Mineralization generates megapascal contractile stresses in collagen fibrils

During bone formation, collagen fibrils mineralize with carbonated hydroxyapatite, leading to a hybrid material with excellent properties. Other minerals are also known to nucleate within collagen in vitro. For a series of strontium- and calcium-based minerals, we observed that their precipitation leads to a contraction of collagen fibrils, reaching stresses as large as several megapascals. The magnitude of the stress depends on the type and amount of mineral. Using in-operando synchrotron x-ray scattering, we analyzed the kinetics of mineral deposition. Whereas no contraction occurs when the mineral deposits outside fibrils only, intrafibrillar mineralization generates fibril contraction. This chemomechanical effect occurs with collagen fully immersed in water and generates a mineral-collagen composite with tensile fibers, reminiscent of the principle of reinforced concrete. Description Key aspects of bone mineralization Bone is a hierarchical material consisting of a organic fibers, mainly in the form of collagen, that are mineralized with inorganic crystals, primarily hydroxyapatite. It is this structure that gives bone its remarkable combination of strength and toughness. Ping et al. examined the deposition of minerals on both the outside and inside of the fibers over time (see the Perspective by Nudelman and Kröger). They found that large contractile forces occur within the collagen during intrafibrillar mineralization regardless of the mineral type, thus giving bone its unusual combination of mechanical properties. This feature is analogous to the reinforcement of concrete using prestressed steel rods. —MSL In bone, the precipitation of various minerals inside collagen causes contraction of the fibrils, thus reinforcing them.