Three-dimensional micro- and nanostructures can protect documents and branded products against counterfeiters. In the past, it was extremely time-consuming to create structures like these using liquids of different colors. Now, physicist Frederik Mayer has developed a chamber capable of making 3D printer-based manufacturing much faster and more precise.
Frederik Mayer’s invention is smaller than a 1-cent coin – and that’s exactly what makes it so interesting. It is a round, stainless steel chamber that’s 1 cm in diameter and 100 micrometers high, which equates to a hundredth of a centimeter. Mayer is a PhD student at the Institute of Applied Physics at Karlsruhe Institute of Technology (KIT). In collaboration with ZEISS, he has developed a special so-called microfluidic chamber. It can handle different liquids in an extremely small space and can be integrated into a 3D laser lithography system. This printer produces the finest 3D structures in the micro- and nanometer range. It’s suitable for producing optical microlenses that are built into the smallest of medical devices, as well as for artificial scaffolds for cell structures. Mayer uses it to print fluorescent structures in several colors, which are then used as security features on banknotes, documents and branded products like spare parts for cars and even jewelry, to prevent counterfeiting. The structures will either be printed on the product itself, or on a security label.
Automated process replaces manual work
The chamber enables the use of several different photoresists during printing. That’s because where security features are concerned, the more complex they are the better they’ll protect objects against counterfeiters. A printing process like this used to involve a lot of technical effort and be very laborious, not to mention prone to inaccuracies. “I used to have to remove my sample from the 3D printer and replace it time and again so that I could manually apply a new photoresist. It took me 30 minutes just to do this for each of the five photoresists I used,” says Mayer. “Thanks to the chamber, this now takes 3–4 minutes – and it’s automated, too!”
As fine as a human hair
At the KIT optics lab, you can hear the sound of ventilators humming; the windows have been darkened and the room is bathed in yellow light. Sensitive materials like the photoresists used here must not be exposed to white light. Mayer shows us a cover glass featuring printed structures; it’s balanced on the tip of his index finger. But, the fine lines cannot be seen with the naked eye. Every structure is around 50 micrometers high and has a side length of approximately 100 micrometers. “Its height is roughly equal to the thickness of a human hair,” he says. Mayer, who’s casually dressed in a light-blue shirt and a pair of jeans, is very good at explaining things. His explanations are purposely not peppered with jargon as he believes it’s essential for people who aren’t seasoned physicists to be able to understand his research.
Before Mayer places his chamber inside the 3D printer, he takes apart the sample holder and cleans everything with isopropanol. It’s now easy to see what his invention comprises: the chamber is the centerpiece of a stainless steel sample holder. It consists of two parts: a type of base and a cover. In the lower part of the chamber is the cover glass, onto which the 3D structure is printed. The upper part contains a round glass window through which the laser beam travels to reach the cover glass. A rubber ring between the two glass coverslips is used to seal the chamber.
Photoresist is overpressurized to flow back into the chamber
The laser lithography system for which Mayer designed the chamber was produced by Nanoscribe, a KIT spin-off. ZEISS offered the startup its technology and advice from day one and acquired shares in the new company in September 2008. The apparatus that conveys the liquids to the chamber consists of an electronic pressure regulator, seven tubes for the different photoresists and development fluids, and a star-shaped selection valve. All the components are connected to each other via tubes and cables, and the processes are controlled by software. “The tubes containing the photoresist are overpressurized,” says Mayer. The selection valve is only opened for one photoresist, which is conveyed to the chamber. Then the laser comes into play: “The laser focus is moved through the photoresist in three dimensions, and it hardens at the laser’s focal point”.
The multicolored, three-dimensional structure slowly takes shape. First, the printer creates the basic structure, a 3D grid that comprises a single, non-fluorescent photoresist. The laser lithography system then prints the color features onto the 3D grid as dots. Mayer has already enriched the photoresists with fluorescent dyes.
“At the end, the chamber is rinsed using solvents to wash away any unhardened photoresist, thus ensuring a clean structure,” he says. Images taken by the fluorescence microscope illustrate just how detailed the printed structure really is. On an area measuring 100 micrometers in diameter, an enlarged image shows there are still several dozen colored dots.
Mayer uses the electronic pressure regulator to control how much pressure is exerted on the photoresist in the containers.
Mayer designed the chamber for this laser lithography system from Nanoscribe.
A computer rendering of the microstructure design and a stack of images taken using fluorescence microscopy show the level of detail at which different photoresist structure elements can be printed. Adapted from: Frederik Mayer, Stefan Richter, Johann Westhauser, Eva Blasco, Christopher Barner-Kowollik, Martin Wegener: Multimaterial 3D laser microprinting using an integrated microfluidic system. Science Advances, 2019. DOI: 10.1126/sciadv.aau9160 Under Creative Commons Attribution License 4.0 (CC BY).
The chamber: the product of an entire year of research
Mayer designed the chamber as part of the “3D Matter Made to Order” excellence cluster, where KIT researchers collaborate with the University of Heidelberg and are supported by the Carl Zeiss Foundation. He published the results of his work in “Science Advances” magazine.
Mayer says he worked solidly for an entire year to develop the chamber. The main challenge was the small space he had to work with. While developing the chamber, he often realized that things didn’t go as planned. For example, he initially used transparent silicon to build several chambers. But, they weren’t viable as they could not be resealed. “While I was able to print the security features onto one chamber, I was unable to remove them. Clearly, that wasn’t the best approach,” he says with a smile. Then, the photoresist hardened while still in a valve, presumably due to heat build-up, and the valve sprang a leak. So Mayer switched to a different type of valve.
When you think about and tweak things a lot, you have to be able to stand back and look at the big picture now and again. When he’s not in the lab, Mayer is normally outdoors swimming in the lakes around Karlsruhe or hiking in the Black Forest. Sometimes, he spends his vacation cycling through the Alps.
Ever closer to industrial production
Prior to the invention of the microfluidic chamber, industrial production of multicolored security features using a 3D laser lithography system was simply unthinkable. The chamber changed everything. ZEISS employee Stefan Richter is in charge of driving new projects at Research and Technology. He was the one who first approached KIT with the idea of a fluorescent 3D security feature and has been following Mayer’s work through regular meetings and advising him as needed. These discussions produced the idea of a microfluidic chamber that would speed up the production of the 3D security features using different photoresists. He’s convinced of the industrial benefit of the microfluidic chamber. “Mayer’s chamber would be an excellent way of expanding an industrial system. It could be used to provide the different photoresists in a cartridge system, just like a color printer,” says Richter. While this is all still speculation, he says: “The results really speak for themselves.” Currently, there are only two versions of the chamber and it is not yet being produced in large quantities.
Mayer is certain the concept will take off – and so is looking beyond his own application. “The microfluidic system doesn’t just make it possible to print multicolored security features.” In the future, he predicts that people will want to print structures using different materials with different properties. “My microfluidic system certainly isn’t the end of this story. Our system is proof that it’s possible to automate the printing of 3D microstructures composed of many different materials.”
Image credits: © Verena Müller