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December 17, 2001
Volume 79, Number 51
CENEAR 79 51 p. 13
ISSN 0009-2347
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AFM study suggests 'sacrificial' bonds form basis of bone's strength


What makes bone so tough? Researchers have long given thought to that question, and in recent times, scientists have succeeded in learning much about the microscopic way in which bone is assembled. But how bone manages to resist fracturing upon impact has more or less remained a mystery.

PUSH AND PULL Santa Barbara's Kindt uses an atomic force microscope to measure mechanical properties of bone
Physicists in California may have unraveled a piece of the puzzle--literally--by poking and prodding bones with an ultrafine probe. Force measurements suggest that weak chemical bonds that readily break and re-form may impart considerable toughness to bone.

The study was conducted by University of California, Santa Barbara, graduate students James B. Thompson and Johannes H. Kindt, physics professors Helen G. Hansma and Paul K. Hansma, and their coworkers.

Using an atomic force microscope (AFM) to measure the forces associated with pulling and stretching molecules of collagen, the principal organic component of bone, the Santa Barbara group finds that the protein contains "sacrificial bonds"--bonds that "allow" themselves to be broken, thereby dissipating energy before the pulling force snaps the collagen backbone. Both collagen molecules supported on a glass slide and those exposed on a bone surface show similar behavior, the group notes. The team also observes that the broken bonds heal (re-form), provided there is a sufficient delay between successive pulling events [Nature, 414, 773 (2001)].

The researchers acknowledge that they do not know the nature of the sacrificial bonds. However, based on the observation that the broken bonds heal readily in buffer solutions containing calcium or other doubly charged ions, but not singly charged ions, the group proposes that the expendable bonds may be ion bridges that form between carboxylate ions on pairs of collagen strands.

Making a connection directly to bone's toughness, the Santa Barbara team used a stiff AFM cantilever to form tiny indentations in bone samples and then studied the recovery process. Thompson tells C&EN that collagen and bone both require about 30 seconds to recover from the physical stresses, "suggesting that the mechanical properties of bone are partially dependent on the weak sacrificial bonds."

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