Cartilage is well lubricated over a lifetime and this phenomenon is attributed to both of the surface hydration lubrication and the matrix load-bearing capacity. Lubricious hydrogels with a layered structure are designed to mimic cartilage as potential replacements. While many studies have concentrated on improving surface hydration to reduce friction, few have experimentally detected the relationship between load-bearing capacity of hydrogels and their interface friction behavior. In this work, a bilayer hydrogel, serving as a cartilage prototype consisted of a top thick hydrated polymer brush layer and a bottom hydrogel matrix with tunable modulus was designed to investigate this relationship. The coefficient of friction (COF, μ) is defined as the sum of interfacial component (μ_Int) and deformation/hysteresis component (μ_Hyst). The presence of the top hydration layer effectively dissipates contact stress and reduces the interface interaction (μ_Int), leading to a stable and low COF. The contribution of mechanical deformation (μ_Hyst) during the sliding shearing process to COF can be significantly reduced by increasing the local mechanical modulus, thereby enhancing the load-bearing capacity. These results show that the strategy of coupling surface hydration layer with a high load-bearing matrix can indeed enhance the lubrication performance of hydrogel cartilage prototypes, and implies a promising routine for designing robust soft matter lubrication system and friction-control devices.

Osteoarthritis (OA) has been regarded as a lubrication deficiency related joint disease. Combination of both joint lubrication and drug intervention may provide a promising nonsurgical strategy for treatment of OA. Developing novel and simple approaches to fabricate superlubricating nanoparticles with drug release property is highly required. Herein, dopamine triggered one-step polymerization method was employed to fabricate polydopamine/poly(3-sulfopropyl methacrylate potassium salt) (PDA–PSPMA) conjugate coating on hollow silica (h-SiO₂) nanosphere surfaces to engineer functional nanoparticles (h-SiO₂/PDA–PSPMA). The as-prepared h-SiO₂/PDA–PSPMA exhibits excellent aqueous lubrication performance on biomaterial substrates as well as natural bovine articular cartilage based on hydration effect of negatively charged PDA–PSPMA coating and "rolling" effect of h-SiO₂ nanospheres. In vitro drug loading-release experiments demonstrate that PDA–PSPMA coating functionalized h-SiO₂ nanospheres show high drug-loading and sustained-release capability of an anti-inflammatory drug, diclofenac sodium (DS). Such h-SiO₂/PDA–PSPMA nanospheres can be potentially used as a synergistic therapy agent for OA treatment combining by simultaneous joint lubrication and drug release.

At present, more and more diseases are associated with the lubrication dysfunction, which requires a systematic study of the complex lubrication behavior of tissues and organs in human body. Natural biomacromolecular lubricants are essential for maintaining ultra-low coefficients of friction between sliding biological interfaces. However, when the surface lubrication performance of tissues or organs destroys heavily, it will bring friction/shear damage for sliding contact interfaces. Therefore, the application of exogenous biological lubricating materials to improve the lubrication situation of damaged tissue or organ interfaces has attracted extensive attention of researchers. In this review, based on a simple summary of lubrication mechanism at sliding biological interface, we systematically introduce the research progress of several kinds of representatively biolubrication materials, including eye drops, tissue anti-adhesion agents, joint lubricants, and medical device lubricants. Meanwhile, the lubrication mechanism and individual advantage and shortcoming for each of these synthetic exogenous lubricated materials are clarified. Correspondingly, the important lubrication application functionality of these biolubricant materials in typically medical surgery scenes, such as dry eye syndrome, tissue adhesion, arthritis, and interventional medical devices, is discussed. Finally, we look forward to the future development direction of artificial biolubricant materials.

Silicone elastomers-based materials have been extensively involved in the field of biomedical devices, while their use is extremely restricted due to the poor surface lubricity and inherent hydrophobicity. This paper describes a novel strategy for generating a robust layered soft matter lubrication coating on the surface of the polydimethylsiloxane (PDMS) silicone elastomer, by entangling thick polyzwitterionic polyelectrolyte brush of poly (sulfobetaine methacrylate) (PSBMA) into the sub-surface of the initiator-embedded stiff hydrogel coating layer of P(AAm-co-AA-co-HEMA–Br)/Fe, to achieve a unified low friction and high load-bearing properties. Meanwhile, the stiff hydrogel layer with controllable thickness is covalently anchored on the surface of PDMS by adding iron powder to provide catalytic sites through surface catalytically initiated radical polymerization (SCIRP) method and provides high load-bearing capacity, while the topmost brush/hydrogel composite layer is highly effective for aqueous lubrication. Their synergy effects are capable of attaining low friction coefficient (COFs) under wide range of loaded condition in water environment with steel ball as sliding pair. Furthermore, the influence of mechanical modulus of the stiff hydrogel layer on the lubrication performance of layered coating is investigated, for which the COF is the lowest only when the modulus of the stiff hydrogel layer well matches the PDMS substrate. Surprisingly, the COF of the modified PDMS could remain low friction (COF < 0.05) stably after encountering 50,000 sliding cycles under 10 N load. Finally, the surface wear characterizations prove the robustness of the layered lubricating coating. This work provides a new route for engineering lubricious silicon elastomer with low friction, high load-bearing capacity, and considerable durability.

Hydrogels as one kind of soft materials with a typical three-dimensional (3D) hydrophilic network have been getting great attention in the field of biolubrication. However, traditional hydrogels commonly show poor tribology performance under high-load conditions because of their poor mechanical strength and toughness. Herein, pure chemical-crosslinking hydrogels mixed with different types of the micron-scale fibers can meet the requirements of strength and toughness for biolubrication materials, meanwhile the corresponding tribology performance improves significantly. In a typical case, three kinds of reinforcement matrix including needle-punched fibers, alginate fibers, and cottons are separately combined with Poly(n-vinyl pyrrolidone)-poly(2-hydroxyethyl methacrylate (PVP-PHEMA) hydrogels to prepare fibers reinforced composite hydrogels. The experimental results show that the mechanical properties of fibers reinforced composite hydrogels improve greatly comparable with pure PVP-PHEMA hydrogels. Among three kinds of fibers reinforced composite hydrogel, the as-prepared composite hydrogels reinforced with needle-punched fibers possess the best strength, modulus, and anti-tearing properties. Friction tests indicate that the fibers reinforced composite hydrogels demonstrate stable water-lubrication performance comparable with pure PVP-PHEMA hydrogels. Besides, the hydrogel-spunlace fiber samples show the best load-bearing and anti-wear capacities. The improved tribology performance of the composite hydrogels is highly related to mechanical property and the interaction between the fibers and hydrogel network. Finally, spunlace fibers reinforced hydrogel materials with high load-bearing and low friction properties are expected to be used as novel biomimetic lubrication materials.