The influence of poly(aspartic acid) on the deformation behavior of calcium oxalate crystals.

In this study, we engineered a modular microfluidic device to grow individual synthetic calcium oxalate monohydrate (COM) crystals in the presence or absence of polyelectrolytes directly onto a glass substrate. The modular design allowed the glass substrate and attached crystals to be removed from the device, which enabled individual COM crystals to be identified and targeted by microscopy for subsequent characterization. Qualitative (morphology, Raman spectroscopy) and quantitative (surface roughness and nanomechanical properties) measurements were performed on these COM crystals to evaluate the influence of modifier-mediated crystallization. We observed major changes to morphology as polyelectrolyte concentration increased, yet Raman spectroscopy validated that all crystals were monohydrate species. The surface roughness of crystals was found to increase for crystals grown in the presence of the polyelectrolyte modifier, though there was no significant difference between the concentrations used in this study. Although the nanomechanical properties (reduced modulus and hardness) were not found to be influenced significantly with the introduction of a polyelectrolyte modifier, poly(aspartic acid), the occurrence of pop-ins decreased as polymer concentration increased. We speculate this behavior can occur in part to the increase of surface roughness due to polyelectrolyte modifiers. In addition, polyelectrolytes may potentially alter stress distributions or inhibit abrupt plastic events during indentation. Altogether, these results demonstrate that polyelectrolyte-mediated crystallization can significantly reduce pop-in events for COM crystals, potentially enhancing their resistance to dislocations and crack propagation events. These results provide new insights into the role of organic macromolecules in the mechanical response of COM kidney stones during fragmentation and removal.