A Biodegradable Piezoelectric Polymer For Vibration Energy Harvesting Applications

Following the development of portable electronics, the requirement for self-powered electronic devices has emerged as one of the hottest scientific topics for next generation of electronics. One approach to scavenge the energy from the environment is utilizing piezoelectric materials to perform the energy harvesting process.

Normally, poly(vinylidene fluoride) (PVDF) based piezoelectric polymer materials are most common materials used in sensors, transducers, and energy harvesting applications. However, PVDF materials suffer from losing their piezoelectricity when the temperature is above 75℃. PLA is a biodegradable polymer which can be synthesized via ring-opening polymerization using the monomer of lactic acid. The monomer can be produced from sugar and starch by bacterial fermentation. From the previous research results, it was found that processed PLLA exhibits excellent piezoelectric property and can be used for sensors, actuators, and energy harvesting applications.

Recently, Ren’s research group in Chinese Academy of Sciences has found that a poly(L-lactic acid) (PLLA) based biopolymer can generate around 10 µW electric power output using double layers of the PLLA films under low-frequency vibration. Additionally, Ren’s group also found that the PLLA polymer even maintained its high piezoelectricity after thermal treatment at 140℃ without an extra-poling step.

At the beginning of the investigation, the PLLA cantilever device was simulated using a theoretical equation which calculated the resonance frequency based on material property and device dimension. The length ratio, thickness ratios between the PLLA layers and the poly(ethylene terephthalate) (PET) layers, and tip loads were calculated to achieve the optimized output conditions and resonance frequency.

Furthermore, a double-layered PLLA based cantilever device was designed and fabricated based on the theoretical result.  After the PLLA films were fabricated by solution casting method, the double-layered PLLA films were sputter coated with Au electrodes on both sides of the film and glued to the top of the PET passive layer using epoxy. A tip load was also attached to the free end of the PET film to form a cantilever structure.

The device was tested using a shaker driven by a function generator and an amplifier. Test results showed that the PLLA based cantilever device generated around 14 µW power for the vibration around ~50 Hz. Next, the output signal was rectified by a lab-designed full-bridge circuit, the rectified PLLA cantilever signal was used to charge an 8 mAhr lithium-ion battery to 3.2 V within 7.5 hours. The electrical energy stored in the lithium-ion battery can light up 30 LEDs for one minute. The paper was published by Advanced Sustainable Systems in 2017.

This study, A Poly(l-Lactic Acid) Polymer-Based Thermally Stable Cantilever for Vibration Energy Harvesting Applications was recently published in the journal Advanced Sustainable Systems.