Researchers from the National Taiwan University have made artificial muscles by gold-plating an onion layer. A study detailing their work is published in the journal Applied Physics Letter.

Chien-Chun Chen, Wen-Pin Shih and colleagues have been making artificial muscles before. They were looking for a means of increasing how much a muscle bends or stretches when stimulated, when the researchers discovered that the onion has dimensions and a cell structure similar to what they have already been making.

Just underneath the surface of an onion is its epidermis, a thin and translucent layer made up of blocky cells laid out in a lattice packed tight. It is this feature that led Chen, Shih and colleagues to believe there is potential in an onion. Artificial muscles so far could only either contract or bend but not both.

Onion cells were treated with acid, removing the hemicellulose, a protein responsible for giving cell walls their rigidity. Both sides of an onion layer were then coated with gold. With current flowing through the gold plating, onion cells stretched and bent just like muscle.

The researchers intentionally gold-plated the onion layer sides at different thicknesses, allowing them to control how the artificial muscle responded. When a low voltage passed through the bottom layer of the onion, the cells expanded and flexed downwards. When high voltage passed through the top layer, the cells contracted and flexed upwards. The bottom side is the thicker layer while the top side is the thinner layer.

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

To demonstrate the artificial muscle they made in action, the researchers put together two of them to form a tweezers. Controlling the voltage, the artificial muscle tweezers was then made to pick up a ball of cotton. Researchers are hopeful they will be able to boost the lifting power of their onion-derived artificial muscles. To do this, they are planning to reduce levels of actuating force and driving voltage.

The study was supported by Taiwan's Ministry of Science and Technology and the Nano-Electro-Mechanical-systems Research Center at the National Taiwan University. Huai-An Jeng, Pin-Chun Huang, Shing-Yun Chang, Hsi-Mei Lai and Pei-Zen Chang also contributed to the study. Jeng is also affiliated with the University of Waterloo.

Artificial muscles can be used in a wide range of applications, including musculature for humanoid robots, comfort-adjusting textiles, prosthetics, temperature-dependent liquid or gas filters and lab-on-chips.

Photo: USDA | Flickr

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