The Impact of Material Surface Characteristics on the Wetting Properties of Silicone Hydrogel Contact Lenses.

This PhD project investigated the ramifications of air-cured and nitrogen-cured manufacturing processes during silicone hydrogel contact lens manufacture in terms of lens surface characterisation and clinical performance. A one-hour contralateral clinical study was conducted for ten subjects to compare the clinical performance of the two study lenses. The main clinical findings were reduced levels of subjective performance, reduced surface wettability and increased deposition. Surface imaging showed the nitrogen-cured lens to have a surface typical of a hydrogel material, whereas the air-cured lens had regions of apparent phase separation. In addition, the air-cured lens had a rougher surface associated with greater adherence of contaminants (often observed in materials with reduced polymer cross-linking). Chemical analysis of the study lens surfaces showed no difference in the dehydrated state due to surface segregation of the silicone components. Analysis of frozen lenses limited surface segregation and showed a higher concentration of silicone polymer components and lower concentration of hydrophilic polymer components at the surface of the air-cured lens, in comparison with the nitrogen-cured lens. Contact angle analysis showed the air-cured lenses had consistently higher advancing and receding contact angle measurements, in comparison with the nitrogen-cured lens. In conclusion, clinical assessment of the study lenses confirmed the inferior performance of the air-cured lens. Surface analysis suggested that the non-wetting regions on the air-cured lenses were associated with elevated level of silicone components, reduced levels of polymer cross-linking and polymer phase separation.