Biophysics: Onions transformed into robotic limbs

What do you get when you plate an onion skin with gold? The world's most flexible artificial muscle.

This unexpected recipe comes from researchers at National Taiwan University who were investigating new materials to power robotic limbs. They found that onion skin - treated with acid, freeze-dried and then coated with thin layers of gold - could be made to expand or contract as varying voltages were applied. The project, which created the first artificial muscle capable of bending in different directions, is published in the journal Applied Physics Letters.

According to Wen-Pin Shih, project leader, the initial aim was to engineer a synthetic microstructure capable of bending or stretching. "One day, we found that the onion's cell structure and its dimensions were similar to what we had been making," he says.

The researchers peeled onions to remove the epidermis, a fragile inner layer of skin with cells packed into a tight lattice. Acid treatment removed the hemicellulose that stiffens cell walls in a live onion and then both sides were coated with gold; the bottom layer was laid down slightly more thickly than the top, to create an asymmetric response in the electrodes. The muscle flexed upwards when a high voltage was applied and downwards when the voltage fell - something never achieved with engineered artificial muscle.

To demonstrate the material in action, the team constructed tweezers with an onion muscle on each side, which they used to pick up a cotton ball. The next stage will be to increase the power and durability of the muscle while reducing the high electrical potential needed to drive this early prototype. Developers of soft robots will be watching their progress with interest.

Photograph: Dreamstime

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