Material lifts 1,000x its mass

Material that changes shape triggered by body heat alone, opens the door for use in medical applications. And a research team led by Chemical Engineering Professor Mitch Anthamatten at the University of Rochester in Rochester, New York, has created a polymer that does just that.

Developed by Anthamatten and graduate student Yuan Meng, the shape-memory polymer can be programmed to retain a temporary shape until it is triggered – typically by heat – to return to its original shape.

“Tuning the trigger temperature is only one part of the story,” Anthamatten says. “We also engineered these materials to store a large amount of elastic energy, enabling them to perform more mechanical work during their shape recovery”

The key to developing the new polymer was figuring out how to control crystallization that occurs when the material is cooled or stretched. As the material is deformed, polymer chains are locally stretched, and small segments of the polymer align in the same direction in small areas – or domains – called crystallites. Those crystallites fix the material into a temporarily deformed shape, and as their numbers grow, the polymer shape becomes more stable, making it increasingly difficult for the material to revert back to its initial – permanent – shape.

Including molecular linkers to connect the individual polymer strands enables tuning of the trigger temperatures. Anthamatten’s group discovered that linkers inhibit – but don’t stop – crystallization when the material is stretched. By altering the number, types, and distribution of links throughout the polymer network, the researchers were able to adjust the material’s stability and precisely set the melting point that triggers shape change.

Heating the new polymer to temperatures near 35°C, just below body temperature, causes the crystallites to break apart and the material to revert to its permanent shape.

“Our shape-memory polymer is like a rubber band that can lock itself into a new shape when stretched,” Anthamatten states. “But a simple touch causes it to recoil back to its original shape.”

Having a polymer with a precisely tunable trigger temperature was only one objective. Anthamatten and his team wanted the material to be able to deliver a great deal of mechanical work as the shape transforms back to its permanent shape. So, they set out to optimize their polymer networks to store as much elastic energy as possible.

“Nearly all applications of shape memory polymers will require that the material pushes or pulls on its surroundings,” Anthamatten says. “However, researchers seldom measure the amount of mechanical work that shape-memory polymers are actually performing.”

Anthamatten’s shape-memory polymer is capable of lifting an object 1,000x its weight. For example, a polymer the size of a shoelace – weighing about 1g – could lift 1L of water.

Anthamatten says the shape-memory polymer could have a variety of applications, including sutures, artificial skin, body-heat assisted medical dispensers, and self-fitting apparel.

The findings were published in the Journal of Polymer Science Part B: Polymer Physics.

University of Rochester

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