Conch Shell Gives Nano Insights into Composite Materials

Researchers at the University of Cambridge use the example of the conch shell as illustration of toughness-by-architecture in the quest for new synthetic materials for engineering, construction and aerospace applications.

The shells of sea snails are composed of about 95% calcium carbonate, 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 illustration of toughness-by-architecture in the quest for new synthetic materials for engineering, construction and aerospace applications.

David Williamson and Bill Proud discovered 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.

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 micrometers long.

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.

Composite Materials