A mechanism to explain the behaviour of spread films of meibomian lipids

The lipid layer of the tear film is difficult to investigate because it is dynamic, transparent, less than a 0.1 μm thick, and made from many components.1 To gain insight into the structure of the lipid layer of the tear film, we have been examining the physicochemical properties meibomian lipid films using a Langmuir trough.2-5 In this device, meibomian lipids are expressed from the eyelids and a film is formed by spreading them (dissolved in a solvent e.g. chloroform or hexane) between two moveable barriers on the surface of a buffer. The solvent is allowed to evaporate and changes in surface pressure of the lipid film are recorded as the barriers are moved towards each other and then away from each other. For the experiments shown here, the meibomian lipids were expressed from a single 56-yearold male with no ocular pathology and collected using a stainless steel spatula, dissolved in chloroform and stored at −80°C. Local ethics permission was obtained and complied with the tenets of the Declaration of Helsinki. Compression and expansion of spread meibomian lipid films results in a complex pressure/area isocycle showing hysteresis whereby the compression part of the isocycle differs substantially from the expansion part of the isocycle (Figure 1). The increase in pressure occurs when molecules at the surface are pushed together and begin to interact with each other (compression part of the cycle) and the decrease in pressure when the molecules are being pulled apart (expansion part of the cycle).