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Composites: Background

The behaviour of interfaces is crucial in determining the properties of regular composites, and is even more important for nanocomposites in which, for biocomposite systems such as bone, enamel and nacre, the filler particles are usually nanoscopic particles of inorganic material, and the matrix consists of an organic polymer. By controlling the structureof both the inorganic phase (e.g. via “templating” or stabilization of charged groups) and organic phase (e.g. via confinement or surface-induced phase transitions) and how it affects the propertiesof the organic-inorganic interface, we seek tounderstand some of the current outstanding challenges in bio-nanocomposite and hybrid systems in areas that are most likely to lead directly to industrial or therapeutic applications.

Bone, arguably the best-studied biocomposite, represents one extreme where an organic phase (collagen) has been stabilised by the secondary in-growth of hydroxyapatite crystallites, comprising 50% by volume of the total material. The tissue is dynamic and undergoes cycles of formation and destruction (bone modelling and remodelling) that may lead to complications notably osteoporosis, a disease which set to ‘ overwhelm’ the NHS with an estimated 4M cases by 2025. A large biopolymer (collagen) controls the overall form of the material, but various types of small molecule can have a significant effect on rate of bone resorption via their binding to the mineral phase.

Coral is a more ‘typical’ biocomposite, in which the organic material plays a role in manipulating the initial growth of the calcium carbonate crystallites, using small molecules (inhibitors) and larger macromolecules (e.g. polysaccharides, chitin). The resulting biomineral which, (unlike bone) is not remodelled contains only small amounts of organic material (< 2%) persisting within a composite mesocrystal. The organic matter may simply be a remnant of the original process of biomineralization or may play a role in altering the material properties of the mature skeleton. We will use the methods developed for bone to study distribution of organic and inorganic material in corallite microstructure using mesoscale methods.