New optofluidic 3D printing technique expands material diversity in micro- and nanofabrication

Additive manufacturing at the micro- and nanoscale has so far been heavily constrained by material limitations. A study published in Nature by researchers from the Max Planck Institute for Intelligent Systems and the National University of Singapore now describes a method that specifically addresses these constraints. The presented optofluidic 3D micro- and nanofabrication approach complements established techniques such as two-photon polymerization, which to date has largely been limited to a small number of photopolymerizable plastics.

“The key idea of this study is to deliberately apply optofluidic interactions—that is, light-driven flows. In this way, we can steer the 3D arrangement of different micro- or nanoparticles into a predefined shape,” explains co-author of the publication Mingchao Zhang. He is an assistant professor at the National University of Singapore.

Classical two-photon polymerization is regarded as a precise method for creating three-dimensional structures with resolutions below one micrometer. Despite its high degree of geometric freedom, however, practical use has often been limited to demonstration objects or polymer-based components. Suitable material options such as metals, oxides, or semiconductors have been lacking for functional applications in medical technology, microfluidics, or robotics.

The newly presented approach uses light-induced flows in liquids to spatially arrange micro- and nanoparticles in a targeted manner. A focused femtosecond laser locally generates a thermal gradient that triggers a directed fluid flow. This flow precisely transports the particles suspended in the liquid into a microform that has previously been printed using a polymer.

“The femtosecond laser induces a localized thermal gradient that generates a strong flow and drives the particles exactly to where we want them. The shape can be arbitrary: from a cube structure to spheres and even a croissant shape—many forms are possible,” says first author of the publication Xianglong Lyu, who conducted research at MPI-IS and now works as a postdoctoral researcher at the Karlsruhe Institute of Technology (KIT). “Once we have assembled all the particles, the polymer outer shell is removed, leaving behind a free-standing structure that consists entirely of the desired material with the desired shape and size. We can now use not just one type of modeling material, but an entire toolbox of materials with different properties.”

In this way, components such as microvalves, particle-sorting elements, or multimaterial microrobots that can be actuated magnetically or optically can be produced.

“Optofluidic assembly overcomes the material limitations of conventional two-photon polymerization. Our new technology enables us to form tiny 3D objects from almost any material. This opens up new horizons for multifunctional microrobots, microtechnology, and many other applications that still sound like science fiction today,” summarizes Metin Sitti the research work. Sitti headed the Department of Physical Intelligence at MPI-IS and is now President of Koç University in Istanbul.

For 3D printing in the micro- and nanoscale domain, the technique opens up new degrees of freedom, particularly in areas where mechanical, electrical, or magnetic properties could previously not be combined.