Direct observation of cholesterol monohydrate crystallization

Cholesterol crystallization is integral to the pathology of diseases such as atherosclerosis and gallstones, yet the relevant mechanisms of crystal growth have remained elusive. Here, we use a variety of in situ techniques to examine cholesterol monohydrate crystallization over multiple length scales. In this study, we first identified a biomimetic solvent to generate triclinic monohydrate crystals, while avoiding the formation of nonphysiological solvates and enabling crystallization at rates where the dynamics of surface growth could be captured in real time. Using a binary mixture of water and isopropanol, with the latter serving as a surrogate for lipids in physiological environments, we show that cholesterol monohydrate crystals grow classically by the nucleation and spreading of crystal layers. Time-resolved imaging confirms that layers are generated by dislocations and monomers incorporate into advancing steps after diffusion along the crystal surface and not directly from the solution. In situ atomic force microscopy (AFM) and microfluidics measurements concertedly reveal abundant macrosteps, which engender a self-inhibition mechanism that reduces the rate of crystal growth. This finding stands in contrast to numerous other systems, in which classical mechanisms lead to unhindered growth by spreading of single layers.