Embedding a critical temperature indicator in a high-frequency passive RFID transponder

The purpose of this work is to demonstate the conversion of an optical critical temperature indicator into a capacitive device and to test the effect of its introduction as a parasitic sensing load into the antenna circuit of an HF passive RFID tag. The resulting indicator-capacitor has been electrically characterized and the proposed methodology tested through the formulation and the study of modified transponders. The physical changes related to the transition from the pristine state of the indicator to its activated state have been recorded by electrical impedance spectrum measurements both through direct impedance analyses on the capacitor and through the characterization of an RFID tag embedding the capacitor in its antenna circuit. Experimental results show that impedance variations introduced in tags by the state transition of the indicator can be acquired both in chipless tags and chip-equipped transponders in terms of deviations from the pristine resonance frequency and that such smart labels could be acquired also in real-life scenarios through low-cost checkpoints. We suggest that more efficient implementations of resonant temperature indicators embedded in RFID tags could take advantage of our preliminary results in order to optimize the performance and functionality of the sensing tag.