From Surface to Solution: Zeta Potential Analysis of Contact Lenses and Cleaning Agents

Contact lenses are worn by many people worldwide for a variety of reasons. Users often cite convenience for sports and other physical activities or aesthetic benefits, and lenses also provide a wide, unobstructed field of view. They are particularly useful when there is a large difference in vision between the two eyes.
Among the many types of contact lenses, soft lenses made from different hydrogel materials are the most widely used. Silicone hydrogel lenses have grown in popularity due to their high wearing comfort and easy handling. This class of materials was specifically engineered to be permeable to oxygen, overcoming the limitations of previous lens materials. Silicone hydrogel lenses are available as both daily disposable and reusable models. [1]
For reusable models, thorough daily cleaning is especially crucial. During several hours of wear, proteins from the tear fluid buildup on the lens surface, which can dull the lens and negatively affect its vision correction performance. Protein deposits may also alter the surface wettability, making the lens less hydrophilic and indirectly affecting oxygen transport by changing how the tear film spreads and moves across the lens. [2]
This is where zeta potential measurements of contact lenses gain relevance. Dynamic streaming potential measurements on the SurPASS 3 instrument can accurately characterize the surface zeta potential and, thus, the hydrophilic or hydrophobic nature of contact lenses. Lens hydrophilicity is a key factor in the degree of protein adsorption, because positively charged tear proteins such as lysozyme are strongly attracted to negatively charged surfaces. [3]
The composition and performance of the cleaning solution are also highly important. For optimal removal of contaminants, peroxide-based solutions are commonly recommended. In these systems, the contact lens is transferred into the solution, and the storage container includes a platinum catalyst that neutralizes the peroxide overnight. This reaction generates gas bubbles, which further help to loosen and detach protein residues from the surface.
Electrophoretic light scattering (ELS) measurements can be used to assess the cleaning performance of the peroxide solution and, in particular, its capability to remove proteins from the lens surface. The underlying assumption is that the zeta potential of a protein-loaded solution differs significantly from the zeta potential of the pristine solution. The magnitude of this zeta potential then serves as an indicator for successful protein removal. The Litesizer DLS is an ideal solution for fast and reliable ELS measurements, thanks to its patented cmPALS technology.