A method for generating ultrasound via light, coupled with 3D printing, has demonstrated the ability to form sound fields with specific shapes, for potential use in biological cell manipulation and drug delivery. This method could provide a simpler, less expensive alternative to the use of piezoelectric arrays to produce sound waves. This schematic illustrates how tailored surface profiles can create patterned optically generated acoustic fields in 3D. Courtesy of Brown et al. To generate acoustic fields focused at multiple points using a single optical pulse, a University College London research team used optically absorbing surface profiles designed to generate user-specified wave fields. The surface profiles were designed using an optimization approach and created using a combination of additive manufacturing and absorber deposition techniques. “One useful feature of the photoacoustic effect is that the initial shape of the sound that’s generated is determined (by) where the light is absorbed,” said researcher Michael Brown. “This can be used to create tightly focused intense points of sound just by depositing an optical absorber on a concave surface, which acts like a lens.” According to the researchers, using their approach would make it possible to manufacture samples of nearly any surface shape using a 3D printer and a transparent material. This is a fabricated sample before absorber deposition. Courtesy of Brown et al. “By depositing an optical absorber on this surface, which can be done via spray painting, a sound wave of nearly any shape can be created by illuminating this sample with a laser,” Brown said. “If you carefully tailor the design of the surface and therefore the shape of the acoustic wave, it’s possible to control where the sound field will focus and even create fields focused over continuous shapes. We’re using letters and numbers.” In theory, the ability to control the shape of the wavefront should lead to a larger degree of control over the resulting field. “But actually designing a wavefront that generates a desired pattern becomes more challenging as the complexity of the target increases,” Brown said. “A clear ‘best’ design is only available for a few select cases, such as the generation of a single focus.” To overcome this limitation, the team developed an algorithm to allow users to input their desired sound field in 3D. The algorithm then outputs a 3D printable surface profile that generates the desire field. “Our algorithm allows for precise control of the intensity of sound at different locations and the time at which the sound arrives, making it quick and easy to design surfaces or ‘lenses’ for a desired application,” said Brown. The team demonstrated the effectiveness of their algorithm by creating a lens designed to generate a sound field shaped like the number 7. After illuminating the lens with a pulsed laser, they recorded the sound field; the number 7 was clearly visible with high contrast. “It was the first demonstration of generating a multifocal distribution of sound using this approach,” Brown said. The tailored optoacoustic profiles have many potential uses. “Highly intense sound can cause heating or exert forces on objects, such as in acoustic tweezers. And similar single-focus devices are already being used for cleaving cell clusters and targeted drug delivery, so our work could be useful within that area,” said Brown. The group is also interested in the effects of propagating through tissue, which introduces distortions to the shape of wavefronts caused by variations in the speed of sound. “If the structure of the tissue is known beforehand via imaging, our approach can be used to correct for these aberrations,” Brown said. “Manipulating the shape and time during which the focused sound is generated can also be useful for maneuvering and controlling biological cells and other particles.” Going forward, the team plans to investigate the use of other light sources and advantages these alternative sources might offer. “One limitation of our work was the use of a single-pulsed laser,” said Brown. “This meant that the temporal shape of the sound generated from the sample was only one short pulse, which limited the complexity of the fields that could be generated. In the future, we’re interested in using alternative modulated optical sources to illuminate these devices.” The research was published in Applied Physics Letters (doi: 10.1063/1.4976942).