The Effect of Lipid Saturation on Tear Film Structure with Molecular Dynamics Simulations

As a protective layer, the lipid layer in the outermost part of the tear film functions to keep the surface of the eye smooth and moist. Alterations in lipid compositions and properties are strongly associated with dry eye. In this study, a tear film composed model of a polar lipid monolayer (phospholipids with different saturations) and a nonpolar lipid layer (triglyceride/cholesterol ester) is constructed to study the changes in its structural properties under controlled surface pressures using molecular dynamics simulations. The results show that saturated polar lipids are less sensitive to surface tension. The existence of double bonds can significantly reduce the ordering of hydrophobic chains in unsaturated lipids, causing the lipid layer to be more loosely structured. The nonpolar lipids are the "adhesive" used to fill the gaps in the polar lipid layer, which inhibits water evaporation. To maintain the deformed structure of lipids under high pressure, the unsaturated lipid chains expand the pinch angle to enhance the interactions between the lipids. It causes the spatial site resistance of water molecules crossing the polar lipid layer to decrease significantly. This work provides molecular-level insights into the influence of lipid composition on dry eye.