Enhancing Superlubricity and Wear Resistance in Mechanically Robust Hydrogel via Microliter‐Scale Subsurface‐Initiated Polymer Brush Grafting

ABSTRACT Hydrogels represent an ideal for articular cartilage replacement, with hydrogel‐polymer brush layered composites emerging as a promising strategy to simultaneously achieve ultra‐low friction and high load‐bearing capacity. However, current approaches for grafting polymer brushes from hydrogels usually require oxygen‐free conditions, large volumes of polymerization solutions, and excessive monomer consumption. Herein, we developed a facile, oxygen‐tolerant subsurface‐initiated polymer brush grafting strategy to fabricate cartilage‐mimicking layered hydrogel‐polymer brush materials, using only microliter volume of monomer solution. To validate this, a mechanically robust, physically cross‐linked poly(vinyl alcohol)‐based hydrogel with subsurface‐initiated polymerization activity was designed by incorporating a tannic acid‐derived cross‐linkable atom transfer radical polymerization (ATRP) initiator, which serves as a robust load‐bearing substrate. Subsequently, polymer brushes were grafted from the subsurface of this robust hydrogel matrix with microliter solutions, yielding cartilage‐mimicking layered structure with an interpenetrated polymer brush‐hydrogel composite lubricating phase. Notably, the resulting materials exhibited synergistic superior lubrication, high load‐bearing capacity, and excellent wear resistance, achieving a stable and ultra‐low friction coefficient (COF∼0.017) over 80,000 cycles under 10 N load. This strategy greatly lowers technical barriers to the fabrication of hydrogel‐polymer brush materials and further advances their practical applications in the field of articular cartilage repair and artificial joint replacement.