Surface pressure measurements of ex-vivo tear lipids:application of the Langmuir technique in lipid-aqueous interfacial studies

This thesis extends previous work utilising the Langmuir trough technique to study tear film lipids towards a new and important area - the effect of contact lens wear on the nature and fate of the lipid layer in the lens-wearing eye. The lipid layer plays a vital part in maintaining tear film stability and the contact lens has a marked influence on the ocular environment. Surface behaviour studies are of particular importance in understanding the physicochemical factors that affect comfort and the occurrence of adverse responses accompanied by the use of this biomaterial device. The measured surface activity (i.e. surface pressure) of individual tear lipids has highlighted the importance of lipid polarity and fatty acid content on the compression and spreading behaviour of the molecule. From this basis, understanding of the behaviour of the whole lipid layer during a blink can be inferred from subsequent studies of ex-vivo tear samples obtained from tear films of individual lens wearers. A particular point of interest in this work is the use of the lens as a probe or carrier to remove lipid from the eye. Differences in key π-A isotherm data were observed due to changes in the collected lipids as a function of lens material and wear modality. It was observed that greater quantities of lipid are deposited as lens hydrophobicity increases. Lipid samples obtained from daily wear and daily disposable lenses showed increased πmax at lower surface concentrations than lipid samples obtained from continuous wear lenses. The potential value of using phospholipids as a supplementative compound in order to increase the surface pressure and stability of native lipid layer. This was examined using a commercial contact lens modified to include an extractable phospholipid. This thesis has examined the use of the Langmuir trough technique to evaluate a variety of factors involved in contact lens wear such as wear schedule, cleaning regimes, lens material and potential phospholipid delivery techniques. These all have potential effects on the surface behaviour and stability of the tear film lipid layer in the lens-wearing eye