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Materials that heal themselves

Overhead view of a material after impact damage. The impact event ejected material from the center of the damage region and created radial cracks around the damage volume. The restorative material is delivered through two isolated fluid streams (dyed red and blue) that infiltrate the cracks and wet the inner surface of the damage. The liquid gels and creates a scaffold for the delivery of new restorative materials and eventual recovery of the entire damage region. This image is halfway through the restoration process.
[Credit: Ryan Gergely, University of Illinois]

Imagine a material that can repair itself after a bullet passes through it. That's what Scott White and colleagues have designed. Until now, polymer materials, or materials made of large molecules that are, in turn, made up of small, repeating subunits, have only been engineered to repair very small defects. But these researchers have improved the self-healing properties of polymer materials to the point that they can now automatically patch holes in themselves that are 3 centimeters in diameter.

The challenge was for White and his colleagues to find a way to deliver liquids to damaged sites in a material and have them stick around long enough for chemical reactions to occur and harden them in place. So they designed a network of channels filled with special liquids that run through their polymer materials like veins. That way, when the material is damaged, a mixture of those liquids quickly forms a gel on the spot.

The researchers used this method to fill holes in a polymer material that were more than 35 millimeters in diameter in just 20 minutes. The material also recovered about 62% of its original strength, they say. This advance paves the way for other polymer materials that are capable of healing themselves after catastrophic damage from ballistic impacts, like those from bullets, bombs or rockets.