If a robot is made entirely out of living tissue, is it still a robot?
That's a question scientists are struggling to answer this week, after the creation of the world's first 'xenobots' - programmable robots grown from stem cells.
"You look at the cells we've been building our xenobots with, and, genomically, they're frogs," said Michael Levin of the Center for Regenerative and Developmental Biology at Tufts University in Massachusetts.
"It's 100 percent frog DNA - but these are not frogs."
The scientists used a supercomputer to run hundreds of simulations of how an entirely biological robot might work, evolving myriad forms and body shapes until they were able to do things like move and carry objects.
Stem cells harvested from African frog species Xenopus laevis were then cut into the shapes the computer came up with - shapes not seen in nature.
"These frog cells can be coaxed to make interesting living forms that are completely different from what their default anatomy would be," said Levin.
"These are novel living machines," says Joshua Bongard, a computer scientist and robotics expert at the University of Vermont. "They're neither a traditional robot nor a known species of animal. It's a new class of artifact: a living, programmable organism."
The millimetre-wide xenobots were not just able to move on their own and carry objects, but heal themselves when damaged.
"We sliced the robot almost in half and it stitches itself back up and keeps going, and this is something you can't do with typical machines," said Dr Bongard.
"The downside of living tissue is that it's weak and it degrades. That's why we use steel. But organisms have 4.5 billion years of practice at regenerating themselves and going on for decades."
Another advantage the xenobots have over their metal and plastic forebears is they don't pollute the environment.
"These xenobots are fully biodegradable. When they're done with their job after seven days, they're just dead skin cells."
The tiny bots had another surprise in store for the scientists, one they're still scratching their heads over. They sometimes appear to coordinate their actions, working together to pile loose cells into piles or moving around in circles - "spontaneously and collectively".
Dr Levin says anyone concerned about where this might lead is right to be, but maintains the research is "an absolute necessity".
"That fear is not unreasonable. When we start to mess around with complex systems that we don't understand, we're going to get unintended consequences... If humanity is going to survive into the future, we need to better understand how complex properties, somehow, emerge from simple rules."
Potential applications for the technology include drug delivery, cleaning up arteries, clearing the oceans of microplastics and mopping up toxic waste.