Fully Printed Flexible Sensor With a Cobalt-Based Metal–Organic Framework Sensing Layer for Humidity Monitoring in Wearable Systems

To advance wearable sweat monitoring, this study introduces a fully printable, flexible moisture sensor based on a novel integration of silver interdigitated electrodes and a cobalt-based metal–organic framework (Co-MOF) sensing layer. Successful synthesis of the Co-MOFs was confirmed through characterization using X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) analyses. The printed electrodes were fabricated using a simple inkjet-printing technique with silver nanoparticles-based ink, and the Co-MOF-based ink was then subsequently deposited to form a uniform sensing layer. Sensor performance was carefully investigated under controlled humidity conditions (25–95% RH) using both resistive and impedance-based measurement methods. A calibration curve with a sensitivity of 0.06 Ω/% in the range of 50-95% humidity was obtained using the resistive technique. In contrast, impedance spectroscopy at an optimized frequency of 100 Hz provided a broader linear range of 35–90% RH, with higher sensitivity of −1.13 Ω/% RH. The impedance-based method also demonstrated good repeatability and reproducibility, with RSDs of 4% and 5.5%, respectively, low hysteresis of 1.3% RH, and fast response/recovery times of 60 s and 100 s. These results highlight the ability of impedance spectroscopy to capture capacitive and dielectric contributions that cannot be measured using conventional resistive testing, particularly at low humidity levels. Overall, the fabricated printable Co-MOF-based sensor shows strong potential for practical applications in non-invasive monitoring of humidity, such as sweat analysis and wearable health devices, although further work is needed to evaluate long-term stability and ultra-low humidity detection on-body.