By wrapping rings of genetically engineered mouse muscle tissue around a variety of soft 3-D printed skeletons, researchers built what they call “bioactuators” that convert energy into motion when stimulated by a specific wavelength of light. The group built a few different varieties of the millimeter-scale bio-bots and got them to jump around by shining flashes of light at them. They say the tiny machines could one day be used to deliver drugs inside the human body.

The research was led by Rashid Bashir, a professor of bioengineering at the University of Illinois at Urbana-Champaign and published today in Proceedings of the National Academy of Sciences. Previously, Bashir led a group that demonstrated bio-bots that responded to an electrical field, but the new optogenetic approach is less invasive and provides more control over their movement. The video below shows a bio-bot made of one muscle ring and two symmetrical, 3-D printed “legs.” Stimulating only one leg makes it move in the direction of that leg.

(Read more: “In Human Test of Optogenetics, Doctors Aim to Restore Sight to the Blind”)