Abstract
Dry eye syndrome (DES), one of the most common ophthalmological diseases, is typically caused by excessive evaporation of tear fluid from the ocular surface. Excessive evaporation is linked to impaired function of the tear film lipid layer (TFLL) that covers the aqueous tear film. The principles of the evaporation resistance of the TFLL have remained unknown, however. We combined atomistic simulations with Brewster angle microscopy and surface potential experiments to explore the organization and evaporation resistance of films composed of wax esters, one of the main components of the TFLL. The results provide evidence that the evaporation resistance of the TFLL is based on crystalline-state layers of wax esters and that the evaporation rate is determined by defects in the TFLL and its coverage on the ocular surface. On the basis of the results, uncovering the nonequilibrium spreading and crystallization of TFLL films has potential to reveal new means of treating DES.
Highlights
The issues of Dry eye syndrome (DES) focus on the eye surface that is covered by a tear film, which is divided into a muco-aqueous gel layer and a tear film lipid layer (TFLL)
Unlike monolayers of polar lipids composed of fatty acids and fatty alcohols that effectively retard evaporation of water,[4] the TFLL is mainly composed of nonpolar cholesteryl esters (CEs) and wax esters (WEs).[5−7] Despite their nonpolar nature, recent studies on model lipid monolayers have shown that WE films can effectively retard the evaporation of water[8,9] and that the properties slowing down evaporation are related to the crystalline structure of WE films.[9,10]
The simulations were carried out systematically over an extensive range of phase space and were complemented by surface potential and Brewster angle microscopy (BAM) measurements applied to model TFLL (Langmuir) monolayers composed of WEs
Summary
Molecular simulations have established themselves as one of the excellent methods for studying the properties of biomembranes.[15]. While determining the effect of different defects is beyond the scope of this study, the results shown in Figure 3 suggest that the evaporation resistance of a solid WE film is determined by the amount of lattice defects and grain boundaries, because permeation through an intact crystal is negligible This is in line with previous works that have shown permeation of water[29] and oxygen[30] through condensed lipid monolayers to occur mainly through interdomain boundaries and intradomain defects. Notes The authors declare no competing financial interest. ¶J.M.H.: Deceased
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