| Summary: | As many biominerals are formed in gel-like media, hydrogel-mediated mineralization is deemed as paradigms of biomineralization and ideal approaches to synthetic minerals with hierarchical architectures and related functions. Nevertheless, the long diffusion distance in hydrogels makes mineralization a diffusion-limited process, leading to isolated crystals instead of uniform hierarchical architectures. In the current study, seeded mineralization in silk fibroin hydrogel matrices is successful in delivering continuous rhombohedral CaCO<sub>3</sub> films. Though the coverage of hydrogel matrices makes mineralization a diffusion-limited process, the presence of seed layers promotes the growth of uniform overlayers in proper conditions. The regulation of the solid content of hydrogels provides a rational route to rhombohedral architectures with tunable morphologies and thickness. In the course of mineralization, the hydrogel matrices are partially occluded in rhombohedral films as inter- and intra-crystalline constituents, as confirmed by scanning and transmission electron microscopy. Our study confirms the availability of synthesizing continuous mineralized films with hierarchical architectures and the structural gradient in hydrogel matrices via self-organized mineralization. These films with the occlusion of hydrogel constituents may exhibit significant strength and resilience, and their formation can deepen our mechanistic understanding of biomineralization proceeding in gel-like media.
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