Body heat bends shape-memory polymer out of shape

Polymers that visibly change shape when exposed to temperature changes are nothing new. But a research team led by Mitch Anthamatten, professor of chemical engineering at the University of Rochester, has now created a shape-change material that can be triggered by body heat alone, opening the door for new medical and other applications.

Developed by Anthamatten and graduate student Yuan Meng, the new material is a type of shape-memory polymer. Such polymers can be programmed to retain a temporary shape until triggered – typically by heat – to return to their original shape.

"Tuning the trigger temperature is only one part of the story," said Anthamatten. "We also engineered these materials to store large amount of elastic energy, enabling them to perform more mechanical work during their shape recovery." The findings are published in the Journal of Polymer Science Part B: Polymer Physics.

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

The ability to control the trigger temperature for shape change was achieved by adding molecular linkers that connect the individual polymer strands. Anthamatten's group discovered that linkers inhibit – but don't stop – crystallization when the material is stretched. By altering the number and types of linkers used, as well as how they're distributed throughout the polymer network, the Rochester researchers were able to adjust the material's stability and precisely set the melting point that triggers the polymer to return to its initial shape.

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," said Anthamatten. "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. Of equal importance to Anthamatten and his team was that the material should deliver a great deal of mechanical work as it transforms back to its permanent shape. Consequently, 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," explained Anthamatten. "However, researchers seldom measure the amount of mechanical work that shape-memory polymers are actually performing."

Their new shape-memory polymer is capable of lifting an object 1000 times its own weight; for example, a polymer the size of a shoelace – which weighs about a gram – could lift a liter of soda. Anthamatten says this shape-memory polymer could have a variety of applications, helping to produce sutures, artificial skin, body-heat assisted medical dispensers and self-fitting apparel.

This story is adapted from material from the University of Rochester, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.