Heat-Sensitive Material Remembers Four Shapes

A substance called a polymer usually used to make fuel cells can "remember" up to four different shapes, and revert to each one at different temperatures, according to new research.

The polymer, or others like it, could be used in fields ranging from biomedical engineering to space exploration.

Perfluorosulphonic acid ionomer (PFSA) is what's known as a shape memory polymer. A polymer is a substance made mostly of repeating identical or similar molecules.

The chemical structure of shape memory polymers allows them to "memorize" shapes under certain conditions of heat, magnetism or moisture. The next time they're exposed to those conditions, the polymers revert back to the memorized shapes.

Shape-memory polymers have been commercially available for decades in the form of insulation for wiring and other industrial uses. Some have been tested for use in medical devices, such as stents that can shape-shift inside blood vessels to clear clots. NASA has considered shape-memory polymers as a way to fold up antennas for launch and then unfold them in space.

But until now, shape-memory polymers have only been able to hold two or three shapes total, and those shapes must be manipulated at certain temperatures.

A memory for shapes

But PFSA is different, said Tao Xie, a research scientist at General Motors and lead author of the new study.

It can hold up to four different shapes, including its original form. And as long as the gap between each temperature is large enough, each shape can be assigned to a temperature of the manufacturer's choice.

Xie coaxed the shape-memory effect out of PFSA by heating thin films of the material, reshaping it to "fix" a shape, then cooling and reshaping again. When heated back up, the material reverted to each fixed shape, recalling up to four shapes total. So far, Xie has been able to twirl, elongate, and shorten strips of PFSA simply by changing the temperature.

PFSA has properties similar to other known shape-memory polymers, Patrick Mather, a professor of biomedical and chemical engineering at Syracuse University who was not involved in the research.

However, PFSA might not qualify under a strict definition of shape-memory polymers because the effect depends heavily on cycles of periodic, rather than continuous, heating, said Mather.

No special chemistry required

From a scientific standpoint, PFSA is "not a unique chemistry at all," said Ken Gall, a materials scientist from Georgia Tech who was also uninvolved in Xie's work.

But on a practical level, the fact that PFSA's shape memory arises more from heating methods than from special chemistry could be important, he said, allowing manufacturers to use less expensive materials.

The study "shows you that the shape-memory property is a lot about the way you process and thermo-mechanically treat the material," Gall told TechNewsDaily.

The next step, Xie said, is to look for shape-shifting materials that also have other useful properties, like biocompatibility for medical devices or ultra-violet radiation resistance for space equipment.

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