Interfacial properties of ocular surface
Keywords: Chemical Engineering.

The ocular surface consists of corneal and conjunctival epithelia covered by a thin, aqueous tear film. A lubricious ocular surface can provide an active barrier against environmental threats. Mechanically, the mismatch between the length scales of corneo-conjunctival epithelium and tear film poses challenges for sustainable lubrication at the ocular surface and dynamic tear film homeostasis. Clinically, lubrication dysfunction plays a significant role in the pathogenesis of many ocular surface diseases, including dry eye disease, which remains the most common reason for ophthalmology visits. However, current treatments and new therapeutic approaches for ocular surface diseases are limited by a lack of fundamental understanding of the lubrication mechanism at the ocular surface and its relation to the pathology of lubrication dysfunction-associated diseases. Despite research efforts since the 1960s, it remains unclear whether boundary lubrication, mediated by epithelial surface molecules (glycoproteins, polysaccharides, and lipids), and hydrodynamic lubrication, mediated by the aqueous tear film, are sufficient to yield a live cell surface with ultra-low friction. This thesis focuses on designing new measurement techniques to investigate the interfacial, adhesive, and tribological properties of live ocular epithelial surfaces and their relations to dry eye disease (DED). Two essential cornerstones for a lubricious ocular surface are a sustainable hydration mechanism and a molecular-morphological design that protects the surface from wear and degradation. A historical and current view of the fundamental lubrication mechanism is presented in Chapter 1. To investigate how the ocular surface sustains constant hydration, the surface tensions of corneal and conjunctival epithelial layers are quantified in Chapter 2. The results showed that the ocular surface exhibits a surface tension gradient stabilized by amphiphilic molecules which ensures constant hydration. Contact angle hysteresis measurements showed that in addition to hydrophilicity, the ocular surface exhibits a high retentive force against fluid films, suggesting a protective mechanism against shear stress-induced tear film dewetting. Contrary to the previous hypothesis, we found that the contact angle hysteresis of corneal epithelium was dominated by its surface morphology rather than its mucin expression. Surprisingly, the high surface tension at the cell-air interface can induce delamination of a stratified corneal epithelial cell layer. Bulk moduli of cell layers were extracted by analyzing stresses at the triple line via Neumann's construction, which showed nonlinearity at high strain values. EDTA treatment showed that the cortical tension of the cytoskeletal components, instead of the adhesive forces at the cell adherens junctions, was the predominant contributor to the viscoelasticity of the epithelial sheets, consistent with previous reports. In addition, a simple model was constructed to demonstrate that a balance among cortical tension, focal adhesion, and cell surface tension is necessary to maintain a mechanically stable ocular surface. To investigate how altered molecular composition of the ocular surface induces DED-related symptoms, Chapter 3 and 4 presents a mucin-deficient cell model, and its shear adhesive properties were investigated with a novel live-cell rheometer. In a subset of dry eye patients, alterations in mucin expression and abnormal glycosylation patterns were found in patients' tear samples. A lack or a reduced amount of mucin molecules on the ocular surface can lead to increased frictional damage. To determine the adhesive and properties of live cell surfaces, we customized a live cell rheometer and performed stress relaxation experiments on a novel in vitro dry eye model that mimics the mucin-deficient dry eye ocular environment. Results showed that mucin-deficient corneal epithelial surfaces exhibited increased adhesive strengths against conjunctival epithelial surfaces. Furthermore, the addition of mucin-like recombinant lubricin molecules rescued the lubrication functions of the ocular surface in a dose-dependent fashion. The results suggested that membrane-bound glycoproteins are sufficient to modify the adhesive properties of ocular surfaces. A simple model was proposed to explain the observed trend in the frictional and adhesive response of live cell surfaces in the presence of additional protein molecules. Overall, this thesis illustrated a framework to measure the interfacial, adhesive, and tribological properties of ocular epithelium that can be adapted to a wide range of epithelial tissues to investigate bio-lubrication and tissue interfacial phenomena