BREAKING BONDS: In unstressed bone, two interacting proteins—osteocalcin and osteopontin—connect clumps of mineral that are in turn fused to collagen fibrils (1). In a bone sample subjected to bending, scores of 100-nanometer-wide elliptical voids appear in the region where tension occurs. The researchers propose that dilatational bands form when the two proteins are pulled apart (2). If the force subsides, the bonds can reform, the nanoscale voids are repaired, and no further damage is done. But if the force continues, bonds between the two proteins break and collagen fibrils are sheared, leading to submicroscopic cracks; a collection of which makes up what is referred to as diffuse damage (3). LUCY READING-IKKANDA
The paper
A.A. Poundarik et al., “Dilatational band formation in bone,” PNAS, 109:19178-83, 2012.
Bone is tough, thanks largely to its complex hierarchical structure. At multiple levels, its constituent materials are arranged in patterns that resist crack propagation. Such mechanisms are well studied at the micrometer scale, but little is known at the nanometer scale about how cracks start, and how further damage is limited.
Deepak Vashishth of Rensselaer Polytechnic Institute in Troy, New York, and colleagues approached the problem by bending small sections of human tibia to its uppermost physiological limits and then using scanning electron and atomic force microscopy to look at the resulting damage. After staining to locate areas of diffuse damage at the microscale, then zooming in to the nanoscale, the ...
