Temperature-induced transitions in the structure and interfacial rheology of human meibum.

Meibomian lipids are the primary component of the lipid layer of the tear film. Composed primarily of a mixture of lipids, meibum exhibits a range of melt temperatures. Compositional changes that occur with disease may alter the temperature at which meibum melts. Here we explore how the mechanical properties and structure of meibum from healthy subjects depend on temperature. Interfacial films of meibum were highly viscoelastic at 17°C, but as the films were heated to 30°C the surface moduli decreased by more than two orders of magnitude. Brewster angle microscopy revealed the presence of micron-scale inhomogeneities in meibum films at higher temperatures. Crystalline structure was probed by small angle x-ray scattering of bulk meibum, which showed evidence of a majority crystalline structure in all samples with lamellar spacing of 49 Å that melted at 34°C. A minority structure was observed in some samples with d-spacing at 110 Å that persisted up to 40°C. The melting of crystalline phases accompanied by a reduction in interfacial viscosity and elasticity has implications in meibum behavior in the tear film. If the melt temperature of meibum was altered significantly from disease-induced compositional changes, the resultant change in viscosity could alter secretion of lipids from meibomian glands, or tear-film stabilization properties of the lipid layer.