Artificial muscles could one day revolutionize fields such as robotics, prosthetics and nanotechnology. So far, we've seen examples made from materials like electroactive elastomers, crumpled graphene, and vanadium dioxide. The problem is, most artificial muscles can only expand in one direction, or contract in the other. Now, however, scientists from National Taiwan University have gotten around that limitation using gold-plated onion cells.

Led by Prof. Wen-Pin Shih and graduate student Chien-Chun Chen, the researchers used cells within an onion's epidermis – this is the thin translucent membrane located right beneath the onion's outer skin. Its blocky cells, which are arranged in a single layer in a lattice structure, are similar in form to an engineered microstructure that the team had already been working on.

A sample of onion epidermis was first treated with acid to remove the cells' hemicellulose, which is a protein that keeps their walls rigid. The membrane was then coated with a thin layer of gold on both sides – one of these layers was intentionally thicker than the other, making the stiffness of the material asymmetric from top to bottom.

When an electrical current was applied to the material, it flowed between the two gold layers, treating them as electrodes. If that current was of a low voltage, it would cause the cells to expand, bending the material down towards the side with the thicker layer. If was a higher voltage, however, it would make the cells contract, bending towards the thinner upper layer.

In order to demonstrate the technology, two of the artificial muscles were combined to form a pair of tweezers, which were used to pick up a cotton ball.

"We found that the single-layer lattice structure can generate unique actuation modes that engineered artificial muscle has never achieved before," says Shih. "Our next step is to reduce the driving voltage and the actuating force."

A paper on the research was recently published in the journal Applied Physics Letters.

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