French scientists have designed an electronic conducting polymer actuator that can extend and contract linearly rather than bending. The fibres could eventually be used to develop artificial muscles. 

Cédric Plesse from University of Cergy-Pontoise, France, and other groups have been investigating polymer actuator devices that bend when electrically stimulated. These lightweight devices can operate in the open air using low voltages, which gives them the potential to replicate operations controlled by arms. But, as Plesse explains, a bending motion is not always useful. 'The challenge is to find some way to develop devices that present "push-pull" linear deformations, just like those of mammalian muscles', he says. 

Conducting polymer actuators translate electrochemical reactions into motion. Traditionally this means having two electrodes on either side of an ionic conducting polymer membrane. When a voltage is applied, ions move between the electrodes as one is reduced and the other oxidised. This makes one electrode expand as the other shrinks, causing a bending motion. 

To avoid this bending motion, Plesse's design consists of two tubular shaped electrodes with the ion conducting polymer membrane sandwiched between them. The inside electrode is very thin and extends as it is oxidised, but the outer electrode is much thicker, so can mop up the charges from the other electrode without undergoing significant redox changes and hence doesn't really contract at all. The rest of the fibre is elastic enough to extend along with the inner electrode, which stops the whole thing from bending and makes it extend and contract linearly. 

Elisabeth Smela, who develops polymer actuators at the University of Maryland, US, commented that Plesse and his colleagues have joined different threads in prior work to achieve a significant advance. She added, 'This work should stimulate others in the field to explore this useful linear actuation mode and to tackle the remaining hard challenges of lifetime, speed, and efficiency - so that these actuators can be widely applied for robotics.' 

These linear actuators will initially be suited for applications in micro-systems which don't involve high forces, explains Plesse. But in the future he hopes they will be used in active prosthetics, robotics and tasks involving repetitive weight lifting. Currently the actuators are limited by a rather low linear deformation of around 3 per cent, but Plesse hopes that by collaborating with other scientists, efficient artificial muscles will be possible in the near future. 

Fay Nolan-Neylan 

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