Chondral lesions caused by stressors, such as injury or inflammation, lead to osteoarthritis (OA). OA is a degenerative joint disease that has become a challenge worldwide. As the articular cartilage is incapable of self-regeneration due to the absence of vessels and nerves, novel cartilage repair techniques are urgently needed. Three-dimensional (3D) bioprinting, which allows the precise control of internal architecture and geometry of printed scaffolds, has stepped up to be a promising strategy in cartilage restoration. With regards to 3D bioprinting, bioinks with proper chemical and mechanical properties play one of the most critical roles in designing successful cartilage tissue constructs. In particular, hydrogels as 3D hydrophilic cross-linked polymer networks are highly recommended as bioinks because of their fine biocompatibility, easy fabrication, and tunable mechanical strength. Herein, we highlight the widely used polymers for hydrogel preparation and further provide a non-exhaustive overview of various functional modified additives (such as cells, drugs, bioactive factors and ceramic) to exploit the unique properties suitable for bioprinted cartilage. Finally, a prospective on future development for 3D-bioprinting in cartilage repair is elucidated in this review.

Current approaches for bone repair predominantly target localized delivery of growth factors that are aimed at the coupling of angiogenesis and osteogenesis. However, delayed revascularization and regeneration of critical-sized bone defects are still challenging. In this study, we engineer an ossification center-like organoid (OCO) that consist of an inner-core bone morphogenetic and neurotrophic spheroid generated via MSCs-loaded 3D printing, alongside the interstitially distributed outer-shell proangiogenic neurotrophic phase. Our results demonstrate that collective implantation of OCOs achieves rapid bone bridging with successive OC-like bone ossicles formation across the bone defect in a “divide-and-conquer” way. Single-cell RNA sequencing analysis unveils a developmentally mimicking stem cell community that dominated with Krt8⁺ skeletal stem cells (SSCs) is uniquely recruited by the pro-regenerative in-situ organoid fusion and maturation. Particularly noteworthy is the specific expansion of Krt8⁺ SSCs concomitant with the simultaneous reduction of Has1⁺ migratory fibroblasts (MFs) post-OCO implantation. Furthermore, cross-species comparisons employing machine learning reveal high resemblance of the relative Krt8⁺ SSCs/Has1⁺ MFs composition in bone regeneration with that in public data from developmental bone tissues. Our findings advocate an approach akin to “divide-and-conquer” utilizing engineered OC-like organoids for prompt regeneration of large-sized bone defects.