Incorporation of amino acids within the surface reactive layers of bioactive glass in vitro: an XPS study

Surface reaction layers grown on bioactive glass (Bioglass^®), by immersion in either simulated body fluid (SBF) or minimal essential medium (-MEM) for 2, 5, 32 and 72 h, were analyzed by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Layers grown in -MEM exhibited delamination when observed in SEM. Low resolution XPS analysis detected nitrogen at the surface of the Bioglass^® exposed to -MEM for 72 h (8 relative at%), whereas insignificant nitrogen was found at the surface of any sample immersed in SBF. XPS depth profiling (argon) showed the presence of nitrogen throughout the depth of the surface layer of the sample incubated in -MEM for 72 h. Deconvolution of the nitrogen envelope in a high resolution XPS spectrum demonstrated nitrogen characteristic of the amine bonds present in amino acids. Carbon concentration also considerably increased over time with exposure to -MEM (24–55%), whereas it remained in the 20–25% range in SBF. These results demonstrate that the amino acids contained in the culture medium were incorporated within the growing calcium phosphate rich surface reaction layer of Bioglass^®