Effects of Molecular Weight and Concentration of Poly(Acrylic Acid) on Biomimetic Mineralization of Collagen

Inspired by nature, poly(acrylic acid) (PAA) and other polyelectrolytes have been used as noncollagenous proteins (NCPs) surrogates for biomimetic intrafibrillar mineralization of collagen fibrils and thus, to model the ultrastructure of bone, to study the mechanism of bone mineralization and, more scarcely to fabricate scaffolds for hard tissue engineering. The objective of this study was to systematically investigate the effect of the molecular weight (MW) and the concentration of PAA on the rate and pattern of biomineralization of collagen matrices. Densified type I collagen films were mineralized in supersaturated PAA-stabilized amorphous calcium-phosphate (PAA-ACP) solutions containing increasing MW (2 kDa, 50 kDA, 450 kDa) and concentrations (10, 25, 50 mg/L) of PAA up to 7 days. The stability and physical properties of collagen-free PAA-ACP solutions were also investigated. In our system, lowering PAA MW and increasing PAA concentration resulted in solutions with increasing stability. Over stable PAA-ACP solutions that fully inhibited mineralization of the collagen matrices were achieved using PAA 2k-50. Conversely, unstable solutions were obtained using high PAA MW at low concentrations. Nucleation and growth of significant amount of extrafibrillar minerals on the collagen fibrils was obtained using these solutions. In a wide range of combined MW and concentration of PAA we obtained intrafibrillar mineralization of collagen with hydroxyapatite crystals aligned parallel to the collagen fibril as in natural tissues. Intrafibrillar mineralization was correlated with PAA-ACP stability and growth of the PAA-ACP particles in solution. Our results support using PAA to surrogate NCPs function as selective inhibitors or promoters of biological mineralization and provide parameters to manufacture new biomimetic scaffolds and constructs for bone and dentin tissue engineering.