Reaction–Diffusion Control of Calcium Carbonate and Calcium Phosphate Crystallization in Hydrogels: Methods, Architectures, and Mechanism

Abstract

Biomineral crystallization occurs under diffusion-limited conditions, where ion transport, local supersaturation, and organic matrix interactions collectively regulate nucleation, phase selection, and hierarchical structure formation. Hydrogel-based reaction–diffusion systems provide versatile experimental platforms for reproducing and systematically probing these processes under spatially and temporally controlled environments. This review summarizes recent advances in diffusion-controlled biomineral crystallization in hydrogels, focusing on methodological strategies, resulting crystal structures, and morphological diversity in calcium carbonate and calcium phosphate systems. We describe the fundamental principles of reaction–diffusion–mediated mineralization, highlighting how polymer networks restrict ion mobility and decouple nucleation and growth compared to bulk solution crystallization. Single- and dual-diffusion approaches enable controlled supersaturation gradients, allowing selective polymorph formation, non-classical crystallization pathways, and the emergence of mesoscale architectures such as Liesegang patterns, spherulites, and composite crystals. The influence of hydrogel properties—including network density, functional groups, and ion-binding capability—on phase stability and growth anisotropy is critically discussed. A major emphasis is placed on the recently developed orthogonal diffusion strategy, which introduces mutually perpendicular ion fluxes within a single hydrogel matrix. This approach generates two-dimensional gradients of ion concentration and pH, effectively transforming the gel into an array of spatially resolved microreactors. Orthogonal diffusion enables the simultaneous observation of multiple crystallization pathways, revealing spatially separated amorphous precursors, metastable phases, and stable biominerals, as well as distinct hydroxyapatite formation routes under near-physiological conditions. Beyond biomineralization, these diffusion-controlled hydrogel systems offer new Earth science perspectives by providing model environments for studying pattern formation, phase transitions, and reaction–diffusion coupling in sedimentary, diagenetic, and geochemical mineralization processes.