Flexible highly-sensitive humidity sensor based on CGO/SMPLAF for wearable human skin humidity detection

Conventional humidity sensors have poor flexibility and low sensitivity, which makes them difficult for wearable electronic device applications. This study aims to fabricate a flexible, high-sensitivity and breathable humidity sensor for skin humidity monitoring. The sensor includes two parts, i.e., shape memory polylactic acid fiber (SMPLAF) as the substrate and crumpled graphene oxide (CGO) membrane as the top humidity-sensitive layer. SMPLAF was firstly stretched and then heated to restore its original shape to obtain the CGO membrane. The unique crumpled morphology of CGO membrane provides a large specific surface area and high capillary force, which facilitates the rapid exchange of water molecules between the sensing CGO membrane and the external environment. Accordingly, the sensor exhibits excellent humidity sensitivity, fast response time (<5 s) and long term stability (15 days). Moreover, the CGO-based sensor demonstrates excellent flexibility. Bent at 120°, the response almost unchanged, whereas the response of the sensor without CGO decreased from 0.89 to 0.66. Though the sensor has the disadvantage of unstable performance at high RH, it could be overcome by controlling the wavelength of the crumpled structure of CGO. Due to the above superior properties, the sensor shows promising potentials for human breath monitoring and skin humidity test.