Injectable engineered micro/nano-complexes trigger the reprogramming of bone immune epigenetics

Imbalance in the local immune microenvironment plays a vital role in the nonunion or delayed healing of bone defects. Additionally, there exists a cross-connection between material design of microfluidics and immune regulation. Therefore, we aimed to investigate whether epigenetic reprogramming of the microfluidic microsphere systems could regulate microenvironmental homeostasis in the acute phase of fracture and facilitate bone regeneration. To this end, we used Gelma hydrogel microspheres as the base carriers to design and fabricate an injectable, macrophage-targeted, engineered micro/nano microsphere reprogramming system (Gelma@Lip@Pla). Liposome (Lip) coating and macrophage targeting were improved by adding DSPE-PEG-CHO and phosphatidylserine (Pla). the surface of Gelma was coated with a lipid membrane, Schiff base, and hydrogel matrix network; the resultant macrophage-targeted liposomes activated AKT signaling in the local microenvironment by releasing the AKT signal activator dihydrocapsaicin and amplifiers (pcDNA3.1-AKT), triggering gene reprogramming in the local microenvironment and restarting the bone healing process. Compared with pcDNA3.1-AKT and dihydrocapsaicin-loaded liposomes (Lip@Pla) and pure Gelma hydrogel microgels (Gelma), the Gelma@Lip@Pla group significantly prolonged the release time of active ingredients. They also promoted the polarization of macrophages to the M2 phenotype, leading to the reactivation of bone marrow-derived mesenchymal stem cell osteogenic differentiation and human umbilical vein endothelial cell angiogenesis. Furthermore, in vivo experiments confirmed that local injection of Gelma@Lip@Pla could significantly accelerate the regeneration of a defective rat femoral condyle. Collectively, the Gelma@Lip@Pla microgel exhibits considerable potential as an extended delivery platform for treating bone defects and other immune-related diseases involving macrophages.