Hydrogels are crosslinked polymer networks that are swollen with water. Because hydrogels have unique properties including permeability to small molecules, low sliding friction, stimuli-responsiveness, etc., they are promising tribology materials with potential applications in many fields, from biomedical engineering and soft robotics to the environmental field. However, conventional hydrogels are mechanically weak in both strength and toughness, greatly limiting their applications. This review will focus on the tough double network (DN) gels and the latest research on self-strengthening and growth of DN gels under mechanical training. A view to application as load-bearing tribology materials such as articular cartilage will also be introduced.

Cellulose nanofibers (CNFs), which are obtained by defibrillating woods, plants and other biomass, have attracted much attention in recent years because they are strong, lightweight, biodegradable, and renewable materials. Herein, we introduce the significance of CNFs for the development of sustainable materials, and their applications in resin/rubber composite materials. Further, an example for understanding and controlling tribological properties on CNF materials is illustrated by our recent results.

Polymer brushes, which are dense polymer arrays whose ends are tethered to substrate or particles, can provide various properties on materials' surface based on their chemical structure, topology, or combined swelling agent, which have been utilized in various research fields like dispersion of particles, separation of biomolecules and antifouling. While tribological applications are also expected due to their super-lubricity based on high osmotic pressure, surface contamination and wear over time remain practical issues. On the other hand, self-healing polymers can repair damaged areas either intrinsically or extrinsically, which are expected to be utilized to suppress the functional deterioration of polymer brushes over time. This paper first describes the basic strategies of self-healing polymers utilizing microencapsulation, dynamic covalent bonding, or non-covalent bonding to perform intrinsic or extrinsic self-healing. Then, the formation and characterization of self-healing polymer brushes utilizing thermal treatment or phase separation to perform self-healing to maintain the surface properties are discussed. Finally, recent examples of tribological application of these self-healing polymer brushes are described.

Ethylene/Styrene/Butadiene terpolymer (ESB), as part of new materials with sustainability and circular economy. It is possible to control the main chain and higher-order structures, and as a result, various physical properties of elastomer materials can be controlled. The precisely controlled structure not only exhibits high specific strength, but also exhibits excellent properties such as self-healing and polymer degradation.

This review will outline self-healing materials inspired by nature and the applied research into tribology inspired by them, with a focus on recent advances in biomimetics. Self-healing materials are of great importance in a society where SDGs are expected to progress, and a variety of self-healing materials and self-healing strategies have been proposed. In this context, biomimetic approaches are attracting attention as a way to address the limitations of current self-healing materials. Here, some researches are introduced on the use of layer-by-layer self-assembly to mimic the repair mechanism of the skin of cephalopods such as squid. By tuning polymer interactions, self-healing performance can be achieved in about one second. When such materials are deployed in tribology, they can again exhibit a low-friction state through self-healing after being worn. The potential for further benefits to tribology will be demonstrated by achieving self-healing low-friction surfaces through the novel incorporation of a biomimetics perspective to improve tribological performance.

Micro-SEIRAS (Surface Enhanced Infra-Red Absorption Spectroscopy) was applied to investigate change in surface concentration of oleic acid from the dilute solution of 0.1 mass% under shearing conditions. The intensity of C=O vibration originating from oleic acid increased with shearing time and was maintained after termination of shearing. SEIRAS spectra revealed that there are two species, monomer and dimer of oleic acid in the concentrated layer. The effect of base oil on the surface concentration was investigated using polyalpha olefin (PAO), mineral oil (150N) and polypropylene glycol (PPG) as base oils. Although the surface concentration was observed with PAO and mineral oil as base oils, the C=O intensity did not increase when PPG, polar compound was used. The concentrated oleic acid at the interface was removed under shearing condition by replacing lubricant with base oil, PPG. The removal of dimer of oleic acid was easier than monomer which adsorbed on the surface. It can be concluded that the formation of concentrated layer of oleic acid under shearing conditions was closely dependent on the solubility of oleic acid in base oil.

To elucidate the mechanism for suppressing wear of high-speed bearing steel (SUJ2) used in automatic transmission fluid (ATF), friction and wear tests were conducted in lubricating oils containing different concentrations of succinimide dispersant. The wear volume of the SUJ2 disk in the base oil was approximately 1.4×10⁴mm³. However, with the addition of 0.75 mass% of succinimide dispersant, the wear volume decreased to about 0.4×10⁴mm³, approximately one-third of the original amount. In Fluid A and Fluid A I₀.₇₅-I₂.₇₅, the reduction in wear volume was observed only in Fluid A I₀.₇₅. This was believed to be due to the presence of succinimide dispersant, which led to the formation of iron oxide within the wear scars, and on the surface, FeS and FeS₂formed as tribofilms. In the case of Fluid A I₀.₇₅, the film hardness was higher, contributing to improved wear resistance. The sole reason for the increased hardness of the tribofilm in Fluid A I₀.₇₅was hypothesized to be the repeated friction of the tribofilm within the surface, leading to the deposition of FeS and FeS₂. This, in turn, was presumed to enhance the density of the tribofilm.