Recently, the application of uniaxially stretched poly(L-lactic acid) (PLLA) films to speakers, actuators, and pressure sensors has been attempted, taking advantage of their piezoelectric performance. However, the shear piezoelectric constant d 14 of uniaxially stretched PLLA film is conventionally 6-10 pC N %1 . To realize a high sensitivity of pressure sensors, compact speakers, and actuators, and a low driving voltage, further improvement of the piezoelectric performance is desired. In this study, we carried out solid-state extrusion (SSE) to stretch and orient poly(Dlactic acid) (PDLA) and verified its effects on piezoelectric performance. By SSE, we were able to improve the mechanical strength and elastic modulus of PDLA samples. Furthermore, the d 14 of the samples was significantly increased to approximately 20 pC N %1 .

By using additives to change the higher-order structure of a poly(L-lactide) (PLLA) film, an improvement in its piezoelectricity was realized. The additive used was a triblock copolymer, which is a pure acrylic symmetric block copolymer consisting of a center block of poly(butyl acrylate) (PBA), corresponding to its soft part, and two side blocks of poly(methyl methacrylate) (PMMA), corresponding to its hard part. The triblock copolymer is hereafter denoted as PMMA-b-PBA-b-PMMA. The piezoelectric e-constant of the PLLA film with added PMMA-b-PBA-b-PMMA (PLLA/PMMA-PBA-PMMA film) was over two times higher than that of the PLLA film without adding PMMA-b-PBA-b-PMMA (reference PLLA film). Also, we found that the glass transition temperature increases with increasing PMMA-b-PBA-b-PMMA content. From atomic force microscopy (AFM) images, it was found that a new higher-order structure was formed in the PLLA/PMMA-PBA-PMMA film with high piezoelectricity. The method of using PMMA-b-PBA-b-PMMA has high productivity and its promising for industrial use.

To realize a new piezoelectric chiral polymer actuator, we studied the motion induced by the shear piezoelectricity of a poly(L-lactic acid) (PLLA) film under the application of an ac voltage. First, PLLA was subjected to a supercritical CO2 treatment to change its high-order structure, resulting in an improvement in its piezoelectricity. Then, we designed a PLLA film roll transducer, which consisted of a piece of the treated PLLA film rolled into a long thin roll. To demonstrate the transducer, we placed it vertically on a fixed stand. A plastic hemispherical container was placed on the upper end face of the transducer. It was found that the container rotated smoothly in the counterclockwise direction when an ac voltage was applied. We emphasize that no special complex mechanical parts were used in this study.

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We have developed a tension sensor with a coaxial structure using a narrow slit ribbon made of a uniaxially stretched poly(l-lactic acid) (PLLA) film for application to a wearable device. The tension sensor is produced as follows. We used tinsel wire as the center conductor of the sensor. The tinsel wire consists of a yarn of synthetic fibers arranged at the center, with a spirally wound rolled copper foil ribbon on the side surface. Next, slit ribbons obtained from a uniaxially oriented film of PLLA are wound helically on the side surface of the center conductor in the direction of a left-handed screw, at an angle of 45° to the central axis. The rolled copper foil is used as an outer conductor and covers the yarn without a gap. The prototype of the fabricated tension sensor has good flexibility, since the sensor is in the form of a filament and consists of a highly flexible material. For the 1 mm tension sensor, it was found that for a tension of 1 N, a charge of 14 pC was output. It was also found that the sensor maintained its room-temperature sensitivity up to 60 °C. Compared with an existing coaxial line sensor using poly(vinylidene fluoride) (PVDF), the sensor using PLLA does not exhibit pyroelectricity, meaning that no undesirable voltage is generated when in contact with body heat, which is a significant advantage as wearable sensors. The result has demonstrated the potential application of the PLLA film to wearable devices for detecting heartbeat and respiration.