Gary Zabow, a researcher, used corn syrup and sugar to transfer the word “NIST”, onto a human hair using gold letters. This grayscale microscope image shows false color.
Credit: G. Zabow/NIST
Gary Zabow, NIST scientist, never intended to use candy within his lab. It was only as a last resort that he had even tried burying microscopic magnetic dots in hardened chunks of sugar — hard candy, basically — and sending these sweet packages to colleagues in a biomedical lab. The sugar dissolves quickly in water, allowing the magnetic dots to be released for research without leaving behind any harmful chemicals or plastics.
By chance, Zabow had left one of these sugar pieces, embedded with arrays of micromagnetic dots, in a beaker, and it did what sugar does with time and heat — it melted, coating the bottom of the beaker in a gooey mess.
“No problem,” he thought. He would dissolve the sugar as usual. But this time, when he rinsed the beaker out, the microdots had disappeared. But they weren’t really missing; instead of releasing into the water, they had been transferred onto the bottom of the glass where they were casting a rainbow reflection.
“It was those rainbow colors that really surprised me,” Zabow recalls. These colors showed that microdots arrays had maintained their unique pattern.
The sweet mess inspired him to come up with an idea. Can regular table sugar be used as a way to extend the power of microchips on new and unusual surfaces? Zabow’s findings on this potential transfer printing process were published in Science In its November 25 issue.
Microprinting is the process of printing precise, but tiny patterns a few meter wide on surfaces to give them new properties. It’s used in electronics, semiconductor chips, and micropatterned surfaces. These tiny mazes of metals, and other materials were traditionally printed on silicon wafers. These intricate patterns, which are small and delicate, need to be printed on non-flat surfaces as more semiconductor chips and smart materials become available.
It is difficult to direct print these patterns on such surfaces. Scientists transfer prints. Flexible tapes and plastics are available that can be used to print these patterns (e.g., using putty for picking up newsprint). However, these solids may have difficulty conforming to corners and sharp curves when the print is laid back. They can also leave behind chemicals and plastics that could make it difficult or dangerous for biomedical use.
REFLEX transferred 1-micron disk arrays onto a sharp point of pin.
Credit:
G. Zabow/NIST
You can also use liquid techniques. The transfer material is laid on the water surface and the target surface is pushed through the fluid. This can prove to be a challenge as it is difficult to position the print exactly where you want it on a new surface when there is a free flowing liquid.
Zabow was surprised to discover that a simple mixture of caramelized sugars and corn syrup could work.
The sugar mixture can then be used to cover micropatterns with a small amount water. After the water has evaporated, the candy will harden and can be removed with the pattern still embedded. The candy with the printed pattern is placed on top of the new surface, and then melted. The combination of sugar and corn syrup maintains a high viscosity during melting, which allows the pattern to retain its arrangement even as it flows along curves and edges. The sugar can then be washed with water to remove it, leaving only the pattern.
This technique is called REFLEX (REflow driven FLExible Xfer). It allows scientists and manufacturers to transfer microcircuit patterns like a stencil. For potential biomedical and microrobotics research, the patterned materials can be transferred to fibers or microbeads, or onto sharp or curved surfaces in new devices.
The technique proved successful for a large range of surfaces, including printing onto the sharp point of a pin, and writing the word “NIST” in microscale gold lettering onto a single strand of human hair. In another example, 1-micrometer-diameter magnetic disks were successfully transferred onto a floss fiber of a milkweed seed. Magnetically printed fibers react to magnetic fields, proving that the transfer worked.
There’s still more to explore with REFLEX, but this process could open new possibilities for new materials and microstructures across fields from electronics to optics to biomedical engineering.
“The semiconductor industry has spent billions of dollars perfecting the printing techniques to create chips we rely on,” Zabow says. “Wouldn’t it be nice if we could leverage some of those technologies, expanding the reach of those prints with something as simple and inexpensive as a piece of candy?”
Paper: G. Zabow. Reflow transfer to conformal microprinting in three dimensions. Science. Published online November 24, 2022. DOI: 10.1126/science.add7023