For some, onions are the key ingredient to a good sauce. For others, the source of a good, sulphuric acid-driven cry. But, in further proof that one person's trash is another's treasure, researchers report in Applied Physics Letters that they've turned onions into artificial muscles superior to anything that's come before.
If onion muscles sound like they'd make great robot parts, well, that's true. The term, though, covers any sort of device that contracts, expands, or bends when someone triggers it, often with heat or an electric current. Until recently, artificial muscles were made out of expensive designer materials such as carbon nanotubes. Most artificial muscles are a bit limited, too; some bend and some contract, but few, if any, can do both at once.
Pondering how to build a better artificial muscle, researchers Chien-Chun Chen and Wen-Pin Shih, along with colleagues at National Taiwan University's Tiny Machine and Mechanics Laboratory, realized that onion epidermis—the thin skin underneath each layer of onion—resembled some of the designer materials they'd come up with.
When the researchers hooked up their gold-plated onion skins to a battery, they found they could bend the device up and down and expand or contract the epidermal cells by controlling the voltage.
But unlike those materials, onion-based artificial muscles were relatively easy to make. After extracting a layer of onion epidermis—all that takes is a chef's knife and a pair of tweezers—the researchers soaked it in a medium-hot sulphuric acid bath to remove hemicellulose, a fibrous material that impedes bending. Next, they cut the skins into five- by 25-millimeter strips, which they coated with gold, 24 nanometers thick on top and 50 nanometers on bottom, so that the top was easier to bend.
When the researchers hooked up their gold-plated onion skins to a battery—actually, a variable electric voltage—they found they could bend the device up and down and expand or contract the epidermal cells by controlling the voltage, something no other artificial muscle has done. Up to about 50 volts, the cells expanded. Combined with the thinner, more elastic gold layer on top, that allowed the device's free end to bend upwards by about 30 micrometers.
But from 50 up to 1,000 volts, the onion cell walls caved in, contracting and bending the device upward by as much as one millimeter. For the fun of it, the team built a pair of tweezers out of their gold-plated onion muscles and used them to pick up a small cotton ball.
So far, onion muscles can't do a whole lot. Chen and Shih's experiments showed they can bend by at most about four percent, and they can't yet lift anything very heavy (that cotton ball weighed only a tenth of a milligram). Still, building an onion muscle costs next to nothing—and with relatively little environmental impact. "Due to the diversity of plants and their cell structures, discovering the use of natural structures in engineering is of interest," for those seeking better, stronger, and environmentally friendly machines, the researchers write.
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