Flowerbed-Inspired Mg-Loaded Scaffold Accelerates Critical-Sized Bone-Defect Repair by Reprogramming the Microenvironment

The repair and reconstruction of critical-sized bone defects pose substantial clinical challenges, particularly owing to the limited self-healing capacity of the central regions. Inspired by the structure and function of a “flowerbed,” we developed a biomimetic composite scaffold by embedding a magnesium-ion-loaded alginate-based (PR/AlgMA/Mg) hydrogel into a triply periodic minimal surface (TPMS) porous Ti6Al4 V framework. This design aims to create a simulated stable osteogenic niche in load-bearing defect areas. The PR/AlgMA/Mg hydrogel exhibited favorable physicochemical properties, controllable degradation, and sustained release of Mg2+. In vitro, it supports the osteogenic differentiation of bone-marrow mesenchymal stem cells (MSCs) in a Mg2+ concentration-dependent manner. In a rat model of critical-sized calvarial defects, the PR/AlgMA/Mg significantly enhanced the formation of bone bridges in the central defect area. Single-cell RNA sequencing identified a unique CCN3+ MSCs subpopulation recruited by the Mg2+-enriched microenvironment, which exhibited high stemness and participated in early osteogenesis via the Wnt/β-catenin signaling pathway. Moreover, CCN3+ MSCs suppressed the M1 phenotype and promoted the M2 polarization of macrophages through secretion of the pleiotropic growth factor PTN, thereby fostering an immune microenvironment conducive to bone repair. The TPMS scaffold demonstrated a uniform stress distribution and excellent permeability, thus facilitating nutrient exchange and cellular infiltration. In a beagle femoral condyle defect model, the PR/AlgMA/Mg–TPMS induced a contact osteogenesis pattern, thus achieving early bone bridging in the load-bearing area and significantly enhancing biomechanical integration. This biomimetic scaffold constructs an osteogenic niche by recruiting progenitor cells and reprogramming the microenvironment, representing a promising strategy for repairing critical bone defects.