Structural and Rheological Properties of Meibomian Lipid

PURPOSE. We explore the unique rheological and structural properties of human and bovine meibomian lipids to provide insight into the physical behavior of the human tear-film lipid layer (TFLL). METHODS. Bulk rheological properties of pooled meibomian lipids were measured by a commercial stress-controlled rheometer; a home-built interfacial stress rheometer (ISR) probed the interfacial viscoelasticity of spread layers of meibomian lipids. Small- and wideangle x-ray scattering detected the presence and melting of dispersed crystal structures. Microscope examination under cross polarizers provided confirmation of ordered crystals. A differential scanning calorimeter (DSC) analyzed phase transitions in bulk samples of bovine meibum. RESULTS. Bulk and interfacial rheology measurements show that meibum is extremely viscous and highly elastic. It is also a non-Newtonian, shear-thinning fluid. Small- and wide-angle x-ray diffraction (SAXS and WAXS), as well as differential scanning calorimetry (DSC) and polarizing microscopy, confirm the presence of suspended lamellar-crystal structures at physiologic temperature. CONCLUSIONS. We studied meibum architecture and its relation to bulk and interfacial rheology. Bovine and human meibomian lipids exhibit similar physical properties. From all structural probes utilized, we find a melt transition near eye temperature at which lamellar crystals liquefy. Our proposed structure for the tear-film lipid layer at physiologic temperature is a highly viscoelastic, shear-thinning liquid suspension consisting of lipid lamellar-crystallite particulates immersed in a continuous liquid phase with no long-range order. When spread over on-eye tear, the TFLL is a duplex film that exhibits bulk liquid properties and two separate interfaces, air/lipid and water/lipid, with aqueous protein and surfactantlike lipids adsorbed at the water/lipid surface.