3D bioprinting is a promising strategy to develop heterogeneous constructs that mimic osteochondral tissue. However, conventional bioprinted hydrogels suffer from intrinsically weak mechanical strength, limited cell adaptability, and no sustained release of biochemical drugs, restraining their use as bioinks to emulate native osteochondral extra cellular matrix. Herein, a novel host-guest modulated dynamic hydrogel is developed for 3D bioprinting heterogeneous cell-laden constructs for osteochondral regeneration. Apart from gelatin methacryloyl (GelMA), this bioink consists of dopamine-functionalized GelMA and acrylate β-cyclodextrin and is crosslinked by host-guest interaction to develop the dynamic network for obtaining promoted cell adaptability, enhanced cell adhesion, reinforced mechanical strength, and tunable modulus. Moreover, based on the sustained drug release provided by the cavity of β-cyclodextrin, a heterogeneous construct is constructed by employing kartogenin (a chondrogenic factor) into the upper zone with lower Young's modulus and melatonin (an osteogenic factor) into the bottom zone with higher modulus to mimic the osteochondral microenvironment. With the favorable regeneration results in vitro and in vivo, a broad application of this bioink in 3D bioprinting for tissues engineering is expected.

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A state-of-the-art review on the design and preparation of hierarchical and heterogeneous hydrogel systems for interfacial tissue regeneration.

The repair of hierarchical osteochondral defect requires sophisticated gradient reestablishment; however, few strategies for continuous gradient casting consider the relevance to clinical practice regarding cell adaptability, multiple gradient elements, and precise gradient mirroring native tissue. Here, a hydrogel with continuous gradients in nano‐hydroxyapatite (HA) content, mechanical, and magnetism is developed using synthesized superparamagnetic HA nanorods (MagHA) that easily respond to a brief magnetic field. To precisely reconstruct osteochondral tissue, the optimized gradient mode is calculated according to magnetic resonance imaging (MRI) of healthy rabbit knees. Then, MagHA are patterned to form continuous biophysical and biochemical gradients, consequently generating incremental HA, mechanical, and electromagnetic cues under an external magnetic stimulus. To make such depth‐dependent biocues work, an adaptable hydrogel is developed to facilitate cell infiltration. Furthermore, this approach is applied in rabbit full‐thickness osteochondral defects equipped with a local magnetic field. Surprisingly, this multileveled gradient composite hydrogel repairs osteochondral unit in a perfect heterogeneous feature, which mimics the gradual cartilage‐to‐subchondral transition. Collectively, this is the first study that combines an adaptable hydrogel with magneto‐driven MagHA gradients to achieve promising outcomes in osteochondral regeneration.