MIT probes secret of bone's strength

New research at MIT has revealed for the first time the role of bone's atomistic structure in a toughening mechanism that incorporates two theories previously proposed by researchers eager to understand the secret behind the material's lightweight strength.

Past experimental studies have revealed a number of different mechanisms at different scales of focus, rather than a single theory. The combination mechanism uncovered by the MIT researchers allows for the sacrifice of a small piece of the bone in order to save the whole, helps explain why bone tolerates small cracks, and seems to be adapted specifically to accommodate bone's need for continuous rebuilding from the inside out.

"The newly discovered molecular mechanism unifies controversial attempts of explaining sources of the toughness of bone, because it illustrates that two of the earlier explanations play key roles at the atomistic scale," said the study's author, Esther and Harold E. Edgerton Professor Markus Buehler of MIT's Department of Civil and Environmental Engineering.

"It's quite possible that each scale of bone--from the molecular on up--has its own toughening mechanism," said Buehler. "This hierarchical distribution of toughening may be critical to explaining the intriguing properties of bone and laying the foundation for new materials design that includes the nanostructure as a specific design variable."

Unlike synthetic building materials, which tend to be homogenous throughout, bone is heterogeneous living tissue whose cells undergo constant change. Scientists have classified bone's basic structure into a hierarchy of seven levels of increasing scale. Level 1 bone consists of bone's two primary components: chalk-like hydroxyapatite and collagen fibrils, which are strands of tough, chewy proteins. Level 2 bone comprises a merging of these two into mineralized collagen fibrils that are much stronger than the collagen fibrils alone. The hierarchical structure continues in this way through increasingly larger combinations of the two basic materials until reaching level 7, or whole bone.

Buehler scaled down his model to the atomistic level, to see how the molecules fit together--and equally important for materials scientists and engineers--how and when they break apart. More precisely, he looked at how the chemical bonds within and between molecules respond to force. Last year, he analyzed for the first time the characteristic staggered molecular structure of collagen fibrils, the precursor to level 1 bone.

In his newer research, he studied the molecular structure of the mineralized collagen fibrils that make up level 2 bone, hoping to find the mechanism behind bone's strength, which is considerable for such a lightweight, porous material.

At the molecular level, the mineralized collagen fibrils are made up of strings of alternating collagen molecules and consistently sized hydroxyapatite crystals. These strings are "stacked" together in a staggered fashion such that the crystals appear in stair-step configurations. Weak bonds form between the crystals and molecules in the strings and between the strings.

When pressure is applied to the fabric-like fibrils, some of the weak bonds between the collagen molecules and crystals break, creating small gaps or stretched areas in the fibrils. This stretching spreads the pressure over a broader area, and in effect, protects other, stronger bonds within the collagen molecule itself, which might break outright if all the pressure were focused on them. The stretching also lets the tiny crystals shift position in response to the force, rather than shatter, which would be the likely response of a larger crystal.

Previously, some researchers suggested that the fundamental key to bone's toughness is the "molecular slip" mechanism that allows weak bonds to break and "stretch" the fabric without destroying it. Others have cited the characteristic length of bone's hydroxyapatite crystals (a few nanometers) as an explanation for bone's toughness; the crystals are too small to break easily.

At the atomistic scale, Buehler sees the interplay of both these mechanisms. This suggests that competing explanations may be correct; bone relies on different toughening mechanisms at different scales.

Buehler also discovered something very notable about bone's ability to tolerate gaps in the stretched fibril fabric. These gaps are of the same magnitude--several hundred micrometers--as the basic multicellular units or BMUs associated with bone's remodeling. BMUs are a combination of cells that work together like a small boring tool that eats away old bone at one end and replaces it at the other, forming small crack-like cavities in between as it works its way through the tissue.

Thus, the mechanism responsible for bone's strength at the molecular scale also explains how bone can remain so strong--even though it contains those many tiny cracks required for its renewal.

This could prove very useful information to civil engineers, who have always used materials like steel that gain strength through density. Nature, however, creates strength in bone by taking advantage of the gaps, which themselves are made possible by the material's hierarchical structure.

"Engineers typically over-dimension structures in order to make them robust. Nature creates robustness by hierarchical structures," said Buehler.

This work was funded by a National Science Foundation CAREER award and a grant from the Army Research Office.

