A Hydration-Entropy Lubrication Coupling Strategy for Superhydrophilicity and Ultralow Friction of Hydrogel Coatings

Hydrogel coatings combining superhydrophilicity with ultralow friction on irregular, load-bearing biomedical surfaces are critical yet elusive. Natural mucus, with its highly glycosylated mucin network, protects complex biological surfaces from friction and wear through a uniquely structured lubrication mechanism. Here, inspired by mucin, we engineer hydrogel coatings that are easy to fabricate on diverse substrates, bear high loads, and maintain enduring superlubricity via pure structural design. We introduce a hydration-entropy lubrication coupling strategy in which long, free polymer chains synthesized in situ on the surface maintain a robust hydration layer and high configurational freedom, thereby generating strong shear-induced steric repulsion. By tuning polymerization kinetics in an oxygen-rich environment, we rapidly (≤90 s) form coatings exhibiting ultralow friction (μ = 0.008, 1/20 that of regular hydrogels) and superhydrophilicity (static contact angle = 7.9°). A densely entangled internal network preserves coating integrity and stress transfer, sustaining lubrication at a pressure of ∼0.25 MPa for over 60 days. Incorporation of degradable cross-linkers endows excellent biocompatibility and controllable degradability, fulfilling sustainability requirements for implantable systems. This strategy provides a versatile and facile approach for integrating robust superlubricity hydrogel coatings into complex biological interfaces.