Xiangkun Piao

By combining theoretical, numerical, and experimental approaches, this study presents a comprehensive investigation into the transmission spectrum of a one-dimensional phononic crystal (PC) composed of periodically stacked glass coverslips separated by water layers. The results reveal bandgaps, defect states, and band edge resonances in the range of 0.6–7.5 MHz. Wideband acoustic waves are excited within the defect layer of the PC and in free space via the photoacoustic effect. Ultrasound radiation is strongly enhanced at the resonant frequency of 0.836 MHz, with an acoustic energy density enhancement of (30.8 ± 0.6)×, while radiation at other frequencies is significantly suppressed. These findings strongly suggest that the enhancement originates from an increased acoustic density of states, although material absorption suppresses the enhancement at higher frequencies. This work provides direct insights for developing MHz ultrasonic emitters for biomedical applications and advances fundamental research in megahertz ultrasound.

Focused ultrasound-induced blood-brain barrier (BBB) opening enables targeted brain drug delivery. However, achieving simultaneous multi-target BBB opening across various depths and regions remains challenging and cost-prohibitive. Here we address these challenges by employing encoded acoustic holograms combined with a single-element plane-wave transducer to generate precise focused acoustic fields. The holograms designed using an iterative angular spectrum approach and fabricated via 3D printing produce single or multiple foci at different depths and regions, which are confirmed by simulations and hydrophone-based measurements. Beam steering capability is demonstrated and further validated in vivo. We design a hologram with less than 10% variation in amplitude across different foci and successfully achieve biplane multi-target ( ≥3) BBB opening in the bilateral hippocampus and medial septum of mice in a single sonication session with no adverse effects. This innovative acoustic holographic approach for simultaneous multi-target BBB opening is time-efficient and cost-effective, presenting broad potential applications in brain drug delivery and neuromodulation.

As a new method of acoustic focusing, metasurfaces have the advantage of achieving high-resolution focusing with compact and planar geometry in a relatively broad frequency band. Among these, the Helmholtz resonator cavity based metasurface has been widely utilized due to its superior performance. However, the research on this metamaterial has focused on the audible frequency band and it remains a challenge to apply this structure to the ultrasonic frequency band for biomedical applications. One reason is that the ultrasonic metasurfaces typically require complex and deep subwavelength microstructures, which is a great challenge to the current state-of-the-art fabrication techniques, and the other reason is that transferring metasurfaces with the conventional metal structure in air to those in water induces a significant transverse wave effect. In this study, we first designed a Helmholtz resonant cavity based metasurface working at 1.5 MHz according to the generalized Snell law, which is the frequency employed in biomedical applications. The resonant cavity unit was made of resin and air, which suppressed the transverse wave effect greatly. The makings and sparse distribution of the unit enabled the easy fabrication of the metasurface by 3D printing. Then, the focusing characteristics were investigated through numerical simulation and good focusing results were achieved, although the unit structure did not meet full phase coverage. Finally, the metasurface was fabricated, and the focusing was demonstrated experimentally. This work paves a way for the application of Helmholtz resonant cavity based metasurfaces in the biomedical ultrasound field.