Eggshell-derived hydroxyapatite as a bioactive filler for toothpaste: rheology and enamel remineralization...

Sustainable and fluoride-free alternatives for oral care products are increasingly sought. This study aimed to valorize eggshell waste as a calcium source for the synthesis of nanocrystalline hydroxyapatite (HAp) and to evaluate its suitability as a multifunctional filler in toothpaste formulations, focusing on rheological, viscoelastic, and enamel remineralization performance. Hydroxyapatite was synthesized from eggshell-derived calcium oxide via wet chemical precipitation and characterized using particle size analysis, FTIR, and XRD. Toothpaste formulations containing 0, 3, 5, and 7 wt% HAp were prepared. Steady shear and dynamic oscillatory rheological measurements were performed and fitted using the Herschel-Bulkley model. In vitro, remineralization was assessed on artificially demineralized bovine enamel using Vickers microhardness testing before demineralization, after lesion formation, and after treatment with HAp-based toothpaste. The synthesized HAp was phase-pure and nanocrystalline, with an average crystallite size of approximately 12-14 nm. All formulations exhibited non-Newtonian shear-thinning behavior with a well-defined yield stress. Increasing HAp content significantly enhanced yield stress, consistency index, storage modulus, and critical stress, indicating progressive reinforcement of the internal microstructure. The formulation containing 5 wt% HAp showed the most balanced viscoelastic properties. Artificial demineralization reduced enamel microhardness by approximately 53% compared with sound enamel. Treatment with HAp-based toothpaste significantly increased microhardness (p < 0.001), restoring values to levels not statistically different from sound enamel. The calculated remineralization percentage reached 115.97%, suggesting effective mineral deposition and surface reconstruction. Eggshell-derived hydroxyapatite acts as both a rheological reinforcing agent and a biomimetic remineralizing material in toothpaste formulations. An optimal loading of 5 wt% provides improved mechanical integrity and controlled flow behavior while promoting substantial enamel remineralization under vitro conditions. This study supports the use of eggshell-derived hydroxyapatite as a sustainable biomimetic remineralizing agent for fluoride-free or fluoride-complementary toothpaste formulations, combining favorable handling properties with significant enamel remineralization potential.