Quantifying the effects of polar lipid proportions on the stability of model tear films

Dry eye Disease (DED) is one of the most widespread ocular surface disorders across the world and is thought to affect hundreds of millions of people worldwide, including costs of over $50 billion annually to the United States alone. However, despite DED’s widespread prevalence, there are very few effective clinical treatments for the syndrome, in part due to the complexity of the thin liquid coating – i.e. tear film – over the surface of the eye.

One hallmark of DED is premature destabilization of the tear film, which can be caused by many different mechanisms. Although we have an extensive knowledge of the various underlying biological causes for DED, we know relatively little about the resulting physical processes that lead to DED, which makes it difficult to design targeted treatment options.

Particularly little is known about tear film lipids – which are thought to sit on the surface of the tear film. Deficiencies in certain types of polar lipids are thought to be linked to an increased risk of DED, but we do not yet understand why this is the case. Finally, despite a wide array of literature on lipid chemistry, structure, and rheology, no clear link has been made yet between lipid composition on the surface of the eye and the severity of DED.

To address these deficiencies in knowledge, this thesis aims to improve our understanding of the role of polar lipids in the stability of the tear film. We test the stability of model tear films supplemented with varying amounts of the phospholipid “dipalmitoyl phosphatidylcholine” (DPPC) and the wax ester “stearyl stearate,” both of which are comparable to common lipids found natively in the eye. We find that model tear films can be stabilized by both DPPC and SS, likely through a combination of lipid-specific physical mechanisms. When model tear films are created with combined DPPC and SS lipid layers, we find that large increases in the DPPC composition can increase the stability of the model films. Finally, we find that mixed lipid layers have physical properties that differ from the pure lipid components, implying that a more nuanced understanding of lipid interactions is required.

Altogether, this data indicates that large changes to lipid compositions in the tear film could play a role in reducing tear film stability, as is thought to occur in patients with DED. However, a more advanced understanding of multi-component lipid layers is necessary before well-targeted therapeutics can be developed. DED remains a pressing health and societal problem, but this work improves our understanding of the lipid layer of the tear film and provides a basis for future research on the role of lipids in dry eye disease.