| Lubrication by PC liposomes, attributed to their exposed surfaces of highly hydrated phosphocholine headgroups, can be understood in terms of the hydration-lubrication paradigm. However, the importance of liposome structure and lipid composition to their lubrication performance remained, until now, unclear. This thesis is a journey into the origins of lubrication by PC layers. First, a systematic study of normal and shear forces between two opposing surfaces bearing PC vesicles/continuous lamellar phases was performed. We have studied the lubricating properties of different liposomes as a function of their acyl chain length, performing measurements in pure water with surfaces that underwent a rinsing stage after adsorption. We demonstrate that overall liposome lubrication ability improves markedly with increasing acyl chain length, and correlates strongly with the liposomes' structural integrity on the substrate surface: DSPC (C18) - SUVs were stable on the surface, and provided extremely efficient lubrication (friction coefficient mu ≈ 10-4) at room temperature at pressures up to at least 18 MPa. DMPC (C14)-SUVs ruptured following adsorption, providing poor high-pressure lubrication, while DPPC (C16) -SUVs behavior was intermediate between the two. A further comparison between DSPC SUVs and DSPC extended lamellar phases showed reduced mechanical strength of bilayers compared to liposomes, as revealed by an AFM force spectroscopy study. In correlation with this, less efficient lubrication of bilayers was observed in SFB measurements. This difference is attributed to the increased mechanical stability of the self-closed, closely packed liposomes, which we believe results from the more defect free nature of the finite sized vesicles. We have next studied the effect of cholesterol addition on mechanical strength and lubrication of lipid layers. We found that addition of cholesterol to DSPC bilayers decreases the breakthrough force required for penetration of an AFM tip through a bilayer. In correlation with this, our SFB experiments showed decreased stability and resistance to normal load with the increase in cholesterol content of a DSPC bilayer. Friction coefficients of 10% cholesterol bilayers were in the same range as 40% cholesterol bilayers (mu~10-3), indicating that chol has a more substantial effect on mechanical properties of a bilayer then on its lubrication performance. We further carried out measurements in the presence of the SUV dispersions and found that in the case a lipid reservoir in the form of SUVs is present, the robustness of adsorbed liposomes no longer provides an advantage, and in this case bilayer-forming vesicles provide reversible and reproducible lubrication that is as good as, and even better then provided by intact vesicles, due to smoothness and uniformity of bilayers at these conditions. |
