Multi-Frequency PMUT Arrays for Blood Pressure Monitoring and Vascular Assessment Using Photoacoustic Sensing

This study presents multi-frequency piezoelectric micromachined ultrasonic transducer (PMUT) arrays operating at frequencies of 5 MHz, 7.5 MHz, and 12.5 MHz in air, respectively, to be designed for vascular diameter measurements over a broad depth range. Simulation results reveal that high-frequency and broad bandwidth are critical for obtaining small-diameter vessels from A-line signals. The PMUTs were fabricated with hexagonal cavities featuring 10 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mu \mathrm{m}$</tex> inter-cavity gaps, resulting in high fill factors and small size. The multi-frequency PMUT arrays demonstrated a broad photoacoustic bandwidth in a gel-type couplant, with 126% at 3.5 MHz (low-frequency PMUT), 103% at 5.5 MHz (medium-frequency PMUT), and 110% at 11.0 MHz (high-frequency PMUT). Using a low energy laser <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(5-6\ \mu \mathrm{J}/\text{pulse})$</tex> that was significantly below safety thresholds, the multi-frequency PMUT arrays successfully acquired A-line signals from vascular phantoms with 1 and 2 mm in diameter and depths between 0.7 to 3.7 cm in an optically clear medium. The integration of PMUT arrays with a portable laser demonstrates the promising potential for portable blood pressure monitors and advanced vascular health applications.