Within these varied applications, a common requirement is that hydrogels need to be durable under varied and dynamic conditions, including changes in ion concentration, pH, or temperature. Traditional hydrogels often lack the needed tensile strength for such applications, and therefore double-network (DN) hydrogels are utilized, owing to their exceptional mechanical properties and contrasting network structures.
In this study, we explore the different properties of a double-network hydrogel system with a poly(acrylic) acid (PAA) phase and a secondary phase, either with poly(vinyl) alcohol (PVA) or chitosan (CHI). PVA is utilized due to its prevalence in current hydrogel technology, while chitosan is used as it is a natural biopolymer derived from crustacean shells, providing good mechanical strength.
In this work, the secondary phase (PVA or chitosan) is varied to study the effects of the physical crosslinking density on hydrogel performance.Specifically, swelling kinetics in solutions of varied pH (2, 7, and 12) are characterized until equilibrium. Additionally, the tensile strength of each hydrogel formulation is measured at equilibrium swelling, thus establishing the effects of water uptake on the network mechanics. Lastly, we characterize adhesive performance (via 180 degree peel test) using a mucus-based hydrogel substrate, which replicates in vivo conditions often explored for these systems
These experiments reveal an increase in the swelling ratio at equilibrium and swelling rate for all formulations with increasing pH. Our data also shows the PAA-CHI hydrogels swell at a faster rate than PAA-PVA hydrogels for all solution conditions. Crosslinking density does also affect the rate of absorption, as hydrogels with lower physical crosslinking densities absorb more water overall. However, hydrogels with a less physical crosslinking between the two networks show higher adhesion to tissue. Interestingly, the materials demonstrated spherical leather-like surface patterning after swelling at high pH, which may have a significant effect on surface adhesion to tissue as it impacts the potential contact area with diverse substrates.
Overall, the interplay between network strength and swelling kinetics provides guidance for the design of hydrogel formulations for varied applications. As one example, our data shows that achieving swelling resistance comes at the expense of decreasing adhesive performance, which is a tradeoff inherent in the properties of the double network hydrogel. This preliminary study provides some preliminary insights into designing biocompatible and durable hydrogels in different conditions, paving the way for its widespread application in biomedical and marine studies.