HEALTH AND MEDICAL PRODUCTS:CONTACT LENS CLEANING SOLUTION.

CONTACT LENS CLEANING SOLUTION

In humans, a contact lens lies in the conjunctival sac of the eye. In a closed eye, this sac is a slitlike space between the conjunctiva-covered eyeball and the eyelids. Contact lenses are small polymer “bowls” that float on tears superficial to the corneal eye layer and correct existing visual deficiencies similarly to glasses. While the idea of the contact lens was formulated as early as 1508, it was not until the 1800s that contact lenses became a reality. Hard plastic contact lenses were invented around 1936, and although soft lenses were invented in 1960, they were not available on the commercial market until 1971.

The two large groups of contact lenses are hard lenses (hard-flexible; rigid, gas permeable) and soft lenses. Modern hard lenses are made from rigid plastic polymers that readily allow for the movement of oxygen to the eye. They are used to correct irregular corneas and are durable, gas permeable, and require simpler cleaning and care than soft lenses. Soft contact lenses consist of “floppy” plastic composition, including a hydrophobic backbone of either polyethylene or polypropylene and attached hydrophilic chemical chain groups. Soft lenses rest on the cornea of the eye and take on the shape of the cornea. They contain up to 75 percent water and are very flexible, porous, and comfortable, but they require more expensive and time-consuming care and are less permeable to oxygen than hard contact lenses.

The human cornea and lens are both avascular (lacking blood vessels); thus, they require proteins and oxygen supplied by natural tear fluid and oxygen available from the surrounding air. For these reasons, the wear- ing cycle of a lens type depends mostly on the lens permeability to oxy- gen. Lens-wearing schedules include lenses formulated for daily wear (cleaned once a day, removed overnight) and extended/continuous wear (remain on the eye for up to thirty days before they require cleaning). Lens replacement cycles include lenses that can be used for up to two weeks and are then discarded (called disposables) and lenses that last for up to three months (called frequent replacement lenses). Lenses require frequent replacement because of the deposition of lens deposits (including proteins) that may potentially disturb vision, cause deterioration of the lens material, or elicit deleterious allergic and immunological reactions in the eye of the consumer. Removal of these deposits requires nontoxic contact lens solutions formulated for cleaning, disinfection (chemical, thermal, or peroxide based), and enzymatic treatment.

For soft contact lenses, a saline solution consisting of boric acid, so- dium borate, sodium (or potassium) chloride, and preservatives (e.g., sorbic acid, edetate disodium, or polyaminopropyl biguanide) is often used as a lens rinse after cleaning or chemical disinfection, as a solution during thermal lens disinfection treatment, or as a lens solution to dis- solve enzymatic cleaning tablets responsible for removing protein deposits. The solution is often a sterile, pH-buffered, and isotonic aqueous solution that is compatible with the chemistry of human tear fluid. Disinfectant solutions are sterile ophthalmic solutions that frequently contain microfiltered hydrogen peroxide and sodium chloride, are stabilized with phosphonic acid, and pH buffered with phosphates. Lenses are exposed to the disinfectant solution, along with an added neutralizer and enzymatic treatment, to neutralize the tissue-damaging hydrogen peroxide solution before lens wearing and to enzymatically remove protein deposits. Multipurpose disinfecting/cleaning solutions for soft lenses are de- signed to eliminate the need for a separate enzymatic or daily cleaner by allowing lenses to remain clean and free of protein deposits when used on a daily basis. They include ingredients such as gentle cleaners (includ- ing surfactants) that remove daily protein deposits and dirt (e.g., sodium citrate, phosphate buffer, poloxamer 237, hydroxyalkylphosphonate, poloxamine, sodium lauroyl lactylate), disinfecting antimicrobial agents (e.g.,polyquaternium-1, myristamidopropyl dimethylamine, polyaminopropyl biguanide, polyhexamethylene biguanide), lubricants (e.g., hydroxypropyl methylcellulose), and agents that assist in balancing the overall solution for lens-wearing comfort and chemically match natural eye tears (e.g., sodium chloride, potassium chloride, borates [sodium borate], mannitol, edetate disodium, boric acid, sorbitol).

Solutions formulated to clean hard gas-permeable contact lenses are sterile and pH buffered. They are soaking solutions and include lens- hydrating (wetting) agents (e.g., cationic cellulose derivatives) and preservatives (e.g., edetate disodium, chlorhexidine gluconate).