Abstract
Purpose: :
To compare the interfacial equilibrium and dynamic rheological properties of whole human tear fluid (WHTF) collected using capillary tubes, human Meibomian lipids (HML), and tear lipids extracted from tears collected using Schirmer strips (SSL).
Methods: :
Samples of WHTF and HML- and SSL-lipids were collected from 10 healthy subjects. Sessile bubble/drop tensiometry was used to examine interfacial properties of WHTF and reconstituted thick films formed using HML and SSL samples. Lipids were dissolved and deposited on the surface of air bubble immersed in buffered saline to form multilayered films with initial surface pressure 50±2.0 mN/m. Lipids films were aged for 3-24 hrs and then subjected to step-strain relaxation experiments at ambient temperature 22 and 36.5 degrees C.
Results: :
The equilibrium surface tension of WHTF measured after 24 hrs of interface aging varied among samples. The minimum tension observed for WHTF was 22.2 ±2.0 mN/m, very close to equilibrium tension of thick films formed by HML and SSL samples, and monolayer of lung phospholipid DPPC alone. Maximum tension was up to 46±3.5 mN/m, typical for lysozyme and other tear proteins in the absence of lipids. Rheological parameters, elasticity modulus (Einf), and relaxation time were dissimilar for WHTF, HML and SSL. Einf for WHTF varied between 32.1 to 54 mN/m, with relaxation time ranging from 950 to 1400 s, whereas Einf for HML was 160±20 mN/m and relaxation time 300-550 s. Einf for SSL varied from 10.7 to 14.8 mN/m, substantially lower than Einf of other samples (p<0.05). Relaxation times for SSL were substantially shorter (p < 0.05), between 80 and 120 s. In temperature interval of 22-36.5 degrees C, no changes in interfacial visco-elastic properties were observed for either HML or SSL.
Conclusions: :
WHTF and human lipid samples collected using different methods exhibited significantly different biophysical properties. We hypothesize that polar phospholipids in SSL are responsible for the differences in interfacial rheological properties between SSL and HML.