Construction Of Nanostructures On Polymer Surfaces And Their Physical Bactericidal Mechanism

Biofilm formed by bacteria adhering to the surface of objects has brought great harm to the natural environment,human health,industrial and agricultural production,etc.Research from the U.S.Centers for Disease Control and Prevention shows that 80% of microbial infections in humans are related to biofilms,and 50% of hospital infections are associated with biofilms on medical devices.According to incomplete statistics,the biofouling resulted from biofilm causes more than $200 billion of loss to various underwater engineering facilities and ship equipment worldwide every year.Therefore,preventing the formation of biofilm is of great significance to protect human life and health,reduce the economic loss of industrial and agricultural production equipment,etc.The research shows that inhibiting the adhesion and growth of bacteria on material surfaces is the key to stopping the formation of biofilm.In 2012,the discovery that the nanostructures of cicada wing surface can mechanically kill adherant bacteria has attracted extensive attention in the research field.Relying primarily on its own physical structure without surface chemistry to lyse bacteria,nanostructures on surfaces provide a novel idea to antimicrobial surfaces and a sustainable and safe option for preventing biofilm formation.In this thesis,cicada-wing-like nanopillar structures were fabricated on polymer surfaces using the technology of anodic oxidation coupled with hot embossing.Then the bactericidal properties of polymer nanostructured surfaces were investigated.Through theoretical calculations and finite element numerical simulations of the mechanical interaction process between bacterial cells and nanostructured surfaces,the mechano-bactericidal mechanism of nanostructures on surfaces was revealed.Details are as follows:Using anodic oxidation coupled with hot embossing technology,the cicada-wing-like nanopillars were prepared on PMMA and PC surfaces.By changing the concentration of sulfuric acid in the electrolyte and the corresponding oxidation voltage in the one-step hard anodizing process,as well as the oxidation time in the multi-step mild anodizing,the precise regulation of pore spacing and height of porous AAO templates was achieved.At the same time,the precise regulation of nanopillar diameter of the polymer surface was also achieved by combining with the wet etching technique.Thus,the regulation of the geometric parameters of nanopillars on polymer surfaces was completed.The spacing,height and tip diameter of the prepared surface nanopillars are in the ranges of 100-320,150-450 and 30-130 nm,respectively.The viability of bacteria adhered to the polymer surfaces was examined by fluorescence staining and scanning electron microscope techniques.The experimental results showed that the prepared nanostructured surfaces have mechano-bactericidal activity against both Gramnegative Escherichia coli and Gram-positive Staphylococcus aureus,and that the subtle changes in nanomorphology could greatly influence the bactericidal performance of the surface.The PC nanopillars with an interpillar spacing of 170 nm,a tip diameter of less than 60 nm and a height of more than 200 nm were able to kill almost all the adherent E.coli cells,with a maximum bactericidal efficiency of 98%.The effect of the spacing,diameter and height of the nanopillars on the bactericidal efficiency was systematically investigated using the control variables method.The results show that a certain nanopillar height is required for an efficient bactericidal nanostructured surface;the smaller the tip diameter of the nanopillars,the better the bactericidal activity of the surface;both larger and smaller interpillar spacing of nanostructured surfaces reduce their bactericidal properties.A mathematical model of the interaction between the nanostructures and the adhered bacterial cells was developed and calculated.It is concluded that the interfacial energy gradient between cells and nanopillars is the driving forces for nanostructured surface killing bacteria,and the magnitude of this driving forces is at the n N level.Additionally,this research reveals the microscopic process mechanism of the nanostructures killing bacteria,i.e.,bacteria migrate toward the interior of nanostructures under the action of this interfacial energy gradient,which forms the pressure on cells.When the pressure is greater than the cell wall critical elastic stress,cells begin to creep and finally rupture.Meanwhile,the deformation of cells pressured by nanostructures was calculated.The results show that the nanostructure parameters,cell volume and its adhesion morphology,and other factors affect the interfacial energy gradient or the pressure on cells,as well as the cell deformation.Some results calculated by this model were compared with the experimental results from literatures and found to be in good agreement,indicating that this model is reasonable and reliable.A three-dimensional thin wall with turgor pressure finite element model(3D-TWTP-FEM)of bacterial cell was established by finite element simulation technique.The reliability of this cell model was verified by the AFM indentation experiment simulation of the cell.The 3DTWTP-FEM is able to simulate suspended bacterial cell and the process of cell adhering to the flat and nanopillar surfaces.The mechano-bactericidal property of the nanostructured surfaces was demonstrated by quantitatively comparing the maximum stresses and strains of cells on smooth surfaces with those on nanostructured surfaces.The simulation results of the mechanical interaction process between bacterial cells and nanopillars reveal the rupture mechanism of the cells.They are damaged firstly at the three-phase contact line of liquid-cellnanopillar.Using the two-stage deformation model of cell on the nanopatterned surfaces,i.e.,elastic deformation stage and creep deformation stage,the effects of nanopillar geometrical parameters on the bactericidal efficiency and bactericidal speed can be calculated.