Fig. 2: Preparation and characterization of uniaxially drawn piezoelectric PLLA films.

a Schematic of uniaxially drawn piezoelectric PLLA films preparation process. The first photograph shows the PLLA chip synthesis, which is polymerized from L-lactic acid that is obtained from the fermentation of renewable and biodegradable plant sources such as corns⁴¹. The second photograph shows the PLLA film extruding. The third photograph shows that PLLA film is stretched at different drawing ratios (DR) of 3.3, 3.7, 4.0, and 4.5 by two drawing rollers, and each sample named DR3.3, DR3.7, DR4.0, and DR4.5, respectively. The thickness of four PLLA samples is about 70 μm. The bottom photograph shows a transparent piezoelectric PLLA film (DR4.5). b Two-dimensional wide-angle x-ray diffraction (2D-WAXD) photographs of four PLLA film samples for crystal orientation analysis; (c) Differential scanning calorimeter (DSC) melting thermograms of the PLLA films with different DRs for crystal form determination. d 1D-WAXD spectrum of the PLLA films for crystal form determination (left) and their crystallinity (right). e Comparison of piezoelectric response of four PLLA film samples under tensioning (1% strain), bending (20° bent angle), twisting (30° twisted angle), and shearing (0.015 shear strain) deformation. The dimension (Length × Width) of each sensor is 4 × 2 cm. Each PLLA film is cut at 45° from the drawing direction for the tensioning and bending test and is cut at 0° for the shearing and twisting test. f Photographs of four unimodal sensors (left) and the sensor flexibility demonstration (right). g Stress-strain curve of a unimodal tension sensor.