(464a) Temperature Dependence of the Viscoelasticity of Ocular Surface Biological Lipids/Surfactants Using Quartz-Crystal Microbalance with Dissipation

The tear film lipid layer (TFLL) is a 100-200 nm layer that covers the tear film on the ocular surface, retarding evaporation. There are several pathologies that affect the composition of the lipids and the integrity of the TFLL and may lead to the development of evaporative dry eye disease.

The TFLL is mainly composed by the secretion of the Meibomian glands, located inside the upper and lower lids. Under certain pathologies, the Meibomian lipids have an increased viscosity, which may obstruct the expression from the glands. The traditional treatment for these diseases consist on the use of warm compresses that heat the lipids, promoting outflow.

Once on the ocular surface, the TFLL cools down due to the natural evaporation of the tear film. Furthermore, the TFLL must remain as a continuous layer, which resists the continuous compression/expansion stresses generated during blinking.

These conditions highlight the importance of the temperature on the viscoelastic properties of the TFLL, and the need to characterize their rheology. However, these materials are scarce, thus the bulk measurement of this properties are challenging.

One suitable technique to measure the rheology of these lipids is the Quartz Crystal Microbalance with Dissipation (QCM-D). In this technique, small amount of material (approximately 5-10 micrograms) forming a thin layer (of around 150 nm thick) can be characterized by quantifying the shift in frequency and dissipation at different subtones and different temperatures. With this information it can be determined if a Kelvin-Voigt model for viscoelastic solids or a Maxwell model for viscoelastic liquids is appropriate, allowing the possibility of estimating the storage and loss moduli of the material. The minimal amount of material needed for the measurements allows the determination of properties of the lipids for single patients, giving the potential to develop novel diagnostics and the testing of therapies.