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By taking bone analysis to a new level of precision, researchers at MIT have revealed an engineering marvel under our skin. Their work may help improve the resilience of man-made materials and enable earlier diagnosis of bone diseases.

Researchers led by Christine Ortiz, associate professor of materials science and engineering, examined the mechani­cal properties of collagen molecules–bone’s building blocks–at a scale of less than 50 nanometers. (To put that in perspective, a human hair has a diameter of about 80,000 nanometers.) Their close inspection showed that the heterogeneous composition of bone at the nanoscale makes it tougher overall.

The researchers used a molecular force probe to poke one-square-­millimeter samples of bovine shin bone hundreds of times in a grid pattern. “It basically tells you where bone is stiff and where it’s not,” says Ortiz. Using the probe’s measurements, they produced maps of the samples’ stiffness and found a dramatically varied terrain. “We weren’t expecting the structure to be so complex,” she says.

Materials science researchers have long known that bone’s structure is heterogeneous at the nanoscale because of variation in the size, shape, and spacing of its primary components, collagen molecules and mineral particles. But they disagreed on one important point, says Subra Suresh, dean of the School of Engineering at MIT and coauthor of the team’s research paper (published in May in Nature Materials).

“If you have a material that’s perfectly ordered and periodic, is that better than a material that has the same average properties, but has high values in some places and low values in others?” says Suresh. “There was no consensus on whether nonuniformity was better or worse.” He believes the MIT team’s research settled that debate. Using the bovine-bone data, the team created a computer model that can virtually bend the bone and predict its behavior. The simulations compared bone with other tough but more uniform materials and showed that it takes more energy to deform a highly heterogeneous material than a uniform one.

These results offer a promising blueprint for stronger composite materials in load-bearing structures. Imitating nature, engineers could enhance a material’s resilience by boosting its nanoscale variability. The project, funded by the MIT Institute for Soldier Nanotechnolo­gies, may help improve body armor and other combat equipment.

The research could also lead to earlier detection of osteoporosis and other bone diseases. “Now that we can measure bone at the nanoscale,” says Suresh, “it might be possible someday to remove a tiny biopsy and to tell if there’s an abnormality in that bone fragment.”

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Credit: Beryl Simon

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