Plasma Confinement-Assisted Carbonization of Ag-Doped Polyimide by a Temporal-Shaped Femtosecond Laser for Mechanical Sensing

Miniature resistance-based portable strain sensors have been widely used for human health monitoring in recent years. Their sensitivities are defined by their resistance variation (ΔR/R), which strongly relies on the conductivity and minimum linewidth of the sensing unit. Laser-induced carbonization is a fast and simple method to fabricate porous sensing structures. However, the resolution of conductive and deformation-sensitive structures is limited by the thermal effect of commonly used laser sources. In this study, with the assistance of femtosecond laser temporal shaping, plasma ejection confinement and silver doping, the conductivity of a sensing structure was improved to 0.0004 Ω/□. A thin line with a lateral resolution of 6.5 μm was fabricated as the sensing unit. The fabricated structures were characterized by electron microscopy, Raman spectroscopy, energy dispersive spectroscopy, X-ray scattering and time-resolved images. The strain sensor had a △R/R value of approximately 40% in a cyclic bending test and showed a prompt response when it detected a human pulse and a bending finger.