Antibacterial nanoparticle - decorated carbon nanotube - reinforced calcium phosphate composites as bone implants

Abstract

Hydroxyapatite (HA) is a biologically active ceramic used in surgery to replace bone. While HA promotes bone growth, it suffers from weak mechanical properties and does not possess any antibacterial property. Multi walled carbon nanotubes (MWCNTs), as one of the strongest and stiffest materials, have the potential to strengthen and toughen HA, thus expanding the range of clinical uses for the material. Furthermore, Silver nanoparticles (Ag NPs) can be decorated to sidewalls of the MWCNTs which could be released over a period of time to prevent infection following surgery. This work sought to develop and characterise Ag NPs- MWCNTs – HA composites in four main areas: 1) production and characterisation of the composite, 2) evaluation of mechanical properties, 3) investigation of antimicrobial property and 4) assessment of biological response to in vitro cell culture.Pristine (p-MWCNTs) and acid treated MWCNTs (f-MWCNTs) were decorated with Ag NPs. In the presence of 0.5 wt % Ag NPs-MWCNTs, HA was precipitated by the wet precipitation method in the presence of either poly vinyl alcohol (PVA) or Hexadecyl trimethyl ammonium bromide (HTAB) as the surfactant. Composites were characterised using various techniques and the diameteral tensile strength and compressive strength of the composites were measured. The antibacterial effect of these composites was investigated against clinically relevant microbe, Staphylococcus aureus. To determine the ability of the HOB cells to differentiate and mineralize in the presence of the composite, HOB cells were cultured on the composites for 21 days. Gene expression studies was performed along with the biochemical assays and scanning electron microscopy was used for qualitative analysis. Pure HA was used as control in all the studies.

The study revealed that both the MWCNTs and surfactants play a crucial role in the nucleation and growth of the HA. XRD and FTIR characterisation revealed that HA was the primary phase in all the synthesised powders. Composites made with f-MWCNTs were found to have better dispersion and better interaction with the HA compared to composites with p-MWCNTs. Although mechanical strength was improved in all the composites, p-MWCNTs composites exhibiting maximum strength. Antibacterial studies showed 80% bacterial reduction in the treatment composites compared to pure HA. The biocompatibility study showed reduced activity of the HOB cells, however, no significant difference was observed between the control and the treatments. This systematic study of the synthesis and properties of the Ag NPs- MWCNTs-HA composites has resulted in improved understanding of the production and processing of these materials and the effect of MWCNTs and silver nanoparticles on primary human osteoblast cells. Additionally, it has yielded interesting biocompatibility result favouring the use of MWCNTs in the development of implants. There is potential to translate Ag NPs-MWCNTs-HA composites into clinically approved product.