Fluoroapatite glass-ceramic coating on alumina: surface behavior with biological fluids.

The results of a surface analysis performed on a fluoroapatite-based glass ceramic (SAF) also coating a full-density alpha-alumina substrate (SAF-alumina coating) are presented. These two materials have also been evaluated after soaking in simulated body fluid to understand their ability to induce hydroxyapatite growth on them. Aiming to understand the fluoroapatite glass-ceramic interaction with some plasma proteins, in the second part of this study, fibronectin, albumin, immunoglobulin G, IgA, and complement factor C3c SAF binding have been evaluated; surface activity on complement activation has also been quantified. SAF-alumina coating provides good sites for the nucleation and growth of an apatite layer, equivalent to the mineral component of bone and binds preferentially plasma fibronectin, which is well known to enhance cell adhesion and spreading. Moreover, SAF-alumina coating reduces alumina complement activation directly or via reduced IgA binding. Alumina was shown to bind the same C3 fragments as Zymosan, used as complement activating control, and to induce increased levels of serum soluble iC3b and Bb. A mechanical resistant material with enhanced bioactivity, bone integration, and reduced inflammatory potential respect to alumina has been obtained.

Fluoroapatite glass-ceramic coatings on alumina: structural, mechanical and biological characterisation.

The aim of this work was to realise bioactive coatings on full density alpha-alumina substrates. An SiO2-CaO-based glass (SC) and an SiO2-Al2O3-P2O5-K2O-CaO-F--based glass-ceramic (SAF) were used for this purpose. Specifically, SAF is a fluoroapatite containing glass-ceramic and previous studies have shown that it is a highly bioactive biomaterial. Furthermore, these fluoroapatite crystals possess a needle-shaped morphology which mimics that of hydroxylapatite found in human hard tissues, particularly in teeth. SAF is a very viscous glass-ceramic and for this reason an intermediate, less viscous, SC layer was interposed in direct contact with alumina aiming to obtain a good coating adhesion. Moreover, this intermediate layer strongly lowers the Al3+ diffusion and thus minimises both compositional changes in the SAF outer layer and the risk of detrimental modifications of the nature of the crystalline phases. A complete characterisation of the coated samples was performed by means of X-ray diffraction, optical and scanning microscopy. Coating adhesion on alumina was tested by comparative shear tests while biocompatibility was investigated on alumina. bulk SAF and on the realised coatings. For this purpose, cytotoxicity, adhesion and proliferation of human osteoblast-like cells were cultured onto the three materials. Results showed that the interposition of the SC layer was successful in allowing a good softening and spreading of the SAF outer layer and in avoiding the crystallisation of undesired crystalline phases maintaining the good bioactive properties of the bulk one. In vitro results on the coatings showed osteoblast-like cell behaviour similar to bulk fluoroapatite glass-ceramic and better respect to alumina substrates, being a promising index of bone material integration and of its in vivo possible applications.

Coatings on zirconia for medical applications.

In order to combine the mechanical properties of a high-strength inert ceramic (yttria-stabilised zirconia, ZrO2-3%Y2O3, defined as zirconia in the text) with the specific properties of bioactive materials, some zirconia samples were coated by two bioactive phosphosilicate glasses and glass-ceramics: RKKP and AP40. Coatings of about 200-300 microm thickness were prepared by a simple and low-cost firing method. They were characterised by optical and scanning electron microscopy (SEM) and compositional analysis (EDS). The adhesion of the coatings on zirconia was tested by shear tests. Vickers indentations at the coating/zirconia interface were performed in order to observe the crack propagation path. The reactivity of glasses and glass-ceramics coatings towards a simulated body fluid (SBF), having the same ion concentration as that of human plasma, was evaluated and compared to that of the bulk glass and glass-ceramics, by examining the morphology of the reaction layer formed on the surface of the coated zirconia after one month of soaking in the SBF at 37 degrees C.