Introducing Water Gradient Technology
Daily disposable contact lenses with water gradient technology represent a new era in contact lens wear.
By Ralph Stone, PhD
|Dr. Stone is one of the foremost experts on contact lens care product design and formulation. A winner of the prestigious Donald Korb Award from the American Optometric Association, Contact Lens & Cornea Section, he is often referred to as the father of modern contact lens care. Dr. Stone is a former VP of research and development for Alcon.|
Contact lens materials have evolved from PMMA lenses of the 1950s to oxygen permeable rigid lenses of the 1980s and beyond. The introduction of hydrogel materials in the early 1970s brought new levels of comfort. Disposable lenses in the 1990s evolved further with the introduction of silicone hydrogels. Material and manufacturing technologies have made possible daily disposable lenses, something that would have been unthinkable in the early days of soft lenses.
As materials evolved, it became useful to find a way to group these materials based on fundamental characteristics including water content and the presence of ionic ingredients for hydrophilic materials and by the hydrophobic components for the rigid hydrophobic materials1. By 1994, FDA listed 34 different soft contact lens materials and 95 brands and 39 hydrophobic contact lenses.1 These grouping systems served the industry well through the both the hydrophobic lens and hydrogel lens eras, but with the introduction of silicone hydrogel materials, the system had to be expanded to include these biphasic materials.
A New Era
Since the introduction of silicone hydrogels in 1998, these materials have become the dominant choice of fitting practitioners.2,3 Today these materials are considered to be a separate class of hydrophilic contact lens materials and experts from around the world are evaluating if additional groups are needed.4 The latest introduction, Dailies Total1 (delefilcon A) water gradient daily disposable contact lenses by Alcon Laboratories, represents a landmark in the continuing revolution in contact lens materials that underscores the need for a new grouping system, because this material is truly in a class by itself as the first water gradient silicone hydrogel contact lens.
Dailies Total1 daily disposable contact lenses have a revolutionary structure based on a silicone hydrogel core and a water gradient surface that creates a transition from core water content of 33% to a surface water content in excess of 80% at the interface with the tear film. The surface of the lens, like the corneal epithelium and glycocalyx, has a brush-like structure that binds water in this region of the lens. A water gradient is created by cross-linked polymeric wetting agents that form a soft, hydrophilic surface gel, which is embedded into the core, creating a smooth transition from the core to the surface. The water gradient provides the lowest silicone surface content of any silicone hydrogel lens material.
Figure 1. Schematic representation of Dailies Total1 water gradient contact lenses.
Figure 2. Atomic Force Microscopy cross-section image of the Dailies Total1 contact lens.
Figure 1 shows a representation of this novel approach. 5 Figure 2 shows an Atomic Force Microscopy (AFM) image of a cross section of the lens demonstrating the change in material properties from the core to the surface. With a center thickness of 90 microns, the water gradient surface represents slightly more than 6% of the central lens thickness on each side.6
The color change in the AFM image represents a change in water content, modulus and chemical composition that is not seen in any other lenses tested. The water gradient provides a lubricous lens surface that is also substantially softer than the core of the lens.6
With the development of materials and lens care systems over time, we have recognized that we can solve some of the issues of contact lens lenses and lens wear with improved technologies. The understanding of the role of oxygen and transmission through the lens has led to the creation of lenses that do not cause oxygen deficiency at the cornea. It is now recognized that enhanced oxygen maintains corneal function and decreases edema and other changes due to hypoxia. Dailies Total1 lenses have a central Dk/t of 156 at -3.00 diopters. This is the highest of the currently available daily disposable contact lens materials. Table 1 shows a comparison of Dk and central Dk/t of several daily disposable lens materials.
Oxygen has not solved all contact lens wear issues. Despite advances made in lens materials to date, it has been reported that nearly 15% percent of patients will stop wearing contact lenses every year.7 The most common reasons reported for discontinuing wear are discomfort and dryness.8-11
Enhancing Comfort: Modulus, Wettability, Lubricity
Three primary approaches have been suggested to enhance comfortable wear of contact lenses: providing a low modulus hydrogel surface, maintaining wettability throughout the day, and minimizing the friction of the lid crossing the front lens surface.
The water gradient of Dailies Total1 lenses address the modulus challenge by providing a soft, cushioning layer on the outer surface of lens. This can be measured using an AFM modulus scan (Figure 3).5
In addition to the modulus gradient observed in the AFM scan, there is an ultra-soft surface gel on the outermost surface of the lens. The high water content at the surface provides an extremely wettable interface with the tear film. Measurements by Menzies and Jones12 showed Dailies Total1 lenses had lower advancing and receding contact angles, as measured by a sessile drop, than other tested daily disposable contact lenses with the exception of nelfilcon A (Dailies Aqua Comfort Plus contact lenses).
