Assessment of in vitro testing approaches for bioactive inorganic materials

This paper deals with non-standard in vitro testing of bioactive inorganic materials shaped as granules or scaffolds. The ISO 23317 standard describes the in vitro test arrangement of bulk bioactive materials under static conditions. However, this norm has not dealt with bioactive materials shaped as granules (with large surface area) that are commonly used in clinical practice. We found that in the case of highly reactive (bioactive) materials, the biogenic elements were exhausted from simulated body fluid (SBF) solution very quickly (within hours) under static conditions. In such exhausted SBF solution the formation of Ca-P layer (hydroxyapatite - HAp) was stopped in agreement with the decrease of Ca and P concentrations. On the contrary, highly soluble materials (glass-ceramic scaffold) induced the formation of a new mineral layer also on the walls of the PE container used. For a non-standard shape of the tested materials the usage of dynamic or static-dynamic in vitro test arrangement was confirmed to be a better option to test bioactivity. However, also for this type of arrangement it is essential to determine the S/V or S/F ratios (the surface area/volume or flow of SBF solution) very precisely. For detailed understanding of the interaction between the tested material and SBF it is important to analyze the leachates (monitoring Ca2+, (PO4)3− and minor element concentrations) and to monitor the pH value. An expected result of the in vitro test (according ISO standard) is the formation of HAp on the surfaces of tested samples in SBF. However, the formation of hydroxyapatite may not be the proof of their potential bioactivity necessarily (e.g. due to the use of TRIS buffer).