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HOW does bone manage to be so tough? It seems that bone contains 鈥渟acrificial
bonds鈥� that are designed to break under stress. Paradoxically, rupturing these
chemical bonds makes bone more durable.

Bone has to satisfy two nearly contradictory requirements鈥攊t must be
stiff without being brittle. To avoid splintering, a stressed bone needs to
dissipate energy, like a shock absorber. And nobody is sure how rigid bone
manages to absorb the energy.

James Thompson and his team from the University of California, Santa Barbara,
think the energy gets used up ripping apart tiny Velcro-like connections between
tangled collagen molecules in the bone. When these sacrificial bonds break they
soak up the energy that might otherwise crack the bone.

To test their idea, the researchers used a delicate device called an atomic
force microscope to repeatedly stretch individual collagen fibres attached to a
glass slide, as well as similar ones sticking out of a sample of rat bone. The
fibres soaked up more energy in each cycle of stretch and relaxation if there
was more time between consecutive pulls. This suggests that when the fibres are
stretched, bonds between the coiled collagen molecules break until the molecules
are more or less straight. As the fibre contracts again, the bonds need time to
reform. If the bonds were simply stretching rather than breaking the fibres
should soak up the same amount of energy no matter how long the interval between
pulls.

The idea of sacrificial bonds is plausible, says John Currey, a biologist at
the University of York, but he鈥檚 not convinced yet. He says it鈥檚 a nice result,
but points out that the Santa Barbara researchers don鈥檛 know for certain that
the fibres sticking out of the bone are in fact collagen. 鈥淭hey鈥檝e got to find
out what it is they鈥檙e pulling on,鈥� he says, 鈥渁nd that may be difficult.鈥