With any product—but especially a medical device—safety is the linchpin of success, as it builds the trust and confidence needed for both doctors and patients to give their assent. The FDA is revising its contact lens solution testing procedures in the wake of the solution recalls of the mid-2000s and new lens material development, particularly silicone hydrogels. Current guidelines cover the following five topics: 

(1) lens compatibility—does the solution alter the parameters of the lens? 
(2) cleaning efficacy, determined through the use of Critical Micelle Concentration (CMC)
(3) microbial testing, with the use of predetermined microbes 
(4) toxicology testing, to determine cytotoxicity, systemic toxicity and ocular irritation 
(5) clinical testing, with a minimum of 60 subjects over a period of one month 

In June 2014, we discussed lens compatibility, where the chief issue is how lens materials affect the uptake and release patterns of solution components. This month, we will look at cleaning efficacy.

Keeping it Clean
Contact lens cleaning is done with a surfactant (more fully, a surface-active agent), a bipolar molecule comprised of both hydrophobic and hydrophilic ends. These polar ends interact with debris or the lens surface and the aqueous tear film, respectively.  

In surfactant-based cleaning products, the hydrophobic ends cluster around debris on the lens to form small clusters known as micelles. The free hydrophilic ends then react with water to help remove the micelles from the lens surface, simultaneously cleaning and moisturizing the contact lens. This process is very similar to what happens when we wash our hands with soap. The molecular weight of the surfactant chain, block copolymer size, solvent composition and temperature affect how tightly it will bind to the lens surface and how micelles are formed. The strength of the bond will determine how long the surfactant will remain on the lens, thus affecting wettability.

The critical micelle concentration (CMC) refers to the concentration above which micelles form and all additional surfactants added to the system form micelles. In effect, CMC can be considered a measure of surfactant efficacy. For example, a lower CMC indicates that less surfactant is needed to saturate interfaces and form micelles. 

Concentrations of surfactants higher than the CMC are needed to achieve micelle formation; in so doing, they provide a reservoir of additional surfactant molecules to form micelles. These micelles solubilize and disperse soils, leading to detergency. CMC values provide a valuable guideline for comparing surfactant detergency. 

The major chemical disinfectants in lens care products on the market are biguanides (primarily poly-hexyl-methyl-biguanide, or PHMB), polyquaterium-1 (Polyquad) and peroxide. While hydrogen peroxide is an effective disinfectant, it is not a surfactant; such agents are added to the system to help with cleaning and wettability.

PHMB is a cationic disinfectant that has antimicrobial efficacy against both gram-negative and gram-positive bacteria, as well as some effect against Acanthamoeba.¹ It works using the electrostatic interaction of cationic sites of PHMB with anionic sites of the bacterial cell membrane, causing the agent to bind to the cell membrane. These interactions result in the disturbance of the membrane structure and the leakage of intracellular components, thus destabilizing the organism.

Polyquad is a cationic surfactant larger in size than PHMB. Polyquad also affects the phospholipid membrane but, because of its molecular size, has a reduced uptake and release pattern in contact lenses and cases.² Uptake and release patterns of surfactant disinfectants affect corneal staining patterns and comfort responses during lens wear.

It was previously believed that CMC is a critical parameter for surfactants to be useful as cleaners. It is uncertain at this time, however, if the FDA plans to use this information or if any changes will be made to the next cleansing protocol.   

1. Imayasu M1, Uno T, Ohashi Y, Cavanagh HD. Effects of multipurpose contact lens care solutions on the adhesiveness of Acanthamoeba to corneal epithelial cells. Eye Contact Lens. 2009 Sep;35(5):246-50. doi: 10.1097/ICL.0b013e3181b4d152.
2. Willcox MD1, Phillips B, Ozkan J, Jalbert I, Meagher L, Gengenbach T, Holden B, Papas E. Interactions of lens care with silicone hydrogel lenses and effect on comfort. Optom Vis Sci. 2010 Nov;87(11):839-46. doi: 10.1097/OPX.0b013e3181f3e2fc.