A. Vladescu

A biodegradable AZ91 magnesium alloy coated with a thin nanostructured hydroxyapatite for improving the corrosion resistance

The main aim of this study was to investigate the properties of an AZ91 alloy coated with nanostructured hydroxyapatite (HA) prepared by radio frequency (RF) magnetron sputtering. The bioactivity and biomineralization of the AZ91 magnesium alloy coated with HA were investigated in simulated body fluid (SBF) via an in vitro test. Scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, and X-ray diffraction (XRD) analyses were performed. The samples were immersed in SBF to study the ability of the surface to promote the formation of an apatite layer as well as corrosion resistance and mass change of the HA-coated AZ91 alloy. Electrochemical tests were performed to estimate the corrosion behaviour of HA-coated and uncoated samples. The results revealed the capability of the HA coating to significantly improve the corrosion resistance of the uncoated AZ91 alloy.

Mechanical properties and biocompatibility of the sputtered Ti doped hydroxyapatite

The hydroxyapatite enriched with Ti were prepared as possible candidates for biomedical applications especially for implantable devices that are in direct contact to the bone. The hydroxyapatites with different Ti content were prepared by RF magnetron sputtering on Ti-6Al-4V alloy using pure hydroxyapatite and TiO2 targets. The content of Ti was modified by changing the RF power fed on TiO2 target. The XPS and FTIR analyses revealed the presence of hydroxyapatite structure. The hardness and elastic modulus of the hydroxyapatite were increased by Ti addition. After 5 days of culture, the cell viability of the Ti-6Al-4V was enhanced by depositing with undoped or doped hydroxyapatite. The Ti additions led to an increase in cell viability of hydroxyapatite, after 5 days of culture. The electron microscopy showed the presence of more cells on the surface of Ti-enriched hydroxyapatite than those observed on the surface of the uncoated alloys or undoped hydroxyapatite.

Enhancement of the mechanical properties of hydroxyapatite by SiC addition.

Improvements of mechanical and anticorrosive properties, as well as superior osseointegration of the hydroxyapatite coated titanium alloy were reported in the last years by the addition of different elements (Si or Ti) into hydroxyapatite structure. The aim of this work was to prepare and to investigate the hydroxyapatite (HAP) coatings enriched with SiC in order to enhance the mechanical properties of HAP films. The coatings were deposited on Ti6Al4V alloy substrates by co-sputtering of HAP and SiC targets, using a magnetron sputtering system. The films were characterized in terms of elemental and phase composition, chemical binding, morphology and mechanical properties by EDS, XRD, FTIR, SEM, AFM, and nanoindentation. Overall, improved mechanical properties were found by adding SiC to the basic HAP structure.

Study on a hydrophobic Ti-doped hydroxyapatite coating for corrosion protection of a titanium based alloy

In the paper, hydroxyapatite coatings enriched with Ti were prepared as a possible candidate for biomedical applications, especially for implantable devices that are in direct contact with bone. The hydroxyapatite coatings with different Ti contents were prepared by an RF magnetron sputtering method on a Ti6Al4V alloy using pure hydroxyapatite and TiO2 targets. The Ti content was modified by changing the RF power fed to the TiO2 target. The formation of the hydroxyapatite compound was not influenced by the addition of Ti. The Ca/P ratio of the Ti-doped hydroxyapatite coatings was found to be in the range between 1.64 and 1.68, which is close to the stoichiometric hydroxyapatite coating. The roughness of the doped hydroxyapatite coatings was augmented by increasing the RF power on the TiO2 cathode. The addition of Ti led to an increase in the contact angle of the hydroxyapatite coatings. The in vitro corrosion performance of the Ti6Al4 alloy was improved significantly by the hydrophobic hydroxyapatite coatings with and without the Ti addition. A surface with a higher Ti concentration and water contact angle exhibited a better corrosion resistance in simulated body fluid at 37 °C. Therefore, the deposition of a hydrophobic Ti-doped HA coating could be a promising surface treatment for the improvement of the electrochemical behaviour of metallic implants.

In Vitro Biocompatibility of Si Alloyed Multi-Principal Element Carbide Coatings.

In the current study, we have examined the possibility to improve the biocompatibility of the (TiZrNbTaHf)C through replacement of either Ti or Ta by Si. The coatings were deposited on Si and 316L stainless steel substrates by magnetron sputtering in an Ar+CH4 mixed atmosphere and were examined for elemental composition, chemical bonds, surface topography, surface electrical charge and biocompatible characteristics. The net surface charge was evaluated at nano and macroscopic scale by measuring the electrical potential and work function, respectively. The biocompatible tests comprised determination of cell viability and cell attachment to the coated surface. The deposited coatings had C/(metal+Si) ratios close to unity, while a mixture of metallic carbide, free-carbon and oxidized species formed on the film surface. The coatings’ surfaces were smooth and no influence of surface roughness on electrical charge or biocompatibility was found. The biocompatible characteristics correlated well with the electrical potential/work function, suggesting a significant role of surface charge in improving biocompatibility, particularly cell attachment to coatings surface. Replacement of either Ti or Ta by Si in the (TiZrNbTaHf)C coating led to an enhanced surface electrical charge, as well as to superior biocompatible properties, with best results for the (TiZrNbSiHf)C coating.

