Diffusion-Controlled Formation and Differentiation of Calcite and Vaterite in a Hydrogel

Two crystal phases of calcium carbonate, calcite and vaterite, were synthesized and controlled within a single hydrogel by using an orthogonal diffusion method. By precise regulation of the diffusion of hydrogen carbonate and hydroxide ions in a hydrogel containing calcium ions, the local pH and ion supersaturation levels were adjusted, optimizing conditions for vaterite formation. Unlike conventional solution-based methods, where rapid precipitation occurs, the hydrogel system facilitated slow diffusion, mimicking biological mineralization and allowing the stabilization of less stable intermediates and crystal phases. This controlled microenvironment enabled the simultaneous observation of spatial variations in calcium carbonate crystallization, providing insights into phase transitions and polymorph selection. The findings highlight the potential of hydrogel-based systems for regulating biomineralization, offering valuable implications for crystallization mechanisms in biomaterials and tissue engineering.