Multipurpose solutions (MPS) now make up the vast majority of the market for contact lens care solutions. Contact lens solutions have a complex role in contact lens wear; the name Multi-purpose Solution (MPS) gives an indication of that point. The influence of silicone hydrogel materials can be seen in an increased complexity in MPS composition. An important aspect of this complexity is the choice of surfactant - used either singly or in combination. Three important elements of solution behaviour are influenced by surfactants: Cleaning, comfort and compatibility.
The removal of lipids and proteins from a lens surface by a MPS can be broken down into three distinct phases. • The lowering of the liquid phase surface tension• Emulsification of the deposit• Surface adsorption to prevent re-deposition
The sequence depicted illustrates the fact that effective lipid removal involves several surface chemical functions and is a dynamic process. Simple static surface tension is important for the first step but is not an adequate predictor of overall cleaning efficacy. Cleaning efficiencies of care solutions are difficult to assess due to the multi-factorial nature of the problem, including patient tear chemistry, wear schedule, compliance and complexity of solution compositions.
In contrast, dynamic surface tension has been shown to be a more useful predictor of other and similar cleaning processes. The third step illustrates one aspect of the importance of surface adsorption – which has a major part to play in MPS behaviour. In addition to its role in minimising lipid accumulation, surface adsorption influences comfort (which is advantageous) and destabilisation of the superficial tear lipid layer (which is disadvantageous). Whereas static and dynamic surface tensions are measured directly and relatively simply, assessment of surface adsorption requires measurements at both liquid/air (surface pressure) and liquid/lens (e.g. friction and surface energy). Additionally, it is important to assess the persistence of surface adsorption effects.