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Heat-Activated Plastic Muscles Can Lift 5000 Times Their Weight

The latest iteration stores almost six times more energy than previous versions.

It's not every day that you get to see robots move their arms by themselves. Now, researcher Zhenan Bao at Stanford University in California is reporting in ACS Central Science that her team has developed a shape memory polymer that allows robots to move their arms on their own when the polymers are heated.

The new polymers can lift objects 5000 times their own weight and store almost six times more energy than previous versions.

Shape memory polymers are materials that can be stretched or deformed only to be returned to their original shapes after heat is applied. These materials could have many applications in robotics but, until now, they haven’t been able to store enough energy to be useful.

The researchers created new more elastic polymers by incorporating a polypropylene glycol backbone with 4-,4’-methylene bisphenylurea units. 

When stretched, hydrogen bonds form between the chains of the new material allowing it to maintain its highly extended form. Then, when heated to 70°C, these bonds break and the material returns to its condensed form.

Experiments have proven that the new polymer can be stretched up to five times its initial length. It can also store up to 17.9 joules of energy per gram in its extended form, six times more energy than previous models.

This makes it very useful in a variety of applications. “It could be used to lift weight or to help to assist or provide an additional push to someone who maybe is having difficulty walking,” Bao told New Scientist.

The team further experimented with making an artificial muscle by attaching the pre-stretched polymer to the upper and lower arm of a wooden mannequin. They then heated the material so it can contract, causing the mannequin to bend its arm on its own.

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Bao and her team noted to SciTech Daily that the polymer is also inexpensive, costing a mere $11 per lb. 

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