A sophisticated alternative to previous 3D printing methods: German researchers have developed a method that uses complex sound fields to form 3D objects from artificial or biological particles. The acoustic forces model the structures without contact, gently and in one step. The scientists say that the process could benefit the development of novel 3D cell culture techniques.
The techniques, applications and materials used are becoming more and more sophisticated: many exciting three-dimensional printing processes have been developed in recent years. Basically, they are mostly based on the fact that certain substances are built up in layers or step by step in order to gradually create three-dimensional structures. Scientists are already working on so-called bioprinting methods, in which cell suspensions are applied through nozzles. However, these are slow processes and the sensitive substances are mechanically or chemically stressed. The team led by Kai Melde and Peer Fischer from the Max Planck Institute for Medical Research in Heidelberg is therefore developing an alternative method that does not encounter these problems.
Under the spell of sophisticated sound wave fields
The concept they are now presenting is based on the power of sound. Everyone is probably familiar with the effect that loud music, for example, can shake matter. High-frequency ultrasound, which is inaudible to the human ear, can also cause this effect - the waves can exert pressure and cause movement. Targeted sonication can also be used to manipulate very small particles such as biological cells. This not only allows individual units to be pushed in certain directions. Special sound wave fields can also cause particles to migrate to specific locations and accumulate there. In earlier work, the research team was already able to use this in a targeted manner. To do this, they used "acoustic holograms": 3D-printed plates with fine structures that produce a specific sound field when exposed to ultrasound. Under its spell, the researchers have already shown that particles arrange themselves into complex two-dimensional patterns.
Now they have successfully transferred this technique to the third dimension. "The crucial idea was to use several acoustic holograms together and thus create a sound field that can capture the particles," says Melde. To do this, the researchers equipped three ultrasonic transducers, each with a specially shaped hologram plate that was able to produce special pressure wave patterns. Together they should result in a system in which a 3D model of an object is encoded. Designing these units was the great challenge of the approach, emphasizes co-author Heiner Kremer: "The digitization of an entire 3D object in ultrasonic hologram fields is very computationally intensive and required new computation routines".
potential for biomedicine
However, the team finally managed to develop appropriate algorithms for optimizing the hologram fields. "With the targeted and shaped ultrasound, we were able to combine the smallest particles into a three-dimensional object in a single step," says Melde. These are hydrogel beads and biological cells suspended in a suspension targeted by the complex acoustic wave field. As it turned out, the particles under his spell actually migrated to the desired positions and accumulated there to form the specified 3D structures. In their experiments, the researchers have so far produced spiral shapes in small vessels typically used in the laboratory. These structures could then be fixed by targeted gelation of the surrounding residual liquid.
According to the team, this is a promising proof of concept. The process could now be further adapted and scaled. By increasing the frequency and the size of the converters, it seems possible to produce more complex objects. The researchers are convinced that there is considerable potential for use: "Due to its versatility and the ability to process different materials, our method is promising for the production of different structures," the researchers write. Above all, however, they see possible applications in biomedicine – specifically in so-called tissue engineering. As an alternative to previous bioprinting methods, tissue structures could also be built up in this way: “The cells used for this are particularly sensitive to environmental influences. However, ultrasound is very gentle,” stresses senior author Peer Fischer.
Source: Max Planck Institute for Medical Research, specialist article: Science Advances, doi: 10.1126/sciadv.adf6182