Photo / Donna Coveney MIT Professor Markus Buehler has helped reveal why bones are so tough. The object on the screen is a triple helical tropocollagen molecule, a fundamental building block of bone. Next to the molecule are nanosized hydroxyapatite chalk-like crystals. In his work he simulates the behavior of the composite of tropocollagen and hydroxyapatite during deformation. Enlarge image CONTACT Elizabeth A. Thomson MIT News Office Phone: 617-258-5402 E-mail: thomson@mit.edu RELATED Model helps students visualize nanoscale problems - An educational experiment during IAP demonstrated that students can learn to apply sophisticated atomistic modeling techniques to traditional materials research in just a few classes, an advance that could dramatically change the way civil engineers learn to model the mechanical properties of materials. 4/2/2007 Markus Buehler - MIT Department of Civil and Environmental Engineering More: Biology More: Civil engineering More: Materials science

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Virtual schooling growing at K-12 level

24 hours news Students get their lessons online and communicate with their teachers and each other through chat rooms, e-mail, telephone and instant messaging

As a seventh-grader, Kelsey-Anne Hizer was getting mostly D's and F's and felt the teachers at her Ocala middle school were not giving her the help she needed. But after switching to a virtual school for eighth grade, Kelsey-Anne is receiving more individual attention and making A's and B's. She's also enthusiastic about learning, even though she has never been in the same room as her teachers.

Kelsey-Anne became part of a growing national trend when she transferred to Orlando-based Florida Virtual School. Students get their lessons online and communicate with their teachers and each other through chat rooms, e-mail, telephone and instant messaging.

"It's more one-on-one than regular school," Kelsey-Anne said. "It's more they're there; they're listening."

Virtual learning is becoming ubiquitous at colleges and universities but remains in its infancy at the elementary and secondary level, where skeptics have questioned its cost and effect on children's socialization.

However, virtual schools are growing fast - at an annual rate of about 25 percent. There are 25 statewide or state-led programs and more than 170 virtual charter schools across the nation, according to the North American Council for Online Learning.

Estimates of elementary and secondary students taking virtual classes range from 500,000 to 1 million nationally compared to total public school enrollment of about 50 million.

Online learning is used as an alternative for summer school and for students who need remedial help, are disabled, being home schooled or suspended for behavioral problems. It also can help avoid overcrowding in traditional classrooms and provide courses that local schools, often rural or inner-city, do not offer.

Advocates say those niche functions are fine, but that virtual learning has almost unlimited potential. Many envision a blending of virtual and traditional learning.

"We hope that it becomes just another piece of our public schools' day rather than still this thing over here that we're all trying to figure out," said Julie Young, Florida Virtual's president and CEO.

Florida Virtual is one of the nation's oldest and largest online schools, with more than 55,000 students in Florida and around the world, most of them part-time. Its motto is "Any Time, Any Place, Any Path, Any Pace."

Struggling students such as Kelsey-Anne, who suffers from attention deficit disorder, can take more time to finish courses while those who are gifted can go at a faster speed.

Casey Hutcheson, 17, finished English and geometry online in the time it would have taken to complete just one of those courses at his regular high school in Tallahassee.

"I like working by myself because of no distractions, and I can go at my own pace rather than going at the teacher's pace," he said.

For all its potential, virtual schooling has its critics and skeptics.

"There is something to be said for having kids in a social situation learning how to interact in society," said state Rep. Shelley Vana. "I don't think you get that if you're at home."

But virtual students get a different kind of social experience that is just as valuable, said Susan Patrick, president and CEO of the North American Council for Online Learning in Vienna, Va.

"We should socialize them for the world that they live in," she said, suggesting that people spend much of their time interacting via computer these days.

Many policymakers approach virtual learning with dollar signs in their eyes, expecting big savings from schools that do not need buildings, buses and other traditional infrastructure.

"We should not, as stewards of public money, be automatically paying the same or even close to the same amount of money for a virtual school day as we pay for a conventional school day," said Florida Senate Education Committee Chairman Don Gaetz.

Florida Virtual this year is slated to get $6,682 for every full-time equivalent student, just slightly less than the average of $7,306 for all of the state's public schools. Young said her school has expenses that traditional schools do not.

"Our data infrastructure is our building," she said.

Teacher unions have opposed spending public dollars on some virtual schools, mainly those that are privately operated or function as charter schools.

Indiana lawmakers this year refused to fund virtual charter schools. Opponents argued they are unproven and would have siphoned millions of dollars from traditional public schools.

Florida Virtual's Young said she plans to recommend that her state follow the example of Michigan, which passed a requirement that students complete some type of online experience to earn a high school diploma.

If "we do not give them an opportunity to take an online course, we're doing them a tremendous disservice," she said. "It's become the way of the world."

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