Frequency-Coded Flexible Microfluidic RFID Sensor for Simultaneous Temperature and Deformation Measurement

This article presents a chipless Radio Frequency Identification (RFID) tag for detecting temperature and deformation. The sensor is constructed using eutectic gallium indium (EGaIn) as a liquid metal patch, polydimethylsiloxane (PDMS) as the substrate, and a copper sheet as the ground layer, which makes the sensor flexible and enables it to detect the curvature or deformation of the surfaces or objects. In the geometry of the sensor, a microchannel-based meander patch is designed utilizing a 3D-printed mold technique, which simplifies the fabrication process of a complex design. The operating principle of the tag involves reflecting part of the incident electromagnetic wave, generating a notch at a specific frequency in the backscattered signal that is responsive to variations in the curvature of the sensor geometry. This work incorporates customizable oil solutions with specific melting temperatures into the sensor's geometry, resulting in the first flexible chipless RFID sensor capable of internally storing temperature sensing data without external memory. The developed sensor detects a temperature threshold crossing at 24°C, showing resonant frequencies of 5.877 GHz and 5.20 GHz below and above the melting point, respectively. Additionally, the shifts in these resonant frequencies vary linearly with different deformation angles of the tag geometry, enabling its use as a deformation detection sensor.