Quentin Cooper takes a look at the new materials that can mend themselves. Researchers are currently developing bacteria in concrete which, once awakened, excrete lime to fill any cracks. In South America you can choose a car paint that heals its own scratches. And there are even gold atoms which can migrate to mend tiny breaks in jet turbine blades. Engineers normally design things so the likelihood of breaking is minimised. But by embracing the inevitability of breakage, a new class of materials which can mend cracks and fissures before you can see them may extend the lives of our cars, engines, buildings and aeroplanes far beyond current capability.