Orchestrating the immuno-chondrogenic microenvironment via a bio-inorganic hybrid microsphere system for tendon-to-bone healing

The structural failure of rotator cuff repair is largely attributed to fibrovascular scar formation driven by a persistent inflammatory microenvironment and insufficient fibrocartilage regeneration at the tendon-to-bone interface. To address this, a microfluidic-generated GelMA microsphere system co-encapsulating Zeolitic Imidazolate Framework-8 (ZIF-8) nanoparticles and bone marrow mesenchymal stem cell (BMSC)-derived small extracellular vesicles (sEVs) is developed. This composite system (ZIF-8/sEVs@MS) enables the sustained release of Zn 2+ and sEVs, which synergistically reprogram macrophages (Mφ) from a pro-inflammatory M1 phenotype to a reparative M2 state and restore the CXCL12/CXCR4 axis for endogenous stem cell recruitment. Transcriptomic analysis elucidates that the system reactivates the PI3K/AKT signaling pathway, thereby reversing inflammation-mediated inhibition and driving chondrogenic differentiation. In a rat rotator cuff repair model, the functionalized microspheres significantly enhance fibrocartilaginous enthesis regeneration, biomechanical structural integrity, and limb function. This study establishes a dual-functional "immuno-chondrogenic" strategy that coordinates immune microenvironment modulation with tissue-specific differentiation to facilitate functional tendon-to-bone healing. Graphical abstract: Schematic illustration of the bio-inorganic hybrid microsphere system orchestrating an immuno-chondrogenic microenvironment for tendon-to-bone healing. A) Synthesis of ZIF-8 nanoparticles. B) Isolation and extraction of sEVs from BMSCs. C) Fabrication of the ZIF-8/sEVs@MS composite microspheres via microfluidic droplet generation technology. The ZIF-8 nanoparticles and sEVs are co-encapsulated within a photo-crosslinkable GelMA matrix to ensure uniform morphology and sustained bioactivity. D) In vivo administration and molecular mechanism driving tissue regeneration. Following local injection into the rotator cuff TBI, the microspheres degrade to release zinc ions and sEVs. These bioactive cues synergistically orchestrate the microenvironment by inhibiting a pro-inflammatory M1 phenotype and promoting a reparative M2 phenotype, facilitating the recruitment of endogenous stem cells. Subsequently, the internalization of these cues reactivates the PI3K/AKT signaling pathway, ultimately promoting chondrogenic differentiation and fibrocartilaginous enthesis regeneration.