Rheology of tear lipid components and their influence on surface properties.

The tear film lipid layer (TFLL) stabilizes the tear film by lowering surface tension and reducing aqueous evaporation, yet its physiology remains unclear. This study examined the surface properties of TFLL lipids. Seventeen lipids (five non-polar, three amphipathic, nine polar) and human meibum (from healthy and meibomian gland dysfunction subjects) were tested. A lipid mixture (LM) mimicking meibum composition was created. Effects of lipid removal or doubling were evaluated using surface pressure-area (π/A) measurements via a Langmuir trough. At equal amounts (1 μg), most individual lipids had significantly lower lift-off areas than the LM (p ≤ 0.05). LM exhibited higher surface pressure and elasticity than pure lipids (p ≤ 0.05). Removing amphipathic lipids (ceramides or (O-acyl)-ω-hydroxy fatty acids), or the polar lipids (phosphatidylcholines or phosphatidylethanolamine), from the LM reduced the surface pressure and elasticity by a minimum of 20% (p ≤ 0.04), while eliminating non-polar lipids cholesteryl ester, wax esters or free cholesterol dropped these parameters by at least 44% (p = 0.02). Amphipathic and polar lipids contributed more to LM surface activity than most non-polar lipids, though wax esters were critical for spreading. Doubling the triacyl glycerol or cholesterol improved surface activity, whereas increasing lysophospholipids impaired it (p ≤ 0.04). Normal meibum showed 53% lower surface pressure than MGD and 66% lower than LM (p ≤ 0.004). All parameters were strongly correlated (r ≥ 0.75; p < 0.001). Lipid classes act synergistically to reduce water surface tension. Amphipathic and polar lipids enhance surface activity, while non-polar lipids aid spreading. LM did not replicate normal meibum behaviour, indicating additional components or structural factors are needed.