Lipid-Bilayer Assemblies on Polymer-Bearing Surfaces: The Nature of the Slip Plane in Asymmetric Boundary Lubrication.

Jacob Klein,Somasundaram Anbumozhi Angayarkanni,Nir Kampf

doi:10.1021/acs.langmuir.0c02956

Jacob Klein, Somasundaram Anbumozhi Angayarkanni + Show 1 more

Open Access

https://doi.org/10.1021/acs.langmuir.0c02956

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Abstract

Phospholipid–macromolecule complexes have been proposed to form highly efficient, lubricating boundary layers at artificial soft surfaces or at biological surfaces such as articular cartilage, where the friction reduction is attributed to the hydration lubrication mechanism acting at the exposed, hydrated head groups of the lipids. Here we measure, using a surface force balance, the normal and frictional interactions between model mica substrates across several different configurations of phosphatidylcholine (PC) lipid aggregates and adsorbed polymer (PEO) layers, to provide insight into the nature of such lubricating boundary layers in both symmetric and especially asymmetric configurations. Our results reveal that, irrespective of the configuration, the slip plane between the sliding surfaces reverts wherever possible to a bilayer–bilayer interface where hydration lubrication reduces the friction strongly. Where such an interface is not available, the sliding friction remains high. These findings may account for the low friction observed between both biological and synthetic hydrogel surfaces which may be asymmetrically coated with lipid-based boundary layers and fully support the hydration lubrication mechanism attributed to act at such boundary layers.

Highlights

Phosphatidylcholine (PC) lipids and their assemblies, such as bilayers and vesicles, have been identified as the active components of boundary layers that are exceptionally efficient in reducing friction between sliding surfaces in aqueous surrounding.[1−5] This arises via the hydration lubrication mechanism:[6,7] the hydration layers coating the phosphocholine groups exposed by such boundary layers are highly tenacious and so resist being squeezed out, yet at the same time they are rapidly relaxing and result in only a very weak frictional dissipation on sliding and shear past similar boundary layers
It is of interest that dynamic light scattering (DLS) from a mixture of the liposomes and the poly(ethylene oxide) (PEO) in water revealed a single peak similar in size to that of the liposomes alone; this suggests that the PEO adsorbs quite strongly onto the lipid vesicles, forming a thin adsorbed layer
Our findings show that surfaces interacting via boundary layers comprising PC lipid aggregates, which may be adsorbed onto or complexed with other molecules, will experience low friction, enabled by hydration lubrication, in both symmetric and asymmetric configurations, whenever sliding may occur across a slip plane between their highly hydrated phosphocholine groups

Summary

■ INTRODUCTION

Phosphatidylcholine (PC) lipids and their assemblies, such as bilayers and vesicles, have been identified as the active components of boundary layers that are exceptionally efficient in reducing friction between sliding surfaces in aqueous surrounding.[1−5] This arises via the hydration lubrication mechanism:[6,7] the hydration layers coating the phosphocholine groups exposed by such boundary layers are highly tenacious and so resist being squeezed out, yet at the same time they are rapidly relaxing and result in only a very weak frictional dissipation on sliding and shear past similar boundary layers. We conclude that slip for this configuration occurs at interface 2, where the hydrated phosphocholine headgroup layers of the PC bilayers repel and slide past each other via the hydration lubrication mechanism This is consistent with the low values of μ seen for configuration I (Figure 3B, as well as Figures 3D and 4A), which are similar to values seen in earlier studies where liposome-bearing mica surfaces slide past each other.[3]. The presence of a single bilayer between the surfaces, as in configurations III and IV (Figures 5A,C), results in very different shear behavior, with much higher friction coefficients (Figures 5B,D and Table 1) The reason for this is clear: there is no longer an available bilayer−bilayer interface which can act as a slip plane. Where the friction coefficients are not too different, say within a factor of 2 or so some combination of slip may occur at the two interfaces, and this may apply to the high-friction slip planes in configurations III and IV

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES