The Development of Dailies Total1 Water Gradient Contact Lenses
Research led to a departure from using a single bulk material for the whole lens.
|Dr. Pruitt is Project Head, Biocompatibility Projects for Alcon Vision Care R&D. He received his PhD in Chemical Engineering from Brigham Young University.|
|Dr. Bauman is project head for Dailies Total1 projects within Vision Care Research & Development at Alcon Laboratories, Inc. He received his doctor of optometry degree with honors from the Southern California College of Optometry in 1982 and an MBA from Coles College of Business in 2001.|
By John Pruitt, PhD and Erich Bauman, OD, FAAO
The development of the delefilcon A lens material that makes up Dailies Total1 contact lenses began more than 10 years ago and involved a dedicated multi-national team of scientists, engineers and clinicians. Early in the development, the team had many intense discussions about what lens properties should be targeted in order to create the most comfortable contact lens possible. The team quickly recognized the many limitations and trade-offs inherent in relying on a single bulk material.
By way of analogy, it is known that the cornea is comprised of different tissue layers, each with their own unique anatomical structure and physiological function. When integrated into one organ, these layers perform multiple functions, for example, light refraction, mechanical support and physiologic protection of the eye. When selecting a target water content, the R&D team recognized from our experience with silicone hydrogel materials that a low water content material with high levels of silicone would provide excellent oxygen transmissibility, good handling characteristics and resistance to dehydration. Conversely, from our experience with (non-silicone) traditional hydrogel materials, we knew that extremely high water content materials without silicone could provide excellent wettability, lubricity, and resistance to lipid fouling.
The R&D team recognized that an extremely low modulus may make a lens easier to fit and help provide improved comfort but at the cost of decreased lens handling performance. There also seemed to be a limit to how soft one can make a lens material. The modulus of corneal epithelial cells1 is less than 0.02 MPa and it seemed impossible to make a contact lens material as soft as that without it simply falling apart.
Figure 1. Cross section illustration of Dailies Total1 water gradient contact lens
Following the paradigm of using multiple materials as in the cornea, we realized that some of these properties, such as oxygen transmissibility and lens handling, could be controlled by a lens ‘core’ material while other properties like wettability and lubricity were only important at the surface. A lens with only a single water content, modulus or chemistry would always entail some level of compromise. The team arrived at the question: “why should patients have to settle for the same water content at the core and surface, or indeed the same material chemistry at the core and surface of a lens?” This led to a radical departure from single bulk material thinking and formed the basis for the development of the delefilcon A water gradient lens material.
A measurable change can be demonstrated in the lens material, the water content and the modulus of delefilcon A from core to surface. This represents the first-ever water gradient contact lens designed to feature an increase from 33% to more than 80% water content from core to surface. To enable this water gradient, the delefilcon A lens material transitions from a highly breathable silicone hydrogel material at the core to a non-silicone hydrophilic polymer structure at the surface (Figure 1). This enables a lens with a Dk/t of 156 (at the center of a -3.00D lens) combined with a surface-water content of over 80%,1 as seen in Figure 2. A gradient occurs between the two areas in which the water content rapidly increases and the material shifts from a silicone-rich core material to an essentially silicone-free surface gel.2 This surface material forms an ultrasoft surface gel and makes up about 10% of the lens thickness. While the average water content of the ultrasoft surface gel exceeds 80%, the water gradient structure allows the water content to approach almost 100% at the outermost surface of the lens.3
Several laboratory techniques have measured this change in lens material properties including Atomic Force Microscopy (AFM), Neutron Reflectometry, and Fluorescent Laser Confocal Microscopy.2 These methods demonstrate the gradient in modulus and other lens properties across the lens cross section, not seen in other lens materials.
Figure 2. The delefilcon A lens material transitions from a highly breathable silicone hydrogel material at the core to a non-silicone hydrophilic polymer structure at the surface, allowing for a lens with a Dk/t of 156 (at the center of a -3.00D lens) combined with a surface-water content of more than 80 percent.
