Primary Investigator & Professor Purdue University, United States
Introduction: Introductions: Biodegradability is an essential parameter in hydrogel design, with implications for drug delivery and tissue engineering. Biodegradable hydrogels permit cell invasion and proliferation, and the eventual replacement of implanted scaffolds with new tissue. The inclusion of hydrolytically degradable functional groups, such as esters, on the polymer backbone has been reported to create hydrogels that degrade in aqueous conditions [1]. However, precisely tuning the degradation rate to range from days to months has been challenging to achieve with these systems. We hypothesized that introducing two hydrolytically susceptible ester groups in distinct regions of the hydrogel network – one on the crosslinker and one on the main polymer backbone – would result in hydrogels with highly tunable degradation rates. We tested this hypothesis using norbornene-modification and partial oxidation of hyaluronic acid (HA) polymers, and performing characterization studies on microgels made with the resulting materials (Fig 1a,b).
Materials and
Methods: Materials and
Methods: We used esterification and oxidation reactions to chemically modify HA to introduce norbornene (Nor-HA) and dialdehyde via sodium periodate (oxidized Nor-HA). HA was separately reacted with carbic anhydride to attach norbornene with a hydrolytically susceptible link (Nor-HACA) and was similarly reacted with sodium periodate (oxidized Nor-HACA) [2]. Two different degrees of oxidation were performed (1% and 2.5%). A total of six hydrogel groups were created: NorHA, 1% Oxidized NorHA, 2.5% Oxidized NorHA, NorHACA, 1% Oxidized NorHACA, and 2.5% Oxidized NorHACA. 1H NMR was used to verify chemical modifications. Using a UV spot curing system, hydrogels were fabricated into microgels using a flow-focusing microfluidic device. Gelation properties were determined using oscillatory shear rheology at a frequency of 1 Hz and 0.5% shear strain with dynamic light exposure. A flow-focusing microfluidics device was used to generate uniformly sized microgels. Microgel morphology was evaluated using fluorescence microscopy and degradation was quantified by measuring the time-dependent release of encapsulated FITC-Dextran (1 MDa) from the crosslinked microgels.
Results, Conclusions, and Discussions: Results and
Discussion: Instantaneous click crosslinking of all hydrogels was confirmed with shear rheology, with storage moduli reaching plateau values within ~5 seconds of light exposure. Uniform spherical microgels with an average size of ~150 µm were obtained using the microfluidic device. A 14-day degradation study revealed that microgels made with oxidized polymers (Nor-HA or Nor-HACA) showed a higher degradation rate compared to non-oxidized counterparts with similar crosslinking and polymer concentrations. Due to its size, FITC-dextran molecules are unable to diffuse out of intact hydrogels; therefore, these variations in FITC-dextran release signify varied loss of hydrogel integrity among modification groups (Fig. 1c). Microgels were not detectable under the microscope after 10 days of incubation in buffered saline for the NorHACA groups, whereas no degradation was observed for the non-oxidized Nor-HA microgels due to the absence of hydrolytically susceptible ester groups (Fig. 1d).
Conclusion: Our data demonstrate that ester groups on the crosslinker afforded by carbic anhydride modification resulted in degradable hydrogels, and modification via partial oxidation further increased the degradation rate. These promising findings warrant further investigation of how different degrees of oxidation, in combination with different crosslinking densities and polymer concentrations, influence hydrogel degradability.
Acknowledgements (Optional): This work was supported by new faculty start-up funds from the Weldon School of Biomedical Engineering to T.H.Q., and a Summer Undergraduate Research Fellowship to Z.C. through Purdue BME, EURO, and the SURF Program at Purdue University.
References: [1] Lueckgen, et al. (2018). Hydrolytically-degradable click-crosslinked alginate hydrogels. Biomaterials, 181, 189–198. [2] Galarraga, et al. (2022). Synthesis, characterization, and digital light processing of a hydrolytically degradable hyaluronic acid hydrogel. Biomacromolecules, 24(1), 413–425.
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