contact lens materials
Thinking Inside the Blister
BY NEIL PENCE, OD, FAAO
Traditionally, soft contact lenses have been packaged in a buffered saline solution. Solutions may have varied as to the buffer used or the tonicity, but nearly all contained simply buffered saline.
Recently, however, this has begun to change. A number of soft contact lenses now come packaged in a solution that contains various polymeric materials.
Blister Pack Additives
The term "polymer" can cover a wide range of materials. The polymeric materials of interest as packaging solution additives are generally water soluble and are typically referred to by their acronyms. The most common are polyvinyl alcohol (PVA), polyvinyl pyrrolidone (PVP), and methylcellulose, which in general have absorptive, film forming, and lubricating properties. The purposes served by these polymeric additives can fall into several categories including conditioning the surface of a contact lens by retaining moisture or preventing the lens from sticking to the package or to itself.
Probably the first time most of us learned about such additives was when speakers, authors, and others began referring to the "wetting agent" that Vistakon added to the packages of its silicone hydrogel lenses. Acuvue Advance and Acuvue Oasys packages contain a type of methylcellulose, which more commonly appears in various artificial tear lubricating products. The concentration in tear products is generally in the 0.25-to-0.50% range, whereas the lens packages contain a much lower concentration added to the buffered saline (0.0005% methyl ether cellulose). Vistakon says that this additive is meant to prevent lens sticking when stored for long periods in various positions.
Bausch & Lomb also added a small amount of PVA to the SofLens 38 package to prevent the lens from sticking or folding.
CIBA Vision has employed polymeric additives in Air Optix Aqua and Air Optix Night & Day Aqua lenses, both of which contain 1% copolymer 845 for its moisture retaining properties.
Daily disposable lenses have experienced the greatest activity with regard to packaging solution additives and eluting or internal wetting agents within the lens. The recently introduced SofLens Daily Disposable (B&L) contains a poloxamine conditioning agent in the packaging solution. In this case, the intent is more to utilize the film or coating properties of the polymer and its ability to attract and retain moisture.
CIBA Vision's Focus Dailies with AquaRelease lenses contain PVA within the lens and possibly in the buffered saline solution. FreshLook One-Day packages contain 0.02% poloxamer 108. CIBA added hydroxypropyl methylcellulose (HPMC) to Dailies AquaComfort Plus as a wetting agent to the saline, which may contain some polyethylene glycol (PEG) as well. This lens also incorporates PEG and PVA into the material.
Likewise, Acuvue 1-Day Moist (Vistakon) lenses have PVP embedded in the lens during manufacturing, but not necessarily added to the buffered saline.
CooperVision does not utilize polymeric materials in their packaging solutions for any products sold in North America. To date the company has not found a need to add wetting agents to its products or has not found additives that offer significant benefit over buffered saline solution.
Look for Future Developments
Adding polymers to the buffered saline solution of contact lens packages is relatively new. It's not clear whether the moisturizing effects simply benefit initial application comfort or if they have longer lasting benefits. Increasing experience may help us further understand their benefits. Judging from the number of recent products that utilize such additives, we can probably expect to see more activity in this area in the future. CLS
Dr. Pence is director of the Contact Lens Research Clinic, Indiana University School of Optometry in Bloomington, Indiana. He is a consultant or advisor to B&L, CIBA Vision, and Vistakon, and has received research funding from AMO.