Ultralight and sensitive ferroelectret films with a human skin-like texture

Biologically inspired by natural skin, polypropylene (PP) ferroelectret films with a human skin-like texture, featuring lightweight, light transmittance, small thickness, stretchability, as well as significant longitudinal and transverse piezoelectric activity, are prepared with a simple procedure, and their piezoelectric properties are characterized. The mechanical structure of the fabricated ferroelectret films is of a double-level cellular structure with distributed tight junctions and dipolar charges deposited on the opposite inner walls of cells. The preliminary results show that the area density, light transmittance, and thickness of the films are 3.5 g/m², 80%, and 10–20 μm, respectively. A quasi-static piezoelectric d₃₃ coefficient of a few thousand pC/N at 3.4 kPa and a dynamic piezoelectric d₃₁ coefficient of −40 pC/N at a frequency of 80 Hz and a stress of 20 kPa are achieved. The improved piezoelectric performance seems mainly owing to the reduction of the elastic modulus for the double-level structure and the enhancement of charges in deep traps in very thin cell walls. The sound pressure level, generated by an ultrasonic emitter made of the PP ferroelectret film, is about 87 dB at 46 kHz as driven by a voltage of 20 V (peak to peak voltage). An ultra-thin tactile sensor based on the fabricated films can localize positions and detect the track and moving speed of applied force. These results indicate that the cost-efficient films may extend the applications of ferroelectrets in flexible electronics.