Low-cost inkjet-printed fully passive RFID tags using metamaterial-inspired antennas for capacitive sensing applications

A fully passive, compact, and low-cost capacitive wireless RFID-enabled sensing system for capacitive sensing and other Internet of Things applications is proposed. The proposed RFID tag antenna based sensor consists of a closely spaced two-element dipole RFID tag antenna array with a printed capacitive sensor connected to one of the tags. A metamaterial-inspired resonator is used to improve isolation among the two antennas and optimize the size of the antenna structure. When high permittivity materials, such as water or human fingers, are close to the on-tag meander line structure, only one of the RFID chips is able to respond due to the capacitance change, and consequently, detuning of the antenna structure. Therefore the system can distinguish capacitance change using just one fixed operation frequency. All components except from the RFID chips are inkjet-printed on photo-paper using a silver nano-particle ink. The tag dimensions are 84mm × 89mm and the tag is compatible with EPC Class 1 Gen 2 (UHF) standard reader at 915 MHz. Measurements using a commercial RFID reader are used to demonstrate the operation of the fabricated prototype.