A multifunctional self-reinforced injectable hydrogel for enhancing repair of infected bone defects by simultaneously targeting macrophages, bacteria, and bone marrow stromal cells

Injectable hydrogels (IHs) have demonstrated huge potential in promoting repair of infected bone defects (IBDs), but how to endow them with desired anti-bacterial, immunoregulatory, and osteo-inductive properties as well as avoid mechanical failure during their manipulation are challenging. In this regard, we developed a multifunctional AOHA-RA/Lap nanocomposite IH for IBDs repair, which was constructed mainly through two kinds of reversible cross-links: (i) the laponite (Lap) crystals mediated electrostatic interactions; (ii) the phenylboronic acid easter bonds between the 4-aminobenzeneboronic acid grafted oxidized hyaluronic acid (AOHA) and rosmarinic acid (RA). Due to the specific structural composition, the AOHA-RA/Lap IH demonstrated superior injectability, self-recoverability, spatial adaptation, and self-reinforced mechanical properties after being injected to the bone defect site. In addition, the RA molecules could be locally released from the hydrogel following a Weibull model for over 10 days. Systematic in vitro/vivo assays proved the strong anti-bacterial activity of the hydrogel against Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli). Moreover, its capability of inducing M₂ polarization of macrophages (M_φ) and osteogenic differentiation of bone marrow stromal cells (BMSCs) was verified either, and the mechanism of the former was identified to be related to the JAK1-STAT1 and PI3K-AKT signaling pathways and that of the latter was identified to be related to the calcium signaling pathway, extracellular matrix (ECM) receptor interaction and TGF-β signaling pathway. After being implanted to a S. aureus infected rat skull defect model, the AOHA-RA/Lap IH significantly accelerated repair of IBDs without causing significant systemic toxicity.

Statement of significance

Rosmarinic acid and laponite were utilized to develop an injectable hydrogel, promising for accelerating repair of infected bone defects in clinic. The gelation of the hydrogel was completely driven by two kinds of reversible cross-links, which endow the hydrogel superior spatial adaption, self-recoverability, and structural stability. The as-prepared hydrogel demonstrated superior anti-bacterial/anti-biofilm activity and could induce M₂ polarization of macrophages and osteogenic differentiation of BMSCs. The mechanism behind macrophages polarization was identified to be related to the JAK1-STAT1 and PI3K-AKT signaling pathways. The mechanism behind osteogenic differentiation of BMSCs was identified to be related to the ECM receptor interaction and calcium signaling/TGF-β signaling pathways.