Ana Conde

In vitro assessment of Staphylococcus epidermidis and Staphylococcus aureus adhesion on TiO2 nanotubes on Ti–6Al–4V alloy

The aim of this study was to evaluate Staphylococcus sp. adhesion to modified surfaces of titanium alloy (Ti-6Al-4V). Specimens of Ti-6Al-4V alloy 6-4 ELI-grade 23 that meets the requirements of ASTM F136 2002A (AMS 2631B class A1) were anodized in a mixture of sulfuric/hydrofluoric acid at 20 V for 5 and 60 min to form nanoporous (NP) and nanotubular (NT) oxide layers with pore diameter of 20 and 100 nm, respectively. The amount of fluorine incorporated in the oxide films from the electrolyte was 6 and 4 wt %, respectively. Bacterial adherence was studied using laboratory strains and six clinical strains each of Staphylococcus aureus and Staphylococcus epidermidis. Lower adherence of laboratory strains was demonstrated on fluoride nanostructured surfaces in comparison with the fluoride-free surfaces. Significant differences between clinical strains and laboratory strains were also found (p < 0.0001, Kruskal-Wallis test) when NP and NT specimens were compared with chemically polished (CP) surfaces. The results of the tests using multiple clinical strains confirmed a decrease in bacterial adherence on F-containing titanium oxide surfaces, suggesting a potential applicability of this surface, with a confirmed added value of decreasing clinical staphylococci adherence, for medical prosthetic devices.

Doped TiO2 anodic layers of enhanced antibacterial properties

Ti-6Al-4V joint replacement implants foster uncemented fixation in orthopaedic surgery. However, bacterial colonization competes with host cells and ultimately may produce implant-related difficult-to-treat infections, justifying the efforts to obtain infection-resistant materials. In a previous work, the authors demonstrated the antibacterial properties of anodic fluoride-TiO2 nanostructured layers on Ti-6Al-4V alloy. In this work, the anodizing bath has been modified in order to grow fluoride-TiO2 barrier layers (FBL). A bacterial adherence protocol, run with reference and six different clinical strains of Staphylococcus aureus and Staphylococcus epidermidis, showed a statistically significant decrease in the percentage of covered surface (p<0.0001, Kruskal-Wallis test) for FBL specimens when compared with non fluoride-containing specimens, i.e. chemically polished Ti-6Al-4V and F-free TiO2 barrier layers. The results obtained on the F-barrier layers allowed discrimination between the effects of the presence of fluoride in the layer and the layer nanostructure on bacterial adhesion.

Influence of the nanostructure of F-doped TiO2 films on osteoblast growth and function

The aim of this study was to evaluate the proliferation and mineralization ability of mouse osteoblastic MC3T3-E1 cells on F-containing TiO2 films with different morphology and nanostructure that previously confirmed antibacterial properties. F-containing TiO2 films were fabricated by anodizing Ti-6Al-4V alloy ELI -grade 23. By using a mixture of H2SO4/HF acid at 20 V for 5 and 60 min, a TiO2 film grows with nanoporous (NP) and nanotubular (NT) features, characterized with a pore diameter of 20 and 100 nm, respectively. Fluoride-TiO2 barrier films (FBL) were produced in 1M NH4H2PO4/0.15M NH4F solution at constant voltage controlled at 20 V for 120 min. The amount of F incorporated in the nanostructured oxide films was 6 at % and of 4 at %, for the NP and NT, respectively, while for the FBL film was 12 at %. MC3T3-E1 cells exhibited different behavior when seeded and grown onto these surfaces. Thus, F-doped TiO2 films with NP structures increased proliferation as well as osteogenic gene expression and the mineralization capacity of these osteoblastic cells. These results confirm that anodizing process is suitable to fabricate multifunctional surfaces on Ti-6Al-4V alloy with improved not only antibacterial but also osteogenic properties useful for bone fixation of prosthetic devices

The role of mechanically activated area on tribocorrosion of CoCrMo

Co-Cr-Mo alloys are among the most used alloys for orthopedic implants because of their excellent corrosion resistance, mechanical properties, and biocompatibility. Although there is extensive literature on corrosion properties of Co-Cr-Mo alloys, fewer articles are focused on the synergistic effect of corrosion and wear in a simulated physiological solution. It is generally assumed that the current density measured during wear conditions for passive materials comes from the active area. However, there are no clear data supporting this statement. The current article correlates electrochemical measurements with the active area generated during sliding wear tests. Open circuit potential and current measurements, potentiodynamic scans, and electrochemical impedance spectroscopy were carried out on samples under static and sliding wear conditions. These measurements showed the importance of the active area, where the current coming from the surface not being abraded is negligible. Finally, by combining the sliding wear and electrochemical tests, the synergistic effect of wear and corrosion was characterized for this alloy, documenting the metal carbides detachment from the cobalt alloy matrix, which leads to a significant increase of total wear volume.

