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A Dual-Sensitive Hydrogel for 3D Printing
so did the influence of salt-out effect on the transition curve of all samples demonstrated a linear decrease in
temperature. More importantly, we found that all the viscosity with the increase of shear rate (Figure 3A),
samples showed similar curves in temperature sweeps indicating their shear-thinning behavior. All the samples
(from 33 to 40°C) regardless of hydrogels composition showed viscosity above 200 Pa∙s at a shear rate of
(Figure 3D). The results indicated that the mechanical 0.01 s , and it decreased to 0.05 Pa∙s when the shear rate
−1
and other properties of the inks can be easily tuned by was increased to 100 s .
−1
varying the fraction of P4DA without compromising its Besides the shear-thinning properties, the rapid
extrudability. elastic recovery from disruption also contributed to high
shape fidelity. Accordingly, elastic recovery tests were
3.3. Assessment of extrudability carried out using DA40 as a model with alternating low
Through the temperature-induced sol-gel transition, the (1%) and high (100%) strain at 100 s intervals (Figure 3B).
complex viscosity (above 10 Pa∙s) of all the samples At a low strain of 1%, G’ of hydrogel was greater than
reached the requirement for a filament formation in G’’, implying a solid behavior, whereas a sharp drop was
the gelation temperature range from 35.2 to 39.1°C, observed when the hydrogel was subjected to a high strain
Figure 3D . To ascertain whether the thermogels were of 100%, and G’ was surpassed by G’’. After removal of
[49]
capable of printing, DA40 was loaded and extruded applied high strain, G’ of hydrogel instantly recovered
from a needle. As displayed in Figure 3F, filaments to its initial value. The recovery of hydrogels remained
with a smooth and uniform morphology were extruded unchanged after five cycles. Furthermore, a three-stage
continuously from the printing nozzle when the printing steady-flow test was set to simulate conditions before,
temperature was set at 37°C. In contrast, only droplets during, and after the printing process (Figure 3C). At
were formed at the nozzle tip at 25°C. This result indicated the first stage, the hydrogel showed a viscosity around
that the extrudability of inks could be easily modulated 200 Pa∙s with a low shear rate (0.1 s ) representing flow
−1
by temperature. To achieve better printing results, all the behavior in the barrel. A high shearing rate (100 s )
−1
printing experiments were conducted at 37°C. was followed to simulate the extrusion of inks, and the
The responsiveness of viscosity to shear rate was viscosity dropped sharply to 1 Pa∙s at this stage. When
examined with rheological characterization. The flow high shearing force was removed as inks were extruded

A B C

F
D E

Figure 3. (A) Viscosity of DA00, DA20, DA40, DA60, DA60, and DA100 as a function of shear rate at 37°C. (B) Elastic recovery
properties of DA40 at 37°C. (C) Shear recovery properties of DA40 at 37°C. (D) Storge modulus of DA00, DA20, DA40, DA60, DA60,
and DA100 after photo-crosslinking as a function of frequency. (E) Degradation profiles of DA00, DA20, DA40, DA60, DA60, and DA100
at 37°C in PBS containing 0.02 mg lipase. (F) Effect of temperature on the filament formation of ink. (G) Photographs of printed construct
with DA40 before and after immersing in water for 24 h (Scale bar: 5 mm).
146 International Journal of Bioprinting (2021)–Volume 7, Issue 3