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One of the many problems with making really, really big creatures is that their bones are, eventually, incapable of supporting their own weight. Eventually, they get to be all leg and no body. This is especially a problem when trying to make something that's humanoid but larger than normal, because you have to make the legs elephantine in order for them to support the weight of everything else. The square-cube law is generally a pain in the rear when doing such things.

Now, the obvious solution to this is to make better bones/supportive structures - ones that can withstand more compressive force per unit of bone mass, meaning that you can use less of them, meaning that you don't need to make disproportionately-wide limbs to counteract a creature's mass.

I considered using the goethite nanofiber/protein mixture that limpet teeth use, which has a tensile strength of 3-6.5 gigapascals, but I don't think that that tensile strength translates into the compressive strength required to make an effective bone, since tensile strength is a measure of how durable a substance is when being pulled on, whereas compressive strength is a measure of how durable a substance is when being squeezed.

So: what are some biological substances stronger than bone?

Quality answers to this question will cite a substance that falls into the following four categories:

  • Either capable of being processed by Earthly biology, or capable of being built out of metabolizable sub-components of itself by Earthly biology. No noble gases or non-reactive substances, here, please, although I wonder how they'd ever be a suitable answer to this. Note that those limpet teeth I previously mentioned are made of such a metabolizable substance, in that the limpet turns iron into ferrihydrite, which is then nucleated and turned into goethite crystals (i.e. those nanofibers I mentioned earlier).

  • Stronger than bone in terms of compressive strength. Bone has a compressive strength of 170 megapascals. I want more. I can worry about shear loads later, or counteract them by wrapping the compressive core of the bone (i.e. what carries the weight) in biologically-grown limpet-tooth spaced armor with bone marrow in-between as a shock absorber.

  • Not water or muscle. I do not want hydrostatic skeletons or muscular hydrostats.

  • Has to be possible on Earth. No frozen hydrocarbons. Especially no neutronium. No, not even a little. Bad Stack Exchange. Bad. Drop. Drop the neutronium. Good boy.

Note that I am not interested in determining what evolutionary pressures might lead to the adoption of such a material as a supportive structure; we're talking mad science and the limits of what's possible here, not boring old evolution. I am also not interested in how this might affect the biology of the creature it's attached to - i.e. things like blood flow issues or increased nutritional needs. All I am interested in is whether there are biomaterials with better compressive strength than bone.

Tagged "science-based" because I want this to be an actual material, and not handwavium, but not "hard-science", because whatever this is has probably never actually been used in a supportive structure in real life.

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    $\begingroup$ Not really a biomaterial-related thing so it's a comment, but you might want to look into tensigrity structures. They won't get around the SCL, but you can get some pretty big self-supporting structures(perhaps bodies?) by dedicating less physical space in the construction of dedicated supports. $\endgroup$
    – Rubrikon
    Dec 7, 2021 at 9:43
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    $\begingroup$ Not toxic, poisonous, cancer-inducing, or otherwise harmful to Earthly biology. No heavy metals. No toxins. - why? if your big humans evolve with this, they're obviously immune. if they're manufactured, then technology is advanced enough to protect against those materials. so i think that's kind of a non-issue. also: how big do you want to get? and what are your thoughts on blood pressure - multiple hearts? actively pumping veins? $\endgroup$
    – Franz Gleichmann
    Dec 7, 2021 at 9:55
  • $\begingroup$ @FranzGleichmann I'm not concerned about blood flow issues. Come to think of it, though, that does seem arbitrary. I'll get rid of it. $\endgroup$
    – KEY_ABRADE
    Dec 7, 2021 at 9:57
  • $\begingroup$ How big do you want them? There are lots of examples taller than 250cm, and they did just fine, leg-bone-wise. -- Also: why is bloodflow unimportant? the sole area is incredíbly compressed, as are any compressively forced joints, and those all need their circulation $\endgroup$
    – bukwyrm
    Dec 7, 2021 at 13:27
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    $\begingroup$ What about a pneumatic energy delivery system such as is found in sauropods, that decreases the structural requirements of the bone material by decreasing the mass of the creature while still allowing it to scale? This is one way of obviating some of the supermaterial requirements. $\endgroup$
    – pygosceles
    Dec 7, 2021 at 23:07

4 Answers 4

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Silica - networks of silicon and oxygen with the general formula SiO2.

Diatoms make their shells out of it. Plants incorporate it into their cell walls. Sponges make their spicule skeletons out of it.

You just need to scale it up. Rather than building microscopic shells or a composite matrix of tiny spicules and collagen / cellulose, just keep depositing larger and larger layers of straight silica.

The easiest form to deposit bulk silica in would probably be opal (hydrated amorphous silica), which would look awesome, but isn't super strong--worse than regular bone. But if you can get the organism to exclude water and just produce macroscopic solid chunks of pure silica, you're looking at a compressive strength of around 1100 MPa.

And if you wanna go a little further out there... Sapphire

Sapphire is aluminum oxide, Al2O3. It has a compressive strength of 2 gigapascals, so even if you accept some losses in incorporating it into a biological composite, you're still starting out way ahead of natural bone. Aluminum is not currently know to play any significant role in biology, but it is bioavailable in ion form (e.g., as aluminum citrate) and accumulates in the biosphere, so it should be available in normal food supplies--and if biology can handle laying down oxidized iron crystals, I'm sure something can be worked out for depositing oxidized aluminum!

