Flexible RFID Tag for Sensing the Total Minerals in Drinking Water via Smartphone Tapping

The recent advances in energy-efficient electronics and sensor fabrication techniques have reshaped the way of sensing and communication, which have nurtured many novel applications. Among the emerging sensing techniques, wirelessly powered sensing and communication enhanced by Radio Frequency Identification (RFID) has gained increasing popularity. In this investigation, we propose a novel flexible RFID sensor tag powered and interrogated by a smartphone with its in-built High-Frequency (HF) RFID module, which can be applied for convenient non-contact analysis of liquids. In this work, the evaluation of total minerals in drinking water is conducted as a proof-of-concept demonstration. The main contributions include: (1) modeling and theoretical analysis of a planar capacitive sensor for liquids analysis by sensing the dielectric properties; (2) design of a flexible capacitive sensor tag that can be conformally mounted on the sidewall of containers for the liquid analysis; (3) a power-efficient hardware scheme for HF RFID powered battery-less sensing via smartphone tapping; (4) a Near Field Communication (NFC) data transmission method for convenient measurement without added software in smartphone devices. Finally, the developed sensor tag is calibrated by multi-piece linear function fitting. Experimental results and quantitative analysis demonstrate the feasibility of the proposed methodology.