The interfacial viscoelastic properties and structures of human and animal Meibomian lipids.

As the interface between the aqueous layer of the tear film and air, the lipid layer plays a large role in maintaining tear film stability. Meibomian lipids are the primary component of the lipid layer; therefore the physical properties of these materials may be particularly crucial to the functionality of the tear film. Surface pressure versus area isotherms, interfacial shear and extensional rheology, and Brewster angle microscopy (BAM) were used to characterize the Meibomian lipids from different species known to have different lipid compositions. The isotherms of humans, bovinae, wallabies, rabbits and kultarrs (a small desert marsupial) were qualitatively similar with little hysteresis between compression and expansion cycles. In contrast, several isocycles were necessary to achieve equilibrium behavior in the koala lipids. With the exception of kultarr lipids, the interfacial complex viscosity of all samples increased by one or two orders of magnitude between surface pressures of 5 mN/m and 20 mN/m and exhibited classic gel behavior at higher surface pressures. In contrast, the kultarr lipids were very fluid up to 22 mN/m; the behavior did not depend on surface pressure. Human lipids were very deformable in extensional flow and the BAM images revealed that the film became more homogeneous with compression as the elasticity of the film increased. The morphology of the kultarr lipids did not change with compression indicating a strong correlation between film structure and behavior. These results suggest that the lipid layer of the tear film forms a gel in vivo, which may aid in mechanically stabilization of the tear film.