Ludwik LeiblerSnap 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.

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 Nature 1.

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.

via BBC