Role of Nanomechanical Properties in the Tribological Performance of Phospholipid Biomimetic Surfaces

Abstract

The role of phospholipid bilayers in controlling and reducing frictional forces between biological surfaces is investigated by three complementary experiments: friction forces are measured using a homemade tribometer, mechanical resistance to indentation is measured by AFM, and lipid bilayer degradation is controlled in situ during friction testing using fluorescence microscopy. DPPC lipid bilayers in the solid phase generate friction coefficients as low as 0.002 (comparable to that found for cartilage) that are stable through time. DOPC bilayers formed by the vesicle fusion method or the adsorption of mixed micelles generate higher friction coefficients. These coefficients increased through time, during which the bilayers degraded. The friction coefficient is correlated with the force needed to penetrate the bilayer with the AFM tip. With only one bilayer in the contact region, the friction increased to a similar value of about 0.08 for the DPPC and DOPC. Our study therefore shows that good mechanical stability of the bilayers is essential and suggests that the low friction coefficient is ensured by the hydration layers between adjacent lipid bilayers.

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).…”

“…In particular, we identify the dissipation pathway and the viscous losses in the bound hydration layers alone, which are characterized by an effective viscosity orders of magnitude larger than that of non-hydration water. This insight largely accounts for the massive friction reduction via the hydration lubrication mechanism recently described in many systems 3,[5][6][7][8][9][10][11] , and has implications for better design of boundary lubricants and their interfacial vectors in biomedical contexts. Moreover, it provides a framework for understanding lubrication processes in living systems where hydration layers are ubiquitous.…”

“…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.…”

Synergies in lubrication

“…Indeed, several reports have described very low friction forces between phospholipid bilayers up to high pressures. 11,37,49,50 Perhaps counterintuitively, it was found that DPPC bilayers in the fluid state have a higher load bearing capacity than such layers in the frozen state. 37 This is illustrated in Fig.…”