The researchers designed the circuit to match the mechanical properties of skin, meaning it can stretch, scrunch, and bend just like skin does. That’s no easy task, given that electronics aren’t usually elastic. To do this, the researchers bent the circuit’s wires into a squiggly shape (see the photo above) that can expand and shrink along with the skin. They also shrank various circuit components to the minute size of the natural bumps and grooves of human skin. Since they wanted to prove that the circuit could have a range of applications, the team included all sorts of different components, from sensors and transistors to wireless antennas. The circuit is made on silicon wafers, then transferred to a sheet of water-soluble plastic, totaling about 40 micrometers thick. It’s then transferred to the skin like a temporary tattoo: dab with water, let it dry, and you’re good to go. (For people who feel the circuit doesn’t match their style, it can be embedded in another temporary tattoo, as the video below demonstrates.) It can stay attached for a few days, and be peeled off at any time. To test out the muscle-monitoring capabilities of the circuit, the researchers tested out how well it could recognize words when pasted on the throat, based on the electrical activity of nearby muscles. The circuit was sensitive enough, they found, that people wearing it could play a voice-controlled computer game—speaking the commands “left,” “right,” “up,” and “down”—with 90% accuracy.