A wearable energy harvester technology is developed for generating electricity from the movement of human joints. A micro-electroplated ferromagnetic nickel cantilever is integrated with a piezoelectric element and bonded on a flexible substrate. Based on the magnetic interaction between the magnetized cantilever and a magnet on the substrate, a novel vertical-vibration frequency-up-conversion (FUC) structure is formed to generate stable amounts of electric energy per cycle from the horizontal substrate stretching/rebounding. The two ends of the flexible substrate are attached on both sides of a limb joint to transform joint rotation into substrate stretching. During limb movement, the flexible substrate is horizontally stretched and rebounded, causing the cantilever to vertically release from and return to the magnet, thereby exciting the piezoelectric cantilever into resonant generation. Since the horizontal low-frequency limb movement is perpendicular to the vertical high-frequency resonance, the stretch has little influence on the resonance of the cantilever. Thus the generated energy is always stable within a wide frequency range of limb movements. The performance of the novel harvester is experimentally verified using a stretching/rebounding movement cycle, where the cycle corresponds to the frequency range of 0.5–5.0 Hz. Within one stretching/rebounding movement cycle, the generated electric energy is stable in the approximate range of 0.56–0.69 μJ for the whole frequency range. Two flexible harvesters are worn on the human elbow and knee for a body kinetic energy harvesting test. Considerable power can always be generated under typical low-frequency limb movements, such as squatting, walking, jogging, and fast running, where the peak-to-peak generated voltages are always approximately 4.0 V. Additionally, energy harvesting under two-directional area stretching is also realized by adjusting the FUC structure layout. The flexible-substrate harvester is promising for various wearable applications.
An integrated micro XY-stage with a 2 × 2 mm 2 movable table is designed and fabricated for application in nanometer-scale operation and nanometric positioning precision. The device integrates the functions of both actuating and sensing in a monolithic chip and is mainly composed of a silicon-based XY-stage, comb-drive actuator and a displacement sensor, which are developed by using double-sided bulk-micromachining technology. The high-aspect-ratio comb-driven XY-stage is achieved by deep reactive ion etching (DRIE) on both sides of the wafer. The displacement sensor is formed on four vertical sidewall surface piezoresistors with a full Wheatstone bridge circuit, where a novel fabrication process of a vertical sidewall surface piezoresistor is proposed. Comprehensive design and analysis of the comb actuator, the piezoresistive displacement sensor and the XY-stage are given in full detail, and the experimental results verify the design and fabrication of the device. The final realization of the device shows that the sensitivity of the fabricated piezoresistive sensors is better than 1.17 mV μm −1 without amplification, and the linearity is better than 0.814%. Under 28.5 V driving voltage, a ±10 μm single-axis displacement is measured without crosstalk and the resonant frequency is measured at 983 Hz in air.
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