Sandra E. Rodil

Abstract:

Orthopaedic and dental implant durability is dependent on successful bone regeneration and osseointegration. Several studies have demonstrated that the implant surface properties like roughness, chemistry, and energy have a significant influence on the biological systems affecting protein adsorption, cell proliferation, differentiation, local factor production and consequently, bone growth and clinical osseointegration. However, very few have provided information about the effect of the atomic arrangement or structure. Using magnetron sputtering deposition, we produced amorphous and polycrystalline TiO2 and ZrO2 coatings. Thin (70-80 nm) oxide coatings were deposited on smooth (PT) and microstructured sandblasted/acid etched (SLA) Ti substrates. The effect of the atomic structure of the oxide coatings on the physico-chemical surface properties was carefully analyzed. The surface roughness, water contact angle (WCA), structure and composition were measured using confocal microscopy, drop sessile drop, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), respectively. XRD confirmed the crystalline or amorphous nature of the films. The nanometer thick coatings presented a well-passivated and uniform TiO2 (Ti4+) and ZrO2 (Zr4+) surface composition, while the substrates (native oxide layer) showed the presence of Ti atoms in lower valence states. The thin films did not alter submicron/micron topography but generated 5-10 nm structures across the surface. Our findings demonstrated that the nano-topography and the surface energy are significantly influenced by the coating structure. The biological response of these coatings was analyzed at different strategic levels: protein adsorption (Albumin), bacterial adhesion and finally, the proliferation and differentiation of human mesenchymal stem cells (MSCs). Two pathogen bacterial strains were tested; Escherichia coli and Staphylococcus aureus. Bacterial adhesion at micro-rough (2.6 �m) SLA surfaces was independent of the surface composition and structure, contrary to the observation in sub-micron (0.5 �m) PT surfaces, where the crystalline oxide (TiO2 > ZrO2) surfaces got the larger amount of bacteria. Particularly, crystalline TiO2, which presented a predominant acid nature, was more attractive for the adhesion of the negatively charged bacteria. Human MSCs were cultured on coated and uncoated titanium surfaces for seven days. Osteoblastic markers (RUNX2 mRNA, alkaline phosphatase activity in cell lysates, and secreted osteocalcin) and related growth factors [secreted vascular endothelial growth factor (VEGF), bone morphogenetic protein 2 (BMP2) and osteoprotegerin (OPG)] were assessed. MSC attachment was higher on both amorphous oxide coatings than on their polycrystalline counterparts or titanium itself; an effect more robust on microstructured SLA surfaces. Cell number, ALP, OCN, OPG, BMP2, and VEGF levels were higher on amorphous than polycrystalline coatings or pure titanium. These results indicated that the HMSCs showed larger differentiation into osteoblasts on the nanoscale amorphous oxide coatings in comparison to the polycrystalline coatings or the native titanium oxide layer. The information provided by this study, where surface modifications are introduced by means of the deposition of amorphous oxide coatings, provides a route for the rational design of implant surfaces to inhibit bacterial adhesion and enhanced the osteoblastic differentiation of HMSCs.