As mineral size decreases (left to right) to reach that found in tooth enamel (inset), the fracture resistance increases.
Gao/NAS
Bones, teeth, and nacre are all biocomposites. A protein matrix absorbs and distributes shocks, but its softness is buttressed by hard, embedded minerals. Somehow the resulting combination is a material that can be thousands of times less susceptible to fracture than the pure mineral.
Fracture of the pure mineral initiates at flaws. “It's inevitable that a mineral will be flawed,” says Gao. “No material is pure. Just from entropic effects you will have impurities.” This is especially true in biocomposites, where proteins will sneak into and disrupt a mineral matrix.
Gao calculated how the fracture strength of such a flawed mineral would vary with changing crystal thickness. After an insult, the amount of energy absorbed by a given crystal depends on the crystal's volume. Much of that energy will be concentrated at the most serious flaw, perhaps leading to its rupture and thus a catastrophic failure.
But at the nanoscale mineral sizes found in virtually all biocomposites, the story is different. “At nanoscale the volume is too small, so the strain energy stored in the mineral is too small to break the bond,” says Gao. “So the crack will not grow—these flaws will not matter.”
Gao says that this principle explains a case of convergent evolution: the similarly small sizes of mineral particles in a wide range of biocomposites. Nanoscale sizing can also be used in making artificial materials. Material processing, he says, “is often like cooking,” with lots of experimenting with ingredients backed up by little theory. So a principle from biology is welcome. He and his colleagues have used the nanoscale theory to study a composite coating—made of grains of titanium nitrate embedded in amorphous silicon nitrate—that is almost as hard as diamond. “That is a perfect application of this principle,” he says. ▪
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