Scientific publications

A novel hybrid sol-gel coating has recently been introduced as an alternative to high toxic chromate-based corrosion protection systems. In this paper, we propose a multiscale computational model to estimate the amount and time scale of inhibitor release of the active corrosion protection coating. Moreover, we study the release rate under the influence of parameters such as porosity and viscosity, which have recently been implicated in the stability of the coating. Numerical simulations obtained with the model predicted experimental release tests and recent findings on the compromise between inhibitor concentration and the stability of the coating.

Hydroxyapatite (HA) is one of the most widely used biomaterials for reconstructing skeleton. HA is mostly used in small devices that bear low mechanical load as well as in coatings since bulk HA-materials generally have poor mechanical properties. A significant effort has been spent towards reinforcing HA matrices with a variety of biocompatible elements and compounds. This study presents the experimental results of mechanical reinforcement of HA originated from biological materials (bovine femur bone, BHA) with nano-yttrium oxide (Y2O3, Sigma Aldrich™, particle size < 50 nm, purity > 99%). BHA-x wt% Y2O3 (x=5 and 10) powder blends were milled for 4 h in a planetary ball-mill with a 7:1 ball-to-powder weight ratio, using ZrO2 milling vials and balls. The milled powder blends were consolidated via cold-pressing (350 MPa) and sintering at different temperatures (1000-1300 °C) for 4 h under air atmosphere. For comparison purposes, similarly produced composites with commercial synthetic HA (CSHA) were also tested. The experimental results of density measurements and SEM observations for both BHA-composites and CSHA-composites showed that sintering satisfactorily occurred at 1300 °C. XRD analyses showed that the reinforcing oxides stabilize the lattice of HA against transformation to tri-calcium phosphate (TCP) even at 1300 °C. However, CSHA was more prone to form glassy phase after sintering at 1300 °C than BHA in the presence of the reinforcing oxides. The highest values of microhardness and compression strength were observed in the samples sintered at 1300 °C. BHA-composites have better mechanical properties than CSHA-composites. In particular, BHA-composites have higher compression strength (for 5 wt% Y2O3 reinforcement, 101.79 MPa) than CSHA (75.81 MPa). Similar results were obtained for 10 wt% Y2O3 reinforcement. However, the best behavior is shown for 10 wt% Y2O3 than 5 wt%. Probably, 10 wt% exceeds the capacity of HA lattice to accommodate the yttrium ions. The extensive formation of glassy phase in the CSHA-composites sintered at 1300 °C (443.57 HV for 5 wt% Y2O3) resulted in harder materials than the BHA-composites sintered at the same temperatures (330.2 HV for 5 wt% Y2O3). Nevertheless, hard biomaterials ruin rapidly the tissues around the implant. Consequently, the experimental results, in conjunction with the important role of Y2O3 in bone forming and healing mechanism, qualify the produced BHA-composites for further in vitro and in vivo studies.

Topography plays a major role on surface-cell interaction beside the surface chemistry. We investigated the effect of the nanotopography on vascular cell adhesion and proliferation in order to improve endothelialisation for restenosis treatment. In this context, Al2O3 nanowires (NWs) composed of a stable Al2O3 shell and an Al core were synthesized by chemical vapour deposition (CVD) of the molecular precursor (tBuOAlH2)2. After the detailed material characterization, human umbilical vein endothelial cells (HUVEC) and human umbilical vein smooth muscle cells (HUVSMC) were seeded and cultivated on these surfaces. Our preliminary results showed that there is a preference of HUVEC adhesion on NWs in comparison to that of HUVSMC. The control of the cell-surface interaction by the topography may represent a key issue for the future stent material design.

Thermal activation analysis of plastic deformation of peacock tail feathers, by temperature changes and stress relaxation, gave for the keratin cortex an activation enthalpy of 1.78±0.89eV and an activation volume of 0.83±0.13nm3, for both the blue and the white subspecies. These values suggest that breaking of electrostatic bonds is responsible for plasticity in feather keratin. These might be bonds between keratin and nonkeratinous matrix or keratin-keratin cross-links. The mechanical properties of the rachis cortex are surprisingly uniform along the length of the feathers.

Biological cells stick together via transmembrane proteins, which are linked to receptor molecules of the extracellular matrix (ECM). This specific biochemical adhesion plays a leading role in many cellular processes, among them cell differentiation, morphogenesis, and wound healing. Various medical applications require endogen cells to bind to an exogene substrate as in the case of an implant. Coatings with proteins that naturally belong to the ECM are known to enhance the cell adhesion. However, the choice of inorganic materials, which promote cell adhesion, is limited. Here, we report on a new engineered surface composed of Al/Al2O3 bi-phasic nanowires (NWs), which promotes the adhesion of fibroblast cells. Fibroblasts grow well on this inorganic layer and keep proliferating. Using the cell monolayer rheology (CMR) technique, we show that the adhesion of fibroblasts on Al/Al2O3 NWs is comparable to fibronectin coated surfaces. To our knowledge, this is one of the strongest cell adhesions on an inorganic surface, which has been reported on so far, since it compares to bio-organic layers such as fibronectin.

Luminescent Eu3+ and Er3+ doped SnO2 powders have been prepared by Sn4+ hydrolysis followed by a controlled growth reaction using a particle's surface modifier in order to avoid particles aggregation. The powders so obtained doped with up to 2 mol% rare earth ions are fully redispersable in water at pH>8 and present the cassiterite structure. Particles size range from 3 to 10 nm as determined by Photon Correlation Spectroscopy. Rare earth ions were found to be essentially incorporated into the cassiterite structure, substituting for Sn4+, for doping concentration smaller than 0.05 mol%. For higher concentration they are also located at the particles surface. The presence of Eu3+ ions at the surface of the particles hinder their growth and has therefore allowed the preparation of new materials consisting of water redispersable powders coated with Eu3+-β diketonate complexes. Enhanced UV excited photoluminescence was observed in water. SnO2 single layers with thickness up to 200 nm and multilayer coatings were spin coated on borosilicate glass substrates from the colloidal suspensions. Waveguiding properties were evaluated by the prism coupling technique. For a 0.3 μm planar waveguide single propagating mode was observed with attenuation coefficient of 3.5 dB/cm at 632.8 nm.

The protective ability of hybrid nano-composite oxysilane coatings, deposited via sol-gel method on AA2024-T3 – aluminium alloy, were studied by linear voltammetry (LVA) and electrochemical impedance spectroscopy (EIS) methods in 0.05 M solution of NaCl. Cerium chloride (CeCl3) was incorporated as an inhibitor into a sol-gel hybrid matrix in two different routes: directly and via filled porous Al2O3 nano-particle aggregates with diameters up to 500 nm. The influences of the inhibitor concentration, as well as the influence of nano-particles on the barrier properties and the susceptibility against corrosion, were evaluated and EIS spectra were fitted by appropriated equivalent circuits. The values for Ccoat, Rcoat, Coxy and Roxy were achieved and their evolution over time was investigated. The investigated coatings possess highly expressed barrier properties (106 to 107 Ω cm2). Despite of the chloride ions inside of the matrix, some samples illustrated a significant durability of over 4000 h during exposure to the corrosion medium before first signs of corrosion appeared. The electrochemical results were compared with the neutral salt spray test. Thus, it was proved that the potential of these coatings is to be used as anticorrosive protective materials and are candidate to replace Cr(VI)-based anti-corrosion coatings.