Soft hydrogel-embedded ceramic skeleton mimicking bone structure <i>via</i> sacrificial bond concept

Bone, consisting of calcium phosphate minerals, rigid collagen fibrils, and acidic proteins, exhibits stiff and tough mechanical properties. On a molecular scale, covalent cross-linking in proteins and ionic interactions within proteins and at the protein-mineral boundary contribute to bone's toughness. In addition, hierarchical structures, like the sponge-like arrangement, are also crucial for the energy dissipation system in bone. Inspired by the multiple sacrificial bonds found in bone, we developed a soft/hard composite made up of two components with contrasting mechanical properties: a porous calcium phosphate skeleton and an acidic polymer hydrogel matrix. The porous ceramic skeleton alone is extremely rigid but brittle. However, the presence of the hydrogel matrix transforms the brittle nature of the porous ceramic skeleton into a soft/hard composite with stretchable and tough characteristics. The composite exhibits significant energy dissipation due to the fracture of the ceramic skeleton under small deformation, while catastrophic failure of the composite is prevented because the hybridized matrix disperses the damage throughout the entire sample. The Ca²⁺-mediated ionic bonding within the matrix hydrogel and at the boundary between the gel and skeleton effectively transfers the stress, enhancing the composite's toughness. Furthermore, the cyclic deformation generates new bare surfaces on the ceramic skeleton, leading to increased interaction between the matrix and these new surfaces, which enhances the composite's healing capability. This study demonstrates that the concept of multiple sacrificial bonds in bone is a smart strategy for designing polymer-ceramic composites with excellent mechanical properties.