Materials Science Forum Vols. 638-642

Abstract: The Stellite 6 hardfacing alloys have been deposited on steel substrate using a Plasma Transferred Arc (PTA) with complex geometry. The residual strain of the PTA technology at the surface of coating layer and the interface were determined by neutron diffraction method. In the present work, a numerical model for the residual stresses formed during the PTA process with physical conditions and mechanical properties using the Abaqus code is analysed. The result reveals that the residual stresses obtained by the numerical simulation are in very good agreement with experimental results by the neutron diffraction.

Abstract: To stimulate bone regeneration, the design of bioactive implants is a great challenge in current orthopedic research. We reasoned that implants should be suitable both to stimulate osteogenic differentiation of mesenchymal stem cells and prevent infections at the site of implantation. Therefore, we focus on copper ions, which are known to exert antimicrobial effects. On the other hand, copper is essential for the cell physiology, including the formation of the extracellular matrix. We studied the influence of copper ions on mesenchymal stem cells at various concentrations and identified the limits of copper concentrations for cell survival. Below the critical concentration for cell survival we analysed proliferation and osteogenic differentiation of the cells in the presence of copper ions. We found that copper stimulated the proliferation of the mesemchymal stem cells at 0.1 mM. Osteogenic differentiation decreased after 14 days at a concentration of 0.05 - 0.1 mM copper ions in osteogenic medium measured by the expression of osteogenic proteins, like alkaline phosphatase (ALP), bone sialoprotein (BSP) and collagen I (COL). We argue that at the implant surface a higher concentration of copper could prevent biofilm formation of bacteria and physiological concentrations in the vicinity of the implant would stimulate stem cell expansion. Together, copper is an interesting agent to control both bacteria and stem cells in the field of implant technology.

Abstract: Materials in contact with tissue and biological fluids affect cell reaction that could eventually lead to clinical complications (i.e. thrombosis, restenosis). Improving the biological performances of the materials used for biomedical applications is the main goal of this study. In particular, cardiovascular devices require excellent haemo- and biocompatibility properties. PTFE is currently the main material used for vascular prostheses. After long contact periods with blood, clinical complications leading to thrombosis and restenosis are often reported. Improving the haematological performances of PTFE could significantly increase its life-time and decrease long-term complications. However, inadequately engineered surfaces could trigger the coagulation cascade with the formation of a clot, the first step towards a thrombosis. Plasma carbon-based coatings with varying nitrogen contents deposited on PTFE have been studied as promising coating to improve the haematological performances of PTFE implants. In this work, several techniques were applied to study the viscoelastic properties of blood after contact with virgin and treated PTFE as well as the presence and the clot morphology eventually formed onto the surfaces. The chemical composition of the surfaces was analysed with XPS and FTIR.

Abstract: Arg-Gly-Asp (RGD) peptidomimetics constructed on the tyrosine scaffold were equipped with an oligoethylene glycol (OEG) spacer-arm for surface grafting. Their in vitro activities as V3 integrin antagonists were in the range of 0.3-0.7 nM. Poly (ethylene terephthalate) (PET) membranes derivatized with peptidomimetics (50-120 pmol/cm2) allowed the adhesion and proliferation of endothelial cells (HSV) in serum-free medium. Iron oxide particles for MR Imaging grafted with peptidomimetics (2-3 molecules/particle of 20 nm diameter) were selectively targeted to activated tumour cells (Jurkat).

Abstract: A fretting fatigue test method in a simulated body fluid is shown to evaluate fatigue properties of metallic materials which are used in the orthopaedics field. Next, fatigue/fretting fatigue behavior in a simulated body fluid is given for 316L stainless steel, Ti-6% Al-4% V alloy, pure Ti for industrial use and Co-Cr-Mo alloy. Finally, we discuss the relationship between the tensile strength and the fatigue strength/fretting fatigue strength of metallic biomaterials at 107 cycles in air and in a simulated body fluid. For all of the biomaterials tested, the fatigue strength at 107 cycles is similar in air and in a simulated body fluid. The fatigue strength is closely correlated to the tensile strength: The fatigue strength increases with increasing tensile strength. However, a correlation is not observed between the fretting fatigue strength at 107 cycles and the fatigue strength or the tensile strength.

Abstract: Since the Medical Device Amendments of 1976 were enacted, the FDA considers Tissue Adhesives as “Transitional Devices” that are classified as Class III medical devices and are marketed in the United States subsequent to the approval of a Pre-market Approval Application (PMA). On February 9, 2006, Regulatory & Clinical Research Institute, Inc. submitted a petition to FDA to reclassify tissue adhesive transitional medical devices for skin approximation from Class III to Class II (special controls). FDA consulted with the General and Plastic Surgery Devices Advisory Panel, and on August 25, 2006, in a public meeting, the panel unanimously recommended that the tissue adhesive transitional medical devices for topical approximation of skin be classified from class III into Class II. Consequently, since June 30, 2008, following the effective date of the FDA Final Rule [1] that reclassified tissue adhesive transitional medical devices for skin approximation, any firm submitting a Premarket Notification [510(k)] for a tissue adhesive for the topical approximation of skin will need to address the issues covered in the published “Class II Special Control Guidance Document: Tissue Adhesive for the Topical Approximation of Skin, dated May 30, 2008” [2]. Accordingly, the firm needs to show that its device meets the recommendations of the published Class II guidance document or in some other way provides equivalent assurances of safety and effectiveness. Also, the author provides a short regulatory description of US FDA, under what laws its operates, how FDA evaluates new medical devices for marketing as Class I, Class II, and Class III [3].

