Title: Ferroelectric Polymer Sensors for Flexible Electronics
Ferroelectric polymers from the PVDF-family have proven to be multifunctional and self-sustaining materials with a broad deployment in printed and flexible electronics. They can be used in large and flexible form factors for detecting mechanical excitations such as pressure variations, force touch and impact, for sensing human-body radiation and proximity, as vibration sensors for structure-borne sound detection and acoustics, as fast and precise strain sensors, as stretchable vital parameter sensors for movement, ECG and respiratory rate monitoring, as well as piezoelectric energy harvesting elements, just to name a few .
The sensors are entirely fabricated by screen printing, which is one of the most common techniques used in printed electronics for the fabrication of large-area flexible components and multifunctional devices. Screen printing is highly tolerant to the type and form factor of substrates, the rheology of ink materials, provides sufficient alignment accuracy for multilayer printing and can be done in a sheet-to-sheet or roll-to-roll scheme. The printed ferroelectric polymer sensors come in two versions; (i) either they have a sandwich-type structure of four layers that are printed onto a flexible or stretchable substrate (e. g. plastic films, paper, and textiles up to A3) and response accurately, fast and reproducibly to pressure and temperature changes over large dynamic ranges or (ii) a two layer structure that is highly sensitive to lateral strain and vibrations .
By optimizing the design, the printing and drying/annealing processes as well as the poling conditions and the source material, functional sensors with a yield of more than 99% with less than ± 2% deviation in the remnant polarization were demonstrated. It is also interesting to note that extended aging tests under definite climate and shock conditions revealed more than 98% preservation of the remnant polarization for high molecular weight PVDF-TrFE polymers. Based on such sensors various applications like flexible and/or 3D user interfaces [3, 4] (Figure 1) and large-area force, impact and proximity sensors, ultrathin object-integrated microphones and condition sensors, smart floors as well as medical patches will be presented either in a passive sensor (array) configuration or as an active sensor matrix with an OTFT-backplane.
Finally, we developed a printable nanocomposite material, which allows reducing the cross-sensitivity between the pyro- and piezoelectric sensing modes. This material is composed of inorganic and preferentially lead-free ferroelectric nanoparticles blended in a ferroelectric polymer matrix , . By exploiting the fact that the piezoelectric coefficient in inorganic ceramics has an opposite sign to that one of the ferroelectric polymer either the piezo- or the pyroelectric activity can be suppressed by independently defining the poling direction of particles and matrix in a two-step poling procedure.
 B. Stadlober, M. Zirkl, M. Irimia-Vladu, Chem. Soc. Rev. 48, 1787–1825 (2019)
 P. Schäffner, M. Zirkl, G. Schider, J. Groten, M. R. Belegratis, P. Knoll, and B. Stadlober, under review (2019)  M. Zirkl, A. Sawatdee, U. Helbig, M. Krause, P. Bodö, P. Andersson Ersman, D. Platt, S. Bauer, G. Domann,and B. Stadlober, Adv. Mat. 23, 2069 (2011)
 C. Rendl, D. Kim, P. Parzer, S. Fanello, M. Zirkl, G. Scheipl, M. Haller, S. Izadi, FlexCase: Enhancing Mobile Interaction with a Flexible Sensing and Display Cover, Proceedings of the 2016 CHI Conference on Human Factors in Computing Systems, San Jose, California, USA, pp. 5138-5150, doi: 10.1145/2858036.2858314 (2016)
 I. Graz, M. Krause, S. Bauer-Gogonea, S. Bauer, S. P. Lacour, B. Ploss, M. Zirkl, B. Stadlober, and S. Wagner, J. Appl. Phys. 106, p. 034503 (2009)
 T. A. Ali, J. Groten, J. Clade, D. Collin, P. Schäffner, M. Zirkl, A.-M. Coclite, G. Domann, B. Stadlober, submitted (2019)
Acknowledgement - The authors would like to thank the FFG for support via funding the projects Smart@Surface and Self-Posh as well as the European Commission for support via funding the KIC Raw Materials project SuperSmart.
Fig. 1 a) Flexible smart card made of a bendable user interface based on printed piezoelectric sensors integrated with a flexible electrophoretic display, b) 3D-curved user interface made of printed and high pressure formed piezoelectric sensors keys with integrated LEDs for backlighting. .
Dr. Barbara Stadlober is Principal Investigator at the Institute for Surface Technologies and Photonics of the JOANNEUM RESEARCH Forschungsgesellschaft mbH (JR) located in Graz/Weiz, Austria. She studied experimental physics at the Karl-Franzens University Graz which she finished in 1992 with a master degree. In 1996 she received her PhD from the Walther Meissner Institute (TU Munich) in low temperature and solid state physics and starting in 1996 was part of the technology development team at Infineon Technologies Austria in Villach. She joined JR in 2002, where she built up a research group dedicated to organic and printed electronics and nanopatterning. Currently, she is head of the Research Group “Hybrid Electronics and Patterning” and her interests range from organic thin film transistors over printed ferroelectric sensors (PyzoFlex® technology) and 3D user interfaces to R2R-nanopatterning of multifunctional surfaces, biomimicry/Bionics, microoptical elements and flexible microfluidics. She has authored more than 90 peer-reviewed papers and is an inventor of more than 10 patents.
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