Structural studies of bioactivity in sol–gel‐derived glasses by X‐ray spectroscopy

Abstract

Extended X-ray absorption fine structure spectroscopy and X-ray absorption near edge structure, X-ray fluorescence spectroscopy, and X-ray powder diffraction have been used to study the local calcium environment in four sol-gel-derived bioactive calcium silicate glasses of the general formula (CaO)(x)(SiO(2))(1-x). The formation of a hydroxyapatite layer on the composition with the highest bioactivity (x = 0.3) with time has been studied, in an in vitro environment, by immersion in simulated body fluid (SBF) at 37 degrees C. The calcium oxygen environment in the four compositions has been shown to be six-coordinate in character. Both the extended X-ray absorption fine structure spectroscopy and X-ray absorption near edge structure show a gradual increase in coordination number and Ca--O bond distance with longer exposure to SBF. X-ray fluorescence show that calcium is quickly lost from the samples on exposure to SBF and the calcium concentration then recovers with time. There is clear evidence that the recovery of calcium content is due to the formation of a CaO-P(2)O(5)-rich layer. Annealing of samples at 650 degrees C shows the presence of what, on the length scales probed by X-ray diffraction, appears to be noncrystalline calcium phosphate after 1 h of exposure to an SBF solution, which becomes more crystalline on longer exposure.

In vitro changes in the structure of a bioactive calcia–silica sol–gel glass explored using isotopic substitution in neutron diffraction

“…it increases), and the silicon content decreases gradually with the dissolution of the glass [4]. This is also seen in previous studies based on the analysis of the ionic content of the SBF [6][7][8][9]. Thus any calcium which is left in the glass post SBF exposure will be a residual amount from the original sample or in the guise of a newly formed (amorphous) calcium phosphate [10].…”

“…This suggests that, on exposure to SBF, the correlation at 2.5 Å (which is seen to reduce) is due to Ca-OH, and there is increase in the Ca-O b correlation and decrease in the Ca-O nb correlation. Approximately 75% of the initial calcium content is lost from the sample within 30 min of reaction time in SBF [8]. It is possible therefore to suggest that the calcium is rapidly and progressively lost from its relatively loosely bound six-fold site in the glass network (Ca-O b :Ca-O nb +Ca-OH = 0.6:3.7) and reappearing in a more tightly bound environment as part of calcium phosphate (where, presumably, Ca-O b bonds are present at higher numbers in an eight-fold coordinated environment).…”

Magnesium incorporation into hydroxyapatite

“…Multiple scattering effects were not considered, since only the nearest coordination shells are probed at these low energies. Such a simplification has already been employed for the study of non-stoichiometric apatites [44] and bioactive glasses [45] and bone [46] by K-edge EXAFS spectroscopy.…”

Characterizing the hierarchical structures of bioactive sol–gel silicate glass and hybrid scaffolds for bone regeneration

“…For example, when undertaking diffraction, the Ca-O correlation overlaps with the O−Si−O correlation. However, combining diffraction data with complementary elementspecific techniques such as extended X-ray absorption fine structure [56], X-ray absorption near-edge structure [57] or isotopic substitution ND can significantly aid data interpretation. These techniques can be further complemented by solidstate NMR [43,47], Raman and Fourier transform infrared (FTIR) spectroscopy and computational modelling [52,58].…”