Glen West

Structural formation and photocatalytic activity of magnetron sputtered titania and doped-titania coatings.

Titania and doped-titania coatings can be deposited by a wide range of techniques; this paper will concentrate on magnetron sputtering techniques, including “conventional” reactive co-sputtering from multiple metal targets and the recently introduced high power impulse magnetron sputtering (HiPIMS). The latter has been shown to deliver a relatively low thermal flux to the substrate, whilst still allowing the direct deposition of crystalline titania coatings and, therefore, offers the potential to deposit photocatalytically active titania coatings directly onto thermally sensitive substrates. The deposition of coatings via these techniques will be discussed, as will the characterisation of the coatings by XRD, SEM, EDX, optical spectroscopy, etc. The assessment of photocatalytic activity and photoactivity through the decomposition of an organic dye (methylene blue), the inactivation of E. coli microorganisms and the measurement of water contact angles will be described. The impact of different deposition technologies, doping and co-doping strategies on coating structure and activity will be also considered.

Mass spectrometric investigation of the ionic species in a dielectric barrier discharge operating in helium-water vapour mixtures

Using advanced mass spectrometry the chemistry of ionic species present in an atmospheric-pressure parallel plate dielectric barrier discharge (DBD) with a single dielectric on the powered electrode have been identified. The discharge was driven in helium with controllable concentrations of water vapour using an excitation frequency of 10 kHz and an applied voltage of 1.2 kV. Both negative and positive ions were identified and their relative intensity determined with variation of water concentration in the discharge, inter-electrode spacing, gas residence time and nominal applied power. The most abundant negative ions were of the family , while the positive ions were dominated by those of the form , with n up to 9 in both cases. Negative and positive ions responded in a similar way to changes in the operating parameters, with the particular response depending on the ion mass. Increasing the inter-electrode spacing and the water concentration in the discharge led to an increase in the intensity of large mass ionic water clusters. However, increasing the residence time of the species in the plasma region and increasing the applied power resulted in fragmentation of large water clusters to produce smaller ions.

Antimicrobial activity of nanocomposite zirconium nitride/silver coatings to combat external bone fixation pin infections.

During external fixation, temporary implants are used to penetrate the skin, muscle and bone to support severely fractured bones. This creates a biologically critical interface at the site of entry, which potentially allows a risk of infection. The aim of this study, therefore, was to investigate potential antimicrobial nanocomposites to combat infection. Magnetron sputtering was used to produce zirconium nitride/silver nanocomposite coatings, which were prepared at two different silver concentrations of 15.5 at.% and 29.8 at.%. These coatings were characterized for morphology, chemical composition, and antimicrobial activity in comparison to pure zirconium nitride and stainless steel. Staphylococcus aureus and Staphylococcus epidermidis were used as in vitro test organisms in a range of antimicrobial assays; retention of the bacteria on the surfaces and their survival using LiveDead™ staining; the use of a metabolic redox dye to indicate a contact kill and zone of inhibition assays to indicate leaching of inhibitory silver ions. Antimicrobial tests demonstrated a significant kill when the bacterial cells came in contact with the coatings containing silver at both 15.5 at.% and 29.8 at.%. No inhibitory leaching from the surfaces occurred. These surfaces demonstrate potential for use as antimicrobial fixation pin coatings.

Quantifying the pattern of microbial cell dispersion, density and clustering on surfaces of differing chemistries and topographies using multifractal analysis.

The effects of surface topography on bacterial distribution across a surface are of extreme importance when designing novel, hygienic or antimicrobial surface coatings. The majority of methods that are deployed to describe the pattern of cell dispersion, density and clustering across surfaces are currently qualitative. This paper presents a novel application of multifractal analysis to quantitatively measure these factors using medically relevant microorganisms (Staphylococcus aureus or Staphylococcus epidermidis). Surfaces (medical grade 316 stainless steel) and coatings (Ti-ZrN, Ti-ZrN/6.0%Ag, Ti-ZrN/15.6%Ag, TiZrN/24.7%Ag) were used in microbiological retention assays. Results demonstrated that S. aureus displayed a more heterogeneous cell dispersion (∆αAS<1) whilst the dispersion of S. epidermidis was more symmetric and homogeneous (∆αAS≥1). Further, although the surface topography and chemistry had an effect on cell dispersion, density and clustering, the type of bonding that occurred at the surface interface was also important. Both types of cells were influenced by both surface topographical and chemical effects; however, S. aureus was influenced marginally more by surface chemistry whilst S. epidermidis cells was influenced marginally more by surface topography. Thus, this effect was bacterially species specific. The results demonstrate that multifractal analysis is a method that can be used to quantitatively analyse the cell dispersion, density and clustering of retained microorganisms on surfaces. Using quantitative descriptors has the potential to aid the understanding the effect of surface properties on the production of hygienic and antimicrobial coatings.

