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Biomaterials can interact with cells directly, that is, by direct contact of the cells with the material surface, or indirectly, through soluble species that can be released to or uptaken from the surrounding fluids. However, it is difficult to characterise the relevance of this fluid-mediated interaction separately from the topography and composition of the substrate, because they are coupled variables. These fluid-mediated interactions are amplified in the case of highly reactive calcium phosphates (CaPs) such as biomimetic calcium deficient hydroxyapatite (CDHA), particularly in static in vitro cultures. The present work proposes a strategy to decouple the effect of ion exchange from topographical features by adjusting the volume ratio between the cell culture medium and biomaterial (V CM /V B ). Increasing this ratio allowed mitigating the drastic ionic exchanges associated to the compositional changes experienced by the material exposed to the cell culture medium. This strategy was validated using rat mesenchymal stem cells (rMSCs) cultured on CDHA and beta-tricalcium phosphate (ß-TCP) discs using different V CM /V B ratios. Whereas in the case of ß-TCP the cell response was not affected by this ratio, a significant effect on cell adhesion and proliferation was found for the more reactive CDHA. The ionic exchange, produced by CDHA at low V CM /V B , altered cell adhesion due to the reduced number of focal adhesions, caused cell shrinkage and further rMCSs apoptosis. This was mitigated when using a high V CM /V B , which attenuated the changes of calcium and phosphate concentrations in the cell culture medium, resulting in rMSCs spreading and a viability over time. Moreover, rMSCs showed an earlier expression of osteogenic genes on CDHA compared to sintered ß-TCP when extracellular calcium fluctuations were reduced.