High-performance stretchable strain sensors, particularly those with high sensitivity and broad sensing range, are highly important for wearable devices. Herein, a novel auxetic bilayer conductive mesh strain sensor (ABSS), composed of multi-hardness silicones, is proposed and fabricated by the direct ink writing 3D printing and ink spraying technique. The bilayer conductive mesh comprises a thin layer of high-conductive and crack-prone single-walled carbon nanotubes (SWCNTs) coated on a stretchable carbon-black-doped Ecoflex silicone rubber (CB/Ecoflex) mesh. The former serves as the dominant sensing material by generating SWCNT cracks in the full strain range, while the latter mainly plays the roles of both generating the resistance change and maintaining the conductive paths under high strain conditions. The presence of high-hardness auxetic frame contributes to the formation of longitudinal SWCNT cracks on transverse meshes, enhancing the sensitivity of the sensors. It is shown that the synergistic effect of the bilayer conductive mesh, strain concentration, and auxetic deformation strategy endow ABSS with a high gauge factor (∼ 13.4) that is 6.6 times larger than that of the common sensor. Additionally, this study demonstrates the superior sensing performance of the ABSS for wearable applications including swallowing recognition, respiration monitoring, and joint movement detection.
Keywords: auxetics; bilayer conductive mesh; direct ink writing; sensitivity; strain sensors.