Engineering vascularized 3D printed scaffolds for bone regeneration through design and biofunctionalization toward clinical translation

The regeneration of critical-sized bone defects remains limited by the inability of current scaffolds to support effective and functional vascularization. Three-dimensional (3D) printing has been used as a fabrication approach to design scaffolds with controlled architecture, mechanical properties, and spatial distribution of bioactive molecules. However, significant gaps persist between experimental results and clinical translation. This review analyzes the biological and engineering challenges associated with vascularizing 3D-printed scaffolds for bone regeneration. A distinction is first established between structural vascularization, defined by the presence of endothelial markers and vessel-like structures, and functional vascularization, characterized by perfusable, anastomosed, and hemodynamically stable networks that sustain long-term tissue survival. Special attention is given to four critical limitations: the oversimplification of vascular architecture in current scaffold designs, the inadequate spatiotemporal control of angiogenic signaling, the disparity between promising in vitro results and poor in vivo integration, and the absence of standardized criteria for vascularization assessment. Osteogenic–angiogenic coupling, immune regulation through osteoimmunomodulation, and additive manufacturing technologies, including extrusion-based bioprinting, stereolithography (SLA), and selective laser sintering (SLS), are discussed in the context of these limitations. Emerging approaches, including computational vascular modeling, stimuli-responsive delivery systems, and hybrid bioprinting strategies, are also examined. This analysis indicates that reproducible strategies capable of generating hierarchical, perfusable, immunocompatible, and mechanically stable vascular networks are still needed to improve the clinical applicability of vascularized scaffolds for the repair of large bone defects.