Crystallization Of Calcium Carbonate On Chitosan And Silk Fibroin Substrates

Biominerals usually have remarkable performances such as mechanical or optical properties, which arise from there intrinsic nanostructures. Generally, the formation of the specific structure in biominerals is regulated by organics. For example, during the formation of nacre, the cooperation ofβ-chitin fibrils, silk-like protein gel withβ-sheet conformation and acidic proteins induces the oriented growth of aragonite, which finally gives rise to the organic/inorganic hybrid material with a highly ordered structure.In nacre, the soft substrates (organics) regulating the crystallization of calcium carbonate are composed of silk-like protein gel and free acidic proteins. Regenerated silk fibroin (RSF), which is derived from natural silk fibers as a main component of silk protein with acidic residues, has a similar primary structure of silk-like protein in nacre. Therefore, as the soluble additive, RSF may replace both of silk-like protein and acidic proteins to play the regulating role during the aggregation and crystallization of calcium carbonate in the mimic system. Moreover, due to the highly ordered microstructure, natural animal silks have been considered as potential templates to induce the oriented growth of crystals. As a derivative of chitin, on the other side, chitosan features numerous functional groups as well as shape adjustability. Thus, it is likely to be employed as an insoluble substrate in biomineralization.It was revealed in our work that the hybrid CaCO3/RSF nanoparticles associated by the electrostatic interaction were adsorbed to chitosan membrane through hydrogen bonds between RSF and chitosan, leading to the growth of disk-like CaCO3 crystals. Oval-shaped crystalline disks of which the long axis was along with the stretching direction were achieved by pre-stretched chitosan membrane, suggesting that the directional adsorption of RSF by arranged functional groups of chitosan induced the growth of the crystal. Furthermore, CaCO3 flakes vertical to the surfaces of crystalline disks formed via epitaxial growth in the supersaturated solution. The solution pH influenced the polymorph of the disk-like crystal through the competition between the changes of negative charge of RSF in bulk solution and that on the membrane, which predominate the vicinity supersaturation. Moreover, higher temperature favored the crystallization of the mineral with a higher surface energy, e.g. aragonite.Additionally, the soluble additive showed regulating role in the ripening of mesostable crystals. In present study, without RSF in the solution, a large number of laminated vaterites precipitated on the chitosan membrane accompanied by a few of sporadic calcites. After ripened, laminated vaterite transformed into thermodynamically stable rhombic calcite via dissolution-recrystallization process. However, when ripened in CaCO3 supersaturated solution containing RSF or polyacrylic acid (PAA), bundle-like aragonite appeared on the membrane, probably due to the reduction of the surface energy caused by the adsorption of additives. The results suggested that some mesophases with potential applications might be obtained through controlling the transformation of unstable phases.Degummed B. mori silks attracted CaCO3/RSF nanoparticles via hydrophobic interactions betweenβ-sheet domains both in silk fiber and RSF, which induced nucleation of aragonite due to the lattice matching effect. The ordered arrangement ofβ-sheet domains in silk fibers resulted in the oriented growth of CaCO3 crystals parallel to the longitudinal axes of silks. With more acidic residues in the amino acid sequences, degummed A. pernyi silks and major ampullate silks of N. edulis spider adsorbed CaCO3/RSF nanoparticles both via hydrophobic interactions between P-sheet domains and electrostatic interactions between hydrophilic domains, thereby generating the mixture of aragonite and vaterite. Sericin proteins with strong hydrophilic properties played the dominant role in the surface mineralization of undegummed B. mori cocoon silks, so RSF were adsorbed onto them only by electrostatic interactions, which produced vaterite on the silk. After investigating the mineralization system with regenerated A. pernyi silk fibroin as soluble additive, we found that it was the increasing of acidic residues in silks caused the increasing of vaterite, rather than the reduction in structural similarity ofβ-sheet domain. The area of amorphous domains in RSF membranes was much larger than that in silk fibers, so the adsorption of CaCO3/RSF nanoparticles through electrostatic interactions onto RSF membranes may occur frequently. Therefore, the minerals grown on RSF membranes were both aragonite and vaterite. Moreover, the P-sheet domains in either regenerated B. mori silk fibroin or regenerated A. pernyi silk fibroin were not in an alignment, leading an isotropic growth of CaCO3 crystals.The primary investigation showed that the mineralization capability of surface of synthetic fibers (e.g. PET fiber or glass fiber) was greatly improved after their surfaces were modified with a RSF coating. As the mineralization capability of a synthetic material should be enhanced when it is applied in bone tissue engineering, while suppressed when it is employed as a soft tissue scaffold, we should either modify synthetic materials properly for improving their biocompatibility or not, according to their specific applications.