Abstract
Crack-based strain sensor systems have been known for its high sensitivity, but suffer from the small fracture strain of the thin metal films employed in the sensor which results in its negligible stretchability. Herein, we fabricated a transparent (>90% at 550 nm wavelength), stretchable (up to 100%), and sensitive (gauge factor (GF) of 30 at 100% strain) strain gauge by depositing an encapsulated crack-induced Ag nanowire (AgNW) network on a hydroxylated poly(dimethylsiloxane) (PDMS) film. Stretching the encapsulated AgNWs/PDMS resulted in the formation of a percolation network of nanowire ligaments with abundant percolation paths. The encapsulating polymer was designed to adhere strongly to both the AgNW and PDMS. The improved adhesion ensured the resistance of the crack-induced network of AgNWs varied reversibly, stably, and sensitively when stretched and released, at strains of up to 100%. The developed sensor successfully detected human motions when applied to the skin.
Highlights
Piezoresistive strain gauges are used to measure the strain experienced by an object under an applied force and are typically fabricated by laminating patterned metal foil or wires on an insulating flexible support[1,2,3,4]
Since no thorough studies on the mechanism of fracture under large strains of Ag nanowire (AgNW) deposited on a stretchable substrate have been reported, we investigated the type of failures that might occur in the nanowire network by applying a large strain to AgNWs-based elastomeric electrodes
Some microcracks were formed in the AgNW forest when we applied a uniaxial strain of 25% to the AgNWs/PDMS, generating an ordered network of nanowire ligaments divided with cracks aligned perpendicular to the stretching direction
Summary
Piezoresistive strain gauges are used to measure the strain experienced by an object under an applied force and are typically fabricated by laminating patterned metal foil or wires on an insulating flexible support[1,2,3,4]. Another method used to produce these devices is to bury the percolated conductive networks at the surface of the cross-linked polymer[22] Since this approach usually employs metal nanowires or CNTs in very low densities, the fabricated structures are typically transparent; this is an advantage for their use in devices. Ultrasensitive strain gauges based on crack-induced sensor systems have recently been reported to achieve gauge factors (GF) as high as 16,000 at 2% strain, which is significantly higher than those of reported composite structures[23, 24] In these crack-induced sensors inspired by the geometry of a spider’s slit organ[23], the metallic nanoscale cracks formed in a polymer layer are disconnected and reconnected with adjacent crack junctions www.nature.com/scientificreports/. The fabricated strain gauge could successfully detect strains induced by muscle movements on the skin of a proximal interphalangeal joint and a human face
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