Geometry-driven immunomodulation in 3D-printed bioceramics: Negative curvature promotes macrophage M2 polarization...

The geometric features of bioactive scaffolds are biophysical cues regulate cell fate, but their immunomodulatory potential in bone regeneration is yet to be determined. Growing evidence suggests that surface curvature is a potent regulator of cellular behaviours and osteogenesis. Therefore, we quantitatively decoded this underlying mechanism by identifying Gaussian curvature(K) as a potent geometric regulator of macrophage polarization, creating a pro-regenerative microenvironment for bone repair. Using a high-throughput β-tricalcium phosphate(β-TCP) bioceramic platform (K = -4.91 to +4.82 mm^-2), we demonstrate that negative gaussian curvature(K ^-, K  < -1.72 mm^-2) promotes M2 macrophage polarization and endothelial CD31 expression. Mechanistically, single-cell transcriptomic RNA sequencing revealed that K ^- scaffold downregulates hypoxia-inducible factor 1-alpha (HIF-1α) via Ras-mitogen-activated protein kinase (Ras-MAPK) inhibition and thus promotes macrophage M2 polarization, consequently elevating BMP2 and VEGF secretion. In vivo, β-TCP scaffolds with K = -1.72 mm^-2 achieved 42.4 % greater bone volume and higher torsional strength at 12 weeks than the scaffolds with K = +4.82 mm^-2 in 15 mm critical-sized segmental defects of rabbit radius. This work indicates a quantitative geometry-immunity relationship for bioceramic scaffolds, contributing to the development of topology-mediated immunomodulatory biomaterials.