Polymers with electrically charged internal air cavities (ferroelectrets) reveal a pronounced piezoelectric response and are regarded as soft electroactive multi-functional materials. This work presents preliminary results on the preparation and piezoelectric effect of ferroelectrets based on the polylactic acid (PLA) polymer. A distinctive feature of the manufactured films is that they are biodegradable. After a microstructure modification of carbon dioxide (CO₂) foamed PLA sheets by hot-pressing treatment and corona polarization, these cellular films reveal large piezoelectric $d_{33}$ and $d_{31}$ responses in both quasi-static and dynamic modes. For freshly charged films, the maximum quasi-static piezoelectric coefficients are about 600 pC/N ($d_{33}$ ) and 44 pC/N ($d_{31}$ ) for a relatively thick film of 360 μm and a nominal porosity of about 60%. During the first 20 days after polarization, the piezoelectric activity decreases by half compared to the primary value, but then remains almost unchanged for a long time. Due to an already established inherent biocompatibility of PLA polymers, these eco-friendly ferroelectrets can be potentially used in various biological applications such as biosensors and microenergy harvesters embedded in tissue and artificial muscles.

During muscle contractions, radial-force distributions are generated on muscle surfaces due to muscle-volume changes, from which the corresponding body motions can be recorded by means of so-called force myography (FMG). Piezo-or ferroelectrets are flexible piezoelectric materials with attractive materials and sensing properties. In addition to several other applications, they are suitable for detecting force variations by means of wearable devices. In this paper, we prepared piezoelectrets from cellular polypropylene films by optimizing the fabrication procedures, and developed an FMG-recording system based on piezoelectret sensors. Different hand and wrist movements were successfully detected on able-bodied subjects with the FMG system. The FMG patterns were evaluated and identified by means of linear discriminant analysis and artificial neural network algorithms, and average motion-classification accuracies of 96.1% and 94.8%, respectively, were obtained. This paper demonstrates the feasibility of using piezoelectret-film sensors for FMG and may thus lead to alternative methods for detecting body motion and to related applications, e.g., in biomedical engineering or structural-health monitoring.

Ferroelectrets have promoted a variety of exciting flexible sensors, actuators, and microenergy harvesters. However, most ferroelectrets have been fabricated from non‐degradable petro‐based resins, and thus the recycling of these materials constitutes a big challenge. This article reports biodegradable and bioabsorbable ferroelectret films made from polylactic acid (PLA) resins for highly sensitive transducer applications, which can operate either in piezoelectric 33 or 31/32 mode. By modification of the microstructure and polarization, pronounced longitudinal and transverse piezoelectric activities are realized in a single material. For samples with a thickness of 400 µm and a bulk density of 350 kg m⁻³, the Young's moduli in thickness and plane direction are ranging from 0.1 to 10 MPa, respectively. After polarization in the thickness direction, quasi‐static piezoelectric d₃₃, g₃₃, d₃₁ (d₃₂), and g₃₁ (g₃₂) coefficients in the PLA films, up to 500 pC N⁻¹, 40 Vm N⁻¹, −44 pC N⁻¹, and −3.6 Vm N⁻¹, are achieved, respectively. The longitudinal piezoelectric coefficients of the PLA films are comparable to non‐degradable polymer ferroelectrets, while the transverse piezoelectric activity is superior, which may be attributed to the reduction of Young's moduli in the plane direction. The preparation procedure of the PLA ferroelectrets is compatible with large‐scale production lines and thus can greatly promote their applications in green electronics.