Effects of hyperosmolarity and temperature on the surfactant properties of human meibomian lipids

![CDATA[Purpose: Human Meibomian lipids are the major part of the lipid layer of the tear film. Their surfactant properties enable their spread across the aqueous layer and help maintain a stable tear film. Their fluidity is affected by temperature (transition 28-32°C) and might also be affected by increased osmolarity in the subphase, such as occurs in dry eye, due to crosslinking of surfactant lipids by divalent ions. The purpose of this study was to investigate effects of hyperosmolarity and temperature on the surfactant properties of human Meibomian lipids in vitro. Methods: Meibomian lipids, squeezed from the eyelids of normal volunteers, were dissolved in chloroform and spread on an artificial tear (AT) solution in a Langmuir trough. The lipid films were compressed and expanded and surface pressure-area (Π-A) profiles were recorded. For studying the effect of osmolarity, the Π-A profiles were recorded on AT solutions of 305, 478, 633, and 920 mOsm/kg. For studying the effect of temperature, the AT subphase was heated or cooled between 20-50ºC and the Π-A profiles were recorded at 10ºC intervals. The surfactant properties were also studied by spreading Meibomian lipids directly on AT subphase (without solvent) at 37ºC, and cooling the subphase to 20ºC. Results: Π-A profiles of Meibomian lipids were unaffected by different osmolarity. The maximum surface pressure (Πmax; ~18mN/m) and Π-A profiles for all subphases were similar. Temperature had a marked effect on the Π-A profiles. Increase in temperature caused lipid films to become fluid, an expected feature with a lowered Πmax (~12mN/m). However, decrease in temperature unexpectedly caused expansion of lipids and an increase in Πmax (~45mN/m), suggesting enhanced surfactant properties. Meibomian lipids spread without solvent at 37ºC behaved identically to those spread with solvent. Conclusions: Hyperosmolarity has no effect on the surfactant properties of human Meibomian lipids in vitro. Meibomian lipids have surfactants that allow them to spread across an aqueous subphase. Expansion of Meibomian lipids at lower temperatures is best explained by higher temperatures allowing lipids to more easily form multilayers.]]