Poly(ethylene glycol)‐interpenetrated genipin‐crosslinked chitosan hydrogels: Structure, pH responsiveness, gelation kinetics, and rheology

Abstract

In the development of pH‐responsive chitosan‐based hydrogels, achieving reproducible porosity and swelling behavior is essential for the design of hydrogel networks. Herein, we enhance the level of control in hydrogel microarchitecture by incorporating poly(ethylene glycol) (PEG) into the chitosan–genipin matrix. Hydrogels, varied in composition, were synthesized under mild conditions (37°C, 1 atm, 24 hr), yielding microporous structures with a pore diameter ranging from 11 to 57 μm and an average cross‐sectional porosity of approximately 40–64%. Compared to chitosan–genipin hydrogels without PEG, presence of PEG in concentrations up to 1.9 mM generated the same effect as would increase in genipin content, yielding structures with a smaller pore diameter, a lower swelling degree in pH 2 buffer and a higher elastic modulus. Considering cost effectiveness and scale‐up, reducing genipin content by the addition of PEG is favorable. Importantly, hydrogel samples containing higher concentrations of PEG (2.9 mM and above) showed a sudden increase in the swelling degree accompanied with a decrease in the elastic modulus. Findings showcase the potential variation in the composition of these hydrogels has in yielding scaffolds with significantly different physico‐chemical behaviors.

Genipin‐crosslinked chitosan hydrogels: Preliminary evaluation of the in vitro biocompatibility and biodegradation

“…This suggests existence of an optimal range of genipin concentration beyond which PEG addition may lead to a reduced number of effective crosslinking between chitosan and genipin. These findings support the results from our previous study, which revealed the link between PEG content and the sudden change in hydrogels' swelling capacity, gelation kinetics and mechanical strengths 29 . Methodology used to follow the degradation of theses gels in vitro is an innovative and yet simple approach to deploy the intrinsic fluorescence of the chitosan‐genipin links for tracking the biodegradation in vitro, and possibly in vivo too.…”

“…Our microscopic images and viability assay show that hydrogels with higher genipin content (3.1 or 4.4 m m ) present better cytocompatibility to fibroblasts, in contrast to the counterparts with 1.7 m m genipin (Figure 1). Our previous study has evaluated the relation between genipin content and elastic modulus using small deformation rheology measurements 29 . We found that increasing genipin content from 3.1 to 6.3 m m significantly increased elastic modulus (from ~ 445 to ~ 1100 Pa) and shortened the linear viscoelastic region by 50%, suggesting a positive correlation between genipin content and hydrogels' rigidity.…”

“…In gels with 4.4 m m genipin, addition of PEG did not further delay degradation as gels F3 and F3P started to degrade at Days 12 and 10, respectively, (Figure 3(f)). It can be postulated that in this case, PEG interacts with chitosan, reducing the number of effective crosslinks between chitosan and genipin, 29 and confining the genipin's suppression effect on degradation. Once the hydrogels start to degrade, the increased hydrophilicity induced by PEG addition appears to accelerate the degradation, resulting in a faster degradation rate observed in PEG‐added hydrogels compared to non‐PEG‐added hydrogels, regardless of genipin content.…”

“…The present study, based on cell viability data (Figure 1(c)), provides evidence to support the addition of PEG. Our previous study has shown that the presence of PEG (1.2 m m ) increased the elastic modulus (from ~ 220 to ~ 445 Pa) and rigidity of the network, 29 which may contribute to the enhanced cytocompatibility of PEG‐added hydrogels. Another key factor that affects cell responses is surface wettability.…”

“…Another major advantage of using genipin is its high selectivity, as primary amino groups (NH 2 ) is the only site targeted by genipin, while the nucleophilic OH groups (OH and COOH) do not react with genipin 28 . Recently, we reported the development of semi‐interpenetrating (semi‐IPN) hydrogels composed of chitosan, genipin and poly (ethylene glycol) (PEG) 29 . The crosslinking reaction between primary amino groups of chitosan and genipin was found to correlate with the development of secondary amine and heterocyclic amine ring‐stretching while PEG addition (up to a critical concentration) may interact with chitosan via attractive intermolecular hydrogen bonding.…”