Intrafibrillar mineralization is crucial in the synthesis of collagen-hydroxyapatite (HAP) composites with hierarchical structure resembling natural bone. The polymer-induced liquid precursor method, which involves immersing the matrix into polymer-stabilized amorphous calcium phosphate (ACP) solutions, has been widely used to regulate collagen mineralization; however, heterogeneous infiltration and crystallization of ACP precursors from the surface to the inner matrix remain a concern. In this study, the polymer-mediated co-organization strategy in which poly (acrylic acid) (PAA)-stabilized ACP solutions were initially introduced to collagen solutions before sol-gel transformation was employed to achieve homogeneous mineralization. Moreover, the effects of PAA with various molecular weights (2, 10, and 450 kDa) and concentrations (40, 80, and 120 mg/L) on collagen mineralization were evaluated. Increasing concentration and decreasing molecular weight both improved the stability of ACP precursors, which affected collagen mineralization. A lower concentration (40 mg/L) of 2 kDa PAA and higher concentration (80 and 120 mg/L) of 100 and 450 kDa PAA effectively retarded ACP crystallization, thereby inducing intrafibrillar mineralization.Extremely unstable ACP precursors failed to infiltrate the interior of fibrils using 100 and 450 kDa of PAA at 40 mg/L, resulting in extrafibrillar mineralization.Inversely, collagen fibrils were unmineralized until 7 days owing to excessive stability of ACP precursors using 2 kDa of PAA at higher concentrations. The present results further explain the mechanisms of intrafibrillar mineralization, and may provide a facile method for producing biomimetic materials.

Collagen self-assembly and simultaneous mineralization by incubating a mixture containing collagen, calcium, and phosphate ions under physiological conditions, is an effective method to prepare bone-like biomimetic materials. The formation of fibrils and minerals is related to pH of system. In the present work, we evaluated the effect of pH (7.9-10.4) on biomineralization process and synthesized composites. Turbidity kinetics and X-ray diffraction (XRD) measurements revealed that increasing pH delayed the crystallization process from nucleation phase to plateau phase because of promoting chelation of Ca 2+ with collagen. Typical peaks of phosphate in Fourier transform infrared spectroscopy coupling with characteristic peaks of hydroxyapatite (HAp) in XRD spectra illustrated the formation of HAp after biomineralization. Scanning electron microscopy measurements indicated that the increase of pH promoted the deposition of spherical minerals in fibrils. Especially, the minerals tended to form cluster-like structure at pH 10.4. The hyp content, Ca and P contents, and gel strength measurements suggested that higher pH promoted the formation of HAp with a Ca/P closed to 1.67 and the prolongation of crystallization gave the time for collagen self-assembly leading to the increase of gel strength at higher pH (9.4-10.4). These results might provide some new ideas for designing biomimetic materials.

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