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bone_steel_Abdelrahman.pdf
1. OPINION
MARCH 2010 | VOLUME 13 | NUMBER 3
6
It’s a starting point for thinking about structural
materials in nature, and in engineering. In works of
fiction, metal is the material of choice for skeletons,
both exo- and endo-, from the bionic man (and
woman) to Dr Who’s cybermen. In the real world we
make bone replacement parts such as artificial hip
joints from metal alloys, including stainless steels,
titanium 64 and cobalt-chromium alloys.
Suppose I was to make you an offer: you can go
into hospital and have all your bones taken out and
replaced with nice shiny new ones, made from an
engineering material of your choice. You could have
metal alloy, or maybe you’d prefer a carbon fibre
composite? I suspect that you would be somewhat
reluctant to take me up on this fine offer. Apart from
a (perfectly natural) suspicion of putting yourself in
the hands of the medical profession, you probably
think that the bones that nature gave you are
likely to be the best possible solution, having been
refined for their purpose through millions of years of
evolution. This view is very common, encouraged by
science writers of the “Nature is Wonderful” school
of thought, who seem to be dedicated to explaining
the marvels of nature and to telling you that us poor
human beings can’t possibly make anything as good.
So let’s look at the facts. The bones in your body
are made from material which has a tensile strength
of 150MPa, a strain to failure of 2% and a fracture
toughness of 4MPa(m)½. For a structural material
that’s not good. We can make alloy steels that are
ten times better in all three of those properties. But of
course there are some other factors we need to take
account of in order to make a valid comparison. Bone
is less dense than metals and this is important because
the weight of our bones strongly affects the energy
needed to move around. To do a quantitative analysis
we need to consider the geometry and loading on the
structure. The major bones are mostly tubular in shape,
loaded in compression and bending. So a rational
comparison is to imagine tubes made from different
materials, all having the same length and diameter,
with their thicknesses adjusted to give them all the
same weight. Putting in some typical dimensions and
material properties we find that the stresses in a bone
made from titanium alloy, for example, would be about
1.3 times higher than in a bone of the same weight,
made from bone. But the titanium alloy is 5 times
stronger so obviously its safety factor is much higher.
There is another important property which
engineering materials don’t have, and certain
biological materials do, which is self repair. A
broken bone will heal, and in fact your bones are
continually being damaged as a result of the cyclic
loads experienced in normal activities. Small fatigue
cracks initiate and grow; you would fall apart from
fatigue within a few years if it were not for the fact
that these cracks are continually being detected and
repaired. This continuous maintenance process is
carried out by living cells; we still don’t completely
understand how it’s done and it’s a fascinating area
of research. But if you had metal bones they wouldn’t
ever need repairing: titanium alloy for example has
a fatigue strength of about 500MPa which is more
than five times greater than the stresses that it would
experience in its life as a bone. And in an impact
situation the metal bone would probably bend, not
break, and could simply be bent back into place.
Another argument that’s often made about bone
is to say that it has a “unique combination of
properties”. What this comes down to is that it
has a relatively high strength, combined with a
relatively low Young’s modulus. It’s true that if we
wanted to make a material with the same modulus
as bone, about 15GPa, we would probably have to
use a composite material and it would be difficult
to match the strength in that case. But what’s the
virtue of this particular combination of properties?
Having a relatively low modulus and high strength
means a large area under the stress/strain curve,
and thus a large amount of energy absorbed in
straining. This energy is of two types, both of
which can be useful. For stresses below the yield
point we have elastic strain energy, which can be
stored and released with relatively little loss. This
is very important in dynamic situations: when you
are walking or, especially, running, energy is stored
during one part of the gait cycle and released a
fraction of a second later. Most of this energy is
stored in your bones, muscles and tendons. At
stresses above the yield point we have energy which
Why are your bones not made of steel?
David Taylor | Trinity College, Dublin | dtaylor@tcd.ie
In science it sometimes pays to ask silly
questions. So let me ask, “Why are your
bones not made of steel?”
The Cybermen: where would they be without a
metal skeleton?
2. MARCH 2010 | VOLUME 13 | NUMBER 3 7
OPINION
is absorbed and not released: this is very useful in
impact fracture situations. So yes, it’s important to
have a large area under the stress-strain curve, but
(you’ve guessed it) many engineering materials are
superior to bone in this respect as well. Steel has
about the same elastic energy but about 25 times
the total energy absorption of bone. A typical carbon
fibre composite has a similar total energy but about
10 times the elastic energy of bone.
If you’re the kind of person who is more convinced
by a real life example, then take Oscar Pistorius, a
South African athlete who has the slight disadvantage
of having no legs. But maybe it’s not such a
disadvantage after all. He runs using legs made from
carbon fibre composite, and was for a time prevented
from competing against able-bodied runners when
scientific analysis concluded that his artificial legs give
him an unfair advantage. I rest my case!
