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Wei Long Ng, Wai Yee Yeong and May Win Naing

10 4 5 G'/G" for 2.5% PGC
2.5% PCG G'/G" for 5.0% PGC
5.0% PCG G'/G" for 7.5% PGC
7.5% PCG tan delta = 1
10 3 4
Viscosity [Pa·s] 10 2 G'/G" 3

10 1 2
1
10 0 20 25 30 35 40
0.1 1 10 100 Temperature ( C)
0
Shear Rate [1/s]
Figure 3. G’/G” ratio of different PGC hydrogels at varying
Figure 2. Rheological behavior of PGC hydrogels at varying temperatures at fixed shear strain of 2%. A high G’/G” ratio (>1)
–1
shear rates (0.1–100 s ) at 27°C (room temperature). All 3
different PGC hydrogels fall within the suitable range of print- would offer good shape fidelity of the printed structures.
ing viscosity (~ 4 to 30 Pa⋅s) at varying shear rates.
one is the deposition of cell-laden hydrogel and the
but both 5% and 7.5% PGC hydrogels have relatively other approach is to print the hydrogel and cells sepa-
more suitable printing viscosities. rately. The latter approach offers better control over
As gelatin is a thermo-sensitive polymer, it is im- the cellular density and distribution across each
portant to evaluate the rheological behaviour of PGC printed layer. As such in our printing process, we fo-
hydrogels at varying temperatures. The storage and cus on the printing of acellular biomaterials using the
loss modulus of PGC hydrogels were evaluated over a extrusion-based printing technique.
temperature range of 20–40°C. Prior to the addition of As shown in Figure 4, the suitable range of printing
NaOH, all the PGC hydrogels were in sol state with pressures for each PGC hydrogel is different. Gener-
low viscosity at temperatures above 25°C, as such it is ally, higher pressures are required to extrude the more
[39]
difficult to achieve good shape fidelity above printing viscous hydrogels . It was observed that the filament
temperatures of 25°C. To analyze sol-gel transition widths of 2.5% PGC hydrogels increase exponentially
state of the PGC hydrogels, the storage (G’) and loss with increasing printing pressures. This is probably
modulus (G”) of the PGC hydrogels were measured. due to the intrinsic low viscosity of 2.5% PGC hydro-
The ratio of G’/G” (tan α) determines the sol-gel state gel which causes higher extent of filament spreading
when a larger printing pressure was used. A similar
of the hydrogel. When tan α is greater than 1, it indi- trend was also observed in 5% PGC hydrogel; the fi-
cates that the material is in a gel state, while a tan α lament widths increase in a linear manner from print-
lower than 1 indicates that the material is in a sol state. ing pressures of 2–2.8 bars and subsequently increase
As shown in Figure 3; only 5% and 7.5% PGC hy- in an exponential manner when the printing pressures
drogels exhibit gel-like behaviour within the tempera- are above 2.8 bars. In contrast, the most viscous 7.5%
ture range of 20–40°C. The tan α of 2.5% PGC hy- PGC hydrogels demonstrated a linear relationship bet-
drogel approaches 1 near 37°C and its tanα value de- ween printing pressures and filament widths through-
creases below 1 at temperatures above 37°C. As such, out 2.6 bars to 3.4 bars. It is likely that the high vis-
2.5% PGC hydrogel will not be used in the bioprinting cosity of 7.5% PGC hydrogel reduces the extent of
process as loss of shape fidelity might occur during filament spreading at higher printing pressures (above
the incubation of the printed construct at higher tem- 3 bars). It was also observed that standard deviation of
perature. Conversely, both 5% and 7.5% PGC hydro- printed filament widths decreases with PGC hydrogels
gels exhibit significantly high G’/G” ratio, which would of higher viscosity. Hence, a more viscous hydrogel
offer good shape fidelity of the printed structures. offers higher printing consistency and better control
over the printed filament widths at increasing printing
3.3 Bioprinting of Biomaterials
pressure.
There are currently two different modes of printing; Generally, a higher feed rate would result in a thinner
International Journal of Bioprinting (2016)–Volume 2, Issue 1 57