This is inspired by our third fortnightly challenge, but a question I've had for a while anyways.

Bones are seriously complex structures. Far more complicated than most structural materials humans use. They are porous, with compact and spongy sections which repair themselves. Bones mostly rely on Hydroxylapatite for their structural strength, but also on collagen to make them more shatter resistant. This results in a good material for making bodies. I would like to do better.

I would like to know how a creature could, assuming earth-like biology and resources, have better bones. Specifically, how can you improve the hardness, compressive strength, and shearing stress of bone without dramatically increasing the weight? This is a reference showing some of the numbers to beat, but I'll list them here for simplicity:

  • Compressive Strength: 170 MPa
  • Hardness of Hydroxylapatite: 5 (Mohs Scale)
  • Shear Stress: 51.6 MPa

Edit: If you feel that a creature could use multiple or different materials for their skeleton (as bone serves many different purposes), please state why in your answer.

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    $\begingroup$ Carbon nano tube based foam would probably do the trick. It has decent compressive and tensile strength while being very light. Finding the interstitial material is a bit trickier. $\endgroup$ Commented Mar 10, 2015 at 2:41
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    $\begingroup$ @IsaacKotlicky I'm excited for your formal answer. You best jump on that before someone else does. $\endgroup$
    – PipperChip
    Commented Mar 10, 2015 at 2:57
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    $\begingroup$ You've given that the mass should not increase, should answers also include feasible self-repair methods? $\endgroup$
    – Samuel
    Commented Mar 10, 2015 at 4:37
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    $\begingroup$ @Samuel Yes, that would be best. If you have a structure similar to bone, something porous, you could assume that blood vessels and other needed cells permeate that area. $\endgroup$
    – PipperChip
    Commented Mar 10, 2015 at 12:54
  • $\begingroup$ What good is hardness in bones? Do you expect them to get scratched or chipped or something? $\endgroup$
    – Mike L.
    Commented Mar 10, 2015 at 14:20

7 Answers 7


You could do this by simply substituting calcium for something else. Maybe carbon as @IsaacKotlicky suggested, as it's pretty light and strong.
For a little more weight you could weave iron into the bone structure. We already use iron in our blood, and blood is manufactured in the bone marrow, so it's not that great of a leap to suggest that in this creature iron could also be stored in the bones until it's needed for blood manufacture as a reservoir.

This would give you a creature that would have really strong bones, and be able to survive blood loss better.


Ok, some numbers... I'm not any kind of ologist, so these are mostly rough estimates.
Calcium is a metal, so we'll assume that this creature uses iron instead of calcium for half it's bone mass in a calcium/iron matrix.

A human male skeleton weights around 13607g (30lb). If you replaced half the weight with iron instead of calcium, it would weigh roughly 16283g (35lb).

Shear modulus: 7.4 GPa
Mohs hardness: 1.75
Brinell hardness 170-416 MPa

Shear modulus: 82 GPa
Mohs hardness: 4
Brinell hardness: 200-1180 MPa

I can't begin to guess what the resulting iron/calcium composite would be like in regards to strength (especially because the structure of bone is already so much stronger than the use of calcium as a building material should make it), but considering how much stronger iron is to calcium I believe it's safe to assume that the resulting structure would be better in almost every way, with only a slight increase in weight.


  • $\begingroup$ Some good ideas, but how light and how strong are these alternatives? I'm looking for specifics, not generalities. $\endgroup$
    – PipperChip
    Commented Mar 10, 2015 at 17:37
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    $\begingroup$ @HSquirrel You are correct, and since I'm not smart enough to design my creature on a cellular level(Onion) I'm going to posit that the resulting iron/calcium alloy will have similar strength enhancing properties. $\endgroup$
    – AndyD273
    Commented Mar 11, 2015 at 19:20
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    $\begingroup$ @John Not an iron/calcium bond, but a lattice. Some kind of iron structure with gaps or holes, in which calcium is stored. If you look at the micro-structure of bone, you'll see that it already has a lattice type shape, which gives it a lot of strength for its weight. If you had an iron lattice, and intertwined a separate calcium lattice around and through it, then you'd have the strength, and still have the calcium. $\endgroup$
    – AndyD273
    Commented Sep 24, 2019 at 15:16
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    $\begingroup$ The "lattice" in bone is calcium phosphate and collagen, replacing it with iron will make the bone far weaker and far heavier. Even if you figured out a way to bond the collagen to iron you don't really gain much strength but add a lot of weight, especially since you need to add the calcium and phosphate back in. for reference the mineral in bone is Hydroxyapatite, the characteristics for metallic calcium are not useful. $\endgroup$
    – John
    Commented Sep 24, 2019 at 15:38
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    $\begingroup$ @John You may know more about the chemistry and structure of bone than I, so thank you for the added info. For an alien creature in a non-hard-science setting, where iron infused bones are evolutionary selected for, it could that life would find a way, especially since life is pretty cool that way. Either way, I'm not able to put a ton of research into an almost 5 year old answer at the moment, so your information is useful. Thanks. $\endgroup$
    – AndyD273
    Commented Sep 24, 2019 at 18:02

