Inkjet printed humidity threshold monitoring sensor solution with irreversible resistance change for passive RFID applications

Continuous monitoring of environmental parameters to determine threshold or dosage violations is of strong interest throughout the life cycle of goods. The task is challenging, when there is no continuous supply of electric energy available and lowest cost sensor solutions are required - two common application constraints in passive RFID applications which are also required to fulfill monitoring tasks. Presented is a humidity threshold violation sensor concept, in which the exceedance of a humidity threshold value leads to a permanent sensor resistance change. The principle is based upon the deliquescence of salts as a threshold detection mechanism, as well as chemical sintering of metal nanoparticles, used as irreversible state change mechanism. The introduced sensor arrangement is completely inkjet printed. Several experiments demonstrate the feasibility of the approach. Due to the strong resistance change, the sensor elements can serve as humidity triggered switches in electric circuits as demonstrated by the integration into a freely programmable RFID UHF transponder tag.