Low-voltage MHz-band air-coupled ultrasound transducer integrated with wire-array acoustic metamaterial for non-contact medical imaging

Non-contact medical ultrasound imaging with a single transducer is desirable in cases where gel coupling or physical contact is impractical. However, most medical ultrasound transducers are optimized for contact operation, and severe acoustic attenuation and impedance mismatch in air limit energy delivery and image quality. Here, we present a non-contact approach that integrates a wire-array acoustic metamaterial (WA_AMM) with an air-coupled ultrasound transducer (ACUT). The WA_AMM enhances acoustic transmission and reception through a compact wire-array structure, while the ACUT employs porous matching, piezocomposite, and lightweight backing layers to operate near 1.1 MHz. The combined WA_AMM_ACUT demonstrated over 100% increase echo amplitude at a 50 mm target distance, supported by FEA simulations and prototype measurements. In ex vivo porcine skin experiments, B-mode imaging showed a 7.3 dB increase in SNR, and acoustic radiation force impulse (ARFI) measurements showed a 181.3% increase in detectable displacement events under low-voltage excitation (3 V_pp for B-mode and 1.5 V_pp for ARFI). These improvements reflect enhanced two-way coupling efficiency rather than higher excitation levels, suggesting suitability for portable or power-limited applications. Overall, the WA_AMM_ACUT offers a promising direction for non-contact B-mode imaging and elastography without coupling media, supporting surface or near-surface tissue assessment under air-coupled conditions.