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Preheating of gelatin improves its printability with transglutaminase
rapid and relatively cell-friendly method for the preheating of gelatin as a simple way to
the making of cellular scaffolds . In DIW 3D influence the printability and the volume change
[21]
printing, a continuous ink filament is extruded of the ink. The insights gained in this study shall
through a nozzle onto a stationary substrate from a be applicable for applications in 3D printing to
microscale syringe tip driven by either pneumatic extend the printing time of the gelatin and TG ink
pressure or mechanical force . Although gelatin to fabricate large cellular scaffolds or intricate
[22]
and TG ink is an ideal material for 3D printing, structures for bioprinting and food printing.
its application in 3D printing remains challenging
because of the difficulty to control the physical 2 Results and discussion
and rheological properties of the ink. 2.1 Justification of approach
In particular, 3D printing with gelatin and TG is
limited by the rate of enzymatic crosslinking; the Several approaches are possible to extend the
rapid crosslinking influenced the gelation time of duration of the printable time for the gelatin inks
the ink . The fast crosslinking would result in the crosslinked with TG, including (1) lowering
[9]
rapid gelation of the liquid gelatin ink. The gelation the concentration of gelatin, (2) lowering the
of the ink increases its viscosity and results in concentration of TG, and (3) altering printing
clumps in ink. These clumps diminish the quality temperature. In this paper, we excluded the
of the printed structures and clog the nozzle during alteration of the concentration of the materials
printing. It was reported that printing was only from our possible approaches to achieve the goal.
possible for approximately 3 min at 37°C before It was previously reported that changing gelatin
the crosslinking reaction prevents ink flow due concentration had a small effect on the gelation
to blockage . Future optimization was therefore time of the gelatin and TG ink . Lowering the TG
[9]
[9]
required to identify other approaches to extend the concentration to increase the duration of printing
printing time. was reported to lower the overall hydrogel
To enhance the usability of the freshly prepared stiffness greatly ; the resulting hydrogels
[9]
gelatin (which we term FG) inks crosslinked possessed poor stability in aqueous environments
with TG in 3D printing, we propose to use due to the reduced degrees in crosslinking
preheated gelatin (which we term PG) inks to between gelatin chains. The uncrosslinked gelatin
extend the printing time. We first evaluated the chains would be washed out in the surrounding
stiffness of hydrogels prepared from FG and PG aqueous environment, destabilizing the printed
crosslinked with TG. Envisaging applications gelatin structure. Because of this reason, we fixed
in bioprinting, we identified the concentrations the concentration of TG as 5% w/w throughout
of gelatin and TG in ink. We then 3D printed the current study. Alteration of the printing
grid patterns for FG and PG possessing similar temperature would influence the viscosity of the
stiffness and assessed their printability, gelation, ink and the stability of TG, and we did not employ
and viscosity profiles. Finally, we incubated that approach to achieve extended printable time.
hydrogels in phosphate-buffered saline (PBS) to We selected to investigate the effect of
determine their swelling profiles. Our experiments preheating of gelatin. Gelatin absorbs water and
suggested that hydrogels prepared from PG (10% swells in liquid media . Preheating of gelatin has
[23]
w/w) offered approximately 4 to 10 times longer been reported to reduce its extent of swelling .
[24]
printing time. Finally, we found that PG inks When the gelatin is preheated, the internal
exhibited greater shrinkage at low concentrations hydrogen bonds responsible for holding the triple
(i.e., 7.5% and 10% w/w) and greater swelling helical structure of gelatin are broken. A previous
at high concentrations (i.e., 20% w/w) than FG study reported that the unfolding of the triple
inks of the same concentrations. The shrinkage helical structure exposes more hydrophobic amino
of PG may allow reducing the size of the printed acids, increasing the surface hydrophobicity of
models by post-processing. Our study suggested the gelatin . However, when gelatin undergoes
[25]
120 International Journal of Bioprinting (2020)–Volume 6, Issue 4