Rosa Espinosa Marzal and her team discovered that positively charged hydrogels can achieve dramatically low friction against oppositely charged surfaces under hydrated conditions — the opposite of what basic electrostatics would predict — because tightly bound water layers at the material's surface generate a repulsive force that outcompetes electrostatic attraction. Published in Materials Horizons, the work advances the design of synthetic cartilage and medical device coatings while also raising important questions about the reliability of electrostatic attraction as a foundation for underwater hydrogel adhesive strategies.
Written by Jackson Brunner
When cartilage in a knee joint begins to fail, the consequences are immediate. A person may deal with pain, stiffness and the grinding sensation of bone moving against bone. Replacing that tissue requires a material that can do two things at once: bear significant mechanical load without breaking down and allow surfaces to glide against one another with almost no friction. Finding a material that genuinely delivers on both fronts has been one of the central challenges of biomaterials research for decades.
Friction maps of the surfaces of PVACs DN hydrogels immersed in water (left), 10mM NaCl (middle), and 10mM NaI (right). Friction in both salts is significantly lower compared to water.
Ivan Racheff Professor Rosa Espinosa Marzal and her team in the Department of Materials Science and Engineering at The Grainger College of Engineering, University of Illinois Urbana-Champaign, are working toward that goal through a careful study of charged hydrogels, which are soft, water-swollen polymer networks that carry a net electrical charge. The resulting paper, "Slippery When Charged: Hydration Lubrication in Hydrogels," published in Materials Horizons from the Royal Society of Chemistry, examines the surface and internal structure of positively charged polyvinyl alcohol-chitosan double network (PVACs DN) hydrogels and connects that structure to the materials' strength and lubricating behavior.
Despite electrostatic attraction, hydration forces govern interfacial behavior in positively charged PVA–chitosan double-network hydrogels, establishing structure–property relationships that enable superlubricity.
The experimental work was carried out by Ming Jun Lee, a PhD candidate approaching graduation, and Isha Bordawekar, an undergraduate researcher who recently completed her Bachelor of Science in Materials Science and Engineering as part of the Class of 2026. Their work advances not only the practical design of biomedical materials, but also the fundamental scientific understanding of what happens at the interface of a charged soft hydrated material and the environment around it.
When a positively charged hydrogel is brought into contact with a negatively charged countersurface, basic electrostatics suggests the two should attract one another, pulling the hydrogel into tight adhesive contact, increasing friction and undermining exactly the kind of slippery interface needed for a cartilage replacement to function. What the team found, however, told a more nuanced story. In sufficiently hydrated conditions and ionic solutions, a phenomenon known as hydration repulsion, arising from tightly bound layers of water molecules at the material's surface, can outcompete that electrostatic attraction entirely at the hydrogel interface.
“The result is a dramatic reduction in friction, driven by a hydration lubrication mechanism that persists even where coulombic attraction might be expected to dominate,” said Espinosa Marzal.
Beyond its implications for tissue engineering, this finding carries an important cautionary message for a related area of research. Scientists designing hydrogel-based underwater adhesives often build their strategies around attractive electrostatic interactions, counting on those forces to hold a material in contact with a target surface. The results from Espinosa Marzal's group suggest that under well-hydrated conditions, hydration repulsion may quietly undermine those attractive forces. It’s a possibility that existing design frameworks may not fully account for.