Microstructure And Formation Mechanism Of Nacre Of H. Cumingii Lea Shell And The Growth Of Nacreous Coatings

Nacre (mother of pearl), a natural biocomposite material, is composed of about 95% inorganic aragonite, with only a few percent of organic biopolymer. It has high ordered microstructures in which the crystals and organic matrix exhibit interdigitating array like brick-mortar in architecture. Nacre has excellent mechanical properties due to its laminated microstructures, particularly in its toughness (work of fracture) being 3000 times higher than that of the artificial aragonite. On the other hand, nacre has a high biocompatibility and an ability of inducing growth of bone, and thus is believed to be a new material for bone repairing and substitution. Therefore, nacre has attracted many scientists to study its microstructure and mechanical properties aiming to unveil the biomineralization mechanism by which nacre is formed in mollusk shell.In the present study, SEM, TEM, XRD, EDS, Raman spectrum and optical microscope were used to systemically research the microstructures and formation process of the nacre of freshwater H. cumingii Lea shells and the corresponding pearls. Based on the studies, a formation mechanism of nacre was suggested. Nacreous biocoatings were fabricated successfully on surface of titanium implants by inserting titanium implants into mantle sacs of shells.SEM observations showed that the cross-section of the normal nacre in the shells exhibits the interdigitating array of tablets and organic biopolymer matrix, about 500nm and 30nm in thickness, respectively. XRD analysis showed that nacre was composed of aragonite crystals with a preferential orientation in [001] direction. SAD showed that the individual tablet was diffracted as aragonite single crystal pattern. However, the disorientation of a- and b-axis exists between adjacent tablets of aragonite in normal nacre. Beside the normal nacre, three kinds of abnormal structure band, columnar nacre structure, needle-like structure and spherulitic structure, were frequently observed to be sandwiched inside the nacreous layer on the cross section of shells. The columnar nacre structure band whose structure is similar with the nacre of abalone is proved to be composed of aragonite tablets with preferential orientation of [001] direction, which is identical with the aragonite tablets in normal nacre. Needle-like structure band and spherulitic structure band were composed of aragonite polycrystals. A spherulitic crystal in spherulitic structure band, whose structure is similar with the growth line of abalone shell, is composed of rod-like crystals. The abnormal structure observed in preset study may be induced by abnormal secretion of organic matrix. To our surprise, the similar abnormal structure bands have never been observed in the nacre of pearls which were obtained from the corresponding shells. This may result from the fact that the formation of pearls' nacre occurs in closed space of mantle sacs.Nucleation of aragonite tablets occurs at the edge of underlying tablets or the boundary region of adjacent tablets. At various stage of growth, the individual aragonite tablets all exhibit laminated structure. The sub-layers are about 20-50nm in thickness. HRTEM showed that many lattice defects existed on the (001) plane of aragonite crystal. These lattice defects are isolated and the spacing in the [001] direction between two neighboring defects is about 5-20nm, a dimension comparable with the thickness of sub-layers, indicating that the laminated structure of individual aragonite tablet may be closely related with these lattice defects. It is believed that these lattice defects may result from the adsorption of organic macromolecules on the (001) plane of aragonite crystal during the growth of individual aragonite tablet. Simulated growth of CaCO₃ in vitro showed that the deposits formed on the native nacre surface were composed of aragonite crystals with laminated structure, while the deposits formed on nacre surfaces which were pre-treated with sodium hypochlorite were composed of aragonite crystals with ridge-like morphology. In the control experiment, the deposits formed on the glass slices were composed of mixture of calcite, aragonite and vaterite. In control experiment, the deposits forming on glass slices were composed of mixture of calcite, aragonite and vaterite. These results suggest that some kind of organic macromolecules that have property to stabilize the (001) plane of aragonite crystals in order to regulate the growth along the c-axis of aragonite crystals, and consequently, form individual aragonite tablet with laminated structure.A new coating technique was developed to fabricate the nacreous biocoatings on titanium implants. All specimens were found to be coated with a layer of natural nacre after implantation for 45 days in spherical mantle sac. However, the mismatching between the local topography of titanium implant surface and spherical mantle sac not only resulted in the formation of a fin along the long-axis of implant, but also led to formation of a gap between the coating and the implant. To solve these problems, we designed a cylindrical pearl sac in order to match the cylindrical titanium implant. Using this cylindrical pearl sac as a reaction room, the nacreous coatings grew smoothly around the whole cylindrical surface of titanium implant. Moreover, the gap between the coating and the titanium implant was disappeared when nacreous coating was fabricated in cylindrical sac, indicating that the close apposition between the coating and the titanium implant was achieved.