Role of Dopants Chemistry and Particle Size on Antibacterial Efficacy of Hydroxyapatite (HAp) Powders for Bone Tissue Engineering Applications

Abstract

Hydroxyapatite (HAp) based bio-ceramics hold high potential in orthopedics as synthetic bone grafts, and fillers due to their compositional similarities with native bone tissues. However, early stage infections and related inflammations are still a major challenge that often limits the wide applications of HAp based bone grafts. To address that problem, this work is focused on developing multifunctional HAp granules doped with different trace metal oxides that can offer combined benefits of improved antibacterial efficacy and enhanced osteo-conductivity. For this purpose, three different dopants (e.g., Ag, Zn and Cu) in form of metal oxide powder were incorporated into HAp at two different concentrations (1 and 5 weight percent) and in vitro antibacterial efficacy was assessed against growth of gram negative bacteria (i.e., E. coli), keeping pure HAp as a control. In this work, we have attempted to evaluate the effect of dopants chemistry as well as dopants particle size on antibacterial efficacy of pure HAp powder and identification of optimal dopants composition for potential therapeutic application as synthetic bone-graft materials. Hence, selected metal oxide powders (e.g., AgO, ZnO, and CuO) were used as dopants of HAp in both micro-sized (particle size less than 200 μm) and nano-sized (particle size less than 100 nm) form in less quantity (i.e. 1 and 5 weight percent). HAp powders were synthesized in-house by co-precipitation route and desired amount of dopants were physically mixed by ball milling and final compositions were made by sintering at 1250oC. Detailed physico-chemical analyses were conducted by XRD, FTIR and FESEM prior assessing in vitro antibacterial efficacy. Qualitative analysis of in vitro antibacterial assessment was done by colony forming unit counting method. Our in vitro antibacterial study results demonstrated that CuO doped HAp exhibits the best antibacterial efficacy, whereas AgO and ZnO doping was found to have minimal effects amongst all groups. CuO doped HAp sample was able to inhibit bacterial growth at 100% level in 20mg/mL concentration. AgO and ZnO doped HAp samples were only able to inhibit bacterial growth by 10 – 20%. Notably nano-sized dopants incorporated HAp has higher antibacterial efficacy than that of micro-sized dopants conjugated HAp granules irrespective of dopants chemistry