Deep hydroxyapatite deposition in porous poly(ethylene glycol) sponge hydrogel via optimized and simplified approach

The controlled mineralization of hydrogels with hydroxyapatite (HAp) offers a promising route for engineering biomimetic scaffolds. However, conventional mineralization methods typically lead to surface-localized precipitation due to rapid ion depletion and limited transport within dense polymer networks, thereby restricting mineral penetration and compromising mechanical performance. Here, we present a simple sequential immersion protocol that achieves deep HAp deposition within a poly(ethylene glycol) (PEG) sponge hydrogel engineered via gel - gel phase separation and freeze - thaw processing. The resulting micron-scale porous architecture significantly enhances mass permeability, enabling bidirectional diffusion of phosphate and calcium ions. Structural and spectroscopic analyses confirm the formation of crystalline HAp throughout the hydrogel, while quantitative mapping of Liesegang ring patterns reveals extended mineral infiltration and nonlinear precipitation dynamics. Mechanical testing further demonstrates that mineralization reinforces the hydrogel without compromising its structure - overcoming the limitations of conventional alternating immersion methods. This work establishes a scalable and chemically straightforward strategy for constructing soft - mineral composites with tunable mineralization depth, advancing the design of bone-mimetic scaffolds and regenerative materials.