Solution–Air Interface Synthesis and Growth Mechanism of Tooth Enamel-like Hydroxyapatite/Chondroitin Sulfate Films

Hierarchically self-assembled hydroxyapatite (HAP) and organic composite resembling tooth enamel has potential biological and surgery applications. Controlled fabrication remains a great challenge. In this work, large-scale translucent hydroxyapatite (HAP) and chondroitin sulfate (ChS) composite films are fabricated by a unique solution–air interface method. The products excellently represent the characteristic hierarchical "prism" structure of tooth enamel. We demonstrate that the films are formed by evaporation-induced nucleation at the interface and subsequent self-confined spherulitic growth. The supersaturation at the interface increases upon solution evaporation, and the local high supersaturation triggers preferred nucleation there. The spherulitic growth is regulated by ChS and limited by the interfaces between the spherulites as well as the solution and air. Consequently, HAP/ChS films with the hierarchically self-assembled prism structure are produced. We emphasize that the mechanism can excellently explain the unique prism structure formation in tooth enamel and in self-assembled nanorod arrays. The findings provide insight into the fundamentals of hierarchical assembly in nature (e.g., assembly of tooth enamel). Moreover, the approach could be expanded to design and grow other sophisticated functional structures.