Articular cartilage covering the joint surface provides an excellent lubrication and load-bearing interface for the daily activities of the human body, which is characterized by high load-bearing, low friction, and wear resistance. Articular cartilage will be damaged and degenerated with age, congenital diseases, trauma, and other factors, however, the vascularization of articular cartilage leads to its weak self-repair ability, which ultimately accelerates the occurrence of osteoarthritis and seriously affects the quality of life of patients. Hydrogels are similar to biological soft tissue and have both solid and liquid properties, which have the characteristics of natural cartilage microstructure similarity, high water content, excellent biocompatibility, stable physical and chemical properties, etc., and have developed into the best alternative material for articular cartilage. However, the mechanical properties and lubricating properties of traditional hydrogels are insufficient, which makes it difficult to meet the application of artificial articular cartilage. Therefore, the development of mechanical enhancement and biomimetic lubrication technology to improve the mechanical properties and lubrication properties of biomimetic cartilage hydrogel materials has attracted extensive attention. In this paper, the research progress of hydrogel-based cartilage replacements is reviewed from the aspects of mechanical enhancement and biomimetic lubrication, and the design strategies and mechanisms of mechanical enhancement such as nanocomposites, multi-network, hydrophobic association, topological structure, supramolecular polymers, and biomimetic ordered structures are introduced, as well as the design ideas and lubrication mechanisms of biomimetic lubrication based on interfacial modulation, polymer brushes, lubricant boundary lubrication, and stimulus-response. Furthermore, based on the structural and functional biomimicry of the natural articular cartilage system, the research progress of high mechanical properties and low-friction biomimetic articular cartilage substitutes was reviewed, and their potential value as articular cartilage substitutes was discussed. Finally, the current problems of biomimetic articular cartilage materials, as well as the future research focus and development direction are discussed.

The changes in the components of the synovial fluid in the human body have an important influence on the tribological behavior of artificial joints. Based on the component concentration of the synovial fluid after arthroplasty, the "hard-soft" joint pair materials composed of cobalt-chrome-molybdenum (CoCrMo) and high crosslinked polyethylene (XLPE) are used as the research object. The composite synovial fluid with different concentrations of albumin (Alb), γ-globulin (γ-Glo), hyaluronic acid (HA), and phospholipids (PLs) are prepared. Based on studying the influence mechanism of single component concentration change on the tribological properties of joint pair materials, the friction and wear behavior of joint pair materials under different composite synovial fluids are systematically explored. The coupling mechanism among the components is clarified, and the wear mechanism of the joint pair materials under different composite synovial fluids is revealed. In addition, the results of 2 million in vitro simulated wear experiments of CoCrMo-XLPE artificial joint in the composite synovial fluid are further studied. Further, it validates the influence of the composition concentration of the composite synovial fluid on the friction and wear properties of artificial joints under actual working conditions. The results show that the four main components in the composite synovial fluid have a great influence on the friction and wear properties of the "hard-soft" joint pair materials. When the concentration of PLs increases from 0.00 mg/mL to 0.45 mg/mL, the wear rate decreases by 69.6% and the coefficient of friction (COF) decreases by 63.3%. The coupling mechanism between PLs, HA, and protein significantly affects the adsorption of the membrane and affects the tribological behavior of the artificial joint. In addition, the simulated wear results of artificial joints in composite synovial fluid are consistent with those of friction and wear tester. It shows that the concentration of each component in the composite synovial fluid significantly affects the lubrication of the artificial joint, and the influence degree becomes more obvious during long-term service. To sum up, this study can provide the theoretical basis for the study of the composite synovial fluid and the improvement of lubrication performance of artificial joints and is of great significance for prolonging the service life of artificial joints.

The polyetheretherketone (PEEK)-highly cross-linked polyethylene (XLPE), all-polymer knee prosthesis has excellent prospects for replacing the traditional metal/ceramic-polyethylene joint prosthesis, improving the service life of the joint prosthesis and the quality of patients’ life. The long-term wear mechanism of PEEK-XLPE knee joint prosthesis is comprehensively evaluated from wear amount, wear morphology, and wear debris compared to that of CoCrMo-XLPE joint prosthesis. After 5 million cycles of in vitro wear, the wear loss of XLPE in PEEK-XLPE (30.9±3.2 mg) is lower than that of XLPE in CoCrMo-XLPE (32.1±3.1 mg). Compared to the XLPE in CoCrMo-XLPE, the plastic deformation of XLPE in PEEK-XLPE is more severe in the early stage, and the adhesive peeling and adhesion are lighter in the later stage. The size distribution of XLPE wear debris in PEEK-XLPE is relatively dispersed, which in CoCrMo-XLPE is relatively concentrated. Wear debris is mainly flake and block debris, and the wear mechanism of XLPE was abrasive wear. The wear volume per unit area of PEEK femoral condyle (10.45×10⁵ μm³/mm²) is higher than that of CoCrMo (8.32×10⁵ μm³/mm²). The PEEK surface is mainly furrows and adhesions, while the CoCrMo surface is mainly furrows and corrosion spots. The PEEK wear debris is mainly in flakes and blocks, and the CoCrMo wear debris is mainly in the shape of rods and blocks. The wear mechanism of PEEK is abrasive wear and adhesion, and that of CoCrMo is abrasive wear and corrosion.

Inspired by the cartilage-bone structure in natural joints, soft-hard integrated materials have received extensive attention, which are the most promising candidates for artificial joints due to their combination of excellent load-bearing properties and lubricating properties. The latest progress showed that the combination of hydrogel and titanium alloy can realize a bionic natural joint lubrication system on the surface of titanium alloy. However, obtaining a tough interface between the hydrogel (soft and wet) and the titanium substrate (hard and dry) is still a great challenge. Here, we designed a "soft (hydrogel)-hard (Ti6Al4V)" integrated material with outstanding combination, which simulates the structure and function of cartilage-bone in the natural joint. The load-bearing properties, binding performance, and tribological behaviors for different forms of the soft-hard integrated materials were investigated. The results showed that the hydrogel layer and Ti6Al4V substrate possess ultra-high interfacial toughness (3,900 J/m²). In addition, the combination of the hydrogel layer and Ti6Al4V substrate provided a good lubrication system to endow the "soft (hydrogel)-hard (Ti6Al4V)" integrated material with high load-bearing and excellent tribological properties. Therefore, this study provided an effective strategy for prolonging the service life of Ti6Al4V in the biomedical field.