A Comparison of Thin Silver Films Grown Onto Zinc Oxide via Conventional Magnetron Sputtering and HiPIMS Deposition

Silver coatings are commonly deposited via dc magnetron sputtering for use in a variety of applications that require high transparency to visible light and good electrical conductivity, including, for example, low-emissivity coatings. The films need to be fully dense in order to achieve the correct optical and electrical properties, although the typical growth mechanism for silver films is by the nucleation and coalescence of islandlike structures that can result in the generation of voids within the coating and uneven surface topography. As a consequence, in order to achieve acceptable electrical properties, the optical transparency of the silver coating is often compromised due to the excessive film thicknesses required for continuous electrically conductive layers. Conventional methods used to enhance the adatom mobility, hence increasing the density of the coatings, include the application of an electrical bias to the substrate or substrate heating, both of which can be problematic for large-area dielectric substrates such as float glass or polymer web. One alternative is the production of a coating using a deposition flux with a high fraction of ionization. This can be achieved via high-power impulse magnetron sputtering (HiPIMS), leading to an enhanced delivery of energy to the adatoms via recombination at the substrate surface. Thin films of silver were deposited onto zinc oxide-coated glass substrates via continuous dc and pulsed-dc magnetron sputtering and also via HiPIMS at the same time-averaged power in order to compare the structure and growth mechanisms via techniques, including AFM, XRD, and Hall-effect measurements for the electrical characterization.

Improved durability of dielectric coatings for large-area applications on glass via ion beam pretreatment of the substrate

Dielectric coatings are commonly deposited onto large-area glass substrates via the magnetron sputtering process. High refractive index materials such as titania and zinc oxide are employed in a wide range of multilayer coating systems, some of which can experience problems with coating durability. A standard magnetron sputtering system has been enhanced via the inclusion of a Hall-current-type ion source, which has been used for the pretreatment of the glass substrate surfaces prior to deposition. Resulting TiO2 and ZnO films exhibited substantial improvements in mechanical durability (up to 80% increase in critical load and 200% increase in wear resistance, as compared with those grown on untreated glass) with no detrimental effect to their optical performance. These improvements to coating performance, along with the suitability of the ion source to inclusion in commercial coaters, demonstrate a potential for significant benefit to the large-area glass coating industry.

Antimicrobial activity of Ti-ZrN/Ag coatings for use in biomaterial applications

Severely broken bones often require external bone fixation pins to provide support but they can become infected. In order to reduce such infections, novel solutions are required. Titanium zirconium nitride (Ti-ZrN) and Ti-ZrN silver (Ti-ZrN/Ag) coatings were deposited onto stainless steel. Surface microtopography demonstrated that on the silver containing surfaces, Sa and Sv values demonstrated similar trends whilst the Ra, average height and RMS value and Sp values increased with increasing silver concentration. On the Ti-ZrN/Ag coatings, surface hydrophobicity followed the same trend as the Sa and Sv values. An increase in dead Staphylococcus aureus and Staphylococcus epidermidis cells was observed on the coatings with a higher silver concentration. Using CTC staining, a significant increase in S. aureus respiration on the silver containing surfaces was observed in comparison to the stainless steel control whilst against S. epidermidis, no significant difference in viable cells was observed across the surfaces. Cytotoxicity testing revealed that the TiZrN coatings, both with and without varying silver concentrations, did not possess a detrimental effect to a human monocyte cell line U937. This work demonstrated that such coatings have the potential to reduce the viability of bacteria that result in pin tract infections.

The effects of blood conditioning films on the antimicrobial and retention properties of zirconium-nitride silver surfaces

External bone fixation devices provide support and rehabilitation for severely damaged/broken bones, however, this invasive procedure is prone to infection. Zirconium nitride/silver (Ti-ZrN/Ag) coatings were characterised for surface topography, chemical composition, physicochemistry and antimicrobial efficacy (against Staphylococcus aureus and Staphylococcus epidermidis), in the presence of a blood conditioning film. The conditioning film altered the width of the microtopography of the surfaces however, the depth of the features remained relatively constant. The conditioning film also altered the coatings from hydrophobic to hydrophilic/partially hydrophilic surfaces. Following the MATH assay, the presence of a conditioning film reduced affinity towards the hydrocarbons for both microorganisms. The addition of a blood conditioning film reduced the antimicrobial efficacy of the Ti-ZrN/Ag coatings but also reduced the number of retained bacteria. This study suggests that the presence of a pre-defined blood conditioning film may result in surfaces with anti-adhesive properties, potentially leading to a reduction in bacterial retention. This, combined with the antimicrobial efficacy of the coatings, could reduce the risk of infection on biomaterial surfaces.