Page 440 - IJB-10-5

International Journal of Bioprinting Bioprinting for large-sized tissue delivery

with the report by Bahney et al. Likewise, 3× EY could Furthermore, the cyclic loading compression tests were
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not attain complete crosslinking, unless the exposure time conducted at 80% strain to characterize the mechanical
was extended over 10 min. For GelMA containing 4× EY, fatigue features of GelMA and GP under white light-
the crosslinking reaction occurred within 7.5 min of light induced photocrosslinking (Figure 2E). The compressive
exposure, and the samples reached full crosslinking under stress of GelMA-EY rapidly decreased to 11% of the
10 min of exposure. GelMA was then cast in cylindrical initial status after 50 deformation cycles, suggesting poor
molds and cultured long-term to assess mechanical fatigue resistance (Figure 2F). However, all GP-EY samples
stability. After a 12-day culture period, severe material endured 100 cyclic deformations without apparent
degradation and irregular edges were observed (Figure fatigue or decrease in mechanical stress (Figure S4,
2B), suggesting suboptimal mechanical behavior when Supporting Information), among which 10G2.5P samples
using GelMA alone. could resist up to 500 cycles (Figure 2G). These results

As an acrylate derivative of polyethylene glycol, PEGDA indicate significantly enhanced fatigue resistance upon
is a soft elastomer that can be crosslinked with GelMA incorporating the specific type of PEGDA into GelMA.
to improve the degradation behavior. Previous studies In contrast, GelMA-LAP and GP-LAP exhibited no signs
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utilizing PEGDA (M < 1000 Da) (Table S2, Supporting of fatigue resistance behavior and fractured rapidly after
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Information) reported improved elastic modulus but 5 cyclic compressions at 70% and 60% strain, respectively
decreased toughness. 15,29 Hence, linear-chained PEGDA (Figure S5, Supporting Information).
with large M (6000 Da) was selected for this study. We The GP hydrogel expressed remarkable superelasticity
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hypothesize that the selected PEGDA will introduce and fatigue resistance. We hypothesized the reinforcement
flexible chains into the GelMA crosslinking network, mechanism of the GP hydrogel from two aspects, i.e.,
thereby enhancing mechanical properties, including (i) white light-induced crosslinking mechanism and
deformation capacity. (ii) microstructure features. Regarding the reaction
mechanism, the EY system requires an extended
Based on the above optimization, we standardized the 22
final concentration of GP prepolymer at 12.5% and the crosslinking time of up to several minutes for facilitating
the full reaction of PEGDA with GelMA to form a hybrid
photoinitiator concentration at 4× EY. During the material crosslinking network. With the outstanding deformation
mixture and photocrosslinking procedures, no signs of capacity of PEGDA (M : 6000 Da), flexible chains can be
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phase separation were observed (Figure S2, Supporting transformed into rigid crosslinking chains of acryl groups
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Information). We discovered a nonlinear relationship in GelMA, thus improving the deformation capacity and
between PEGDA concentration and exposure time, reinforcing the GP hydrogel (Figure 2I). Nevertheless, the
with a minimum exposure time observed for 10% (w/v) LAP system is characterized by rapid photocrosslinking
GelMA and 2.5% (w/v) PEGDA (referred to as 10G2.5P) within tens of seconds, which complicates achieving a
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(Figure 2C). To assess the mechanical properties of GP complete reaction among long-chain PEGDA with the
hydrogel, we analyzed the mechanical behaviors of fully GelMA matrix, thereby compromising mechanical fatigue
crosslinked GP under different prepolymer concentrations features. Similarly, He et al. and Li et al. have reported
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and compared them across different hydrogel types (GP that long-chain PEGDA/GelMA mixtures exhibited
and GelMA) and photoinitiator systems (EY system enhanced toughness under the LAP photoinitiator system
for white light-induced crosslinking and LAP system (Table S2, Supporting Information). However, neither
for UV-induced crosslinking) (Table S3, Supporting superelasticity nor fatigue resistance was mentioned.
Information). LAP is widely used as a UV/near-UV For the microstructure features, SEM analysis displayed
photoinitiator with higher crosslinking efficiency and interconnected macroporous structures similar to cryogel
lower cytotoxicity compared to Irgacure 2959 (I2959). In scaffolds in the GP hydrogel, with an average pore
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cyclic compression tests, all GP-EY and GelMA-EY groups
displayed superior endurance when the strain increased diameter of 33.0 ± 9.0 μm and pore wall thickness of 4.2 ±
from 10 to 80% (Figure S3A, Supporting Information; 0.8 μm (Figures 2J and S6, Supporting Information). The
Video S1, Supporting Information), suggesting remarkable macroporous structure with thin walls likely contributes to
superelasticity. The corresponding stress–strain curves of the reinforced mechanical behavior of GP. The large pores
10G2.5P are presented in Figure 2D, indicating a maximum serve as deformation units to absorb and release elastic
strain of 80% and a maximum stress of 192 kPa. Conversely, energy, while the thin walls provide sufficient stretchability
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UV-induced crosslinking led to decreased elasticity, under large deformations.
with GelMA-LAP fracturing at 70% strain (Figure S3B, At this stage, we conducted a comprehensive literature
Supporting Information) and GP-LAP fracturing at 60% review on previous reports involving GP mixture in the
strain (Figure S3C, Supporting Information). PubMed database. To our knowledge, all the reports

Volume 10 Issue 5 (2024) 432 doi: 10.36922/ijb.3898