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D. L. Leiske, S. Y. Nishimura, H. A. Ketelson, G. G. Fuller; Rheological Characterization of Model Tear Films Under Shear and Extensional Flow. Invest. Ophthalmol. Vis. Sci. 2008;49(13):80.
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The rheological contributions of different tear film components can help to determine ideal formulations for dry eye products. This study explores the interfacial properties of model tear films both in shear and under extensional deformations. Methodologies: Cholesteryl myristate (CM), 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and lysozyme films were characterized using surface tension measurements; Brewster angle microscopy (BAM), interfacial shear rheology (ISR) and interfacial extensional rheology. First, a series of CM:DPPC films were tested to explore the differences between a two-component model tear film and a pure DPPC film. Following this characterization lysozyme was injected into the subphase to monitor the effect on the physical properties of protein adsorption into a lipid film.
Isotherms showed that films containing ≥ 90 mole % CM have a greater percent area loss between the first and second compressions than the films with less CM. BAM images clearly showed that CM films did not expand after compression, and solid-like regions extending 1-2 mm were observed at low pressures (1 mN/m). Conversely, lipid films with ≤ 70 mole % CM were primarily fluid. It was also demonstrated that lysozyme adsorption increases the ductility and decreases the isotherm hysteresis for CM:DPPC films, indicating that the addition of proteins can rescue the film from breakage and improve recovery under imposed strains. These results were confirmed with both shear and extensional rheology.
CM increased the elasticity of the lipid films, but also caused them to become brittle and incapable of expansion following compression. Lysozyme adsorption onto these films was shown to improve post-deformation recovery in high mole percentage CM films.
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