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While the question has been asked multiple times on this site. They almost all universally boil down to, "Use Carbon nanotubes!"

While CNT do have impressive material properties and can be produced by the body, the issue is that while CNTs are great for elastic strength, it is very rare for a bone to be stretched to the breaking point while compression and shear forces are what commonly cause bones to facture. Forces that CNTs are not the best at preventing or optimized for.

So that leaves us in a bit of a pickle. What material can we use to strengthen the skeleton that doesn't leave it more brittle than before (Iron/ceramic matrixes) or is ultimately redundant (Above example)?

Requirements for this question.

  • Numbers would be greatly encouraged to support the answer.
  • Does not necessary have be biological in nature, but it would be greatly appreciated.
  • Pros and cons of the solution should be presented.
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    $\begingroup$ Tensile strength is valuable to prevent bending. Plenty of fractures start with crack formation on the outside of a bend due to tensile failure of the material under maximum strain, then the crack propagates through and the bone snaps. Healthy bones don't generate fail due to the material on the inside of the bend being crushed. $\endgroup$ Commented Apr 2, 2022 at 7:12
  • $\begingroup$ You're using terms like "elastic strength", "stretched", "compression", and "shear" but I don't think those words quite mean what you think they mean in a physics or material-science sense. Even in a simple bend, internal strain in both the tension and compression directions exists. Something like so: ecourses.ou.edu/ebook/mechanics/ch04/sec041/media/d4123.gif $\endgroup$
    – Dragongeek
    Commented Apr 3, 2022 at 13:02

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Carbon fibers, like CNT, also have great elastic behavior, yet they are used also to resist to shear forces, e.g. in automotive race frames and components. How is it done?

Proper winding of the fiber orientation can ensure the desired behavior under the expected loads. Apply the same trick to your bones, and you have your very light and yet very capable bones.

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  • $\begingroup$ How would I get my hands on some carbon nanotubes? $\endgroup$
    – Daron
    Commented Apr 2, 2022 at 15:05
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CNT's.

When we are young, our bones are mostly made out of elastines that make the bones flexible. This makes it hard to break children's bones as they'll bend more. This isnt ideal for the body, so as you age more Calcium is added for a better combination of stiffness and elasticity.

Then as you grow older the elastines dissappear and leave an abundance of calcium. This is the point where it gets brittle, when the elasticity goes away.

CNT's would be great, their elasticity helps absorb impacts and reduces the chance of breaking. Even when broken, CNT's could help the bone to still be somewhat attached, making it easier to keep it in line and heal. Also if pre-loaded under stress such CNT's could very well be used for stiffness and stability. Perhaps there could even be a system that detects if the calcium underneath is broken, and will activate something that winds up the CNT strings to stiffen it, push the broken parts together and provide some emergency support.

There is another consideration: in humans breaking bones is GOOD. Our bones are relatively fast in regenerating and arent as important for survival. When breaking at the point they do they absorb a lot of the force, which protects your other vitals like bloodvessles, liver, kidney, lung tissue etc*.

This is why it always irks me when Supersoldiers like Halo's Spartans are said to have "nigh unbreakable bones". Its as if they are trying to get them killed rather than make them better. An intermediary solution would be needed: the bone breaks, but has one solid core that can still function partially as a support for the body.

*it increases survivability, it does not guarantee it. In circumstances with the right angle and excessive force bone shards can rip through the body. "Excessive force" being key. It is questionable if an unbreakable bone that transfers more force to the vitals would have been better in such circumstances.

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Even if you would make bones absolutely indestructible, the survivability of such a person would not change much.

A lot of damage we receive in traumas is not related to bones breaking. Small traumas like tendons, cartilage will all remain exactly the same, and is often worse in terms of healing time than an actual fraction.

Big traumas that do cause fracture no matter what, like fall from a hight, are questionable too, if they will be less damaging if bones wouldnt fracture. Bone fracture dissipates some of the impact energy, and makes it so that body parts location and orientation is acceptable. In particular blood vessels and nerves - their path is near the bone. If bone breaks, assuming bone shards dont cut the nerves and blood vessels, it allows blood vessels and nerves to stay connected to the rest of the body. Deformation of this type is accounted for. If bones would not fracture after a fall, then energy will be spent on destroying the tendons, cartilage, nerves and blood vessels connection in a joint. All of them are much more valuable than a bone. Lack of any of them makes a limb completely useless, as they rarely heal.

Would I want indestructible bones? No. I value other systems of my body as much, if not more, than bones. Bones are easier to replace or fix than cartilage, tendons, blood vessels or nerves.

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Bone

enter image description here

Carbon nanotubes are nice and all. But how many of us has actually SEEN a carbon nanotube.

On the other hand we are all familiar with bones since they make up 15% of our bodymass. Reinforce the bones by making them thicker with more bone. Thicker bone is stronger bone.

Indestructible skeletons are overkill. If you fall from orbit or get run down by a lorry, your internal organs are pulverised and you die. An intact skeleton is no help.

Using biological material is great since it makes the bones self-repairing. Artificial materials will be subject to heat and chemical degradation, which they have no way to repair. That carbon nanotube has expanded by 0.3% every day and then shrunk every night. This creatures micro fissures and the body has no way to repair them.

Normal bones are subject to these pressures too, but the body already knows how to repair bones. Thick bone strong bone.

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Even if the bones are so tough to the point to be indestructible by conventional risks of death if the skin, muscles and other organs are not upgraded your upgraded skeleton will not be very useful.

The skin and muscles and fat are soft so a squishy material that is very strong and tough( better than steel) will be necessary. They will also protect the organs of the digestive system which the bones cannot cover to protect.

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Bioglass. It's a real life type of glass that turns into bone when exposed to in vivo conditions (basically, it's fractionally soluble and partially made of calcium and phosphate, and so the body ossifies it easily). Not only is it stronger than bone but when installed right, even the junction between it and real bone is much stronger than a real bone.

The main downsides are that it has no bone marrow in it and is heavy.

https://en.wikipedia.org/wiki/Bioactive_glass

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  • $\begingroup$ Note: You'll want composite materials to get the best from bioactive glasses. I'm not sure that your CNT skepticism is warranted; CNT ceramic composites can have great fracture toughnesses, which is possibly what you care about the most in bones. The CNTs deflect cracks, leading to improved toughness and graceful failure. $\endgroup$
    – user86462
    Commented Apr 2, 2022 at 11:22
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You can make the bones denser (having less voids inside of them) and make them much stronger and resistant to compression. There are actual people in real life with a mutation that makes their bones 8 times denser than ordinary bones and this makes their bones much stronger than normal bones. This mutation was discovered after a man with this mutation survived a severe car crash without any fracture in his bones.

Materials with good tensile strength like CNT can definitely protect bones from breakage. So it is possible to make bones even stronger by replacing some of the collagen or elastin in the bones with a material with good tensile strength that can be produced biologically like spider silk, which is very strong and is actually being considered to be used in high strength composites. I know CNT is stronger than spider silk but I know that spider silk can be produced and decomposed biologically and it is important if you want the bones to remain strong in the long term without losing strength.

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Is metal too prosaic?

metal repair

http://www.animalmedicalandsurgical.com/fractures-of-humerus-radiusulna-femur-and-tibia/

Metal has advantages. It is strong. It lasts long. Technology is mature. It can hold bone in position so the bone can heal. It is cheap. It is biocompatable.

Depicted: repair with steel rod, plate and screws. And staples.

Or you could use titanium. Or tantalum!

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