4D BIOPRINTING OF VASCULARIZED AND INNERVATED TISSUE-ENGINEERED BONE FOR REPAIRING LARGE SEGMENTAL BONE DEFECTS

Large segmental bone defects remain an unsolved clinical challenge because of the lack of effective vascularization in newly formed bone tissue. 4D bioprinting is a fabrication technology with the potential to create vascularized and innervated bone grafts with biological activity for repairing bone defects in response to patients' specific needs. Traditional bioinks have shown low mechanical strength, poor osteoinductive ability, and lacking a suitable micro-environment for laden cells. Recent advances of in situ vascularized and innervated tissue-engineered bones constructed using 4D bioprinting technology have a potential of being used for repairing large bone defects. 3D bioprinting is an additive manufacturing technique which involves the sequential, typically layer- by-layer, deposition of biocompatible materials and cells to create a 3D construct. When a printable biomaterial contains a biologic (a cell, nucleic acid or biomolecule) it is commonly referred to as a bioink. Although 3D bioprinting theoretically enables the creation of cell-laden constructs of any size and shape, maintaining the viability and function of embedded cells both in vitro (during maturation of the engineered tissue) and in vivo (following its implantation into the body) remains a central challenge in the clinical application. The use of 4D bioprinting to fabricate scaffolds with effective internal vascularization and osteoinductive bioactivity for bone defect repair has become a medical research hotspot. Combining nanocomposite bioinks and multicellular 4D bioprinting technology, biomimetic neural-bone construct can also be developed for innervated bone regeneration. A method was designed for in situ 4D bioprinting of vascularized Tissue Engineered Bone to induce osteogenesis. The in vitro evaluation of osteogenic differentiation of BMSCs has shown excellent osteogenic mineralization.Chinese researchers have developed a biomimetic neural-bone construct delivering with BMSCs and SCs. With inorganic calcium silicate nanowires incorporated, the cells bioprinted scaffolds could simultaneously support the osteogenic and neurogenic differentiation of encapsulated cells.Furthermore, by implanting the multicellular scaffolds into the cranial defects, the authors confirmed their abilities of achieving innervated bone regeneration.In combination with the nanocomposite bioinks and multicellular 4D bioprinting technology, vascularised neural-bone construct can be successfully developed for vascularized and innervated bone regeneration. Vascularised neural cells and bone-related cells were orderly printed to imitate the simplified spatial distribution of bone, blood vessel and nerves. Appropriate drugs can be incorporated into the bioinks to serve as “bioactive factors” to regulate multicellular behaviors, enabling the development of multicellular multiphasic tissue-engineered scaffolds, capable of delivering personalised precision medicine for patients with segmental bone defects.