IF YOU want to see some inspired design, look no further than the back
garden. From tree trunks to hedgehog spines, nature has come up with some of the
strongest and lightest materials known. Now, researchers are starting to mimic
the natural world to find materials for everything from bullet-proof vests to
jet engines.
Many natural products are hard to beat. Wood can support buildings for
centuries, and is more widely used for making buildings, bridges, furniture and
tools than all other materials put together. Also, natural materials are often
made under harmless conditions, at ambient temperatures and from water-based
solutions, while their synthetic counterparts, such as ceramics or polymers,
often need scorching heat or poisonous solvents.
But biological materials have their disadvantages too. Some, like wool,
shrink at relatively low temperatures, while others are not waterproof, and have
a tendency to rot if kept even slightly damp. Then there is the obvious
inconvenience that they do not come in the ideal sizes and shapes. Nature did
not have tables and floorboards in mind when wood evolved, so around 10 per cent
of all wood ends up as sawdust.
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Pearly king
This is where the science of biomimetics comes into its own. It copies
nature鈥檚 most useful features, and it is now starting to flourish. For instance,
by mimicking the structure of nacre鈥攎other-of-pearl鈥擝ill Clegg at
the University of Cambridge is developing materials for super-tough jet engine
blades. The nacre on the inner layer of many mollusc shells is 95 per cent
chalk, yet it is up to 3000 times tougher than bulk chalk鈥攖read on a
limpet shell and the chances are that the limpet will survive. Nacre owes its
toughness to its composite structure. Most of it is aragonite, a dense and
crystalline form of calcium carbonate, arranged in layers of microscopic
鈥減latelets鈥� about 8 micrometres across and 0.5 micrometres thick. But joining
the platelets is a tenuous 鈥渕atrix鈥� of sticky silk-like protein.
This combination provides toughness in two ways. First, when a heavy weight
is placed on nacre, cracks go through the platelets but are deflected as they
try to cross the protein layers. This dissipates the force, and can stop a crack
in its tracks (see
diagram). Andrew Jackson, working at
Reading University in collaboration with ICI, showed a few years ago that there
is a second strengthening factor. As a crack forms, the protein matrix stretches
out into strands across the fracture. This process absorbs the energy that is
essential for the crack to continue.
Nacre does fall down on one count, though: at around 60 掳C the silk-like
matrix starts to break down. But by combining nacre鈥檚 ingenious structure with
heat-resistant materials, Clegg managed to improve on nacre鈥檚 poor heat
resistance. He rolled out layers of ceramic paste 150 micrometres thick, and
piled them on top of each other. He separated the layers with a dusting of
graphite, and baked the ceramic like a pot in a kiln. This produced a material
that can withstand temperatures of around 1500 掳C, and has all the
durability of a ceramic baking dish. It has also vastly improved toughness
because, as in nacre, cracks are deflected between the layers. In Clegg鈥檚 early
tests, the toughness was three or four times better than nacre.
Spiral wonder
Clegg is now developing the material for the turbine blades of jet engines.
Today, metal turbine blades are linked to cumbersome cooling systems to stop the
blades melting as the engine heats up, often to well over 1000 掳C. Blades
made from Clegg鈥檚 ceramic mother-of-pearl may one day make these cooling systems
redundant (see Technology, 14 January 1995, p 22).
Wood is another of nature鈥檚 wonder material. Weight-for-weight, it is as
strong and stiff in tension as steel. The Second World War鈥檚 Mosquito, one of
the most damage-tolerant aircraft ever, was made of light balsa wood sandwiched
between layers of denser plywood. Wood is also very tough, which makes it a very
鈥渟afe鈥� material. As cracks move through it, the wood breaks relatively slowly so
you can see or hear it happening and have time to do something about it.
