Combining High Sensitivity and Dynamic Range: Wearable\nThin-Film Composite Strain Sensors of Graphene, Ultrathin Palladium,\nand PEDOT:PSS

Abstract

Wearable mechanical\nsensors have the potential to transform healthcare\nby enabling patient monitoring outside of the clinic. A critical challenge\nin the development of mechanicale.g., strainsensors\nis the combination of sensitivity, dynamic range, and robustness.\nThis work describes a highly sensitive and robust wearable strain\nsensor composed of three layered materials: graphene, an ultrathin\nfilm of palladium, and highly plasticized PEDOT:PSS. The role of the\ngraphene is to provide a conductive, manipulable substrate for the\ndeposition of palladium. When deposited at low nominal thicknesses\n(∼8 nm), palladium forms a rough, granular film which is highly\npiezoresistive (i.e., the resistance increases with strain with high\nsensitivity). The dynamic range of these graphene/palladium films,\nhowever, is poor and can only be extended to ∼10% before failure.\nThis fragility renders the films incompatible with wearable applications\non stretchable substrates. To improve the working range of graphene/palladium\nstrain sensors, a layer of highly plasticized PEDOT:PSS is used as\na stretchable conductive binder. That is, the conductive polymer provides\nan alternative pathway for electrical conduction upon cracking of\nthe palladium film and the graphene. The result was a strain sensor\nthat possessed good sensitivity at low strains (0.001% engineering\nstrain) but with a working range up to 86%. The piezoresistive performance\ncan be optimized in a wearable device by sandwiching the conductive\ncomposite between a soft PDMS layer in contact with the skin and a\nharder layer at the air interface. When attached to the skin of the\ntorso, the patch-like strain sensors were capable of detecting heartbeat\n(small strain) and respiration (large strain) simultaneously. This\ndemonstration highlights the ability of the sensor to measure low\nand high strains in a single interpolated signal, which could be useful\nin monitoring, for example, obstructive sleep apnea with an unobtrusive\ndevice.