The application of nano-hydroxyapatite (HA) in the repair of early caries lesion has received considerable attention. Neither the effects of the size of HA nor the effects of the effective pH range of nano-HA on remineralization have been investigated comprehensively, and the protective mechanism is still open for debate. To address these factors, the remineralization effect of nano-HA on demineralized bovine enamel is investigated under pH cycling conditions through surface and cross-sectional microhardness (CSMH) tests and polarized light microscopy (PLM). The percentage of surface microhardness recovery and integrated mineral loss obtained from CSMH tests demonstrated that nano-HA provides better remineralization than micro-HA. However, detailed investigation using CSMH tests and PLM indicated that nano-HA helped mineral deposition predominantly in the outer layer of the lesion and only had a limited capacity to reduce lesion depth. Nevertheless, the remineralization effect of nano-HA increased significantly when the pH was less than 7.0. Clearly, nano-HA has potential as an effective repair material and anticaries agent. Our findings also suggest that both the particle- and ion-mediated remineralization pathways in nano-HA may contribute to the repair of demineralized enamel.

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