If you ask most kids what bones are made of, they would quickly answer “calcium!” Though it’s not incorrect, years of us telling them to drink their milk for strong bones has hidden an amazing feature of nature, biomineralization by proteins. Calcium is only half the story of bones. Though the semi-crystalline mineral that contains calcium, oxygen, hydrogen, and phosphorus, called hydroxyapatite, provides strength and rigidity, our bones also need to be elastic to help absorb shock. This is where proteins come in. Your bones are made of living tissue that spew out networks of protein that trap and crystallize the calcium in your body. Together they form the strong, resilient biomaterial we know as bones.
You can feel the flexibility of the protein network for yourself by reacting and dissolving the hydroxyapatite from the bones in vinegar. What’s left are entertaining, wiggly bones the kids will love to play with (except my daughter, who was too grossed out and didn’t want to touch them which is why all these pictures are of my hands ¯\_(ツ)_/¯ ).
- Bones (I recommend thinner bones, like the skinny one from chicken wings above. They will take less time to become bendy and they get super bendy, whereas the larger bones kind of fold more than bend (see below).)
- Sealable container
- Clean all the meat off the bones. We boiled ours to remove the stubborn bits.
- Place in the sealable container and cover with vinegar.
- Leave them in vinegar until bendy. Our thinnest bones from chicken wings took just a few days to become bendy, whereas the bones from the chicken leg took over 3 weeks.
Biological mineralization, when an organism causes inorganic salts to crystallize or precipitate, happens more frequently than you think! Humans are able to make bone and teeth. Other examples of biomineralization include sea shells, pearls, and egg shells. There are even little bacteria that can make magnetic crystals to help them navigate!
All of these organisms have specialized proteins that guide the formation of the various minerals. Some organisms, such as diatoms, have such exquisite control that they can create intricate microscopic structures from the minerals.
Our bones are made of a complicated network of cells that work together to mineralize organic tissue. The actual crystallization happens on a ubiquitous protein called collagen and is controlled by several specialized accessory proteins. You may have heard of collagen before in beauty product commercials. It is a protein made of three strands that wrap around each other, forming a triple helix, and is responsible for giving tissues elasticity.
Osteoblasts, a type of cell in bones, send out calcium phosphate over the collagen network. Calcium phosphate is a water soluble form of calcium derived from the foods you eat. The proteins attract the calcium phosphate and arrange it to form semi-crystalline hydroxyapatite. This whole process is called ossification.
What is happening to the bone in vinegar?
Vinegar is a solution of water and 3-5% acetic acid. The acetic acid reacts with the hydroxyapatite in the bones. The products of the reaction are soluble in water, so they are removed from the bone. The large collagen framework is not soluble in water so that remains and retains the shape of the bone. Since the material that was making the bones stiff was dissolved, this allows you to feel the usually cloaked elasticity of the collagen in bones!
Free Coloring Page!
We hope you have fun trying this out! It is so cool to see bones bend like this. To add to the experience for kids, here is a free coloring page of apatite crystals on a collagen framework. If you post your pictures of the project to social media, tag us (@Brave.The.Elements on Instagram) and include the hashtag #IBravedTheElements for a chance to be featured!
For the coloring page, either click on the link below to download the PDF, or just save the JPG image below it.
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Nature Materials. 2010; 9:1004–1009.