The production of synthetic crystals with controlled shapes and properties is an enticing prospect, yet, the production of such materials is an ongoing challenge. Here, we present a strategy for chemo-mechanically directing the growth of crystals with non-equilibrium structures using a custom-designed double-diffusion cell. We combine chemical additives (e.g., Mg²⁺ ions) and mechanical confinement (e.g., hydrogel networks) to modulate the growth of calcium carbonate crystals. Specifically, the combination of Mg²⁺ ions with a strong agarose gel results in calcitic structures, at the gel-glass slide interface, with distinct fried egg-like morphologies and radial or Maltese-cross extinction patterns. In contrast, precipitation with only Mg²⁺ or agarose results in aragonite spherulites or squished calcite rhombohedra, respectively. Raman spectroscopy and energy dispersive spectroscopy of the “fried eggs” reveals that they are composed of Mg-calcite, which becomes less disordered over time, and the “egg whites” make this transition before the “yolks”. We propose that the “fried eggs” form due to a spherulitic growth process molded by the crystallization-induced delamination of the gel away from the glass slide at the gel-glass interface. In support of the importance of the gel-glass interface, the “fried eggs” do not form when the glass slide is treated with a hydrophobic silane, suppressing heterogeneous nucleation and weakening the interfacial adhesion between the gel and glass, making it easier for the gel to delaminate, thus reducing the confinement effect. As such, this work highlights the important chemo-mechanical role that gel environments can play in crystal formation.

The promise of crystal composites with direction-specific properties is an attractive prospect for diverse applications; however, synthetic strategies for realizing such composites remain elusive. Here, we demonstrate that anisotropic agarose gel networks can mechanically "mold" calcite crystal growth, yielding anisotropically structured, single-crystal composites. Drying and rehydration of agarose gel films result in the affine deformation of their fibrous networks to yield fiber alignment parallel to the drying plane. Precipitation of calcium carbonate within these anisotropic networks results in the formation of calcite crystal composite disks oriented parallel to the fibers. The morphology of the disks, revealed by nanocomputed tomography imaging, evolves with time and can be described by linear-elastic fracture mechanics theory, which depends on the ratio between the length of the crystal and the elastoadhesive length of the gel. Precipitation of calcite in uniaxially deformed agarose gel cylinders results in the formation of rice-grain-shaped crystals, suggesting the broad applicability of the approach. These results demonstrate how the anisotropy of compliant networks can translate into the desired crystal composite morphologies. This work highlights the important role organic matrices can play in mechanically "molding" biominerals and provides an exciting platform for fabricating crystal composites with direction-specific and emergent functional properties.