The impact of lipid composition on the stability of the tear fluid lipid layer

The tear fluid protects the corneal epithelium from drying and pathogens and it also provides nutrients to these cells. Tear fluid is composed of an aqueous layer as well as a lipid layer that resides at the air-tear interface. The function of the lipid layer is to lower the surface tension of the tear fluid and probably prevent evaporation. We have studied the impact of lipid composition on the structural and dynamical properties of the tear lipid film using Langmuir films, X-ray diffraction, and coarse-grained molecular dynamics simulations. Based on recently published lipidomic data, we have selected a number of model systems comprised of phospholipids, cholesteryl oleates, triglycerides, and free fatty acids to study how the organization, stability, and dynamics of the lipid layer depend on its composition. In particular our aim is to unravel how excess neutral lipids affect the stability and dynamics of the lipid layer, since physiologically this condition resembles the tear fluid of chronic blepharitis patients. Our results indicate that neutral lipids residing on top of phospholipids and facing the air phase are needed to produce a stable lipid film at the air-water interface for a wide range of surface pressures. Decreasing the phospholipid-neutral lipid ratio, however, decreases the stability of the lipid film. This turns out to stem from the changed organization of the lipid film that varies from a layered structure to an oil droplet-like structure with decreasing phospholipid-neutral lipid ratio. The results provide a plausible rationale for the development of dry eye syndrome in blepharitis patients.