## 3 Energy Harvesting
### Summary of Energy Harvesting  
Various studies have tackled energy harvesting challenges through innovative piezoelectric and hybrid systems. Researchers have tested different ways to convert ambient mechanical energy--ranging from footstep pressure to vibrations and even raindrops--into electrical power that can run low-energy devices such as pedestrian signals, street lights, and IoT sensors. Materials such as PZT, ZnO, PVDF, and advanced composites (e.g., PVDF/BaTiO\({}_{3}\)/Graphene) have been applied in setups including walkway-embedded sensors, cantilever harvesters, and multi-modal devices that capture both bending and torsional vibrations. While some configurations have generated promising voltage outputs--as high as 60.5 V- overall energy yields remain relatively small and are often sensitive to environmental conditions and inconsistent mechanical forces. Common hurdles include low power output, sensitivity to pressure and weather changes, installation challenges, and high production costs. This illustrates the potential and limitations of today's energy-harvesting approaches, pointing to the need for further refinement to make them more reliable and scalable for sustainable power. Studies show that hybrid energy systems using multiple materials generally outperform single-material setups. For example, Refs. [80; 85] created hybrids by combining solar photovoltaic panels with piezoelectric elements. These configurations improve overall energy production by compensating for each source's variability, resulting in peak outputs of 9.59 V [80] and 3.29 V [85], though both remain limited to low power uses. Another hybrid approach from [86] uses a composite of PVDF, BaTiO\({}_{3}\), and graphene, achieving an open-circuit voltage of 60.5 V and a short-circuit current of 654.2 nA, retaining stability over 600 cycles. Despite increasing structural complexity and production costs, these multi-material systems hold great promise for scalable energy harvesting technologies. Figure 4 shows our ratings for power output, harvesting style, design complexity, and environmental suitability across the reviewed studies. The rating for energy harvesting, design complexity, and environmental suitability was based on the authors' judgment for the context of this study (Al-Haram). Meanwhile, the power output column is based on the power output reported in this study. This heatmap relies on factors such as material resilience, device performance, and energy output. A quick look reveals which studies rank high or low in each area: deep reds and oranges highlight strong performance in a specific category, whereas darker blues mark lower scores. [PERSON] et al. [6] and [PERSON] et al. [3] achieved the top ratings overall. Piezoelectric materials, in particular, offer high conversion efficiency under cyclic mechanical stress. Modified piezoelectric polymers--such as PVDF blends--show improved flexibility and durability, making them ideal for wearable devices and structural applications [85]. Figure 4: Comparison of the required criteria of footstep harvesting across studies ([PERSON] et al. [4], [PERSON] et al. [5], [PERSON] et al. [6], [PERSON] et al. [16], [PERSON] et al. [18], [PERSON] and [PERSON] [19], [PERSON] et al. [87], [PERSON] and [PERSON] [88], [PERSON] et al. [89]). One major challenge in creating effective energy-harvesting devices is finding materials that are both comfortable and efficient [33]. Recent studies [90, 91] have examined textiles such as polyester, nylon, and cotton for their triboelectric properties and their compatibility with standard textile manufacturing methods [92]. For example, [PERSON] et al. [93] reported a polyester-based t-TENG capable of producing 35 \(\upmu\)W/cm\({}^{2}\), making it suitable for low-power applications such as health-monitoring sensors. In another study, [PERSON] et al. [94] developed a multi-layered t-TENG using interwoven conductive threads, which substantially improved both durability and energy output.