The modulus of the lenses also changes with the material becoming much softer at the surface of the lens with the outer surface having a compression modulus of only about 0.025 MPa.4 The modulus of the lens core is maintained at 0.7 MPa for excellent insertion handling. The surface modulus of delefilcon A is almost as soft as the corneal epithelial cells1,5 yet this ultrasoft surface gel is able to maintain its integrity because it is supported by the lens core material. In a similar manner the ultrasoft corneal epithelial cells are supported by collagen fibrils, giving the cornea an overall bulk modulus that is much higher than the modulus of the individual cells.
Highly Breathable Daily Disposable Lens
Why do we need such a high Dk/t for a daily disposable contact lens? There are several factors to consider. Stated Dk/t values (and even theoretical flux or equivalent oxygen estimations) are normally provided only for the center of a -3.00D lens. The greater peripheral lens thickness means oxygen transmissibility is lower with minus powers, but in plus powers, the central oxygen transmission values will be lower than the stated value for -3.00D. Figure 3 illustrates this with the color oxygen maps across the whole lens in different materials and powers. The blue end of the spectrum is used to indicate high Dk/t and red shows areas of lower Dk/t. It has become apparent that a single Dk/t value isn’t sufficient to completely characterize the oxygen transmission in contact lenses. Furthermore, the published Dk/t values for the lenses only represent a small portion at the center of a -3.00D lens. The peripheral oxygen transmissibility is generally much lower as seen in the colored maps. Research demonstrates that peripheral oxygen transmission is equally important to ocular health as that in the center.6 In addition, different patients have different oxygen demands, some of which may not be predictable during a routine examination, even when using a slit lamp.7 Thus, starting with the highest available central Dk/t value is in the best way to avoid hypoxic concerns and meet the needs of various patient lifestyles.
Oxygen transmissibility in delefilcon is largely determined by the core of the lens because it comprises the majority of the lens thickness. Of particular interest are the unique properties at the surface of the lens, because these are key to a comfortable lens-wearing experience.
Figure 3. Oxygen transmissibility in delefilcon is largely determined by the core of the lens because it comprises the majority of the lens thickness.
The Importance of Surface Lubricity
The comfort of a contact lens may be influenced by many factors ranging from the modulus of the material, lens thickness, and water content to lens design and parameters. The surface coefficient of friction, or lubricity,8 has been shown to have a high correlation with lens comfort scores. Lubricity is the inverse of friction and, for a contact lens, is described as how easily the components of the ocular surface, such as the palpebral conjunctiva, can slide across the lens surface. We blink about 14000 times per day.9 With each blink, the superior lid has to slide down, then back up over the lens surface. As such, it makes sense that lubricity is highly predictive of lens comfort. Lubricity can be detected with the fingers as a slippery feeling and it can be measured by using either an inclined plane or a micro tribometer.10 Regardless of the method used to measure lubricity, it is important that the pressures reflect those found in the ocular environment — in other words, matching those exerted by the eyelid against the lens on eye). This is especially important when measuring the lubricity of delefilcon A because the extremely soft water gradient surface structures can be artificially crushed if measured at pressures that exceed those found in the eye thereby giving erroneous lubricity results at high testing pressures.11
Using kinetic coefficient of friction, measured by the inclined plate method, delefilcon A has been shown to have extremely low friction (excellent lubricity).5
A new contact lens with different core and surface properties meant that detailed research was needed to optimize the chosen design parameters. Numerous studies were conducted to co-optimize the base curve, diameter and lens design. Ultimately, the combination of an 8.5mm base curve with a 14.1mm diameter was selected to give optimal centration and lens movement. The full technical specifications and range of parameters can be seen in Table 1.