Characterization of the Ti-10Nb-10Zr-5Ta Alloy for Biomedical Applications. Part 2: Wettability, Tribological Performance and Biocompatibility

The Ti-10Nb-10Zr-5Ta alloy, prepared in a levitation melting furnace, was investigated as a possible candidate for replacing Ti6Al4V alloy in medical applications. The sessile drop method, pin-on-disc and in vitro tests were used to analyze wettability, wear resistance, and biocompatibility of the new alloy. The characteristics of the Ti-10Nb-10Zr-5Ta alloy were assessed in comparison to those of the Ti6Al4V alloy. The Ti-10Nb-10Zr-5Ta system was found to have hydrophilic characteristics with similar contact angle as the Ti6Al4V alloy. In all environments (deionized water, simulated body fluid and Fusayama Meyer artificial saliva), the friction coefficient showed a stable evolution versus sliding distance, being similar for both alloys. On overall, the wear resistance of Ti-10Nb-10Zr-5Ta alloy was lower than that of Ti6Al4V for all testing environments. The Ti-10Nb-10Zr-5Ta alloy exhibited good biocompatibility characteristics at in vitro test compared to Ti6Al4V alloy. The cell viability on Ti-10Nb-10Zr-5Ta surfaces was higher than the one observed on Ti6Al4V samples, regardless the number of days spent in osteoblast-like cells culture. A high degree of cell attachment and spreading was observed on both alloys.

Radio Frequency Magnetron Sputter Deposition as a Tool for Surface Modification of Medical Implants

The resent advances in radio frequency (RF)‐magnetron sputtering of hydroxyapatite films are reviewed and challenges posed. The principles underlying RF‐magnetron sput‐ tering used to prepare calcium phosphate‐based, mainly hydroxyapatite coatings, are discussed in this chapter. The fundamental characteristic of the RF‐magnetron sputtering is an energy input into the growing film. In order to tailor the film properties, one has to adjust the energy input into the substrate depending on the desired film properties. The effect of different deposition control parameters, such as deposition time, substrate tem‐ perature, and substrate biasing on the hydroxyapatite (HA) film properties is discussed.

Evaluation of Ag containing hydroxyapatite coatings to the Candida albicans infection

In this research work, the synthesis of Ag doped hydroxyapatite coatings for dental or orthopedic implants was performed. The main goal was to determine the influence of Ag content on the roughness and antimicrobial performance of the prepared thin films. The films were deposited on Ti6Al4V alloy by means of RF magnetron sputtering. Those coatings were characterized by X-ray diffraction (XRD) and 3D surface profilometry. The antifungal activity after 1 and 7days of culture was evaluated in the presence of Candida albicans (ATCC - 10231). The increase of Ag content increased roughness and reduced the antifungal activity. The results showed that the Ag doped hydroxyapatite coatings can be a potential solution for the improvement of the antifungal activities of Ti based alloy.

Influence of the electrolyte’s pH on the properties of electrochemically deposited hydroxyapatite coating on additively manufactured Ti64 alloy

Properties of the hydroxyapatite obtained by electrochemical assisted deposition (ED) are dependent on several factors including deposition temperature, electrolyte pH and concentrations, applied potential. All of these factors directly influence the morphology, stoichiometry, crystallinity, electrochemical behaviour, and particularly the coating thickness. Coating structure together with surface micro- and nano-scale topography significantly influence early stages of the implant bio-integration. The aim of this study is to analyse the effect of pH modification on the morphology, corrosion behaviour and in vitro bioactivity and in vivo biocompatibility of hydroxyapatite prepared by ED on the additively manufactured Ti64 samples. The coatings prepared in the electrolytes with pH = 6 have predominantly needle like morphology with the dimensions in the nanometric scale (~30 nm). Samples coated at pH = 6 demonstrated higher protection efficiency against the corrosive attack as compared to the ones coated at pH = 5 (~93% against 89%). The in vitro bioactivity results indicated that both coatings have a greater capacity of biomineralization, compared to the uncoated Ti64. Somehow, the coating deposited at pH = 6 exhibited good corrosion behaviour and high biomineralization ability. In vivo subcutaneous implantation of the coated samples into the white rats for up to 21 days with following histological studies showed no serious inflammatory process.

Mechanical, Corrosion and Biological Properties of Room-Temperature Sputtered Aluminum Nitride Films with Dissimilar Nanostructure

Aluminum Nitride (AlN) has been long time being regarded as highly interesting material for developing sensing applications (including biosensors and implantable sensors). AlN, due to its appealing electronic properties, is envisaged lately to serve as a multi-functional biosensing platform. Although generally exploited for its intrinsic piezoelectricity, its surface morphology and mechanical performance (elastic modulus, hardness, wear, scratch and tensile resistance to delamination, adherence to the substrate), corrosion resistance and cytocompatibility are also essential features for high performance sustainable biosensor devices. However, information about AlN suitability for such applications is rather scarce or at best scattered and incomplete. Here, we aim to deliver a comprehensive evaluation of the morpho-structural, compositional, mechanical, electrochemical and biological properties of reactive radio-frequency magnetron sputtered AlN nanostructured thin films with various degrees of c-axis texturing, deposited at a low temperature (~50 °C) on Si (100) substrates. The inter-conditionality elicited between the base pressure level attained in the reactor chamber and crystalline quality of AlN films is highlighted. The potential suitability of nanostructured AlN (in form of thin films) for the realization of various type of sensors (with emphasis on bio-sensors) is thoroughly probed, thus unveiling its advantages and limitations, as well as suggesting paths to safely exploit the remarkable prospects of this type of materials.