We developed a prototype system with a simple structure for energy-harvesting as humans walk in their daily life, using piezoelectric electrets as piezoelectric-power-generating elements. We prepared a porous poly(tetrafluoroethylene) (p-PTFE) film with a thickness of 12 μm and an average pore size of 0.7 μm sandwiched by two films of tetrafluoroethylene–hexafluoropropylene copolymer (FEP) with a thickness of 6 μm (FEP/p-PTFE/FEP film) as an electret film used for the energy-harvesting device. A corona discharge system used to fabricate an FEP/p-PTFE/FEP film with an area of 20 × 20 cm2 that generates piezoelectricity (electret FEP/p-PTFE/FEP film). The electret FEP/p-PTFE/FEP film had a piezoelectric constant d33 of more than 100 pC/N. Then, we fabricated a multilayer film by stacking the metal foil and the electret FEP/p-PTFE/FEP film without forming wrinkles or streaks. The voltage, current, and power generated by the electret FEP/p-PTFE/FEP multilayer film during an exercise involving a research subject repeatedly stepping on the film placed in the floor were evaluated. The maximum instantaneous generated power was about 4500 μW each time the subject stamped up and down. The energy consumed in transmitting an 8-byte signal using a Bluetooth Low Energy (BLE) device is known to be about 600 μW. Considering the electricity consumption of BLE devices, the above result strongly indicates that the power generated by the electret FEP/p-PTFE/FEP multilayer film has great potential for use in BLE devices.
A new super-multilayer alternating laminated film in the shape of a rectangle with round corners has been developed. The super-multilayer film, which comprised piezoelectric poly(l-lactic acid) (PLLA) and poly(d-lactic acid) (PDLA) films, was wound with the number of turns on the order of from 100 to 1000 to form piezoelectric rolls. These piezoelectric rolls could generate an induced voltage of more than 95% of the initial voltage for over 10 s when a constant load was applied. The desired duration and magnitude of the piezoelectric response voltage were realized by adjusting the number of turns of the piezoelectric rolls. Similarly to many other conventional piezoelectrics, the piezoelectric rolls enable instantaneous load-dependent voltage generation and attenuation. The piezoelectric rolls are also lighter than conventional piezoelectric ceramics and can be used as a novel pressure sensor.
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