Tissue engineering, with the ultimate goal of engineering artificial tissues or organs to replace malfunctioning or diseased ones inside the human body, provides a substitute for organ transplantation. Driven by the growing, tremendous gap between the demand for and the supply of donated organs, tissue engineering has been advancing rapidly. There has been great success in engineering artificial organs such as skin, bone, cartilage and bladders because they have simple geometry, low cell oxygen consumption rates and little requirements for blood vessels. However, difficulties have been experienced with engineering thick, complex tissues or organs, such as hearts, livers or kidneys, primarily due to the lack of an efficient media exchange system for delivering nutrients and oxygen and removing waste. Very few types of cells can tolerate being more than 200 μm away from a blood vessel because of the limited oxygen diffusion rate. Without a vasculature system, three-dimensional (3D) engineered thick tissues or organs cannot get sufficient nutrients, gas exchange or waste removal, so nonhomogeneous cell distribution and limited cell activities result. Systems must be developed to transport nutrients, growth factors and oxygen to cells while extracting metabolic waste products such as lactic acid, carbon dioxide and hydrogen ions so the cells can grow, proliferate and make extracellular matrix (ECM), forming large-scale tissues and organs. However, available biomanufacturing technologies encounter difficulties in manufacturing and integrating vasculature networks into engineered constructs. This work proposed a novel 3D bioprinting technology that offers great potential for integration into thick tissue engineering. The presented system offered several advantages, including that it was perfusable, it could print conduits with smooth, uniform and well-defined walls and good biocompatibility, it had no post-fabrication procedure, and it enabled direct bioprinting of complex media exchange networks.

Vascularization of thick engineered tissue and organ constructs like the heart, liver, pancreas or kidney remains a major challenge in tissue engineering. Vascularization is needed to supply oxygen and nutrients and remove waste in living tissues and organs through a network that should possess high perfusion ability and significant mechanical strength and elasticity. In this thesis, we introduce a fabrication process to print vascular conduits directly, where conduits were reinforced with carbon nanotubes (CNTs) to enhance their mechanical properties and bioprintability. The generation of vascular conduit with a natural polymer hydrogel such as alginate needs to have improved mechanical properties in order to biomimic the natural vascular system. Carbon nanotube (CNT) is one of the best candidates for this goal because it is known as the strongest material and possesses a simple structure. In this thesis, multi-wall carbon nanotube (MWCNT) is dispersed homogenously in the hydrogel and fabricated through an extrusion-based system.In vitro evaluation of printed conduits encapsulated in human coronary artery smooth muscle cells was performed to characterize the effects of CNT reinforcement on the mechanical, perfusion and biological performance of the conduits. Perfusion and permeability, cell viability, extracellular matrix formation and tissue histology were assessed and discussed, and it was concluded that CNT-reinforced vascular conduits provided a foundation for mechanically appealing constructs where CNTs could be replaced with natural protein nanofibers for further integration of these conduits in large-scale tissue fabrication. It was concluded that MWCNT has a significant effect on mechanical properties, vascular conduit swelling ratio and biological characterization in short-term and long-term cellular viability.