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Hendrik Walther, Lakshman N Subbaraman, Shawn Wettig, Lyndon William Jones; In vitro surface pressure measurements of various tear film lipids. Invest. Ophthalmol. Vis. Sci. 2014;55(13):43.
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The elasticity of tear film lipids is essential for retaining its stability and can be determined by plotting isotherms and by increasing the pressure of a surface. Thus, the aim of this study was to determine the surface pressures of individual tear film lipids and the effects of various lipid combinations on isotherms.
Six different tear film lipids (cholesterol, cholesteryl oleate [CO], oleic acid [OA], oleic acid methyl ester [OAME], triolein and phosphatidylcholine [PC]) were examined using a Longmuir-Blodgett trough (KSV NIMA). In addition, the lipid domains on the air-water interface were captured using a brewster angle microscope (BAM). Initially, each lipid was solubilized in cholesterol in various concentrations (3.6, 4, 3.6, 4, 4 and 1mg/mL, respectively). Furthermore, various solutions with mixtures of two lipids and one consisting of all six lipids were prepared; the concentrations were kept consistent. Based on lipid concentration and molecular weight, between 5µL and 40µL aliquots of the lipid solutions were gently applied onto air-water interface of de-ionized aqueous sub-phase (Milli-Q), using a gastight micro-syringe. Lipid layer and solvent were allowed to equilibrate and evaporate for 10 min before measuring surface pressures. Subsequently, the surface (768.5 cm2) was compressed at a rate of 21 cm2/minute and the pressures were determined (mN/m) with a platinum Wilhelmy plate.
The maximum surface pressure (πm) ranged from ~13 mN/m to ~47 mN/m: cholesterol ~46 mN/m, CO ~13 mN/m, OA ~30 mN/m, OAME ~15 mN/m, triolein ~27 mN/m, and PC ~47mN/m. The πm of the six-lipid stock was 26 mN/m. The PC lipid layer collapsed and formed a bilayer after reaching the πm and a surface area of ~125 cm2. For the examined lipid mixtures, the πm and lift-off areas (AL) of the isotherms changed, affecting the elasticity of the lipids. With rising surface pressures, uniform lipid layers started to solidify and precipitate, however, this effect was reversible.
The πm and AL of the tested lipids vary based on the applied lipid concentration. The elasticity of lipids changes when mixed with others. The transitions between gaseous and solid phase is reversible and, thus, might have a key influence in the spreading of the tear film. It will be valuable to conduct further research to investigate the role of tear film lipid surface pressures and to determine the effects of degraded lipids on isotherms.
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