Surprise! Gold-plated onion cells make great artificial muscles

You can use an onion to flavor your stir fry, keep an avocado from turning brown, and make you cry when you aren’t sad. Now, according to a new study, you can also use it to make an artificial muscle.

To be clear, no one is talking about a fist-sized onion pumping away like a heart. The first onion muscle prototypes are very small -- just a few onion cells long. They were created from the thin, translucent layer of epidermal cells that lie just below the dry outer skin of the average store-bought onion.

But it turns out that if you freeze-dry these cells, coat them in gold, and then hook them up to an electric current, they will contract or elongate depending on how much voltage you use. As the cells contract or elongate, they cause the entire length of cells to bend.

“Overall, the cells contract and bend like human muscle,” said lead researcher Wen-Pin Shih of National Taiwan University.

According to the study, this is an engineering first.

The authors define an artificial muscle as one that bends in response to an external stimulation. Prior to this study, researchers had tried to make artificial muscles with various types of polymers that could contract or expand, but none that could also bend at the same time.

The polymers were expensive to make, and they weren’t working so well anyway, so the researchers decided to experiment with more easily accessible, and significantly cheaper biomaterials.

“When we were in elementary school, we studied plant cell structure by observing onion cells under the microscope,” Shih said. “So, when we recently searched for plant cells to replace our engineered artificial muscles, we thought about onion cells again.”

The cells in the thin epidermis of an onion are already handily arranged in a single layer. To collect them for this work, the researchers used a pair of tweezers to remove the thin “skin” from a fresh, peeled onion and then washed it in water.

Next the researchers freeze-dried the cells for 24 hours to remove water from the interior of the cell. This step kept water from spontaneously evaporating later, something that could cause unwanted corrugations or pin holes in the cell walls.

After the freeze-drying, the onion cells were stiff and brittle, so the scientists added an acid treatment process to loosen them up. Finally, to make the cells able to respond to an electric current, they were coated with gold.

The researchers found that when an electrical current between 0 and 50 volts was applied to the onion cell muscle, it causes the top part of the cells to elongate, in turn making the whole muscle bend downward. When the current was between 50 and 1,000 volts, the cell walls become concave and the muscle bends upward.

So far, the researchers have demonstrated how two of these small artificial muscles made of onion could be used almost like a pair of tweezers to pick up a small piece of a cotton ball. But Shih said more intriguing applications may be on their way.

“Gold electrode and onion muscles are both biocompatible, so this work could be applied for biomedical manipulations,” he said.

He added that in the future, these artificial muscles could be bundled for use in robots.

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