Researchers from the Max Planck Institute for Intelligent Systems, ETH Zurich, KTH Royal Institute of Technology, the National University of Singapore, and Koç University have developed an optofluidic 3D microfabrication and nanofabrication technique that enables the creation of fully three-dimensional microstructures from a wide range of materials, including metal nanoparticles, metal oxides, diamond nanoparticles, and quantum dots.
Reported in Nature, the method combines two-photon polymerization with light-driven fluid flow to assemble nanoparticle building blocks into volumetric structures, addressing long-standing material limitations in high-resolution micro-scale 3D printing.
Using light-driven flow to assemble matter in 3D
Conventional two-photon polymerization is widely used for micro- and nanoscale 3D printing due to its high spatial resolution, but it is largely restricted to cross-linkable polymers. While recent research has expanded printable materials through specialized photoresists or post-processing strategies, these approaches typically remain material-specific.
The newly reported method separates geometric definition from material composition. In the process, a hollow polymer microtemplate is first fabricated using 2PP. The template is then immersed in a suspension containing nanoparticles or microparticles. A femtosecond laser is applied near an opening in the structure, generating a localized thermal gradient that induces strong convective flow within the surrounding fluid. This optofluidic flow transports particles into the confined volume of the template, where they accumulate and assemble into the prescribed 3D geometry.
After assembly, the polymer template is removed using plasma treatment, leaving a mechanically stable, free-standing microstructure composed entirely of the densely packed target nanoparticles, held together primarily by van der Waals forces.
Predictable assembly governed by colloidal physics
The researchers show that successful assembly depends on the balance between particle–particle interactions and particle–fluid interactions. Inter-particle attraction, described using DLVO theory, must overcome hydrodynamic drag forces generated by the laser-induced flow.
By varying parameters such as ionic strength, solvent composition, surfactant concentration, and laser scan speed, the team established predictable regimes for particle clustering versus dispersion. Experimental results closely matched theoretical phase diagrams, allowing the assembly process to be tuned for stability and efficiency. For instance, they identified a critical flow speed threshold of approximately 300 µm/s for the model SiO₂ system, below which clustering reliably occurs.
Assembly rates on the order of 10⁵ particles per minute were reported, exceeding typical optical assembly techniques and approaching practical throughput for microscale device fabrication.
Broad material compatibility demonstrated
Using the optofluidic approach, the team assembled complex 3D microstructures from a wide range of materials, including silica particles of various sizes, titanium dioxide nanoparticles and nanowires, iron oxide nanoparticles, tungsten oxide nanowires, aluminum oxide nanowires, silver nanoparticles, diamond nanoparticles, and cadmium telluride quantum dots.
The method supports particles ranging from tens of nanometers to several micrometers in size, as well as mixed-particle assemblies. Site-selective and sequential assembly was also demonstrated, enabling multi-material structures to be fabricated on a single substrate without cross-interference. This capability culminated in the fabrication of a single L-shaped microrobot integrating four distinct functional materials.
Surface quality was shown to improve with narrower particle size distributions, while post-processing steps such as thermal annealing further enhanced mechanical robustness through inter-particle bonding.
Microfluidic and microrobotic device demonstrations
Beyond structural fabrication, the study demonstrated functional microdevices enabled by the technique. Particle-assembled microvalves embedded within 3D printed microfluidic channels were used to selectively filter and enrich nanoparticles based on size, allowing solvent flow while retaining solid particles.
The team also fabricated microrobots with multimodal actuation. These included magnetically actuated iron oxide structures, light-driven titanium dioxide–gold micromotors, and multi-material robots capable of responding to magnetic fields, ultraviolet light, and chemical fuels. By controlling geometry and spatial material distribution, distinct motion modes such as tumbling, linear propulsion, and rotational motion were achieved.
Moving beyond polymer-limited micro 3D printing
Recent reporting has highlighted efforts to scale two-photon polymerization through standardized testing and improved benchmarking, with the aim of increasing repeatability and comparability in micro-scale 3D printing processes. These developments reflect growing process maturity for high-resolution polymer-based fabrication. However, as noted by the authors of the present study, such advances do not address the underlying material compatibility constraints of two-photon polymerization, which remains largely limited to cross-linkable polymers. The optofluidic assembly approach described here targets this remaining limitation by enabling volumetric microstructure fabrication from a broader range of particulate materials.
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Featured image shows Schematic illustration of the optofluidic 3D microfabrication/nanofabrication process Image via the authors, published in Nature.
