An Irreversible Single-Use Humidity-Threshold Monitoring Sensor Principle for Wireless Passive Sensor Solutions

Monitoring environmental parameters, which have negative effects on goods (e.g., temperature and relative humidity) to determine threshold or dosage violations, is of strong interest throughout their life cycle. The task is challenging if there is no continuous supply of electric energy available, and lowest cost monitoring solutions are required. These are two common application constraints when passive sensor-enhanced RFID tags are used to fulfill this monitoring task. A sensor principle is proposed in which the exceedance of a humidity threshold followed by critical exposure leads to a permanent change in an electric sensor parameter, i.e., a pronounced electric resistance decrease in the presented case. The sensor principle is based on the phenomena deliquescence of salts for humidity threshold detection and sensor activation mechanism and salt solution dissemination processes in porous media as well as chemical liquid phase sintering of metal nanoparticles under ambient temperature conditions as an irreversible state change mechanism. The sensor principle deduced arrangement is fully compatible with printed electronics manufacturing technologies as exemplified by a sample fabrication via inkjet printing. The proposed sensors can serve as humidity-triggered switches in electric circuits, which permanently close due to critical humidity exposure above a humidity threshold. Presented are experimental results demonstrating the feasibility of the proposed sensor concept. Also given are application examples not only demonstrating the suitability of the sensor concept for sensor enhanced RFID tags via an integration into a freely programmable RFID UHF transponder tag but also for wireless passive sensors exemplified via integration into an inductively coupled resonant sensor tag.