Scientists and engineers are eager to understand the secret behind bone's lightweight toughness so they can mimic it in the design of new materials, but previous studies have revealed a number of different strength mechanisms at different scales of focus, rather than a single theory.
New research from MIT appearing in a recent issue of Nanotechnology reveals for the first time the role of bone's atomistic structure in a toughening mechanism that incorporates several previously proposed theories. This mechanism allows for the sacrifice of a small piece of the bone in order to save the whole, helps explain why bone tolerates small cracks, and seems to be adapted specifically to accommodate bone's need for continuous rebuilding from the inside out.
“The newly discovered molecular mechanism unifies controversial attempts of explaining sources of the toughness of bone, because it confirms that two of the earlier explanations play key roles at the atomistic scale,” said the study's author, Esther and Harold E. Edgerton Professor Markus Buehler of MIT's Department of Civil and Environmental Engineering. “It's quite possible that each scale of bone—from the molecular on up—has its own toughening mechanism. This hierarchical distribution may be critical to explaining the intriguing properties of bone. This knowledge may lay the foundation for new materials design.”
Unlike synthetic building materials, which tend to be homogenous throughout, bone is heterogeneous living tissue whose cells undergo constant change. Scientists have classified bone's basic structure into a hierarchy of seven levels of increasing scale.
Buehler scaled down his model to the atomistic level, to see how the molecules fit together—and equally important for materials scientists and engineers—how and when they break apart. More precisely, in order to find the mechanism behind bone's strength, which is considerable for such a lightweight, porous material, he looked at how the chemical bonds within and between molecules respond to force.
He found that the mineralized collagen fibrils in level 2 bone are made up of strings of alternating collagen molecules and consistently sized hydroxyapatite crystals. These strings are “stacked” together in a staggered fashion such that the crystals appear in stair-step configurations. Weak bonds form between the crystals and molecules both in the strings and between the strings.
When pressure is applied to the fabric-like fibrils, some of the weak bonds between the collagen molecules and crystals break, creating small gaps or stretched areas in the fibrils. This stretching spreads the pressure over a broader area and protects other, stronger bonds within the collagen molecule itself, which might break outright if all the pressure were focused on them. The stretching also lets the tiny crystals shift position in response to the force, rather than shatter, which would be the likely response of a larger crystal.