Reconfigurable liquid-based acoustic lens for controlling the focal depth of focused ultrasound

Focused ultrasound (FUS) enables noninvasive treatment. Conventional FUS transducers with fixed geometries lack depth control, whereas dynamic focusing with multi-element phased arrays requires complex electronics that limit scalability. This study introduces a reconfigurable, liquid-based acoustic lens that attaches to a single-element planar ultrasound transducer to provide tunable focal-depth control. The lens consisted of a deformable low-density polyethylene membrane filled with perfluorooctyl bromide (PFOB), a biocompatible liquid with matching acoustic impedance and low reflectivity, and was coupled to a 500-kHz transducer. Adjusting the internal PFOB volume changed the lens curvature and acoustic wavefront, thereby controlling focal depth. Acoustic pressure fields were mapped under three conditions: no lens, 2 mL PFOB, and 3 mL PFOB. Cross-sectional analysis showed volume-dependent plano-convex lens deformation, with central heights of approximately 3.6 mm and 4.7 mm at 2 mL and 3 mL, respectively. Acoustic field maps showed that increasing the PFOB volume shortened the focal depth from 52 mm without a lens to 37 mm and 26 mm, with improved focal localization. Numerical simulations closely reproduced the experimental pressure maps. These findings demonstrate controllable focusing with a reconfigurable, liquid-based acoustic lens in a compact, single-element FUS system.