Preparation And Antibacterial Ability Of Silver Ions-substituted Hydroxyapatite/Titania

Microbes exist universally in the nature and can propagate in appropriate conditions. Although most of them are harmless to human beings, some are even beneficial and essential, a small proportion of them are still harmful which are breed by the intimate sweat and scurf and thus causes diseases to body.Many metallic ions existing in the nature, such as silver, copper and zinc, have the ability to inhibit and sterilize bacterium. Of all the metallic ions, silver has the strongest antibacterial ability. Therefore, it is well fit for the preparation of antibacterial material because it is safe and harmless.Hydroxyapatite (HA) is the main inorganic composition of bone tissue and has been used for the ideal material for hard tissue repair because of its good bio compatibility and bioactivity, which makes HA be able to chemically bond to bone tissues. HA belongs to hexagonal crystal and it possesses a pathway in structure parallel to c axis which is easy to absorb metallic ions such as silver, copper and zinc. Moreover, the metallic ions can even directly exchange with calcium ion of hydroxyapatite .In this paper, therefore, nano-hydroxyapatite (n-HA) was prepared using Ca (NO₃)₂ and Na₃PO₄ in aqueous solution. The obtained n-HA has a shape of thin pin whose structure is very similar to that of bone apatite. Furthermore, the crystal lattice parameters of prepared silver ions-substituted HA (Ag-HA) is bigger than that of n-HA, which indicates that some of the calcium ions are substituted by silver ions in the crystal lattices. The Ag-HA displays good inhibitory effects against staphylococcus, Escherichia coli and Candida albicans. The antimicrobial textile loaded Ag-HA has also good characteristics of antibacterial effect and resisting lavation.Titania (TiO₂) is a kind of light induced catalyst and thus has the ability of light-catabolic action which produces active oxides to sterilize bacteria under the light or environmental energy. Since TiO₂ has the antibacterial effect only when it is under violet light and silver is expensive as noble metal.. Based on the advantages both of silver and TiO₂, Ag-HA/TiO₂ was prepared in this paper as a new antibacterial material which possesses inhibitory with the result of both light catalysis and metal catalysis to produce synergistic action. The minimal inhibitory concentration (MIC) of Ag-HA/TiO₂ (0.51wt%Ag⁺5.3wt%TiO₂) against staphylococcus and Escherichia coli both are 390.5μg/ml, but the content of Ag⁺ ions in the material is only 0.51wt%, which is much smaller than that of the general loaded Ag-HA materials in which the Ag⁺ ions content is 3wt% . Furthermore, the changeable color, i.e. the deoxidization of the Ag⁺ ions can also be effectively inhibited by adding some amount of TiO₂. The Ag⁺ ions can produce the greatest antibacterial effect induced by the light-catabolic action of Titania under the light, but the antibacterial effect can still continue while there is no light.When studying the ultrastructure changes as the antibacterial material of Ag-HA/TiO₂ against Escherichia coli, worked two mechanisms for Ag-HA/TiO₂ to sterilize Escherichia coli: one is that the antibacterial particles contact the Escherichia coli cells and damage the cells wall and plasmolemma, in some situation they can even penetrate the cell wall and plasmolemma to enter into the core to make cytoplasm agglomerate and thus cause the cells dead; the other is that the antibacterial particles of Ag-HA/TiO₂ may produce active oxides which act on Escherichia coli lose their proliferation ability. It is also found that the shape of some DNA molecules of Escherichia coli changes from a related state into a condensed form when their cell walls and plasmolemma are still integrated and cytoplasm and periplasmic space are homogenized. In this situation DNA molecules lose their replicating ability which is the unique characteristic of silver ions.The antibacterial material of Ag-HA/TiO₂ can also kill some oral bacteria, but it is different for the antibacterial effect. The antibacterial effect against Gram negative bacteria is better than that against Gram positive bacteria. It is probably due to the peptidoglycan which is much more in the cell wall of Gram positive bacteria than that in the cell wall of Gram negative bacteria restrains silver ions into the inner cell from penetrating the cell wall because of its negative charge character.Furthermore, the influence of Ag-HA/TiO₂ on the biological behavior of osteoblasts and human gingival fibroblasts was also studied in this paper. The results showed that the antibacterial material of Ag-HA/TiO₂ had some inhibitory effect on the proliferation of osteoblasts and increased its activity, the ultrastructure of the cells after absorbing the antibacterial particles had no change. There had also no significant differences about proliferation, activity and ultrastructure for osteoblasts between Ag-HA/TiO₂ and the control of HA. However, it is indicated that there was no toxicity for cells after adding silver and titania into the materials, and the biocompatibility of Ag-HA/TiO₂ which was similar to that of HA was also verified in vitro.In the culture medium containing less than 100μg/ml of Ag-HA/TiO₂, there were no significant changes about growth and proliferation of human gingival fibroblasts and also no significant differences about its ultrastructures after phagocytizing antibacterial particles compared with the control.