Development of Hydroxyapatite Coated Titanium Alloy with Improved Corrosion Resistance and Biocompatibility for Bone Implant

Abstract

The ability of a material to carry out its intended task by extending desirable functionalities after a medical intervention being performed without getting rejected by the host is known as biocompatibility. The basic criteria for a material to be considered as an implant material is its biocompatibility (cytocompatibility, hemocompatibility, non-immunogenicity), higher fatigue and shear strength, and corrosion resistance. Usually, metallic biomaterials possess appreciable mechanical strength and are nontoxic, but they suffer from inherent shortcomings such as bioinertness and corrosive wear. These problems can be dealt by adopting various surface modification techniques to alter their surface topography. Hydroxyapatite (HAP) is one of such popular coating materials for metallic implants. However, the most undesirable problem with HAP coatings is their lower adhesion to the implant. The resistance to bending, chipping and delamination during the medical procedure and post operative activities are also serious concerns associated with the HAP based coatings. In the present study, the surface morphology of implant grade Ti6Al4V alloy sheets was successfully altered by applying bioactive hydroxyapatite (HAP) coating. This was attempted mainly because HAP possesses the closest structural analogy to human bone and allows chemical functionalization for tailoring its properties. The coating was achieved by two approaches namely, biomimetic mineralization and microwave irradiation. The microwave irradiation process also yielded cationic doped HAP coating by incorporating Niobium (Nb) as dopants in HAP. Three major input parameters viz. irradiation power (W), doping % of Nb and time of exposure (min) were considered for the experimental runs and targeted biological responses such as hemolysis %, clotting time and protein adsorption from blood plasma were optimized using Grey-Taguchi analysis. The sample coated with this optimal factor setting was further used for other assays. Additionally, an attempt was made to co-dope the HAP with Strontium (Sr) along with Nb via microwave irradiation and these samples were also assessed for their physical and biological properties. The surface characterizations revealed that both approaches yielded homogeneous surfaces free from flaws. However, it could be observed that the morphology of the HAP particles altered after the cationic doping in case of microwave assisted coating. The characteristic peaks of HAP could be confirmed in the crystallographic studies performed on the coated samples. After performing various assays, it was observed that both the treatments rendered higher hardness to the substrate, but an insignificant increase of surface roughness was present. However, the wettability of the Ti6Al4V sheet was considerably increased after the coating was applied which could facilitate better host-implant interaction. Potentiodynamic polarization studies revealed that the barrier action offered by the HAP coating had significantly reduced electrochemical corrosion in the form of ionic dissolution. The treated samples showed lesser hemolysis as compared to untreated sample and did not induce clotting of the blood, making them more hemocompatible. The protein adsorption capacity was enhanced by the application of coating, which also significantly enhanced biocompatibility. Reduced cytotoxicity was also evident from cell culture assay and the treated surface also prevented bacterial growth. The cell viability post doping found to have increased by more than 30%. Moreover, there was no evidence of cytotoxic effect of either Sr or Nb on human cells as confirmed from the cell viability assay. The co-doping found to help cell proliferation. So, it can be concluded that the incorporation of metallic ions like Sr and Nb in HAP coating can protect the implant surface from the body fluid-induced wearing as well as bacterial infection and thus, prolong its life. Therefore, microwave-assisted coating can be considered as a one-step solution for simultaneous synthesis and deposition of reinforced HAP coating on metallic implants.