The biphasic concentration-dependent effect of strontium on the mineralization of reconstituted type I collagen and demineralized dentin

Objectives To investigate the concentration-dependent effects of strontium on the mineralization of reconstituted type I collagen and demineralized dentin remineralization. Methods Sr²⁺ at varying concentrations (0, 0.1, 0.25, 0.5, 0.75, 1 mM) was incorporated into polyacrylic acid-stabilized amorphous calcium phosphate (PAA-ACP) mineralization solutions. The mineralization solutions with 1 mM Sr²⁺ were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD). Cytocompatibility was assessed using the CCK-8 assay. Reconstituted type I collagen and demineralized dentin models were employed to explore mineralization differences; high-resolution TEM (HRTEM) and selected area electron diffraction (SAED) were applied to investigate the underlying biphasic mineralization mechanism of Sr²⁺. Results In the reconstituted type I collagen model, early crystalline mineralization was detected at 12 h in the 0.25 mM group, while 0.5 mM Sr²⁺ induced more ordered apatite crystal alignment along the collagen long axis. The observed lattice expansion (d-spacings of 0.35-0.37 nm and 0.29 nm) was consistent with the formation of Sr-containing apatite-like mineral. At ≥ 0.75 mM Sr²⁺, the minerals deposited preferentially on the fibril surface replaced intrafibrillar mineralization. In the demineralized dentin model, a consistent biphasic concentration-dependent pattern was observed: 0.1-0.5 mM Sr²⁺ was associated with increased density of the remineralized layer and enhanced dentinal tubule occlusion, the enlarged lattice spacing (d-spacings of 0.35 nm) was consistent with that observed in the reconstituted type I collagen model. In contrast, 0.75-1 mM Sr²⁺ was associated with reduced remineralization and intratubular mineral filling. Both the collagen and dentin models suggested the biphasic effect of strontium on biomimetic mineralization. Significance Sr²⁺ shows a biphasic concentration-dependent response on collagen mineralization and dentin remineralization. These findings provide preliminary concentration-response information for the rational design of Sr-containing biomimetic materials.