Mechanical Properties and Heterogeneous Growth of Uric Acid Crystals
The mechanical and structural properties of the anhydrous (UA) and the dihydrate (UAD) forms of uric acid crystals were investigated. UA, the more stable form, was found to have a substantially higher Young’s modulus and hardness than UAD. Load–displacement curves and AFM post-indent images revealed slip planes in preferred crystallographic directions and oriented crack formation at higher loads. UAD exhibited substantial creep and some “self-healing” in response to quick and shallow indents.A correlation between the mechanical properties and defect densities was also established in both phases. Methylene blue (MB) and Bismarck brown Y (BBY) additives inclusion in single crystals was used as a means to intentionally create quantifiable defects. The observed trends in UA indicate two competing effects which are a function of defect concentration. Growth sectors with the lowest % inclusion had a lower modulus than pure UA due to increased dislocation movement at defect sites. At defect concentrations above a critical threshold, material strengthening was observed as a result of impeding such motion. Statistical analysis of the pop-ins observed on samples with different dye concentrations corroborates the dislocation-governed strengthening mechanisms. Macro-defects including Carlsbad-type twinning were also observed in UA-BBY.In a comparison of UAD and UAD-BBY, kinetic modeling suggested both dehydrate via similar solid-state mechanisms, though SEM showed the dehydrated phases have different topographies. The increased thermal stability and modulus of UAD-BBY compared to UAD suggests that BBY dopants likely serve as both a physical block to water loss and reinforce the host lattice dimensions, delaying collapse to the anhydrous lattice.UAD crystals grown from DI water (UAD-W) and artificial urine (UAD-U) were found to have microstructural differences. In situ AFM dissolution studies on (001) revealed UAD-W surfaces are smooth and faceted, reflecting classical layer-by-layer assembly, while UAD-U surfaces showed large granular regions, suggesting non-classical assembly by colloidal precursors. Dynamic light scattering in supersaturated urine confirmed the presence of ~200 nm particles, which are similar in size to the grain sizes observed by AFM.
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Presores, Janeth (Georgetown University, 2012)Uric acid, a product of protein metabolism, is the most abundant organic component in human kidney stones. At least six different crystalline phases of uric acid have been identified in kidney stones. Anhydrous uric acid ...