Interfacially Engineered Alginate/Hydroxyapatite Biocomposites With High Strength and Toughness

ABSTRACT Inspired by the high toughness of cortical bone, we aimed to develop biomass‐based structural materials by hybridizing sodium alginate (ALG‐Na) with hydroxyapatite (HAP) through in situ crystallization. Two protocols were designed: Method A, crystallizing HAP from ALG‐Na in a Na 2 HPO 4 /NaOH mixture; and Method B, crystallizing HAP after ALG‐Na pre‐crosslinking with Ca 2+ in CaCl 2 . Composites synthesized at 25°C–90°C were characterized by thermogravimetric–differential thermal analysis, Fourier‐transform infrared spectroscopy, X‐ray diffraction, three‐point bending, and scanning electron microscopy. Method B, particularly at 70°C, yielded higher thermal stability, stronger organic–inorganic interactions, and improved mechanical properties. In Method A, ALG‐Na carboxylate groups (COO − ) preferentially bound free Ca 2+ , suppressing HAP interactions. Method B enabled mild HAP crystal growth near calcium alginate, promoting COO − interaction with positively charged HAP a ‐planes. The optimized composite showed 13‐, 8.6‐, and 22.5‐fold higher bending strength ( σ b ), elastic modulus ( E b ), and modulus of toughness ( K f ) than pure ALG‐Na. σ b matched poly(methyl methacrylate) and nylon 66; E b matched glass fiber–reinforced poly(butylene terephthalate). K f remained comparable to softwoods, balancing rigidity and toughness. This study provides guidelines for interfacial control in carboxylate biopolymer/HAP composites and has the potential to contribute to the rational design of high‐performance biomass structural materials.