title: Structural and rheological properties of meibomian lipid.
abstract: OBJECTIVE
  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 wide-angle 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.
authors.affiliation: Department of Chemical Engineering, Stanford University, Stanford, California 94305-5025, USA.
authors.forename: Liat
authors.initials: L
authors.lastname: Rosenfeld
authors.forename: Colin
authors.initials: C
authors.lastname: Cerretani
authors.forename: Danielle L
authors.initials: DL
authors.lastname: Leiske
authors.forename: Michael F
authors.initials: MF
authors.lastname: Toney
authors.forename: Clayton J
authors.initials: CJ
authors.lastname: Radke
authors.forename: Gerald G
authors.initials: GG
authors.lastname: Fuller
mesh_terms: D000818:Animals; D002152:Calorimetry, Differential Scanning; D002417:Cattle; D017357:Cyclotrons; D006801:Humans; D008055:Lipids / Q000737:chemistry*; D008537:Meibomian Glands / Q000737:chemistry*; D008859:Microscopy, Polarization; D012212:Rheology; D012542:Scattering, Radiation; D013666:Tears / Q000737:chemistry; D055682:Viscoelastic Substances; D014783:Viscosity; D014961:X-Ray Diffraction
chemical_list: D008055:Lipids; D055682:Viscoelastic Substances