Published online 20 April 2011 | Nature | doi:10.1038/news.2011.251

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Ultraviolet radiation erases scratches in rubbery materials.

Plastic that can be healed in a flash. Gina Fiore for Adolphe Merkle Institute, Case Western Reserve University, US Army Research Laboratory

Cracked your mobile-phone display? In the future, the remedy might be as simple, and cheap, as shining some light on it.

That is according to Christoph Weder, a materials scientist at the University of Fribourg in Switzerland, and his colleagues, who have developed a type of rubbery plastic that 'self heals' when exposed to ultraviolet light. The material might one day be incorporated into varnish, paint or mobile-phone covers.

"Just try to imagine how the lifetime of many items could be improved if damages could be easily fixed," says Weder.

Self-healing materials are not totally new. Many rubbery plastics can be heated until they begin to melt, so that liquid plastic flows into the damaged area before hardening again. But this is a crude approach: if the heat and melting can't be focused or directed, the entire piece of plastic could be put out of action until the repair is complete. In some situations — say, if the plastic forms a structural element in a device — taking it out of commission might be impossible.

Weder and his colleagues use a much more delicate method. Having made a deep scratch in their plastic, they simply illuminate it with focused ultraviolet light. The light heats only the region around the scratch, and within a minute the damage has been erased. The findings are published today in Nature1.

Localized repair

The trick to the self-healing ability lies in the plastic's structure. Normal plastics consist of long, chain-like polymers bound together by strong covalent bonds. Weder's plastic is different: it is made up of shorter molecules with interspersed with zinc or lanthanum ions.

Where irradiated with ultraviolet light, the originally solid material is liquefied and can quickly fill up cracks. After the light is switched off, the material solidifies and the original properties are restored. courtesy Marc Pauchard for Adolphe Merkle Institute, Case Western Reserve University, US Army Research Laboratory

In solid form, these molecules are not covalently bonded, but instead make sticky 'metal coordinate' bonds, forming long chains linked by the metal ions. When ultraviolet light shines on the plastic, it is absorbed by the metal ions and converted to heat — enough to melt the local area and repair a fracture.

In tests, Weder's group made a scratch more than 200 micrometres deep in a 400-micrometre-thick piece of their plastic. After two 30-second exposures of ultraviolet light, the cut had healed.

Richard Wool, a materials engineer at the University of Delaware in Newark, says that one drawback with Weder's method is that it only works when the plastic is thin, so that the light can penetrate it. But he adds that in future, engineers might get around this problem by creating a composite of the plastic that contains a network of implanted fibre-optic cables to deliver light throughout.

"Overall, it's a clever, novel self-healing method which could find many applications in medical, electronic and composite applications," he says.

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Weder's plastic is just one of several self-healing materials in development. In 2008, for example, scientists at the Industrial Physics and Chemistry Higher Educational Institute in Paris reported2 a material that could self-heal when broken pieces were simply pressed back together. That material was far more rubbery than Weder's plastic, and so was more suited to flexible, non-structural applications, but it had the advantage that it could repair a total fracture, not just a scratch.

Weder is hesitant to face up to such competition. "Perhaps it is appropriate to compare our work with the cool concept cars that you see at auto shows … they show feasibility of attractive new features," he says. "Sometimes these make it into commercial products, sometimes not."