Our bones are light, tough, and fracture resistant. That’s because of the hierarchical way in which they are built. On the nanoscale, tiny collagen fibers have a laminatelike arrangement, with different layers of fibers oriented in different directions. On larger scales, bones have a latticelike structure and different patterns of voids that make them light and strong. These structures ensure that bone resists the propagation of cracks in any one direction. Metallurgists have now followed nature’s lead, creating steel with a similar bonelike nanostructure. In addition to harboring a layered structure that tends to keep cracks from spreading beyond the layers where they start, the material has different alloy components with different degrees of hardness. That way if a crack does begin to form, it has to follow a different path to propagate, which reduces the chance a small crack will grow. Also, some areas within the steel are more flexible than others, which can help absorb the energy of repeated stresses applied to the steel and even close up cracks after they occur. The upshot, the researchers report today in Science, is that the new steel (with its different components shown in red, green, and black dots in the image above) is far more resistant to cracks created by metal fatigue than conventional steels. That could allow engineers to use the material to build everything from bridges to spacecraft that are less susceptible to catastrophic failure, which can happen when a tiny crack becomes a full-blown fracture.