Onion artificial muscles can either expand or contract to bend in different directions depending on the driving voltage applied, says a group of scientists led by Dr Wen-Pin Shih of National Taiwan University.

Artificial muscles are strong and lightweight actuators with the capability of either bending or contraction/elongation.

However, there are currently no artificial muscles that can accomplish these actions simultaneously.

Dr Shih and co-authors thought that onion epidermal cells might be a viable candidate for the tricky task of creating a more versatile muscle that could expand or contract while bending. The onion epidermis – fragile skin found just beneath the onion’s surface – is a thin, translucent layer of blocky cells arranged in a tightly-packed lattice.

“The initial goal was to develop an engineered microstructure in artificial muscles for increasing the actuation deformation,” said Dr Shih, who is a co-author on the study published in the journal Applied Physics Letters. “One day, we found that the onion’s cell structure and its dimensions were similar to what we had been making.”

The scientists found that the single layered, latticed microstructure of onion cells after acid treatment became elastic and could simultaneously stretch and bend when an electric field was applied.

“By modulating the magnitude of the voltage, the artificial muscle made of onion epidermal cells would deflect in opposing directions while either contracting or elongating,” they said.

To transform the onion cells into artificial muscles, Dr Shih and his colleagues from National Taiwan University and the University of Waterloo, Canada, treated the cells with acid to remove the hemicellulose, a protein that makes the cell walls rigid.

“After the acid pretreatment, only a small amount of hemicellulose is left in the cell wall of the onion epidermal cells.”

They then sputtered the onion epidermal cell layer with different thicknesses of gold on both sides (top – 24 nm; bottom – 50 nm).

The gold layers were intentionally deposited at different thickness as to generate different bending stiffness on the upper and bottom cell walls. This was to make the bending actuation more prominent.

“We intentionally made the top and bottom electrodes a different thickness so that the cell stiffness becomes asymmetric from top to bottom,” Dr Shih explained.

The asymmetry gave the scientists 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.

To demonstrate their device’s utility, Dr Shih and co-authors combined two onion muscles into a pair of tweezers, which they used to pick up a cotton ball.

In the future, the team plans to increase the lifting power of their artificial muscles.

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Chien-Chun Chen et al. 2015. Onion artificial muscles. Appl. Phys. Lett. 106, 183702; doi: 10.1063/1.4917498