Bonds make a sacrifice for tough bones

Most bumps and falls don’t break bones, but it’s not entirely clear why these structures are so resilient. Now, scientists have discovered molecular details that may help explain skeletal toughness.

Bones are composite materials made of both brittle hydroxyapatite crystals and a toughening network of fibers, composed mainly of the natural polymer known as collagen. The researchers suggest that bone’s resilience comes from bonds in or between collagen molecules that, in effect, sacrifice themselves for the overall good of the fibers. These bonds break easily in an impact and quickly dissipate energy before forces build up to break collagen molecules’ chainlike backbones.

Because of the sacrificial bonds, “the polymer network doesn’t fail,” says molecular biologist Daniel E. Morse.

In their experiments, he and his colleagues at the University of California, Santa Barbara (UCSB) repeatedly pulled on collagen molecules sitting on a glass slide. To do this, the team used the styluslike tip of a specially designed atomic-force microscope. In the process, the researchers measured both the force exerted by the tip and the extension of the collagen molecules.

The investigators then calculated how much energy the molecules can dissipate, they report in the Dec. 13 Nature.

The team also took measurements in a setting closer to collagen’s natural venue. The scientists used the microscope tip to pull on the surface of rat bones that had been polished in an effort to expose underlying collagen fibers. They also used the tip to make indentations in the surface of rat bones.

The energy dissipated during the pulling and indentation experiments indicated that chemical bonds had broken, says coauthor James B. Thompson, a UCSB physicist. When the researchers waited 100 seconds and pulled or pressed again, they found that most of these bonds had reformed.

The sacrificial bonds reform best when collagen or bone has been soaked in a solution containing certain multiply charged ions, such as calcium, which seem necessary for the reconnections to form, Thompson adds.

The Santa Barbara team still has to identify exactly what’s being stretched in the tests on rat bone, notes biologist John Currey of the University of York in England.

Thompson says that the team is now working to determine whether the tip is pulling on one or more molecules of collagen or on some other material.

In time, Currey predicts, the team’s work on the fundamental mechanism of resilience could prove useful in studies of how bones become brittle as they age.

Beyond contributing to the understanding of age- or disease-related changes in bone resilience, the work might also lead to better synthetic polymers for bone and dental repair, says Morse.