Vertical and Horizontal Double Gradient Design forSuper-Slippery and High-Bearing Hydrogel Skeleton

Hydrogels with excellent hydrophilicity, high water content, outstanding biocompatibility, and lubrication properties are among the promising articular cartilage replacement materials. Despite extensive research focus on lubrication of hydrogels, realization of bioinspired materials with high load-bearing, low-friction, extraordinary lubrication, and anti-wear properties
is challenging. Herein, robust lubrication can be realized by structuring vertical and horizontal dual gradient surfaces, with structural design in vertical direction gradually transitioning from an anisotropic tubular to a compact body to provide load-bearing, and a surface rigid porous polymer skeleton and a filled loose lubricating polymer structural design to provide
lubrication in horizontal direction. What is more, gradient transition region in vertical direction greatly weakens gradient layer interface stress to increase load-bearing, and gradient surface in horizontal direction is constructed by interconnected surface rigid polymer skeleton containing “pockets” that house highly hydrated and mechanically fragile soft lubrication polymer to prevent removal of lubrications under high load and wear. The designed hydrogel has an extremely low coefficient of friction (COF) with hard steel (COF ≈0.0036, 50 N, 35,000 cycles) and achieves a balance between lubrication and load bearing. The work opens up an avenue for rational design of high load-bearing and ultralow friction soft material surfaces.