Stability of mesoporous oxide and mixed metal oxide materials under biologically relevant conditions

The dynamic behavior of nanoscale mesoporous oxide materials exposed to aqueous solutions under biologically relevant conditions is shown to be highly dependent on composition, porosity, and calcination temperature. Dynamic processes were followed as a function of exposure on thin oxide films amenable to environmental ellipsometry porosimetry for the analysis of mechanical strength and pore size distributions as a function of exposure. Additionally, X-ray photoelectron spectroscopy was used for the elucidation of compositional changes as a function of exposure. Combined, this approach gives the first detailed, quantitative information of the degradation of nanoscale oxide materials under biologically relevant conditions. This approach also shows the utility of using film geometry as a convenient model system for the study of dynamic properties, as films are amenable to sensitive ellipsometric characterization. Pure silica films underwent a rapid degradation, occurring on the time scale of hours, while silica films mixed with 10% or less of zirconia or alumina were significantly more stable. These mixed metal oxide films showed structural changes on two time scales, undergoing a rapid partial degradation followed by a stabilization of the structure as the composition of the films evolved toward a depleted silica state. The time scales of these two processes were on the order of hours and days, respectively, and could be tuned by varying the composition and the calcination temperature of the films. These time scales are especially relevant to the culture and growth of mammalian cells and for drug release applications. Titania materials were shown to be stable under all conditions studied, making them suitable candidates for applications where the scaffold functions as a permanent support. These results yield unprecedented levels of detail on the kinetics of degradation and the dynamic structural and compositional changes occurring in these nanostructured materials.