Sung-Soo Kim

The surface architecture of hydrogels determines not only their lubrication performance, but also the time-dependent pathway through which friction evolves under sliding contact. Here, we show that oxygen-mediated surface structuring during free-radical polymerization can program hydrogel surfaces into dynamic interfacial water reservoirs that regulate time-dependent lubrication. Polyacrylamide hydrogels were synthesized under controlled oxygen concentrations and compared with hydrogels prepared using conventional glass and polystyrene molds. Although all hydrogels exhibited nearly identical micrometer-scale elastic moduli of approximately 27 kPa, their poroelastic relaxation, kinetic friction, and triborheometric hysteresis differed markedly depending on the oxygen environment during polymerization. Oxygen-unaffected hydrogels formed dense surface networks with limited water-storage and water-supply capability, resulting in reduced poroelastic relaxation, higher kinetic friction, and torque responses with negligible hysteresis during angular speed sweeps. In contrast, oxygen-affected hydrogels developed water-retentive, loosely crosslinked surface architectures that enhanced force relaxation, reduced friction, and produced pronounced frictional hysteresis. These results indicate that the oxygen-affected layer does not merely act as a static low-friction coating, but serves as an active water-regulating surface region that stores mobile water and supplies it to the contact interface during compression and sliding. Hydrogels prepared under 20% oxygen exhibited surface and lubrication characteristics similar to those of polystyrene-molded hydrogels, whereas oxygen-free hydrogels resembled glass-molded hydrogels, suggesting that the conventional mold effect is closely associated with local oxygen-mediated surface structuring. This study establishes a direct connection between oxygen-regulated polymerization, near-surface water-supply capability, and sliding-history-dependent hydrogel friction, providing a simple strategy for designing hydrogel interfaces with controlled time-dependent lubrication.