Interfacial dilatational viscoelasticity of human meibomian lipid films

The meibomian lipid layer is able to withstand the enormous stresses and deformations that occur during blinking due to the combination of its elastic and viscous properties. The purpose of this study was to measure the dilatational viscoelasticity of in vitro meibomian lipid films and compare how these properties differ between room temperature and physiological temperatures. Viscoelasticity was also compared with meibomian lipid films seeded with cholesterol or β-carotene (the levels of these lipid species change in disease states).

Dilatational viscoelasticity (E) was measured using an oscillating pendant drop method. Measurements were carried out on spread films at the air-water interface as a function of frequency (0.1256-12.56 rad/s) at various temperatures between 18-43 °C.

Generally, E gradually decreased as the overall temperature was increased. At both 37 and 20 °C, films demonstrated that the elastic modulus (E') was more dominant than the viscous modulus (E″), indicating films were more solid-like than fluid-like, regardless of temperature. E' and E″ were also dependant on frequency, indicating some molecular rearrangements of the lipid molecules as films were compressed and expanded. Films seeded with cholesterol or β-carotene showed a modest increase in the moduli.

These results are consistent with previous findings which have predicted and indicated that the meibomian lipid layer is a viscoelastic film at the air-liquid interface. These properties are integral to how the tear film lipid layer is able to maintain its structure, and hence integrity of the ocular surface.