The conch shell, symbol of juvenile democratic power on a desert island in William Golding's Lord of the Flies, also harbours a secret power of its own. The shells of sea snails are composed of about 95% calcium carbonate, the same material as crumbly chalk, yet they are a thousand times tougher. The other 5% is mainly organic matter. Writing in the International Journal of Materials Engineering Innovation, researchers at the University of Cambridge use the example of the conch shell as an illustration of toughness-by-architecture in the quest for new synthetic materials for engineering, construction and aerospace applications.
David Williamson and Bill Proud review how these organisms build such tough shells from such a seemingly weak substance. They discover that the key to conch strength lies in the small size of the calcium carbonate crystals from which it is formed by the sea snail. The crystals are below a threshold size known as the Griffith flaw size, any bigger and the crystals would be large enough for cracks to propagate through them under stress, the team explains. This makes the shells tough enough to cope, to some extent, with the crushing jaws of predatory turtles and the vice-like grip of crab claws. Weight for weight the shells are as tough as mild steel.
In the early twentieth century, engineers were preoccupied with the premature failure of materials used in shipping and railways. Concepts such as stress magnification and the propagation of tiny cracks that grow to form big cracks were beginning to be understood. Civil engineer Charles Edward Inglis Inglis devised a mathematical equation to help explain the process. And, in 1920, Alan Arnold Griffith built on the Inglis work to explain for the first time that the reason materials in the real world are not as strong as theoretical calculations would suggest is that the presence of tiny flaws magnify the applied stress in a manner according to the Inglis analysis leading to premature failure.
"Griffith pointed out that the effective strength of technical materials might be increased many tens of times if these flaws could be eliminated," explain Williamson and Proud. Little was known at the time of biomaterials and how their properties might one day copied to create biomimetic materials of much greater strength than their industrial counterparts. Griffith's work has now been used to improve our understanding of conch shells and other biomaterials to allow scientists to produce novel composite biomimetic materials. Research in this area has seen almost exponential growth in the last decade.
The team explains that in the archetypal conch shell material, the queen conch (Strombus gigas) uses a crossed layered, or lamellar, structure. At the smallest length scale the shell is made from tiny crystals of calcium carbonate in the so-called orthorhombic polymorphic form of aragonite. Each single crystal is a mere 60 to 130 nanometres thick and about 100 to 380 nanometres across, although they can be several micrometres long. A nanometre is a billionth of a metre; a micrometre is a thousand times bigger, a millionth of a metre. These dimensions, the Cambridge team explains are below the critical flaw size described by Griffith almost a century ago.
To make a biomimetic material, researchers might first adopt the small crystal size for their composites as well as the crossed layered structure of the conch shell. However, to be truly biomimetic, such materials will also have to incorporate another critical feature of the living material: the ability to self-heal. Attacked by a hungry turtle the shell of a queen conch might be strong enough to deter the predator, but damage will occur, but living tissue can carry out repairs. Materials scientists have discovered that certain polymers can be heat treated so that they undergo self-healing, extended research might allow crystalline composites that mimic conch shell to be made that have the same property.
The team concludes that, it is important to treat these biomaterials as sources of inspiration, rather than prototypes to be replicated in exquisite detail. After all, if nature had access to a modern, high-tech material like the extremely tough ceramic titanium boride used in aluminium smelting equipment and electrical discharge machining, would seashells look the same as they do now?
- David Williamson and Bill Proud. The conch shell as a model for tougher composites. Int. J. Mater. Eng. Innovat, 2011, 2, 149-164
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