Molecular Basis for Elasticity and Viscoelasticity in Chitosan-Surfactant Hydrogels

The carbohydrate polymer, chitosan, is a biocompatible material commonly used in biomedical applications. Chitosan's chemistry enables its switching from a stable, net-neutral, elastic hydrogel at basic pH, to a soluble polycationic state at acidic conditions. Experiments show that the addition of the anionic surfactant sodium dodecyl sulfate (SDS) to polycationic chitosan gives rise to electrostatically linked hydrogels displaying viscoelasticity. In this work, we employ multiscale molecular dynamics to identify the molecular phenomena that underpin these distinct mechanical behaviours. ~20nm-scale percolated hydrogels are first assembled with coarse grained molecular dynamics, and later back-mapped to atomic resolution. We subjected the atomistic structures to tension-compression cycles to study how the intermolecular interactions respond to mechanical stimuli. Direct chitosan-chitosan interactions were found to recover elastically with deformation. But, SDS-chitosan interactions in electrostatic gels increase in number dramatically, while simultaneously showing viscoelastic stress response. These results unify bulk mechanical properties with fine intermolecular phenomena, and promise to aid the engineering of customizable chitosan materials in the pH-counterion design space.