|Dailies Total1 Parameters & technical specifications|
|Material Surface Water Content (%)||Delefilcon A <80%|
|Base Curve (mm)||8.5|
|Center Thickness (mm) (-3.00D)||0.09|
|Core Water Content (%)||33%|
|Dk/t||156 @ -3.00D|
|Core Modulus (MPa)||0.7|
|Packaging||5 (trials), 30 pack, 90 pack|
|Handling Tint||VISITINT lens|
|Manufacturing||Newest generation of LightStream Technology enables the creation of the water gradientLenses contain phosphatidylcholine, an ingredient also found in natural tears.|
|Power Ranges (at launch)|
|Diopters||-0.50 to -6.00 (in 0.25 steps)
-6.50 to -10.00 (in 0.50 steps)
Exceptional Comfort Throughout the Day
The outcome of this superior lubricity is outstanding wearer comfort through the end of the day. In a clinical study with 104 subjects, cumulative comfort scores were superior for Dailies Total1 contact lenses in comparison with other silicone hydrogel daily disposable lenses.12 In a group of 53 symptomatic subjects, 100% of them could wear Dailies Total1 contact lenses for at least 8 hours and 85% were able to wear them up to 12 hours. In comparison to their habitual lenses, the majority of the subjects were able to wear Dailies Total1 contact lenses for clinically significant longer periods of time.13
The Start of a New Era
Since soft contact lenses were first introduced, there have been numerous incremental changes to materials to improve water retention and wearer comfort. Ciba Vision introduced the first silicone hydrogel contact lens in 1998, ushering in a new era in lens material technology that has triggered extensive steps to improve oxygen transmission for patients worldwide. The creation of the first water gradient contact lens, featuring an increase from 33% to over 80% water content from core to surface, marks the start of yet another new era in contact lenses, and with it, hope for a new era in comfort for contact lens wearers around the world. CLS
Editor’s note: A similar article appeared in the April 5, 2013 edition of Optician.
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1. Straehla J, Limpoco F, Dolgova N, Keselowsky B, Sawyer WG. Tribology Letters 2010;38(2):107-113.
2. Thekveli S et al. Structure-property relationship of delefilcon A lenses. BCLA Annual Clinical Conference, 2012.
3. Angelini TE, Nixon RM, Dunn AC, Uruena JM, Pruitt J, Sawyer WG. Viscoelasticity and mesh-size at the surface of hydrogels characterized with microrheology. Invest Oph Vis Sci 2013;54(E-Abstract 500).
4. Dunn A, Urueña J, Huo Y, Perry S, Angelini T, Sawyer WG. Lubricity of surface hydrogel layers. Tribology Letters 2013;49(2):371-378.
5. Rudy A, Huo Y, Perry SS, Ketelson H. Surface mechanical and tribological properties of silicone hydrogels measured by atomic force microscopy. Invest Oph Vis Sci 2012;53(E-Abstract 6114).
6. Papas E, Willcox M. Reducing the consequences of hypoxia: the ocular redness response. Contact Lens Spectrum, special edition 2006; pages 32-37.
7. Bonanno JA, Nyguen T, Biehl T, Soni S. Can variability in corneal metabolism explain the variability in corneal swelling?. Eye Contact Lens 2003;29(1 Suppl):S7-S9;discussion S26-29, S192-194.
8. Brennan NA. Contact lens-based correlates of soft lens-wearing comfort. Opt Vis Sci Abstract 90957, November 2009.
9. Inoue K, Okugawa K, Amano S, et al. Blinking and superficial punctate keratopathy in patients with diabetes mellitus. Eye (Lond). 2005;19(4):418-421.
10. Subarraman L, Jones LW. Measuring friction and lubricity of hydrogel contact lenses & A review. Contact Lens Spectrum; 2013 (in press).
11. Angelini TE, Dunn AC, Uruena JM, Ketelson H, Sawyer WG. Stress induced frictional transitions in cross-linked surface gels. Invest Oph Vis Sci 2012;53(E-abstract 6113).
12. Keir N, Richter D, Varikooty J, Jones L, Woods C, Fonn D. End of day comfort using a novel cumulative comfort score. Invest Oph Vis Sci 2012;53(E-Abstract 4728).
13. Varikooty J, Keir N, Richter D, Jones L, Woods C, Fonn D. Subjective comfort with three silicone hydrogel daily disposables in symptomatic contact lens wearers. BCLA Annual Clinical Conference, 2012.