A Study Of Hydroxyapatite-contained Composite Coatings On Titanium Substrates

Bioactive hydroxylapatite (HA) coatings are commonly used to improve the biocompatibility and osteoinductivity of the titanium. HA coated titanium has the advantages of metals and ceramics and has been considered as one of the most promising bone replacement materials. Recently, there is a tendency to develop HA-contained composite coatings to obtain new type of coatings that satisfy the different requirement of clinic applications. One type of the composite coatings is to incorporate biomacromolecules, such as chitosan (CS) and gelatin, into HA coatings in order to promote the biological performance of the coatings. The other is to add inorganic materials, such as ZrO2 and TiO2, into HA coatings in order to improve the mechanical properties of coatings. The homogeneity of composite coatings is still a big challenge. In this study, three types of composite coatings, HA/CS, HA/geltin and HA/ZrO2, were prepared and the processing parameters was optimized. The microstructure, composition and biological performance of coating was evaluated. The main findings were summarized as follows.Chitosan (CS) is a biopolymer that has good biocompatibility, antibacterial properties and the suitability for cell growth. In the present study, the electrochemical deposition method was-employed to produce HA/CS composite coatings on titanium substrates. The results indicated that pulse voltage mode can improve the CS content in the coatings and can obtain uniform coatings, compared with constant voltage mode. Pulse voltage mode also promoted the interaction between Ca and CS according to XPS analysis. Pulse voltage affected the morphology of the coatings and efficiency of the deposition. The suitable pulse voltage is-1.3 V. The concentration of CS in the electrolyte affected the CS content in the coatings and the efficiency of the deposition. The suitable one is 33.1g/l. The antibacterial test indicated that HA/CS coatings had good bactericidal ability. Osteoblasts were cultured on the coatings to evaluate the biocompatibility of coatings and the results indicated that the composite coatings could favor the attachment and proliferation of osteoblasts.HA/CS composite coatings were also prepared on titanium surfaces by hydro-thermal synthesis and sol-gel method. The composition, structure, morphology and biocompatibility of composite coatings were characterized. Alkali treatment converted Ti surfaces into super-hydrophilic surfaces, which was the essential prerequisite for preparing uniform HA/CS composite coatings. XRD analysis revealed that the coatings were mainly composed of HA and CS and the content of composition were determined by TG analysis. SEM observation found the coatings with different HA content had different morphologies. Osteoblasts were cultured on the coatings to evaluate their biocompatibility. Alamar Blue assay indicated that cells on the composite coatings had higher proliferation rate than those on pure titanium. Also the ALP activity of the cells on the composite coatings was higher than those on the pure titanium samples. These results demonstrated that as-prepared HA/CS composite coatings have good biocompatibility.Gelatin is the denatured form of collagen and is expected to be beneficial for hard tissue applications. The CaP/gelatin composite coatings could combine the bioactivity and osteoconductivity of CaP with the good characteristics of gelatin. In the present study, the pulsed electrochemical deposition method was employed to deposit CaP/gelatin composite coatings on titanium substrates. The main purpose of the study is to optimize the processing parameters of the composite coating. Electrolyte was the mixture of the Ca-P and gelatin aqueous solution with different concentration. SEM, XRD and FTIR were used to characterize the composition and morphology of the coating. The coatings prepared under constant voltage and pulse voltage electrochemical deposition were compared. It was found that the pulse voltage mode was more suitable for preparing CaP/gelatin coatings. The results indicated that the optimized experimental conditions were:pulse voltage range, 0～-1.3 V; the concentration of Ca2+,5.0×10-4mol/l; gelatin solution,0.5 g/l; pH value,5.0; temperature,50℃. Osteoblasts were cultured on the coatings to evaluate the biocompatibility of the coatings. Alamar Blue assay indicated that the composite coatings could favor the proliferation of the osteoblasts.In recent years, HA/ZrO2 composite coatings have attracted extensive research attention because they could combine unique biocompatibility and bioactivity of HA ceramics with excellent mechanical properties of ZrO2. In this study, dense and uniform HA/ZrO2 composite coatings was fabricated on titanium substrates by pulsed electrochemical deposition. The composition, morphologies, biocompatibility and physiological stability of the composite coatings were studied. X-ray diffraction showed that OCP in the coating was converted into HA and the coating was composed of HA and ZrO2 after heat treatments. Scanning electron microscopy indicated that ZrO2 was uniformly distributed in the coatings and the coatings became dense after heat treatments. Simulated body fluid immersion test proved that the as-prepared composite coatings had good bioactivity to induce calcium phosphate formation under simulated physiological environment. Atomic absorption spectrometry analysis was used to measure Ca ion release rate of the coatings immersed in PBS. It was found that the release rate of Ca ion of HA/ZrO2 was lower than that of pure HA coatings. Nanoscratching tests and tensile tests revealed that HA/ZrO2 coatings had better interfacial bonding strength than that of pure HA coatings. Osteoblasts were cultured on the coatings to evaluate the biocompatibility of coatings. The results of the Alamar blue and ALP test indicated that the composite coatings could favor the proliferation and differentiation of the osteoblasts, which indicated that the as-prepared HA/ZrO2 nanocomposite coatings had good biocompatibility.In summary, three types of HA-contained composite coatings, HA/CS, HA/geltin and HA/ ZrO2, have been successfully produced on titanium substrates. The results of present study provided valuable reference for the further research in developing composite coatings for biomedical applications.