3D printing with ultrasound

3D printing enables the production of complex parts from various materials, even biological ones. Traditional 3D printing can be a slow process in which objects are built up layer by layer. Researchers in Heidelberg and Tübingen now show how to form a 3D object from smaller building blocks in just one step.

„Using targeted and shaped ultrasound, we were able to assemble the smallest particles into a three-dimensional object in a single step,

says Kai Melde, postdoc in the group and first author of the study. „This can be very useful for so-called bioprinting. The cells used there are particularly sensitive to environmental influences and ultrasound is a gentle method," adds Peer Fischer, professor at the University of Heidelberg.

Sound waves exert forces on matter - a fact that every concert-goer who experiences the pressure waves of a loudspeaker knows. With high-frequency ultrasound, which is not audible to the human ear, the wavelengths can be shifted below one millimetre into the microscopic range, which researchers use to manipulate very small building blocks such as biological cells.

In previous studies, Peer Fischer and colleagues showed how ultrasound can be generated using acoustic holograms - 3D-printed plates designed to encode a specific sound field. They demonstrated that these sound fields can be used to assemble materials into two-dimensional patterns.

With their new study, the team was able to take the idea one step further. In the sound fields, they capture particles and cells floating freely in the water and assemble them into three-dimensional shapes. In addition, the new method works with a variety of materials, including glass or hydrogel beads and biological cells. First author Kai Melde says that „the key idea was to use several acoustic holograms together to form a sound field that can trap the particles“. Heiner Kremer, who wrote the algorithm to optimise the hologram fields, adds: „Digitising an entire 3D object into ultrasound hologram fields is very computationally intensive and required new computational routines.“

The scientists believe that their technology represents a major advance in the formation of cell cultures and tissues in 3D. The advantage of ultrasound is that it is gentle on cells and can penetrate deep into the tissue. This means that the new method can be used to manipulate cells remotely without causing damage.