I'm first going to reply to your question as it is. Then I'll elaborate and try to answer the question in a more general sense.

Reply to question as is

If all you care about is hardness, compressive strength and shear strength, you could take something like diamond (or a similar material that is easier to produce biologically) Diamond (see references 1,2,3 and 4):

Moh's hardness: 10

Tensile strength: 60 GPa (perfect crystals could be up to 225 GPa)

Shear strength: 95 GPa

Compressive strength 223 - 470 GPa

So far we're looking at twice as hard and approx 1000 times as strong.

Enter toughness at: 2.0 MPa m1/2

Compared to bone toughness (see reference 5) is more like 3.6 MPa m1/2

So diamond bones would actually break faster. They would potentially allow you to exert significantly greater forces if you did it slowly enough and if the rest of your body could handle it.

More general response

First we'd need to look at what we use bones for and what properties we'd need for that.

Bones are used for attaching muscles and taking the stresses induced by using muscles. In general these forces don't happen to sudden, so we can use tensile strength and shear strength to see how much the bones can take. Try to go for the yield values in both cases as you want the stresses to stay in the elastic range (would not be good to have your bones permanently deform as you use them). Somewhat less important is the amount of deformation that happens in the bone. You want this to be low enough to not bother you in movement and also low to reduce the energy expense to bend the bone so most of the energy expended goes to the intended movement (energy for bending is stress times the amount of deformation). So that comes down to having a high Young's modulus. So we're looking at mainly high strength and to some extent high stiffness.

Bones are also used to keep the shape of your body. This means they should deform little under various stresses (elastically). This is again the stiffness.

Bones protect you from damage. Bones should deal with life (falling, impacts, ...). Both of these require high toughness. You can look at total toughness (energy absorbtion up till breaking) or to elastic toughness (energy absorbtion up to permanent deformation).

It would also be less than ideal if your bones easily deform permanently (bending bones back in shape seems harder than mending broken bones). This implies having the yield strength be very close to the actual strength (so breaking occurs instead of bending). I guess this one is somewhat more debatable.

Bones have to able to grow organically (so slowly accumulate, not just appear finished at once).

Bones have to live in your body. So they should corrode minimally in the conditions present in your body and have little negative impact.

So now we've set the scene:

High enough (yield) strength to take the load of muscle usage, weight of body.

High enough stiffness to keep your shape and optimal energy usage.

High enough toughness to deal with sudden impacts such as falling, getting hit, ...

On top of those requirements we'd also have to look at what's biologically feasible.

Metals would be a nice start (I'm thinking hardened steel, which is both stronger and tougher than bone, but plastic deformations are minimal). Metals however have the disadvantage of being hard to make into the required composition by means of biological process. They may also corrode and be poisonous. If we could purify them somehow and we could coat them to keep them separate from the body, it might be an option. In any case, other answers have treated this sufficiently, so I don't feel the need to add much more.

Otherwise I'm thinking about all kinds of biological polymers. Some of the strongest and toughest materials known to man are biological polymers. Spider silk for example has a tensile strength up to 2000MPa and is very tough. Unfortunately it deforms rather easily as it has a low stiffness. So having bones of something like spider silk would be like having very strong rubber bones. Stiffening it up with extra crosslinks between protein chains might help make it stiffer (though also less tough).

In general, I'd look at reinforced polymers. Life is good at making polymers and reinforced polymers (example: wood are strong fibers in a polymer matrix). So you'd just need to find the correct fiber and the correct polymer matrix to go around it. Other examples of reinforced structures are carbon fiber with a glue matrix (high strength, low weight bike frames). The entire structure can be stiffened (at the cost of toughness) by adding hard particles (like calcium salts). Note that if you increase your stiffness, you usually lower your toughness (unless you also manage to increase your total strength or your maximum deformation).

