Crystal Growth in Confined Volumes

A fundamental characteristic of biological systems is that their organisation and function are based on compartmentalisation. Indeed, there is increasing realisation that biological and chemical reactions can be dramatically affected by confinement, stimulating a rapidly growing interest in this effect. Biomineralisation processes are no exception to this, and it is well-recognised that a key step in the biological control of mineralisation is the initial construction of a “privileged environment” within the organism in which mineralisation occurs. Within these localised microenvironments organisms actively select the mineral phase, and determine the morphology, orientation and location of the biomineral product through control of precursor ions and phases, and via interaction with soluble organic macromolecules and insoluble organic matrices. However, despite the fact that crystallisation invariably occurs within such confined volumes, experiments modelling biomineralisation are invariably carried out in bulk solution.

As a major research effort, we are investigating the effects of confinement on crystallisation over length scales ranging from the micrometer to the nanometer level. A range of experimental systems are being used including controlled-pore glasses, track-etch membranes, droplet arrays, gels, and droplets created within microfluidic devices. These experiments have demonstrated that confinement can have a significant effect on crystallisation even at remarkably large length scales, enabling control over features including nucleation and growth rates, polymorph, orientation, single crystal/ polycrystalline structure and morphology. The mechanisms underlying such control are currently being investigated. Therefore, while precipitating minerals within localised environments is fundamental to biologically-controlled biomineralisation processes, it can be suggested that confinement – and specifically the large ratio of mineral/ organic surface area to mineral volume – also provides organisms with an additional mechanism of control over mineral formation.

Figure. Calcium sulfate crystallised within 50nm diameter titania nanotubes.