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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. [ABSTRACT FROM AUTHOR]

Despite the excellent lubricity of conventional hydrogel materials due to their wet-soft properties, they produce severe mechanical elastic deformation at higher interfacial contact stresses. Balancing the load-bearing capacity and lubricating properties of hydrogel material is the difficulty of the current research work for articular cartilage substitutes. Great progress has been made in developing bionic joint materials with high load-bearing and low-friction hydrogels based on gradient designs. However, most bionic materials are based on sliding friction greatly limiting the improvement of lubrication performance. Herein, we designed and prepared a new hydrogel material with high load-bearing capacity and stable lubrication performance, breaking through the traditional friction method and turning to “sliding” for “rolling”. The network on the hydrogel surface was dissociated by UV irradiation and the pores on the surface were filled with SiO2 nanoparticles. The dense network structure of the underlying layer endows the hydrogel material with good load-bearing properties, while the high degree of hydration of the surface layer and the rolling friction effect of SiO2 nanoparticles greatly enhance the lubrication property. With the synergistic effect of these designs, the multi-layered hydrogel with nanoparticles on the surface achieved an ultra-low average coefficient of friction (COF) of ∼0.00809 at a high load of 50 N during 30,000 cycles. This idea of hydrogel material design provides a new strategy for the replacement of biomimetic articular cartilage materials.

Double-layer hydrogel materials combine excellent hydrophilicity, outstanding lubricating properties, high load-bearing modulus, and good biocompatibility, and are often used to achieve high load-bearing (high modulus) and low frictional (low modulus) properties of articular cartilage, but which also makes it difficult for them to form efficient, stable and long-term adhesion underwater. Herein, we constructed a kind of novel underwater Janus hydrogel patches with strong adhesion performance by robustly anchoring mussel-protein-inspired poly (dopamine methacrylamide-co-methoxyethyl acrylate) p(DMA-co-MEA) copolymer onto the surface of poly(acrylic acid-co- acrylamide) p(AAc-co-AAm) hydrogel with bilayer modulus structure analogous to articular cartilage. Due to the coupling of the top lubrication layer and the bottom load-bearing layer, the constructed Janus hydrogel patch not only has fatigue resistance and low friction but also high load-bearing [coefficient of friction (COF) ∼0.0076, 5 N; COF∼0.048, 30 N]. Moreover, the Janus hydrogel also exhibits excellent bonding strength underwater (bonding strength ∼41.6 kPa, Fe substrate). Our strategy for constructing a novel underwater Janus hydrogel patch will provide valuable guidance for the realization of interface bonding of soft material of bionic articular cartilage and promote the practical application of tissue engineering.

Understanding on the lubrication properties under charged conditions is not only one of the important fundamental research directions in the lubrication field, but also is of great significance in some newly-emerging applications. This paper presents a review of important research progresses on charged lubrication in recent years. First, the lubrication and friction behaviors in different lubrication states under charged conditions, as well as representative lubrication instabilities at charged interfaces are reviewed. Subsequently, two relevant applications, i.e., charged bearings and electric contact lubrication systems, are introduced. Finally, a brief summary and future outlook of the researches on charged lubrication are given. [ABSTRACT FROM AUTHOR]