Bone has finally given up one of its deepest secrets. Electron microscopy has revealed precisely how bone mineral organizes at the nanoscale, a discovery that could help scientists better understand bone disorders and develop artificial bone implants (Science 2018, DOI: 10.1126/science.aao2189).
As a biocomposite of organic and inorganic materials, bone combines stiffness and toughness, offering robust support for the body while deforming enough to take an impact without fracturing. The interplay between two of bone’s key components—collagen and carbonate-substituted hydroxyapatite mineral—is fundamental to these properties, says Roland Kröger at the University of York, who along with Molly M. Stevens at Imperial College London led the team behind the research. “Bone somehow manages to put [stiffness and toughness] together, when they’re usually mutually exclusive,” Kröger says.
Although the organic structures in bone are well understood, the mineral components have proved more perplexing. Bone mineral contains many crystalline and amorphous phases, which have foiled attempts to pin down its nanostructure unambiguously. “Bone is probably the most studied tissue in the body, yet we still don’t understand how it’s put together,” says Melinda Duer, who studies the chemistry of bone and tissue at the University of Cambridge. “The structure is the link between the biology and the chemistry, and that’s been missing.”
To solve that structural mystery, Kröger and Stevens’s team prepared 100-nm-thick slivers of femur bone using focused ion beam milling, which can carve high-quality samples with nanometer precision, avoiding preparation problems that have stymied previous studies. Electron microscopy highlighted three distinct patterns within the bone mineral: slightly curved filaments, lacy strands of mineral surrounding voids, and rosettes with a left-handed twist.
Using state-of-the-art scanning transmission electron microscopy, the team then produced a series of images of the bone samples from different angles, which they combined to create a three-dimensional view with nanometer resolution. These 3-D images revealed that the three patterns were simply different projections of the same structure. Needle-shaped nanocrystals, roughly 50–100 nm long, curve gently around collagen fibrils and pack together into platelets that are slightly twisted like a fan blade. These platelets form stacks that aggregate and ultimately span the gaps between adjacent collagen fibrils, producing an interlinked network. “It’s a beautiful piece of work,” Duer says. “The detail they have been able to extract is transformative.”
The helical shapes seen in the nanocrystals and platelets are a common theme in bone structure. Most bones have a visible curvature, or twisted grooves running along their shaft. At the microscale, tightly packed mineralized collagen fibrils form helical structures. And each collagen fibril is built from triple helices of collagen molecules. The new experiments show that bone mineral relies on the same motif. “We see the helical organizational pattern all the way from the macroscopic scale to the nanoscale,” Kröger says.
The team’s electron microscopy techniques could now be used to study how bone changes in diseases such as osteoporosis and osteoarthritis, Duer says. “There are a lot of chemists out there trying to understand bone mineral formation, but we haven’t had enough evidence,” she says. “This removes a lot of blocks.”
Understanding the nanostructure of bone could also prompt a more general approach to building nanocomposites, Kröger adds, particularly among researchers trying to develop artificial bone implants to replace diseased or fractured bone. “You want a material that is as close as possible to real bone,” he says. “Our work could guide that kind of research.”