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.

In the more than 100 years since the Liesegang phenomenon was discovered, intensive studies have been conducted to understand and control the characteristics of the periodic precipitation patterns in which the outer electrolyte diffuses into a hydrogel containing the inner electrolyte. Between fields of physics and chemistry, the periodicity of the precipitate has been investigated restrictively by spatial analyses and numerical simulations at macroscopic scales and it has been considered as a result of simple precipitation. In this work, calcium ion diffusion into gelatin hydrogels containing phosphate ions, a biomimetic system for bone formation, resulted in typical Liesegang patterns at macroscopic scales, but the asymmetric growth of the crystal was found in every single band at microscopic scales, which has not been observed or overlooked in the previous reports. The pattern consists of three characteristic bands: a continuous band, a split-fin band, and an intact-fin band. While the continuous band has a uniform crystal density, the split-fin and intact-fin bands have asymmetric crystal densities along the single band. We investigate the formation process of individual bands as well as the whole pattern by combining microscopic and spatiotemporal analyses based on the nucleation theory. Formation processes of asymmetric bands are explained by the unique stability and the diffusive property of amorphous precursors depending on the rate of calcium ion delivery. This is the first study to focus on the inhomogeneity of a single band in Liesegang patterns and the time-dependent mechanism of its growth.

Diffusion-controlled crystallization in a hydrogel has been investigated to synthesize organic/inorganic hybrid composites and obtain a fundamental understanding of the detailed mechanism of biomineralization. Although calcium phosphate/hydrogel composites have been intensively studied and developed for the application of bone substitutes, the synthesis of homogeneous and integrated composites remains challenging. In this work, diffusion-controlled systems were optimized by manipulating the calcium ion flux at the interface, concentration gradient, and diffusion coefficient to synthesize homogeneous octacalcium phosphate/hydrogel composites with respect to the crystal morphology and density. The ion flux and local pH play an important role in determining the morphology, density, and phase of the crystals. This study suggests a model system that can reveal the relation between local conditions and the resulting crystal phase in diffusion-limited systems and provides a synthetic method for homogeneously organized organic/inorganic composites.