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Poly(lactic acid) (PLA) has a high piezoelectric constant due to molecular orientation by uniaxial elongation and crystallization by heat treatment. The piezoelectric constant of PLA yarn is approximately 10 pCN−1. In our previous study, we reported the strong antibacterial effect of piezoelectric PLA fabrics that we had developed with fabric extension and contraction [Ando et al., Jpn. J. Appl. Phys. 56, 10PG01 (2017)]. In this study, we simulated the generating voltage of the yarn under tensile stress, considering filament slippage among adjacent filaments. Measurements were conducted to confirm the generating voltage and resulting electric field strength. Further, we confirmed that the growth of bacteria was suppressed, based on the electric field that formed in the fabric. Testing of the fabrics across multiple production lots repeatedly revealed a strong antibacterial effect, and scanning electron microscope image of dead bacteria in the fabric was different from those under the heat, drying and acid test.

A lactic acid monomer has an asymmetric carbon in the molecule, so there are optical isomer l- and d-type. The most widely used poly(lactic acid) (PLA) for commercial applications is poly(l-lactic acid) (PLLA). PLLA is the polymerization product of l-lactide. Certain treatments of PLLA can yield a film that exhibits shear piezoelectricity. Thus, piezoelectric PLLA fiber can be generated by micro slitting piezoelectric PLLA films or by a melt spinning method. We prepared left-handed helical multi fiber yarn (S-yarn) and right-handed helical yarn (Z-yarn) using piezoelectric PLLA fiber. PLLA exhibited shear mode piezoelectricity, causing the electric polarity of the yarn surface to be reversed on the S-yarn and Z-yarn when tension was applied. An SZ-yarn was produced by combining the S-yarn and Z-yarn, and fabric was prepared using the SZ-yarn. This study demonstrated that the fabric has a strong antibacterial effect, which is thought to be due to the strong electric field between the yarns. The field is generated by a piezoelectric effect when the fabric was extended and contracted.

Poly(lactic acid) (PLA) is a well-known, environmentally-friendly polymer. Piezoelectric poly(L-lactic acid) (PLLA), a type of chiral polymer, exhibits a high shear piezoelectric constant, is highly transparent, and is not pyroelectric. New human machine interface devices with intuitive control can be fabricated using piezoelectric PLLA as a deformation sensor. Specifically, for projected capacitive touch panels widely used in smartphone technology, the PLLA film as a pressure sensor with multiple electrodes allows pressure-sensitive functions to be added to the touch panel display.

Poly(lactic acid) (PLA) is a widely used biomass-derived polymer. It is chiral because the lactic acid monomer has an asymmetric carbon. If the L-lactide is polymerized, then the PLA polymer is an L-type PLA or poly(L-lactic acid) (PLLA); if the D-lactide in PLA is polymerized, then the polymer is a D-type PLA (PDLA). When these polymers undergo drawing or elongation, they exhibit shear piezoelectricity. PLA films are highly transparent and do not exhibit pyroelectricity because of the lack of intrinsic polarization. Therefore, if a PLLA film is used for a touch panel, which is operated by pressure, there is no spurious signal due to heating from the fingers. This suggests that PLLA films may be suitable for touch panels using pressure detection. We used PLLA as the base film of a projected capacitive touch panel with multiple electrodes, and demonstrated a multitouch gesture screen that was sensitive to pressure applied on the screen. This touch panel technology has potential applications for smart phones and tablet personal computers.

Synthetic piezoelectric polymer films produced from petroleum feedstock have long been used as thin-film sensors and actuators. However, the fossil fuel requirements for synthetic polymer production and carbon dioxide emission from its combustion have raised concern about the environmental impact of its continued use. Eco-friendly biomass polymers, such as poly(L-lactic acid) (PLLA), are made from plant-based (vegetable starch) plastics and, thus, have a much smaller carbon footprint. Additionally, PLLA does not exhibit pyroelectricity or unnecessary poling. This suggests the usefulness of PLLA films for the human–machine interface (HMI). As an example of a new HMI, we have produced a TV remote control using a PLLA film. The intuitive operation provided by this PLLA device suggests that it is useful for the elderly or handicapped.