Electric Field-Guided Biomimetic Mineralization of Enamel via Interfacial Engineering of Nanostructured HAp/PDA Coatings with Anisotropy and Enhanced Hardness

Human enamel is a hierarchically organized hydroxyapatite biomineral, but it lacks regenerative capacity, motivating interfacial engineering strategies that control mineral growth at the enamel surface. This work examines an electric-field-assisted biomimetic mineralization approach that forms organized organo-mineral coatings composed of nanocrystalline carbonate-substituted hydroxyapatite (ncHAp), amino acids (AAs), and polydopamine (PDA). The method utilizes isolated electrodes to apply an electrostatic field, eliminating direct current through the substrate and directing the assembly at the interface. Crucially, this configuration with fully insulated electrodes prevents Faradaic current flow-through biological tissue, establishing a safe foundation for this in vitro proof-of-concept study. Grazing-incidence X-ray diffraction confirms the hydroxyapatite phase and shows a strong preferred orientation, which is reflected in a markedly increased crystallinity index due to the formation of a textured, highly oriented architecture. Electron microscopy, atomic force microscopy (AFM), and synchrotron nano-IR imaging (SINS) reveal densely packed ncHAp/AA/PDA nanoagglomerates with a core-shell architecture. Machine-learning clustering of SINS hyperspectral maps identifies nanoscale chemical heterogeneity and phosphate-rich oriented domains. Crucially, the coating exhibits enhanced mechanical properties: Vickers microhardness measurements and AFM-based nanoindentation show that the coating-substrate system exhibits a higher apparent surface hardness compared to that of native enamel under the tested indentation conditions, demonstrating the reinforcing effect of the textured, electric field-assisted composite layer. This enhancement in the composite coating-substrate system is attributed to the textured assembly of ncHAp nanocrystals reinforced by a PDA/AA interphase. The work elucidates the mechanism of electric-field-guided assembly, establishing a route for fabricating structurally organized, hard biomimetic coatings on enamel. This study establishes fundamental principles of electric-field-guided mineral assembly on enamel surfaces and provides a versatile platform for engineering high-performance biomimetic interfaces, with potential relevance to future noninvasive enamel restoration approaches.