Environmentally friendly Hanji cellulose paper-based humidity sensor for biomedical applications

Flexible and environmentally sustainable humidity sensor are increasingly required for wearable and biomedical monitoring. In this work, we present a fully biodegradable humidity sensor based on traditional Korean Hanji cellulose paper, which serves simultaneously as both the substrate and the active sensing layer. The device is fabricated through a simple two-step process by attaching silver-coated aluminum foil electrodes onto Hanji paper, eliminating the need for cleanroom processes or complex synthetic sensing materials. Owing to the intrinsic porosity and the hydrophilicity of the Hanji fibers the sensor exhibits a strong humidity response of 8.52 × 10 5 ( I 91.8% / I 7.6% ) across a wide relative humidity range of 7.6%–91.8% while operating at an ultralow bias voltage of 0.3 V with excellent linearity ( R 2 = 0.9912). The device also demonstrates mechanical flexibility and stable performance under bending cycles. Furthermore, the sensor enables practical demonstrations such as non-contact fingertip humidity detection and breath intensity monitoring, highlighting its potential for wearable biomedical sensing. These results demonstrate that Hanji paper provides a promising platform for low-cost, eco-friendly, and disposable humidity sensing technologies.