Scientists at Duke University, Durham, NC, report that microscopic stresses and tears in a new kind of man-made material could help the substance bulk up like an athlete building stronger muscles. They predict that the stress-induced strength from the new materials could be used to make better fluids or soft-structure substances like artificial heart valves.

Materials like this tend to wear out over time because they experience repeated mechanical forces during use. But, if a material had properties to slow down its destruction, it would greatly improve its lifecycle.

The researchers say that this is the first time scientists have used force-induced chemistry within a material to make it stronger in response to stress. The new man-made materials the Duke team is making have characteristics already in place so that when a stress triggers a bond to break, it breaks in a way that triggers a subsequent reaction forcing the busted atomic bonds to reform new ones.

The researchers first stressed one of the test materials by pulsing high-intensity sound waves through them. The sound waves create bubbles, which typically collapse and break the bonds of the molecules in the material. The forces breaking atoms in the new materials, however, triggered the formation of new bonds, which strengthened the liquid by transforming it into a soft gelatin-like consistency.

To test the strength-building ability of a putty-like material, the team used a twin-screw extruder, pulling the material through, and destroying some of its molecular bonds. They found that the material formed more new bonds than those destroyed, becoming more solid and stronger.

The one drawback to the new materials, they discovered, is that the forces used deformed the material's initial structure, so that while it is stronger, it is no longer the same shape.

Their work will continue to create synthetic materials that can repair themselves after stress while retaining the original shape, and they would also like to see the material respond quicker. Currently, it takes minutes to strengthen; they would like to see it take milliseconds.

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