Here's a critter that would be a showstopper in your aquarium: By layering rat heart cells over a gold skeleton, scientists have built tiny swimming artificial stingrays that can be driven and guided by light. These little ray-bots, described in the journal Science, may offer insight into building soft robotics, studying the human heart -- and perhaps even building an artificial one from scratch. Senior author Kit Parker, a Harvard bioengineer, first got the idea for these tiny ray-bots when his young daughter tried to pet a stingray at an aquarium and it quickly and gracefully evaded her hand. Parker watched the rippling body, which reminded him of the stringy cord-like trabeculated muscle on the endocardial surface of the heart, and a thought struck him: He could probably build something that moved like that. "It kinda hit me like a thunderbolt," he said.

Or at least a coin-sized cyborg stingray made from rat heart cells that can be controlled underwater using light. Designed by Kevin Kit Parker from Harvard University's Wyss Institute and his team, the 16-millimetre-long soft robot has a gold skeleton overlaid with a flexible polymer. Its muscles are made up of about 200,000 rat heart cells laid down in layers. "My building material is alive," says Parker. To get the tiny robot to move, the team tweaked the rat-cell genes to make them light-sensitive.