Review 3D-Printed hydrogels for tissue engineering: a review.

Three-dimensional (3D) bioprinting of hydrogels has emerged as an important strategy in tissue engineering because it enables the fabrication of scaffolds with controllable architectures, tunable properties, and biomimetic microenvironments. This review primarily focuses on recent advances in hydrogel-based 3D bioprinting over the past 2 decades, while also incorporating selected landmark studies to provide historical context for the evolution of the field. Major hydrogel printing techniques, including light-based, extrusion-based, and inkjet-based systems, are discussed and compared in terms of printing resolution, structural fidelity, material compatibility, cytocompatibility, and practical limitations. In addition, the roles of natural and synthetic hydrogels are examined, highlighting their distinct yet complementary advantages in bioactivity, printability, and mechanical performance. Recent progress in tough and composite hydrogels is further reviewed, with emphasis on strategies such as nanocomposite reinforcement, supramolecular interactions, double-network formation, gradient structures, and hydrogel-polymer hybrids for improving the mechanical durability and functional performance of printed constructs. The review also summarizes emerging applications in wound healing, cartilage repair, bone and osteochondral engineering, vascularized tissue fabrication, and other tissue-specific systems, while addressing major translational challenges, including scalability, vascularization, reproducibility, manufacturing standardization, and regulatory considerations. Overall, this review provides an updated and integrated perspective on the design, fabrication, functional optimization, and clinical translation potential of 3D-printed hydrogels for tissue engineering applications.