A time-lapse photo
of a new shape-memory polymer reverting to its original shape
after being
exposed to body temperature. (University of Rochester photo / J. Adam Fenster)
(February 9, 2016) Material
Can Lift 1000 Times Its Mass
Polymers that visibly change shape when exposed to
temperature changes are nothing new. But a research team led by Chemical
Engineering Professor Mitch Anthamatten at the University of Rochester created
a material that undergoes a shape change that can be triggered by body heat
alone, opening the door for new medical and other applications.
The material developed by Anthamatten and graduate student
Yuan Meng is a type of shape-memory polymer, which can be programmed to retain
a temporary shape until it is triggered—typically by heat—to return to its
original shape.
The findings are
being published this week in the Journal of Polymer Science Part B: Polymer
Physics.
“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 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, 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.
“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.”
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—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 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.