Contact Lens Design & Materials
The Technology Behind Daily Disposable Contact Lenses
BY RONALD K. WATANABE, OD, FAAO
Contact lenses usually feel the best on the first day, even if they are cleaned and disinfected properly. Also, clean lenses are less likely to cause microbial- or deposit-related complications. In addition, without the added potential reactions to care solution components, daily disposable lenses have been ideal for sensitive patients.
The first generation of daily disposable (DD) lenses consisted of low-oxygen-permeable traditional hydrogel materials, which was fine for most patients and didn’t cause clinically significant complications. But with newer hyper-Dk silicone hydrogel (SiHy) materials, the original hydrogel DD lenses could be considered a compromise as far as oxygen is concerned. And, patients needing toric or multifocal designs originally could not use DD lenses due to design limitations. Fast forward to today, and it seems that DD lenses are leading the way in soft lens technology.
With the advent of SiHy materials, oxygen has been minimized as a contact lens problem. For years, DD lenses were not available in a high-Dk SiHy option. But now, there are several high-Dk DD options. In fact, many of the newest high-Dk soft lens materials are being developed for the DD segment; of the two lenses with the highest available Dk value, one is a DD. Even non-silicone-based materials are being developed with very high water contents to provide good oxygen transmission without having to deal with the inherent hydrophobic properties of silicone. Gone are the days when we would have to try to convince patients that DD lenses are their best option when a monthly or biweekly lens can deliver significantly more oxygen.
Manufacturers are constantly working to improve contact lens comfort, especially at the end of the day. One of the biggest factors for contact lens dropouts is discomfort, and discomfort is often equated with “dryness” (Nichols KK et al, 2013). What constitutes a feeling of dryness with contact lens wear is complicated. Most efforts to decrease dryness aim to maintain as wettable a lens surface as possible. By maintaining a wet surface, the lens is more slippery, which allows the lids to slide over the lens with less resistance or friction. With less resistance working against the lids, the lens feels more comfortable (Jones et al, 2013).
All soft lenses start out with excellent wettability, but over time, surface wetness decreases due to evaporation and other factors. Lens developers have come up with different ways to try to maintain a wet lens surface throughout a patient’s wear time. One example of novel surface technology is the use of a surfactant on the lens surface that resists dehydration, similar to the lipid layer of the tear film. Another example is the binding of a layer of highly hydrophilic polymer chains to a SiHy core. Yet another way to maintain comfort is to infuse the lens matrix with wetting agents. The latest example of this is an electrolyte-balanced wetting agent that mimics the composition of tears; this aims to be more compatible with the eye and, therefore, more comfortable. SiHy materials have also been developed with decreased silicone content that can maintain high Dk while allowing more moisture to be retained in the lens matrix.
Many of the latest material innovations are being developed in the DD category; they should be considered a first-line option for more of our patients. CLS
For references, please visit www.clspectrum.com/references and click on document #242.
Dr. Watanabe is an associate professor of optometry at the New England College of Optometry. He is a Diplomate in the American Academy of Optometry’s Section on Cornea, Contact Lenses and Refractive Technologies and is in private practice in Andover, Mass. You can reach him at firstname.lastname@example.org.