Eco-Friendly High-Sensitive Piezoelectrets for Force Myography

Piezoelectrets (or ferroelectrets) are polarized cellular polymers exhibiting outstanding piezoelectric activity. Recently, a new manufacturing route has been introduced by combining 3-D-printed elastomeric soft grids and bulk fluorinated ethylene propylene (FEP) thin films to build an artificial ferroelectret sandwich exhibiting outstanding ${d} _{{33}}$ coefficients. In this work, a similar approach is taken, however, replacing the environmentally harmful fluorinated polymer FEP by eco-friendly polylactic acid (PLA). The goal of this study is thereby threefold: 1) to investigate the temporal charge stability of PLA films charged at 25 °C and 80 °C, respectively; 2) to determine the stability of the piezoelectric activity when assembled as ferroelectret; and 3) to study the applicability of this easy assembled eco-friendly sensors for force myography (FMG). The surface potential of the PLA films was recorded over 180 days and showed promising stability. Because of the high flexibility of the spacer material, a large piezoelectric ${d} _{{33}}$ coefficient of up to 2850pC/N was recorded directly after charging and stabilized at about 1500pC/N after $\approx 50$ days under ambient environmental conditions. The FMG was conducted using four similar ferroelectrets placed on a human forearm. The generated signal shapes and magnitudes of the sensors can be clearly distinguished from each other using simple machine learning algorithms known as support vector machine (SVM) with a classification accuracy (CA) of 89.5%. This work can be regarded as a milestone for these sensors to be accepted in the field of human–machine interfaces (HMIs) as an eco-friendly alternative to the conventional piezoelectric sensors.