Biodegradable 3D-printed ferroelectret ultrasonic transducer with large output pressure

Ferroelectrets are typical cellular polymer foams that exhibit high flexibility and an excellent matching to air. Recently, biodegradable cellular polylactic acid (PLA) ferroelectrets have been introduced, exhibiting a large longitudinal piezoelectric coefficient. In this work, we present a combination of a PLA film and a printed PLA backplate using fused deposition modeling to build a ferroelectret air-coupled ultrasonic transducer. The ultrasonic transducer consists of three main components all made from PLA: the film presenting the vibrating plate, the printed backplate with well-defined groves, and the printed holder. The PLA film and the printed backplate build together the ferroelectret with artificial air voids. The printed holder clamps the film on the backplate and fixes the ferroelectret together. After poling, the trapped surface charge density serves as electrical bias as well as a mechanical pre-stress due to the permanent electrostatic forces. The resulting sound pressure is measured with a calibrated microphone (Type 4138, Bruel & Kjaer) at 30 cm distance. The sound pressure of the transducer is measured at the resonance frequency for different applied AC voltages (from 30 to 70 Vrms). The biodegradable ultrasonic transducer exhibits a large bandwidth of approximately 45 kHz and fractional bandwidth of 70 %. The resulting sound pressure at the resonance frequency can be increased from 98 dB up to 106 dB for driving voltages from 30 to 70 Vrms. respectively.