Effect of the stability of the poly(gamma-glutamic acid)-ACP dispersion system on biomimetic mineralization of type I collagen†

Objective: To explore the relationship between the stability of poly(gamma-glutamic acid) (γ-PGA) dispersion systems with γ-PGA of different molecular weights (MWs) and concentrations and type I collagen mineralization. Methods: γ-PGA was used as a noncollagenous protein (NCP) analogue to regulate the stability of supersaturated γ-PGA-stabilized amorphous calcium phosphate (PGA-ACP) solutions by changing the γ-PGA MW (2, 10, 100, 200 and 500 kDa) and concentration (400, 500 and 600 μg mL⁻¹). Then, the optical density (OD) at 72 h was measured to determine the PGA-ACP solution stability. Recombinant type I collagen films were mineralized in different PGA-ACP solutions for 3 d and observed via transmission electron microscopy (TEM) to confirm the occurrence of intrafibrillar mineralization. The collagen scaffolds were mineralized for 7 d and observed via scanning electron microscopy (SEM) to determine the collagen mineralization pattern and degree. X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and thermogravimetry (TG) were used to analyse the mineralized collagen scaffold composition. Results: The PGA-ACP solutions with γ-PGA of different MWs and concentrations had different stabilities and type I collagen mineralization. Except for the 100 kDa group, which neither stabilized the supersaturated calcium phosphate solution nor induced intrafibrillar mineralization, the groups stabilized the solutions for at least 10 h and induced different intrafibrillar mineralization patterns and degrees. Conclusion: In our system, the PGA-ACP solution stability and occurrence of intrafibrillar mineralization are directly correlated. Thus, we suspect that the same correspondence exists in other biomimetic mineralization systems and that a relatively stable supersaturated calcium phosphate solution may be a necessary condition for intrafibrillar mineralization.