A Miniature Photoacoustic System for Skin Melanin Detection

The bulky size of conventional photoacoustic systems limits portable and wearable applications. Many studies focused on device miniaturization but lacked optimization of spatial arrangement to achieve higher integration and better signal-to-noise ratios (SNR). In this paper, we present a high-fill factor piezoelectric micromachined ultrasonic transducer (PMUT) array. By integrating the PMUT array with a compact vertical-cavity surface-emitting laser (VCSEL), a miniature photoacoustic sensing system, measured to be <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$4.6\ \text{mm} \times 2.0\ \text{mm} \times 5.2\ \text{mm}$</tex>, was proposed. Electrical and acoustic characterization results reveal that the PMUT exhibits a center frequency of 4.89 MHz in air and 3.50 MHz in a gel-type couplant, with a photoacoustic bandwidth extending up to 146% (–6 dB). Based on the system, we successfully recognized the photoacoustic signals from phantoms at various depths, highlighting the advantages of the PMUT, especially for close-range applications. These advancements show great potential for portable photoacoustic devices suitable for continuous monitoring in diverse biomedical and industrial applications.