Glenn Cassar

Influence of cross-rolling on the micro-texture and biodegradation of pure iron as biodegradable material for medical implants.

Iron-based biodegradable metals have been shown to present high potential in cardiac, vascular, orthopaedic and dental in adults, as well as paediatric, applications. These require suitable mechanical properties, adequate biocompatibility while guaranteeing a low toxicity of degradation products. For example, in cardiac applications, stents need to be made by homogeneous and isotropic materials in order to prevent sudden failures which would impair the deployment site. Besides, the presence of precipitates and pores, chemical inhomogeneity or other anisotropic microstructural defects may trigger stress concentration phenomena responsible for the early collapse of the device. Metal manufacturing processes play a fundamental role towards the final microstructure and mechanical properties of the materials. The present work assesses the effect of mode of rolling on the micro-texture evolution, mechanical properties and biodegradation behaviour of polycrystalline pure iron. Results indicated that cross-rolled samples recrystallized with lower rates than the straight-rolled ones due to a reduction in dislocation density content and an increase in intensity of {100} crystallographic plane which stores less energy of deformation responsible for primary recrystallization. The degradation resulted to be more uniform for cross-rolled samples, while the corrosion rates of cross-rolled and straight-rolled samples did not show relevant differences in simulated body solution. Finally, this work shows that an adequate compromise between biodegradation rate, strength and ductility could be achieved by modulating the deformation mode during cold rolling.

The effect of benzalkonium chloride additions to AH Plus sealer. Antimicrobial, physical and chemical properties

OBJECTIVES The aim of this study was to determine the antimicrobial and antibiofilm activities and physicochemical properties of AH Plus sealer mixed with different concentrations of benzalkonium chloride (BC). METHODS AH Plus was tested alone and mixed with 1%, 2% and 3% of BC. The antimicrobial and antibiofilm activities of the sealers against Enterococcus faecalis were evaluated by the direct contact test (DCT) and by confocal laser scanning microscopy, respectively. Setting time, flow and solubility were assessed according to ANSI/ADA specifications. Microhardness and contact angle tests were also performed. The chemical changes of the sealers were evaluated by X-ray diffraction analysis, and both Fourier transform infrared spectroscopy (FT-IR) and attenuated total reflectance Fourier transform infrared (ATR FT-IR). RESULTS AH Plus+3% BC was the only sealer to promote total elimination of E. faecalis and the biovolume in this group was significantly lower than in the rest of the sealers (p>0.05). The physical properties of the sealers were according to the ANSI/ADA specifications. The microhardness decreased significantly when BC was added and a significant reduction in contact angle was obtained when incorporating 2% and 3% BC (p<0.05). No phase changes were observed with the modified sealers. CONCLUSIONS The addition of 2% or higher concentrations BC to AH Plus showed antimicrobial and antibiofilm activities without affecting the properties specified in ANSI/ADA standards. However, additives to the root canal sealer altered other physical and chemical properties that are not commonly found in the literature to evaluate filling materials. CLINICAL SIGNIFICANCE The present study highlights that the antimicrobial properties of AH Plus can be significantly improved with the addition of BC. Testing beyond what is specified in standards may be indicated.

Mechanical properties of pristine and nanoporous graphene

We present molecular dynamics simulations of monolayer graphene under uniaxial tensile loading. The Morse, bending angle, torsion and Lennard-Jones potential functions are adopted within the mdFOAM library in the OpenFOAM software, to describe the molecular interactions in graphene. A well-validated graphene model using these set of potentials is not yet available. In this work, we investigate the accuracy of the mechanical properties of graphene when derived using these simpler potentials, compared to the more commonly used complex potentials such as the Tersoff-Brenner and AIREBO potentials. The computational speed up of our approach, which scales O(1.5N), where N is the number of carbon atoms, enabled us to vary a larger number of system parameters, including graphene sheet orientation, size, temperature and concentration of nanopores. The resultant effect on the elastic modulus, fracture stress and fracture strain is investigated. Our simulations show that graphene is anisotropic, and its mechanical properties are dependant on the sheet size. An increase in system temperature results in a significant reduction in the fracture stress and strain. Simulations of nanoporous graphene were created by distributing vacancy defects, both randomly and uniformly, across the lattice. We find that the fracture stress decreases substantially with increasing defect density. The elastic modulus was found to be constant up to around 5% vacancy defects, and decreases for higher defect densities.

Effect of Build Orientation of Electron Beam Melting on Microstructure and Mechanical Properties of Ti-6Al-4V

Build orientation influences thermal activity during the EBM process, thus affecting the resultant bulk material properties of the part being produced. This work focuses on EBM build orientation with respect to the X, Y and Z axes and its effect on microstructure and mechanical performance of Ti-6Al-4V parts. A series of EBM Ti-6Al-4V specimens were fabricated using an Arcam S12 setup in different build orientations: XY, ZX, ZY, XY 30° and XY 60° inclination to the start plate. Using conventionally wrought Ti-6Al-4V as a benchmark, EBM specimens were tensile and impact tested. Furthermore, microhardness measurements, optical and electron microscopy were used for characterization. Horizontally oriented EBM Ti-6Al-4V parts develop finer lamellar microstructures as a result of higher cooling rates, however exhibiting comparable strength and rather lower ductility and toughness when compared to vertically oriented parts. Solidification defects, resulting from inconsistencies in melting due to high cooling rates but also related to specimen geometry, have countered the influence of a finer microstructure. This study has also shown that EBM parts develop columnar prior-β grains which follow build direction. Benchmark wrought Ti-6Al-4V specimens show higher tensile properties while offering increased resistance to crack nucleation due to their homogeneous equiaxed microstructure.

Surface Microstructural Changes and Release of Ions from Dental Metal Alloy Removable Prostheses in Patients Suffering from Acid Reflux

PURPOSE To investigate the surface microstructural changes and the release of ions from metal alloys used in removable dental prostheses and the potential effects of acidic reflux found in patients suffering from gastroesophageal reflux disease (GERD). MATERIALS AND METHODS Thirty-seven (37) patients were recruited. Data were gathered through a questionnaire and clinical examination. Samples of metal alloy from the dentures and patients saliva were collected. GERD was confirmed using the GerdQ questionnaire. Denture samples were characterized using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), while salivary samples were tested for trace metal ions using inductively coupled plasma (ICP). RESULTS Characterization of denture samples revealed the presence of nickel, cobalt, and chromium. Nickel-chromium exhibited an etched surface appearance, while cobalt-chromium exhibited no noticeable surface microstructural changes. Higher mean salivary levels of chromium and cobalt in patients wearing any metal alloy-based denture and of chromium and nickel in patients wearing Ni-Cr prostheses were found to be significant. No differences were found in salivary metal ion levels of patients suffering from GERD. CONCLUSIONS Nickel-chromium alloy is prone to acid etching in the oral cavity, while cobalt-chromium alloy appears to be more resistant. Cobalt, chromium, and nickel are leached in saliva of patients using cast removable prostheses. The impact of gastric acid on metal ion release from dental metal alloys deserves further investigations. CLINICAL SIGNIFICANCE This preliminary study suggests that metal-based removable prostheses leach trace metal ions in saliva. Nickel-chromium-based dentures exhibit an etched appearance unrelated to GERD.