Tailoring Calcite Growth through an Amorphous Precursor in a Hydrogel Environment

The precipitation of calcium carbonate in hydrogel-like environments is used by certain living organisms to build functional mineral–organic composite structures. Here, we investigate a pathway for calcium carbonate mineralization in agarose hydrogels with a wide range of polymer networks. The experimental investigation demonstrates that the formation of amorphous calcium carbonate (ACC) throughout the agarose hydrogels is a diffusion-limited process, and therefore, it is affected by the supersaturation of the solution and by the hydrogel network. In contrast, the inclusion of the polymer into the calcite crystals and their morphology as well as the rate of crystal growth are controlled by the amorphous precursor, and thereby, they are quite unaffected by the initial supersaturation. The nucleation rate of calcite in agarose is sufficiently high to hinder ion diffusion limiting the calcite growth rate, so that a uniform mineralization takes place in the hydrogel, in the absence of concentration gradients. This work demonstrates that the precipitation of ACC affords a tight control of calcium carbonate mineralization in the hydrogel over a wide range of calcium carbonate concentrations and hydrogel microstructures. The results of this work not only reveal an important mechanism underlying (bio)mineralization, but they can also inspire a new avenue to craft biomimetic materials with a high degree of precision.