https://qz.com/1323970/mit-engineers-developed-tiny-magnetic-robots-that-might-someday-crawl-inside-you/

Engineers at MIT have developed tiny magnetic robots that could one day be used as biomedical devices inside humans.

The robots, which are about the size of a quarter coin, are made of a novel type of 3D-printer ink, infused with tiny magnetic particles arranged in such a way that when a magnet is passed beneath them, they constrict in a specially-designed pattern.