Boundary Lubricants with Exceptionally Low Friction Coefficients Based on 2D Close‐Packed Phosphatidylcholine Liposomes

Origins of hydration lubrication

“…Earlier findings on lubricated sliding between hydrated surface layers including hydrated ions [3][4][5] , surfactants [6][7][8] , polymers [10][11][12] or liposomes 9 could not reveal the underlying process. This is because the measured friction was a convolution of the hydration lubrication with other dissipation pathways, such as the activated processes described above, or deformation [6][7][8][9] or entanglement [10][11][12] effects in the boundary layers themselves.…”

“…This is because the measured friction was a convolution of the hydration lubrication with other dissipation pathways, such as the activated processes described above, or deformation [6][7][8][9] or entanglement [10][11][12] effects in the boundary layers themselves. The present study elucidates this-for the simplest case of trapped, hydrated ions-by unravelling this convolution and separating the dissipation modes (and in future work it would be interesting to examine different aspects such as the case of different ions and of the transition between the regimes).…”

“…However, there is little microscopic understanding of this mechanism, especially of the frictional dissipation within the subnanometre hydration shells that form its basic elements. This is because in all relevant studies to date, including refs 4-11, the measured friction may have been dominated by other dissipation pathways, such as polymer chains disentangling past each other 10,11 or distortion of sheared liposomes 9 .…”

“…Thus, friction coefficients of order 10 À 3 , at pressures exceeding 100 atm, typical of articulating cartilage surfaces in healthy human joints 1 , cannot be reproduced by any synthetic surfaces, and its molecular-level understanding remains elusive 2 . Over the past decade, the concept of hydration lubrication has been invoked to account for the extremely low sliding friction observed at high pressures between charged surfaces in high-salt solution [3][4][5] , or when coated by surfactants [6][7][8] , liposomes 9 or hydrated polymer brushes [10][11][12] boundary layers resembling those at articular cartilage surfaces 12,13 . According to this, hydration shells formed by water molecules are tenaciously attached to the charges they surround, and so cannot be easily squeezed out on compression [14][15][16][17][18] , yet are labile and so respond to shear in a fluid manner.…”

Multidirectional Wear and Impact-to-wear Tests of Phospholipid-polymer-grafted and Vitamin E-blended Crosslinked Polyethylene: A Pilot Study

“…As a result, efforts to improve the lubrication of bearing materials have focused on reducing the number of PE wear particles. The fluid film lubrication provided by the hydrated layer is essential in natural synovial joints, and a phospholipid layer that covers the joint cartilage surface provides hydrophilicity and works as an effective boundary lubricant [9,14]. Therefore, grafting a phospholipid-like layer onto the surface may realize comparable hydrophilicity and lubricity resembling that of the physiological joint surface.…”