Material scientists make bones crack up

A team of German and Austrian material scientists has discovered why bones fracture more easily in some directions than others.

Bone is a composite of a stretchy fibrous polymer (collagen) and hard ceramic nanoparticles (carbonated hydroxyaptite), making it both tough and stiff. ’In principle, it is quite difficult to combine stiffness and toughness,’ said lead research Peter Fratzl of the Max-Planck Institute of Colloids and Interfaces, Potsdam. ’Bone combines the two properties by a complex hierarchical structuring which uses practically all known structural "tricks" to make the progression of cracks more difficult.’

Despites this toughness, cracks propagate much faster through bone in some directions than others. To find out why, Fratzl and his team subjected a sample of human bone to a controlled crack extension experiment, which allowed them to determine the energy required to propagate a crack in different directions. They found that they needed to apply over 25 times more energy to propagate a crack running at right angles to the collagen fibres than one running parallel to the fibres. Furthermore, the parallel crack was straight and smooth, whereas the right angle crack was much more distorted.

This implies that bone actually changes from a brittle (fractures easily) to a quasiductile (bends before it fractures) material depending on the angle of the crack. Fratzl and his team found that this change occurs at an angle of around 50? between the crack and the collagen fibres.

This work could help in the design of novel composite materials that are both stiff and tough, says Fratzl. ’There is also an implication for understanding bone fragility in certain diseases, since a modification of the intricate structure of bone material will increase fragility,’ he told Chemistry World. Jon Evans