Flexible resistive strain sensors for application in wearable electronics

We report flexible piezoresistive strain sensors containing two different layers of materials such as graphene and graphite, fabricated by a simple spin coating method on flexible polydimethylsiloxane (PDMS) substrates. In-house synthesized graphene and graphite-PDMS nanocomposites are used as active layers into the devices. The microstructure analysis of graphene and graphite-PDMS composites are carried out using scanning electron microscopy (SEM) technique. Small flakes having a thickness of 10-30 nm are clearly seen from the SEM images. Fundamental physical properties of graphene and graphite-PDMS nanocomposites are studied using Raman spectroscopy technique. In the Raman spectra, different bands, i.e. D, G and 2D of graphene, are located at 1340, 1577 and 2690 cm−1, respectively. The characteristic G peak of graphene is raised due to planar vibration of sp2 hybridized carbon. Two-phonon 2D peak determines the carbon stacks present in graphene. Existence of defects in graphene are determined by the D peak. The electrical characteristics are studied in PDMS based flexible devices with two different kind of device structures, viz. Ag/graphene/PDMS and Ag/graphite-PDMS. The gauge factor and sensitivity for the devices containing graphene are determined to be 6.260 and 2.155, whereas those values are found to be 0.906 and 0.051 for graphite-PDMS nanocomposite-based devices, respectively. Graphene based flexible devices are observed to be superior than its graphite-PDMS nanocomposite counterparts. Thus, simple device structure, easy device fabrication, affordable, portable and accessible including reasonably good sensitivity; it finds great potential for the manufacturing of wearable sensors.