Advancing Bone Regeneration: Pioneering the Future with 3D-Printed Scaffolds and Serum Exosomes

Large bone defects, commonly resulting from trauma, infections, tumors, or congenital disorders, pose a substantial challenge in orthopedic surgery. Traditional treatments involve autologous or xenogenous bone grafts, which often suffer from limitations like donor scarcity, potential disease transmission, and immune rejection. In a new study published in ACS Applied Materials & Interfaces Journal by Dr. Hao Liu, Ranli Gu, Wei Li, Lijun Zeng, Yuan Zhu, Boon Chin Heng, Yunsong Liu, and led by Professor Yongsheng Zhou from Peking University, the authors introduced an innovative “cell-free scaffold engineering” strategy, employing a three-dimensional (3D) printed titanium (Ti) scaffold integrated with strontium (Sr) and serum exosomes (sEXOs). This combination demonstrates significant potential in maintaining the morphological characteristics of bones during critical bone defect (CBD) repair and enhancing bone formation while suppressing fibroblast activity. The team created a 3D-printed Ti scaffold based on the morphology of bone defects, followed by integration of Sr into the scaffold’s surface layer. The scaffold’s structure was examined using various techniques, including micro-CT and SEM, confirming the successful incorporation of Sr. The SrTi scaffold maintained excellent biomechanical strength and demonstrated the controlled release of Sr, critical for bone regeneration.

The authors assessed the scaffold’s ability to promote bone formation and inhibit bone resorption in human bone marrow stem cells (hBMSCs) and osteoclast-like cells which were exposed to the scaffold’s leach liquor. They found that SrTi scaffold significantly enhanced osteogenic differentiation (evidenced by ALP and ARS staining) and reduced the formation of osteoclast-like cells, indicating its dual role in promoting bone growth and inhibiting resorption. The exosomes used in their study were isolated from the serum of rabbits during the bone fracture healing phase. These exosomes, labeled BF EXO, were analyzed using transmission electron microscopy and Western blotting, confirming their typical exosomal markers and structure. The team conducted various assays including ALP/ARS staining, migration, and tube formation assays using hBMSCs and endothelial cells to examine the osteogenic and angiogenic capabilities of BF EXO and found it significantly promoted osteogenesis and angiogenesis compared to control exosomes. They tried to understand the molecular mechanisms behind BF EXO’s effects and used next-generation sequencing to analyze the miRNA profiles of BF EXO and identified specific miRNAs in BF EXO that potentially contribute to its osteogenic and angiogenic effects.

The research team conducted invivo experiments where they implanted the composite of SrTi scaffold and BF EXO into radial bone defects in rabbits and afterward, they monitored bone healing using micro-CT, histological analysis, and serum biomarker evaluation. The results were astonishing and the composite significantly enhanced bone repair, evidenced by improved bone density and structure, as well as accelerated revascularization in the defect area. The new study also investigated the pathways and molecular interactions facilitated by the miRNAs in BF EXO and found to be involved in multiple signaling pathways crucial for bone and vascular tissue regeneration.

Indeed, the authors engineered a novel SrTi scaffold that exhibits several key features: 3D-Printed Titanium Scaffold: The base of the scaffold is a 3D-printed Ti structure, designed to mimic the bone defect area’s morphology, providing a customized treatment approach. Strontium known for its dual role in promoting bone formation and inhibiting resorption, is incorporated into the scaffold. This controlled release of Sr from the scaffold’s surface is pivotal in facilitating bone regeneration. The authors innovatively uses  sEXOs, particularly those derived during the fracture healing phase (termed BF EXO), to enhance osteogenesis and angiogenesis. These exosomes are rich in RNAs and proteins crucial for bone and vascular growth. The approach has also the advantage that it does not rely on direct cell transplantation, which mitigates risks associated with immune responses and disease transmission. The SrTi scaffold integrated with BF EXO demonstrated several therapeutic actions. Firstly, the scaffold supported new bone growth by providing a matrix for bone deposition (osteoconduction) and stimulating the differentiation of precursor cells into osteoblasts (osteoinduction). Secondly, BF EXO promoted the formation of new blood vessels  (angiogenesis), essential for supplying nutrients and oxygen to the regenerating bone tissue. The authors also investigated the mechanism where miRNAs in BF EXO facilitate osteogenesis and angiogenesis, highlighting the complex molecular interplay in bone healing.

The new study opens new avenues in regenerative medicine, particularly for treating large bone defects. The cell-free, biomaterial-based approach offers a promising alternative to traditional bone grafts, with the potential for fewer complications and improved outcomes. Future research could focus on refining the scaffold design, exploring the long-term implications of Sr and BF EXO integration, and clinical trials to establish efficacy and safety in humans. In summary, the study by Dr. Liu and colleagues represents a significant step forward in orthopedic regenerative medicine. By combining advanced materials science with a deep understanding of bone biology and the therapeutic potential of exosomes, this research paves the way for innovative treatments for complex bone defects. The integration of Sr and sEXOs in a 3D-printed Ti scaffold offers a novel, potentially more effective approach to bone regeneration, marking a notable advancement in the field. In conclusion, Peking University scientists developed a powerful approach for treating large bone defects, offering benefits in osteogenesis, angiogenesis, and revascularization. The study establishes a foundation for future research and potential clinical application of scaffold-based, cell-free approaches in bone regeneration.