Critical-sized bone defects, especially for irregular shapes, remain a significant challenge in orthopedics. Although various biomaterials are developed for bone regeneration, their application for repair of irregular bone defects is limited by the complicated preparation procedures involved, and their lack of shape-adaptive capacity, physiological adhesion, and potent osteogenic bioactivity. In the present study, a simple strategy of precipitation by introducing tannic acid (TA) with abundant phenolic hydroxyl groups and Fe 3 O 4 nanoparticles, as metal-phenolic networks (MPN), is developed to easily prepare a fast gelling, shape-adaptive, and highly adhesive regenerated silk fibroin (RSF)/TA/Fe 3 O 4 hydrogel system that can respond to a static magnetic field (SMF). The RSF/TA/Fe 3 O 4 hydrogel exhibits sufficient adhesion in biological microenvironments and good osteogenic effect in vitro and in vivo, under an external SMF, and thus, can be applied to repair critical-sized bone defects. Moreover, bioinformatics analysis reveals that the synergistic mechanism of Fe 3 O 4 NPs and SMF on osteogenic effects can be promotion of osteoblast differentiation via activation of the cyclic guanosine monophosphate (cGMP)/ protein kinase G (PKG)/extracellular signal-regulated kinase (ERK) signaling pathway. This study provides a promising biomaterial with potential clinical application for the future treatment of (irregular) critical-sized bone defects.

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Oxidative stress can lead to nucleus pulposus cell (NPC) apoptosis, which is considered to be one of the main contributors to intervertebral disc degeneration (IVDD). Procyanidin B2 is a natural antioxidant that protects against oxidative stress. However, whether procyanidin B2 protects NPCs from oxidative stress remains unknown. In this study, we demonstrated that procyanidin B2 could reduce tert-butyl hydroperoxide-induced reactive oxygen species in rat NPCs and attenuate rat NPC apoptosis. Further experiments revealed that procyanidin B2 upregulated the expression of both nuclear factor erythroid 2-related factor 2 (Nrf2) and phosphorylation of protein kinase B (Akt). We then used silencing of Nrf2 and LY294002 to silence Nrf2 expression and block the phosphatidylinositol 3-kinase (PI3K)/Akt pathway, respectively, and found that the protective roles of procyanidin B2 in NPCs were inhibited. Therefore, we demonstrated that procyanidin B2 alleviated rat NPC apoptosis induced by oxidative stress by upregulating Nrf2 via activation of the PI3K/Akt signaling pathway. This study provides a potential therapeutic approach for procyanidin B2 in IVDD, which might help in the development of new drugs for IVDD treatment.