SLIPPING CLEANING FLUID into one's eye is not likely to be high on anyone's list of hobbies, but contact lens wearers do it daily. After spending the night in cleaning solution, a dripping contact lens is inserted into the eye—ideally, without causing any discomfort.
Historically, putting lens cleaning solution in the eye wasn't an option. Twenty years ago, cleaning contact lenses was generally a multistep process requiring a cleaner, a disinfecting agent, saline for rinsing off the cleaner or disinfecting agent, and an occasional enzyme tablet to remove protein buildup from tear fluid.
But the more steps there are, the more likely wearers won't follow them correctly, or at all—increasing the likelihood of sight-threatening infections. Teenagers just starting to use contact lenses are particularly likely not to comply with complicated systems, says Srini Venkatesh, senior director for lens care product development at Bausch & Lomb.
In response, manufacturers developed one-step systems for the soft lenses that dominate the market today. These systems come in two types, peroxide and multipurpose solutions. Both contain cleaners, such as bisphosphonate compounds, to break down proteins adhering to lenses or surfactants, such as block copolymers, to interfere with the protein-lens interactions. The solutions also typically contain moisturizing or conditioning agents such as cellulose, propylene glycol, or polyvinyl pyrrolidone, ingredients that Ciba Vision's global head of research, Lynn Winterton, hopes will make users say, "Wow, this feels great!" There are also buffers to maintain an eye-friendly pH and preservatives to maintain shelf life.
The difference between the peroxide and multipurpose solutions lies in how they disinfect. Peroxide systems, as their name would indicate, use peroxide as the disinfection agent. They typically involve two components: a 3% peroxide solution and a neutralization catalyst. While the peroxide does a fine job of killing whatever microbes might have latched onto lenses, it's not very eye-friendly, so a platinum, palladium, or silver catalyst is used in the contact lens case to completely neutralize the solution by reducing the peroxide to form oxygen and water.
The peroxide neutralization reaction itself, as well as the O2 product bubbling through the solution, may further help to kill microbes, Venkatesh says. Typically, a lens wearer puts their lenses in the case with the peroxide solution and catalyst, and returns four to six hours later to find their lenses ready to wear.
Peroxide, of course, will oxidize many things other than microbes, so one tricky aspect to solution formulation is to find cleaners or conditioning agents that can survive storage in a peroxide solution without degradation.
In contrast to peroxide solutions, multipurpose solutions typically use a polymeric disinfection agent such as polyhexamethylene biguanide or polyquaternium, which incorporates quaternary ammonium centers. As with peroxide solutions, the disinfectant presents the biggest challenge to solution developers, although for a different reason: The solution, including the disinfectant, must be gentle enough that lens wearers can put their lenses directly into their eyes without rinsing.
The polymers, therefore, are a compromise. They're made from monomeric building blocks that are effective at killing microbes but are too harsh to go in the eye in that form. Using polymers instead of monomers provides effective antimicrobial activity while still being gentle to the eye.
One active area of contact lens solution research, Winterton says, involves finding ways to eliminate preservatives from solutions so there's one less thing going into the eye. Another is to develop dual-purpose conditioning agents that not only will keep lenses comfortable but will also repel microbes.
Venkatesh notes that contact lens solutions offer chemists unique challenges. The solutions have six or seven ingredients whose compatibilities must be balanced, but overall more than 95% of the content is water. In addition to being mindful of chemical interactions, solution chemists must also be cognizant of ingredients adsorbing to bottles and pipes. "When you start at a really low concentration, if it drops even by 10% you can have huge differences in performance," Venkatesh says. "There's an art to developing solutions and a lot of skill."