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Some context, right off the bat: I am a mad scientist. This means that evolution has been defenestrated; as such, I am not interested in determining what evolutionary pressures might lead to the adoption of such a material as a supportive structure, and nor am I interested in where this thing might find aluminum to keep its bones intact; we're talking mad science and the limits of what's possible within the laws of physics and mortal biology here, not boring old evolution.

I, being a mad scientist, just so happen to be planning on making a creature with aluminum bones; specifically, bones whose primary material is aluminum oxide, rather than carbonated hydroxyapatite.

Now, before we get any further, the question: What are some structural or biological weaknesses of using aluminum oxide monocrystaline whiskers suspended in a collagen matrix as a material for bones - i.e., why shouldn't I?

Please note that the core of these bones is plain 'ol bone marrow. You gotta have that to live if you're Earthly life; I might be a mad scientist, but I'm not a mad scientist, y'know?

However, the other parts of these bones - i.e. where there would normally be carbonated hydroxyapatite - are made of aluminum oxide. Here's why that's great:

Aluminum oxide has several properties that make it superior to bone as a supportive structure.

Now, as I mentioned further up this post, it might seem that aluminum oxide has a few glaring weaknesses in comparison to bone; it likely has a lesser ductility and elastic limit, and I had to handwave its fracture toughness and Poisson's ratio.

While I cannot find figures on the elastic limit - how much force per unit of area it can withstand without being permanently deformed - and ductility - how much it can be elastically deformed without fracturing - I'm willing to bet that bone's elastic limit is greater than aluminum oxide's elastic limit of 69 to 665 megapascals, and that bone's ductility is greater than aluminum oxide's ductility of of 0.00018.

The only relevant areas in which I know for a fact that it's possible for bone to beat aluminum oxide are fracture toughness - how hard it is for an already-established crack in the substance to grow further - and Poisson's ratio - how much a substance squishes out to the sides when compressed.

  • Cortical bone has a fracture toughness of 2 to 12 MPa.m^(-1/2), whereas aluminum oxide has a fracture toughness of 3.3 to 5 MPa.m^(-1/2). This is enough for a handwave.

  • Cortical bone also has a Poisson's ratio of 0.12 to 0.63 (if you want to find that particular bit, use control-F to find it, since it's a long source), as opposed to aluminum oxide's 0.21 to 0.33, meaning that aluminum oxide might be squishier. Again, a handwave is completely possible here, but these things will become irrelevant once I implement my solution to aluminum oxide's flexibility and brittleness problems below.

I have a solution to these things, you see; you structure the aluminum oxide bones like limpet teeth. Limpet teeth contain monocrystalline whiskers of goethite - that is, crystals of goethite that are so small that they're flaw-insensitive, meaning that they do not have structural impurities that make larger crystals more susceptible to structural failures. Moreover - and this is the important point - these goethite crystals are embedded in a matrix of collagen, which allows the teeth of a limpet to be flexible, non-brittle, etc.

In addition to that, these goethite crystals have a low critical fiber length relative to their total, meaning that they're very good at transferring loads to the collagen matrix - in other words, they don't need to be very long to act as good shock absorbers.

My solution to this, therefore, is to suspend crystals/fibers of aluminum oxide within a collagen matrix - much like a limpet's teeth are crystals/fibers of iron(III) oxide-hydroxide (goethite) within a collagen matrix - to bring the flexibility of these bones up to a level more comparable to more conventional Earthly life.

All in all, I'd say that aluminum oxide monocrystalline whiskers suspended within a collagen matrix are - mechanically speaking, at least - a much better bone than the ones that actually exist in real life, but of course I'd say that, because I came up with 'em. It remains to be seen whether there are actually problems with them, which is where you come in:

What are some structural or biological weaknesses of using aluminum oxide monocrystaline whiskers suspended in a collagen matrix as a material for bones - i.e., why shouldn't I?

Assume that this creature with aluminum oxide bones is designed to operate under "normal" Earth-standard conditions, on land, at sea level, etc, and that, other than its unique supportive structures, it is essentially a tiger in all other aspects of its biology.

Good answers will point out a problem with these types of bones, and have a sense of biology, physics, and chemistry at least as strong as my rudimentary ones.

