http://www.sciencemag.org/news/2018/08/these-tiny-stretchy-speakers-and-microphones-let-your-skin-play-music

If you’re prone to forgetting your headphones, new wearable technology that could turn your skin into a speaker should be music to your ears. Created in part to help the hearing and speech impaired, the new “smart skin” could be embedded into the ears—or into a patch on the throat. A similar device, described in the same study, acts as a microphone, which can be connected to smartphones and computers to unlock voice-activated security systems.

To build the speakers and the microphone, which are thinner than a temporary tattoo, the researchers needed to design electronics flexible enough to stretch and bend with the skin, without losing their capacity to conduct electricity and heat—both necessary to transmit audio signals.