These "smart skin" stickers are also speakers

These stickers, thinner than temporary tattoos, have a grid of silver wires in them that make sound when electrified. They also work as microphones.

From ScienceMag:

After receiving an electric audio signal from a music player, the tiny loudspeaker heats up the wire grid to about 33°C, which replicates the sound pattern by changing the pressure of the surrounding air. Our ears pick up these changes in air pressure as sound waves.

The microphone operates in reverse, converting speech sound waves back into an electric signal, which can then be stored and played back by a smartphone or computer. It can detect sound waves coming from the mouth, but it can also recognize words simply from the rumbling of the vocal cords through the skin, the team reports today in Science Advances.