Artificial Muscles That Remember

Materials scientist combines artificial muscles and shape memory polymer for future sensors, robots, prosthetic limbs

Illustration: Jeff Hall/Stoyan Smoukov/University of Cambridge

Muscle memory is often touted by people trying to teach some physical skill, such as piano playing or yoga. But for Stoyan Smoukov, a researcher in materials science at the University of Cambridge in England, muscle memory is something he’d like to impart to polymers to make them smarter.

So-called smart polymers already exist. One simple type is an ionic electroactive polymer, a kind of artificial muscle. Place two electrodes on certain dielectric materials and turn on the voltage, and the electrodes will attract each other, deforming the polymer. These can be used as soft actuators for manipulating fragile objects, perhaps placing a stent or steering a catheter through a small space in the body. Or, they could form the basis of an artificial muscle in a robot or a prosthetic limb.

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But these polymers could be even smarter if they incorporated another capability, shape memory. Shape memory polymers can be deformed to take on a particular shape—say, a spiral—then return to their original shape—flat, for instance—when triggered by a stimulus such as a change in temperature.

Smoukov is working on developing ionic actuators that also have shape memory, to make muscles that move back and forth between two, or possibly even three, states. “We can actually design about any function or any combination of functions that you like,” he told a session of the Materials Reseach Society’s fall meeting in Boston last week.

Shape is programmed into a polymer by heating it, deforming it into a desired new shape, then letting it cool to lock the polymer molecules in place. It retains that shape until heated again to the right temperature to unlocked molecules, whereupon it reverts to its original shape. Smoukov and his colleagues used Nafion, a commercially available electroactive polymer that also has shape memory capabilities. Using platinum electrodes, they applied a voltage to stretch the polymer, then used the same electrodes to heat it to 60°C, then cooled it to imprint that amount of stretch. They could program in several different amounts of stretch by repeating the process multiple times, each time heating the material to a different temperature above 60°C, stretching it a different amount, then allowing it to cool.

How much they stretched the polymer determined the limit of how far it would move as an actuator. To change the movement, they simply had to change the temperature at which it was locked into place. For instance, heating a chunk of the programmed material to 70°C and applying a voltage caused it to bend to an angle of 17 degrees. Heating it to 90° caused it to shift to a different amount of stretch, and applying a voltage now made it bend to an angle of 25 degrees.

Smoukov says this is an early demonstration of a general way of controlling the shape and movement of a polymer, and he’s not sure what specific uses it might be put to. “I haven’t thought exactly of the application,” he says. But it doesn’t have to be used simply as an actuator. Because it can retain a physical memory of conditions it was exposed to, it might be used as a sensor for temperature or pH, for instance. “It’s quite completely general,” he says.

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