The robotics world just got a new kind of artificial muscle to flex, crafted from an unexpected material: onion skin. Today in Applied Physics Letters, researchers from National Taiwan University unveiled a new artificial muscle design, made by layering gold on top of the skin of an onion. It's a new tactic in the search for a working artificial muscle, a search that could have important implications for robotics and healthcare in the years to come.

Because of the onion's unique cell structure, the new artificial muscle is able to remain soft and bendable while it's contracting, a trick that has eluded previous artificial muscle design. The researchers had been attempting to make a similar structure out of polymers when they realized the onion provided a cheaper natural alternative. "Like human muscles, it can bend and contract simultaneously," said Wen-Pin Shih, a mechanical engineer who worked on the project. "That's something previous models couldn't do."

The muscle is built on the epidermis of the onion, the filmy layer underneath the outer shell. Like real muscles, that film is both stretchy and responsive to electricity, thanks to the single-layered lattice structure of its cells. Still, getting the film to work as a muscle took a lot of preparation. The team freeze-dried the skin to remove internal water and dipped it in dilute sulfuric acid to make the skin more elastic. Then the onion skin was dipped in two layers of gold and an electrode was attached.

A diagram of the study's onion tweezers.

The result is a working machine built on top of the onion's basic properties. The upper portions of the skin expand under low voltages, bending the muscle downward, but contract under higher voltages, curving the muscle upward. The team even used it to pick up a tiny ball of cotton, combining two muscles for a pair of onion tweezers. The muscles are still only strong enough to pick up about 2 milligrams at a time, but Shih says the next step is strengthening the muscle and lowering the voltage. Along the way, he hopes to discover more about the cellular structure that allows the muscle to stay elastic while contracting. "We still don't fully understand the structure of the cell walls and its associated properties," Shih says. "We're just reporting what we have so far to exchange ideas."

If the new muscle becomes viable for production, it could be a valuable tool for the emerging world of soft robotics, which focuses on squishy bots that can be set loose without damaging their surroundings. Currently, the bots are limited to simple motor functions like contracting, expanding, and rotating, but robotics experts hope that artificial muscle technology might one day combine all three of those functions into a single all-purpose muscle. And if today's findings pan out, it might be the film of an onion skin that finally gets us there.