Surface pressure measurements of human tears and individual tear film components indicate that proteins are major contributors to the surface pressure

Purpose: Tear film stability has been associated with a low surface tension (high surface pressure), which has been attributed to a variety of tear film components. In this study, we examined the contribution of various tear proteins, mucin, and meibomian lipids to the surface pressure of human tears. Methods: A Langmuir trough was used to measure and compare the surface activities of albumin, lipocalin, beta-lactoglobulin, lactoferrin, lysozyme, secretory IgA, mucin, meibomian lipid, and tears. Results: All proteins exhibited surface activity. The surface pressure-area (Pi-A) profiles of most protein films at equilibrium surface pressure (Pi(eq)) were sigmoidal and showed hysteresis between the expansion and compression phases of the cycle. Pi(eq) of most proteins took 4-9 hours to occur. By contrast, the Pi-A profiles for meibomian lipid films were hyperbolic rather than sigmoidal and had little hysteresis, and Pi(eq) was attained within 1 hour. The Pi-A profiles of mucin films showed mostly hyperbolic characteristics with small hysteresis. The Pi-A profiles of films of tears were sigmoidal, showed strong hysteresis, and reached Pi(eq) at about 5 hours. Partitioning of the proteins and whole tears into the subphase also occurred. Conclusion: Comparison between the dynamic Pi-A profiles of tears and those of individual tear film components shows that tear film proteins not only are capable of surface activity but also are major contributors to the surface activity of the tear film.