Phosphate-treated siwak as a calcium phosphate–bearing bioactive material for fluoride-free oral care: physicochemical characterization and in vitro enamel microhardness recovery

Phosphate-treated siwak ( Salvadora persica ) was developed as a calcium phosphate-bearing bioactive material and evaluated for its in vitro enamel remineralization potential when incorporated into a fluoride-free toothpaste. Siwak powder was phosphate-treated with phosphoric acid under controlled conditions and characterized by X-ray fluorescence (XRF), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM/EDS), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD). Fluoride-free laboratory toothpastes were formulated with phosphate-treated siwak or hydroxyapatite (HAp) as the active material. Twenty independent bovine teeth were allocated to four groups ( n = 5 per group). Surface Vickers microhardness was evaluated at the tooth level, with five indentations averaged per specimen, and analyzed using Welch’s one-way ANOVA followed by Games–Howell pairwise comparisons. Phosphate treatment produced Ca/P-bearing signatures by XRF/EDS together with phosphate-associated FTIR and XRD features. Mean surface microhardness was 265.93 ± 2.12 HV in sound enamel, 127.93 ± 5.93 HV in the post-demineralization reference group, 300.19 ± 5.08 HV after the 7-day phosphate-treated siwak protocol, and 295.25 ± 13.31 HV after the HAp protocol. Overall group differences were significant (Welch ANOVA: F(3, 7.71) = 813.21, p < 0.0001), while no statistically significant difference was detected between the two treatment groups ( p = 0.863). Enamel FTIR showed qualitative condition-associated differences in phosphate-region features. These findings support preliminary proof-of-concept evidence that phosphate-treated siwak can participate in a treatment protocol associated with enamel surface microhardness recovery. Because the post-demineralization reference was measured immediately whereas the treated groups underwent an additional 7-day period in artificial saliva, the microhardness comparisons represent the net effect of the complete treatment protocol rather than the isolated effect of the active toothpaste component.