Different MEMS piezoelectric energy harvesting devices including (**a**) thin-film PZT cantilever, (**b**) high-aspect-ratio microbeam, (**c**) cantilever array, (**d**) silicon oxide wafer-based cantilever, (**e**) microfluidic harvester, and (**f**) harvester with rectangular hole [34]. This reliability ensures stable performance over an extended operational lifespan. The materials used in their construction, such as silicon substrates and piezoelectric thin films, exhibit excellent mechanical properties and resistance to environmental factors like temperature fluctuations, humidity, and vibration stresses. This inherent durability enables silicon MEMS harvesters to maintain their energy conversion efficiency and functionality over an extended operational lifespan, making them ideal for applications requiring continuous and reliable energy harvesting capabilities, such as in industrial, automotive, aerospace, and environmental monitoring environments. * _Scalability_: Silicon MEMSs' fabrication processes enable the production of harvesters in large quantities, ranging from individual devices for specialized applications to arrays of harvesters for increased power output. Scalability allows for cost-effective manufacturing and deployment in various scenarios, which is demonstrated in Figure 6, where scalability refers to the ability of a MEMS energy harvester to be produced and deployed with a minimal quantity of materials and energy (low resource input) while also maintaining its efficiency even for critical parameters, such as low frequencies with no need for using a macrobeam. By being scalable, the technology can be easily adapted and manufactured in varying quantities and sizes (micro or nano) without significantly increasing costs. This means that as the production volume or deployment scenarios change, the cost per unit or efficiency does not increase proportionally, making it an attractive and economically viable solution for mass production and deployment in various scenarios, which is visible in Figure 7, because it is a small version of millimetric beam design. Despite their numerous advantages, MEMS piezoelectric energy harvesters face challenges in fabrication complexity, requiring specialized manufacturing processes and expertise (Figure 7). While silicon-based MEMS devices are generally considered stable and reliable, the intricate nature of their structures can introduce challenges that require careful design considerations for long-term stability. In essence, MEMS devices have the potential to be stable and reliable, but achieving and maintaining that stability over an extended period may necessitate specific design approaches to address factors such as mechanical fatigue and environmental susceptibility (temperature variations, humidity levels, exposure to contaminants, and mechanical stresses) [39]. Therefore, stability can be achieved with the appropriate design measures in place to ensure the resilience and longevity of MEMS devices in operation. Figure 6: Complex MEMS piezoelectric energy harvester, which reaches ultralow-frequency resonance [38]. The integration of MEMS piezoelectric energy harvesting elements into wearable devices presents exciting prospects for self-sustaining electronics. These devices can power smart textiles, health monitoring systems, and IoT gadgets, offering a seamless and continuous energy supply without the need for external charging. Using a flexible foil as a substrate for MEMS piezoelectric energy harvesting offers several advantages in certain applications. One primary reason for choosing a flexible foil over other substrates, such as rigid materials or MEMS structures, is the ease of fabrication and flexibility in design. Flexible foils are lightweight and cost-effective and can be easily integrated into various wearable and flexible electronics without compromising comfort or mobility.