Abstract

The human tear film lipid layer (TFLL) is a duplex lipid layer film comprising the outermost layer of the tear film, responsible for surface tension reduction of the tear film in the blinking function. Exposure of the tear film lipid layer to increased ground-level concentrations of air pollutants, for example tropospheric ozone, can impact the chemical composition, structure and the function of the tear film lipid layer by impacting its surface activity, stability, respreadability and viscoelasticity, important TFLL characteristics in the blinking function. This compromise in these characteristics, leads to the emergence of dry eye disease (DED). In this research, Langmuir films spread at the air-water interface are used as TFLL mimicking model membranes. In addition to the study of the functional role of each component in the TFLL, the impact of ozone exposure on their biophysical properties is also investigated using Langmuir balance, Brewster angle microscopy, a profile analysis tensiometer, and mass spectrometry. Moreover, the crystallinity, lateral ordering, and vertical structure of the cholesteryl oleate film, as an important cholesteryl ester used in TFLL model membrane studies is investigated using Grazing Incidence X-Ray Diffraction (GIXD), X-ray reflectivity (XR) and Grazing Incidence Off-Specular Scattering (GIXOS), respectively. Crystallinity of the cholesteryl oleate film was attributed to the cholesterol ring packing and a flat, monolayer film was observed at lower surface pressures. Oxidation impacts the phase transition behaviour of the model membranes as well as their multilayer formation. It also leads to expansion of the films to higher molecular areas, fluidization of the films, significant morphological changes, reduction of their respreadability and a composition-driven impact on their viscoelasticity. These findings can help better understand the roles of TFLL components in its function as well as the impact of prolonged ozone exposure on the mechanical and biophysical properties of human TFLL.