Effect of Zn and Co doping on antibacterial efficacy and cytocompatibility of spark plasma sintered hydroxyapatite

Abstract

Hydroxyapatite [Hap, Ca-10(PO4)(6)(OH)(2)] is one of the most preferred bioceramic material for orthopedic implants and coatings due to its stoichiometric similarities with human hard tissues. However, foreign body implantation inside human body sometimes leads to bacterial film formation over the implant surface causing the implant failure, thereby needing a revision surgery. This study attempts to select the better dopant between zinc (Zn) and cobalt (Co) as per the antibacterial efficacy when doped in Hap. To prepare antibacterial transition-metal-doped Hap, Zn and Co are doped in Hap as per the chemical formula Ca10-x M-x(PO4)(6)(OH)(2), (M = Zn or Co and x = 0.24) to improve antibacterial efficacy. Phase and microstructural characterization by Rietveld refinement, scanning electron microscopy (SEM), and Fourier transformation infrared spectroscopy (FT-IR) confirms the doping. Evaluation of antibacterial activity against E coli reveals that both Zn- and Co-doped Hap shows antibacterial property with the latter being more effective (zone of inhibition similar to 3 mm more) for the same level of doping. Inductively coupled plasma-mass spectrometry confirms the presence of similar to 676 ppb Co+2 and similar to 303 ppb Zn+2 after leaching. In addition, cytotoxicity assay with NIH3T3 cell line reveals cytocompatibility of both the compositions with either dopant. The effect of spark plasma sintering on densification and mechanical properties of Co-doped Hap is investigated for the first time and compared with Hap with the same level of Zn doping. It appears that Co-doped Hap attains higher densification (similar to 7% more) and fracture toughness (similar to 2 times better) as compared to that of Zn-doped counterpart (densification: 86% and fracture toughness: 0.75 +/- 0.12 MPa root m). Thus, this study suggests that Co- and Zn-doped Hap are promising candidates for bone tissue engineering with improved antibacterial properties and in addition, Co-doped Hap can attain higher density and offer better fracture toughness than that of Hap doped with Zn.