Revitalizing bioinert PCL: A biomimetic 3D-Printed gradient-pore scaffold with CSMA hydrogel-mediated BMP-2/QK delivery for vascularized bone regeneration

Critical-sized bone defects exceed bone’s regenerative capacity and require space-maintaining implants that support vascular and bone-tissue ingrowth. Three-dimensional (3D)-printed polycaprolactone (PCL) scaffolds are widely investigated for bone repair owing to controllable geometry, interconnected macropores, and structural stability. However, uniform-pore designs cannot recapitulate heterogeneous bone porosity, while hydrophobic PCL has limited bioactivity. Inspired by the transition from dense cortical to porous cancellous bone, we designed a stepwise gradient-pore PCL framework combining compact regions for structural support with progressively more open regions, providing multiple cell- and tissue-relevant environments. An in situ photocrosslinked methacrylated chondroitin sulfate (CSMA) hydrogel containing bone morphogenetic protein-2 (BMP-2) and the vascular endothelial growth factor-mimetic QK peptide was formed within the printed pores as a hydrated delivery phase. The scaffold retained sequential dense, intermediate, and loose regions; CSMA improved wettability; and PCL confinement slowed the release of both factors relative to hydrogel alone, with QK reaching approximately 80% cumulative release by day 14 and BMP-2 approximately 65% by day 28. Relative to PCL and PCL-CSMA, PCL-CSMA-QK-BMP enhanced endothelial responses, rat bone marrow mesenchymal stem cell proliferation and migration, and osteogenic differentiation and mineralization. Single-factor comparisons supported QK-related endothelial and BMP-2-related osteogenic effects. In rat calvarial defects, PCL-CSMA-QK-BMP increased bone volume fraction, trabecular number, bone mineral density, collagen-rich matrix deposition, and osteogenic and angiogenesis-associated marker expression at 4 and 8 weeks relative to PCL and PCL-CSMA. These findings support coupling graded macroporosity with a hydrated delivery phase for vascularized bone regeneration.