Boron-doped nano-hydroxyapatite for enamel demineralization prevention and biofilm inhibition

This study evaluated the effects of boron-doped nano-hydroxyapatite (nHA) formulations, with and without sintering, on enamel surface hardness and Streptococcus mutans biofilm formation under in vitro conditions. Ninety-eight sound human premolars were randomly allocated into seven groups (n = 14): pre-sintered nHA, sintered nHA, 2% boron-doped pre-sintered nHA, 2% boron-doped sintered nHA, 4% boron-doped pre-sintered nHA, casein phosphopeptide-amorphous calcium phosphate (CPP-ACP), and artificial saliva. Enamel demineralization was assessed using Vickers microhardness testing and scanning electron microscopy following a 10-day pH-cycling protocol. Antibiofilm activity was evaluated by cultivating S. mutans biofilms on orthodontic brackets (n = 7 per group). The 2% boron-doped sintered nHA group demonstrated significantly higher enamel surface microhardness compared with all other groups (P < 0.05), with qualitative scanning electron microscopy observations showing a relatively more uniform and intact surface appearance. Significant differences in S. mutans biofilm formation were observed among the groups (P < 0.001). The pre-sintered 2% and 4% boron-doped nHA groups showed significantly lower OD values than artificial saliva, whereas the sintered 2% boron-doped nHA group did not differ significantly from the negative control. Boron incorporation combined with sintering enhanced the capacity of nano-hydroxyapatite to preserve enamel surface hardness under acidic challenge in vitro, with the sintered 2% boron-doped formulation demonstrating the highest final microhardness among the tested materials.