Abstract
Biophysical properties of the tear film lipid layer are studied at the molecular level employing coarse grain molecular dynamics (MD) simulations with a realistic model of the human tear film. In this model, polar lipids are chosen to reflect the current knowledge on the lipidome of the tear film whereas typical Meibomian-origin lipids are included in the thick non-polar lipids subphase. Simulation conditions mimic those experienced by the real human tear film during blinks. Namely, thermodynamic equilibrium simulations at different lateral compressions are performed to model varying surface pressure, and the dynamics of the system during a blink is studied by non-equilibrium MD simulations. Polar lipids separate their non-polar counterparts from water by forming a monomolecular layer whereas the non-polar molecules establish a thick outermost lipid layer. Under lateral compression, the polar layer undulates and a sorting of polar lipids occurs. Moreover, formation of three-dimensional aggregates of polar lipids in both non-polar and water subphases is observed. We suggest that these three-dimensional structures are abundant under dynamic conditions caused by the action of eye lids and that they act as reservoirs of polar lipids, thus increasing stability of the tear film.
Highlights
Tear film is important to the health and optics of the human eye [1,2]
The macroscopic view studies have an abridged interpretation of the underlying biochemistry while those focusing on biochemical compositions and molecular level biophysics often do not fully cover the effects their results have on tear film macroscopic scale function
Encouraged by the results of recent molecular dynamics studies, our objective was to increase the scale of the tear-air interface and include in the study both lipids present in the tear film and those originating from the meibomian gland
Summary
Tear film is important to the health and optics of the human eye [1,2]. It refreshes with every blink, undergoes several phases of its kinetics, and eventually ruptures if blinking is suppressed [3]. Tear film has been studied for decades, there is still much to be discovered in relation to its composition and function at both macroscopic and microscopic scales. There is no unified theory that would provide comprehensive foundations for explaining the microscopic and macroscopic behavior of the human tear film. This is attributed to the inherent complexity of the tear film and technical difficulties in measuring its characteristics in vivo [16]
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