Jul 3, 2007
Bone provides engineering paradigm
Hierarchical structures appear in abundance in the formation of nature's structural protein materials, such as spider silk, skin, hair and bone. Bone, for instance, is an extraordinary material that features seven hierarchies, from nanoscale to macroscale, creating a fascinating material that combines strength with robustness, while being lightweight, constantly repairing itself and adapting to environmental conditions. Breakthrough research at the Massachusetts Institute of Technology (MIT), reported in a recent issue of Nanotechnology, has shown that the characteristic nanostructure of bone is crucial in achieving these traits.
By a combination of theoretical analysis and atomistic simulation, a previously unknown toughening mechanism of bone that operates at the nanoscale – the level of individual collagen molecules and nanoplatelets of hydroxyapatite – was discovered. All bone starts to develop as a pure collagen phase. It was shown that the addition of a second, hydroxyapatite mineral phase creates the characteristic material properties of bone, that is, strong, stiff and yet very tough and robust against failure.
The MIT researcher has shown that the characteristic nanostructure is vital to bone's physiological role, since it ensures that bone can sustain cracks without harm, a paradigm in contrast to many engineering materials. Tolerance against cracks is crucial for the operation of bone molecular units – small elliptical inclusions inside bone that are responsible for bone remodelling. The characteristic nanostructure of bone allows the material to tolerate cracks of several hundred micrometer size, without causing any fracture. The intrinsic nanostructure of bone shields it from major damage due to large stresses. The way bone does this is intriguing: in case of emerging damage, bone fails locally, that is, a local piece of the material is sacrificed in order to protect the larger structure. In other words, a small failure is tolerated for the “greater good”.
The findings have wide-ranging implications for biomimetic material design. Operating at multiple, hierarchical scales enables one to reach physical realities that would not be accessible to single-scale design, a phenomenon found widely in biology. This new paradigm could be crucial for nanotechnology applications since it enables the linking of nanoscale and macroscale applications.
About the author
Markus J Buehler is the Esther and Harold Edgerton assistant professor of civil and environmental engineering at the Massachusetts Institute of Technology, where he also directs the Laboratory for Atomistic and Molecular Mechanics. His lab focuses on modelling of mechanical properties of hierarchical protein materials, with the aim to develop structure-function relationships and insight into their deformation and fracture behaviour.