Danielle L. Leiske

The interfacial viscoelastic properties and structures of human and animal Meibomian lipids.

As the interface between the aqueous layer of the tear film and air, the lipid layer plays a large role in maintaining tear film stability. Meibomian lipids are the primary component of the lipid layer; therefore the physical properties of these materials may be particularly crucial to the functionality of the tear film. Surface pressure versus area isotherms, interfacial shear and extensional rheology, and Brewster angle microscopy (BAM) were used to characterize the Meibomian lipids from different species known to have different lipid compositions. The isotherms of humans, bovinae, wallabies, rabbits and kultarrs (a small desert marsupial) were qualitatively similar with little hysteresis between compression and expansion cycles. In contrast, several isocycles were necessary to achieve equilibrium behavior in the koala lipids. With the exception of kultarr lipids, the interfacial complex viscosity of all samples increased by one or two orders of magnitude between surface pressures of 5 mN/m and 20 mN/m and exhibited classic gel behavior at higher surface pressures. In contrast, the kultarr lipids were very fluid up to 22 mN/m; the behavior did not depend on surface pressure. Human lipids were very deformable in extensional flow and the BAM images revealed that the film became more homogeneous with compression as the elasticity of the film increased. The morphology of the kultarr lipids did not change with compression indicating a strong correlation between film structure and behavior. These results suggest that the lipid layer of the tear film forms a gel in vivo, which may aid in mechanically stabilization of the tear film.

Structural and Rheological Properties of Meibomian Lipid

PURPOSE We explore the unique rheological and structural properties of human and bovine meibomian lipids to provide insight into the physical behavior of the human tear-film lipid layer (TFLL). METHODS Bulk rheological properties of pooled meibomian lipids were measured by a commercial stress-controlled rheometer; a home-built interfacial stress rheometer (ISR) probed the interfacial viscoelasticity of spread layers of meibomian lipids. Small- and wide-angle x-ray scattering detected the presence and melting of dispersed crystal structures. Microscope examination under cross polarizers provided confirmation of ordered crystals. A differential scanning calorimeter (DSC) analyzed phase transitions in bulk samples of bovine meibum. RESULTS Bulk and interfacial rheology measurements show that meibum is extremely viscous and highly elastic. It is also a non-Newtonian, shear-thinning fluid. Small- and wide-angle x-ray diffraction (SAXS and WAXS), as well as differential scanning calorimetry (DSC) and polarizing microscopy, confirm the presence of suspended lamellar-crystal structures at physiologic temperature. CONCLUSIONS We studied meibum architecture and its relation to bulk and interfacial rheology. Bovine and human meibomian lipids exhibit similar physical properties. From all structural probes utilized, we find a melt transition near eye temperature at which lamellar crystals liquefy. Our proposed structure for the tear-film lipid layer at physiologic temperature is a highly viscoelastic, shear-thinning liquid suspension consisting of lipid lamellar-crystallite particulates immersed in a continuous liquid phase with no long-range order. When spread over on-eye tear, the TFLL is a duplex film that exhibits bulk liquid properties and two separate interfaces, air/lipid and water/lipid, with aqueous protein and surfactantlike lipids adsorbed at the water/lipid surface.

Molecular Structure of Interfacial Human Meibum Films

Meibum is the primary component of the tear film lipid layer. Thought to play a role in tear film stabilization, understanding the physical properties of meibum and how they change with disease will be valuable in identifying dry eye treatment targets. Grazing incidence X-ray diffraction and X-ray reflectivity were applied to meibum films at an air-water interface to identify molecular organization. At room temperature, interfacial meibum films formed two coexisting scattering phases with rectangular lattices and next-nearest neighbor tilts, similar to the Ov phase previously identified in fatty acids. The intensity of the diffraction peaks increased with compression, although the lattice spacing and molecular tilt angle remained constant. Reflectivity measurements at surface pressures of 18 mN/m and above revealed multilayers with d-spacings of 50 Å, suggesting that vertical organization rather than lateral was predominantly affected by meibum-film compression.

Insertion mechanism of a poly(ethylene oxide)-poly(butylene oxide) block copolymer into a DPPC monolayer.

Interactions between amphiphilic block copolymers and lipids are of medical interest for applications such as drug delivery and the restoration of damaged cell membranes. A series of monodisperse poly(ethylene oxide)-poly(butylene oxide) (EOBO) block copolymers were obtained with two ratios of hydrophilic/hydrophobic block lengths. We have explored the surface activity of EOBO at a clean interface and under 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) monolayers as a simple cell membrane model. At the same subphase concentration, EOBO achieved higher equilibrium surface pressures under DPPC compared to a bare interface, and the surface activity was improved with longer poly(butylene oxide) blocks. Further investigation of the DPPC/EOBO monolayers showed that combined films exhibited similar surface rheology compared to pure DPPC at the same surface pressures. DPPC/EOBO phase separation was observed in fluorescently doped monolayers, and within the liquid-expanded liquid-condensed coexistence region for DPPC, EOBO did not drastically alter the liquid-condensed domain shapes. Grazing incidence X-ray diffraction (GIXD) and X-ray reflectivity (XRR) quantitatively confirmed that the lattice spacings and tilt of DPPC in lipid-rich regions of the monolayer were nearly equivalent to those of a pure DPPC monolayer at the same surface pressures.

