Structure−Function Relationship of Calcium Alginate Hydrogels: A Novel Crystal-Forming Engineering

Hydrogels of calcium alginate are the cross-linked networks containing a large fraction of water, which were used as the precursors of calcium carbonate (CaCO₃) mineralization for the first time. The well-defined geometry, the permeability, and the ion-exchange property of these pregels favored the facile fabrication of calcite superstructures through the slow inpouring of ammonia and carbon dioxide gases. When calcium alginate hydrogels sponged up a relatively high amount of liquid, the resulting products with the outside calcite sequences transcribed the spongelike pregel beads with the outside nucleation sites of carboxyl groups. The inner characteristic of these CaCO₃ superstructures showed the endocentric growth trends of calcite, indicating the permeability of hydrogel beads and the diffusing directions of permeated gases. In the presence of low liquid content, the pregels favored the formation of calcite superstructures with relatively smooth surfaces, demonstrating the inside lamellar array of calcite nanoparticles. This strategic approach indicated to a great extent the biologically controlled mineralization mechanism, dealing with (1) the preadsorption of calcium ions by the functional groups of biomolecules, (2) the confined crystallization within the three-dimensional networks, and (3) the proper arrangement of nanosized calcites by association with the organic architectures. Surprisingly, even the apparently “single-crystalline” CaCO₃ was proven to comprise tiny calcite rhombohedrons. Furthermore, these building blocks coaligned each other with respect to the polymers’ conjugated backbones. Therefore, these suggest a novel pathway of multivalent metal pregelation phases for the biomimetic fabrication of functional materials.