Printed Humidity Sensor With Memory Functionality for Passive RFID Tags

This paper presents a printed UHF radio frequency identification (RFID) sensor solution that indicates whether a passive RFID tag has been exposed to a certain degree of humidity. The printed sensor operates as a write-once read-many-times resistive memory as it permanently changes its resistance from about 2 k Omega to less than 50 Omega after exposure to humidity or water. The change of the sensor resistance is used to modulate the properties of a UHF RFID tag antenna by changing antenna input impedance and also introducing or removing ohmic losses in the antenna structure. The final result is reflected in the change of the minimum transmit power required to power-up the RFID tag. Both the sensor and the tag antenna are printed horizontally on a paper substrate. Different geometric combinations of sensor and antenna structures are investigated. Typical applications include noninvasive methods for detecting wetness or humidity levels in hidden locations, such as within construction structures. The sensor setup can also serve to provide information as to whether a package sent over a supply chain has exceeded a certain level of humidity during its route.