SADLY for engineers, inanimate objects cannot yet repair themselves. But work by Claudia dos Santos at the Fraunhofer Institute for Manufacturing Engineering and Automation, and Christian Mayer at Duisburg-Essen University in Stuttgart, has brought the day when they will be able to do so a little nearer. They and their colleagues have invented a way for damaged metals to heal themselves.

The surfaces of many metal objects are coated with other metals for protection. Iron, for instance, is frequently galvanised with zinc. The basic idea of the new technology is to infiltrate this coating with tiny, fluid-filled capsules. When the metal coating is punctured or scratched, the capsules in the damaged area burst and ooze restorative liquids, in the form of compounds called trivalent chromates. These react with nearby metal atoms and form tough, protective films a few molecules thick to ameliorate the damage.

The idea of doing this has been around for years, but it has proved difficult in practice because the capsules used were too big. Surface coatings tend to be about 20 microns (millionths of a metre) thick. The capsules were 10-15 microns across—large enough to disrupt the coatings, and thus do more harm than good. The trick worked out by Dr dos Santos and Dr Mayer is how to create capsules a few hundredths of this size.

The capsules the researchers have come up with are made by mixing butylcyanoacrylate, a chemical found in superglue, with an oil carrying the healing compounds. This mixture is then mixed with dilute hydrochloric acid. The result is an emulsion of droplets between 100 and 300 nanometres (billionths of a metre) across. Each droplet has an oil core surrounded by a thin layer of butylcyanoacrylate. To make the droplets stable, phosphate is added to the emulsion. This triggers the polymerisation of the butylcyanoacrylate into a tough plastic, which forms the outside of the capsule.

The greatest challenge for the team, however, was not making the capsules in the first place, but stabilising them during the plating process. Though galvanisation is often done by dipping steel in liquid zinc, it is sometimes done by electrolysis—nickel and copper plating are normally done this way. The capsules, though, tend to stick together in the liquids used as electrolytes during electroplating, and are also destroyed by the extreme acidity or alkalinity that is often involved in the process. To overcome these problems, Dr dos Santos and Dr Mayer used special detergents that stick to the polymerised butylcyanoacrylate shell around each capsule, which stops them sticking together and protects them from the electrolytes.

The researchers have now proved their techniques in electroplated layers of copper, nickel and zinc, and believe that self-repairing metals should commonly be available in the years ahead. Moreover, their nanocapsules may have other applications. Lubricants such as silicone oils can be included in them, to make the damaged surfaces of ball-bearings that have run out of oil more slippery, so that they are not scratched too rapidly. Anti-fouling compounds can be placed in capsules on the surfaces of metals intended for use in marine environments. And, in a nod to butylcyanoacrylate's origins in superglue, capsules containing chemicals that will react to form adhesives when two surfaces are put together are also on the horizon.