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Bioactive glass coatings obtained by thermal spray techniques

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Universitat de Barcelona. Facultat de Química

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[eng] Bone tissue has an excellent healing response, recovering functional and structural properties when damaged. Notwithstanding, severe damage to the bone implies the need for surgery to recover. Different orthopaedic devices are used to replace long-term hard bone tissue and are designed to last as long as possible. However, despite the significant advances in the design of implants, there is still a failure rate that cannot be underestimated. The implant failure, mainly due to aseptic loosening or infection, leads to revision surgery, which is expensive and implies more pain and time for the patient's recovery. This thesis studies surface modifications strategies by incorporating bioactive glass coatings to produce implants with materials capable of replacing bone and stimulating its regeneration to promote more efficient osseointegration.

Atmospheric plasma spray is a coating process in which materials are deposited as fine particles by applying thermal and kinetic energy to form a coating on a prepared substrate. The poor bond strength of bioactive glass coatings to the metallic substrates, partly because of the thermal expansion coefficients mismatch, is one of the main challenges faced through different strategies. To evaluate the bond strength of the developed coatings, AST C633 has been followed. By modifying the morphology of the bioactive glass powders and creating agglomerated particles, the cohesion between coating and substrate can be increased. Also, using hydroxyapatite in combination with bioactive glass, as a mixture of both powders or as an anchor layer, can improve the affinity of the coating with the substrate. Furthermore, applying a pre-heating to the substrate immediately prior to the deposition enhances the coating adhesion of bioactive glasses. An improvement in adhesion can be achieved if a thermal post-treatment is performed on the coatings. The different strategies evaluated have increased the bond strength of bioactive glass coatings with respect to the standard powder deposition. Some of the strategies evaluated have provided coatings with high bonding values that comply with the regulations for implantation.

In order to understand the role of the elements forming the structure of the glass and their proportion, bioactive glass coatings with different compositions have been deposited by atmospheric plasma spray onto titanium alloy substrates. The different elements forming the glass affects the mechanical and biological properties of the coatings. These elements produce a change in the coating's microstructure, affecting the coating's porosity and thus its cohesion. Also, some of the elements contained in the glass structure are able to stimulate bone cell response. Moreover, the coating's reaction in physiological solution is also affected by the content of network-forming oxides and the crystalline phases present on the coating. The analysis of the ability of the coatings
 
to form a hydroxyl carbonate apatite and the evaluation of their dissolution behaviour has clarified how the composition of the glass can influence the osseointegration process. In vitro tests with osteoblasts have shown that the non-commercial 62W composition has a better cellular response and stronger bond to the substrate than the commercial glass formulations. Thus, this formulation can be optimal for replacing the current hydroxyapatite-coated implants.

Cold gas spray can generate coatings with relatively low heat, making it possible to maintain the feedstock powder's material chemistry and phase composition in the resulting coatings. However, the brittleness of the glasses and the low temperatures involved during the process make it a challenge to produce glass coatings by this technique. Mixing the bioactive glasses with PEEK powder have allowed the deposition of a composite coating by low-pressure cold gas spraying, where brittle particles are embedded in a polymeric matrix. The amount of glass in the mixture affects the coating's properties and influences the deposition efficiency of the coatings. Another approach considered is to design formulations with adequate viscosity for the operating temperatures of the cold gas technique, which makes it possible to deposit the coating. However, the high degree of dissolution compromises the bioactive behaviour of these glass coatings.

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Bioactive glass coatings obtained by thermal spray techniques