Flexible sensor and energy harvesting technology based on ferroelectret for the acquisition of mechanical and physiological parameters close to the body

With increasing digitization, the need for highly integrated sensor systems that ensure long-term recording of physical data is also growing. Smart materials are ideal for implementing such systems, as they have the ability to convert mechanical energies in the form of stresses, deformations and vibrations into an electrically measurable quantity and thus fulfil a sensory task. This can be achieved using the direct piezoelectric effect (the inverse or indirect piezoelectric effect can enable an actuator function). In addition, the electrical charge energy generated by the piezoelectric effect can also be fed to an electrical storage device (accumulator, capacitor, etc.) in order to supply energy to a decentrally operating system. In this case, we speak of energy harvesting. The combination of these two technical principles (sensor and energy harvesting) is particularly suitable for the design of decentralized energy-autonomous, highly integrated monitoring systems. The best-known materials with a piezoelectric effect are ceramic or crystalline materials enabling the design of a wide variety of sensor and harvesting concepts. Due to their high young's modulus, their brittle material behavior and their high specific density, these materials are only suitable to a very limited extent for technical applications in which solutions are sought for integration into flexible systems such as textiles. Alternatives are offered by polymeric materials of which PVDF (polyvinylidene fluoride) has already been considered in several papers. However, these materials are disadvantageous in terms of transducer coefficients. More advantageous are piezoelectric electrets (also called piezoelectrets or ferroelectrets) due to their high piezoelectric coefficients and low mass density. They allow the realization of lightweight transducers with high output power. In this paper, the results from recently completed research projects are presented and possible applications for ferroelectrets to implement sensors and energy harvesters, especially for smart wearables, are outlined.