Flexible Passive Temperature Sensor Label with Contactless Interrogation

A passive inductive-capacitive (LC) resonant temperature sensor embedded onto a flexible label is presented. The sensor exploits a patterned copper foil on adhesive paper for the inductive coil and a ceramic capacitor as the temperature sensing element. Temperature affects the value of the capacitance and hence of the resonant frequency of the LC tank resonator. The sensor can be contactless interrogated through an external readout coil electromagnetically coupled to the on-label coil. The interrogation technique is based on the measurement of the reflected impedance at the readout coil to monitor the variations of the resonant frequency of the LC pair on the sensor label. Preliminary tests have demonstrated a temperature sensitivity of 330 Hz/°C from room temperature up to 80°C. The proposed flexible label opens future uses as a cost-effective device for sensing applications in enclosures and packages.