Thesis Abstract - Kulovesi Pipsa

Tear Film Lipid Layer

The tear film covers the cornea and conjunctiva providing nutrients to the corneal cells and protecting it from the external environment. Tear film is composed of three intermixed layers. Mucous matrix covers the epithelial cells and is gradually mixed with the aqueous layer which is covered by a thin lipid layer. The lipid layer consists of both polar and nonpolar lipids. Lipids are thought to prevent the collapse of the tear film onto the ocular surface and to retard evaporation from the tears.

In this study several in vitro methods were used to study the surface properties and organization of different lipid mixtures resembling those in the tear film lipid layer. Phosphatidylcholine (PC), phosphatidylethanolamine (PE), and freefatty acids (FFA) were used as the polar lipids and cholesterol oleate (CO), triglycerides (TG), and wax esters(WE) were used as the nonpolar lipids. Langmuir film technique was used to examine the behavior of the lipid films during compressions and de-compression. Brewster angle microscopy (BAM) and atomic force microscopy (AFM) were utilized for visualizing the films. Grazing incidence X-ray difraction was used for surface structure studies. The results of experimental studies were compared with coarse-grained molecular dynamics simulations. Custom built system was used to evaluate the evaporation retarding effect of several lipid mixtures containing wax esters.

Compression isotherms showed one to two kinks in the compression curve that account for the rearrangement of the lipids in the film for all mixtures studied. Hysteresis was small meaning that these films are very stable and little, if any, solubilization of lipids into the aqueous phase takes place. All lipid films studied were more or less inhomogeneous when viewed with BAM, especially in higher surface pressures. This is most likely caused by the nonpolar lipids aggregating on the lipid film surface. This was also seen in the simulation studies where CO and TG formed circular aggregates on top of the polar lipids. Nonpolar lipids stabilized the films under high compression by arranging so that the lipid film could have a lower surface pressure than would be expected for small surface areas. However, an excess of nonpolar lipids caused the films to be more inhomogenous and to have less stable structure. Lipid mixtures that contained wax esters did not retard evaporation, which suggests that lipids' function in the tear film may have more to do with maintaining a thin tear film and preventing its collapse rather than preventing evaporation.

The task of TFLL is still under debate, although it has been regarded to be the evaporation barrier, hindering the movement of water molecules out from the tear fluid. This, however has been questioned in this thesis project. Although some WEs have been shown to retard the evaporation of water, this effect is lost when different lipid species are mixed, as was done in this study where several WE species were mixed with PC, CO, and TG. No evaporation retarding effect was detected for any mixtures studied. Evaporation retarding lipid films must be very tightly packed but this is not possible for TFLL-like films because the tear film lipids must also be suitable for the environment they are in where they must adapt to fast changes in surface pressure and must also be fluid. This cannot be achieved by the films that have been shown to retard evaporation as these films are rather stiff and cannot be compressed.