Macrophage polarization-regulating hydrogels for bone regeneration

Bone regeneration represents a complex and highly orchestrated process that involves not only osteogenic cells but also the immune system, particularly macrophages. Macrophage polarization plays a pivotal role in bone healing, and hydrogel-based immunomodulatory strategies offer promising approaches to guide this process. However, the design of such hydrogels must incorporate bone-specific requirements, such as mechanical support, biodegradation synchronized with bone remodeling, and spatiotemporal regulation of macrophage phenotypes. This review systematically examines advanced hydrogel systems engineered to direct macrophage polarization for bone repair. We highlight key material design principles-including polymer selection, crosslinking methods, and bioactive functionalization-that govern macrophage phenotypic switching. A focal point is the essential macrophage-osteoblast crosstalk promoted by these hydrogels, whereby M2 macrophage-secreted factors (e.g., BMP-2, TGF-β, VEGF) establish a pro-osteogenic microenvironment. Furthermore, we evaluate therapeutic outcomes across diverse bone defect scenarios, from standard critical-sized defects to pathological conditions such as infected, osteoporotic, and diabetic bone defects, where tailored hydrogels address distinct immunopathological barriers. The discussion extends to the potential application of these immunomodulatory hydrogels in cartilage and tendon repair, illustrating the broader relevance of macrophage-targeted strategies. By integrating material science with immunology, this review provides a framework for designing next-generation hydrogel therapies that harness macrophage plasticity to enhance bone regeneration and advance immunomodulatory tissue engineering.