Surface wettability (i.e., wetting angle or contact angle) describes how a liquid spreads across a surface. It is the relationship of the liquid’s surface tension with the contact lens material. A high wetting angle (greater than 90°) means that the surface does not wet well and has many dry areas. Conversely, a lower wetting angle (less than 90°) means that moisture is more evenly dispersed across the surface. A surface active agent, or surfactant, decreases surface tension, significantly reducing dry areas.
The Lens Material
While wettability describes how a liquid spreads, hydrophilicity determines the strength of attraction that a liquid has to the lens surface. Hydrogel contact lenses are made up of complex polymeric matrices that contain hydrophobic and hydrophilic ends. As a contact lens surface dries out, the hydrophilic ends move inward toward the center of the contact lens inner matrix to seek moisture and the hydrophobic ends turn outward toward the dry lens surface. Silicone contact lenses have dry, non-wetting areas due to the physical properties of the silicone itself. Because of the extremely hydrophobic nature of silicone, it preferentially attracts hydrophobic substances, such as lipids, onto the lens surface.
Adding a Wetting Agent
Simply adding an aqueous solution will not rewet the lens surface adequately because the aqueous component will be pushed away by the hydrophobic polymer ends. A surfactant agent is required to wet the surface of both hydrogel and silicone lenses, so the moisture is retained on the lens surface. A moist lens surface is required for physiology, comfort, deposition prevention and optical clarity.
A surfactant has both hydrophilic and hydrophobic ends to its structure. In the case of surfactant cleaning products, the hydrophobic ends cluster around debris to form micelles. The free hydrophilic ends are able to react with water, and then the micelle can be whisked off the lens surface. This also is what happens when we wash our hands with soap. In the case of a surfactant wetting agent, the hydrophobic end interacts with the dry hydrophobic lens surface, allowing the lens surface to regain hydrophilicity.
The Role of Biocides
Many of the organic biocides in current use are based on molecules having a relatively limited variety of functional groups, which impart varying degrees of hydrophilicity to the molecules in which they are incorporated. In addition to these functional groups, broad-spectrum biocides may require hydrophobic elements in order to penetrate biological membranes and achieve their full potency. Thus, many organic biocides have both a lipophilic and hydrophilic component.
Biocides need to provide a desired balance between lipophilicity and hydrophilicity to produce desirable results. In contact lens solutions, this balance can be highly challenging due to the extreme sensitivity of the ocular tissue. Disproportionately hydrophobic biocides can be adsorbed onto the surfaces of contact lenses or lens cases, which may cause wearer irritation and/or result in biocidal compromise. On the other hand, biocides that are excessively hydrophilic typically exhibit insufficient antimicrobial activity altogether.
Adding polypropylene oxide and polybutylene oxide (EOBO) to Polyquad/Aldox (Opti-Free Puremoist, Alcon) imparts a preferred balance of hydrophobic and hydrophilic qualities. This allows the solution to retain biocidal efficacy, while reducing the undesirable attributes of hydrophobicity. In designing this new solution, the relative ratio of EOBO was chosen for its particularly strong affinity and positive interactions with silicone (e.g., keeping the lens clean, increasing wettability and decreasing the wetting angle).
Understanding the interaction between surface wetting, wetting agents, biocides and lens materials helps us prescribe solutions appropriately to patients—keeping them comfortable and loyal to our practices.