A lot also depends on the exact structure of your material (molecular buildup, orientation, crosslinks, crystal structure, ...).

You could try limpet teeth (See reference 6) which have a tensile strength of 3 - 6.5 GPa, if necessary combined with some kind of longer fiber to keep it strong in larger sizes.

You can probably get away with just attributing it to a change in micro-structure (even without added elements or replacements), though that might not be as cool as fancy new molecules :-).

  • $\begingroup$ Keep in mind that Calcium, the main component of the bones is indeed a metal. Actually, the human body uses tons of different metals to do a lot of things. Our blood is mostly made of Iron, for example. Biological processes are really good in using metals, just not in the most common "steel" sense. $\endgroup$
    – Mermaker
    Commented Mar 13, 2015 at 20:06
  • $\begingroup$ Yes, our bodies are really good at using metals (in the sense of elemental metals) but really bad at using metals (in the sense of metallic bonding). This due to the fact that metallic bonds are chemically not that stable (f.e. metallic bonded iron would rust), it's why we use the more expensive titanium in prostheses (pretty stable in our bodies). See en.wikipedia.org/wiki/Metal for various definitions of metal. Metal in the human body is usually bound as an ion or covalent bond in some chemical structure. $\endgroup$
    – HSquirrel
    Commented Mar 15, 2015 at 10:52

Even humans have a way for strengthening bones. Take a look at the Wolff's law. Bone in a healthy person or animal will adapt to the loads under which it is placed. Martial arts takes advantage of this.


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    $\begingroup$ Care to say by how much bones get strengthened? Would it be comparable to, say, steel? Is it a 50% improvement, and to what attribute? $\endgroup$
    – PipperChip
    Commented Mar 12, 2015 at 14:54
  • $\begingroup$ I can not tell you the numbers as I am no biologist. I can give you an example of people who pursued their "Iron body" training as chinesee call it. Thai boxer Melchor Menon for example strengthened his shin to the level that he is now able to break baseball bat with a kick. $\endgroup$
    – Januson
    Commented Mar 12, 2015 at 15:47
  • $\begingroup$ @Januson I would think that technique is more important than bone strength for that. Does he break it at at the handle or on the barrel? $\endgroup$
    – KSmarts
    Commented Mar 12, 2015 at 21:35
  • $\begingroup$ Somewhere along the handle. You are right. Technique is important to generate enough force to break through, but without bone strength his shin would shatter upon the impact. $\endgroup$
    – Januson
    Commented Mar 13, 2015 at 6:03
  • $\begingroup$ +1 for using the resources we have. I assume you could easily train by using heavy gravity (as opposed to the bone problems we get with lighter gravity). As an extra benefit of bone training is that you usually also train the other structures (tendonds, ligaments, muscles) so you get a net increase in strength. It would be interesting to know the theoretical limits of this strengthening and if there are any trade-offs. $\endgroup$
    – HSquirrel
    Commented Mar 18, 2015 at 7:42

Well, carbon nanotubes are obviously the stuff of the future, but I don't really know much about those, so I'll be leaving that up to @IsaacKotlicky.

Instead, I'm going to go down a different route, and see if we could integrate steel into your creatures' bones. Now, I'll be talking about endoskeletal bones (ie. the ones on the inside that primarily support your posture and locomotion); there are quite different considerations on exoskeletal bones (eg. turtle shell) and similar not-quite-bone structures (teeth), which I'll touch on at the end.


The word "steel" actually refers to a rather wide variety of (principally) carbon-iron alloys, with an equally wide variety of properties. I wasn't able to find figures for shear stress of steel (presumably because it depends heavily on the geometry), but the yield point (eg. the stress beyond which the material permanently bends, or breaks if it's hard/brittle) seems to be about 10x higher compared to bone for a decent bar of steel you can make into something in your forge.

This is a good thing, because flexibility (expressed as a high yield point) will be a principal requirement for an endoskeletal bone - you want it to be able to take as much stress as possible while flexing, so that (in trauma situations) it absorbs as much of the schock as possible without getting irreversibly bent or broken.

Note that this is the kind of flexibility that we generally talk about in structural engineering - a good bone will be flexible much like a good sword, rather than limp like rubber.

Organic manufacture of steel

Obviously, we won't be able to melt down iron (or iron ore) inside your typical organism. The good news is, we might not have to.

