A Wireless Passive Humidity Threshold Monitoring Solution Based on a Permanent Resistance Change

AbstractA wireless passive sensor solution is presented, which permits continuous monitoring of a humidity threshold viola- tion, but does not require a permanent supply of electric energy. It is based on an inkjet printed resistance changing element, which incorporates three mechanisms: the deliquescent behavior of salts, transport processes in porous media and chemical sintering of silver nanoparticles under ambient temperature conditions. Upon exceeding a salt specific deliquescence humidity a salt solution forms in a reservoir (humidity threshold activation), which is transported through a porous substrate (delay mechanism) into a silver nanoparticle region, triggering a chemical sintering under ambient temperature conditions (permanent state change). In dependence of the design and manufacturing parameters compar- atively large permanent resistance changes (i.e., 1MΩ and above) can be realized. These humidity triggered switches can be integrated into inductively coupled resonant (ICR) sensor tags, in the presented case, a double planar sensor coil arrangement