Gary Zabow, a scientist at the National Institute of Standards and Technology (NIST) in the US, never intended to use candy in his experiments. But already desperate, and as a last resort, he buried small magnets in pieces of hard candy, since sugar dissolves easily in water, releasing these magnets leaving no harmful plastics or chemicals behind. While he was preparing this package for the lab of some biomedical colleagues, he left one of these ‘trinkets’ in a beaker.
With the heat and time, the caramel melted. To clean the glass, she rinsed it with water, thinking that this action would dissolve the sugar. However, he saw that at the bottom of the glass there was a strange rainbow color that surprised him: the micromagnets had been transferred to the bottom of the container, as a kind of ‘sticker’. “The colors indicated that the microdot arrays they had retained their unique pattern,” explains Zabow.
That gave him an idea: was it possible to use regular table sugar to bring the power of microchips to new and unconventional surfaces? Zabow’s findings on this potential transfer printing process have just been published in the journal Science.
The difficulty of printing circuits in different materials
Semiconductor chips, micropatterned surfaces, and electronics are based on the microprinting, the process of placing precise but minuscule patterns from millionths to billionths of a meter wide on surfaces to give them new properties. Traditionally, these tiny labyrinths of metals and other materials are printed on flat wafers of silicon – traditional motherboards. But as the possibilities of semiconductor chips and smart materials expand, these tiny, intricate patterns are imprinted on new, curved, and even malleable surfaces.
Directly printing these patterns onto such surfaces is tricky, so scientists transfer the prints from other materials: there are tapes and flexible plastics that can do this job, but they also sometimes have trouble conforming to curves or corners. In addition, they could leave behind plastic residues or other chemicals that are difficult to remove or unsafe for biomedical uses.
There are liquid techniques, in which the transfer material floats on the surface of the water and the target surface is pushed through it, as seen in multiple videos on social networks in which a colorful pattern is printed thanks to this method . But that can be tricky, too: With a free-flowing liquid, it can be difficult to position the print exactly where you want it on the new surface.
But, as Zabow discovered to his surprise, a simple combination of caramelized sugar and corn syrup can do the trick. When dissolved in a small amount of water, this mixture can be poured over micropatterns on a flat surface. Once the water evaporates, the caramel hardens, leaving the embedded pattern. The imprinted candy is then placed on the new surface and melted. The combination of sugar and corn syrup maintains a high viscosity as it melts, allowing the pattern to maintain its layout as it flows over curves and edges. Then, using water, the sugar can be removed, leaving only the pattern.
The REFLEX technique is born
Using this technique, called REFLEX (REflow-driven FLExible Xfer), patterns of microcircuits could be transferred as a template to allow scientists or manufacturers to etch and fill the materials they need in the right places. Or, the patterned materials could be transferred from your chip original to fibers or microspheres for potential biomedical or microrobotics studies, or on sharp or curved surfaces inside new devices.
The technique proved successful for a wide range of surfaces, including printing on the sharp point of a pin and writing on the word “NIST” in micro-scale gold letters on a single strand of human hair, as seen in the image that accompanies the text. In another example, magnetic disks 1 micrometer in diameter were successfully transferred to a fiber of dental floss from a milkweed seed. In the presence of a magnet, the magnetically printed fiber reacted, showing that the transfer had worked.
Using sugar and corn syrup (i.e. candy), researcher Gary Zabow transferred the word “NIST” in gold lettering onto a human hair, shown in false color in this black-and-white microscope image.
There is still more to explore with REFLEX, but this process could open up new possibilities for new materials and microstructures in fields ranging from electronics to optics to biomedical engineering.
“The semiconductor industry has spent billions of dollars perfecting printing techniques to create chips we trust,” says Zabow. “Wouldn’t it be nice if we could take advantage of some of those technologies, extending the reach of those prints with something as simple and inexpensive as candy?”