Enhancing Bacterial Adhesion with Hydro-Softened Chitosan Films

In applications ranging from microbial fuel cells to targeted drug delivery, bacterial adhesion is critical for surface interactions and functional performance. Current strategies for modulating bioadhesive properties of chitosan largely rely on biochemical functionalization - ligand grafting, surface charge manipulation, and polymer blending. Here, we introduce a mechanically driven framework based on hydro-softening - a physical process that modulates adhesion outcomes by tuning elasticity and interfacial energy without introducing foreign chemical species. Hydro-softened chitosan thin films entropically entrap interfacial water during substrate-mediated condensation, forming pseudosolid water domains that lower both the elastic modulus and effective work of adhesion. We integrate changes in these mechanical effects into a Griffith-criterion-derived theoretical adhesion model, coupled to a stochastic simulation incorporating extended Derjaguin-Landau-Verwey-Overbeek (DLVO) interactions. The resulting predictions of enhanced bacterial adhesion were validated experimentally through quantitative Scanning Electron Microscopy (SEM) analysis and morphological classification. Hydro-softened chitosan thin films exhibited over 5-fold greater adhesion compared to unsoftened chitosan thin films, primarily through increased single-cell attachment. These findings demonstrate that substrate mechanics alone can govern quasistatic bacterial attachment in <i>in vitro</i> settings. This work establishes hydro-softening as a chemically passive yet effective process-driven strategy for engineering bioadhesive interfaces. It further demonstrates that mechanically induced changes can influence biological interactions even at the cellular scale.