Repurposing traditional China Xuan paper for versatile humidity sensing

Xuan paper (also known as Chinese rice paper), traditionally used for calligraphy and painting, has rarely been explored as a functional material. In this study, Xuan paper is repurposed for the first time as a humidity-sensitive material, exhibiting state-of-the-art sensitivity over a wide humidity range. A humidity sensor with a thickness below 0.09 mm and a mass below 0.012 g was fabricated using only Xuan paper, sodium chloride (NaCl) aqueous solution, and conductive carbon ink through a simple three-step process. Characterization of the sensor shows that NaCl crystals are combined with sparse cellulose fibers, facilitating moisture absorption and forming an electrochemical sensing system. To investigate the electrochemical properties of the sensor, electrochemical impedance spectroscopy was measured. The results reveal a transition in the conduction mechanism across a wide relative humidity range (11-97%), with an output variation as high as 2.65 × 10⁴ times. The large output variation enables easy readout without sophisticated circuits, paving the way for versatile applications. To enable humidity readout and wireless transmission, a flexible eight-channel readout circuit was developed based on a microcontroller (Arduino). The readout circuit and supporting smartphone application facilitated the practical tests of the humidity sensor, demonstrating its capabilities in environmental humidity monitoring, humidity-based touch sensing, urination monitoring, and motion state detection. This study attempts to address the longstanding trade-off between high performance and complex fabrication processes in humidity sensors and reveals the potential of Xuan paper as a functional material.