Paper-based flexible strain and pressure sensor with enhanced mechanical strength and super-hydrophobicity that can work under water

Paper-based flexible sensors have attracted much attention due to their abundant resource, low cost and good degradability in the natural environment leaving no electronic waste (e-waste). However, the mechanical properties of paper still need to be strengthened and the output stability may be impacted by water or high humidity because of the hydrophilicity of cellulose fibers. In order to solve these problems, for the first time, a paper-based flexible sensor with available running data under water without encapsulation was fabricated using softwood cellulose fibers and graphite nanoplates through a papermaking technique in this work. The cellulose nanofibers (CNFs) were added to reinforce the paper and the alkyl ketene dimer (AKD) emulsion was coated on the paper to endow it with super-hydrophobicity. The sensor can be used for both strain and pressure detection. As a strain sensor, the gauge factor (GF) was 18.99 and the response time was 0.3 s. It can be used to monitor human motions such as the bending of fingers, the wrist, and elbow. Interestingly, it was proven to be able to monitor finger bending at high humidity and under water due to its super-hydrophobicity. As a pressure sensor, it exhibited sensitivity S1 = 0.019 kPa^-1in the range of 0 to 316.5 kPa, and S2 = 0.01 kPa^-1in the range of 316.5 to 1421 kPa, with the response time of 0.3 s. The detecting range is much wider than that of reported sustainable flexible pressure sensors. Furthermore, this sensor can be easily disposed of by burning, thus avoiding the production of e-waste. This multifunctional flexible paper-based sensor may find potential in sport and health monitoring, soft robotics and human-machine interfaces.