Crystallization Behavior of Calcium Oxalate in Silica Hydrogels Using a Single Diffusion Technique

The deposition of inorganic biominerals such as Calcium Oxalate (CaOx) typically takes place in gel-like extracellular matrix environments composed of organic macromolecules, leading to complex aggregated morphologies. In the kidney, crystal aggregation in renal tubules and attachment to epithelial cells are major contributors to urinary stone retention in stone formers. In this study, silica hydrogels were employed as an in vitro crystallization medium to predominantly grow Calcium Oxalate Monohydrate (COM) and to investigate its aggregation behavior. A single diffusion technique was used to examine COM aggregation under conditions of excess oxalate ions relative to calcium. Beyond its relevance to kidney stone formation, druse-like COM morphologies, naturally occurring in plant systems, were successfully reproduced, demonstrating that silica hydrogels are suitable model media for studying biomineral formation. Calcium Oxalate Dihydrate (COD) crystals were observed at hydrogel locations of higher relative supersaturation, closer to the solution reservoir-gel interface. To further modulate calcium oxalate morphologies and better mimic biomineralization environments, a negatively charged polyelectrolyte, poly(sodium 4-styrenesulfonate) (PSS), was introduced as an additive. In the presence of PSS, COM crystals nucleated at higher relative oxalate concentrations within the hydrogel column and exhibited larger (100) crystal faces.