Doped zirconia mesoporous coatings for biomedical applications

A. D’Agostino, F. Tana, S. Cometa, E. De Giglio, A. Cochis, L. Rimondini, R. Chiesa, L. De Nardo
Polytechnic University of Milan,
Italy

Keywords: mesoporous zirconia, dental, surface coating

Summary:

Material composition and the quality of the implant surface are among the major factors that influence osseontegration and sequential long term clinical achievement of endosseous dental implants(1). Bulk zirconia components have been widely used in dentistry as a valid alternative to metals due to their mechanical reliability and aesthetic similarity with natural teeth(2). Inorganic mesoporous materials (e.g. TiO2, CeO2, ZrO2) have attracted widespread interest as a fundamental and technological strategy to tune the cell fate and improve tissue interaction underneath biomaterials(3,4). Porous structures have reported to support and control cell adhesion, growth and proliferation on different materials, proving that cell fate is influenced even by pores as small as a few nanometers in diameter. Here we report a straightforward strategy based on the deposition of an overlying mesoporous doped zirconia coating aimed at (i) stabilizing tetragonal phase of zirconia; (ii) enhancing cell adhesion and proliferation thanks to doping with Ca ions; (iii) exerting an antibacterial activity through doping with Ga ions(5). Mesoporous thin films have been prepared by a sol-gel/dip-coating method (Figure 1A), employing ZrCl4 as a non-expensive inorganic precursor and non-ionic block copolymer (Pluronic F127) as templating agent. Different concentrations of dopant agents have been used and thermal treatment have then performed at different temperatures (T= 400°C-800°C) to remove the template and studying the crystalline phase evolution. Several techniques have been carried out in order to investigate the physicochemical properties of mesoporous surfaces such as TEM, XRD, static contact angle, XPS. Long term release of doping agent has been evaluated by ICP-OES analysis. Preliminary in vitro assays and bacterial viability tests have been assessed to study the overall biocompatibility of substrates and the ability to inhibit biofilm formation of the oral pathogen Aggregatibacter actinomycetemcomitans (Gram-negative) and Streptococcus salivarius (Gram-positive). Homogeneous mesoporous coatings have been obtained (Figure 1B and 1C). The XRD analysis have revealed different grain structures depending on thermal treatments; XPS and ICP-OES analysis have shown an effective doping of ZrO2 films. The stability range of the mesoporous structures has been successfully extended by the presence of Ca and Ga dopants at even high calcination temperature. In vitro findings have suggested a possible active role on cell proliferation exerted by Ca-doped mesoporous surfaces(6); moreover, mesoporous Ga-doped zirconia coatings have provided encouraging results in the various in vitro tests, showing no cytotoxic effect and exerting antibacterial activity on A. Actinomycetemcomitens strains. The resulting surfaces were fully characterized to achieve a correlation between their physicochemical properties and biological activity. The formation of a porous structure and the addition of dopants, induced modification in surface morphology, chemical composition, and wettability. The presence of calcium dopant not only improved the stability of the mesoporous structure, but also lead to higher in vitro cell viability. Furthermore, Ga-doped coatings have been shown to be effective in reducing the bacterial viability than non-doped samples.