Surfactant interaction and persistence at the contact lens surface

Abstract

The interaction of poloxamer and poloxamine (Pluronic® and Tetronic®) surfactants with hydrogel and silicone hydrogel contact lens surfaces is of interest for this thesis. The persistence of surfactant molecules at the lens surface can indicate how long the surface has been modified. It is therefore important to observe and characterise the surface and surfactant behaviour separately. Characterisation of the contact lenses was carried out through dehydrated sessile drop measurements and surface energy calculations. Silicone-containing materials tended to be most hydrophobic regardless of water content. Static and dynamic surface tension measurements were used to assess the surfactants and the critical micelle concentration was also observed. Pluronics® and Tetronics® do not behave as simple low molecular weight surfactants; their structure and size mean they are less mobile in solution and may be able to form mono molecular micelles. Surfactants with different molecular structure, molecular weight and hydrophobicity were used to observe how these properties affect surface tension behaviour and influence surfactant persistence. The aim of the work was observe the persistence of surfactants at the lens surface, any difference between the surfactant persistence, and the possibility to predict surfactant persistence on a lens. The ex vivo work presented here shows little distinction between surface tension measurements over time or between treated and untreated materials. It is not possible to measure in vivo surfactant persistence with surface tension techniques and therefore necessary to create in vitro models to assess surfactant behaviour. A simplified in vitro eye model was created to assess preliminary observations. These results and observations were used to progressively alter the model and create a more ‘eye-like’ system. Large hydrophobic Tetronics® were most persistent at the lens surface; hydrophobic drive was considered the most influential factor. In addition to this, the contact lens material and condition prior to surfactant treatment also had an effect on persistence. Materials containing PVP showed increased surfactant persistence, which was increased further when the lenses were dehydrated prior to surfactant treatment. Lens dehydration had no effect on persistence if PVP was not present in the lens material