Body temperature triggers polymer to change shape
09 February 2016
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.
Developed by the University of Rochester's Professor Mitch Anthamatten and graduate student Yuan Meng, the shape-memory polymer is capable of lifting an object one-thousand times its weight and might have a variety of applications, including sutures, artificial skin, body-heat assisted medical dispensers, and self-fitting apparel.
“Tuning the trigger temperature is only one part of the story,” says Anthamatten. “We also engineered these materials to store large amount of elastic energy, enabling them to perform more mechanical work during their shape recovery”
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, 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 tune the trigger temperature was achieved by including molecular linkers to connect the individual polymer strands. Anthamatten’s group discovered that linkers inhibit, but don’t stop, crystallisation 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 at which the shape change is triggered.
Heating the new polymer to temperatures near 35°C, just below the 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,” says Anthamatten. “But a simple touch causes it to recoil back to its original shape.”