Purpose. To evaluate the interfacial properties of ex vivo tear lipid multilayers with controlled and varying thickness. The influence of lysozyme and surfactant-containing multipurpose lens care solution (MPS) on interfacial rheology of lipids and mixed lipid-protein films were studied.
Methods. Lipids were extracted from lotrafilcon A lenses worn continuously for 1 month. Interfacial properties of the lipids without and with lysozyme in the aqueous phase were examined using tensiometry and step-strain relaxation. Lipid-lysozyme multilayers were exposed to either diluted opti-free express (OFX) or opti-free replenish (OFR) for 30 min, and then MPS was displaced from the bulk phase. Surface tension and rheological parameters before and after MPS exposure were measured and compared.
Results. Thick lipid films exerted high surface pressure at the air-aqueous interface, 50 ± 2 mN/m, with little inter- and intrasubject variability. The rheological storage modulus (E∞; 25.3 ± 2 mN/m) and relaxation time (τ; 87 ± 25 s) were similar among subjects. Neither lysozyme nor MPS changed the surface tension of the lipid multilayers. Lysozyme adsorbed irreversibly onto multilayers without changing E∞, but increased τ 2.5 times. Exposure of mixed multilayers to OFX reduced E∞ to less than a half of its original value (13 ± 4.5 mN/m; p < 0.001), whereas after OFR exposure, small but statistically significant changes in E∞ were found (21 ± 3.0 mN/m; p = 0.0044). MPS-treated mixed films relaxed much slower than an untreated one, τ increased 4.5 times after OFX and 7 times after OFR exposure.
Conclusions. Thick multilayers of the ex vivo tear lipids have exhibited surface tension 15 ± 1 mN/m lower than reported in the literature for meibomian lipids. The lipid-lysozyme interaction altered the interfacial rheology of the ex vivo lipids. OFX and OFR changed rheological properties of the mixed films to different extents.