Abstract
Purpose: :
Pure 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) is often used as a model for the lipid layer of the tear film. However, Meibomian lipids (ML), the major component of the lipid layer, are composed of a rich mixture of lipid species which are primarily non polar. It is questionable whether a single phospholipid can represent the diversity of physical and chemical properties of Meibum. To explore the mechanical validity of this model, we have compared the interfacial rheology of DPPC and ML monolayers as a function of surface pressure and temperature
Methods: :
ML was collected from healthy individuals. Both pure lipids and Meibomian lipids were spread at an air-aqueous buffer interface in the interfacial stress rheometer (ISR). Interfacial viscous and elastic modulus were monitored as the films were compressed. For temperature sweeps, the mean molecular area was held constant as temperature was increased at 0.2°C/min from room temperature to 37°C. During the temperature sweep, the ISR was utilized to monitor changes in the mechanical properties of the monolayer as a result of increased temperature and melting of the lipids.
Results: :
In general, ML was more elastic and exhibited larger modulus values at the same surface pressure compared to DPPC. Between 4 mN/m and 8 mN/m the interfacial viscous modulus of ML increased from around 0.05 mN/m to 0.3 mN/m, and by 9 mN/m the film became dominated by elastic (solid) behavior. In comparison, the mechanical properties of DPPC monolayers were below the sensitivity limit (0.02 mN/m) of the ISR until 11 mN/m. The films remained fluid up to the collapse pressure. At starting surface viscous modulus of 1-2 mN/m, the Meibomian lipids became fluid around 30°C, and reached 0.02 mN/m by 35°C. The mechanical properties of DPPC were slightly less sensitive to temperature, reaching 0.02 mN/m at 37°C.
Conclusions: :
The strong elasticity of Meibomian lipids compared to DPPC indicates that DPPC does not represent the mechanical properties of the lipid layer of the tear film. While both monolayers exhibit fluidization as temperature increases, Mebiomian lipids are more sensitive to temperature than DPPC.