Abstract
Acoustic waves exert forces when they interact with matter. Shaping ultrasound fields precisely in 3D thus allows control over the force landscape and should permit particulates to fall into place to potentially form whole 3D objects in "one shot." This is promising for rapid prototyping, most notably biofabrication, since conventional methods are typically slow and apply mechanical or chemical stress on biological cells. Here, we realize the generation of compact holographic ultrasound fields and demonstrate the one-step assembly of matter using acoustic forces. We combine multiple holographic fields that drive the contactless assembly of solid microparticles, hydrogel beads, and biological cells inside standard labware. The structures can be fixed via gelation of the surrounding medium. In contrast to previous work, this approach handles matter with positive acoustic contrast and does not require opposing waves, supporting surfaces or scaffolds. We envision promising applications of 3D holographic ultrasound fields in tissue engineering and additive manufacturing.