Frequency-locked Wireless Multifunctional Surface Acoustic Wave Sensors.

Surface acoustic waves (SAWs) have shown great potential for developing sensors for structural health monitoring (SHM) and lab-on-a-chip (LOC) applications. Existing SAW sensors mainly rely on measuring the frequency shifts of high-frequency (., >0.1 GHz) resonance peaks. This study presents frequency-locked wireless multifunctional SAW sensors that enable multiple wireless sensing functions, including strain sensing, temperature measurement, water presence detection, and vibration sensing. Our sensors leverage SAW resonators on piezoelectric chips, inductive coupling-based wireless power transmission, and, particularly, a frequency-locked wireless sensing mechanism that works at low frequencies (., <0.1 GHz). This mechanism locks the input frequency on the slope of a sensor's reflection spectrum and monitors the reflection signal's amplitude change induced by the changes of sensing parameters. The proof-of-concept experiments show that our wireless sensors can operate in a low-power active mode for on-demand wireless strain measurement, temperature sensing, and water presence detection. Moreover, our sensors can operate in a power-free passive mode for vibration sensing, with results that agree well with laser vibrometer measurements. We anticipate that the designs and mechanisms of our frequency-locked wireless SAW sensors will inspire researchers to develop future wireless multifunctional sensors for SHM and LOC applications.