Engineers have created a material that changes shape when exposed to body heat alone, but that's not the only thing that makes this new shape-memory polymer (SMP) special.
It also has ability to store large amount of elastic energy, enabling it to perform impressive mechanical work during its shape recovery.
Adjusting The SMP Trigger Temperature
Controlling the crystallization that occurs when the material is cooled or stretched was key for this research.
As the material deforms, polymer chains are locally stretched and small polymer segments align in the same direction in small areas called crystallites. These cause the material to become temporarily fixed in shape. Additional crystallites increases the polymer’s stability, making reversion to its original shape increasingly difficult.
Previous SMPs required relatively higher temperatures to change their shape—up to 100C.
In order to make the material responsive to mere body heat, researchers used molecular linkers to connect the individual polymer strands. These linkers inhibit the crystallization that results from stretching the material without stopping it entirely.
By altering the number, types and distribution of linkers, the researchers were able to adjust the material’s stability and precisely set the melting point which would trigger a shape change.
Heating the new polymer at 35C causes the crystallites to break apart, restoring its original shape.
A Polymer that Changes Shape and Lifts Heavy Weight
“Our shape-memory polymer is like a rubber band that can lock itself into a new shape when stretched,” said Mitch Anthamatten, chemical engineering professor at the University of Rochester. “But a simple touch causes it to recoil back to its original shape.”
Anthamatten and his colleagues also wanted the polymer to be capable of doing significant mechanical work as it changed shape. Consequently, they endeavored to optimize the polymer network 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," said Anthamatten. "However, researchers seldom measure the amount of mechanical work that shape-memory polymers are actually performing."
The new polymer is capable of lifting an object 1,000 times its weight.
Applications for Shape-Changing Polymer
Since the new SMP is triggered by body temperature, it offers a number of possible medical applications, including artificial skin, improved sutures and body-heat-assisted medical dispensers.
The low production costs of SMPs could also enable the new polymer to be used in self-repairing auto-parts, such as fenders or auto choke elements for engines.
The research findings are published in the Journal of Polymer Science Part B: Polymer Physics.