Onions have so many layers — and there's a great deal they can teach us. Not only do they not have to make us cry anymore, a fresh idea suggests they could actually make us stronger. A team in Asia has shown how the specific structure of the onion can be used to create artificial muscle.
Researchers at the National Taiwan University have discovered how to create artificial muscles by using the cells of onions. Though synthetically created muscles themselves aren't new, unlike previous attempts, the Taiwanese prototypes are significantly more versatile in that they can expand and contract while bending. This is an important development and promises big things for future applications. Their creation is described this week in the journal Applied Physics Letters.
"The initial goal was to develop an engineered microstructure in artificial muscles for increasing the actuation deformation" — the amount by which a muscle can bend or stretch — said 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 particular qualities of the onion's structure make it, literally, a lot more flexible.
The important part of the onion — the epidermis — is the thin, fragile skin just below the surface. It is made up of a translucent layer of "blocky" cells that are packed tightly together into a lattice arrangement. This layer of the onion protects the bulb against unwanted attention and damage from viruses and fungi. The useful similarities between onion cells and human cells has long been acknowledged and, because of the simplicity of their structure, they are often used to introduce students to plant cell biology. Because of the parallels between onion epidermal cells and human epithelial cells, Shih and his colleagues thought that research into what the onion had to offer might prove them to be potentially useful for creating a more versatile muscle.
Typically, muscles have three basic ways of moving: contraction, expansion and rotation. These simple actions — which are technically known as actuation responses — can be combined to achieve more complex actions like bending. To date, Shih says, artificial muscles can either bend or contract, but not at the same time.
First, the scientists used acid to remove hemicellulose from the onion; this substance makes the cell walls rigid. They then coated both sides of the onion layer with gold, a metal that has very low resistance to an electrical current. When electricity was passed through the now flexible onion, the cells bent and stretched like a muscle. "We intentionally made the top and bottom electrodes a different thickness so that the cell stiffness becomes asymmetric from top to bottom," said Shih. This asymmetry meant the team could exercise greater control over the response of the "muscle." A low voltage made them expand and flex downwards, towards the thicker bottom layer. A higher voltage caused the cells to contract and flex upward, toward the thinner top layer.
Although this doesn't seem to herald a direct medical breakthrough as such — after all, we're not talking about a replacement for human muscle here — it has great potential for other technological arenas. Artificial muscle has a great many potential uses in biomimetic machines — that is, those that attempt to imitate the actions and functions of living things. The science of robotics is used widely in (among many others) the aerospace, automotive and entertainment industries. Perhaps most exciting is the potential for use in the developing field of telesurgery where specialist doctors can remotely participate in operations from hundreds of miles away when it is impractical for them to be present.
Though this is a first step using this new medium, Shih's team is optimistic about the future. They hope that, with more research, they will be able to increase the lifting power of their artificial muscles by refining the efficiency of their system. "Our next step is to reduce the driving voltage and the actuating force," said Shih. "We found that the single-layer lattice structure can generate unique actuation modes that engineered artificial muscle has never achieved before," says Shih. To show just how dexterous their new muscles can be, they attached two of them to a pair of tweezers and then used them to pick up a cotton ball. Hey, even scientists need a party trick.