Effect of Bilayer Nano-Micro Hydroxyapatite on the Surface Characteristics of Implanted Ti-6Al-4V ELI

Ti-6Al-4V ELI is a well-known, popular medical-grade titanium alloy due to its biocompatibility and excellent mechanical properties. However, like other metal implants, it is less bioactive that affects tissue regeneration around the implant, and may lead to implant failure. So, a bioactive substance such as hydroxyapatite (HA) has usually been coated on metal implants to improve its bioactive properties. However, a single layer of HA was reported to be dissolved into body fluid after a long time of exposure in the human body. In this study, bilayer HA was deposited on the surface of screw-type implants made of Ti-6Al-4V ELI through electrophoretic deposition (EPD method. The bottom layer consists of micro-size of HA, and the second layer contains nano-size HA. The suspension contains each micro and nano size of HA powder was homogenized for 1 h followed by sonication for 2 h using a magnetic stirrer. The coating layer was subsequently sintered at 700oC for 1 h. The bilayer-coated screw implant was then implanted into the proximal tibia of health rattus novergicus under proper surgical procedures. Some screws without HA deposition were also implanted into rattus novergicus for comparison. The implanted screws were then taken out via surgery after 2, 3 and 4 weeks, and they were subsequently observed by optical microscope, SEM and XRF. The results showed that organic material is found on each coated specimen, and few HA layer is disintegrated from the surface of the screw. The disintegrated HA remained in the surface of the screw, and the amount of HA increased with increasing implantation time, which indicates the increase of osseointegration between the bone and HA layer. XRF showed a significant difference in Ti and titanium oxide contents on the surface of the coated samples and the non-coted ones, where it is only 0.66%Ti (0.39% TiO2) on the surface of the screw with HA layer and 70%Ti (67% TiO2) for without HA. When TiO2 is formed as a fast self-healing reaction while the screw is exposed to body fluid, the HA acts as an interface against body fluid that may contain aggressive ions. So, HA layer is not only effective against corrosion attack but also inhibits the formation of TiO2 on the implant surface. The coated screws also revealed a strong bonding between the HA layer and the surface of the implant screw. Besides, the ratio between Ca and P elements on the screw surface is in the range of 0.58 – 2.04, which is in the range of bone characteristics.