Site-specific antibacterial efficacy and cyto/hemo-compatibility of zinc substituted hydroxyapatite

Hydroxyapatite [HA, Ca-10(PO4)(6)(OH)(2)] is among the most sought after bioceramics having chemical structure similar (with Ca/P ratio of 1.67) to that of human bone and other hard tissues. Zn doping in HA based implants not only enhances bone mineralization, biological calcification but also introduces antibacterial efficacy that reduces the chances of implant failure due to periprosthetic infection. This is for the first time, dopant site specific biological properties of Zn doped HA has been investigated. In this work, Zn doping in HA is performed as per the stoichiometric formula Ca-10(PO4)(6)Zn-x(OH)(2-delta) and Ca10-xZnx(PO4)(6)(OH)(2), (x = 0, 0.1 and 0.25) to investigate the effect of different doping sites on the lattice parameter, cytocompatibility, hemocompatibility and antibacterial efficacy. Phase analysis has confirmed Zn substitution in Ca site and incorporation in OH channels of HA. Zn doping in Ca site of HA up to the level 2.5% shows cytocompatibility and no cytotoxic effects have been found after altering the doping site and incorporating Zn in the OH channel. Zn substitution either in Ca site or incorporation in OH channel in improving the antibacterial efficacy of HA has been confirmed against E. coil, and S. aureus. The antibacterial test reveals that Zn doped HA shows antibacterial efficiency only when Zn is substituted in Ca site of HA, which can be attributed to restricted Zn+2 leaching (similar to 5.28 ppb) when Zn is incorporated in OH channels of HA. But, when Zn is substituted in Ca site of HA, Zn+2 leaching increases to similar to 413 ppb (confirmed by inductively coupled plasma-mass spectrometry). Thus, site specific doping of Zn in hydroxyapatites can serve as potential bioceramic for bone implant applications with tailored biological properties and controlled antibacterial efficiency depending on the doping site.