Rationale: Mineral particles have been widely used in bone tissue engineering scaffolds due to their osteoconductive and osteoinductive properties. Despite their benefits, mineral particles can induce undesirable inflammation and subsequent bone resorption. Aspirin (Asp) is an inexpensive and widely used anti-inflammatory drug. The goal of this study is to assess the synergistic effect of Asp and optimized mineral particle coating in macroporous scaffolds to accelerate endogenous bone regeneration and reduce bone resorption in a critical-sized bone defect model. Methods: Four commonly used mineral particles with varying composition (hydroxyapatite v.s. tricalcium phosphate) and size (nano v.s. micro) were used. Mineral particles were coated onto gelatin microribbon (µRB) scaffolds. Macrophages (Mφ) were cultured on gelatin µRB scaffolds containing various particles, and Mφ polarization was assessed using PCR and ELISA. The effect of conditioned medium from Mφ on mesenchymal stem cell (MSC) osteogenesis was also evaluated in vitro. Scaffolds containing optimized mineral particles were then combined with varying dosages of Asp to assess the effect in inducing endogenous bone regeneration using a critical-sized cranial bone defect model. In vivo characterization and in vitro cell studies were performed to elucidate the effect of tuning Asp dosage on Mφ polarization, osteoclast (OC) activity, and MSC osteogenesis. Results: Micro-sized tricalcium phosphate (mTCP) particles were identified as optimal in promoting M2 Mφ polarization and rescuing MSC-based bone formation in the presence of conditioned medium from Mφ. When implanted in vivo, incorporating Asp with mTCP-coated µRB scaffolds significantly accelerated endogenous bone formation in a dose-dependent manner. Impressively, mTCP-coated µRB scaffolds containing 20 µg Asp led to almost complete bone healing of a critical-sized cranial bone defect as early as week 2 with no subsequent bone resorption. Asp enhanced M2 Mφ polarization, decreased OC activity, and promoted MSC osteogenesis in a dosage-dependent manner in vivo. These results were further validated using in vitro cell studies. Conclusions: Here, we demonstrate Asp and mineral particle-coated microribbon scaffold provides a promising therapy for repairing critical-sized cranial bone defects via immunomodulation. The leading formulation supports rapid endogenous bone regeneration without the need for exogenous cells or growth factors, making it attractive for translation. Our results also highlight the importance of optimizing mineral particles and Asp dosage to achieve robust bone healing while avoiding bone resorption by targeting Mφ and OCs.

Carl Zhang

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