contact lens materials
The Evolution of Contact Lens Wetting Agents
BY KATHY DUMBLETON, MSC, MCOPTOM, FAAO, & LYNDON JONES, PHD, FCOPTOM, FAAO
Without question, the biggest challenge to contact lens wear remains the elusive goal of longer "comfortable" wearing times. Lens manufacturers have begun incorporating a number of additives into lens materials and packaging solutions in an attempt to improve end-of-day comfort.
Additives in Daily Disposables
Daily disposable lenses were the first products to use these "additive" technologies. In the Focus Dailies (CIBA Vision, nelfilcon A) family of lenses, the original lens material was created from cross linking functional, polymeric polyvinyl alcohol (PVA) (Buhler et al, 1999). Adding additional non-functionalized PVA into the material created a slow-release system in which PVA is gradually released into the tear film and acts as a wetting agent (Winterton et al, 2007). In the latest iteration of this technology (Dailies AquaComfort Plus), hydroxypropylmethylcellulose (HPMC) and polyethylene glycol (PEG) are also added to the packaging solution. The HPMC is added to aid comfort on application, and the PEG associates with the PVA within the lens matrix after which both products are "squeezed out" by the blinking process throughout the day to aid lens wettability (Winterton et al, 2007).
In 1-Day Acuvue Moist (Vistakon, etafilcon A), polyvinylpyrrolidone (PVP) is firmly embedded into the etafilcon A (pHEMA + methacrylic acid) material and assists with surface wettability and comfort enhancement (Veys and Meyler, 2006).
Bausch & Lomb's SofLens Daily Disposable (hilafilcon B) utilizes the surfactant poloxamine in its packaging solution to assist with lens comfort (Mack, 2009).
Hyaluronic Acid as an Additive
Hyaluronic acid (HA), which is commonly used as a contact lens wetting agent (Szczotka-Flynn, 2006), has also been used as a lens additive in traditional hydrogel materials (Atkins, 2008). Incorporation of this lubricant into hydrogels helps reduce deposition and enhances surface wetting properties (Van Beek et al, 2008). A two-week replacement and a daily disposable lens with an HA additive have become available in Europe (Safegel, Safilens). In these lenses, the HA is reportedly released into the tear film during lens wear to maintain lubrication. (Atkins, 2008).
Additives in Silicone Hydrogels
While silicone hydrogel lenses reportedly provide longer periods of comfortable wear than traditional hydrogels do, late-day discomfort is still reported by some wearers. Consequently, some manufacturers are using additives in their silicone hydrogels. Air Optix Aqua (CIBA, lotrafilcon B) and Air Optix Night & Day Aqua (CIBA, lotrafilcon A) have 1% copolymer 845 (PEG and PVP) added to their packaging solution. This additive is designed to release throughout the day similar to the Dailies AquaComfort Plus additives to enhance lens comfort.
PVP has also been incorporated into Acuvue Advance (Vistakon, galyfilcon A) and Acuvue Oasys (Vistakon, senofilcon A) as well as into the new silicone hydrogel daily disposable lens Acuvue TruEye (Vistakon, narafilcon A). The PVP is firmly bound into the material and acts as an internal wetting agent.
More manufacturers will likely explore the potential benefits of bound or released additives, not only to improve comfort, but possibly also to provide antimicrobial properties and even drug delivery benefits (Jones, 2009). CLS
For references, please visit www.clspectrum.com/references.asp and click on document #167.
Dr. Dumbleton is a senior clinical scientist at the Centre for Contact Lens Research in Ontario, Canada. Dr. Jones is the associate director of the Centre for Contact Lens Research and a professor at the School of Optometry at the University of Waterloo, Ontario, Canada. He has received research funding from Alcon, AMO, B&L, CIBA Vision, CooperVision, Johnson & Johnson and Menicon.