Flexible sensors are fundamental devices for human body monitoring in application areas ranging from health care to soft robotics. During the last decade the possibility to couple sensing of mechanical strain and physiological parameters have attracted ever increasing interest to design novel, robust and low-cost wearable sensing units. Stretchable and pH sensible piezoelectric strain sensors made by blending intrinsically conductive polymers and polymeric electrolyte can serve this purpose. In this work, we specifically investigate a Crosslinked Nanofibers (NFs) Flex Sensor able to detect mechanical flection and pH change. The optimized sensitive element of the NFs Flex Sensor is based on crosslinked electrospun NFs mats made of a blend of (polyethylene oxide) PEO as the polymeric electrolyte, and poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS) as the intrinsically conductive polymer. The NFs Flex Sensor has been obtained by directly collecting the nanomaterial on a flexible and biocompatible polydimethylsiloxane (PDMS) slab and thermally treating it to promote electrical conductivity of the PEO/PEDOT:PSS NFs. The thermal treatment was optimized to crosslink PEO and PSS while preserving the nanostructuration, to optimize the mechanical coupling with PDMS substrate and to improve water resistance. In this work, we demonstrate excellent mechanical sensing of the NFs Flex Sensor coupled to electrochemical pH detection. Change of pH was detected by Electrochemical Impedance Spectroscopy (EIS), obtaining a linear dependence of the capacitance with the pH value. The piezo-resistive caracterization of the Crosslinked NFs Flex Sensors demonstrated the ability of nanomaterials to recover their initial configuration after release of the mechanical strain in both compression and traction mode. The Gauge Factors (GFs) values were 45.84 in traction and 208.55 in compression mode, reflecting the extraordinary piezoresistive behavior of our nanostructurated PEO/PEDOT:PSS NFs.

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