Bone indentation recovery time correlates with bond reforming time

Despite centuries of work, dating back to Galileo(1), the molecular basis o f bone's toughness and strength remains largely a mystery. A great deal is known about bone microsctructure(2-5) and the microcracks(6,7) that are pre cursors to its fracture, but little is known about the basic mechanism for dissipating the energy of an impact to keep the bone from fracturing. Bone is a nanocomposite of hydroxyapatite crystals and an organic matrix. Becaus e rigid crystals such as the hydroxyapatite crystals cannot dissipate much energy, the organic matrix, which is mainly collagen, must be involved. A r eduction in the number of collagen cross links has been associated with red uced bone strength(8-10) and collagen is molecularly elongated ('pulled') w hen bovine tendon is strained(11). Using an atomic force microscope(12-16), a molecular mechanistic origin for the remarkable toughness of another bio composite material, abalone nacre, has been found(12). Here we report that bone, like abalone nacre, contains polymers with 'sacrircial bonds' that bo th protect the polymer backbone and dissipate energy. The time needed for t hese sacrificial bonds to reform after pulling correlates with the time nee ded for bone to recover its toughness as measured by atomic force microscop e indentation testing. We suggest that the sacrificial bonds found within o r between collagen molecules may be partially responsible for the toughness of bone.