Wearable sensors are attracting great attentions due to their potential applications in human health monitoring and caring systems. The wide utilization of wearable electronics may cause great burden to the environment, due to the vast contaminants generated in their fabrication and after their usage. Consequently, great efforts are devoted to the eco-friendly strategies for the material and fabrication in wearable sensors. Herein, recent advantages in developing wearable sensors with eco-friendly materials and green manufacturing approaches are reviewed. The functional materials with accessibility, recoverability and degradability have participated in the sensors as substrates, sensing elements and conductors. The fabrication strategies, from facile manual schemes to low-emission automated techniques are also introduced with their merits and demerits. Finally, the existing challenges and future opportunities in this field are summarized.

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This paper proposes a multidirectional piezoelectric vibration energy harvester based on an improved U-shaped structure with cross-connected beams. Benefitting from the bi-directional capacity of U-shaped beam and additional bending mode induced by cross-connected configuration, the proposed structure can well capture the vibrations in 3D space at the frequencies lower than 15 Hz. These features are further validated by finite element analyses and theorical formulas. The prototype is fabricated and the experiments under different conditions are carried out. The results show that the proposed harvester can generate favorable voltage and power under multidirectional vibrations at a low operating frequency. Practical applications of charging capacitors and powering a wireless sensor node demonstrate the feasibility of this energy harvester in supplying power for engineering devices.

This Note proposed a multi-direction piezoelectric energy harvester with a wide bandwidth and low working frequency, which is distinguished by the multiple working modes of the U-shaped beam and the high capacity of the pendulum in collecting arbitrary vibrations. The structural features are evaluated by finite element simulation and verified by experiments. At least three voltage peaks are generated in the frequency range of 0 Hz–25 Hz, and favorable harvesting consistency in different directions is achieved. The designed structure is adaptable in collecting energy from arbitrary vibration in ambient environments.

To validate the potentials of unequal-length section-varied geometry in developing a orthoplanar spring-based piezoelectric vibration energy harvester (PVEH), a modified spring with quad-trapezoidal-leg configuration is designed, analyzed, and fabricated. A basic quad-trapezoidal-leg orthoplanar spring (QTOPS) is theoretically analyzed, and the structural effective stress and eigenfrequency are formulated to determine the main dimension parameters. Then, an improved QTOPS with additional intermediations is constructed and simulated. Porotypes with different leg geometries and mass configurations are fabricated and tested. The results of QTOPS and a conventional rectangular-shaped spring are compared. It is verified that the proposed approach provides the structure with an enlarged effective stress and lower resonant frequency, which makes it more suitable to construct a high-performance PVEH than the orthoplanar spring with equal-length or rectangular legs.