**4. Conclusions**

92 Ceramic Coatings – Applications in Engineering

dissolution rate of silica in the first steps of Hench's chemical growth mechanism, favouring the bioactivity processes of the films by a higher rate of biomineralization (Serra et al., 2002; Liste et al., 2004). This hypothesis is in agreement with our results, the stronger biomineralization, expressed by the thickest CHA film grown, was obtained for the BG3 sample. Although BG compositions with very high network modifier content have a native poor stability, we can tailor their in-vitro behaviour by thermal treatments of crystallization

The mechanism of calcium phosphates formation onto the surface of bioactive glasses is widely accepted, and involves the dissolution of cations from the surface of bioactive glasses and the consequent increase of the supersaturation degree in the surrounding fluid, with respect to HA components. Hench's theory states that the first stage of a bioglass mineralization upon immersion in SBF involves the rapid exchange of Na+ ions from the glass for H+ and H3O+ ions from the solution, which shall initialize the breaking of the Si-O-Si bonds of the glass structure and the subsequent formation of silanol groups. In a next step the silanol groups polycondensate forming a silica-rich layer at the bioglass surface, which favours the growth of Ca-P type layers. Homogenous nucleation (precipitation) of biomimetic apatite occurs spontaneously in a solution supersaturated with respect to hydroxyapatite. Heterogeneous precipitation, on the other hand, takes place on the sample's surface. Both, homogenous and heterogeneous nucleation, could be considered in

Due to their designed low thickness (lower than 700 nm), even in case of total dissolution, the amount of BG film that could be released in the SBF solution is around few micrograms. Thus, intuitively the bioactivity processes will not be governed in case of thin BG films preponderantly by the supersaturation of SBF solution in Na, Ca and P and further precipitation of Ca-P phases, but by the pH evolution at the sample-solution interface, surface energy and electrostatic interactions between electrically negative charged sample surface and

A reinterpretation of the classical biomineralization process based on the electrical double layer effect will be presented. The electrical double layer refers to two parallel layers differing in charge surrounding an object (BG sample). The increased number of surface hydroxyl groups could act as nucleation sites for HA. Thus the first layer, with negative surface charge, comprises Si–OH- groups primarily formed during chemical interactions with the fluid. Ca2+ ions and with them also anions, such as (HPO4)2-, are present in the electrolyte space charge facing the sample surface enriched in polymerising silanol groups. Thus, the second layer is composed of calcium ions attracted to the surface charge via the Coulomb force, electrically screening the first layer. This second layer could be seen in the first stages as a diffuse layer being loosely connected with the silica-rich one, as it consists of free calcium ions which move in the fluid under the influence of electric attraction and thermal motion. As the (HPO4)2- anions interact and react with this second layer, the heterogeneous nucleation of CaO–P2O5 layer starts, which is anchoring progressively into the sample's surface, resulting in a hydrated precursor cluster consisting of calcium hydrogen phosphate, and in time by consuming and incorporating various ions [(OH)-, F- or (CO3)2-] from the surrounding media crystallize into a carbonated-flour-hydroxyapatite mixed layer. Consequently in case of BG thin films one should consider the heterogeneous

the cations and anions from stagnant solution in the proximity of the BG surface.

to obtain the best bioactive outcome (G.E. Stan et al., 2010b).

competition during the SBF immersion.

nucleation as the dominant mechanism of HA growth.

The effects of sputtering pressure, composition of working atmosphere and deposition parameters on the compositional, morpho-structural and mechanical properties of the coated implants were emphasized, and their influence on the *in vitro* behaviour of the coatings in simulated body fluid (SBF) was studied.

The adhesion strength at the coating – substrate interface is a critical factor in successful implantation and long-term stability of such implant-type structures. The chapter presented few technological RF-MS derived approaches to surpass the low adherence drawback when dealing with BG having high CTE: by introducing intermediate buffer layers [BG1-xTix (x=0-1)] with compositional gradient, or by strong inter-diffusion phenomena induced via heat-treatments, with the aim in the nucleation of BG-Ti mix phases at the substrate-coating interface. The importance of a proper *in situ* substrate cleaning, performed by argon ion sputtering processes which will enhance the ad-atoms diffusion, should be also emphasized. Also when using BG cathode target with CTE closely matching the Ti substrates, the RF-MS deposition conditions exert great influence on the adherence of films as well as on their composition. The application of the above mentioned technological algorithms conducted to pull-out adhesion values higher than the usual ones reported in literature, opening new perspectives in oral and orthopaedic implantology.

Peculiarities of the *in vitro* behaviour of the BG thin films were discussed in correlation with the widely accepted Hench bioactivity mechanism. Based on our *in vitro* studies it was concluded that the chemical composition, bonding configuration and crystallinity could play important roles in BG films' reactivity in SBF. For achieving bioactive properties an optimal ratio of network formers/network modifiers is a necessary, but not sufficient, condition. BG compositions with very high network modifier content have a poor native stability, and their *in vitro* behaviour can be tailored by thermal treatments of crystallization to obtain the best biological outcome.

By varying the sputtering pressure and working atmosphere we attempted to tailor the films' composition and structure, hinting optimal mechanical performance and bioactive behaviour. Understanding these correlations is important for biomaterials science, implantology, as well as from an applied physics and technological point of view, opening new perspectives in regenerative medicine.
