Forming Anisotropic Crystal Composites: Assessing the Mechanical Translation of Gel Network Anisotropy to Calcite Crystal Form

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.