Acoustic Hologram Lens Made of Nanoparticle-Epoxy Composite Molding for Directing Predefined Therapeutic Ultrasound Beams

We present an acoustic hologram lens fabrication method for prototyping nondeformed hologram lenses with a tailored acoustic impedance. A pixelized hologram pattern is typically manufactured by photo-curing 3D printing methods, such as stereolithography (SLA) printing. However, SLA printing has major limitations for lens fabrication: vulnerability to deformation during photo-curing of a thin-plate shape lens structure and limited controllability of acoustic impedance. To overcome these limitations, we adopted a synthesized epoxy composite molding technique in this work. The used alumina nanoparticle (300 nm)-epoxy composite contains 22.5% alumina particles in volume. The characterized acoustic impedance of the composite was 4.68 MRayl whereas the conventional photopolymer exhibited 3.13 MRayl. We used these acoustic properties in lens modeling and acoustic hologram simulations. In simulations, the composite lens generated 145% pressure amplitude of the photopolymer lens due to improved acoustic impedance matching between a piezoelectric ceramic and water medium. We prototyped a composite lens through 1) 3D printing a lens cavity, 2) silicone rubber molding, and 3) epoxy composite lens molding. We observed no deformation of the prototyped composite lens whereas the photopolymer lens showed deformed edges. The beam mapping result using the composite lens showed 17% improved structural similarity with the designed pressure pattern compared to the photopolymer result. Due to the air bubbles trapped in a composite lens, the expected improvement of pressure amplitude over a photopolymer lens was not experimentally demonstrated. The additional degassing procedure will be included for future prototypes and pressure transmission will be evaluated.