Surface acoustic wave sensing chip based on acoustic impedance effect: a method for rapid gas leakage detection and respiratory monitoring

Abstract The transformation of acoustic impedance (AI) in different media regulates the transmission and reflection characteristics of acoustic waves between media, thus enabling many interesting acoustic applications. Here we introduce for the first time a surface acoustic wave (SAW) chip based on the AI effect, which utilizes its piezoelectric effect and unique interdigital transducer to excite a mechanical wave (SAW) propagating along the surface of the piezoelectric crystal, and it features high sensitivity to surface-loaded media such as gases and humidity changes due to the SAW energy being localized on the surface. On this basis, we theoretically established the relationship between surface load AI and SAW propagation loss, and analyzed the influence of AI on acoustic propagation loss under different gas/humidity media using mass conservation and ideal gas state equations. Experimental measurements using SAW chips reveal that the differences in AI generated by different gases trigger different acoustic propagation loss signals, and can achieve wide-range (1-100 v/v%) gas monitoring, with fast response and recovery speeds reaching sub-second levels (t90<1 s, t10<0.5 s). This capability can also be perfectly utilized for human respiratory monitoring, accurately reflecting respiratory status, frequency, and intensity. Therefore, the SAW sensing method and chip based on the AI effect proposed in this work provide a new solution for in-situ detection of gas leaks and precise monitoring of human respiration.