It looks like C-3PO, the gold-covered robot in Star Wars, was on the right track. Researchers have engineered artificial muscle made from gold-plated onion cells, which they say can expand or contrast to bend in various directions.
"The initial goal was to develop an engineered microstructure in artificial muscles for increasing the actuation deformation [the amount the muscle can bend or stretch when triggered]," said engineer and lead researcher Wen-Pin Shih. "One day, we found that the onion's cell structure and its dimensions were similar to what we had been making."
The onion epidermis
More specifically, Shih is referring to the onion epidermis (the thin, fragile layer of skin that surrounds the onion). He realized that those cells could potentially create artificial muscle that was versatile and capable of contracting or expanding while bending. According to Shih, this was something that hadn’t been done before.
Initially, the cells were treated with acid to clear out a protein called hemicellulose that causes the cell’s walls to become rigid. The next step involved coating the onion with gold, which helped the cells behave similar to muscles when current flowed through the gold electrodes.
"We intentionally made the top and bottom electrodes a different thickness so that the cell stiffness becomes asymmetric from top to bottom," Shih explained.
The importance of asymmetry
The asymmetry is key because it offers control of the muscle’s response. For instance, a low voltage makes the muscles expand and flex downwards, while a high voltage makes the muscles flex and contract upwards.
"We found that the single-layer lattice structure can generate unique actuation modes that engineered artificial muscle has never achieved before," Shih said.
What’s next for the team? Shih and his researchers from National Taiwan University hope to increase the artificial muscle’s lifting power. "Our next step is to reduce the driving voltage and the actuating force," said Shih.
A detailed account of their research has been published in the journal Applied Physics Letters.