Microstereolithography of Three-Dimensional Polymeric Springs for Vibration Energy Harvesting

The inefficiency in converting low frequency vibration (6∼240Hz) to electrical energy remains a key issue for miniaturized vibration energy harvesting devices. To address this subject, this paper reports on the novel, three-dimensional micro-fabrication of spring elements within such devices, in order to achieve resonances and maximum energy conversion within these common frequencies. The process, known as projection microstereolithography, is exploited to fabricate polymer-based springs direct from computer-aided designs using digital masks and ultraviolet-curable resins. Using this process, a micro-spring structure is fabricated consisting of a two-by-two array of three-dimensional, constant-pitch helical coils made from 1,6-hexanediol diacrylate. Integrating the spring structure into an electromagnetic device, with a magnetic load mass of 1.236 grams, the resonance is measured at 61Hz, which is within 2% of the theoretical model. The device provides a maximum normalized power output of 9.14 μW/G (G = 9.81ms−2) and an open circuit normalized voltage output of 621mV/G. To the best of the authors knowledge, notable features of this work include the lowest Young’s modulus (530MPa), density (1.011 g/cm3), and “largest feature size” (3.4mm) for a spring element in a vibration energy harvesting device with sub-100Hz resonance.