What Makes Bone So Strong?

Even this electron microscope image of hydroxyapatite crystals in bone doesn't reveal its amazing secret.
(Public domain image)

Bone is a truly incredible substance. It is as strong as steel but at as light as aluminum. Not only is it strong, but it is surprisingly flexible as well. As is the case with most things God made, human technology cannot come close to producing something with bone’s amazing properties. Consider, for example, the work of Antoni Tomsia at Lawrence Berkeley National Laboratory in California. He and his colleagues are trying to artificially produce something with the characteristics of bone, but they simply cannot come up with anything as elegant and sophisticated as bone. He says:

People want a strong, light, and porous material, which is almost a contradiction in terms, but nature does it…Bone is made from calcium phosphate and collagen, which are both extremely weak. But nature mixes them together at room temperature and without toxic chemical [sic] to create something that is very tough — this fascinates us.

What makes bone so special? The short answer is that we don’t really know. However, we are learning. For quite some time now we have known that bone is a mixture of many things, principal among them a protein called collagen and a calcium compound called hydroxyapatite. The collagen gives bone its flexibility, while the hydroxyapatite gives bone its strength.

However, the hydroxyapatite in bone is stronger than hydroxyapatite made in the lab. Why? It has to do with the size of the crystals. When hydroxyapatite is made artificially, the individual crystals that form are very large. In bone, the crystals are very small, on the order of 3 billionths of a meter long. These nanocrystals have long been thought to be the reason that hydroxyapatite in bone is so strong. However, scientists haven’t been able to understand why the nanocrystals stay so small in bone.

Now Klaus Schmidt-Rohr and his colleagues might just have figured that part out!

Using nuclear magnetic resonance spectroscopy (the same basic technology used in medical magnetic resonance imaging devices)1, the team found that these nanocrystals of hydroxyapatite are studded with another molecule, called citrate.2 The citrate seems to latch on to the outside of the crystal, and that stops the crystal from growing.

So the process that makes bone in living creatures utilizes nanoengineering. It is designed to add citrate when necessary to keep the crystals nano-sized, and that makes the bone strong. It seems that this is an integral part of the whole bone-making process, as this study indicates that the citrate covers one sixth of each hydroxyapatite crystal’s surface.

Now that’s fascinating enough, but there is another fascinating aspect to this story. As a commentary on this study says:3

Scientists have long known that citrate is abundant in bone, but they thought it was involved in calcium leaching and demineralization.

Indeed, this is what I was taught at university. Since citrate is what results when citric acid acts as an acid, and since hydroxyapatite is a base, it was thought that the citrate in bone was the result of citric acid reacting with hydroxyapatite. This would destroy the hydroxyapatite, but it would allow the body to pull calcium out of the bones and into the blood. This is something the body does from time to time, especially when it is low on calcium. Well, that idea is most likely quite wrong. The citrate is not there because hydroxyapatite was destroyed. Instead, it is there to nanoengineer the hydroxyapatite so as to keep the bones as strong as possible.

God is not only an amazing engineer, He is an amazing nanoegineer!


1. It turns out that when MRIs were first developed, the process was called by its proper name, “Nuclear Magnetic Resonance Imaging.” This is the proper name, because the scanner looks at how the nuclei of atoms absorb and emit energy in the presence of a magnetic field. This depends on the nuclei around them, so it allows you to figure out some of the atomic details of what is being analyzed. However, people were so afraid of the word “nuclear,” that they would not agree to have the scan done. As a result, the name was changed to “Magnetic Resonance Imaging” to make the scans less frightening.
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2. Y-Y. Hu, A. Rawal, and K. Schmidt-Rohr, “Strongly bound citrate stabilizes the apatite nanocrystals in bone,” Proceedings of the National Academy of Sciences of the USA, 107:22425-22429, 2010.
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3. CHA, “Citrate Stabilizes Bone Apatite,” Chemical and Engineering News, December 6, 2010, p. 36.
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  1. Enoch Heise says:

    Dr. Wile,
    I liked the article! I am unceasingly fascinated at the way God designed nature to produce substances so advanced that humans, even with the power of reverse-engineering at their command, have been unable to even approach there effectiveness. Do you think it is possible to use our knowledge of how bones are formed to recreate the material for our own purposes? If not, is it possible to use non-embryonic stem cells to grow the bone material in a controlled environment? Also, at a molecular level, what is it that makes biological substances (such as bone, beetle shells, or spider silk) so strong? Is it the advanced molecules, their complex arrangement, or a combination?

    1. jlwile says:

      Thanks for your comment, Enoch. It is not possible for us to use our knowledge of how bones are formed to recreate the material. For example, while we now know HOW bone limits the size of its apatite crystals, we cannot do it ourselves. We don’t know how to add the citrate so that it binds to the crystals at the right time. This is obviously a well-choreographed engineering process, and we don’t know most of the steps!

      Now it might be possible to use stem cells to do the trick, as they contain the programming that we don’t understand. Obviously, non-embryonic stem cells would be the ethical choice. However, the hurdle to overcome there is to give the cells the right environment so that they become osteoblasts (the kinds of cells that produce the bone matrix).

      In general, what makes biological substances like bone and spider silk so strong is the PHYSICAL characteristics of the chemicals. In the case of bone, it isn’t the chemical itself, it is the size of its crystals. God’s engineering allows for the crystal size to be regulated to give bone its strength. In the case of spider silk, it is the particular way the proteins fold to form their final shape. We can actually take the genes that we know spiders use to make their silk and transplant them into goats, for example. That allows goat milk to be full of spider silk. The proteins are chemically identical to spider silk proteins, but even so, when we “spin” that silk, what results is not as strong as spider silk, because the silk glands that “spin” spider silk impart a particular shape to the proteins. We don’t know how to do that, so even though we can force goats to make chemically identical proteins, the way those proteins fold is different, and as a result, the silk just isn’t as strong.

  2. The Black Sheep says:

    I was fascinated when during a conversation with my doctor, she explained to me that running actually makes my bones stronger. She explained in pretty basic terms that when I run the bone actually breaks down a bit, and then rebuilds itself stronger than before. Pretty cool!

    On the subject of running, perhaps you can answer a question for me. A friend and I are always puzzled by the way our bodies, or rather minds, react after a run. This really only tends to happen when we do longer runs, like when we were training for a half marathon. After we finish, whether it be a race or just a training run, we feel completely stupid. We’ve discussed that it seems our minds are not functioning properly. Very simple tasks or questions take great effort to complete or answer. This obviously wears off after a little while, but after our half marathon we were both grateful to have a friend to drive us home because we weren’t sure it was safe for either of us to drive. I’ve recently started reading a book called “Born to Run” by Christopher McDougall. I started to feel a little better when the author mentioned that it’s not uncommon for ultra-runners to not just lose mental capacity, but actually hallucinate. Unfortunately, he did not explain why this happens. I get that pushing your body will make it physically hurt, that makes sense. But why the diminished mental capacity? And is there anything to help prevent it from happening or speed up the recovery process when it does?

    1. jlwile says:

      Thanks for commenting, Black Sheep. I’ll make that the topic of my next post. Despite the fact that it will be “sciency and nerdy,” I hope you will enjoy it!

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