In addition to adding flavor to food and bringing tears to your eyes, onions are now being credited with creating artificial muscles. 

The groundbreaking achievement was published today in the journal Applied Physics Letters, a publication of the American Institute of Physics (AIP). 


These muscles are unique to existing artificial muscles because they can either expand or contract to bend in different directions, depending on the driving voltage applied. Before this discovery, artificial muscles could bend or contract but not at the same time.

Artificial muscles can be used to help improve the usefulness of prosthetic limbs, robots, and implanted medical devices.

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Failure Leads to Unexpected Discovery

National Taiwan University researchers’ original goal was to use onion cells to develop an engineered microstructure in artificial muscles for increasing the amount a muscle can bend or stretch when triggered.

Because the fragile skin found just beneath an onion's surface (called epidermis) is a thin, translucent layer of blocky cells arranged in a tightly-packed lattice, researchers thought that onion epidermal cells might help in creating a more versatile muscle that could expand or contract while bending.

To their dismay, lead researcher Wen-Pin Shih, Ph.D., and colleagues found that the onion's cell structure and its dimensions were similar to what they had already been making.

The researchers moved forward by treating the onion cells with acid to remove a protein that makes the cell walls rigid. Then, they coated both sides of the onion layer with gold. When current flowed through the gold electrodes, the onion cells bent and stretched similar to a muscle.

Shih told the AIP that the team made the top and bottom electrodes a different thickness so that the cell stiffness became asymmetric from top to bottom. This gave the researchers control over the muscle's response so that a low voltage made them expand and flex downward, toward the thicker bottom layer. A high voltage caused the cells to contract and flex upward, toward the thinner top layer.

This accomplishment has never been achieved with engineered artificial muscles before, Shih stated.

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How the Research Can Be Put to Use

The researchers showed the AIP what their artificial muscle can do by combining two artificial muscles to create a pair of tweezers. Then they used the tweezers to pick up a cotton ball.

The researchers hope to increase the lifting power of their artificial muscles so they can move heavier items. 

Shih said his next step is to reduce the driving voltage and the force needed to put the artificial muscles in motion. 

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