Here are three answers that I already have solutions for, and that I would not like people to answer with:

  1. Weight. Aluminum oxide is 3.95 grams/cm^3, whereas bone is ~0.92-1.4 grams/cm^3. These bones will be ~2.75-4 times heavier per unit of mass. I know this, and have found a way around it.

  2. Availability. Aluminum has to be extracted from substances like bauxite, and new metabolic pathways need to be developed to process and handle it. I know this, and have found a way around it.

  3. Toxicity. Aluminum oxide fibers are apparently bad for you; I personally imagine that they're an inhalant risk, but have no hard sources on that. I don't consider this a problem, for various reasons outside of the scope of this question; i.e. toxic bones are awesome.

I was inspired by Logan R. Kearsley's answer to a previous question of mine.

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    $\begingroup$ What're the dimensions of the fibres, the patterns of distribution? Here's a picture of how a bone is constructed to optimise for various factors, not the least of which is blood circulation. How are your bones optimised? $\endgroup$ Dec 9, 2021 at 3:46
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    $\begingroup$ Imagine picking a car and swapping out its crumple zone for a nigh indestructible material and then claiming the car is safer for it. Our bones break when they do for protection. For humans Its easier to survive and heal a bone than it is to survive and heal an organ like a liver, lung or perhaps shattered bloodvessles. The breaking absorbs kinetic energy that would otherwise damage more important organs. You would need some kind of composite material that can both break but partially remain intact for structure somehow. $\endgroup$
    – Demigan
    Dec 9, 2021 at 5:31
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    $\begingroup$ Similarly, an elastic modulus helps to absorb shocks. $\endgroup$
    – Anon
    Dec 9, 2021 at 6:46
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    $\begingroup$ @KEY_ABRADE "Aluminum hydroxide's lack of an ..." Aluminium hydroxide and aluminium oxide are two different substances (and both different from the aluminium oxyhydroxide) $\endgroup$ Dec 9, 2021 at 9:08
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    $\begingroup$ Might run into integrity issues if it's not one solid piece, and your whole structure would only then be as strong as the connective tissues, which as far as I know won't be much if collagen is anything to go by $\endgroup$
    – Lemming
    Dec 9, 2021 at 10:20

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In hydrated environs, aluminium will form preferably its hydroxide

To get it to form aluminium oxide, one needs to calcine it as high as 1100C. This is why one will find the crystalline form of aluminium oxide mainly in geologies which had some way or another to do with heating and pressure (metamorphic or ultramafic). If you get to volcanic rocks, it will be mainly associated with various forms of silicates (together with other metals).

In the context of the question - I doubt you'll find a biochemistry path able to convince aluminium hydroxides to part with their beloved water and be happy with the oxygen only - you simply need too much energy.

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    $\begingroup$ "you simply need too much energy"? I smell a challenge. However, IIRC, you can also react aluminum hydroxide with hydrochloric acid to turn it into aluminum chloride and water, and then turn that into aluminum oxide and hydrochloric acid - hence my comment about gibbsite and repurposed parietal cells. +1 anyway. $\endgroup$
    – KEY_ABRADE
    Dec 9, 2021 at 11:03
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    $\begingroup$ @KEY_ABRADE "aluminum hydroxide with hydrochloric acid to turn it into aluminum chloride and water". Surprise! It won't work. The only route you can prepare anhydrous AlCl3 is by directly burning aluminium in chlorine. The moment you let water get into the picture, the anhydrous AlCl3 hydrolyzes spontaneously, forming HCl and aluminium hydroxide. That is to say: Al is happier with 3 OH than with 3 Cl. (ctnd) $\endgroup$ Dec 10, 2021 at 0:36
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    $\begingroup$ (ctnd) Sure, you can dissolve Al directly in an aqueous solution HCl, what you'll get is $Al(H_{2}O)_{6}Cl_3$. Which is impossible to get dehydrated by simple heating, as it goes on the path of $Al(H_2O)_6Cl_3 → Al(OH)_3 + 3 HCl + 3 H_2O$. You need dry conditions and other substances melted together at 150C+ to recover some anhydrous AlCl3 $\endgroup$ Dec 10, 2021 at 0:47

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