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Solvent-based extrusion 3D printing
is associated with elastic energy stored by the modulus values and biological functionality .
[44]
alginate chains. As shown in Figure 2D, filament Ouyang et al. studied thermal cross-linking by
[45]
formation with several concentrations of laponite investigating the influence of printing temperature
was evaluated; filaments were formed at laponite on ink printability and the viability of embryonic
concentrations greater than 5 wt%. The ink solution stem cells. They defined the bioink printability
with higher viscosity is relatively more difficult based on the printing of a square shape. The
to flow or spread; this phenomenon can serve to temperature was shown to affect the gelation
maintain the printed filament shape. degree. Samples at 30°C were proper gelatin; on
the other hand, 25°C resulted in over gelation.
3.4 Ink cross-linking mechanisms for SBE 3D It should be noted that gels formed by thermal
printing cross-linking usually lack sufficient mechanical
Apart from increasing the ink concentration and strength; furthermore, the incorporated cells
[40]
adding ceramic nanoparticles , ink cross-linking would be sensitive to temperature changes.
[39]
is a method to improve the ink elastic modulus Photo-cross-linking is the photoinduced
and facilitate filament formation. Particularly formation of a covalent bond between
in the case of a hydrogel-based ink, the cross- macromolecules to form a cross-linked network
linking procedure can facilitate the transition with illuminated light. Ultraviolet (UV) light is
from solution to gel. The most common used commonly used for polymer cross-linking [38,46] .
[47]
cross-linking procedures include ionic, covalent, Schuurman et al. investigated the stiffness of
thermal, and photo-cross-linking. Ionic cross- gelatin methacrylate after exposure to various UV
linking occurs when a water-soluble and charged light doses. The results shows that compressive
polymer undergoes cross-linking with an ion of modulus increases with UV exposure time.
opposite charge . It is an important mechanism Hyaluronic acid is a material that is commonly
[38]
in bioprinting since it enables mild and instant used for cartilage tissue repair due to its structural
gelation of hydrogel bioinks. The cross-linking and biological properties. However, unmodified
solution can be mixed, immersed, or sprayed with hyaluronic acid exhibits poor stability. To solve
[48]
the ink during the printing process. Freeman and this issue, photo-cross-linkable dextran derivates
[30]
Kelly investigated alginate ink stiffness by or acrylate pluronic were added to improve
[41]
varying the calcium chloride (CaCl ) crosslinking the mechanical properties and printability of
2
ratio. The results indicate that the spatial hyaluronic acid ink. The incorporated chondrocytes
microenvironment was found to have a significant showed good compatibility with the formulated
effect on the differentiation of MSCs within the inks and high cell viabilities. They indicate that
alginate bioinks; stiffer regions of the printed the photo-cross-linking procedure can be used
construct preferentially supported osteogenesis with cell-containing inks since light exposure is a
over adipogenesis. Chung et al. incorporated minimally invasive process.
[42]
gelatin within an alginate solution; the alginate/ 3.5 The processing parameters for SBE 3D
gelatin scaffold showed better print resolution printing
than the alginate scaffold after CaCl -crosslinking.
2
In addition, alginate hydrogels can form covalent In addition to the ink material, ink rheology,
crosslinking through reacting with poly (ethylene and cross-linking mechanism, the processing
glycol)-diamine . Liu and Li combined a parameters also affect the performance of the SBE
[43]
[40]
covalent cross-linking polyacrylamide and ionic 3D printing process. The processing parameters
cross-linking k-carrageenan to synthesize the in SBE 3D printing include the extrusion rate,
k-carrageenan/polyacrylamide double-network needle-moving speed, substrate, and parameters
hydrogels. Their results showed that newly of the scaffold. The extrusion rate can be affected
synthesized hydrogels displayed high elastic by several factors, including the ink rheology,

34 International Journal of Bioprinting (2020)–Volume 6, Issue 1