Artificial Muscles Made from Onion Skin and Gold

National Taiwan University published in an Applied Physics Letters, a publication of the American Institute of Physics (AIP). Researchers discover a way to create artificial muscles from the skin of onions.

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This discovery is unique to existing artificial muscles because they can either expand or contract to bend in different directions, depending on the driving voltage applied. In the past, artificial muscles could bend or contract but not at the same time.

Artificial muscles can be used for use in prosthetic limbs, robots, and implanted medical devices. The original goal was to use onion cells to develop an engineered micro-structure in artificial muscles for increasing the amount a muscle can bend or stretch when triggered. They thought this would create a more versatile muscle.

Fragile skin found just beneath an onion's surface (called epidermis) is a thin, translucent layer of blocky cells arranged in a tightly-packed lattice. Treating the onion cells with acid to remove a protein that makes the cell walls rigid. Then coated both sides of the onion layer with gold. When current flowed through the gold electrodes, the onion cells bent and stretched similar to a muscle. 

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By making the top and bottom electrodes a different thickness. The cell stiffness became asymmetric from top to bottom. This gave the researchers control over the muscle's response. Low voltage made them expand and flex downward, toward the thicker bottom layer. A high voltage caused the cells to contract and flex upward, toward the thinner top layer.

This never been achieved with engineered artificial muscles before. This will make artificial muscles more versatile. The hope is to increase the lifting power of artificial muscles so they can move heavier items.