Hard-soft matter interfaces: from understanding to engineering

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The term material is extremely broad, so for simplicity's sake, materials are often described as either hard or soft . While hard materials such as ceramics are strong, they are often brittle. In contrast, soft materials such as polymers are often mechanically weak, but can show valuable elastic properties. Combining these two in one new composite can therefore give rise to remarkable new materials, which benefit from the advantages of both components. This is just one benefit of combining hard and soft materials. In fact, interaction between hard and soft materials occurs in all walks of life. Whether a medical implant is accepted in the body depends on how cells recognise and interact with the hard implant surface. Controlling this requires that we understand molecular-scale processes, which govern how soft biomolecules interact with surfaces - and also processes occurring on much larger length-scales, most importantly how cells interact and recognise a hard surface. In this case, the soft matter must adapt to the hard surface, potentially changing its shape and chemical properties. This is important for many applications - from the toxicology of nanoparticles to strategies for environmental remediation. Perhaps surprisingly, it is not only hard materials which control the soft - the converse also occurs. Biomineralisation - the formation of mineral structures such as bones, teeth and seashells by organisms - shows this beautifully. It is through interaction of growing minerals with soft, organic matter that Nature produces these materials with their remarkable shapes and properties. Biominerals are often very different from synthetic minerals. While a crystal of calcite (calcium carbonate) precipitated in the lab has a regular, geometric form, in the spines of a sea urchin a calcite single crystal is sponge-like, with curved surfaces replacing flat crystal planes. Biominerals are also almost always composites - soft organic molecules are embedded within the crystal. It is this structure which gives biominerals such wonderful mechanical properties - indeed, tooth enamel is one of the hardest materials known. Soft matter not only affects the properties of biominerals, but controls almost every stage of their formation - from the earliest stages of nucleation, through growth, to production of the final biomineral. Insoluble organic molecules define the special environments in which biominerals form and nucleate, while small, soluble organic molecules bind to a crystal during growth, influencing its shape. Clearly, understanding how soft and hard materials interact and control each other is of great importance, and has applications spanning disciplines from medicine to geology, from climate science to nanotechnology. The strategies used by biology to produce biominerals can be applied to the design and fabrication of new materials - where the structure can be controlled at the atomic scale, and the synthesis carried out under mild conditions. If we can design molecules to attach to surfaces strongly, we can use them to inhibit crystal growth. Crystals growing where they should not - in boilers, heating systems and oil wells - remains a major problem in industry and domestic life. Finally, many biomaterials are carbonates. They are a part of the planet's carbon cycle - a major way in which carbon dioxide is removed from the atmosphere for long periods. In the oceans, structures such as coral reefs are under threat due to changes in oceanic conditions; we need to understand the mechanisms of their growth to understand fully why. Removing carbon dioxide from the atmosphere and converting it into carbonates is a possible carbon capture strategy. The research carried out in this grant will use both experiment and theory in a unique way to shed light on the fundamental mechanisms behind this most fascinating and essential capability of the biosphere and to harness this knowledge to develop of novel materials.