https://techcrunch.com/2018/07/25/nanotech-powers-this-super-sensitive-microphone/

The trouble with microphones is that they don’t just hear — they have to listen. Powering the mic and its signal processor means using energy, and energy means a battery, and a battery means charging. This new microphone-like system hears more like the way our own ears do, requiring little or no power, and could help fill the world with voice-responsive machines. (If that’s something we really want.)

The device is called a “triboelectric auditory sensor,” and it works via what’s called the triboelectric effect — essentially when two surfaces rub together and create a charge. They’re still trying to figure out why this happens, but what matters to engineers is that it happens reliably.