Fabrication of optically transparent ultrasound transducers to integrate light and sound in multimodal biomedical systems.

Optical and acoustic technologies are extensively utilized across sectors for imaging purposes and in theranostic applications. The integration of coaxially aligned optical and acoustic pathways results in synergistic systems that overcome the limitations of each approach and provide complementary advantages such as increased precision and efficiency, housed in a compact instrument. However, commercial ultrasound transducers are optically opaque and therefore necessitate oblique configurations that compromise system simplicity, spatial alignment and signal quality. The use of optically transparent materials for the acoustic and electrical components results in transparent ultrasound transducers (TUTs) that can be used in biomedical setups and do not require oblique pathways or complex alignment. Here we describe a comprehensive and customizable pipeline for TUT development, covering material selection, simulation, fabrication and performance characterization. The procedure includes the materials' selection best suited for various target applications, the design optimization via acoustic simulation and the fabrication steps required for surface processing, electrode deposition and the integration of the acoustic layers. We explain how to characterize the fabricated TUTs to ensure their optical and acoustic functionality and to confirm that their performance matches that predicted in the simulations. The entire procedure requires ~3 weeks for users with moderate experience in piezoelectric device fabrication, acoustic measurement and microfabrication techniques. This protocol could facilitate future developments in multimodal biomedical systems, particularly those requiring seamless and high-fidelity opto-ultrasound integration.