We human beings should be proud of ourselves,
especially those of us who are materials scientists
and engineers. After a long and glorious history,
metallurgy and materials science reached the point
where, some time early in the twentieth century,
natural organisms started to make materials that
were actually better than those in their own bodies.
Now this is all very well, I hear you say, and if I do
happen to lose an arm or a leg it’s comforting to
know that I could get a good replacement, but my
body can’t make steel so what’s the use? This brings
me on to my next question: why have our bodies, and
those of other animals, evolved so as to make the
particular structural materials that they do make, and
not to make others, especially metals? This is a bit of
a mystery. As far as I know, there are no organisms
that make use of metals as structural materials, but
we all have lots of metallic elements in our bodies.
There doesn’t seem to be any fundamental reason
why an animal couldn’t evolve which makes steel,
for example. It would get its raw material like we do,
from iron ore. The activation energies for oxidation
and reduction of iron are of the order of 30-60KJ/
mole, comparable to the figure of 57KJ/mole for ATP,
a molecule which is commonly used for delivering
energy around our bodies. We normally make iron
from its ore at very high temperatures, because the
rate-limiting process is diffusion in the solid state.
But the body makes materials in a very different
way, from the bottom up, atom by atom, molecule
by molecule. And of course the fact is that you are
already oxidizing and reducing iron inside your body
all the time. Haemoglobin, which is the molecule that
carries oxygen around in your blood, works by having
a single Fe ion at its centre, whose oxidation state
can be changed to allow the molecule to take up, or
release, oxygen atoms.
So it seems that there are no fundamental reasons
why animals could not evolve a metal skeleton.
Maybe they already exist on some other planet. If
so, what would they be like? Well, assuming they
had the same body form as we do, and were subject
to the same gravity, then they could afford to be a
lot bigger. With a bone material that’s four times as
strong as ours, one can use scaling laws to estimate
that the entire body could be four times taller, 64
times heavier. These seven-metre tall giants would
get their raw materials by eating rocks, which would
be no problem since their teeth would be made of
case-hardened steel. Let’s hope they are friendly!
Evolution is wonderful of course, but it has its
limitations, and it’s better at doing some things
than others. For example, evolution is very good at
changing the shapes of animals. Take mammals for
instance; all mammals have basically the same set
of bones, the only thing that distinguishes me from
a mouse or an elephant is the size and, to a lesser
extent, the shape of each bone in our respective
bodies. The breastbone of a bird is, relatively speaking,
much larger than yours because it’s the point of
attachment for the major muscles used in moving
the wings during flying. These kinds of morphological
changes can happen gradually from one generation to
the next, allowing species to adapt. When it comes to
materials, however, nature is much more conservative.
Virtually all biological materials, whether found in
animals or plants, insects or fish, are fibre composites
made up of proteins and polysacchrides, reinforced
with ceramic particles based on calcium or silicon
compounds. As far as we know it has been thus since
the dawn of time. For a couple of billion years there
were no hard materials, at least if they were they left
no record in terms of fossils. Around a half a billion
years ago nature seems to have discovered the trick of
making hard materials by the process of precipitation.
Bones, for example, are very soft when first made,
consisting largely of the protein collagen. Over a
period of months they gradually harden, thanks to
precipitation of the calcium compound hydroxyapatite
(HA), a process which is monitored and controlled by
cells living in the bone. Exactly how the cells do that
we’re not sure, and when things go wrong it leads
to crippling diseases such as osteoporosis, so it’s a
matter of great interest.
Probably I shouldn’t be so hard on nature, because
there is something about bone which is quite
remarkable. It has reasonable properties considering
the terrible stuff that it’s made from. The materials
involved – collagen and HA – are very poor compared
to engineering polymers like epoxy and reinforcing
fibres such as carbon or glass. Researchers have
tried to make artificial bone using nature’s material
and the results are nowhere near the mechanical
properties of real bone. The trick, and this is where
nature’s bottom-up approach is so successful, is to
make a nanocomposite. The size of the HA crystals
(a few microns thick) is similar to the critical defect
size for this brittle material, thus optimising its use.
There is also some important structure at the hundred
micron scale: features called osteons perform a similar
function to grains in other materials, acting as barriers
to crack growth and thus improving toughness. There
are some tricks here that we can learn from when
developing nanomaterials for structural purposes.
If you would like to know more about bone
and other structural biological materials, I can
recommend two excellent books: John Currey’s
Bones: Structure and Mechanics and Julian Vincent’s
Structural Biomaterials, both published by Princeton
University Press. Two recent review articles of mine
provide more detail about current research on bone
biomechanics (Taylor, D., Hazenberg, J. G., and Lee,
T. C., Living with cracks: damage and repair in human
bone. Nature Materials (2007) 6, 263, and Taylor,
D., Fracture and repair of bone: a multiscale problem.
Journal of Materials Science (2007) 42, 8911.).
Oscar Pistorius, complete with carbon fibre legs.