Researchers have developed a new flexible postage-stamp-sized generator device that converts muscle movements into enough power for small electronics, using human skin as one of its charge-collectors.

This friction-inducing phenomenon is called the triboelectric effect, and electrical charge builds up on two dissimilar surfaces when they are put in close contact. A potential difference is generated and a current starts flowing between when they are pulled apart or flexed. This current can be collected using an electrode.
Researcher Lokesh Dhakar said, "Skin, the most abundant surface on a human body, is a natural choice for one of the triboelectric layers and also skin as a triboelectric material has a high tendency to donate electrons or get positively charged which is important in improving the performance of the device if the other triboelectric layer intentionally chosen as the one with a tendency to get negatively charged." The device could light up twelve commercial LEDs.
Source-Medindia