Slippery when charged: hydration lubrication in hydrogels.

Hydrogels are hydrated polymer networks with promising biomedical applications due to their microstructural similarity to biological tissues at sliding interfaces, such as the articular cartilage. These tissues are highly hydrated, charged, and operate in ionic environments. A fundamental understanding of lubrication at charged hydrogel interfaces is thus essential to advance practical applications. Hydration lubrication has recently been proposed to govern hydrogel lubrication. Here, a thin film of water remains confined under pressure, enabling friction coefficients below 0.01, a condition referred to as superlubricity. Yet, studies of hydration lubrication in hydrogels remain limited. This work investigates positively charged polyvinyl alcohol-chitosan double-network hydrogels as a model system for hydrophilic, positively charged hydrogels. When equilibrated in salt solution, these hydrogels exhibit stronger surface zeta potential and weaker storage modulus. Using atomic force microscopy, we directly image the sliding interface in water and salt solutions while measuring friction with a negatively charged probe. Friction coefficients are highest in water and smallest in NaI solution. The results reveal that hydration (repulsive) forces can dominate over electrostatic attraction and activate superlubricity. This study provides new insights into the mechanisms that govern hydration lubrication in charged hydrogels.