Wood is composed of parallel columns of long hollow cells joined end-to-end,
around which fibres of cellulose are wound in spirals and embedded in a matrix
of lignin, a complex polymeric resin. This spirally wound layer contributes up
to 80 per cent of the total thickness of the cell wall, and is the major
load-bearing component. Back in the 1970s, Jim Gordon and George Jeronimidis of
the University of Reading analysed the way this layer contributes to the
toughness of wood, and their work ultimately led to a new type of composite
material.
Their starting point was this: if wood were a simple composite of fibres in a
resin matrix, the toughness would be mainly down to friction, making it
difficult for fibres to be pulled out from the surrounding matrix. But they
discovered that wood is at least ten times tougher than you would expect if this
were the only mechanism at work. Jeronimidis found that wood鈥檚 success is down
to the way that it uses up the energy available for cracking by encouraging the
formation of complex cracks that weaken the material as little as possible. It鈥檚
like pulling the ends of a spirally wound paper art straw. When it breaks, it
does so partly by buckling inwards so that its diameter is reduced, and partly
by the development of a spiral fracture running some distance along the
straw.
Cracked but intact
Wood fractures by the same sort of mechanism. When a wood cell buckles
inwards it breaks away from the surrounding cells, absorbing energy in the
process. A long spiral crack forms parallel to the wound fibres, dissipating the
energy available for cracking over a short length of the wood. And since the
crack runs along between the fibres, within the lignin, the fibres stay intact
so the wood does not fall apart. Although it鈥檚 鈥渂roken鈥�, the wood can still
support a significant load.
Jeronimidis started modelling wood using straws of spirally wound glass
fibres stuck together with resin. Weight-for-weight, this turned out to be 50
times tougher than any other currently available synthetic material. However,
the glass fibres were not very easy to wind and glue together, so Jeronimidis
came up with a cheap and easier way. He used sheets of parallel glass fibres in
unhardened resin which were then folded and glued in a mould to produce a
corrugated structure, on a similar scale to corrugated cardboard (see
below).
In this design, the long gaps between the corrugations mimic the long columns
of hollow cells in wood. The cunning part of the plan was to arrange the fibres
at an angle of about 15 degrees to the corrugation ridges, giving something like
the spiral windings in the wood cell wall. This new material seems to fracture
using a similar mechanism to wood, and has greatly increased resistance to
impact. In Reading we are currently developing this material for protection
against high-velocity particles such as bullets or shrapnel from bombs. We hope
eventually to develop it into lightweight bullet-proof clothing.
Nature has also worked out the best designs for tube-like
structures鈥攑lant stems, say. To have a high degree of stiffness, it is
best to concentrate the material far from the centre. Nature has applied this
idea in the feather shaft, porcupine quills, hedgehog spines and mushrooms.
Nature rarely puts absolutely all the material at the edge, however. Plant
stems and porcupine quills have light foam-like substances in the centre, which
help to support the thin outer walls of the tubes against local buckling; the
structure can then bend further than if it were empty. Take hedgehog spines, for
instance. In the late 1980s, working with Paul Owers, then a student at Reading,
I showed that the internal honeycomb-like structure of the spines of the
European hedgehog, Erinaceus europaeus, are extraordinarily strong and
flexible. The honeycomb supports the spine so well that the hedgehog can fall
several metres without harm, and simply bounces as its spines hit the ground.
Inspired by the bouncing hedgehog, Gordon invented a new type of puncture-proof
wheel-covering where the tyre is replaced by lots of little hedgehog spines
which then have the shock-absorbing properties of rubber. You could use the same
idea to replace the rubber in running shoes.
Last year, Lorna Gibson of the Massachusetts Institute of Technology in
Cambridge showed, by comparing mathematical models with experimental results,
that this honeycomb structure is much more effective even than the foam-like
structure in plants. This should be of great interest to designers of
lightweight structures from racing cars to space stations. The tubes for the
front suspension of racing cars are sometimes made of foam-filled aluminium
alloy tubing, but the message from hedgehogs is that a honeycomb is better than
foam. Once again nature leaves us behind, but at least we are on the trail.