Geometry-Optimized Flexible Ferroelectret Fabricated by Laser Patterning of an Adhesive Spacer

The growing demand for continuous cardiac signal monitoring has propelled the development of highly sensitive flexible sensors with reduced power requirements. Proposed flexible ferroelectrets/piezoelectrets are voided polymers that can be polarized/poled, acquiring a piezoelectric behavior that is suited for sensing applications. Here, ferroelectret preparation was achieved by laser-patterning of an adhesive-coated polypropylene (PP) spacer with low Young's modulus, resulting in improved piezoelectric coefficient while avoiding high-temperature thermal bonding between the different ferroelectret layers. The spacer pattern and thickness were optimized and compared to a suitable analytical model. The optimized piezoelectret showed a low compressive modulus (~300 kPa), a maximum dynamic piezoelectric coefficient <tex-math notation="LaTeX">d_33 </tex-math> = 81 ± 5 pC/N, and a quasi-static piezoelectric coefficient of ~8552 pC/N. In the short term, the sensors were stable in a wide range of temperature and relative humidity, but after 70 days at room temperature, the <tex-math notation="LaTeX">d_33 </tex-math> decreased to ~36% of the initial value. Finally, the optimized ferroelectret device was demonstrated in the measurement of the human pulse and extraction of cardiovascular parameters [heart rate (HR) and carotid-femoral pulse wave velocity (PWV)].