Characterization of Wrist Motions and Bionic Energy Harvesting for Wrist Wearables

Wrist-worn smart wearable devices, such as smartwatches and wristbands, are showing a high growth trajectory for decades in the wearable technology market. However, most wrist-worn generators are confined into watch bodies, which undermines the functionality of the wearables. In this study, we first characterize the dynamic properties of wrist motions and propose four rules for the design of wrist-worn energy harvesters. Based on the design rules, we then present a bionic piezoelectric energy harvester, composed of an inner band, an outer band with a piezoelectric array and a watch body. The two-layer band structure mimics two branches of a Y-shaped hyoid bone of the woodpecker’s head. A finite element model is built to study the mechanical and electrical responses of the design. The model is validated by an experiment and is utilized to find the optimal thickness and positions of piezoelectric elements. Based on the optimizations, we fabricate a prototype with a piezoelectric array as the energy harvesting unit. With an AC-DC rectifier, the prototype is tested to characterize the electrical responses in four wrist motions. The result shows that the average power output of bionic harvester is 2.10 mW in shaking arm motion, higher than previously reported wrist-worn generators. Finally, we demonstrate that the prototype enables to sustainably power a screen of a smartwatch, a wireless temperature monitoring system and a commercial electronic thermo-hygrometer, respectively.