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Stretchable polymer reverts back to original shape when triggered by body heat

Shape memory materials that can revert back into a desired form after being bent, twisted and stretched are finding their way into a number of applications, ranging fromsports bras to more efficient refrigerators. One team of scientists is now examining potential biomedical applications, with a polymer that can revert to its original form when it comes into contact with heat from the human body.


Heat-activated shape memory materials have shown promise in things like cars andbuilding materials, but this has typically involved high temperatures and therefore external mechanisms to activate the conversion. If the trigger point can be lowered to a point where practical materials change shape when making contact with skin, it could mean big things for the medical world.
To this end, researchers at the University of Rochester have developed a polymer elastomer that can be stretched out and revert to its original shape when heated to around 35° C (95° F), which is just below body temperature. Achieving this required the team to manipulate the crystallization that takes place as the polymer is deformed.
When the material is stretched, so too are the polymer chains that form it. This causes small segments of polymer to crystallize, which in turn stabilizes the material and helps it retain its deformed shape. The team found that adding molecular linkers to individual polymer strands inhibits, but doesn't prevent, this crystallization effect . So by fine-tuning the makeup of the material, it was possible to determine its stability and the precise temperature at which the crystals would melt and return it to its normal state.
But the team was looking to do more than develop a material that responds to body heat. It focused equal attention on enabling the material to release a useful amount of energy as it goes through this process. In doing so, they optimized the polymer networks to enable the material to lift an object one thousand times its own weight.
"Nearly all applications of shape memory polymers will require that the material pushes or pulls on its surroundings," says lead research Professor Mitch Anthamatten. "However, researchers seldom measure the amount of mechanical work that shape-memory polymers are actually performing."
Anthamatten imagines that the heat-activated polymer might find its way into applications such as wound treatments, artificial skin, medical dispensers and self-fitting apparel. In testing, the researchers used the mechanical strength of the heat-activated polymer to tow a model truck, lift toy construction materials and crush a seed pod. 


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