CALL it the yoga polymer: Nafion, a material used in some fuel cells, has an unrivalled memory for contortions.
Tao Xie at General Motors in Warren, Michigan, has twisted and stretched a Nafion strip into three distinct shapes, and found that it will revert to each shape at the appropriate temperature.
Nafion becomes softer as it is heated. At 140 °C Xie stretched it into a particular shape, which was locked in the polymer's "memory" as it cooled to 107 °C and stiffened. Stretching and cooling it twice more allowed two other shapes to be memorised, so that when heated to the appropriate temperature the Nafion formed the corresponding shape (Nature, DOI: 10.1038/nature08863).
Previously the best shape-memory polymers were able to remember only two shapes.
By Ian Kempton
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