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Mechanical metamaterials and phononic crystals enable localizing, focusing, and guiding of elastic or acoustic waves in various ways. Here, we describe the physical mechanisms underpinning wave manipulation and then review the most recent energy harvesting methods for converting localized mechanical wave energy to useable electrical energy. Due to the exceptional wave-matter interactions enabled by the man-made structures, energy is collected more efficiently than through conventional methods. Artificially designed mechanical structures are versatile, especially when used in renewable and ecologically-benign energy transformation, and have a wide array of potential applications. Judicious design of metamaterials and phononic crystals permits the realization of novel localization and wave-guiding properties. Here, recent developments and strategies for applying these structures to piezoelectric energy harvesting are reviewed. [ABSTRACT FROM AUTHOR]

The applicability of piezoelectric energy harvesting is increasingly investigated in the field of renewable energy. In improving harvester efficiency, manipulating elastic waves through a geometric configuration as well as upgrading harvester elements is important. Periodic structures, such as phononic crystals and metamaterials, are extensively employed to control elastic waves and enhance harvesting performance, particularly in terms of wave localization and focusing. In this study, we propose a double-focusing flexural energy harvesting platform consisting of a gradient-index lens and elastic Bragg mirror. Based on the design process, the frequency and time response of the harvesting platform are analyzed. The results indicate that the output voltage and power calculated at 1800 Ω are 7.9 and 62 times higher than those observed in the bare plate, respectively. Even when compared to the existing gradient-index system, they are 1.5 and 2.3 times higher, respectively. These findings can facilitate the usage of periodic structures as geometric stimuli to significantly enhance harvesting performance. [ABSTRACT FROM AUTHOR]

• A trampoline metamaterial is designed for realizing multiband energy localization and harvesting. • Bragg-scattering and local resonance is exploited to control the bandgap at high and low frequencies. • Flexural wave energy is localized through defect modes by breaking the periodicity. • High-performance piezoelectric energy harvesting is achieved at multi-frequency range. In this study, we investigate high-performance piezoelectric energy harvesting at multiband frequencies. Previous harvesting platforms were limited to a high-frequency range and exhibited poor electrical performance. Our proposed piezoelectric energy-harvesting platform, which is based on a periodically arranged trampoline metamaterial, exhibits a remarkable electrical output at both low and high frequencies. The proposed harvesting platform broadens the bandgap induced by Bragg scattering in the high-frequency range, and the local resonance is critical in opening a wide bandgap in the low-frequency range. By breaking the periodicity, we generate multiband defect states for flexural wave trapping under fundamental physical phenomena. We experimentally demonstrate flexural wave localization in the defect cavity, while a confined wave is converted into high-performing electrical energy using a piezoelectric element. We measure 1.37 V and 4.05 V as output voltage, and 24.4 μ W and 1.28 m W as output power at the 1st and 2nd defect modes, respectively, which are 188% and 400% compared with the bare plate. Our high-power energy-harvesting platform has potential applications as a renewable and sustainable energy source in various fields, such as structural health monitoring, signal processing, and sensors. [ABSTRACT FROM AUTHOR]