Mineralization of Calcium Oxalate in Silica Hydrogels
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The main objective of this thesis is to investigate the crystallization of calcium oxalate, the primary mineral constituent of kidney stones, in silica hydrogels. Hydrogels serve as excellent matrix models for biomineralization media. The gel matrix acts as a template for crystallization, while at the same time controls the diffusion of lattice ions, regulates the nucleation and growth rate, and directs the hierarchical organization of the minerals. The morphology of calcium oxalate crystals formed in the hydrogels was quite different from that of crystals grown in aqueous solutions. There was also variation between the morphology of the crystals grown in silica hydrogel (chemical gel) and gelatin hydrogel (physical gel). This morphological change in the two different hydrogels can be attributed to gel-crystal/lattice ion pair interactions. The polymorphic form of calcium oxalate did not seem to be influenced by the hydrogel matrix; only the thermodynamically stable monohydrate was formed. Further the effects of varying silica gel pH on the morphology and polymorphism of calcium oxalate crystals were studied. The nucleation, growth, and morphological evolution of calcium oxalate crystals as influenced by a negatively charged polyelectrolyte, sodium polystyrene sulfonate (PSS) (as a model polymeric additive) were also investigated. PSS was found to affect the morphology of the produced crystals and to preferentially promote the crystallization of metastable calcium oxalate dihydrate. A systematic investigation toward studying the formation mechanisms of biominerals in an extracellular matrix-like environment is essential for understanding biomineralization processes and for developing bio-inspired materials with superior properties.