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  • $\begingroup$ This sounds a bit like you want bones made of glass or concrete. How are they going to handle the stresses real bones do? Some citations would be good. $\endgroup$
    – DWKraus
    Dec 14, 2021 at 17:08
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    $\begingroup$ @DWKraus Not my problem. The question is specifically and narrowly about compressive strength. $\endgroup$
    – Logan R. Kearsley
    Dec 14, 2021 at 17:22
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Abalone-chitin composites (and dinosaur engineering):

Without making the individual materials, it's hard to promise that the resulting substances will be super-materials. But research into artificial bone graft materials suggests a likely composite. Hydroxyapatite tends to be brittle, so more flexible materials are being researched. Another excellent article on biomaterial ceramics like bone and nacre can be found HERE.

  • NACRE, also known as mother-of pearl, has great composite strength. Artificial nacre is being developed as a material due to these properties. It has an extremely high Young's modulus, which is a general evaluation of strength, and has a compressive strength of 300-500 MPa. It is composed of small plates, bricks, hexagons or disks of aragonite that are then separated by elastic biopolymers. In a study of nacre versus whale bone, nacre outperformed bone in studies of toughness, with bone being weaker and more brittle.
  • Your material will need a viscoelastic layer between the plates (platelets) of aragonite, composed of a polysaccharide-type material. I would suggest a silk or chitin material, since it already has a long history of being used for this function, and chitin can surround and fill in where the nacre stacks don't.

Unlike more exotic materials like graphene, these are biologically produced with current biology and don't require your organisms to reinvent the wheel (biologically speaking).

  • Design from Dinos: I suspect (but I'm not a bio-engineer) that the square-cube law will hit your giants in the joints before this material would fail. Pressure on joints is already a constraint on humans, and a regenerative and arthritis-resistant joint design will be important to your answer's long-term success. You may also want to look to dinosaur bone structure to understand how the construction of bone in dinosaurs may have contributed to their ability to support greater weight.

Nacre microstructure

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A quick Wikipedia search returns a list of biomaterials with related compressive strength.

Cortical bone has a reported compressive strength of 100-230 MPa, while Hydroxyapatites is listed at 500-1000 Mpa.

Hydroxyapatite, also called hydroxylapatite (HA), is a naturally occurring mineral form of calcium apatite [...] Up to 50% by volume and 70% by weight of human bone is a modified form of hydroxyapatite, known as bone mineral. Carbonated calcium-deficient hydroxyapatite is the main mineral of which dental enamel and dentin are composed. [...] Hydroxyapatite is present in bone and teeth; bone is made primarily of HA crystals interspersed in a collagen matrix—65 to 70% of the mass of bone is HA. Similarly HA is 70 to 80% of the mass of dentin and enamel in teeth. In enamel, the matrix for HA is formed by amelogenins and enamelins instead of collagen.

Wrapping up, it looks like nature has done already a good job at tuning the bone overall performance.

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    $\begingroup$ And I'd wager that if you increade the HA-part of the bone you loose out on a lot of other necessary traits like flexibility and so on $\endgroup$
    – Hobbamok
    Dec 8, 2021 at 10:54
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    $\begingroup$ Yep. Bones are reasonably bendy and need to absorb shock as well as stress. If you break a bone it can fix itself but if you shatter a tooth it's gone forever. Having bones made of tooth enamel would be a crippling disability. $\endgroup$
    – J...
    Dec 8, 2021 at 15:04
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Stone.

https://commons.wikimedia.org/wiki/File:Acropolis_-_column_of_the_Propylaea.jpg

enter image description here

https://pubs.naturalstoneinstitute.org/pub/2c4ec57c-aef5-8a85-16e0-106c5cada13c#:

A higher compressive strength indicates that the stone can withstand a higher crushing load. The required values range from 1,800 psi (12.45 MPa) for marble to 19,000 psi (131 MPa) for granite.

Your animals would of course not make stones, but would find suitable stones and put them in their bodies. Other animals use environmental objects chosen for hardness - hermit crabs come to mind, using found shells (or anything else of suitable size and shape) as carapaces.

Your creatures will find stones and incorporate them as weight bearing elements. I envision them stacked like the above column, much as our own vertebral column is stacked. Stones will be held in place by tendons and ligaments just as our bones are. They could be endoskeletons or exoskeletons.

The stone - stone interface will naturally wear into shape as the animal moves, forming interfaces the one to the next. A consequence of this is that previously used stones from dead giants will already have wear on them and so would work better as skeletal elements than fresh stones with only abiogenic wear.

I like the idea of technologically sophisticated aliens bringing presents to these giants: ceramic coated metal skeletal elements. Now the giants can get really big!

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    $\begingroup$ Normal ordinary bone is essentially bio-stone. $\endgroup$
    – Zeiss Ikon
    Dec 7, 2021 at 15:34
  • $\begingroup$ @ZeissIkon Agreed. But biostone is not as strong as granite. $\endgroup$
    – Willk
    Dec 7, 2021 at 15:48
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    $\begingroup$ So basically what you need for this answer to work is... an amorphous and hyper adaptive flesh/organ mass that tends to be rather small and unspecialised in form but can take in solid objects for use as structural support and basically construct its own skeleton? $\endgroup$
    – Rubrikon
    Dec 7, 2021 at 18:38
  • $\begingroup$ The highest compressive strength I've found for granite is 200 MPa; most references place it around 130-150 MPa. That's slightly weaker than bone: the human femur, for example, has a lengthwise compressive strength of around 205 MPa. $\endgroup$
    – Mark
    Dec 8, 2021 at 0:36
  • $\begingroup$ @Mark you know it pains me when people accurately comment about how I was not right, and I feel that pain now. So thanks to you these giants cannot use just any rock. It must be quartz crystals with a compressive strength of 2.2 - 2.7 Gpa.. Quartz is not super rare. Crystal skeletons are more fantastic anyway. Jade might be better if they can get it. $\endgroup$
    – Willk
    Dec 8, 2021 at 1:15

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