Advancements in Wireless Passive Surface Acoustic Wave Sensors and Their Applications: A Review

Wireless passive surface acoustic wave (WP-SAW) sensors enable battery-free sensing in sealed, rotating, high-temperature, or otherwise inaccessible environments where wired instrumentation and powered sensor nodes become impractical. Despite rapid progress, the literature remains fragmented across device architectures, functional coatings, and reader implementations, and many prior surveys do not reconcile system-level deployability constraints with reported sensing performance explicitly for wireless passive sensing applications. This review addresses that gap by consolidating and benchmarking representative passive wireless SAW demonstrations spanning temperature, strain, pressure, humidity, and gas sensing, and by explicitly mapping device structures (one/two-port resonators, delay lines, reflective delay lines) to interrogation/readout workflows (pulsed, FMCW, and SFCW) and the associated DSP used to extract resonance, phase, and time-delay observables. Reported operating frequencies span 10 MHz-4.74 GHz, with a strong clustering around the 434 MHz band. Among the surveyed works, temperature and strain dominate, with temperature sensitivities ranging between 7.62 kHz/°C (YX/128° LiNbO 3 -based One-Port SAW device) and 170 kHz/°C (YX-cut quartz-based One-Port SAW device) and reported temperature coefficients of ~24-47 ppm/°C (GaN Substrate One-Port SAW and AlN/Sapphire One-Port SAW respectively). Representative performance figures include humidity phase sensitivities of ~11.8°/%RH (YZ-cut LiNbo3 Reflective Delay Line SAW) near 433 MHz, and pressure sensitivities up to ~124.9 kHz/bar (ST-Cut Quartz One Port SAW) and ~5.9 kHz/Pa (GaN substrate One Port SAW) depending on the operating regime and readout observable. By unifying these quantitative benchmarks with practical reader and packaging considerations, this review provides design guidance and research priorities intended to accelerate scalable, robust WP-SAW deployments.