Bioactive Scaffolds For Bone Tissue Engineering: A Comprehensive Review Of Materials, Fabrication, Vascularisation, And Smart Scaffold Strategies

Abstract

Bone tissue engineering (BTE) has emerged as a transformative discipline integrating materials science, cell biology, and clinical medicine to address the global burden of large bone defects. Traditional repair strategies, including autografts, allografts, and synthetic implants, are constrained by donor-site morbidity, immune rejection, inadequate vascularisation, and limited volumetric availability. This review comprehensively examines bioactive scaffolds for BTE across four interconnected domains: (i) scaffold material classes encompassing ceramics, natural and synthetic polymers, metals, and composites; (ii) fabrication technologies including 3D bioprinting, electrospinning, and freeze-drying; (iii) vascularisation strategies incorporating growth factor delivery, co-culture systems, and prevascularised constructs; and (iv) smart and stimuli-responsive scaffold platforms exploiting reactive oxygen species (ROS), pH gradients, and matrix metalloproteinase (MMP) activity as autonomous therapeutic triggers. The global clinical and economic burden is substantial: approximately 22.3 million orthopaedic procedures are performed worldwide annually, generating costs exceeding US$25 billion in the United States and approximately 28 billion EUR in Europe. Quantitative assessment of scaffold material performance demonstrates that dense sintered hydroxyapatite (HAp) achieves a compressive strength in the range of approximately 100–300 MPa, representing the highest value among characterised bioceramic biomaterials, while porous ceramic scaffolds produced via wax-template-assisted sintering yield compressive strengths in the range of 0.5–50 MPa for typical porous BTE scaffolds (50–90% porosity), with values reaching 100–200 MPa only for low-porosity (<30%) specimens. The Ryshkewitch exponential model confirms a strong inverse relationship between porosity and compressive strength, with strength declining from approximately 60–100 MPa at 30% porosity (dense sintered HAp achieves 100–300 MPa at ≤5% porosity) to near-zero at 90% porosity. [1] Prevascularised scaffold constructs incorporating mesenchymal stem cells (MSCs) with co-cultured endothelial cells demonstrate bone volume fractions approaching 60%, substantially exceeding acellular controls. Dual growth factor delivery systems achieve sequential vascularisation and osteogenic induction, with VEGF approaching complete release by days 10–12 and BMP-2 sustaining a prolonged release profile over 28 days, reaching approximately 95% cumulative release by days 27–28. Among piezoelectric biomaterials, PVDF/BaTiO3 composites exhibit the highest d33 piezoelectric coefficient at approximately 75 pC/N. Translational barriers, including GMP-compatible manufacturing, ATMP regulatory classification, and the absence of standardised aged preclinical models, are critically appraised. The convergence of multi-responsive material chemistry, osteoimmunology, nanomedicine, and artificial intelligence is identified as the defining research frontier of the field.