Calcium carbonate shows polymorph-specific bioactivity, reactivity, and Ostwald–Lussac ripening in simulated body fluid which can be conveniently tuned via incorporation of trace elements, such as Mg.

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Calcareous biominerals typically feature a hybrid nanogranular structure consisting of calcium carbonate nanograins coated with organic matrices. This nanogranular organisation has a beneficial effect on the functionality of these bioceramics. In this feasibility study, we successfully employed a flow-chemistry approach to precipitate Mg-doped amorphous calcium carbonate particles functionalized by negatively charged polyelectrolytes—either polyacrylates (PAA) or polystyrene sulfonate (PSS). We demonstrate that the rate of Mg incorporation and, thus, the ratio of the Mg dopant to calcium in the precipitated amorphous calcium carbonate (ACC), is flow rate dependent. In the case of the PAA-functionalized Mg-doped ACC, we further observed a weak flow rate dependence concerning the hydration state of the precipitate, which we attribute to incorporated PAA acting as a water sorbent; a behaviour which is not present in experiments with PSS and without a polymer. Thus, polymer-dependent phenomena can affect flow-chemistry approaches, that is, in syntheses of functionally graded materials by layer-deposition processes.

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For bioactive biomaterials such as bioceramics and bioglass, it is generally accepted that, apart from acting as heterogeneous nucleators, it is their solubility and the resulting release of relevant ions such as calcium or basic anions which mainly governs the biomaterial's bioactivity. This contribution reveals that this bioactivity, as assessed by simulated body fluid (SBF), can also be considerably modified by the bioceramic's morphology, i.e., bioactivity is also governed by microstructure and surface morphology. When crystals are forced to adopt out‐of‐equilibrium crystal habit, this simple change in morphology converts an essentially bioinert material, here calcite, into a bioceramic which shows bioactivity in SBF. On larger length scales, already simple morphological changes, such as scratches, can have inverse effects. Limited mass transport into grooves and pits on a bioceramic surface can lead to local ion depletion which, in turn, causes reduced bioactivity of bioceramics which, otherwise, show distinct bioactivity in SBF. This contribution emblematically illustrates the unforeseen importance of even minor morphology changes on different length scales when assessing and designing a biomaterial's bioactivity through SBF assays.