Tracking the initial stage of bioactive layer formation on Si-Ca-Na-P oxide glasses by nanoindentation
Bioactivity of a newly formulated glass composition is assessed based on qualitative information of X-ray diffraction, spectroscopic or electron microscopic data of the surface layer formed during soaking in simulated body fluid (SBF) under physiological temperature, pH and CO2 partial pressure. In our experiments we use an alternative method, nanoindentation testing to reveal compositional dependence of bioactivity in a series of SiO2(45)CaO(25)Na2O(30-x)P2O5(x) glasses, where x = 0,1,3,5. According to our results, analytical evaluation of nanoindentation data, namely the variation of Vickers hardness and Young modulus as function of composition, allows quantitative description of bioactive layer formation already in the very early stage of the reaction. These results can be reproduced from the data provided in this contribution.
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In these data files we share raw nanoindentation data on untreated surfaces and SBF soaked surfaces together with optical and scanning electron microscope (SEM) images of the sample surfaces before and after SBF soaking. The glass samples were prepared by melt quenching as described in . The in vitro bioactivity test was performed according to the protocol described in . Nanoindentation tests were performed on the flat surfaces using an UMIS nanoindentation device with Vickers indenter by applying a maximum load of 50 mN at a loading rate of 0.5 mN/s. 160 measurements were performed on each sample in a 4 x 40 matrix with 40 μm separation between the neighboring indents. The Vickers hardness (HV) and the Young’s modulus (E) from each test were determined by the Oliver-Pharr method . Secondary electron (SE) and back scattered electron (BSE) images were taken using an FEI Scios 2 DualBeam system SEM. The composition of the surface layers was determined by energy dispersive spectroscopy (EDS) within the SEM. To reveal the cross-section of the surface layers formed during the immersion of the glasses into the SBF and study the penetration depth of the solution as well as the related compositional changes, ca. 5-10 µm x 10-30 µm sized and approximately 6-12 µm deep trenches were cut perpendicular to the sample surface using the focused ion beam gun (FIB) of the dual beam system. To protect the sample surface during FIB milling of the trench, the milling was preceded by the deposition of a 2 µm thick Pt protective layer onto the surface using 2 kV electrons and subsequently, 30 kV Ga-ions. Imaging of the surface of the trenches was performed in the same SEM by tilting the sample to 45 deg from the normal vector of the surface. The effect of tilting was compensated in the corresponding SEM images.  T. Kokubo, H. Takadama (2006) https://doi.org/10.1016/j.biomaterials.2006.01.017  M. Fabian, Zs. Kovacs, J.L. Labar, A. Sulyok, Z.E. Horvath, I. Szekacs, V.K. Kis (2020) https://doi.org/10.1007/s10853-019-04206-z  W.C. Oliver , G.M. Pharr (2004) https://doi.org/10.1557/jmr.2004.19.1.3