New artificial muscle made by applying a voltage across gold-plated onion cells
A part of soft robotics research, artificial muscle technologies have witnessed a boom in recent years as scientists look at new ways to interact with and manipulate our environment – be it through medical implants or industrial robotics. Such technologies are often manufactured using polymers or pneumatic cylinders, but recent attention has focussed on exploiting natural products such as cellulose.
‘The use of biodegradable materials is definitely a growing field,’ explains Andrew Conn from the Bristol Robotics Laboratory, University of Bristol, UK, who was not involved in the work. ‘I think it’s a very exciting field for novel electronic and robotic devices.’
Wen-Pin Shih and his colleagues at the National Taiwan University were initially conducting research in this field when they made a strange discovery. ‘We used micro machining to fabricate a polymer muscle in the beginning,’ says Shih. ‘Then we found that the lattice structure of onion cells was similar to what we have been trying to make. So we started to investigate using onion cells.’
The single-layer hollow lattice of an onion skin cell makes it an effective muscle actuator, allowing it to contract or expand, according to Shih. But the presence of hemicellulose in the cell wall stops it from being soft and elastic. The team removed this unwanted compound by pre-treating the onion skin with sulphuric acid and freeze drying it.
After depositing gold of different thicknesses onto each side of the cells, the researchers applied a voltage across their artificial muscle. At low voltages, from 0–50V, the cells expand and bend down, but they contract and bend upwards between 50–1000V. ‘Overall, the cells contract and bend like human muscle,’ comments Shih.
The bending is so pronounced in both directions, Shih and his colleagues can construct simple tools made from the muscle. ‘We have made a pair of tweezers from onion muscles – this device can grip and move a small [cotton] ball of 0.1mg,’ he says. ‘We can also make the muscle vibrate and will try to use it to deliver acoustic waves.’
Conn is impressed with work, but believes the real impact lies in the very material used to make the artificial muscle. ‘There will be groups very interested in this work,’ Conn tells Chemistry World. ‘[What will] get their attention is the fact that they’re using a readily available, natural material and it doesn’t require huge amounts of processing to get a functional actuator.’