Answer: Directly onto the skin.
The key is in the ink. Most inks used in 3-D printing cure at very high temperatures and would burn human skin. This particular ink, however, cures at room temperature, making it harmless. The ink is made of silver flakes and is conductive, allowing for the use of electronics.
Since our bodies still move even when we do our best to hold still, a series of markers must first be placed on the skin in order to keep the printer in alignment. The skin is then scanned with a 3-D scanner, and the printer uses computer vision to keep on track while printing.
“We are excited about the potential of this new 3-D-printing technology using a portable, lightweight printer costing less than $400,” said Michael McAlpine, the University of Minnesota professor who led the project. “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. It would be like a 'Swiss Army knife' of the future with everything they need all in one portable 3-D-printing tool."