(V) Deadhesion Mechanics of S. aureus Bacteria on Hip-joint Biosurfaces

Post hip-joint arthroplasty trauma of bacterial infection becomes very painful for patients, especially due to its load-bearing nature. A perspective of mechanical engineer in selecting antibacterial materials and eliciting enhanced resistance to bacterial adhesion is addressed. In this work, deadhesion strength of S. aureus gram positive bacteria on various substrates is quantified, i.e. metallic (316L SS ~ 15 nN and Ti-6Al-4V ~11 nN), polymeric (ultra-high molecular weight polyethylene, UHMWPE ~4 nN), and ceramic (hydroxyapatite, HA ~8 nN). The adhesion strength of bacterium is evaluated by gluing S. aureus on the cantilever of tip-less atomic force microscope. Lowest adhesion strength of bacterium observed on UHMWPE surface contrast with conventional results of lowest colony forming units (with most %dead bacteria on HA surface. Molecular dynamics has been utilized to visualize the adhesion of bacterial adhesin proteins (i.e. Staphylococcal protein A (SpA), Clumping factor A (ClfA), and Serine-rich S. aureus protein (SraP)) with biomaterial surfaces. Poisson’s regression has elicited lower long-range and short-range adhesion forces in UHMWPE compared to that of HA and other biosurfaces. Further, tribological analysis has revealed that the sliding wear rate of UHMWPE (9.54×10^-5mm³) decreased with synergistic reinforcement of Ag and ZnO (5.65×10^-5mm³). During micro-scratch test, ZnO reinforcement elicited an increased wear volume (~0.29 mm³) due to its poor interfacing with UHMWPE when compared to that of Ag reinforcement (~0.16 mm³), which is attributed to change in wear mechanism from sliding to ploughing. An optimal Ag/ZnO combination has elicited an enhanced tribological resistance, and enhanced bioactivity while providing antibacterial efficacy in potential UHMWPE polymeric acetabular cup liner. Thus, synergistic engineering of tribological damage tolerance and controlling bacterial infection can render an enhanced implant life and provide comfortable post- hip-joint arthroplasty experience of patients.