Diffusion-Controlled Crystallization of Calcium Carbonate in a Hydrogel

Biominerals possess characteristic structures and superior properties originating from exquisitely controlled nucleation and growth of constituent crystals. There have been many attempts to mimic the processes and consequent structures of biomineralization by introducing soluble polymers or insoluble organic matrices into the crystallization process. However, the importance of slow and controlled ion transport in living organisms has been overlooked. Hydrogel is an attractive medium for emulating ion transfer in organisms because ion transfer is restricted by hydrogel polymer networks. It is expected that the role of the ion supply rate on crystallization in organisms could be inferred by controlling ion diffusion and investigating the crystal morphologies formed in a hydrogel medium. In this study, carbonate ions were diffused into agarose hydrogel containing calcium ions in order to make the flux of the outer ion a prevailing parameter. The flux of the carbonate ion gradually decreased as a function of diffusion distance, and various morphologies of calcites were formed along the diffusion direction. The formation of various morphologies was explained by the interaction between the hydrogel polymers and crystals, which differed by the supply rate of ions. This study suggested a new understanding of the role of ion supply rate in biomineralization as well as the role of the hydrogel medium in mimicking biomineralization.