Polysaccharides like chitosan (CHT) can sustainably replace synthetic polymers in many material applications, but their native mechanical properties are often subpar. Addition of ionic surfactants like the anionic sodium dodecylsulfate (SDS) can bring about dramatic mechanical enhancements in polysaccharide materials, including those of CHT. At basic pH, CHT is neutral and forms elastic hydrogels, but the cationic nature of CHT at acidic pH enables ionic cross-linking with SDS, leading to viscoelastic hydrogels with superior strength. Thus, SDS:CHT has emerged as a promising platform for spatial and dynamic programming of hydrogels with unique responses to mechanical loads, but the nanoscale origins of their load-bearing mechanisms remain elusive. To address this gap, CHT hydrogel networks were self-assembled at varying pH values and SDS concentrations and mechanically tested using a multiscale modeling pipeline. In addition to yielding self-assemblies with mechanical properties consistent with experimental reports, our methods revealed distinct pH-and SDS-dependent load-bearing mechanisms. We found that basic CHT networks underwent loaddependent crystallization, similar to stretched rubber, while SDS micelles shouldered the load response in acidic SDS:CHT networks by merging into larger micelles. These findings may enable the adaptation of these programming mechanisms for other polysaccharide−surfactant combinations, lead to the improvement of mechanical robustness of existing SDS:CHT applications, and inspire the development of new applications.
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Chitosan is a gel-forming polysaccharide biopolymer. The amine groups on chitosan monomers ionize with a pKa 6.5 which facilitates pH-responsive transition from an insoluble physically crosslinked hydrogel at basic pH, to a soluble polycationic state at acidic pH. However, addition of anionic surfactants such as sodium dodecyl sulfate (SDS) to cationic chitosan allows the formation of electrostatically linked hydrogels. In contrast to the elastic nature of chitosan gels at basic pH, these acid-stable SDS:chitosan gels have been shown to display viscoelasticity and self-healing properties. This tunable nature of chitosan makes it an attractive biomaterial for bioelectronics fabrication, and other biomedical applications. Here, we use a multiscale molecular modeling approach to identify the molecular interactions responsible for the orthogonal dependence of mechanical properties on pH and SDS concentration. We use coarse-grained molecular dynamics to selfassemble large, space-spanning SDS:chitosan networks. We then back-map these networks to atomistic resolution to further investigate the finer differences between their interaction profiles. Our findings show that water mediated contacts are the predominant crosslinking interaction at basic pH, but not at acidic pH. Instead, SDS-chitosan salt bridges and hydrogen bonds are the dominating crosslinks at acidic pH. Furthermore, direct chitosanchitosan hydrogen-bonds appear to have a much smaller role in hydrogel structure than previously thought. These findings offer valuable insights into the fine molecular interactions that stabilize these polysaccharide-surfactant hydrogels. Taken together with the known mechanical behaviours of these hydrogels, our results offer directions for the rational design of chitosan materials from the bottom up.
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