Influence of the base preheating on cracking of the laser-cladded coatings

Coatings of stellite SF6 are prepared on the chromium steel base by means of a direct laser cladding. A 1.2 kW CO2 laser is applied and the original material is delivered into the processing zone coaxially with the laser beam. The samples are produced with and without a controlled preheating of the substrate and are investigated by means of metallographic techniques. It is found that the observed micro-cracking susceptibility decreases markedly with increase of the base preheating temperature up to 750 K and the crack-free coatings are produced for preheating around 950 K. The substrate-coating interface reveals a metallic bond and the microstructure is characterized by a fine-grained, dendritic structure. The nearly constant concentration dependence of Fe, Co, Cr and Ni on the distance from interface indicates on homogeneous chemical composition of the produced coatings.

Microstructural and Wear Behavior Characterization of Porous Layers Produced by Pulsed Laser Irradiation in Glass-Ceramics Substrates

In this work, wear behavior and microstructural characterization of porous layers produced in glass-ceramic substrates by pulsed laser irradiation in the nanosecond range are studied under unidirectional sliding conditions against AISI316 and corundum counterbodies. Depending on the optical configuration of the laser beam and on the working parameters, the local temperature and pressure applied over the interaction zone can generate a porous glass-ceramic layer. Material transference from the ball to the porous glass-ceramic layer was observed in the wear tests carried out against the AISI316 ball counterface whereas, in the case of the corundum ball, the wear volume loss was concentrated in the porous layer. Wear rate and friction coefficient presented higher values than expected for dense glass-ceramics.

Bacterial and fungal biofilm formation on anodized titanium alloys with fluorine

Orthopaedic device-related infections are closely linked to biofilm formation on the surfaces of these devices. Several modified titanium (Ti-6Al-4V) surfaces doped with fluorine were studied in order to evaluate the influence of these modifications on biofilm formation by Gram-positive and Gram-negative bacteria as well as a yeast. The biofilm studies were performed according to the standard test method approved by ASTM (Designation: E2196-12) using the Rotating Disk Reactor. Four types of Ti-6Al-4V samples were tested; chemically polished (CP), two types of nanostructures containing fluorine, nanoporous (NP) and nanotubular (NT), and non-nanostructured fluorine containing samples (fluoride barrier layers, FBL). Different species of Gram-positive cocci, (Staphylococcus aureus and epidermidis), Gram-negative rods (Escherichia coli, Pseudomonas aeruginosa), and a yeast (Candida albicans) were studied. For one of the Gram-positive (S. epidermidis) and one of the Gram-negative (E. coli) species a statistically-significant decrease in biofilm accumulation for NP and NT samples was found when compared with the biofilm accumulation on CP samples. The results suggest an effect of the modified materials on the biofilm formation.Graphical Abstract

Microstructural and Micromechanical Effects of Cold Roll-forming on High Strength Dual Phase Steels

In this work correlation between the 1000 MPa dual phase (DP) steel microstructure and the strain gained after roll-forming process have been studied by both microstructural and micromechanical analysis. The scanning electron microscope (SEM) inspection in the bent area reveals changes in the ferrite-martensitic microstructure. The plastic deformation of DP steels originates defects at the edges of bent sheet make them partly responsible for the damage caused. In addition, electron backscatter diffraction (EBSD) measurements have been carried out for an in-depth characterization after roll-forming. A high density of misorientation of the crystal lattice within the ferrite strained grains is observed, mainly concentrated in the ferrite/martensite grain boundaries. Furthermore, the ultramicrohardness tests exhibit little dependence between mechanical parameters and the material properties.

Laser cladding of Ni- and Co-based coatings for turbine industry applications

Laser cladding by means of direct remelting of allow powders Stellite SF6 and Metco 16C was applied for production of protective coatings on chromium steel substrates of composition close to that of the gas turbine blades which operate in extremely aggressive environment. For sample preparation a 1.2 kW cw CO2 laser with powder jet coaxial to the laser beam was used. Compared to previous experiments the controlled preheating of substrate at temperatures around 500-720°C was applied and resulted in crack-free coatings.

Direct laser cladding of stellite coatings and selection of process parameters

The direct laser cladding technique is applied for preparation of Co-based stellite SF6 protective coatings on the chromium steel base of a composition designed for the steam turbine blades. The cw 1.5 kW CO2 laser stand equipped with the multi-stream nozzle characterized by a powder feeding coaxial with the laser beam is used. It is observed that the increase of the base preheating temperature and the average temperature of the coating reduces the cracking susceptibility of the laser clads. This is accompanied by the decrease of the corrosion and wear resistances.