Mechanical and sensing performance under hydrothermal ageing of wearable sensors made of polydimethylsiloxane with graphitic nanofillers

The effect of hydrothermal ageing on flexible wearable sensors made of PDMS reinforced with graphitic nano -fillers (carbon nanotubes, CNTs, and graphene nanoplatelets, GNPs) is explored. It is observed that the nanofiller addition, especially in case of GNPs, promotes a much higher water uptake at saturation due to the higher water diffusion as the generalized porosity is much more significant. However, it does not harm the mechanical properties in comparison with unaged samples, as these water molecules are not prevalently incorporated into the polymer network (around 0.3% water uptake in neat resin). Furthermore, the electrical properties at un-strained state are not severely affected but the electromechanical ones, which are much more sensitive to any change in the electrical network, drastically change with hydrothermal ageing. The gauge factor significantly increases in the case of GNP-doped samples, due to the higher separation between neighbouring nanofillers induced by the slight polymer swelling. The ultra-sensitivity of GNP-doped sensors (up to 100 at very low strains and above 10(6) at high strain levels) makes them very applicable for the detection of small human movements. For this purpose, a trial with a 2-month-aged GNP-doped sensor was successfully carried out in terms of neck and wrist pulse monitoring.