title: Surface properties of artificial tear film lipid layers: effects of wax esters.
abstract: OBJECTIVE
  The tear film lipid layer is believed to stabilize the tear film and to retard evaporation. Based on previous simple in vitro studies, the evidence for the latter property is scarce. In this study, we used complex lipid mixtures including various wax esters to study their physical properties and evaporation retarding effect.
  
  
  METHODS
  Twelve samples of artificial tear film lipid layer mixtures composed of (L-α)-phosphatidylcholine, cholesterol oleate, and triglycerides were mixed with wax esters. A Langmuir balance was used to analyze the compressibility and rheological properties of these mixtures. In addition, a custom-built system was used for the evaporation studies used at 35°C. Lipid films were imaged with Brewster angle microscopy.
  
  
  RESULTS
  None of the studied lipid mixtures decreased the evaporation rate. All lipid mixtures had similar compression isotherms and viscoelastic properties regardless of the wax ester species or its concentration. The results suggest that the overall properties of these mixtures are independent of individual lipid species and that these films are very cooperative and showed minor variation depending on the wax ester species. Brewster angle microscopy images revealed that the lipid films assembled into multiple layers.
  
  
  CONCLUSIONS
  Wax ester-containing lipid mixtures resembling the tear film lipid layer are organized in a layered fashion so that amphiphilic lipids are adjacent to the aqueous phase and the nonpolar lipids are layered on top of these. This organization does not retard evaporation and raises overall questions about the role of lipids in the tear film.
authors.affiliation: Helsinki Eye Lab, Department of Ophthalmology, University of Helsinki, Helsinki, Finland.
authors.forename: Pipsa
authors.initials: P
authors.lastname: Kulovesi
authors.affiliation: Helsinki Eye Lab, Department of Ophthalmology, University of Helsinki, Helsinki, Finland.
authors.forename: Antti H
authors.initials: AH
authors.lastname: Rantamäki
authors.affiliation: Helsinki Eye Lab, Department of Ophthalmology, University of Helsinki, Helsinki, Finland.
authors.forename: Juha M
authors.initials: JM
authors.lastname: Holopainen
mesh_terms: D002788:Cholesterol Esters / Q000737:chemistry*; D008853:Microscopy; D009883:Ophthalmic Solutions / Q000737:chemistry*; D010713:Phosphatidylcholines / Q000737:chemistry*; D012212:Rheology; D013499:Surface Properties; D014280:Triglycerides / Q000737:chemistry*; D014783:Viscosity; D014885:Waxes / Q000737:chemistry*
chemical_list: D002788:Cholesterol Esters; D009883:Ophthalmic Solutions; D010713:Phosphatidylcholines; D014280:Triglycerides; D014885:Waxes; C014409:cholesteryl oleate
keywords: Langmuir films; dry eyes; rheology; tear film lipid layer; wax ester