HF RFID Tag as Humidity Sensor: Two Different Approaches

This paper describes printed resonant structures on a flexible substrate for humidity detection. The devices consist of a screen-printed spiral inductor working as radio frequency identification (RFID) antenna and two different inkjet-printed planar capacitive structures forming LC resonators. The sensitive material is directly the chosen substrate: a polyimide whose electrical permittivity is dependent on the moisture content. Therefore, no extra sensitive layer is required to functionalize these structures. In particular, two different approaches can be followed to use it. On one hand, the resonant structure can be used as a threshold tag including an RFID chip. The tags will be detected or not by an RFID reader depending on the humidity. On the other hand, these structures can be directly used as quantitative humidity sensors where the moisture content is associated with the resonant frequency of the structure.