Toward the understanding of poly (Acrylic Acid) on amorphous calcium phosphate mediated collagen intrafibrillar Mineralization: Surface adsorption versus bulk incorporation

Poly (acrylic acid) (PAA), as a noncollagenous proteins (NCPs) mimic, is considered to play a crucial role in biomimetic collagen mineralization. However, its regulation mechanism in collagen mineralization is yet not fully elucidated. Here, we especially have focused on what is the effect of amorphous calcium phosphate (ACP) associated with surface adsorption or bulk incorporation of PAA on collagen intrafibrillar mineralization. We found that no matter PAA is adsorbed on or incorporated into ACP, it facilitates intrafibrillar mineralization in a concentration-dependent manner. But, importantly, in the presence of 60 & mu;g/mL PAA, almost fully intrafibrillar mineralization can be achieved when PAA is adsorbed on ACP surface (ACPS), which can just obtain partial intrafibrillar mineralization when PAA is incorporated into ACP (ACPB). The reason lies in the high electrostatic interaction of ACPS with fibrils and creating a local supersaturation around collagen-I fibrils when PAA is adsorbed on the ACP surface. Conversely, ACP nanoparticles were almost not charged when PAA is incorporated into ACP. This finding demonstrates that the spatial distribution of additives with ACP nanoparticles is crucial for collagen intrafibrillar mineralization in vitro and may provide new insights for the regulation and design of biomimetic materials.