Researchers Create Scalable System for 3D Printing on Micro- and Nanoscale

Researchers from Germany and Australia give us insight into a new method they have created in ‘Multimaterial 3D laser microprinting using an integrated microfluidic system.’ As authors Frederik Mayer, Stefan Richter, Johann Westhauser, Eva Blasco, Christopher Barner-Kowollik, and Martin Wegener explored the ever-expanding field of 3D laser micro- and nano-printing further, they saw the need for structures that could be created beyond those with the typical ‘single constituent material.’

The research team created a scalable system allowing users to 3D print on both the microscale and nanoscale, for use in applications like photonic crystals, wire bonds, free-form surfaces, optical technology, mechanical metamaterials, microscaffolds, and much more. Previously such applications were catered to with 3D printed microstructures of only a single material, produced in a much more time-consuming manner. The ability to create structures with material materials at once means an obvious and substantial savings in time and money, as the researchers explain:

“… as resist systems and cycles increase, such a process performed by humans rapidly becomes not only very tedious and time consuming but also quite unreliable. Therefore, it is highly desirable to avoid having to go back and forth between the chemistry room and the 3D laser printer numerous times and instead integrate all steps and components into one compact tabletop machine tool.”

Seven different liquids are used within their system in this study:

• Nonfluorescent photoresist for the structure’s backbone
• Four photoresists containing fluorescent semiconductor quantum dots and organic dyes with different emission colors
• Two developers (mr-Dev 600 and acetone)

“The scaling-up to a yet larger number of chemicals is straightforward,” state the scientists.

See an example of their microfluidic chamber scheme in Figure 1A, below, with the example photoresist in Fig 1B, and an expanded view of the stainless-steel microfluidic chamber in Figure 2A. Other features include:

• Optical access through a round glass window (diameter, 25 mm; thickness, 170 μm)
• Another round glass window (diameter, 10 mm; thickness, 170 μm) acting as the substrate for printing samples
• A distance of 100 μm between the two windows

Fig. 1 Scheme of the microfluidic chamber. (A) A high-NA oil-immersion microscope objective lens focuses femtosecond laser pulses into a chamber, which is clad by two thin glass windows (light blue). One of them serves as the substrate for the samples. The selection valve shown in Fig. 3 allows for switching between different photoresists (here, one nonfluorescent and four fluorescent) and solvents (acetone and mr-Dev 600), which are injected into the microfluidic chamber. For clarity, the scheme is not to scale. A to-scale technical drawing is shown in Fig. 2B. (B) Structure formulae of the components of one of the fluorescent photoresists containing Atto dye molecules.

Fig. 2 Microfluidic sample holder for 3D laser lithography.(A) Left-hand side: Scheme of the complete sample holder, which can be placed into a commercial 3D laser lithography machine. Right-hand side: Explosion drawing of the microfluidic chamber, which hosts a small coverslip (diameter, 10 mm) inside the chamber, onto which structures can be 3D-printed. The chamber is sealed using a solvent-resistant O-ring, and the top part features a circular glass window for the high-NA oil-immersion objective to focus inside the chamber. (B) Cross-sectional scale drawing of the sample holder. The sample holder features connectors for liquid tubing and channels for the liquids to be guided in and out of the microfluidic chamber. The liquid flow path is indicated using red arrows.

Fig. 3 Scheme of the system connected to the microfluidic chamber.
(A) It consists of an electronic pressure controller connected to a nitrogen bottle, up to 10 containers for the photoresists and solvents for development, and the star-shaped selection valve. Pumping individual liquids is possible by applying a pneumatic pressure to all liquid containers and opening the flow path for a single liquid using the selection valve. Following the selection valve, the liquid flow is guided through an overpressure valve and our homebuilt sample holder. Last, it is directed into a waste container. (B) Cross section through our homebuilt selection valve assembly. The assembly consists of commercial solenoid valves and a homebuilt 10-to-1 manifold that connects the 10 liquid containers to 10 solenoid valves, and the valve outputs to one manifold output port. An example flow path for one liquid is indicated with red arrows.

The new design means that larger samples can be printed, resolution can be tuned, and overhanging structures are possible. The substrate can be removed, and the top part included a groove designed for a solvent-resistant O-ring. The researchers added this feature to seal the fluidic sample holder, making it leakproof, and they also added measures to prevent the internal setup from exploding due to pressure in the chamber. An electronic pressure controller was added, along with five different photoresists for 3D security features.

“It is conceivable that these microfluidic systems will become widely established for the manufacture of complex 3D micro- and nanostructures composed of multiple materials, with applications in diverse fields such as 3D scaffolds for cell culture, 3D metamaterials, 3D micro-optical systems, and 3D security features. As we have shown, the system can even be integrated into commercially available state-of-the-art 3D laser lithography machine tools,” conclude the scientists.

It doesn’t take long to realize the world of 3D printing includes doors continually opening from one realm of progression to the next, with each innovation building on the last, and new ones continually making impacts in a wide range of industries and applications. The study of materials and ongoing research has resulted in many other intricate customizations and open systems, along with great advances in miniaturization and microfluidics, and new methods on the microscale.

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(A) On the left-hand side, a computer rendering of the design for the microstructure is shown. It consists of a nonfluorescent 3D support structure (gray) with fluorescent markers with different emission colors printed into it. On the right-hand side, a stack of images taken by using fluorescence microscopy is shown. (B) The designs of the test patterns were printed into the five different marker layers of the microstructure. (C) Measurement data from fabricated microstructures taken using fluorescence microscopy. Insets show the level of detail at which different photoresist structure elements can be printed.