A Fully Flexible Wireless Passive Strain Sensor Utilizing an Ultra-Thin Quartz Surface Acoustic Wave Resonator

Conventional wireless passive surface acoustic wave (SAW) strain sensors often employ rigid materials, limiting their application on curved surfaces. This letter presents a fully flexible wireless passive strain sensor, implemented by the flip-chip integration of an ultra-thin SAW device with a flexible antenna. The SAW sensor, fabricated on a 50- $\mu $ m ST-cut quartz substrate with 200-nm aluminum electrodes, is coupled with a flexible antenna made by patterning copper electrodes on a polyimide substrate. Experimental evaluations include both wired and wireless tests, demonstrating the system’s sensitivity of 121.63 Hz/ $\mu \varepsilon $ within a dynamic range of $5000~\mu \varepsilon $ and a wireless transmission range exceeding one meter. Besides, the system also exhibits excellent linearity and repeatability. The fully flexible sensor design enhances the system’s versatility, enabling deployment in challenging environments, such as confined or curved surfaces, particularly for structural health monitoring applications.