26 July 2012
The material, known as nacre, is found on the outside of pearls and the inside of some molluscs. It is a composite made up of inorganic layers of calcium carbonate and separated by organic polymers, such as chitin. This layering gives nacre its iridescent sheen and makes it damage proof.
The Cambridge scientists recreated the biological steps that form nacre in molluscs to create a material that has a similar structure, mechanical behaviour, and optical appearance to naturally occurring nacre.
Prof Ullrich Steiner from Cambridge University’s Physics Department told The Engineer that the development of the material came about following a student project.
‘My student, Alex Finnemore, conducted a literature study to learn what is known about the growth of nacre in molluscs,’ said Steiner. ‘He then translated each of these steps into procedures that can be carried out in a fashion that does not require the help of biology.’
First, they had to try and stop calcium carbonate, which is the primary component of nacre, from crystallising when precipitating from a solution.
‘Somehow you have to tell the material to actually form a layer,’ said Steiner. ‘Nature does this by controlling the environment in such a way that the material does not crystallise when it precipitates from water.’
The Cambridge team relied on a mixture of ions and organic components in the solution that mimicked how a mollusc prevents crystallisation. The precipitate was then able to be absorbed by surfaces, forming layers of well-defined thickness.
The precipitate layer was then covered by a porous organic layer with the help of a synthetic procedure developed by Finnemore.
Finally, crystallisation was induced and the steps were repeated to create a stack of alternating organic and inorganic layers.
‘Nacre inherits the hardness of calcium carbonate and the ductility of the polymer/protein,’ said Steiner. ‘Whether or not it is interesting as an engineering material is debatable.’
As in nature, the process of developing nacre takes many hours and Steiner believes this could limit its applicability.
‘The advantage of our approach is that it happens at ambient temperatures in water, using very cheap materials,’ he said.
The automised procedure relies on a dip-coating robot to cycle the sample through the various stages. ‘This could in principle be scaled up to an industrial level,’ said Steiner. ‘However, the intrinsic slowness of this process limits the applicability of this type of nacre manufacture.’