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As Hard as Snails?

Posted on 3 May 2016

International study involving York researchers shows how hardness of calcite crystals can be modified, providing new insights into the mechanical properties of inorganic / organic nanocomposites.

Bea Demarchi and Kirsty Penkman

An international study involving Bea Demarchi & Kirsty Penkman, researchers at York,  has shown how the hardness of calcite crystals can be tuned by incorporating different amino acids, providing new insights into the mechanical properties of inorganic / organic nanocomposites.  Not only is this important for our understanding of biomineralisation, but opens up the possibility of using this strategy to tailor the mechanical properties of a wider range of materials.

Despite being formed from brittle minerals and flexible polymers, biominerals (e.g. shells, corals, teeth) are inorganic / organic composites that exhibit remarkable strength and toughness.  In order to better understand these properties, and in an attempt to control them, an international research project grew and analysed calcite crystals which had incorporated amino acids, organic molecules which have been implicated in biomineralisation.  
The project was led by researchers in Leeds, who grew crystals of calcite within solutions of varying amounts of 2 amino acids, while analyses at York (Bea Demarchi & Kirsty Penkman) and Cambridge showed that the amino acids were incorporated within single crystals as individual molecules.  Computer simulations at Sheffield determined how the amino acid molecules fitted into the crystal lattice, which showed remarkable concordance with the experimental data from York.  Researchers at Technion and the Diamond Light Source characterised the distortions formed by this incorporation of the amino acids within the crystal lattice.  The hardness of the crystals were the largest yet reported in man-made synthetic calcite, and similar to those measured in biomineral calcite.  A team from Cornell determined how far apart the molecules were from each other, and then showed that the greater hardness in the modified crystals (compared to pure calcite) was determined by the force needed to break the covalent bonds within the amino acids.  
This collaborative study therefore provides a window into how organic materials incorporated into minerals can provide such an increase in their toughness, an aspect that has been exploited by nature for the last 541 million years.  The “tuning” of the mechanical properties made possible by this understanding could pave the way for using similar strategies to tailor-make other composite materials.

The research, "Tuning hardness in calcite by incorporation of amino acids" was published on May 2nd in Nature Materials (http://www.nature.com/nmat/journal/vaop/ncurrent/full/nmat4631.html).