Biophysical interactions of essential fatty acids with human tears

Purpose: Dietary intake of essential fatty acids (EFAs) is recommended for relieving symptoms of dry eye disease as it is thought to reduce inflammation at the ocular surface, although conflicting reports also exist. Topical application of EFAs to the ocular surface is also being investigated. Fatty acids are surfactants and their topical delivery into tears can alter the tear stability which plays an important role in dry eye. While previous studies investigated interactions of EFAs with meibomian lipids, this study aimed at investigating the biophysical interactions of EFAs with human tears to determine the efficacy of EFAs for topical ocular application. Methods: Human tears from asymptomatic volunteers were collected with informed consent. Pressure-area profiles and rheology of surface films of human tears, EFAs (LA - linoleic acid and ALA - α-linolenic acid), and mixtures of EFAs with tears were studied using Langmuir trough technology on an artificial tear solution at the physiological pH and temperature. The compressibility and elasticity of surface films were determined from pressure-area profiles. Results: Pressure-area profiles indicated that tears formed a highly compressible, non-collapsible surface film with a maximum surface pressure of 35mN/m. The surface film of LA was also highly compressible, comparable with that of tears, with a maximum surface pressure of 30mN/m but the surface film of ALA was very expanded and showed very low surface pressure with a maximum of 5mN/m. The elasticity of surface films of tears and LA increased with the compression of the films till phase transition followed by a decrease. Elasticity of ALA remained very low. LA and ALA when mixed with tears did not increase the surface pressure or elasticity of the mixed films in comparison with that of tears alone. Conclusions: Whole tears possess surface active properties better than those reported for meibomian lipids. This is likely due to the contribution from surfactant proteins in addition to the lipids in tears. Addition of small amounts of EFAs does not give a profound effect on the biophysical properties of tears because whole tears have their own robust surface active characteristics. Future studies with tears from dry eye patients, higher amounts of EFAs, and interactions of EFAs with tear proteins will help in determining possible benefits of topical application of EFAs to the ocular surface.