Some of Mother Nature's inventions continue to baffle scientists. Take bone, for instance. Although researchers have a handle on what makes the substance so stifftiny mineral crystals imbedded in its organic materialthey are still unsure of what imparts its ability to absorb impacts and resist being weakened by small scratches and holes. Now a report published today in Nature suggests that some of bone's molecular bonds sacrifice themselves to absorb stress.

James B. Thompson and colleagues at the University of California, Santa Barbara, studied both collagen polymers, the main organic constituent of bone, and rat femurs. Using an atomic force microscope, they stretched the collagen molecules and found so-called sacrificial bonds that can break and reform, which means the collagen keeps extending and absorbing energy without rupturing the backbone of the structure. Tests on the surface of polished bone fragments yielded similar results, prompting the authors to conclude that "the sacrificial bonds found within or between collagen molecules may be partially responsible for the toughness of bone."

The findings, though potentially important for understanding the behavior of bone, are still very preliminary cautions John Currey of the University of York in an accompanying commentary. Scientists will no doubt keep investigating collagen's strengths though because, as the authors point out, it "is the most abundant protein in the human body and serves as a structural component of a variety of tissues including bone, tendon and skin.