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Clayton Radke, Liat Rosenfeld, Colin Cerrretani, Danielle Lieske, Michael Toney; A New Paradigm for the Structure of the Tear-Film Lipid Layer. Invest. Ophthalmol. Vis. Sci. 2013;54(15):947.
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We explore the unique rheological and structural properties of human and bovine meibomian lipids (ML) to provide insight into the on-eye physical behavior the human tear-film lipid layer (TFLL).
Human ML was collected from healthy individuals. Bovine ML was exuded from freshly excised eyelids. 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 wide-angle X-ray scattering detected the presence and melting of dispersed crystal structures. A differential scanning calorimeter (DSC) analyzed phase transitions in bulk samples of bovine meibum.
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 DSC, confirm the presence of suspended lamellar-crystal structures at physiologic temperature. A melt transition is detected in the bulk and interfacial rheology between 29-36 °C. Disappearance of crystalline particles was detected by SAXS, WAXS, and DSC over the same temperature range as that of the melt transition in the rheological measurements.
The TFLL is classically thought to be a parallel stack of molecular layers. Based on our new findings, the proposed structure for the TFLL shown at physiologic temperature in Fig. 1 is a highly viscoelastic, shear-thinning liquid suspension consisting of lipid lamellar crystallites immersed in a continuous liquid phase with no long-range order. The duplex lipid film exhibits two separate interfaces, air/lipid and water/lipid, with aqueous protein and surfactant-like lipids adsorbed at the water/lipid surface. Minor amounts of water and protein are present in the film. This new picture of the TFLL overturns the current accepted viewpoint.
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