Development of hybrid sol-gel coatings for the improvement of metallic biomaterials performance

Ti implants osteointegration is widely recognized. However, silicon deficiency in animals leads to bone defects, since this element plays an important role in bone metabolism. Thus, hybrid (organic-inorganic) sol-gel coatings synthesis has been performed to create a material able to release silicon compounds under in vivo conditions, to promote a fast and good osseointegration. Synthesis procedures included acid-catalysed hydrolysis, sol preparation and the subsequent gelation and drying. To this end, methyl-trimethoxysilane (MTMOS) and 3-glycidoxypropyl-trimethoxysilane (GPTMS), alkoxide precursors with different molar ratios were used. After the determination of the optimal synthesis parameters to obtain homogeneous films, the materials were physicochemically characterized by 29Si nuclear magnetic resonance (29Si NMR), Fourier transform infrared spectroscopy (FT-IR), contact angle measurements and electrochemical impedance spectroscopy (EIS) tests. The materials were assayed in vitro for their ability to release Si in a controlled manner. The sustained release of Si over long periods was demonstrated. Electrochemical analysis revealed the formation of pores and water uptake during the degradation. The degradation kinetics and Si release of coatings was mainly influenced by the amount of GPTMS. Among the cell types involved in bone regeneration, human adipose tissue-derived mesenchymal stem cells (AMSCs) are included; thus, the attachment and proliferation of these cells onto the coatings was analyzed. Furthermore, the osteoinduction capacity of the coatings was evaluated by establishing the mineralized extracellular matrix production by quantification of calcium-rich deposits. MSCs had good cell proliferation onto the hybrid coatings and could be able to produce mineralized extracellular matrix, evidencing an active osteoinduction process. After the in vitro tests, one formulation was selected to coat titanium implants and perform an in vivo test in rabbits. Although the in vivo results were not as good as those obtained in vitro, we demonstrated that the ability to utilize sol-gel coating processes on titanium implants opened up the opportunity to tailor surfaces to clinical requirements. Thus, a further research is proposed to include other precursors that enhance the coating degradation kinetics in order to obtain an early release of Si compounds that accelerate the osseointegration.