In vitro analysis of the antibacterial effect of nanohydroxyapatite-ZnO composites

Hydroxyapatite (HA) is a biocompatible and bioactive synthetic material for biomedical applications as it binds to bone and enhances bone tissue formation. Particularly, nanophased HA can mimic the dimensions of constituent components of natural tissues; can modulate enhanced osteoblast adhesion and resorption with long-term functionality of tissue engineered implants. However, HA does not inhibit bacteria from adhering onto its surface, and this has implications in the bone healing process required for patient recovery, since infection can lead to the implant failure. In the present work a composite that combines the favorable biological characteristics of nanohydroxyapatite (nanoHA) and, simultaneously, possesses antimicrobial activity as expressed by ZnO was synthesized. To determine whether the size of ZnO particles was playing an important role in inhibiting bacterial growth, ZnO particle of different sizes (from the microscale down to the nanoscale) and concentration were incorporated into nanoHA and tested. The composite samples were characterized by SEM, FT-IR, XRD, XPS and zeta potential. The antibacterial activity of the composites was investigated, as well as the biofilm formation, using both Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli) microorganisms. The characterization revealed that ZnO particles were dispersed homogeneously within the nanoHA matrix. The composites antibacterial activity increased with decreasing ZnO particle size and increasing concentration. Biofilm formation tests revealed that the nanoHA-ZnO composites exhibit a strong effect against the common pathogens S. aureus and E. coli.