A wireless passive humidity threshold monitoring solution based on a permanent resistance change

A wireless passive sensor solution is presented, which permits continuous monitoring of a humidity threshold violation, 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 comparatively large permanent resistance changes (i.e., 1M 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.