Fabrication of humidity sensor using 3D printable polymer composite containing boron-doped diamonds and LiCl

Humidity sensing is of significant interest to monitor and control the moisture sensitive environments. Here, we developed a novel 3D printable composite consisting of boron-doped diamond (BDD) (60 wt %) and LiCl (2 wt %) in acrylonitrile butadiene styrene (ABS). SEM analysis of the composite material confirmed the uniform distribution of the BDD and presence of a thin layer of LiCl distributed throughout the matrix. The developed composite material was employed for simple and quick (∼2 min) fabrication of the humidity sensor using low cost fused deposition modeling (FDM) 3D printer. The unique composite material allowed the fabrication of one-piece 3D printed sensor in comparison to traditional multicomponent (e.g., support, sensitive film, and electrodes) humidity sensing devices. The resulting humidity sensor showed excellent sensitivity with up to 125-fold change in resistance for the range of 11–97% relative humidity. The quick response (60 s, n = 3, RSD= 18.7%) and the recovery time (120 s, n = 3, RSD = 16.6%) is attributed to the uniform distribution of the BDD electrode material and strong networking with the LiCl layer distributed throughout the matrix. Long-term stability and repeatability was evaluated, with relative standard deviation of the response of less than 15% obtained over a test period of 14 days. When applied as a sensor for humidity in human breath, the response curves obtained for 12 consecutive breath cycles with post-breath compressed air-drying, showed excellent repeatability and sensitivity, with quick response and recovery (13 s, n = 12, RSD = 15%). The developed 3D printable humidity sensing material was also used to fabricate a customized 3D printed sensor for monitoring the humidity of the N₂ supply.