Artificial muscles know their onions

Researchers at the National Taiwan University have turned to the epidermal cells of the onion, Allium cepa, to help them make an artificial muscle. The new muscle responds to an applied voltage like a pair of tweezers pincing or opening depending on the direction of the voltage and so expanding or contracting in different directions, a first for artificial muscles. The team reports details in the journal Applied Physics Letters [Chien-Chun Chen et al 2015, 106, 183702; DOI: 10.1063/1.4917498]

"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]," explains project leader 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 is the soft and tissue-like layer beneath the onion's surface. This fragile and translucent layer is composed of block-shaped cells that form a tightly-packed lattice. Shih and his colleagues recognized that these epidermal cells might be useful for accomplishing a difficult task in making devices that could act as artificial muscle because these cells can both contract or expand when they are bent, a phenomenon not observed in polymer gels, nanotubes and other materials tested for devices based on artificial muscle.

Before they could construct an artificial muscle from onion epidermal cells, the team had to treat the biostructure with dilute sulfuric acid to remove the hemicellulose, a protein that gives the cells some rigidity. Then were then able to coat these newly flexible cells with a layer of gold to give them an electrode layer. A current applied to the gold electrode caused the onion cells to bend or stretch like a muscle cell, the team reports.

"We intentionally made the top and bottom electrodes a different thickness so that the cell stiffness becomes asymmetric from top to bottom," explains Shih. This anisotropy gives the team control over how the artificial muscle structure responds electrically. A lower voltage (less than 50 V) makes them expand and flex downwards just 30 micrometers, towards the thicker bottom electrode layer. Conversely, a high voltage (50-1000V) causes the cells to contract and so flex upwards, up to 1 millimeter, towards the thinner top layer.

"We found that the single-layer lattice structure can generate unique actuation modes that engineered artificial muscle has never achieved before," adds Shih. By combining two onion muscles the team was able to fashion a pair of "tweezers" which they could then use to grab hold of a tiny cotton ball.

"For this onion muscle project, the next step is to lower the driving voltage for better integration with driving circuits and to increase actuation force for broader applications (currently 20 micronewtons at 1000 V," Shih told us. "The ultimate aims of developing muscles from biomaterials are to develop biomedical devices [including microelectromechanical systems (MEMS)] and robotics."

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".