AFM image of a nanoindentation in the outer layer of an adult bovine bone.

Our bones have to be able to withstand many types of impact. Daily activities such as walking, for instance, require a certain amount of elasticity, but the bones in our heels and back are often subject to sudden jolts that compact bone fibrils. Understanding more about the mechanisms that prevent our bones from fracturing under such compressive loads will help in the treatment of problems that result from old age, disease and injury. This is why Christine Ortiz and colleagues at the Massachusetts Institute of Technology are exploring the nanostructural origins of bone strength (Nano Lett. doi:10.1021/nl061877k; 2006).

The carbon-based mineralized platelets that coat the collagen fibrils in our bones are known to provide increased strength under tension (pulling). But, how do these minerals affect the elastic response of bone when the fibrils are squeezed together? Ortiz and co-workers wanted to check if the frictional interactions between these minerals helped bones to resist cracks and failure under compression. They combined nanoindentation — which involves pushing a sharp tip into a material — and atomic force microscopy to study how bone responds to compressive forces on sub-10-nm length scales (see image). Their results show that normal bone has a greater resistance to compressive stress than demineralised bone, and that cohesion and friction between the mineralized platelets help them to compress easily, rather than slip. Ortiz's findings are consistent with what is observed in nature: tendons, which respond to tension, contain no minerals, whereas whale bones, which must sustain large compression forces, are almost entirely made of minerals.