Crystallization of Calcium Minerals Modulated by Macromolecules
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Biologically mineralized materials fulfill various vital functions and exhibit superior structural and functional properties arising from their inorganic-organic composition, complex morphology, orientation, and hierarchical organization. Undesired mineralization and demineralization can lead to various pathological problems. In-depth understanding of the biomineralization process is crucial for addressing such pathological issues and designing superior inorganic-organic hybrid materials for biological and industrial applications. Organisms have developed additive-controlled and organic matrix-controlled crystallization for producing mineralized superstructures. Fundamental questions pertaining to the mechanism of additive-controlled-crystallization and organic-matrix-controlled crystallization remain open and motivate the present research. We consider crystallization of calcium oxalate (CaOx) and calcium carbonate (CaCO 3 ) in solution media in the presence of additives to determine the role of such additives and investigate the mechanisms of additive controlled crystallization. CaOx and CaCO 3 are selected for their biological relevance: CaOx is the primary component of kidney stones and CaCO 3 is the most abundant biomineral. Molecules that are involved in modulating crystallization in biological environments are mainly macromolecules rich in acidic groups. Therefore, we selected sodium polystyrene sulfonate (PSS), an anionic polymer, as a model additive. Evidence of various interactions between additive-crystalline/amorphous phase, additive-precursor ions, and additive-additive is presented and their influences on crystallization processes and final crystalline products are discussed. PSS was found to promote nucleation, reduce growth rate, and favor formation of calcium oxalate dihydrate (COD) over the thermodynamically stable calcium oxalate monohydrate (COM). At high concentrations, PSS completely inhibited the precipitation of CaOx. PSS can modulate the morphology, polymorph selectivity (favors formation of unstable vaterite), and crystallization kinetics of CaCO 3 . The implications of the addition of an oppositely charged surfactant (cetyltrimethylammonium bromide, CTAB) on the effectiveness of PSS in controlling the crystallization of CaOx are also discussed. The effects of organic matrix on the crystallization of CaOx have been investigated by considering crystallization of CaOx in gelatin hydrogels. We demonstrated that the crystal morphology, composition, and properties of CaOx-gelatin composites can be modulated by controlling the crosslinking density of hydrogels.