Abstract
AbstractThe 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.
Highlights
Attempts to reduce friction between sliding surfaces have been documented since antiquity [1, 2], as in the ancient Egyptian wall paintings at Saqqara or in references in the Hebrew Bible [2]
Boundary lubrication [10, 11] concerns molecularly thin films of materials that are attached to the mutually-sliding substrates, so that when the surfaces slide past each other the slip occurs at the interface between these boundary layers
We describe a number of systems where this effect has been observed and studied: lubrication by charged and zwitterionic polymer brushes; boundary lubrication by classical surfactants under water; and the remarkable effects observed when liposomes—vesicles consisting of phospholipids, the building blocks of living cell membranes—are attached as boundary layers at surfaces
Summary
Attempts to reduce friction between sliding surfaces have been documented since antiquity [1, 2], as in the ancient Egyptian wall paintings at Saqqara or in references in the Hebrew Bible [2]. We describe a number of systems where this effect has been observed and studied: lubrication by charged and zwitterionic polymer brushes; boundary lubrication by classical surfactants under water; and the remarkable effects observed when liposomes—vesicles consisting of phospholipids, the building blocks of living cell membranes—are attached as boundary layers at surfaces. We follow this by discussing the relation of hydration lubrication to biological friction, in particular for the major load bearing joints such as hips or knees. We conclude this review by emphasizing the challenges and opportunities that lie in the understanding and utilization of this new lubrication paradigm
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