Capillary force and concentration gradient promote bioprocessing-inspired formation of ultralong fluorapatite nanorods under confinement

Crystallization within small volumes rather than bulk solutions is a common phenomenon found during materials synthesis and biomineralization processes. However, the driving forces for mass transport and crystallization under confined environment remain elusive. Here, inspired by the intrafibrillar collagen mineralization process, we investigate the infiltration and crystallization mechanisms of fluorapatite (FAP) within confined channels by comparing anodic aluminum oxide and track-etched template with different surface properties. The results demonstrate that, similar to intrafibrillar collagen mineralization, the capillary force, along with specific interaction between confined channel surface and mineral precursors, is the main driving force for initial infiltration of liquid precursors into confined channels, leading to the nucleation FAP nanocrystals on the surface of channels. We elucidate the critical role of negatively charged polyacrylic acid in promoting the formation liquid precursors for successful infiltration into confined channels and controlling the crystallization kinetics within channels. The formation of FAP nanorods proceed by further promoting the ion diffusion via a concentration gradient resulted from the lower local concentration surrounding FAP crystals. Furthermore, FAP nanocrystals exhibit progressive alignment along the channel direction during subsequent crystal growth stage and ultralong FAP nanorods with a length of more than 25 μm could be obtained. The collective findings underscore the pivotal role of the structure and surface properties of nanoscale confined environment in controlling the infiltration and crystallization pathways of inorganic crystals and establish a foundation for controlled synthesis of biomimetic materials under confinement.