The centimeter-long "biobot" was made by attaching heart muscle cells onto a flexible structure, or body, of hydrogel—the same material used to make contact lenses for human eyes.

To make the biobot's body, the team used a 3-D printer, which creates solid objects by laying down successive layers of soft materials that fuse together and harden.

Gathering the heart cells was a bit trickier. The researchers removed whole hearts from anesthetized newborn rats, cut the organs into tiny pieces, and then processed the fragments to loosen and separate the heart cells. The cells were then added to the robot body—each bot contains between a few thousand and a few hundred thousand. (Read "Heart Cells Can Regenerate, Nuclear-Bomb Evidence Shows.")

"In a few days