Influence of surface rheology on dynamic wetting of droplets coated with insoluble surfactants

Wetting phenomena are crucial to many problems, including wetting and dewetting in the tear film in the eye. Here we introduce insoluble surfactants to the surface of advancing droplets of pure water. Three pure surfactants were chosen with similar Gibbs elasticity but distinct interfacial shear properties. The effects of arachidyl alcohol, a Newtonian monolayer, on dynamic wetting are compared to 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and cholesteryl myristate, which both form monolayers with shear elasticity. In addition, meibomian lipids, a natural mixture of lipids found in the tear film, were also studied. We show that while droplets covered with arachidyl alcohol follow classical hydrodynamics, surface shear elasticity introduces unique behavior including stick–release phenomena at low velocities and non-ideal behavior at higher velocities.

Interfacial shear rheology of highly confined glassy polymers

We present results on interfacial shear rheology measurements on Langmuir monolayers of two different polymers, poly(vinyl acetate) and poly(methyl methacrylate) as a function of surface concentration and temperature. While for the high glass transition poly(methyl methacrylate) polymer we find a systematic transition from a viscous dominated regime to an elastic dominated regime as surface concentration is increased, monolayers of the low glass transition polymer, poly(vinyl acetate), remain viscous even at very high surface concentrations. We further interpret the results in terms of the soft glassy rheology model of Sollich et al. [P. Sollich, F. C. Lequeux, P. Hebraud and M. E. Cates, Phys. Rev. Lett., 1997, 78, 2020–2023] and provide evidence of possible reduction in glass transition temperatures in both poly(methyl methacrylate) and poly(vinyl acetate) monolayers due to finite size effects.

3-Hydroxybutyric acid interacts with lipid monolayers at concentrations that impair consciousness.

3-Hydroxybutyric acid (also referred to as β-hydroxybutyric acid or BHB), a small molecule metabolite whose concentration is elevated in type I diabetes and diabetic coma, was found to modulate the properties of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) monolayers when added to the subphase at clinical concentrations. This is a key piece of evidence supporting the hypothesis that the anesthetic actions of BHB are due to the metabolites abilities to alter physical properties of cell membranes, leading to indirect effects on membrane protein function. Pressure-area isotherms show that BHB changes the compressibility of the monolayer and decrease the size of the two-phase coexistence region. Epi-fluorescent microscopy further reveals that the reduction of the coexistence region is due to the significant reduction in morphology of the liquid condensed domains in the two-phase coexistence region. These changes in monolayer morphology are associated with the diminished interfacial viscosity of the monolayers (measured using an interfacial stress rheometer), which gives insight as to how changes in phase and structure may contribute to membrane function.

Monoclonal Antibody Interfaces: Dilatation Mechanics and Bubble Coalescence

Monoclonal antibodies (mAbs) are proteins that uniquely identify targets within the body, making them well-suited for therapeutic applications. However, these amphiphilic molecules readily adsorb onto air-solution interfaces where they tend to aggregate. We investigated two mAbs with different propensities to aggregate at air-solution interfaces. The understanding of the interfacial rheological behavior of the two mAbs is crucial in determining their aggregation tendency. In this work, we performed interfacial stress relaxation studies under compressive step strain using a custom-built dilatational rheometer. The dilatational relaxation modulus was determined for these viscoelastic interfaces. The initial value and the equilibrated value of relaxation modulus were larger in magnitude for the mAb with a higher tendency to aggregate in response to interfacial stress. We also performed single-bubble coalescence experiments using a custom-built dynamic fluid-film interferometer (DFI). The bubble coalescence times also correlated to the mAbs aggregation propensity and interfacial viscoelasticity. To study the influence of surfactants in mAb formulations, polyethylene glycol (PEG) was chosen as a model surfactant. In the mixed mAb/PEG system, we observed that the higher aggregating mAb coadsorbed with PEG and formed domains at the interface. In contrast, for the other mAb, PEG entirely covered the interface at the concentrations studied. We studied the mobility of the interfaces, which was manifested by the presence or the lack of Marangoni stresses. These dynamics were strongly correlated with the interfacial viscoelasticity of the mAbs. The influence of competitive destabilization in affecting the bubble coalescence times for the mixed mAb/PEG systems was also studied.