This millimeter-scale robot designed by researchers at the Max Planck Institute for Intelligent Systems could enable "applications in microfactories such as the construction of tissue scaffolds by robotic assembly, in bioengineering such as single-cell manipulation and biosensing, and in healthcare such as targeted drug delivery and minimally invasive surgery" with bots inside the body controlled by magnets. From their scientific paper in Nature:
Here we demonstrate magneto-elastic soft millimetre-scale robots that can swim inside and on the surface of liquids, climb liquid menisci, roll and walk on solid surfaces, jump over obstacles, and crawl within narrow tunnels. These robots can transit reversibly between different liquid and solid terrains, as well as switch between locomotive modes. They can additionally execute pick-and-place and cargo-release tasks.
"Small-scale soft-bodied robot with multimodal locomotion" (Nature)
