The world's first living robots have been built using stem cells from frog embryos, in a strange machine-animal hybrid that scientists say is an "entirely new life-form".
Dubbed "xenobots" because they are constructed of biological material taken from the Xenopus laevis frog, the little bots are the first to be constructed from living cells.
Researchers are hopeful they could be programmed to move through arteries scraping away plaque, or swim through oceans removing toxic microplastic.
And because they are alive, they can replicate and repair themselves if damaged or torn.
"These are novel living machines," said Dr Joshua Bongard, a computer scientist and robotics expert at the University of Vermont, who co-led the new research. "They're neither a traditional robot nor a known species of animal. It's a new class of artefact: a living, programmable organism."
Living organisms have often been manipulated by humans in the past, right down to their DNA code, but this is the first time that biological machines have been built from scratch.
Scientists first used the Deep Green supercomputer cluster at the University of Vermont to create an algorithm that assembled a few hundred virtual skin and heart cells into myriad forms and body shapes, for specific tasks.
Based on the blueprints, a team of biologists from Tufts University, Massachusetts, then assembled the cells into living bots, just 1mm wide.They gathered stem cells from the embryos of African frogs and using tiny forceps and a miniature electric knife to cut and join the cells under a microscope into a close approximation of the designs specified by the computer.
Assembled into forms never seen in nature, the cells began to work together. The skin cells formed a "body" while contractions of heart muscle cells were repurposed to create a forward motion. These organisms were able to explore their watery environment for days or weeks, powered only by embryonic energy stores.
Later tests showed that groups of xenobots would move around in circles, pushing pellets into a central location - spontaneously and collectively.
Others were built with a hole through the centre, in which drugs could be placed, so they could be carried to specific parts of the body.
"You look at the cells we've been building our xenobots with, and, genomically, they're frogs. It's 100 per cent frog DNA - but these are not frogs. Then you ask, well, what else are these cells capable of building?"
The results were published in the Proceedings of the National Academy of Sciences.