Wettability is only one measure of ways to help keep contact lenses comfortable. Brennan reported that lubricity as measured by coefficient of friction is highly correlated to comfort (r=0.90, p<0.01).13 Korb and colleagues14 reported that there is a correlation (p<0.00001) between lid wiper epitheliopathy and dryness symptoms associated with wear of contact lenses. In studies of 105 symtomatic and asymptomatic lens wearers, Korb and colleagues14 found 80% of symptomatic patients showed the presence of lid wiper staining compared to 13% of asymptomatic patients within the first 4 hours of wear.
|Comparison of Oxygen Permeability and Transmissibility of Daily Disposable Lens Materials|
|Delefilcon A (Dailies Total1 contact lenses, Alcon)||140*||156|
|Narafilcon A (Acuvue TruEye, Johnson & Johnson Vision Care — not available in US)||100||118|
|Narafilcon B (US version Acuvue TruEye, Johnson & Johnson Vision Care)||55||65|
|Etafilcon A (1-Day Acuvue Moist, Johnson & Johnson Vision Care)||28||33|
|Omafilcon A (Proclear 1 Day, CooperVision)||33||36|
|Nesofilcon A (Biotrue, Bausch + Lomb)||42***||42|
|*Alcon data on File 2010
**Dk/t calculated using manufacturer’s published center thickness for -3.00 diopter lenses when available. Others as published in Tyler’s Quarterly
***Data from FDA 510(k) K113703 Summary June 6, 2012
|Comparison of Ocular Physiology* and Surface Appearance of Two Silicone Hydrogel Daily Disposable Lenses and Dailies Total1 Daily Disposable Lenses with a Water Gradient Surface.|
|Dailies Total1 Contact lenses (delefilcon A)||Clariti 1-Day Contact lens (7ettab II 3)||p value compared to Dailies Total1 lenses||Acuvue TruEye Contact lenses (narafilcon A)||p value compared to Dailies Total1 lenses|
|Corneal Staining (0-10,000)||25+54||38+ 70||p<0.01||74+117||p<0.01|
|Conjunctival Staining (0-100)||4+5||30+12||p<0.01||10+8||No difference|
|Graded wettability compared to Dailies Total1 lenses||.27||p<0.01||.34||p<0.01|
|Difference in observed deposition compared to Dailies Total1 lenses||.31||p<0.01||.13||p<0.01|
|*Mean plus/minus standard deviation|
The novel water gradient surface of the Dailies Total1 lenses provides a highly lubricious surface. Contact lenses can provide hydrodynamic lubrication during the fastest part of a blink as the tear film is maintained between a contact lens and the eyelid. At lower ocular movement speeds, however, a contact lens is in direct contact with the ocular tissues resulting in much higher friction. The soft, high water surface of the Dailies Total1 lenses acts as a boundary lubricant during these low speed ocular movements providing excellent lubricity (low coefficient of friction). Measurements by Sawyer and co-workers15 found that the coefficient of friction of Dailies Total1 lenses was comparable to or even lower than the coefficient of friction of epithelial cells. Additional qualitative and quantitative methods have been used to estimate the lubricity (or its inverse the coefficient of friction) and have demonstrated the highly lubricious nature of the Dailies Total1 lenses.16When measuring the lubricity or friction of Dailies Total1 lenses it is important to utilize pressures similar to those found in the eye (<7 kPa) because the surface of the lenses is so soft that higher pressures could actually crush the surface giving erroneous test results.17
Figure 3. Modulus comparison of a cross section of Dailies Total1 contact lenses.
Of course the presence of a water gradient, high oxygen permeability, low surface modulus, enhanced surface wettability and low friction/high lubricity are only really meaningful if they impact clinical performance. Keir and colleagues18 reported recently on a clinical trial with 104 subjects using Dailies Total1 lenses and two competitive silicone hydrogel daily disposable lenses. While the other daily disposable lens tested were acceptable, Dailies Total1 lenses showed the least impact on ocular physiology and the least change in lens surface appearance when evaluated after 8 hours of wear. The results are shown in Table 2.
In a separate report by Varikooty and colleagues,19 the subjective comfort of 53 enrolled symptomatic patients from this study was evaluated. The comfort was measured at 4, 8 and 12 hours. At each time point Dailies Total1 lenses was numerically superior to other silicone hydrogel daily disposable contact lenses.
Dailies Total1 lenses provide a new concept in contact lenses combining the benefit of a high permeability oxygen silicone hydrogel core and a water gradient transition from the low water silicone hydrogel core to a high water content surface. This new technology is made possible by the next generation of Alcon’s patented LightStream Lens Technology. The high water surface provides a low modulus, highly wettable and lubricious surface. This revolutionary technology does not fit the current classification approaches for either the conventional hydrophilic lens materials or the biphasic silicone hydrogels, and stand alone as a novel approach to contact lens chemistry. CLS
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17. Angelini TE, Dunn AC, Uruena JM, Ketelson H, Sawyer WG. Stress induced frictional transitions in cross-linked surface gels. Invest Ophthalmol Vis Sci 2012;53(E-abstract 6113).
18. Keir NJ, Varikooty J, Richter D. Evaluation of lens surface appearance and ocular physiology with three silicone hydrogel daily disposables. Cont Lens Ant Eye 2012; BCLA abstract 15.
19. Varikooty J, Keir N, Richter D, et al. Subjective comfort with three silicone hydrogel daily disposables in symptomatic contact lens wearers. Cont Lens Ant Eye 2012; BCLA abstract 21.