Onion skin cells make an effective muscle actuator

The use of artificial muscle technology in industrial and medical applications has expanded greatly in recent years as we seek new ways to interact with and manipulate our environment. We tend to think of robots as hard, rigid metal structures driven by powerful motors but to harness something like the manual dexterity of humans we need to look at what nature offers in the way of hard and soft structures, actuators, sensors and grippers. Research into what is known as ‘soft robotics’ has so far concentrated on pneumatic devices manufactured using a variety of polymers although recent attention has begun to focus on the use of natural, preferably biodegradable materials.

Until now, artificial muscles have been made that can either contract or bend but not both at the same time. However, that could change following research at the National Taiwan University where a team using micro-machining to fabricate polymer muscle realised that the single-layer hollow lattice structure of cells just beneath the skin of onions was similar to what they had been trying to make.

The research showed that, while the onion skin cells made an effective muscle actuator which allowed it to contract or expand according to the stimulation, hemicellulose in the cell wall stopped it from being soft and elastic enough to bend. This protein was removed by pre-treating the onion skin with sulphuric acid and freeze drying it. Then each side of the onion layer was coated with gold to make electrodes. When current flowed through the gold electrodes the onion cells bent and stretched much like a human muscle.

The next stage was to deposit gold of different thicknesses on to each side of the onion cells which caused the cell stiffness to be asymmetric from top to bottom. This asymmetry allowed the researchers control over the muscle’s response to different voltages of current. Whereas applying a low voltage (up to 50V) caused the cells to expand and bend downwards towards the thicker layer, they would contract and bend upwards towards the thinner layer above 50V. The bending was so pronounced in both directions that the researchers were able to construct a pair of tweezers from their onion muscles that could grip and move a small cotton ball. Furthermore, they found they could also make the muscle vibrate — something that could be used to deliver acoustic waves.

Although demonstrating that the single-layer lattice structure would generate the sort of unique actuations that artificial muscles had not achieved before is impressive in its own right, the researchers think the real impact could be in the fact that they used a readily available, natural material that did not require huge amounts of processing to make a functional actuator.