The two chemical processes you need to master is first getting the iron in the first place, and then creating steel and making it into something useful.

Your body can already get iron from food - it's what your red blood cells are made of, among other things - but for volume production, you might want to look for other sources. Iron is one of the most abundant elements, mostly in form of various oxides, so you could concievably have your creatures eat something that contains hematite powder. Extracting atomic iron from hematite is a question of reduction - in industrial practice this is done in a furnace using coke as the reducing agent, but I'm pretty sure any number of organically-available reagents would do the trick.

So now that you have iron, what do you do next? Well, iron (and all other metals, really), have the interesting property that if you simply put together two pieces of it in a favourable chemical environment (such as vacuum, but anything that will prevent the surface from oxidizing will do the trick), they will get cold-welded. This way, you could build up small particles of iron into larger structures - interspersing them with some cementite to up the carbon content - until you get steel.

Altering the properties

The properties of steel are largely dependent on the size and configuration of the monocrystals that make it up (besides the exact chemical composition, of course). In general, larger crystals make harder, more brittle steel, while smaller ones make it softer and more flexible; you want to strike a balance here.

The human body, among others, seems to be capable of growing crystalic substances; that's how it makes enamel out of hydroxylapatite, so I'm going to run with that and assume that we can design an organic process that will grow microcrystals of different sizes.

Building bones

By depositing and welding them together, you could concievably construct matrices of desireable properties, much like we have inside our bones, but made of steel. If you add cells with the ability to selectively oxidize some parts of that matrix away, you now have osteoblasts and osteoplasts and your steel bones have the same self-repair and adaptation capability as normal bones do.

Exactly what the properties of such a bone would be is anyone's guess, and i depends heavily on the geometry of the bone mesh and the applied forces. We can take the factor of 10 as the ideal case, but other things to consider are that clever geometry can actually make a steel item stronger than its weight/volume would suggest (that's how I-beams work, basically), plus if you can play around with carbon content and crystal sizes, you can mix it up to either mimic pattern welding, or tempering/quenching without the (usually) necessary heat.

Final notes

I promised to mention something about teeth here, so here goes: where flexibility is needed and you are not expecting to have the object directly scratched or chipped by something really hard, you don't really care about high hardness. Quite the contrary, hardness usually corellates with brittleness, which in turn decreases flexibility, so it's something you might want to avoid.

In some cases however, it might be beneficial. Teeth might be one such case (they are not properly bone, but the enamel - the hard outer part - is also based mainly on hydroxylapatite), where in striking the balance between hardness and brittleness, you might want to go a bit higher on the hardness scale (although not hign anough that you can break your own teeth biting down hard).

Depending on how it's made, steel runs the mohs scale from 4 all the way up to 8. Otherwise, there is plenty of minerals harder than hydroxylapatite that you might grow crystals of using a similar process, although it should be noted that unlike bone, a broken tooth can not be repaired (there are no osteoplasts or osteoblasts that would do that inside a tooth). You could, of course, go the way of the shark and just grow a new one, if you're so inclined.

Finally, note that stronger bones alone do not mean you can put your creature through unlimited abuse - heavy wear or significant shock might damage joints, and you also need good muscle to support the skeleton and help absorb the forces. Also, I hear that bones are supposed to break in order to absorb shock that would cause greater harm to other body parts - making them unbreakable might cause other problems later on.

Well, this was a fun mental excercise; I hope I didn't write anything outrageously wrong:)

  • $\begingroup$ I'd say that if the body can grow microcrystals organically, it would probably have way better control over the microstructure than we have with current metallurgy. So for material characteristics in this case, you can pick any existing steel and our body will probably have those, but just slightly better (less unwanted impurities to reduce strength). $\endgroup$
    – HSquirrel
    Commented Mar 12, 2015 at 7:05
  • $\begingroup$ In terms of you getting things outrageously wrong, are you sure about small crystals and large crystals (I thought large crystals meant a softer and more ductile material). I thought the avoidance of large crystals in practice had more to do with the desire for uniform crystal size (more uniform material properties instead of local variations). I might have misunderstood though. $\endgroup$
    – HSquirrel
    Commented Mar 12, 2015 at 7:10
  • $\begingroup$ @HSquirrel Yeah, that's kinda what I was going for, I just didn't want to overestimate the capabilities of a "biofactory" like this. I may have gotten the thing with the crystals mixed up, but I do remember that fiddling around with the size of component crystals is how tempering and quenching work. $\endgroup$
    – Mike L.
    Commented Mar 12, 2015 at 8:17
  • $\begingroup$ Fair enough for the biofactory. I looked up the crystal size thing and it turns out it depends. I based myself on a traditional material science coursebook where they only know about decreasing grain size increases strength. People tried to reduce grain size even more, but grain sizes smaller than 10nm seem to no longer increase strength or even decrease strength (en.wikipedia.org/wiki/…). Thanks for having me learn something new :D. $\endgroup$
    – HSquirrel
    Commented Mar 13, 2015 at 14:39
  • $\begingroup$ @HSquirrel No problem, anytime:) $\endgroup$
    – Mike L.
    Commented Mar 13, 2015 at 14:47

