They call the technique "mechanically sintered gallium-indium nanoparticles," because, well, they need to turn liquid metal into nanoparticles first before it can function as ink. That's made possible by immersing the alloy into a solvent such as ethanol and subjecting the mixture to ultrasound in order to disperse the liquid metal.

The printed circuits end up being protected by a "skin" that prevents electrical conductivity. But that can easily be removed by applying light pressure (like stamping or scraping) on the printout, allowing designers to choose which parts of the circuits to activate. The team plans to explore the technique further and perhaps put it to the test, but you can read more about their study when their paper gets published in the April 18th issue of Advanced Materials.