Ionic Control of Microstructure and Lubrication in Charged, Physically Cross‐Linked Hydrogels

Abstract Charged, physically cross‐linked hydrogels provide a versatile platform for designing responsive materials with programmable interfacial properties, with applications in drug delivery, biomedical devices, and biosensing. Here, poly(methacrylamide‐co‐methacrylic acid) hydrogels stabilized by a short‐range attractive, long‐range repulsive potential is investigated. By integrating microstructural, mechanical, and tribological characterization across multiple length and time scales, this work uncovers how salt addition alters not only swelling, but also the microstructure and dynamics, near‐surface stiffness and charge, and ultimately, its lubricity. Friction measurements reveal a velocity‐dependent response with distinct mixed, transition, and elastohydrodynamic regimes. Microscopic friction imaging reveals that adhesive interactions are promoted by salt and play a critical role. Unlike neutral hydrogels, where increased water content correlates with reduced friction, salts introduce additional dissipation mechanisms that can dominate over hydration effects, offering enhanced control of functional behavior. These findings provide new insights into the salt‐induced responsive behavior of poly(methacrylamide‐co‐methacrylic acid) hydrogels‐including lubrication mechanisms‐and challenges conventional interpretations of salt effects in polyelectrolyte systems. This knowledge will inform design principles for synthetic hydrogel interfaces and advance understanding of the functional behavior of hydrogel‐like materials such as biological tissues, whose lubricity‐in salinity conditions similar to those studied here‐is essential to their function.