When we want to strengthen a material, like concrete, putting in a bit of metallic structure into the mix helps a lot...as in reinforced concrete.

An animal could in theory, do this to its own bones..depositing a metal into the bone structure in fibers, producing an internal web of stronger material. Think of a fiberglass like result, but with metal.


Limpet teeth are the strongest biological material to date and are made of goethite, an iron compound. A composite of limpet teeth and silk hydrogel would be the strongest and toughest composite I can think of while still being 100% possible according to known biology.

The goethite for hardness, the silk for tension and the oil hydrogel for the spongy material.

You can have the skeleton be around 20% heavier at max depending on density or have it weigh less for equal strength to normal bones.


One interesting thing I find about humans is the common perception that we're weak in the animal kingdom - that as we evolved for intelligence, we lost muscle (and presumably) bone strength. And technically this is true - you can compare us to Chimpanzees, which are roughly twice as strong as humans pound for pound.

But the fact is that there's no free lunch in evolution or body design. Stronger muscles and bones take more energy and take longer to heal. Humans are "weak" because weak is efficient, and with technology we can prioritize efficiency over strength and still come out ahead, enhancing our overall ability to survive. And human bones can break, but we can heal that broken bone. Think about taking a fall - if your bone doesn't break, but your muscles aren't any stronger, your bone could literally rip itself off of the muscle structure. That could turn a 6-week fracture into an amputation or death scenario.

Now, this isn't to say you can't strengthen bones in your creatures - lots of excellent answers here, from carbon nanotubes to titanium or steel. But you shouldn't strengthen them in isolation. A stronger skeleton necessitates a stronger and more robust muscular system, higher energy requirements, and might result in a tougher time recovering from injuries. You might also want to change the very structure of your animal so that it can use those super strong bones to protect the rest of the body - for example, maybe the animal can "lock" the skeleton in place, so that most of the shock of a fall or impact is taken by the skeleton, therefore protecting the comparatively weaker muscles and ligaments.

  • $\begingroup$ Interesting, but I do not think this actually answers the question. We're addressing bone improvement here, not why things are the way they are. As a matter of fact, there are only two answers so far. Answering in comments is not the SE way. $\endgroup$
    – PipperChip
    Commented Mar 10, 2015 at 21:47
  • $\begingroup$ @PipperChip: The last paragraph is intended to address that. I think there are several obvious ways to improve bones (using known stronger materials) but I wanted to address the side effects and the design characteristics (in other words you don't just want to plug stronger bones into a creature, you want to consider the side effects and design changes that are possible with stronger bones). But it might be out of scope for an answer, I wasn't 100% sure when posting it. $\endgroup$ Commented Mar 10, 2015 at 21:52
  • $\begingroup$ I'm well aware of the design issues of simply stronger bones without proper joints to go along with them. There was a discussion in chat this morning about this, too. $\endgroup$
    – PipperChip
    Commented Mar 10, 2015 at 22:00
  • $\begingroup$ The bone is connected to muscles by tendons. So, when a bone is literally ripped off of the muscle structure, you have a ruptured (torn) tendon. While this can be painful and debilitating, I don't see how it is significantly worse than a bone fracture. Recovery times are pretty comparable. It certainly isn't lethal. $\endgroup$
    – KSmarts
    Commented Mar 11, 2015 at 16:11
  • $\begingroup$ @KSmarts: I'm thinking of a situation where the bone is ripped out and then goes through muscle - if the bone can't break and act as a shock absorber, that energy has to go somewhere. So think about what would be an open-air fracture, but one end of the bone is entirely ripped out of the body and is no longer connected at all. $\endgroup$ Commented Mar 11, 2015 at 16:13

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