Force-controlled 3D mechanical stretching to enhance the exosome secretion of bone mesenchymal stem cells for bone repair

Exosomes derived from bone mesenchymal stem cells (BMSCs) show promising potential for treating bone defects. However, their clinical application is hindered by low yield and insufficient repair ability. Three-dimensional (3D) mechanical stimulation has been a well-known method for enhancing exosome secretion; however, the traditional stimulation process is always achieved by controlling the displacement of manipulators, which may induce uneven loading distribution and degradation of stimulation strength. Here, we propose a micro-stretching manipulator that automatically controls the stretching force applied to gelatin methacryloyl (GelMA)/hyaluronic acid methacryloyl (HAMA) hybrid hydrogel sheets containing BMSCs within an incubator. To ensure the structural stability of the sheets after long-term stretching, the mixing ratio between GelMA and HAMA was optimized according to the mechanical property response of the sheets to cyclical loading. Subsequently, force-controlled mechanical loading was applied to the BMSC-laden sheets to produce exosomes. Compared with displacement control, force-controlled loading provides a more stable force stimulation, thereby enhancing exosome secretion. Furthermore, continuously stimulated exosomes exhibited a stronger capacity for promoting osteogenic differentiation of BMSCs and facilitating the repair of bone defects in a rat model. These findings suggest that force-controlled loading of cell-laden hydrogels offers a novel approach for the production of BMSC-derived exosomes and their application in bone repair.