Tear film lipid layer

The tear film lipid layer (TFLL) is a 40–100 nm thick lipid layer residing at the outermost air–water interface of the tear film. Its principal physiological function is to retard evaporation from the underlying aqueous tear film. Excessive tear evaporation is widely recognized as a central pathological mechanism in evaporative dry eye disease (DED) and meibomian gland dysfunction (MGD), contributing to tear film instability, hyperosmolarity, ocular surface inflammation, and visual disturbance. Consequently, quantitative assessment of TFLL evaporation resistance is of substantial clinical and biophysical significance for elucidating tear film dysfunction and advancing lipid-based therapeutic strategies for evaporative DED. 

We have developed a next-generation droplet-based platform, termed the controlled-area droplet evaporimeter (CADE), to overcome the limitations of existing in vitro methodologies. By integrating three independent mechatronic feedback control loops that actively regulate droplet surface area, surface temperature, and environmental relative humidity, CADE enables direct and quantitative measurement of TFLL evaporation resistance under physiologically relevant conditions. In addition, CADE enables versatile biophysical characterization of the TFLL, including assessment of surface activity, interfacial rheology, and interfacial ultrastructure and topography using atomic force microscopy (AFM). These studies provide direct quantitative evidence linking TFLL interfacial organization to its evaporation barrier function, thereby establishing CADE as a physiologically relevant, mechanistically resolved platform for investigating tear film evaporation and providing new insights into the structure–function relationships of the TFLL in evaporative dry eye disease.