Low-temperature direct writing fabrication of flexible humidity sensors

The low-temperature， high-bonding direct writing of three-dimensional conductive structures poses a challenge to the manufacturing of flexible electronic composite devices. Based on the silver-ammonia complexation mechanism， a particle-free direct writing solution suitable for forming three-dimensional conductive structures has been developed in this study. The problem of metal particle aggregation was overcome， achieving long-term stability. Under low-temperature sintering conditions at 50 °C， the conductive fibers prepared using the silver-ammonia complex solution exhibit excellent conductivity， with resistance as low as 365 Ω/mm， surpassing those made with silver nanoparticle solutions. The fiber structures produced using the silver-ammonia complex solution have strong adhesion to flexible substrates and possess good self-stacking characteristics. By adjusting the number of fiber stacking layers from 10 to 50， the resistance of the conductive fibers can be controlled within the 6.7-34.1 Ω/mm range. A humidity sensor designed based on the silver-ammonia complex solution was tested in different saturated salt solutions to establish humidity environments. The sensor demonstrated excellent humidity responsiveness， with hysteresis as low as 3.2%， and exhibited ideal real-time response even under high humidity conditions. The particle-free direct writing solution holds promising prospects in the fabrication of flexible electronic products and finds wide applications in fields such as motion detection and health management. © 2023 Chinese Academy of Sciences. All rights reserved.