Multiscale structural analysis of nacre-like HAp-based composites using small-angle X-ray scattering tensor tomography

OBJECTIVES: Biomimetic hydroxyapatite (HAp)-based composites are promising materials for dental restorations due to their hierarchical structure and similarity to natural dental tissues. This study aims to investigate the three-dimensional crystallographic organization of HAp within nacre-inspired composites and to evaluate how different polymers infiltrations influence the structural orientation. METHODS: Nacre-inspired HAp ceramic scaffolds were fabricated via bidirectional freeze-casting and subsequently infiltrated with different polymers, including Polyurethane (PU), Poly(methyl methacrylate) (PMMA), Epoxy, and Urethane dimethacrylate (UDMA). The three-dimensional structural organization and crystallite orientation of these composites were investigated using synchrotron-based 3D SAXS tensor tomography (3D SASTT), complemented by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). RESULTS: The results reveal distinct differences in crystallite alignment among the composites. HAp/PU exhibits the highest degree of preferred orientation (∼0.7-0.8), whereas HAp/PMMA and HAp/Epoxy show lower alignment values (∼0.2-0.4). The HAp/UDMA composite displays heterogeneous orientation with localized regions of moderate alignment. SEM and EDX analyses confirm variations in lamellar morphology, polymer infiltration, and porosity distribution across the composites. SIGNIFICANCE: These findings demonstrate that 3D SASTT enables quantitative mapping of nanoscale crystallite orientation within bulk biomimetic scaffolds and provides new insights into the hierarchical structure of composites, supporting structural design of advanced dental restorative materials.