Water shifts rubber's shape

Researchers from Nanyang Technological University in Singapore and the College of Transportation Engineering in China have developed a material that can be shaped, but changes back to a permanent shape when immersed in water.

A polyurethane shape memory polymer wire with a permanent straight shape can be heated and bent into a circle, for example, and will remain a circle as long as it remains dry. It will relax into its permanent straight shape within about an hour and 15 minutes, however, when immersed in room-temperature water.

The polyurethane shape memory polymer could eventually be used in medical devices, according to the researchers. They have made prototype retractable stents and self-unraveling knots from the material.

Key to the method is that hydrogen in the water weakens the polyurethane's chemical bonds so that the material relaxes into its permanent shape at a lower temperature than usual. The material's usual transition temperature is 36 degrees Celsius. The longer the material remains in water the lower the transition temperature.

The material is ready to develop into practical applications now, according to the researchers. The work appeared in the March 14, 2005 issue of Applied Physics Letters.