To do so, the researchers took a different approach from the usual open-loop "calibrate-then-print" process used in conventional 3D-printing technologies. Instead, they modified a low-cost 3D printer to allow it to adjust to the smallest movements of the body during printing.
They achieved this by first scanning temporary markers that are placed on the skin, and then using the printer's built-in computer vision to adjust to movements in real time. This adaptive 3D printing approach is a closed-loop method that uses real-time feedback control.
"No matter how hard anyone would try to stay still when using the printer on the skin, a person moves slightly and every hand is different,” says Michael McAlpine, the lead author of a study on the research. "This printer can track the hand using the markers and adjust in real time to the movements and contours of the hand, so printing of the electronics keeps its circuit shape."
The researchers' printing approach also uses a specialized ink - made of silver flakes - that can cure and conduct at room temperature. Standard 3D printers need to cure at temperatures of around 100°C (~212°F) - a temperature that would burn a person's skin.
The resulting skin-printed electronics can also be easily removed, say the researchers. It can be peeled off with tweezers or even just washed off with water.
"We imagine that a soldier could pull this printer out of a backpack and print a chemical sensor or other electronics they need directly on the skin," says McAlpine. "It would be like a Swiss Army knife of the future, with everything they need all in one portable 3D-printing tool."
The technique, say the researchers, is not limited to electronics. It could, for example, be used to print cells to help people with skin diseases.
"I’m fascinated by the idea of printing electronics or cells directly on the skin," McAlpine says. "It is such a simple idea