![]() ![]() ![]() ![]() 1 – 6 A common theme in most models is that lipids are present in a layer at the air interface of the tear film. However, in recent years, a number of models have been proposed to challenge the traditional three-layer concept ( Fig. 1). Based on extrapolation from simplified mockups and speculation, the tear film was originally modeled as three separate tiers composed of lipid, aqueous and proteins, and tethered mucins, respectively. An abnormal tear film is implicated in dry eye disease, yet the tear film structure is unknown. The human tear film is an example of a critically important biologic film that is dynamic with the surface continually repopulated with each blink. The data provide strong evidence that phospholipids are present at the surface of tears. Absorption peaks attributable to phospholipids were detected by PM-IRRAS on the human tear film surface and were augmented by the addition of phospholipid. Phosphate signals were shown to reflect relative concentrations. Surface selection of PM-IRRAS was demonstrated by suppression of water and phosphate signals in buffers with monolayers of oleic acid. Nulling ellipsometry was used to confirm the presence of monolayers and surface thicknesses when surface active reagents were added to solutions. In order to constrain the depth of signal detection to the surface, an extreme grazing angle of incident radiation was employed. Polarization-modulated Fourier transform infrared reflective absorption spectroscopy (PM-IRRAS) was used to study the presence of phosphate signals at the tear surface. ![]() Herein, the possibility that phospholipids are adsorbed at the air-surface interface of tears is interrogated. Although surface active, phospholipids have been presumed to be bound entirely to protein in the aqueous portion of tears or reside at the aqueous-lipid interface. The most controversial components are phospholipids. While many lipid and protein species have been identified in tears by mass spectrometry, the localization of the major components within the tear film structure remains speculative. The structure of tears has been theoretically considered three tiers with lipids at the air interface, aqueous and proteins in the subphase, and anchored mucins on the corneal epithelial surface. ![]()
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