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.