A researcher at Massachusetts Institute of Technology has revealed the role of bone's smaller structures in making bones tough and lightweight. The body seems to sacrifice a small piece of bone to save the rest, which helps explain why bones tolerate small cracks and seem to be continuously rebuilding from the inside out.

“It's possible that each scale of bone — from molecular on up — has its own toughening mechanism,” says study author Markus Buehler of MIT's Department of Civil and Environmental Engineering. “This hierarchical distribution of toughening may be critical to explaining the intriguing properties of bone and laying the foundation for new material designs that include the nanostructure as a specific design variables.”

Unlike conventional building materials, which tend to be homogenous, bone is heterogeneous living tissue with cells constantly changing. Scientists classify bone's basic structure into a hierarchy of seven levels of increasing size. Level 1 bone consists of chalk-like hydroxyapatite and collagen fibrils, which are strands of tough proteins. Level 2 comprises a merging of these two into mineralized collagen fibrils that are much stronger than the collagen fibrils alone. The hierarchical structure continues in this way through increasingly larger combinations of the two basic materials until level 7, or whole bone.

At the molecular level, mineralized collagen fibrils are made up of strings of alternating collagen molecules and consistently sized hydroxyapatite crystals. These strings are “stacked” together but staggered so that crystals resemble stairs. Weak bonds form between the crystals and molecules in and between strings.

Pressing the fabric-like fibrils breaks some weak bonds between the collagen molecules and crystals, creating small gaps or stretched areas in the fibrils. Stretching spreads the pressure over a broader area, and in effect, protects other, stronger bonds within the collagen molecule, which might break outright if the pressure was focused on them. Stretching also lets the crystals move in response to the force, rather than shatter, which would be the likely response of a larger crystal.

Buehler also discovered something notable about bone's ability to tolerate gaps in the stretched fibril fabric. These gaps are of the same magnitude — several hundred micrometers — as the basic multicellular units associated with bone's remodeling. The units are a combination of cells that work together like a small boring worm that eats away old bone at one end and replaces it at the other, forming small crack-like cavities in between as it moves through the tissue.

Thus, the mechanism responsible for bone's strength at the molecular scale also explains how bone can remain strong, even though it contains the many tiny cracks required for its renewal. Bone creates strength by taking advantage of the gaps, which are made possible by the material's hierarchical structure.