A multiple-exposure image of a new shape-memory polymer reverting to its original shape after being exposed to body temperature. Photograph by J. Adam Fenster, courtesy of the University of Rochester.
A research team at the University of Rochester has created a material that undergoes a shape change that can be triggered by body heat. The material is a type of shape-memory polymer, which can be programmed to retain a temporary shape until it is triggered to return to its original shape. The material may be used for artificial skin, sutures, and body-heat assisted medical dispensers, among other uses.
“Our shape-memory polymer is like a rubber band that can lock itself into a new shape when stretched,” said Mitchell Anthamatten, PhD, a chemical engineering associate professor at the university who led the research. “But a simple touch causes it to recoil back to its original shape.”
The key to development of the new polymer was controlling the 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 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, permanent shape.
The ability to tune the trigger temperature was achieved by including molecular linkers to connect the individual polymer strands. The team discovered that linkers inhibit crystallization when the material is stretched, and by altering the number and types of linkers used, as well as how they are distributed throughout the polymer network, the 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 just below body temperature causes the crystallites to break apart and the material to revert to its permanent shape.
Having a polymer with a precisely tunable trigger temperature was only one objective. Anthamatten and his team also wanted the material to be able to deliver a mechanical work as the shape transforms back to its permanent shape. Consequently, they set out to optimize their polymer networks to store as much elastic energy as possible. Anthamatten’s shape-memory polymer is capable of lifting an object 1,000 times its weight. For example, a polymer the size of a shoelace, which weighs about a gram, could lift a liter of soda.
Editor’s note: This story was adapted from materials provided by the University of Rochester.