Self‐powered Wearable Humidity Sensor for Accurate Respiratory Monitoring

Two-dimensional transition metal carbides are promising materials for humidity sensing because of their excellent hydrophilicity and metallic conductivity. However, the inherent self-restacking and weak interactions between the nanosheets usually lead to sluggish ion transport and unsatisfying mechanical strength. Here, an ionic cross-linking MXene nanocomposites have been developed with fast ion kinetics and superior mechanical properties for high-performance flexible humidity sensors. The sodium alginate/calcium ion cross-linking network acts as both an interlayer spacer and a binder point. It not only suppresses the restacking effect of nanosheets, but also creates hydrophilic nanochannels that facilitate water adsorption and ion transport. As a result, the flexible sensor based on ionic cross-linking MXene nanocomposites generates a spontaneous voltage over a broad humidity range with a maximum output of 0.577 V and rapid response/recovery speed. The performance improvement thanks to redox reactions mediated by the dissociation of water molecules inside the nanocomposites. Furthermore, the wearable humidity sensors are demonstrated to be employed for the early detection of obstructive sleep apnea that is of great commercial value. The study presents an insight into the design of structural materials for wearable sensors toward advanced medical diagnosis.