Hydration lubrication

The hydration lubrication paradigm, whereby hydration layers are both strongly held by the charges they surround, and so can support large pressures without being squeezed out, and at the same time remain very rapidly relaxing and so have a fluid response to shear, provides a framework for understanding, controlling, and designing very efficient boundary lubrication systems in aqueous and biological media. This review discusses the properties of confined water, which-unlike organic solvents-retains its fluidity down to molecularly thin films. It then describes lubrication by hydrated ions trapped between charged surfaces, and by other hydrated boundary species including charged and zwitterionic polymer brushes, surfactant monolayers, liposomes, and biological macromolecules implicated in synovial joint lubrication. Finally, challenges and prospects for future development of this new boundary lubrication approach are considered.