Polymer-based lubricating materials for functional hydration lubrication

The hydration lubrication was proposed by Jacob Klein, a famous tribologist, in 2001. The core of this mechanism is the formation of a strong hydrated layer surrounding polar groups. Over the past 20 years, various lubricating materials have been developed according to this mechanism. Among them, polymer-based lubricating materials have attracted much attention because of their facile tunability of structures and functions. Especially, they have played a tremendous role in functional hydration lubrication, which means that other interesting functions can be integrated into hydration lubrication during water-based friction, such as stimuli-responsive property, biomedical performance and biomimetic design. This review outlines the recent progress of functional polymeric materials employed for realizing versatile hydration lubrication, which are categorized as: (i) stimuli-responsive lubricating materials, through external stimuli to control the interfacial friction; (ii) biomedical lubricating materials, integrating lubricating ability into biomedical materials; (iii) biomimetic lubricating materials, designed by learning from living organisms and nature.