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Hyeong-jin Lee, Young Won Koo, Miji Yeo, et al.

has limitations, such as a relatively low printing reso-

lution owing to the microsized extruding nozzle and
comparatively low cell viability caused by severe wall
shear stresses within the nozzle using viscous bioink.
Therefore, researchers using microextrusion-printing
systems are striving for an advanced microextrusion
printing technology that creates a precise print with a
high cell viability [14,16,34,35] .

3. Modified Cell Printing Processes
3.1 3D Cell Printing with Modified Crosslinking
Processes
The 3D cell printing process with natural-polymHU-
based bioink usually contains a cr osslinking proFHVV
owing to low mechanical properties or low visFRVLW\
of the bioink. In this section, a few applications RI PR
dified crosslinking processes during printing are LQWUR
duced.
In recent, Ahn et al. [36–38] developed a modified 3D
cell printing technology with an aerosol crosslinking
process (Figure 2a) that finely sprayed the crosslinked
solution creating a coagulation of the bioink to fabri-
cate the desired form and structure. They reported that
the fabrication of a 3D cell-laden porous mesh struc-

Figure 1. Basic techniques of 3D cell printing, (a) laser-ass- ture using an alginate bioink can produce adequate
isted 3D cell printing techniques with and without an absorbing cell growth, and it was successfully achieved by
layer, [17,22] (b) thermal, piezoelectric, and acoustic inkjet 3D spraying aerosols of calcium chloride (CaCl 2) solution
cell printing systems, [22,28] and (c) microextrusion 3D cell pri- during the printing process. Spraying the aerosol
nting systems and products [14,35] .
cross-linked solution induced a high printability of
unsolved issues, it is expected to be a v ersatile tool the bioink owing to the hardening of the structure sur-
in broad tissue engineering application [22,28] . faces during the crosslinking process and increased
the coherence between the printed cell-laden struts.
2.3 Microextrusion-based Cell Printing
Throughout the process, the amount and position of
Cell-embedded 3D printing with microextrusion in- the cells were controlled within the scaffold.
cludes a d ispensing system that uses pneumatic or The submerged-in-crosslinker cell printing process,
mechanical forces to extrude bioink in a line (Figure referred to as drop-on-demand printing, has been ap-
1c) [29–33] . It is one of the most common cell printing plied to the inkjet [39,40] , laser-assisted [41] , and extru-
methods owing to its accessibility and versatility in sion-based [42,43] cell printing processes to build 3D
printing 3D structures. Microextrusion can be per- structures with relatively low-viscosity bioinks. Xu et
formed using various bioinks with a broad property al. [39] and Boland et al. [40] built the drop-on-demand
range, and especially the viscosity of the bioink in printing apparatus shown in Figure 2b, which uses a
microextrusion is usually much higher than in other layer-by-layer-sinking plate in the crosslinker-filled
3D cell printing methods. This allows for the fabrica- chamber, and the alginate-based bioink was printed on
tion of a complicated 3D structure. Another main ad- the surface of the crosslinking liquid. Through their
vantage of the microextrusion process is its capacity modified method, they overcame one of the limita-
for loading cells at a high density. Using dense cells in tions of the inkjet printing process, the low 3D printa-
the 3D structure can be more effective in the forma- bility, and fabricated a 3D structure with a height of
tion of engineered tissues. However, this process also approximately 12 mm [40] . In 2015, Xiong et al. [41]

International Journal of Bioprinting (2017)–Volume 3, Issue 1 31