A self-healing elastic band
It looks just like an elastic band or a very flexible piece of rubber. But if it breaks, you just have to press the two ends together for a few minutes for it to repair itself. No need for glue, just light pressure from your fingers at room temperature. In a few minutes, the elastic band is back in its original state. You can stretch it without it breaking again. And "this process of breaking and repair can be repeated over and over again", as the French researchers who published their discovery in the journal "Nature" last Thursday, 21 February, explain.
The invention was developed by the Soft materials and Chemistry laboratory, part of the French National Scientific Research Centre and the Advanced School of Industrial Physics and Chemistry (ESPCI) in Paris. The laboratory was set up by Pierre-Gilles de Gennes, who was awarded the Nobel Prize for Physics in 1991.
"An amazing feat", explains Professor Ludwik Leibler, the Polish scientist who heads the ESPCI laboratory, because the great innovation is in the chemical composition of the material. The ingredients of this magical elastic are simple: fatty acids, such as those found in vegetable oils, and urea, a compound produced from the breakdown of amino acids, found in urine, which can be synthesised.
Unlike rubber, which is made of long chains of large molecules connected to each other by strong bonds, the new elastic is made up of a group of small fatty acid molecules. These are the active element in the self-healing process. The network of molecules is strengthened by hydrogen bonds that allow the material to be stretched to several times its length and return to its original shape. Experiments have shown that healing can still take place even several hours after the material has been cut or broken.
The research in the Paris laboratory forms part of what is known as "supramolecular chemistry". Because they use natural, commonly available and renewable materials and were keen to apply their discoveries on a large scale, the laboratory researchers formed a partnership in 2000 with a private company, Arkema, which has taken over the chemicals business of the French oil and gas group Total. In 2004, Arkema began to develop industrial applications for this discovery, some of which are nearly ready and could be on the market within two years. Arkema envisages manufacturing "all kinds of articles which could be reused after they have split or broken, thanks to their self-healing properties". Examples include fabrics for clothing that can mend its own holes, shoe soles that will not lose their shape, self-repairing children’s toys and spare parts for engines that can repair themselves without a trip to the mechanic.
"If you drill through a seal in a wall, it will repair itself. This would work with all kinds of things involving compression, such as structural joints and coatings that are liable to scratching," explains the head of the laboratory, Ludwik Leibler. In reality the applications are almost infinite.
For now, Arkema says it has "two product families ready to launch on the market" in a year or two, giving it time to make a few improvements to the rubbers, which are currently still somewhat lacking in resistance.
The first family of products, according to Manuel Hidalgo, a researcher at Arkema, relates to bitumens which, like rubbers, use molecules of plant origin. And to give the bitumen a higher level of resistance than materials manufactured from hydrocarbons, "we combine molecules, from vegetable oils for example, to give them a solid form at ambient temperatures," explains Manuel Hidalgo. This product family also includes varnishes, adhesives and paints, which can be manufactured more economically because they can be produced at lower temperatures than when using hydrocarbon derivatives.
The second family concerns plastics that are also made from combinations of plant molecules offering better biodegradability, the advantage of which lies in their improved resistance to solvents. The icing on the cake, emphasise Justin Maynar and Tazuko Aida, from the University of Tokyo, in an article accompanying the one by the French researchers, is that the material is truly ecological. Indeed, it can be destroyed by heat, as it dissolves in water heated to 100°, and it is recyclable because it reconstitutes itself on cooling whilst retaining its properties.