Credit: © AAAS 2008

Natural materials such as bone, wood or shell attain remarkable strength and toughness through complex combinations of brittle minerals and softer materials, which scientists would like to replicate. Now, Robert Ritchie and co-workers at the Lawrence Berkeley National Laboratory and the University of California in Berkeley have made1 incredibly tough new hybrid materials by mimicking the structure of nacre (mother of pearl), found in the shells of molluscs such as abalone.

Nacre consists of calcium carbonate platelets, which provide strength, bonded by a thin layer of organic material that acts to dissipate strain. The composite is far tougher than normal calcium carbonate. Ritchie and co-workers replicated the nacre structure by carefully controlling the freezing of a ceramic suspension in water. They managed to make ice crystals grow in two dimensions, which forced the ceramic (alumina) into platelets just five micrometres thick. The ceramic platelets were then stuck together by adding a polymer (polymethyl methacrylate), in the same manner as bricks and mortar.

This new hybrid material matches the strength and toughness of the best aluminium alloys, and is even more resistant to fracture than nacre. It shows the great potential of imitating biological materials, producing toughening mechanisms that act from the nanoscale to the microscale.