Sericin/Nano-Hydroxyapatite Hydrogels Based on Graphene Oxide for Effective Bone Regeneration via Immunomodulation and Osteoinduction

Background: Immune responses and osteogenesis differentiation induced by implants are crucial for bone tissue regeneration. Consideration of only one of those properties is not sufficient. To investigate the synergistic actions, we designed alginate/graphene oxide/sericin/nanohydroxyapatite (Alg/GO/Ser/nHAP) nanocomposite hydrogels with both osteoimmunomodulatory and osteoinduc-tive activities. This study aimed to explore the effect of hydrogel with osteoimmunomodulatory properties on promoting osteogenesis of bone marrow stem cells (BMSCs).Methods: Alg/GO/Ser/nHAP nanocomposite hydrogel was fabricated and was characterized by SEM, FTIR, XRD, stress-strain, rheology, swelling and degradation. After the impact of sericin on M2 macrophage polarization was identified, the BMSCs viability and adhesion were evaluated by CCK8 assay, live/dead staining, cytoskeleton staining. The cell osteogenic differentiation was observed by ALP/ARS staining, immunofluorescence staining, RT-PCR, and Western blotting, respectively. Rat cranial defect model was used to assess osteoimmunomodulatory effects of scaffolds in vivo by micro-computed tomographic, histological, and immunohistochemical analyses after 8 weeks of healing.Results: In vitro experiments revealed that the hydrogel presented desirable mechanical strength, stability, porosity, and biocompatibility. Significantly, sericin and nHAP appeared to exert synergistic effects on bone regeneration. Sericin was observed to inhibit the immune response by inducing macrophage M2-type polarization to create a positive osteoimmune microenvironment, contributing to improving osseointegration at the bone-implant interface to promote osteogenesis. However, the osteogenic differentiation in rat BMSCs was further enhanced by combining nHAP and sericin in the nanocomposite hydrogel. Eventually, the hydrogel was implanted into the rat cranial defect model, assisting in the reduction of local inflammation and efficient bone regeneration.Conclusion: The nanocomposite hydrogel stimulated bone formation by the synergistic effects of immunomodulation of macrophage polarization by sericin and direct osteogenic induction by nHAP, demonstrating that such a scaffold that modulates the osteoimmune microenvironment to promote osteogenesis is a promising approach for the development of bone tissue engineering implants in the future.