Wooden robot arm is powered by plastic muscles

The polymer can change its shape by heating and lift objects up to 5000 times their own weight. This could be used in robotics.
Shape-memory plastics can flip between their normal state where molecules are rigid and disordered and their deformed state where they bind to each other after being stretched. The polymer can be unstratified once it is in its stretched or deformed state and returned to its normal state by heating or using light.

Traditional shape-memory plastics don't store much energy when they are stretched. This means that they don't release as much energy while unstratched, which makes them less useful for tasks such as lifting or moving objects.

Zhenan Bao from Stanford University, California and her collaborators have created a shape-memory plastic that can store and release significant amounts of energy. They used a polypropylene glycol backbone and added 4-,4-methylene bisphenylurea.


The polymer chains are physically aligned when stretched. Intermolecular and intramolecular hydrogen bonds form between them. These bonds help maintain the material's highly stretched state. When heated to 70C, hydrogen bonds are broken and the material can be reverted to its original untightened form. This releases energy from the bonds.

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The polymer can be stretched up to five times its original length, and it can store up to 17.9 joules per gram in its extended version six times more than other shape-memory plasticmers. Bao says it could be used to lift weight, or to provide extra push for someone with difficulty walking.

The team created an artificial muscle using the material between the upper and lower arms of a wooden mannequin to demonstrate its potential. The polymer was stretched when the elbow joint of the mannequin was straightened. By heating the polymer, it contract again, causing the elbow joint of the mannequins to bend.

An artificial muscle made of a stretched shape-memory polymer contracts upon heating, bending a mannequins arm Adapted from ACS Central Science 2021, DOI: 10.1021/acscentsci.1c00829

Jonathan Aitken, University of Sheffield, UK, said that the reliable properties and strength of the material allow for new methods of designing joints that can lift multiple times the material's weight. Aitken, however, questions whether heat should be applied to the polymer.

Bao plans to study the properties of polymers to determine if there is an automatic elongation and shrinkage in response to temperature changes that are smaller and easier to attain.

Journal reference: ACS Central Science, DOI: 10.1021/acscentsci.1c00829