Bio-printing process:
The bio-printing starts with obtaining the anatomical structure of the target tissue by a proper imaging technique such as CT and MRI. Then the image is translated to a CAD drawing of cross-sectional layers using specialized software such that the printing device will be able to add them in a layer-by-layer process. Next, the bioprinting device constructs the tissue using a specific printing method such as inkjet 3D bio-printing, micro-extrusion 3D bio-printing, Laser-assisted 3D bio-printing and stereo-lithography by employing a combination of printing materials such as scaffold, bioink and other additive factors. The deposition accuracy, stability and tissue viability vary through these processes. Finally, the constructed tissue is post-processed in a bioreactor to recreate the required in vivo environment to maintain tissue viability during the maturation period. Dababneh and Ozbolat published an excellent review on the 3D bioprinting process [Dababneh and Ozbolat, 2014]. The ultimate aim of the 3d bio-printing is to develop a technology that will be able to realize 3D functional complex organs as a source for tissue grafts, full organ transplants and animal-alternative models for drug screening. This technology still in the very early stage but rapidly moving forward with a plethora of research span from printing engineering to tissue engineering and cell sciences being done on bio-printing technology. Bioprinting technology has been evolved from the traditional 2D printing on paper and later the 3d printing, including inkjet and laser printing as well as 3d printing of the non-biological materials using extrusion and stereolithography. Therefore, it is not surprising that the engineering aspect is more advanced than the bioink materials technology. However, each printing technology, since it is developed for no-biological material printing, still suffering from several limitations related to the material compatibility when replacing other building materials with bioink. For example, inkjet printing requires low viscosity bioink [Moon et al, 2010]. This makes the deposition of several effective highly viscous hydrogels and extracellular matrix (ECM) more difficult. The excessive heat generated in the laser-assisted 3D bioprinting process may damage cells and affect the cell viability in the printed tissue [Gudapati et al 2014]. The pressure used to expel the bioink in the extrusion-based bioprinting results in imposing high shear stress on the bioink components and may lead to loss of cellular viability and distortion of the tissue structure [Chang et al, 2008 and Ozbolat and Yu, 2013]. In stereolithography-based bioprinting, the high intensity of ultraviolet radiation needed for the cross-linking process and the lengthy post-processing requirements also impose limitations to use the technology in tissue printing. Bioink development, on the other hand, is the most challenging task which still delaying the progress of bioprinting technology. The ideal properties for bioink must meet both the physical and biological material requirements to enable in vivo-like cellular behaviour, such as proliferation, differentiation, migration, and maturation. The physical properties are the viscosity structural strength, the printing capacity, degradation and functionality. Biological properties include cytocompatibility, biocompatibility and bioactivity [Malmonge and Santos, 2018]. Diverse bioink composition are existed to meet the requirements of specific printing technology. Bioink viscosity is a crucial parameter that always needs optimization to adjust the bioink flow and cell encapsulation efficiency and tissue structure stability [Jang et al, 2018]. Hydrogels are promising candidates for the development of bioinks thanks to their biocompatibility, low cytotoxicity, hydrophilicity and the ability to form networks of polymers allowing them to acquire ECM similar a structure.