Diffusion-Controlled Crystallization of Calcium Carbonate in a Hydrogel

Abstract

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.

Effect of poly(acrylic acid) on crystallization of calcium carbonate in a hydrogel

“…Since hydrogels not only slow the diffusion rate of ions but also build a biomimetic environment, crystal growth in such an environment has been observed. [18][19][20][21][22][23][24][25][26][27][28][29][30][31] In particular, it has been found, by us and others, that slow diffusion dramatically affects the crystallization process, as well as crystal morphologies. 19,26,31 In hydrogels, local supersaturation can be controlled using the diffusion rate of ions, which affects nucleation and growth mechanisms, resulting in the alteration of the internal structure, morphology, and polymorphs.…”

“…[18][19][20][21][22][23][24][25][26][27][28][29][30][31] In particular, it has been found, by us and others, that slow diffusion dramatically affects the crystallization process, as well as crystal morphologies. 19,26,31 In hydrogels, local supersaturation can be controlled using the diffusion rate of ions, which affects nucleation and growth mechanisms, resulting in the alteration of the internal structure, morphology, and polymorphs. [31][32][33][34] The hydrogel systems for CaCO 3 crystallization are classified into 3 categories with respect to diffusing of ion species.…”

“…19,26,31 In hydrogels, local supersaturation can be controlled using the diffusion rate of ions, which affects nucleation and growth mechanisms, resulting in the alteration of the internal structure, morphology, and polymorphs. [31][32][33][34] The hydrogel systems for CaCO 3 crystallization are classified into 3 categories with respect to diffusing of ion species. 1) Carbonate ions are diffused into a hydrogel containing calcium ions.…”

“…1) Carbonate ions are diffused into a hydrogel containing calcium ions. 18,31,[35][36][37][38][39] 2) Calcium ions are diffused into a hydrogel containing carbonate ions. 40,41 3) Calcium and carbonate ions are simultaneously diffused into a native hydrogel in the opposite direction.…”

“…In our previous work, it was found that the diffusion limited systems can alter the local environments, including the ion concentration, supersaturation, and pH of the medium. 31 Here, the pH change in a hydrogel system was focused because it affects the chain conformation of anionic polyelectrolytes. 47,48 We speculated that the CaCO 3 structures could be controlled by manipulating the chain conformation of polyelectrolytes.…”