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Gold-plated onion cells make artificial muscles

05 May 2015

An artificial muscle created from onion epidermal cells can either expand or contract to bend in different directions depending on the driving voltage applied.

Artificial muscle 'tweezers' made from gold-plated onion epidermal cells (image: Shih Lab, National Taiwan University)

The onion epidermis - the fragile skin found just beneath the onion's surface - is a thin, translucent layer of cells arranged in a tightly-packed lattice. Wen-Pin Shih and his colleagues from National Taiwan University thought that onion epidermal cells might be a viable candidate for creating a more versatile muscle that could expand or contract while bending. Hitherto artificial muscles were able to bend or contract, but not at the same time.

"The initial goal was to develop an engineered microstructure in artificial muscles for increasing the actuation deformation [the amount the muscle can bend or stretch when triggered]," says lead researcher, Wen-Pin Shih from National Taiwan University. "One day, we found that the onion's cell structure and its dimensions were similar to what we had been making."

The researchers treated the cells with acid to remove the hemicellulose, 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 much like a muscle.

Onion epidermis being peeled away and a microscopic view of onion epidermal cells (image: Shih Lab, National Taiwan University)

"We intentionally made the top and bottom electrodes a different thickness so that the cell stiffness becomes asymmetric from top to bottom," says Shih. The asymmetry gave the researchers control over the muscle's response: a low voltage made them expand and flex downwards, towards the thicker bottom layer. A high voltage, on the other hand, caused the cells to contract and flex upwards, towards the thinner top layer.

"We found that the single-layer lattice structure can generate unique actuation modes that engineered artificial muscle has never achieved before," Shih claims.

To demonstrate their device's utility, the researchers combined two onion 'muscles' into a pair of tweezers, which they used to pick up a cotton ball. In the future, they hope to increase the lifting power of their artificial muscles. "Our next step is to reduce the driving voltage and the actuating force," said Shih.

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