Broadband Multi-frequency PMUT Arrays for Miniature Photoacoustic Sensing

Continuous and cuffless blood-pressure (BP) monitoring has attracted considerable attention in recent decades. Photoacoustic (PA) technology, which offers high vascular selectivity and encodes vessel diameter intrinsically, estimates BP by measuring vessel diameter and converting it to pressure, showing great promise. Here, we present miniature (2.5 mm × 5 mm) piezoelectric micromachined ultrasonic transducer (PMUT) arrays with tri-band operation that optimizes the depth–resolution trade-off in PA vascular sensing. The high-fill-factor, hexagonal-cavity PMUT arrays were fabricated via a cavity silicon-on-insulator (CSOI) process and exhibited broadband responses centered at ~ 3.5 MHz, 5.5 MHz, and 11 MHz, with − 6 dB fractional bandwidths of 126%, 103%, and 110%, respectively. This multi-frequency design balances sensitivity and resolution: low-frequency elements yield high sensitivity (noise-equivalent pressure, NEP of 2.9 \(\:mPa/\sqrt{Hz}\) at 3.5 MHz) for detecting deep vessels, while high-frequency elements resolve sub-millimeter vessels. Using ~ 5 \(\:\mu\:\)J 532 nm laser pulses, the system is able to successfully detect a 1 mm diameter vessel phantom at a depth of 5 mm in an optically clear medium. The vessel diameter was extracted from the temporal separation between wall echoes and matched theoretical predictions. Vessel phantoms were detected successfully at depths up to 40 mm in clear media and up to 6 mm in a tissue-mimicking phantom. This approach offers high spatial resolution together with extended penetration depth, demonstrating the feasibility of integrating multi-frequency PMUT arrays into compact PA systems. This work represents a significant step toward miniature photoacoustic sensing systems that are suitable for wearable, real-time, cuffless BP monitoring.