Injectable hydrogels in bone tissue engineering: from bench to bedside

Large segmental bone defects resulting from trauma, tumor resection, infection, or congenital abnormalities often exceed the innate regenerative capacity of bone, necessitating effective therapeutic strategies. Conventional treatments, such as autografts and allografts, are associated with significant limitations, including donor site morbidity, immunogenicity, limited availability, and risk of disease transmission. Bone tissue engineering (BTE) has emerged as a promising alternative, integrating scaffolds, bioactive molecules, and stem cells to promote bone regeneration. Among various scaffold systems, injectable hydrogels have gained considerable attention due to their excellent biocompatibility, adaptability to irregular defect sites, and tunable physicochemical properties. These hydrogels provide a three-dimensional, biomimetic microenvironment that is conducive to cellular adhesion, proliferation, differentiation, and tissue integration. Moreover, their ability to encapsulate and deliver therapeutic agents and stem cells in a controlled and localized manner further enhances their regenerative potential. Recent advancements, including the incorporation of bioactive nanoparticles, osteoinductive factors, and the development of smart, stimuli-responsive, and self-healing systems, have significantly expanded the functional capabilities of injectable hydrogels in bone repair. This review provides a comprehensive overview of recent progress in the design and application of injectable hydrogels for bone tissue engineering, with a focus on material innovations, gelation mechanisms, and biological performance.