Published online 20 February 2008 | Nature | doi:10.1038/news.2008.611

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Broken bits of rubber can stick themselves back together again.

Ludwik Leibler and a bit of self-healing rubber. NATURE

Snap this stretchy piece of rubber in two, and it will heal itself back into a stretchy piece of rubber again. The material's eerie ability to 'heal' itself means that it can broken, stuck and stretched time and time again.

The self-healing rubber was made in the lab of Ludwik Leibler, at the Industrial Physics and Chemistry Higher Educational Institution in Paris. Leibler and his colleagues built up their rubber from simple starting materials — fatty acids and urea (that's vegetable oil and a component of urine).

The resulting material is a cross between silly putty and a rubber ball — it can stretch, but when snapped in two it can be stuck back together again. The team hasn't yet tried to mold the stuff into a ball, so they don't know for sure how bouncy it is. But, Leibler adds, “knowing what I know about it, it will bounce.” Their research is published in Nature1.

The material is more than just a curiosity, says Takuzo Aida, a chemist at Tokyo University. “The discovery is very close to [being used in] business,” he says. Self-healing rubber could have applications in anything from adhesives to bicycle tyres.

Stretchy stuff

Conventional rubber is made of a single, continuous, stretchy molecule, held together with strong chemical links called covalent bonds. Once these bonds are cut by a break in the material, that’s it — the rubber can’t be reassembled.

Leibler's approach was to use small molecular groups instead: the fatty acids from vegetable oil. Reacting these molecules with urea in a two-step process stuck nitrogen-containing chemical groups (amides and imidazolidones) onto the ends of the fatty acids. The fatty acids link to each other using hydrogen bonds — a strong attractive force between hydrogen and another atom, and the bond responsible for holding water molecules to each other.

The resulting molecular system is very non-uniform: some acids have three protruding groups and some have two. This means that the compound can't crystallize into a hard, shatterable material. Instead it can be stretched to five times its original size and then return to normal, albeit slower than an elastic band would — it takes around a minute.

Sticky stuff

Press two snapped bits of rubber together... NATURE

If rubber is cut, the end groups on the acids become exposed, and the hydrogen bonds to neighbouring groups are broken. It is in the amide group's nature to seek out a partner to link up to, and this happens when the cut surfaces are brought back in contact — the hydrogen bonds can form again. The longer the cut ends are held together, the more of these partnerships are made, and the more completely healed the rubber is.

...and after a few hours you can stretch it again. NATURE

A freshly cut sample can heal enough that the re-stuck rubber can be stretched to twice its size again after just 15 minutes. The system isn't perfect yet — healed rubber that hasn't been left long enough will break again at the original 'wound' site.

The broken rubbers will stick only to the other broken piece, and the rubber doesn’t have to be mended immediately — it can still efficiently reform up to 18 hours after being severed.

Simple stuff

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Other self-healing materials exist, but their bonds knit back together only when heated or a strong force is applied. Leibler’s material works at room temperature, and just requires the two broken bits to be held together gently.

"It is a very simplistic concept, and using renewable resources is really nice," says Oren Scherman, a chemist at the University of Cambridge who will be running a session on self-healing materials at the upcoming American Chemical Society meeting in New Orleans. "It's a good indication that you can rip things apart and put them together."

Leibler has a deal with French chemical company Arkema to develop and commercialize the material. Leibler would like to see the rubber used in toys: "children like to break things, if you could heal them it would be very nice."

Watch a video of the rubber.