Super shells
by Jonathan Sarfati
The prestigious evolutionary journal Nature marvelled
at the giant conch shell. Nature said that this shell is ‘one of
nature’s greatest engineering masterpieces’.1
So what makes it so special?
The animal grows its shell by first depositing an organic outer layer called the
periostracum. This forms a base for tiny elongated crystals to grow, pointing
at right-angles to the membrane. This layer is only one micron (1/25,000 inch)
thick. So more mineral grows on top to a thickness of a few millimetres (about
1/8 inch).
This has a three-layered criss-crossing structure. It comprises rods of aragonite,
a form of calcium carbonate (CaCO3), held together by protein glue. This
makes a shell with 99% mineral and 1% protein. The rods in each layer line up at
90° to those in the adjoining layer. Further, each rod is composed of even smaller
rods, and these comprise rods that are smaller still. And so on,
down to individual crystals.2
Dr Roberto Ballarini, Material Science Engineer at Case Western Reserve University,
investigated the shell’s strength. He showed that its arrangement makes it
hard for a crack to travel through the entire structure. So although aragonite is
very brittle, the architecture means that it is ‘one of the toughest brittle-natured
composites known to man’. In fact, it’s 30 times stronger
and about 1,000 times tougher (more resistant to fracturing) than the pure mineral.3 He hopes to be able to copy this structure ‘for
small electronic hardware to make it tougher and more resilient’.2
And the living conch does something no man-made material can do—repair itself.
Dr Ballarini’s colleague, Dr Su Xiao-Wei, has shown how the conch repairs
holes. Within 24 hours, the conch seals a wound with a transparent membrane. Then
it deposits tiny aragonite crystals, forming many fine layers. Only then, after
6–8 days, does the conch deposit the elongated crystals followed by the amazingly
tough cross-layered structure.1
The repair process requires fine coordination of the organic and mineral layers.
Dr Su and colleagues hope that their research will show how to design tough man-made
materials. However, they still need to find out how this process is controlled at
the molecular level. And when they discover the way the genes program this, they
will learn yet another lesson from the Programmer (cf.
Romans 1:18–23).
Further reading
References
- Daw, R., Give a shell a break, Nature 427(6976):691,
19 February 2004. Return to text.
- Ballarini, R.,
Cracking the conch conundrum: tough ceramics at the seashore, Distinguished
Lecture Series, Cornell University, 11 February 2003. Return
to text.
-
Queen conch shell suggests new structure for ceramics, 30 June 2000; based on
Kamat, S., Su, X., Ballarini, R. and Heuer, A.H., Structural basis for the fracture
toughness of the shell of the conch Strombus gigas, Nature
405(6790):1036–1040, 29 June 2000. Return to text.
|
