Enabling a Paper-Based Flexible Sensor to Work under Water with Exceptional Long-Term Durability through Biomimetic Reassembling of Nanomaterials from Natural Wood

Paper-based sensors have many distinguishing advantages; however, how to improve their working durability especially under water is a critical issue in many applications and unfortunately remains a huge challenge. In this work, we design and develop an innovative strategy enabling paper-based strain and pressure sensors to work under water with exceptional long-term working durability by biomimetic reassembling of nanomaterials from natural wood. A composite paper consisting of softwood fibers and 22 wt % graphite nanoplates was prepared based on a papermaking protocol. Cellulose nanofibers were added to strengthen the composite paper, and lignin nanoparticles were applied onto the paper surface via the paper coating technology to obtain its superhydrophobicity. Subsequently, the as-obtained superhydrophobic composite paper was assembled into a flexible sensor that can be used to detect strain and pressure changes both in air and under water. As the strain sensor, its gauge factor was 10.9 and 14.6 in air and under water, and the corresponding response time was 0.3 and 0.15 s, respectively. Surprisingly, an exceptional working stability was achieved even after more than 10,000 bending–unbending cycles under water. As the pressure sensor, its sensitivity (S) was 0.02 and 0.38 kPa–1 in air and under water, and the corresponding response time was 0.46 and 0.3 s, respectively. Its long-term durability can also exceed 10,000 pressing–releasing cycles. This novel strategy developed in this work can be an effective approach for biomimetic re-engineering of biomass-derived nanomaterials, aiming to develop highly stable paper-based flexible sensors that can find applications in wearable systems and underwater equipment.