Effect of hierarchical porous scaffold on osteoimmunomodulation and bone formation

Abstract The immune system plays an indispensable role in the process of tissue regeneration following scaffold implantation. Macrophages are one of the key immune cells in regeneration, and their depletion inhibits blastema granulation tissue formation and tissue repair. Increasing evidence indicates that different pore sizes of scaffolds can affect macrophage polarization and tissue regeneration. However, the accurate disciplinary is still misty. To address these issues, we sought to explore how diverse apertures affected the response of macrophages and the subsequent bone regeneration. We designed three types of hierarchical porous scaffolds with diverse pore combinations based on PLGA, bioactive glass (BG) and mesoporous bioactive glass (MBG). Each scaffold has the same composition but disparate porous structure: simplex PB-1 scaffold with conventional macro pores, dual-structured PMB-2 scaffold with macro‑meso pores, and the ternary combinational structured PMB-3 scaffold with macro-micro‑meso pores. We found the hierarchical PMB-3 scaffolds drove macrophages to significantly polarize towards M1 at early stage (0~3 days), while significantly reduced M1 polarization and increased M2 polarization at later stage (3~7 days). This fast-responsive feature can evoke resonate fluctuation of relevant pro-inflammatory and anti-inflammatory genes and cytokines. Moreover, enhanced angiogenic potential of HUVECs and osteogenic capacity of BMSCs were acquired by ternary PMB-3 scaffolds. Further in vivo implantations in mice demonstrated the immune microenvironment induced by PMB-3 scaffolds promoted the optimal neovascularization and ectopic bone formation. These findings indicated that scaffolds with complex macro-micro‑meso porous structure facilitated bone regeneration by endowing favorable osteoimmunomodulatory properties.