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

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.