(201b) Viscoelastic and Structural Changes of Human Meibomian Lipids with Temperature

The outermost layer of the tear film may play an important role in tear film stability via lipid mechanical properties. These mechanical properties are a result of lipid structure and organization. We seek to establish the range of viscoelasticity and structure found in stable tear films to ultimately identify which properties are important for stability in the tear film. Samples of human meibomian lipids (ML) were collected via manual expression of ~15 glands in the lower lid of healthy volunteers with no clinical symptoms of dry eye. ML was stored at -20°C. Material was spread without solvent in a Langmuir trough and interfacial mechanical properties of ML monolayers were monitored using an interfacial stress rheometer. After compression monolayers were held at a fixed surface area while temperature was increased at 0.2°C/min. Crystalline structure of bulk meibum was probed by small angle x-ray scattering (SAXS) between 25 and 45°C; grazing incidence x-ray diffraction (GIXD) measured order of ML monolayers at 25°C. At 15 mN/m and 18°C, ML monolayers were elastic with high surface moduli values, 2-7 mN/m. As the monolayers were heated to 35°C the surface viscous modulus decreased to 0.02 mN/m and the surface elastic modulus disappeared ~30°C. SAXS showed that bulk ML are crystalline with lamellar packing ~49Å, roughly the length of the lipids. As temperature increased peak intensity decreased and was absent by 45°C. Preliminary GIXD showed that ML monolayers form a tilted lattice with spacing ~4.85 Å. We have shown there is an association between structure and mechanical properties of ML. Moreover, the dramatic changes in interfacial viscoelastic properties are clinically relevant. If the melt temperature of lipids in dry eye individuals is altered (because, for example, of compositional differences), delivery of the lipids from the gland to the tear film and mechanical stabilization of the tear film may be modified in a manner that leads to instability.