Abstract: Microtubules (MTs) are cellular supramolecular structures that, in combination with actin and intermediate filaments, form the cell cytoskeleton. Within cytoskeleton filaments, MTs exhibit the highest bending stiffness. Up today, experimental techniques have not been able to investigate the origin of MTs flexural rigidity, despite the many experimental efforts done to estimate MT mechanical properties. Molecular Dynamic (MD) and Normal mode Analysis (NMA) show the potentiality for getting insight into this topic. However, these standard molecular modelling techniques are not yet able to simulate large molecular structures as MTs. In this work we developed a multiscale Coarse Grain (CG) model of an entire MT up to 180 nm long, by integrating information from MD and NMA molecular modelling. In particular, MD models were used to obtain information about the molecular conformation and arrangement of the tubulin dimers inside the MT lattice structure and Normal Mode Analysis (NMA) was used in order to study the mechanical behaviour of a MT modelled as an elastic network. MT macroscopic properties, such as bending stiffness (kf), bending modulus (Yf), stretching modulus (Ys), and persistence length (lp) were calculated on the basis of the bending and stretching modes, and results were directly compared to experimental data. Starting from the stretching modes calculated for MTs with lengths up to 180 nm, we found a non-length dependent Ys of about 0.5 GPa, which is in the range of the experimental values (Ys~0.1-2.5 GPa), and a Yb in the range of 0.13-0.35 GPa depending on MT length. These results strongly confirm the anisotropy of the MT mechanical properties.

Abstract: Low modulus β Ti alloys are attractive for biomedical application. This work examines the mechanical properties of Ti-Cr-Sn-Zr system alloys, especially the effect of the varying alloy composition on the microstructure, the Young’s modulus and the deformation mechanism.The Young’s modulus of the alloy varies with the composition, which variation is caused mainly from the competition between the meta-stable β phase and ω phase.The deformation modes of the Ti-Cr-Sn-Zr alloy, which are the mechanical twinning, the deformation by slip and the deformation-induced transformation, also change depending on the composition of the alloy. The minimum of the Young’s modulusof the Ti-Cr-Sn-Zr alloy in this experiment was shown in the composition where the microstructure of the alloy changes from the martensitic structure to the meta-stable β structure.

Abstract: Mechanical behavior and fracture mechanisms of plasma sprayed hydroxyapatite coatings on Ti-6Al-4V substrate were assessed taking into consideration two variables: the coating thickness and the substrate roughness. The results show that the specimens having a substrate arithmetic average roughness parameter Ra = 2.29 µm is favorable with respect to Ra = 1.23 µm. For coating thickness above 105 µm, cracks can be observed in the coating/substrate interface and the higher critical load Pc2 (used generally in comparative evaluation of adherence) decreases. A 90 µm coating thickness sprayed on a substrate having an arithmetic average roughness parameter Ra equal to 2.29 µm seems to be the best compromise between microstructure, mechanical resistance (high critical loads and fairly good contact quality) and long term stability in the physiological medium (low dissolution rate) for an orthopedic application.

Abstract: The titanium-osteoblast-interaction can be influenced both by surface roughness and by chemical modifications. We have ascertained that a positively charged titanium surface boosts osteoblast cells adhesion due to their negatively charged cellular hyaluronan coat. In current experiments, chemical surface modifications were combined with different topographies. Titanium disks of technical purity were modified (i) in their roughness by polishing (P), machining (M) and corundum blasting (CB), and (ii) by subsequently chemical functionalization by a thin film (d≤0.1 µm) of microwave plasma polymerized allylamine (PPAAm). In addition, collagen I was immobilized on PPAAm via the bifunctional linker polyethylene glycol diacid or glutar dialdehyde, respectively. The cell shape and material's contact of human osteoblasts was analyzed by FE-SEM and time dependent cell adhesion measured by flow cytometry. The cell dynamic of the adhesion component vinculin was observed in living cells. Amino-functionalization (PPAAm) considerably enhances the adhesion of osteoblasts in combination with topographical features, which was in contrast to collagen modified surfaces. PPAAm allows the cells to literally melt into the groove structure of the titanium. The bone cells lie over a large area and very close to the surface, so that the edges of the cells can hardly be distinguished from the structure of the surface. The combinatory effect of topography and plasma modification could improve bonding of the implant to the bone tissue.