Injectable Co-assembled microgels for programmed endogenous regeneration of nucleus pulposus

Endogenous regeneration of the nucleus pulposus (NP) is hindered by inefficient recruitment of stem/progenitor cells, limited control over differentiation, and poor matrix maintenance. Here, we developed a microsphere-based platform (GBKE) that transforms upon mild hydration into a dynamic, self-healing hydrogel, allowing in situ gelation and stable retention within soft tissues. The hydrogel forms a highly porous, interconnected network that facilitates nutrient transport and deep cellular infiltration. The microspheres form an interpenetrating network structure by co-assembling through β-sheet/π-π stacking driven self-assembly, combining gelatin methacrylamide (GelMA) with the self-assembling small molecule BPAA-GFF. The chondrogenic-inducing small molecule kartogenin is bound to the BPAA-GFF nanofiber network through non-covalent interactions such as hydrogen bonds and hydrophobic forces, achieving sustained inducing effect. The MSC affinity peptide E7 is covalently coupled to the surface of the microspheres to achieve targeted cell homing. Thanks to the enhanced effect of the nanofiber network induced by BPAA-GFF, the storage modulus (≈1700 Pa) and critical shear stress (≈35 Pa) of the microgels are significantly higher than those of pure GelMA microgel. In vivo, GBKE microgels significantly improved disc height index and extracellular matrix restoration in rat models, maintaining NP regeneration for at least 8 weeks. In rabbit models, MRI analyses and histological results confirmed durable structural and functional repair for up to 12 weeks. This system enables the programmed regulation of endogenous cell behavior, offering a promising approach for minimally invasive intervertebral disc repair.