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M. C. Lin, T. F. Svitova; Temperature and Dilatational Strain Effects on Interfacial Rheology of ex vivo Human Lipids Differ Between Asian and Non-Asian. Invest. Ophthalmol. Vis. Sci. 2010;51(13):4155.
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To study the effects of temperature and interfacial strain on the rheological properties of ex vivo human lipids collected from subjects of different ethnicity.
Sessile bubble tensiometry was used to examine interfacial properties of reconstituted ex vivo lipids. The lipid samples were extracted from Focus N&D lenses worn on continuous-wear basis for 1 month by 12 healthy subjects, 6 Asians (A) and 6 non-Asians (NA), with average age of 21±2.9 years. The lipids were deposited on the surface of an air bubble immersed in buffered saline to form a multilayered film of 15-20 nm thick with initial surface pressure of 50 ± 1.0 mN/m. The lipid films were aged for 16-24 hours in aqueous media and then subjected to step-strain relaxation experiments at various temperatures (22º, 30º, 36º- 44º C, in an increment of 2º±0.5º steps) and dilatations (dA) from 3 to 30%.
There was little inter- and intra-subject variability in the equilibrium surface tension for the thick films formed by the reconstituted ex vivo lipids (Mean ± SD = 22±0.5 mN/m). The rheological parameters such as elasticity modulus (E), and interfacial viscosity (υin,) were found to be significantly different between lipids collected from subjects of A and NA ethnicity. At low dilatations (dA< 10%), the average E was 19.4±3.6 mN/m for NA, 1.4 times higher than that for A subjects (11.6±3.8 mN/m; p < 0.05). Interfacial viscosity was 15±2.5% higher for A compared with NA (p < 0.05) and did not depend on dilatation. No changes in interfacial rheological properties were observed for both A and NA lipids between 22º and 36º C. The melting region of interfacial films was determined as a temperature interval at which the elasticity modulus started to decrease significantly; these were 42- 44º C and 38- 40ºC for NA and A, respectively.
Relaxation of A lipids was slower than that of NA lipids, occurring mostly via viscous dissipation. Higher elasticity of NA lipids indicated stronger impact of structural forces in relaxation process. The differences in interfacial rheology of A and NA lipids are likely responsible for observed variations in tear breakup time. We speculate that this dissimilarity in rheological behavior is caused by higher content of cholesterol and its derivatives in NA lipids.
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