Effect of silk sericin on assembly of hydroxyapatite nanocrystals into enamel prism-like structure

Taking the inspiration from the biomineral, the silk sericin was selected to modulate the assembly of nanosized hydroxyapatite crystals via a modified co-precipitation method. The effect of silk sericin concentration and mineralization time on the formation of hydroxyapatite nanocrystals, including their nucleation, growth, aggregation, especially assembly dynamics, were investigated using the transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectroscopy. The results revealed that both the concentration of silk sericin and the mineralization time could affect the size, morphology and assembly of hydroxyapatite crystals. Higher concentration of silk sericin in the reaction system was apt to form the larger hydroxyapatite crystals and longer mineralization time helped to improve the crystallinity of hydroxyapatite crystals together with an increase of crystal size. More interested thing was that an arrangement of nanocrystals in order was observed when the concentration of silk sericin was kept at 1(w/v)% in the reaction system. The larger rod-like hydroxyapatite crystals of 300-500 nm in length and 50-80 nm in diameter were assembled along the c-axis by the smaller crystals of ca. 20 nm under the control of silk sericin, whose arrangement method was similar to the natural enamel. A proposed formation mechanism of the special structure was elucidated. In addition, the biocompatibility of obtained crystals was evaluated in vitro and their strong ability to promote the cell differentiation and proliferation was proved in the paper, which may be important for the biomedical applications and also for the fundamental studies of cell-matrix interactions.