Page 538 - IJB-9-5

International Journal of Bioprinting GelMA/PEG-TA IPN networks for 3D bioprinting

Table 1. Water uptake and gel content of GelMA and and GelMA/8PEGTA physically crosslinked hydrogels
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GelMA/8PEGTA ‑IPN hydrogels. were optimal at a temperature of 22°C. Subsequently,
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the rheological properties of the hydrogels formed after
Network Photograph a Gel Water
content (%) uptake (%) UV crosslinking (GelMA-UV) and UV and enzymatic
GelMA-UV 86±3 1730±180*** crosslinking (GelMA/8PEGTA -IPN) were determined. In
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these experiments, the GelMA/8PEGTA solution contains
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both LAP as a photoinitiator and a low concentration of the
enzyme HRP. Incubating the photo-crosslinked hydrogel
in a hydrogen peroxide solution, the second crosslinking
step afforded the IPN (Scheme 3).
GelMA/8PEGTA -IPN 91±3* 665±6
5 The kinetics of UV irradiation in the physically
crosslinked GelMA/8PEGTA hydrogel at room
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temperature was determined by measuring the changes
in the rheological properties upon in situ UV irradiation.
With increasing the irradiation time from 20 s to 120
a Scale is in mm. *P < 0.05, ***P < 0.001, n = 5 s, the storage modulus of IPN gels increased from
1.67 KPa to 3.35 KPa (Figure 3A). Interestingly, after
a decreased water uptake. Interestingly, the GelMA-UV shutting down each UV irradiation, the storage modulus
gel was transparent after gelation and photo-crosslinking was still increasing as function of time sweep. This
whereas the GelMA/8PEGTA -IPN was turbid. The IPNs indicates with insufficient UV irradiation, the double
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showed a significantly lower swelling (665%) compared to bonds presence in precursor could not fully reacted. As
GelMA-UV networks (1730%). depicted in Figure 3A, dash line, an optimal time for UV

3.5. Rheology crosslinking using LAP as a photoinitiator appeared to
be 2 min, as indicated by plateau that was observed after
At the optimized concentration and temperature for irradiation. Longer times did not significantly increase
deposition of stable printed fibers, mechanical properties the storage modulus of the gel. A temperature sweep
of the physically crosslinked, photo-crosslinked, and from 10 to 40°C of the GelMA/8PEGTA hydrogel after
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IPN hydrogels were determined by rheology. First, strain UV crosslinking revealed a drop in the storage modulus
and frequency sweeps of physically crosslinked GelMA in the range of 25 – 30°C due to loss of the gelatin
and GelMA/8PEGTA were recorded at 5°C. Strains of physical crosslinks (Figure 3B). At a temperature of
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maximally 0.5% could be applied before deformation 37°C, the photo-crosslinked GelMA/8PEGTA hydrogel
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occurred and a gel-sol transition at higher strains was showed G’ and G’’ values close to each other indicating
observed for both gels. At a constant strain of 0.5%, the a soft viscous gel. Such printed scaffolds were expected
physically crosslinked gels showed minor dependence of G’ to have low shape stability upon implantation [37] . In a
and G” on the frequency (Figure S3, Supplementary File). control experiment, within the temperature range of 10
The storage (G’) and loss modulus (G”) of these physically – 40°C, an enzymatically crosslinked 2 wt% 8PEGTA
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crosslinked hydrogels as a function of temperature are hydrogel showed no changes in the storage and loss
presented in Figure 2A. The complex viscosity of GelMA modulus (Figure 3C). The GelMA/8PEGTA gel was
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and GelMA/8PEGTA solutions showed no difference subsequently submerged in 0.03 wt% H O solution in
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2
2
as a function of temperature (Figure S4, Supplementary PBS to enzymatically crosslink the 8PEGTA conjugate
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File). The gel point, G’ = G”, for both systems is observed to form the IPN. The enzymatic crosslinking is very
at 27°C. The appropriate printing temperature can be fast and within seconds the storage modulus reached a
visualized by the loss tangent (tanδ) of the inks, the ratio of maximum value of 6 kPa. IPN gels showed mechanical
loss modulus (G”) and storage modulus (G’), representing properties independent of temperatures up to 37°C
the plasticity and elasticity of materials . As shown in (Figure 3D).
[40]
Figure 2B, at higher temperatures (G’ < G”), the ink will
show a typical liquid-like behavior, and no filaments can 3.6. Compression and tensile properties
be formed during printing. On cooling, G’ increases and Representative compressive and tensile stress-strain curves
at temperatures below the gel point, a temperature window for all gels are depicted in Figure 4. A reduced swelling
for optimal printing is present. behavior and increased mechanical properties of hydrogels
By the foregoing experiments, it was shown that the are expected with increasing concentration and degree of
rheological properties and printing behavior of the GelMA crosslinking.

Volume 9 Issue 5 (2023) 530 https://doi.org/10.18063/ijb.750